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1 AM Antenna Systems W.C. Alexander Director of Engineering Crawford Broadcasting Company Atmospheric noise, natural and manmade, Abstract Over the years, the collective affect the signal-to-noise ratio at the knowledge of AM antenna systems and the receiver, but it does not affect the level of principles behind them has faded somewhat. signal arriving from the transmit antenna. Most of the tomes used by the last AM antenna systems are vertically generation of AM consulting engineers date polarized . This is done for a number of back to the 1940s and are no longer in print. reasons, including superior groundwave It is the purpose of this paper to propagation and simplicity of antenna provide the interested reader with a general systems. The downside of vertical body of knowledge of AM antenna systems. polarization is that most atmospheric noise is also vertically polarized. Still, vertical AM Antenna Basics 1.0 polarization is a better choice for AM The purpose of any AM antenna is to broadcast than horizontal and virtually all radiate the power generated by the AM radiators are vertical. Not only are transmitter. Some antennas do this better horizontal dipole antennas mechanically than others, and there are many ways to get impractical, their radiation on the horizon is a signal into the air. not nearly as good as that of a vertical Non-directional antennas radiate radiator. Since it is necessary to erect two equally in all directions, providing the relatively tall towers to support a horizontal simplest way to get a signal out in an dipole, why not just drive one of the towers efficient manner. Directional antennas are directly and forget about the other tower and used to concentrate signal in some directions dipole? (toward population centers, for instance) Earlier, we mentioned that several while suppressing signal in others (toward things influence the amount of signal other stations which must be protected from received at a particular point from a given interference). antenna system. The first of these was the The antenna system is the last point amount of radiation toward the receiver. The in a broadcast system where the broadcaster amount of radiation toward a particular has any control over the signal. After that point is influenced by the transmitter power, point, environmental factors, receiver system losses, antenna efficiency and characteristics and other factors have sway antenna directivity. Transmitter power is over what the listener hears. The amount of self-explanatory. System losses come in signal received at a given point is dependent several areas — resistive losses in on the amount of radiation toward that point conductors, ground system and tuning from the antenna, the distance to the components, and transmission line losses. receiver, the conductivity of the earth Antenna efficiency is really defined in two between the transmit and receive locations, ways: One has to do with the vertical the character of the terrain between antennas radiation characteristics of the antenna; the and, sometimes, the ionospheric conditions. other has to do with the radiation resistance.

2 AM Antenna Systems Alexander/2 We’ll look at both of these in more detail used internationally, the reason for this later. The efficiency of a non-directional having to do with treaties that predate the AM antenna is expressed in millivolts per adoption of the current set of US meter at one kilometer (mV/m/km), and this groundwave curves. We previously mentioned the figure is referred to as the inverse distance efficiency expressed as the inverse distance field IDF . or field of a non-directional antenna. You have Another factor that influences the probably already figured out that the amount of signal received at a particular conductivity of the ground in the region point is attenuation. Over perfectly between the antenna itself and the receive conducting earth, the amount of signal point 1 km away will cause the field received at a distance would be inversely strength at that point to be attenuated below proportional to the distance from the what it would be over perfectly conductive transmit antenna. This relationship is known earth. How, then, can one accurately as the inverse distance rule. For example, if measure the efficiency of an antenna? The at a distance of 1 km a field strength of 100 answer is with many measurements taken mV/m is present, at 2 km the field strength radially, beginning very close to the antenna would be 50 mV/m. At 4 km, the field (usually at the point where the first on-scale strength would be 25 mV/m, and at 8 km it reading can be taken). The very close-in would be 12.5 mV/m. If you were to graph measurements establish the unattenuated this relationship as field strength versus IDF while measurements farther away from distance on log-log graph paper, it would the antenna establish the conductivity of the plot as a straight diagonal line. ground between the antenna and the last In the real world, the earth is not point measured. Ground conductivity perfectly conductive. varies from very good (seawater) to very Non-Directional Antennas 2.0 poor (rock and certain soils). The more Non-directional antennas can come conductive the ground is, the less a signal in several forms, but by and large these are from an AM antenna will be attenuated and simple vertical radiators. One type is base the more the field strength versus distance insulated and series-fed; the other is plot will resemble the inverse distance line. grounded base and shunt fed. We’ll look at Over ground that is less conductive, the both these types in detail later. more a signal from an AM antenna will be electrical length Ideally, the of an attenuated and the more the field strength AM antenna will be 90 electrical degrees versus distance plot will curve away from (1/4 wavelength) or more. Antennas of this the inverse distance line. length provide adequate efficiency and is groundwave curves A family of bandwidth. Sometimes, though, aeronautical published by the FCC for each group of or structural considerations force frequencies, showing the effects of different broadcasters to use shorter towers. The ground conductivities. These curves are the apparent electrical length of short towers basis for predicting distance to a field can be increased through the use of top strength and thus the entire US allocation loading . Top loading increases the system. A slightly different set of curves is

3 AM Antenna Systems Alexander/3 capacitance to ground, and is usually signal radiated above the horizon. While such an antenna may appear to be more (a flat, achieved through use of a top hat efficient than a shorter, non-sectionalized horizontal disk attached to the top of the antenna, the spherical (total) radiation in tower) or using bonded guy wires. both antennas will be the same for a given Because of mechanical amount of input power, assuming all losses considerations, the use of top hats is not as are the same. Sectionalization simply puts common as other methods. The top hat must the signal where it is needed, toward the usually support its own weight and horizon, in much the same manner as a withstand wind, ice and other environmental multi-bay FM antenna achieves antenna hazards unsupported, so top hat size (and “gain.” thus effectiveness) is limited. By far, the most common method of top loading is through use of bonded guy 3.0 Current Distribution An insulated, non-top loaded tower wires. This method uses sections of guy wires bonded to the top of the tower that are (or current loop will typically have a usually bonded above the first insulators to maximum) 90 electrical degrees down from adjacent guy wires. Often, other non- the top of the tower. If the tower is shorter structural guys are added and bonded to long, the current loop will occur at ° than 90 structural guys to increase the effectiveness the tower base. Current distribution on a of the top loading. It is not unusual to see six single, insulated, uniform cross-section or more guy wires bonded together in a radiator will be more or less sinusoidal in “spider web” fashion in a top loading nature and is approximately defined as arrangement. follows: The advantages of top loading are increased base resistance, reduced base = i I G y sin( ) − A A capacitive reactance, lower Q and improved bandwidth. While all this sounds very = current in amperes at height y w here: i A attractive, it is almost always better to I = Maximum current in amperes A achieve these qualities with increased tower G = Tower height in degrees height rather than top loading. y = height in degrees of current Sectionalization is a method of element increasing the groundwave efficiency of a vertical radiator, improving groundwave There is always a (or current node performance and reducing skywave minimum) and voltage loop at the top of any radiation. In a sectionalized tower, an tower that does not employ top loading. As insulator is placed near the center of an we move down the tower, the voltage will electrically long radiator and a network is decrease and the current will increase in an placed between the sections. In simple approximately sinusoidal fashion until a terms, the current in the upper section can be current loop and voltage node occur at the adjusted to be in phase with the signal in the point 90 ° below the top of the tower, if the lower section, thus focusing the signal tower is greater than 90 ° tall, or at the tower radiated toward the horizon and reducing the base if it is shorter than 90 ° . At the point

4 AM Antenna Systems Alexander/4 where the voltage or current nodes occur, insulated bracket/handle so that all the voltage or current does not pass through measurements can be made with the loop the zero but rather reach minimum values and same distance from the tower leg. in phase in ° shift approximately 180 Vertical Radiation Characteristics 4.0 traversing the node region. On a tall tower Generally speaking groundwave ), more than one node will ° (greater than 180 radiation (and apparent efficiency) from a occur along the tower’s length. vertical radiator will increase as the current Many things influence current loop moves up from the base. The optimum distribution on a tower. Cross-section, electrical length of a vertical radiator is 225 ° uniformity, and nearby conductors are or 5/8 wavelength. At this electrical length, among a few of these. In some cases (which current loops is occur at 45 and 135 degrees we will explore in detail in the future), there above the base. Radiation on the horizon is are actually two currents flowing on a maximized and radiation above the horizon particular frequency on a tower — the is minimized. current that contributes to radiation and the Shorter towers have more radiation current that is induced from another nearby above the horizon and thus produce more radiator. This is the norm in directional skywave radiation and less groundwave arrays, where a tower will have current flow radiation. Towers considerably shorter than from its own excitation and current flow from radiation arriving from other elements ° produce so much radiation above the 90 in the array. The current distribution on horizon that much of the power is wasted towers such as these is sometimes hard to into space. Nighttime power is usually much predict using conventional methods. more limited when using an electrically For most purposes, we assume short radiator, although considerably more sinusoidal current distribution on a radiator, daytime power may be allowed as a result of and it usually works fairly well. Modern the reduced groundwave efficiency. computer modeling using moment method The vertical radiation characteristic analysis can, however, do an excellent job of of a vertical radiator that is not top loaded or predicting the current flow and distribution function of theta sectionalized, or the , is on a radiator and this gives the designer a defined as follows: much better picture of what is happening. Knowing the current distribution is − G ) cos( G cos sin θ ( f = ) θ important to the vertical radiation ) ( cos − cos θ G 1 characteristics of an antenna. Knowing where the current loop is on an antenna is essential when detuning a radiator for the purpose of eliminating reradiation. Occasionally, it is beneficial to where: measure the current distribution on a tower. f( θ ) = function of theta To do this, a small sample loop is G = height of the antenna in constructed out of copper tubing or degrees aluminum angle and fitted with some sort of θ = vertical angle

5 AM Antenna Systems Alexander/5 determine how much skywave field a This equation returns a multiplier by particular station would produce at a given which the inverse distance field of an location. The design engineer would find the antenna is multiplied to find the radiation at appropriate vertical angle for the distance to a particular vertical angle ( ). For example, θ the receiver, find the function of theta using if a tower is 90 electrical degrees tall, has an skywave multiplier this formula, find the inverse distance field of 300 mV/m at one from the FCC formula and multiply that by km and you wish to find the radiation from ), or the radiation at the pertinent θ /, E(0 the antenna at a vertical angle of 20 degrees azimuth and vertical angle from the station. above the horizon, the above formula gives I mention all this now only to show the need us a function of theta or multiplier of 0.914. to know the function of theta in some Multiplying this by the antenna’s circumstances. groundwave inverse distance field of 300 One way an electrically short tower mV/m, we find that the radiation at 20 can be electrically lengthened is using top degrees above the horizon is 274.3 mV/m at loading . If a tower employs top loading, the one km. function of theta is computed as follows: This information would be used in engineering a nighttime allocation to B A B A A B ( cos ) sin ( sin sin sin ) sin ( cos cos ) θ θ θ − − + f ( ) = θ − + B A B [cos )] θ ( cos cos where: electrical height based on current ) = function of theta f( θ distribution achieved through top loading, A = the physical height of the tower the function of theta is higher than the same in degrees electrical height achieved without top B = the difference, in degrees, loading. between the apparent electrical Another type of antenna we briefly height (based upon current discussed in Part 1 was the sectionalized distribution) and the actual tower. Sectionalization is used to increase physical height the groundwave efficiency (and reduce θ = vertical angle skywave radiation) by placing an insulator near the center of an electrically long If, for example, a particular tower is radiator and controlling the current flow on only 60 degrees tall (A) but employs top each section with a network between the loading that makes it seem 30 degrees taller sections. For sectionalized towers, there is (B) based upon current distribution, and we yet another formula to determine the wanted to find the function of theta for a function of theta: vertical angle of 20 degrees, the above formula yields 0.923. Note that for the same

6 AM Antenna Systems Alexander/6 θ θ θ θ θ C B A ∆ − + − − ∆ D C D B G A sin sin cos [cos sin ] cos ) sin cos( [cos sin )] sin ) sin sin ( sin ) cos cos ( ( = f ( ) θ G B B D + cos ∆ cos [sin (cos θ cos ) sin (cos − )] − ∆ where: A = the physical height, in electrical degrees, of the lower section of the tower B = the difference between the apparent electrical height (based on current distribution) of the lower section of the tower and the physical height of the lower section of the tower C = the physical height of the entire tower, in electrical degrees D = the difference between the apparent electrical height of the tower (based on current distribution of the upper section) and the physical height of the entire tower. D will be zero if the sectionalized tower is not top loaded. G = the sum of A and B (A + B) H = the sum of C and D (C+D) = the difference between H and A (H-A) ∆ towers are fed across the base insulator. This By way of example, if we have a sectionalized tower that has a lower section type of tower is said to be , since series fed that is 120 electrical degrees tall (A) and the the excitation is, in essence, fed in series current distribution makes the lower section with the tower base. seem as if it is 20 degrees taller than that Occasionally, it is advantageous to (B), the overall height is 220 electrical use a grounded base tower as an AM degrees tall (C) and top loading of the upper radiator. Such circumstances may include section results in current distribution that mounting of an FM or other antenna on the makes the tower seem as if it is 15 degrees tower, use of an existing grounded-base taller (D) and we want to find the function tower for AM or, from time to time, of theta for a vertical angle of 30 degrees, proximity of the tower base to a populated the above formula yields 0.593. This building or structure. In these cases, it is formula is, obviously, quite cumbersome possible to use a grounded base tower with and difficult to solve using a pocket good results as an AM radiator if the guy calculator and paper. However, it lends itself wires are all insulated. easily to codifying into computer language The base impedance of a grounded- or a programmable calculator. base tower is essentially zero, but that impedance rises with height above the base. At some location up the tower (assuming it Insulated and Grounded Towers 5.0 By and large, the vast majority of is of adequate height), a point will exist that AM antennas consist of insulated base will provide an acceptable feed impedance. towers. Almost all directional arrays use The easiest way to feed a grounded base insulated base towers. There are several tower is with a slant wire , which is attached advantages to using an insulated base tower, at the aforementioned point and returns to chief of which is control of the current the transmitter building at an angle distribution on the tower. Insulated base approaching 45 degrees. This forms what is

7 AM Antenna Systems Alexander/7 essentially half of a “delta” match. The tower base, and this is where the excitation location of the best attachment point for the is applied. This type of grounded base slant wire is usually determined by a cut- . folded monopole antenna is called a and-try method, although experience on the The skirt wire conductors form the part of the field engineer and modern outer conductor of a transmission line. Were computer modeling techniques can point to the short located at the quarter-wave point, it a starting point that should be close to the would transform the short at that end to an desired impedance. In most situations, if the open at the other end, creating a virtual base feedpoint is properly selected, the only insulator at the bottom of the tower. By matching needed will be a series capacitor to adjusting the location of the short between cancel out the inductive reactance of the the skirt wires and the tower, the impedance slant wire. at the feed point can be adjusted to a Because the excitation in a slant wire favorable value. This arrangement forms fed grounded base tower is shunted across what is essentially a “gamma” match. the grounded tower base, this type of Current flow in a folded monopole is antenna is said to be up on the skirt wires and down on the tower . The current shunt fed structure. Such antennas perform much like flow is up the slant wire and then on up the base insulated towers. Radiation current tower to the top, where a current node and flows up the skirt wires to the tower and voltage loop will exist. Some current, then on up to the top of the tower, where a however, also flows down the tower from current node and voltage loop will exist. the slant wire feed point and contributes to radiation. Because there are two radiated fields below the feedpoint (one from the 6.0 Base Impedance slant wire and the other from the tower The base impedance of an below the feedpoint), some suppression of insulated-base tower is determined primarily radiation occurs, usually on the side of the by the electrical length of the antenna, cross- tower where the slant wire is located. section, the extent of the ground system and Seldom is this a problem, however, and the the elevation of the feed point above ground. antenna is still considered to be a non- Short towers have much lower base directional radiator. resistance, and the reactance becomes quite Another way to feed a grounded base capacitive. When base resistance is quite tower is to mount an insulated skirt on it low, the fixed ground loss of one to three consisting of three or more wires suspended ohms becomes a significant part of the off the tower on insulators and parallel to radiation resistance of the antenna. This, in the length of the tower. The skirt wires are part, is the reason that short towers are less bonded together at the top of the tower and efficient than tall towers. then bonded to the tower itself at a point that Taller towers have higher base produces a desired driving point resistance, and at some point (usually around 80 electrical degrees), the reactance , usually somewhere below the impedance crosses over and becomes inductive. There 90 degree point (0.15 wavelengths is a is some benefit to selecting a radiator with common location for this bond). The skirt close to a zero reactive component in its wires are also bonded together near the

8 AM Antenna Systems Alexander/8 system as is commonly used in one form or base impedance when designing an antenna another in all AM broadcast antenna system. Empirical tower base impedance systems. data is available and published in many While it is certainly possible to use places, both in tabular and graph format. materials other than copper in a ground Through the use of moment method system, copper is by far the best computer modeling, the base impedance of compromise for both performance, many different tower configurations can be durability and economy. Copper is used accurately predicted. This is particularly almost exclusively for this application, so useful with tapered or free-standing towers, for the remainder of this discussion, we will where the cross-section is great and the assume bare, soft-drawn copper wire to be parallel capacitance of multiple base the material of choice. insulators is significant. In grounded-base shunt-fed towers, the feed point impedance is more dependent 7.1 Ground Currents on feed point location. Since this is usually Did you ever wonder what was the at least to a degree within the control of the return path for all that current flowing in user, a desirable base impedance can often your AM tower? These currents leave the be obtained. This impedance often has a antenna as displacement currents, flow relatively large reactive component; the through space and finally into the ground, at resulting Q is usually higher and bandwidth which point they become conduction lower than that of an insulated tower of the currents. Due to skin effect, these currents same height. usually flow very close to the surface as they flow radially back to the antenna base. With copper ground radials in place, 7.0 Ground Systems Beneath every AM antenna system is the is made up of two parts: ground current a One part of this current flows through the (or at least there is ground system earth itself; the remainder flows in the supposed to be). The ground system is every buried wires. This can generally be viewed bit as important to the operation of an AM as a parallel resistive circuit. As the ground antenna as the tower that is the vertical current flows inward through the ground or radiator. the wire toward the antenna base, it is Without a copper (or otherwise continually added to by additional conductive) ground system, the losses in a displacement currents from the antenna that vertical AM antenna are very high. These are flowing into the ground. Because these losses are largely due to the conduction of additional currents differ in phase from the currents through the earth, which at best has components already flowing in the ground a high resistance. By placing a number of and buried wires, the ground currents do not wires in the earth from near the tower base necessarily increase. to radial points some distance away, we provide a relatively low resistance path for the ground currents to return to the ground 7.2 Ground Losses in the vicinity of the tower base. These It has been established through radial wires make up the classic ground studies and years of experience with such

9 AM Antenna Systems Alexander/9 result of these experiments: systems that the ground currents at a distance are proportional to an antenna’s The distribution of currents depends • field strength at one kilometer, or the on the wire size in a logarithmic inverse distance field (IDF). In these studies, fashion, making the size of wire used power was fed to a vertical radiator and the relatively non-critical actual current flowing in the buried wires at The losses in a ground system are • a distance from the tower base was inversely proportional to the number measured. The results further established of radial wires used that the current flowing in the ground radials • The resistance of an antenna of a at distances greater than 0.3 wavelengths given height is reduced as the from the tower base remain relatively number of radial ground wires is constant for varying heights of antennas increased with a constant input power. This indicates • The reactance of an antenna varies that the power lost in the ground at distances only slightly with differences in the greater than 0.3 wavelengths will generally ground system be the same, regardless of the antenna • The efficiency of an antenna is height. increased with the number of ground On the other hand, close to the radial wires used, with 120 wires antenna, ground currents of electrically short being the optimum balance between antennas become quite large. Knowing that cost and efficiency the power lost in a conductor is directly The optimum length of a ground • proportional to the resistance of that system is 140 electrical degrees conductor, the importance of maintaining a The presence of a ground screen in • good array of ground conductors within a the vicinity of the antenna base quarter wavelength or so of a short antenna makes no difference in antenna is apparent. It may be surprising to you that resistance or efficiency the losses in a ground system for a 1/4 A buried radial ground system • wavelength tower can easily reach 3 dB or functions equally well as the same more if many radials are missing, cut or number and length of radials deteriorated. The shorter the tower, the more installed above the surface of the pronounced the ground system losses will ground. be. This is one reason that electrically longer antennas are more desirable than short ones. It was largely as a result of these experiments that the standard non- The “Standard” Ground System 7.3 directional AM antenna ground system was Many years ago, experiments were defined as 120 radials, 90 electrical degrees conducted to determine the effects that the in length, composed of #10 soft-drawn bare number, size and length of radial ground copper wire buried 6" to 8" below the wires had on the antenna resistance and field surface. With performance being roughly strength at a distance (efficiency) of AM equal between surface and subterranean antennas of varying heights. Some systems, it is more desirable to bury the interesting conclusions were reached as a ground system in order to protect it from

10 AM Antenna Systems Alexander/10 element arrays often flow in a spiral fashion damage. toward the individual tower bases. In the The conclusion regarding the early 1990s, a complex rooftop ground ineffectiveness of the copper screen system for a five-tower directional array was notwithstanding, a copper screen 25' to 50' constructed in grid rather than radial square (or 120 interspersed 50' radials) in fashion. The resulting efficiency and the vicinity of the tower base is also part of stability of this unusual and unconventional the standard ground system. The purpose of ground system proved to be quite the screen or additional short radials is to satisfactory. stabilize the resistance of the antenna and the capacitance across the base insulator with changing ground conditions due to 7.5 Lightning Protection weather, moisture content and the like. A buried copper radial ground Experience has shown that a screen or system, while very effective as part of a additional radials close in are very effective vertical antenna system, is seldom at stabilizing an antenna’s impedance. satisfactory as a mechanism to dissipate the For directional antennas with energy from lightning that strikes a tower. multiple towers, the same basic ground Individual radials can be burned in two at system elements are used, except that where the point of connection to the tower base radials from different towers intersect, they strap when large lightning currents flow are terminated into and bonded to a through them. A separate, large conductor (0 AWG or larger) wire is needed to discharge . For example, in a transverse copper strap strike currents. This conductor should be two-tower array with 1/4 wavelength connected to an array of at least four eight- spacing, a transverse copper strap would be foot copperweld ground rods near the tower installed halfway between the two towers base pier. and all the radials that would intersect the Insulated-base towers need some other tower’s radials would terminate onto method of discharging static across the base the transverse strap. insulator, such as a that static drain choke has a high impedance at the RF operating 7.4 Current Flow frequency but a very low impedance to DC. As was mentioned earlier, in a Towers that use sample loops at tower single-element AM (non-directional) potential must use sample line iso-coils to antenna, ground currents return to the tower isolate the loops from the sample line at the base radially. In the case of multi-element operating RF frequency, and such iso-coils directional arrays, displacement currents serve well as static drain chokes. will arrive at every point on the surface of Static drain chokes, while serving to the ground from each element of the array, keep dangerous static potentials from and all these currents components will all building on an insulated tower, are not at all have different phases. The current flow in effective at dissipating lightning strike such a multi-element array will not, then, be currents. In fact, they present a very high entirely radial in nature. impedance to the ultra-fast rise times of a It has been shown in recent typical lightning current. A spark gap of experiments that ground currents in multi-

11 AM Antenna Systems Alexander/11 is cut and the wire end is pushed back into some sort is needed directly across the tower the furrow. base insulator to provide a path for such In this manner, the actual installation lightning currents. Gaps of this sort most of a radial ground system can proceed quite often consist of two horizontally separated rapidly. A typical crew can install a full 120- galvanized steel balls, with an air gap radial system for a single tower in about one between them, located just below the base day, assuming a clean site and that all the plate of the tower. “Horn” type gaps are surveying/marking of the radial endpoints used in other installations, and from time to has been done. time, a “needle” gap is used. Both the ball It is important for the engineer in and horn type gaps are of a design where charge of the construction and maintenance any arc across the elements is self- of a station to observe the installation of the extinguishing. As the sustained arc climbs ground system. As we have previously farther and farther out on the gap, the established, the performance of an AM spacing gradually increases until a point is antenna system is directly related to the reached where the voltage across the gap is condition of the ground system. Mistakes less than the breakdown voltage of the air made in the installation process will be between the points where the arc is around long after the installation crew is occurring. When that point is reached, the gone and the performance of the station will arc is extinguished. suffer as a result. Gap spacing is usually set by trial The selection of a ground system and error, but a starting point can be installation crew should be made carefully calculated. The installation, maintenance and not necessarily be based upon the lowest and adjustment of spark gaps will be dealt bid. Choose a reputable contractor with with in a later chapter. good (and recent) references. Carefully review the amount of materials needed and keep track of the material as it is used in the 7.6 Ground System Installation installation. Copper wire, screen and strap Ground radials are usually installed are costly and can disappear quickly from a by use of a specially adapted plow. Such site. Spot check individual radials by implements are equipped with a spindle for digging into the furrow and locating the the spool of wire and a tube or conduit that wire. More than once, such spot checks have conveys the wire from the spool to a point revealed an empty furrow. beneath the surface just behind the plow At the tower base, the ends of the blade. The end of each radial is secured to ground radials are terminated into a copper the copper strap or ring of copper tubing at strap or piece of heavy copper tubing. One the tower base and then the plow blade is method is to build a square frame around the lowered into the ground at a point ten feet or tower base pier out of wooden two-by-fours, less from the tower base. It is then pulled then to lay a length of four-inch copper strap radially away from the tower to the desired over the wood. A galvanized roofing nail is radial endpoint previously established by then partially driven into the wood through survey and marked with a stake. When that the copper strap for each radial, and the end point is reached, the plow is raised, the wire

12 AM Antenna Systems Alexander/12 Many installations use a copper strap of the radial is wrapped around the nail. from every tower to every other tower and Once all of the radials are plowed in, the to the transmitter building ground system. radial ends are silver-soldered to the copper While there is nothing wrong with doing strap and the nails are either removed or this, it is generally redundant and driven all the way in. The straps from the unnecessary. The outer jacket of the underside of the tower base insulator are transmission line feeding each tower serves also silver soldered to this strap. well as a ground strap between the tower It is important to insure that silver and transmitter building or phasor. There is solder, not tin/lead solder, is used where no real reason to connect the tower bases making bonds in a ground system. It will together with a strap, as the ground currents quickly deteriorate if buried, and the are carried adequately by the radials and the mechanical strength of tin/lead solder joints ground itself. is inferior to that of silver soldered joints. Another method calls for a piece of one-inch diameter copper tubing shaped into 7.61 Special Circumstances a circle around the tower base pier with the What happens when a special two ends silver soldered together. The circumstance exists at a site that does not individual radial ends are wrapped one time permit a “conventional” ground system to be each around the tubing and silver soldered installed? Where there is a will, there’s a into place. way, and there are just about as many If the installation uses short, variations on the basic ground system interspersed radials close in, the ends of scheme as there are engineers. Some of these radials are attached to the strap or these variations are quite creative. tubing in the same manner. Where a copper Many AM transmitter sites are screen is used, the individual pieces are laid located in flood planes or flood prone areas. out on top of the ground, cut to the proper In these cases, the transmitter building, size and shape and silver soldered together tuning houses and tower base insulators are and to the strap or tubing. Where a screen is often elevated so that the highest predicted used, it is best to cover it with large gravel level of flood water will not reach the to a depth of at least two inches to protect buildings or base insulators. In these and secure it. Pea gravel can be used, but a circumstances, the buried radial ground typical copperweld screen is likely to system is still used, but the ground screen or eventually work itself up through small interspersed short radials are mounted on a gravel and become exposed in places. at base insulator level. A counterpoise Typically, large gravel is less expensive frame of some sort is usually constructed in than pea gravel, so it is a better choice for a hexagon or octagon shape with a 20 - 25 more than one reason. foot radius, and the screen or short radials Where radials intersect a transverse are installed from a strap or tubing ring strap, they should be cut to length and below the base insulator to the edges of the bonded to the strap by silver soldering. counterpoise. The outer edges of the Strap intersections should be secured counterpoise are lined with copper strap, and mechanically and then silver soldered. the edges of the screen or the ends of the

13 AM Antenna Systems Alexander/13 that go into the creation of a successful short radials are silver soldered to this strap. cathodic protection system are well outside A copper strap is then run down each the realm of broadcast engineering. support leg of the counterpoise to ground In other installations, it may be level, where it is bonded to the buried necessary to extend a ground system across ground system, which has been installed in a a creek, river or canal. Unless the waterway normal manner below the tower base pier or can be lined and capped, it is usually not support pylon. A set of ground straps advisable to string ground radials across the (usually three or four) is run from the inner top. Plowing radials in through the bed of ring of the counterpoise down the tower the waterway is likewise inadvisable. support pylon to join the inner ring of the Flooding, dredging, erosion and the like are buried ground system. In the same manner, a prone to displace the radials and damage or number of large gauge wires (0 AWG or destroy that part of the ground system. larger) should be run from the bottom side A very effective way to cross a small of the spark gap to the ground rod array waterway is to install a strap, usually four around the base pier or pylon for lightning inches in width, along and parallel to either protection. side of the waterway. All the radials that It is important to note that if the intersect this strap should be trimmed to counterpoise support frame is constructed of length and silver soldered to the strap in the a conductive material, cathodic protection same manner as with a transverse strap. On must be used to protect it. Cathodic the side of the waterway opposite from the protection is a method of inducing a tower(s), the radials start at the parallel strap negative DC current into a metal object that and continue to their full length. Then, in at is in contact with the ground or another least three evenly spaced locations, a large conductive object, thereby preventing strap is connected across the waterway to corrosion. In the case of a steel frame each parallel strap, This strap, which is at counterpoise without cathodic protection, least six inches wide, is then buried a safe corrosion of the frame will eventually take depth beneath the waterway invert (usually place, both below the ground and at the at least three feet). points where the copper ground material contacts the steel of the frame. If cathodic protection is used, it may be worthwhile to Deterioration and Damage 7.7 The ground system is often blamed use insulated rather than bare copper wire in for signal problems. Since it is out of sight the buried portion of the ground system. and somewhat difficult to observe in place, Were bare wire to be used, a good part of it is easy for station personnel to assume that the cathodic protection current would flow there must be something wrong with it when in the copper ground radials and not in the coverage is not what it should be. steel where it is needed, thus reducing the The truth of the matter is that a effectiveness of the cathodic protection. properly installed and buried ground system Should you find that the situation will last many, many years. Except in the may call for cathodic protection, it is wise to most extreme circumstances, a buried retain an engineering firm that specializes in ground system will not deteriorate such systems. The variables and calculations

14 AM Antenna Systems Alexander/14 careful inspection of the antenna site appreciably in place. Stations that have been property should be made. The condition and in place for fifty or more years have integrity of the ground system should be operated with the original ground system checked by use of a field strength meter or without difficulty. Spot checks of radials metal detector if damage is suspected. have revealed them to be in excellent It is a good idea to periodically condition. inspect the ends of the radials where they The biggest dangers to buried ground connect to the base strap or ring to be systems are damage from construction or certain that they have not been burned open acts of God and vandalism/theft. A by lightning. Periodically check the construction company installing a pipeline, lightning ground wires and connections as sewer, or underground cable across an AM well, particularly at the beginning and end station’s site property can cut many radials of the thunderstorm season. and cause significant damage. When the Vandalism and theft are perhaps the ditch is filled back in, it may well become most common dangers to a buried ground impossible to locate the cut ends of the system. Copper is valuable, and a radials and the more economic fix may be to determined thief can rip an entire ground plow in new radials in those directions. It system out in a matter of hours. He may not may be impossible to avoid having the get more than $500 from the scrap value of construction work take place across the the copper, but the cost to replace the system property, but with careful planning and can exceed $50,000. supervision, the cut radials can be spliced Perhaps the best way to protect a before the ditch is filled in. The result will ground system from thieves and vandals is be no reduction in effectiveness of the to keep it properly buried. “Out of sight, out system. of mind” is a maxim to remember in Acts of God present a more difficult protecting your ground system. If thieves do picture. Erosion, floods, earthquakes and the not know the buried copper is there, they like are usually beyond our ability to predict will be unlikely to look underground for it. accurately, and they can leave a ground Good overall site fencing is important, and system uncovered and exposed. An ounce of nothing can substitute for good relations prevention is the best medicine in cases with site neighbors. Keep that in mind when where such calamities are likely. Radials can the farmer next door complains about station be plowed in deeper, for example, to prevent audio in his telephone. Fix or replace his them from becoming unearthed in a flood or phone and enlist him to help you keep an from erosion. At some antenna sites where eye on the site. erosion is a constant problem, a tractor with several implements is kept at the site and used to keep the ground system covered. Shared Site Use 7.8 Constant vigilance is necessary in such More and more, AM antenna site cases. Even a short length of radial that property is becoming too valuable to use as becomes exposed can easily be broken or only an AM antenna site. Open real estate is further unearthed by wildlife or livestock. scarce in many locations, and the big, open Following a flood or earthquake, a field with the tower on it is often very

15 Alexander/15 AM Antenna Systems the ground system is important, however, attractive to developers. While an in-depth and the use of interconnected perimeter discourse on shared AM site use is beyond straps serves this purpose nicely. the scope of this discussion, the treatment of ground systems in such situations does bear looking at. Directional Antenna Systems 8.0 One common shared site use calls for In an ideal world, all AM antennas placement of a parking area or roadway would be non-directional (circular radiation across an AM station’s site property. There pattern). Transmitter site locations would be is nothing wrong with paving over a ground selected so that all the population to be system. In fact, this can serve to protect the served would be covered with a strong buried radials from damage and theft or signal with a minimum waste of signal into vandalism. The important things to unpopulated areas. Population centers would remember are to work carefully with the be evenly spaced, with medium and large contractor to insure that the radial wires are cities being a good distance apart. In this not broken during construction, and have a ideal world, the number of stations would be surveyor make an accurate set of as-built low enough that all the stations could live drawings. It is easy to break a radial during together on the band without causing the paving process, and to prevent this from interference to one another. happening, it is a good idea to place some In the world we live in, though, sand around each radial to give it a cushion . things just aren’t that way. Transmitter site The as-built drawings will allow you or locations are selected as a compromise others to accurately locate buried radials between cost (land is much cheaper away during future excavations. from the city), location and permissible use. Buildings can be placed on top of a As Mr. Murphy usually has it, seldom is a station’s ground system, but this may not be site available where it will work the best for a good idea if RF power densities are high the station. Population centers are often or the antenna system is a directional array. clumped together, as on the east coast, and If a building must be placed on the ground the number of stations is very high, over system, depending upon the size of the 4800 at last count. With only 107 channels building, it may be best to treat the building on the regular AM band, we all know the in the same manner as you would a canal or band is way overcrowded. Interference waterway. Run a copper strap around the between stations is common. All this trouble outside of the building’s foundation, started when the second AM station signed terminating intersecting radials onto the on the air! strap. Several straps can then be run under To help overcome some of these the building’s foundation, connecting problems, when the broadcasting industry between the straps along the building sides. was in its infancy, directional antennas were Very little in the way of displacement introduced. The very first directional currents will enter the ground through the antennas were used to direct the signal into building, so having radials in place beneath areas where coverage was desirable. Later, the building is not important. Providing a directional antennas were used both for radial path for currents already flowing in directing coverage and for protecting co-

16 Alexander/16 AM Antenna Systems and interference protection. and adjacent-channel stations from The problem gets even more interference. As the band became more and complex with respect to nighttime more crowded, directional antennas were allocations. During the day, in most used to “shoehorn” new stations in wherever circumstances only the groundwave signal there was a hole. The result... well, the must be considered. At night, however, the current AM band with all its overcrowding skywave signals become a factor. Not only and interference problems is the result. The must radiation be limited in the direction of FCC terms this condition as a “mature” protected stations, but the vertical angle or band. Consider that a 20:1 groundwave theta (the angle above the horizon that will protection ratio (26 dB) is required between result in a reflection of the signal off the co-channel stations, and 6 dB is required ionosphere so that it arrives in the protected between first adjacent channel stations. In station’s vicinity) must be considered as the middle of the AM band with average well. Limitations on nighttime radiation values of conductivity, the 0.5 mV/m (still called MPR) are almost always contour of a typical station transmitting with expressed in mV/m at 1 kilometer at a 1 kW will lie at a distance of about 45 miles specific value or range of values of theta. from the antenna site. The same or a similar For example, the MPR from a particular station’s 0.025 mV/m contour (1/20 of 0.5 nighttime facility toward another might be mV/m) will lie at a distance of about 140 35 mV/m at 1 km at a theta range of 16 to 24 miles from the antenna site. Assuming two degrees. That would indicate that the typical 1 kW co-channel stations both have radiation on that azimuth could not exceed non-directional radiation patterns, they 35 mV/m at 1 km for any vertical angle cannot, then, be located closer than 185 between 16 and 24 degrees. This is where miles to one another and still maintain the the function of theta of the antenna, required groundwave protection to one discussed in Part 2 of this series, comes into another. If one of these stations uses a play. directional antenna to reduce its radiation From all this, you can see that toward the other station, the stations can be determining what a directional pattern must located much closer together. The maximum look like is a complex affair. Seldom is this radiation permissible from one station process as simple as protecting a single toward another is expressed in mV/m at 1 station. Allocation pictures often look like kilometer at the appropriate azimuth, and jigsaw puzzles, and with different engineers call this value the MPR conductivities Having an effect along the (maximum permissible radiation). various radials, coming up with a pattern to Seldom is a groundwave allocation fit the situation can be very difficult. problem so simple as one station protecting just one other co-channel station — usually 8.1 A Simple Two-Tower Pattern there are many stations that are entitled to In a directional antenna, the radiation protection. By using a directional antenna pattern is created by controlling the and carefully selecting the operating power, amplitude and phase of the RF current in a balance can be achieved between coverage each element of the array. The resulting

17 AM Antenna Systems Alexander/17 from the northern tower. This results in field at any point is the vector sum of the complete cancellation of the fields from the individual element radiation components. two elements. In locations where the fields from Now let’s consider the same array the various elements are in phase with one but move our observation point to a location another, the fields add; where they are out of due south of the array. At this observation phase, they subtract. In most locations, the point, equal fields will arrive from each of fields are not perfectly in or out of phase, the two elements, and in the case of arrays but because the with more than two southern element is elements, there is a 90 degrees closer, combination of addition its field will arrive and cancellation at most 90 degrees ahead of points. the field from the Let’s consider a northern tower (90 simple two-tower degree leading directional array. Let’s assume our array’s ). space phasing elements are spaced 90 Since the current Figure 1 degrees apart on a north- fed to the southern south bearing, and that the elements are element is delayed by 90 degrees, that driven with equal currents (1.0 ratio) with cancels the 90 degree leading space phasing, the phase of the current to the southern and the resultant field from the southern tower lagging by 90 degrees. If we stand at element arrives in phase with the field from an observation point some distance from the the northern element. The fields completely array on a bearing of zero degrees True (due add, so the resultant field at the observation north), equal fields will arrive at the point is equal to F . + F 2 1 observation point from each point. Because At other points around the array, the elements are spaced 90 degrees apart, neither complete addition nor complete the field arriving from the southern element cancellation will occur, and vector addition will arrive 90 degrees later than the field of the arriving fields is used to determine the from the northern element (90 degree resultant field. Figure 1 shows the resulting lagging space phasing). Add to that the 90 directional pattern of our two tower array. degree phase delay in the southern element The theoretical parameters for such current and the field arriving at our an array would normally be listed as observation point from the southern element follows: is 180 degrees out of phase with the field Tower Ratio Phase Spacing Orient Height 0.0 0.0 0.0 90.0 1.000 1 -90.0 90.0 180.0 90.0 2 1.000

18 AM Antenna Systems Alexander/18 west side of the array. If the phase of the two elements is If we move to an observation point reversed (i.e. the phase to the southern north of the array, equal fields will arrive element advanced 90 degrees rather than from the two elements, but the field from the retarded), the pattern will be reversed, with southern element will arrive 180 degrees complete cancellation to the south and behind that of the field from the northern addition to the north. element. Because the currents in the two Now let’s consider the same simple elements has the same two-tower array, but phase, the fields let’s increase the spacing arriving at the to 180 degrees and feed observation point will the two elements in be completely out of phase with one another phase and will thus (0 degree phase shift).At cancel. The same is an observation point due true at an observation east of the array, the point due south of the equal fields from the two array. Figure 2 shows elements will arrive at the resulting the same time, since the Figure 2 directional pattern of observation point is this two-tower array. equidistant from both elements. Since the The theoretical parameters for such phase of the currents in the two elements is an array would normally be listed as the same, the fields from the two elements follows: arrive in phase and thus completely add. The same is true at an observation point on the Tower Ratio Phase Spacing Orient Height 1.000 0.0 0.0 0.0 90.0 1 1.000 180.0 180.0 90.0 0.0 2 elements (current phase plus By altering the space phase) will vary, currents in the two elements producing patterns of and making them unequal, different shapes. The same is there will be neither complete true of changing the spacing cancellation no complete of the elements. addition at any point around The design engineer the array. The result is that can vary all these parameters nulls are somewhat filled and — current, phase and spacing lobes are not as large. — in addition to moving the By altering the phase bearing of the line of towers of the current in the elements, to achieve the desired pattern. the resulting instantaneous If more nulls or broader nulls phase of the fields from the Figure 3

19 AM Antenna Systems Alexander/19 of a distant station, where are needed, additional elements radiation toward that station are added to the array. For each needs to be suppressed without additional element, another pair necessarily placing a null in that of nulls is created. By placing direction. nulls close together, a broad arc We can combine these where the radiation is two patterns — multiply them, suppressed can be created. The if you will — to achieve a four- possibilities are limitless. tower parallelogram array that produces a pattern with nulls at 8.3 Pattern Multiplication 280, 345, 350 and 105 degrees In a more complex (see Figure 5). The nulls at 345 example, if we start with a and 350 degrees combine to simple two-tower array with a produce, in effect, a broad 315-degree tower line, 90- Figure 4 single null, which is useful in degree spacing and 106 degree protecting a large contour or a phasing, this will result in nulls national border. at 280 and 350 degrees or 35 We multiply patterns degrees either side of the tower through the use of vector line (see Figure 3). arithmetic. Since we already Now let’s suppose we know what the parameters for need additional nulls at 345 and both of the two-tower patterns 105 degrees. A two-tower are, we have the parameters for pattern with a 45-degree tower towers 1, 2 and 3. All that line, 180-degree spacing and remains is to find tower 4's 90-degree phasing will produce parameters. As shown in Figure the pattern shown in Figure 4. 5, simply multiply the field This pattern was ratios of towers 2 and 3 and selected to show that with a add the phases of towers 2 and wide-spaced array, maximum Figure 5 3 to find the parameters for radiation does not occur on the tower 4. The parameters for this azimuth of the tower line. This type of parallelogram array would then be: pattern is often useful in daytime protection Tower Ratio Phase Spacing Orient 0.0 0 0 1 1.000 106.0 315 1.000 90 2 90.0 180 45 3 1.000 196.0 201 18 1.000 4

20 AM Antenna Systems Alexander/20 While scientific principles govern this art, 8.4 Pattern Size experience through trial and error is what So far, for simplicity of illustration, makes an engineer proficient at pattern all the patterns shown have equal radiated design. There are many tricks to the trade fields, or field ratios of 1. We can fill the that simplify the pattern design process. A nulls of our patterns by making the currents novice designer, for example, may take unequal. As we fill the nulls, we also reduce many times longer to produce a desired the size of the pattern major lobe. The pattern than an experienced designer using amount of energy in a pattern remains the the basic principles we have discussed here. same, regardless of the depth of the nulls or Computer directional antenna design the size of the lobes. The size of a pattern is programs are a big help and time saver, but found by integrating the hemispherical it is entirely possible (and even easy) to energy flow (the power radiated on and design a terrible pattern with one. The above the horizon in all directions). For a computer cannot substitute for knowledge of given power input and loss, this will remain the craft. the same, regardless of pattern shape. We For those building and maintaining can liken this to a balloon filled with air. directional arrays, it helps to understand the You can squeeze it in the middle, but the basic principles of array design. Faced with ends bulge out. As you reduce pressure in the adjustment of an array, without a good the middle, the ends return to their normal understanding of the designer’s intentions, size. Squeeze both ends and the middle the adjusting engineer may well find himself bulges out. Still, no matter where or how in an iterative trial-and-error situation that hard you squeeze, the total size of the produce the proper pattern. A never may balloon does not change. good understanding of vector arithmetic and As a pattern design progresses, it is antenna design principles will speed the important to keep an eye on what is going process by allowing the engineer charged above on the horizon. It is easy, particularly with adjusting the array to make educated with more complex patterns, to wind up decisions as the tuneup proceeds. In the next with a good bit of energy being radiated part of this series, we will examine in detail above the horizon and into space. This the use of vectors in DA adjustment. power is, for most intents and purposes, lost, and in the case of most nighttime antennas, it is harmful as it causes skywave 8.5 Real World Parameters interference to other stations. Because the Anyone who has ever looked at the total hemispherical energy in a pattern never FCC license for an AM directional station changes, by keeping an eye on the size of has probably seen that there are two sets of the horizontal pattern, the designer can directional antenna parameters listed. One generally tell if he has a vertical radiation set is theoretical; the other is operating problem. If the horizontal pattern size or parameters. There are usually significant RMS begins to shrink, that power is going differences between them. somewhere, and that somewhere is up. The indicate theoretical parameters Directional pattern design is a the radiated field ratios and the phases of the complex art that takes years to learn well. radiated fields. If the towers are of equal

21 AM Antenna Systems Alexander/21 radiation, and the current induced by mutual current ratios heights, the theoretical loop coupling from other elements. The current will be equal to the radiated field ratios; if that contributes to radiation tends to be the tower heights are different, they will be sinusoidal, but the effect of the induced different. current tends to distort that sinusoidal Assuming sinusoidal current flow, a current distribution. The position of the current maximum or loop will occur 90 current loop can be quite a distance from electrical degrees below the top of a vertical where sinusoidal current distribution would radiator. If that radiator is less than 90 place it. electrical degrees tall, this loop will exist at There are many methods of the tower base. If the tower heights are the determining the correct operating same, the theoretical parameters which are parameters for a directional array, but all, to shown on the station license are those the one degree or another, rely on trial-and-error designer used to mathematically create the adjustments and field measurements. pattern and they reference this current loop. Knowledge of the design, experience, and When the towers are of unequal height, computer modeling can all help to make these ratios reference the radiated fields. each trial an educated trial (rather than a The operating parameters , on the random guess) and shorten the tuneup other hand, indicate the values shown on the process. antenna monitor when the array is properly adjusted to produce the correct pattern. The operating parameters often deviate 8.6 Standard Patterns In all the examples we have used so significantly from the theoretical far in this series, on certain azimuths, the parameters, and this is due, in large part, vectors from the array elements completely sample system errors and to the mutual cancel one another and the resultant coupling between towers. radiation on that azimuth is zero. This is No matter how careful the installer difficult to achieve in the real world. is, there be errors in the sample system. will Reradiation, scatter and drift in the phasing, It is difficult to make all the runs of sample coupling and sampling systems limit how line exactly the same length, and each close to zero the radiation on a null radial individual run will have a slightly different can be adjusted and maintained. In January characteristic impedance. Differences in the values of antenna monitor terminating of 1981, the FCC instituted the standard resistors, differences in the sample loops or pattern , which increases the size of the current transformers, even slight differences theoretical pattern (or calculated pattern) in the locations of the sample pickups all by a specific amount. In another part of this contribute to sample system error. series, we will look at how the standard Mutual coupling, on the other hand, pattern is calculated. distorts the assumed sinusoidal When the standard pattern was characteristics of the current flow on each instituted, the FCC calculated the standard tower. There are two on-frequency current pattern for all existing stations and components in each element of a directional authorized them by modification of each array — the current that contributes to station’s license. Today, when the FCC

22 AM Antenna Systems Alexander/22 authorizes a directional pattern for a station, the voltage vector reference it is the standard (and not the theoretical) axis.) pattern that is authorized. The FCC proscribes radiation that exceeds the Φ - is the space phasing cos( ) Φ S i i standard pattern value on any azimuth. The due to the location of portion of β i th designer must use the standard pattern as he tower and W is the electrical the i i designs a pattern to fit a particular .) phasing portion of β i application as the FCC requires use of the standard pattern in all calculations of = electrical length of spacing of S Where: i th interference and coverage. tower in i the the horizontal plane from the space reference point. The Directional Antenna Formula 8.7 The equation to calculate the pattern = true horizontal azimuth Φ shape in the horizontal plane for a i th tower with orientation of the i directional array of n towers is: respect to the space reference = n i axis. β θ ) ( Σ = f E E i i i Φ = true horizontal azimuth angle of = 1 i the direction to the reference point (P) measured clockwise = total effective field strength E Where: from true north. vector at unit distance (P) for the antenna array with respect to the = time phasing portion of β due W i i voltage vector reference axis. to the electrical phase angle of the th th tower in the directional i = i tower radiated field of the i antenna taken with respect to the voltage array. vector reference axis. n = total number of towers in the This equation, when applied to a array. directional array, will yield a complex number that represents the field strength and = magnitude of the field strength E i phase of the signal arriving at a particular at unit distance in the horizontal observation point from each element in the th tower i plane produced by the array. This is no more than we were doing in acting alone. our heads in Part 5 of this series with simple two-tower arrays. With more complex = (phase ) + W Φ - S Φ cos( β i i i i arrays, things get a bit harder, but the basic relation of the field strength at principle is the same. While this formula the observation point (P) for the may not readily lend itself to working out th tower taken with respect to i with pencil and paper, computer types will

23 AM Antenna Systems Alexander/23 find that it is easily written into computer code. Φ = true horizontal azimuth angle for which the vector is being Envision the Vectors 8.8 calculated. To see the vectors for a particular azimuth, all we really need to know are the = true horizontal azimuth Φ theoretical parameters and the following i i orientation of tower from the formula: reference tower. Φ = + S θ − β Φ [ )] ( cos i i i i If you look closely, you will see that this is actually the rotational portion of the = phase relation of the field from Where: β i other formula. At first glance, it looks a lot tower i on the specified azimuth. easier, but what are all those variables? Let’s look at a typical set of directional = phase of tower i with respect to θ i antenna theoretical parameters and see if the reference tower. things start to make sense. Look at Table 1. = electrical length spacing of S i tower i from the reference tower. Phase Tower Spacing Field Orientation 0 0 0 1 1.000 2 90 0 90 1.000 - This is the spacing of the tower, These are the theoretical parameters S i in electrical degrees, of a typical two-tower . from the reference tower directional array, similar In this case, the spacing to other examples we of the reference tower have used in this series. with respect to itself is Let’s plug these zero, and the spacing of parameters into our tower #2 with respect to vector formula and see the reference tower is what happens. - This is the θ ° 90 . i Figure 6 of the tower with phase - This is the Φ i . In this case, respect to the reference tower of the tower, in electrical orientation the phase of the reference tower with respect . In this degrees, from the reference tower to itself is zero, and the phase of tower #2 case, the orientation of the reference tower ° with respect to the reference tower is 90 . with respect to itself is zero, and the

24 AM Antenna Systems Alexander/24 orientation of tower #2 with respect to the The conventional way to plot vectors has zero azimuth at 3 o’clock and angles plotted , or true north. ° reference tower is 0 counterclockwise. For the sake of simplicity from the Φ - This is the azimuth and those that have been taught that north is center of the array to the observation point. always at the top of the page, the examples Another way to express this is as the here will place zero degrees at 12 o’clock azimuth on which we wish to compute the and angles will be plotted clockwise. If you vectors. wish to do it the conventional way, do so. It ° = 0 Φ Let’s start with true. When makes no difference in the length of the we plug in all the parameters for tower 1 resultant which way you do it. (the reference tower), we come up with a Starting near the center of the page, . This is the same as the phase of ° value of 0 make a dot to represent the starting point. tower 1, and this will always be true of the Using the protractor and ruler, draw a line reference tower. If the phase of the reference 1.0 inches long at an angle of 0 , or straight ° , the formula tower had been other than 0 ° up on the page. This would have yielded represents tower #1's whatever the phase of vector. Now, move the the reference tower protractor to the end of was. In other words, it this line (the north end) is not necessary to run and draw the vector for the reference tower tower #2. This line, 1.0 through the formula — inches long at an angle only the other towers in (or straight of 180 ° the array. Just know the down), will lie back field and phase of the over and completely reference tower. The cover the tower #1 complex number vector, with the end representing the vector falling back at the for tower #1 at an starting point. The ° true, azimuth of 0 vector is the resultant Figure 7 0 then, is 1.000 ∠ ° . distance and angle Now, let’s run the parameters for between the starting point and the end of the tower #2 through the formula. When I did tower #2 vector. Since these points coincide ° this, I came up with a phase of 180 , so the ∠ . 0 ° in this case, the resultant vector is 0 vector for tower #2 at an azimuth of 0 ° true This vector diagram for an azimuth of 0 ° 180 ° . ∠ is 1.000 true, shown in Figure 6, shows graphically With these numbers in hand, let’s how at that azimuth the signals from the two plot them on a piece of paper. You’ll need towers completely cancel one another. nothing more than a ruler and protractor.

25 AM Antenna Systems Alexander/25 Now let’s try the same thing at an What if you have more than two towers? It’s simple. Calculate the vector for ° true. Remember that the azimuth of 180 tower and draw it onto the end of the each vector for the reference tower is always the preceding vector. The resultant vector will ∠ 0 ° . Plugging tower same, in this case 1.000 still be from the starting point to the end of azimuth into our ° #2's parameters and a 180 the last vector. When formula, I get a vector dealing with arrays ° of 1.000 ∠ 0 . When I with more than two draw the two vectors towers, it is important on paper, it is easy to the vectors as to label see that the vector from you draw them. If you tower #2 completely dn’t, it is easy to lose adds to the vector from track of which vector tower #1, yielding a belongs to which resultant vector of tower. 0 ° (see Figure 2.000 ∠ Three-tower 7). vectors are fairly easy We’ve seen the to draw, but when four extreme cases of or more towers are complete cancellation Figure 8 involved, it becomes and complete addition. more difficult. It should be understood that Now let’s try something in between. Plug in the angle of the resultant vector is not an azimuth of 90 true and tower #2's ° important. Only the length is important. As parameters and you should come up with a you experiment and plot different vectors, true of 1.000 90 ° . tower #2 vector for 90 ° ∠ don’t get hung up on the resultant angle. The Draw tower #1's vector of 1.0" at an angle of field strength meter does not care what the vector onto ° , then draw tower #2's 1", 90 ° 0 incident phase of the resultant field strength the end of that. If you measure the resultant vector is. vector, you’ll find it is 1.414" long at an If you wish to plot the directional ° angle of 45 (see Figure 8). You can see pattern of the array on a piece of polar graph from this graphical representation that the paper, simply plot a dot representing the signals from towers 1 and 2 partially add to scaled length of the resultant vector on the produce a field that is greater than the field azimuth at which the vector was calculated. of either one, but less than the sum of the In the case of the 180-degree azimuth in the two combined. If you run the same problem example above, you would plot a point at true, you’ll find you for an azimuth of 270 ° distance from the center of 2 times the scale come up with the same vector. Go ahead and value at an azimuth of 180 degrees. For the try this at other in-between azimuths. You’ll 90-degree radial you would plot a point at a find that when you get closer to 0 ° , you get distance from the center of 1.414 times the more cancellation of the two signals; when scale value at an azimuth of 90 degrees. you get closer to 180 , you get more ° Calculating the length of the resultant and addition. plotting it at five-degree increments all the

26 AM Antenna Systems Alexander/26 way around, you can then smoothly join the lines commonly used today to feed power points together in a graphical representation from the transmitter to the antenna system. of the pattern shape. One of these is the open-wire feeder, of which there are several variations. 8.81 Using the Vectors With the vectors in hand, we have at 9.1 Open-Wire Feeders Transmission lines or feeders created our disposal a method of graphically from open (unshielded) wires supported on depicting what is happening on any given glass or ceramic insulators are commonly azimuth. This is particularly helpful when referred to as open-wire feeders. Amateur setting up an array or working to correct a radio operators often use ladder line, which problem. By plotting the vectors on a given consists of parallel conductors separated by radial, you can see how the different towers Plexiglass rods, to provide a balanced feed interact to create the resultant field on that for high-frequency (HF) antennas. Twin- radial. Many monitor points are on null lead, which used to be the feedline of choice radials, where some degree of cancellation for television receiving antennas, is another occurs. By plotting the vectors, you can variation of the balanced open-wire feeder visualize how towers pair-up on the radial. theme. Neither of these has any practical use In parallelogram arrays, pairs of towers in broadcasting because of the relatively low often cancel other pairs of towers. Using power handling capability, but there are vectors, you can get a mind picture of the some other types that were commonly used best way to go about adjusting such an in radio’s early days. Three and six-wire array. unbalanced systems are the most common of these, and some vestiges of these systems 9.0 Transmission Lines remain today. No new systems have been Just as the electrician must choose constructed using open-wire feeders in many the correct size and type wire for a particular years. wiring job, and the plumber must select the Three-wire systems, where two appropriate pipe, as radio engineers we must ground conductors supported on telephone choose the right size transmission line. If the pole cross-arms bracket the center RF wrong line is chosen, the results can be with conductor are one such type. This type of catastrophic. Proper transmission line transmission line has a relatively high selection can make the difference between impedance, can withstand high peak having a system that is reliable and capable voltages and is relatively easy to maintain. of continued operation under adverse Cracked and broken insulators as well as conditions, or having a borderline system deteriorated or damaged support structures that is a time-bomb primed for failure. are the biggest problems with this type of Sometimes the choice is clear and the open-wire feeder. differences are obvious; other times the The six-wire feeder consists of four limitations are hidden. In this installment of relatively small ground conductors our AM Antenna series, we'll look at feed bracketing two parallel center RF lines and explore these limitations. conductors in a cubical fashion. In effect, There are three types of transmission

27 AM Antenna Systems Alexander/27 this creates a coaxial cable of sorts. Six-wire Under the broad heading of coaxial cable are systems are typically supported on short several sub-categories. In broadcast creosote poles. The impedance is usually transmission applications, we primarily use lower than three-wire open feeders. semi-flexible cable, sometimes referred to Disadvantages are lower peak voltage by the trade names “Heliax®”or capacity, generally lower power handling “Flexwell®,” and rigid line, sometimes ability and greater mechanical complexity. referred to as "hard line." With so many more wires — usually 8- or 10-gauge soft-drawn copper — and six glass 9.21 Semi-Flexible Cable insulators on each rather complex support Semi-flexible cable is by far the structure, there are many points for potential most common transmission line in use in problems. Ice poses a particular hazard for AM stations. It is ideal for a wide range of this type of open-wire feeder. One low and medium power uses, and its cost is environmental problem that is often caused considerably lower than that of comparably by this type of feeder occurs when the rated rigid line. The term "semi-flexible" is spacing between ground and center used because the bending radius of the line conductors is sufficient to allow small birds is quite large. For a 1-5/8" line, the to perch on the center conductor wires. minimum bending radius is 20". Anyone When the birds take to flight, their wings who has wrestled line such as this into a can come into contact with the outer wires. tight space or through a conduit will attest The results of this inadvertent shunting of that “semi-flexible” is indeed an appropriate the transmission line are predictable. The moniker. feeder survives; the birds do not. Under the heading of semi-flexible Open-wire feeders were used in the cable are both foam- and air-dielectric lines. early days of broadcasting for a number of Foam lines are designed for applications that reasons. One was that they could be made do not require a pressurization path to the on-site out of readily available materials. antenna. Their average power handling Another was that they were capable of capability is lower, and loss is higher than withstanding large standing wave ratios. In the same size air-dielectric line, but their the days before the operating impedance peak power ratings are higher. This is bridge, driving point impedances in multi- primarily due to the higher losses of foam as element arrays were difficult or impossible a dielectric. Foam-dielectric cable is to measure with any accuracy. Transmission available in sizes up to 1-5/8". lines that could withstand the inevitable high Air-dielectric lines utilize a spiral VSWR were needed. polyethylene spacer to keep a constant spacing between the inner and outer conductors. To maintain safe operation, this Coaxial Transmission Lines 9.2 standard de facto Coaxial cable is the type of line must be kept under constant transmission line today and it has been for pressurization with dry air or nitrogen. quite some time. As its name implies, Usually, a dehydrator or nitrogen regulator coaxial cable is constructed using two is connected to the line at the transmitter concentric conductors on a single axis. building end of the run to provide

28 AM Antenna Systems Alexander/28 pressurization. Weekly checks of the such an occasion. amount of pressure in the line should be part of the engineer's routine inspection. Leaks 9.3 Characteristics that develop are a sure sign of impending Several properties determine the trouble and should be investigated quickly. suitability of a given line for a particular Air is, by far, a superior dielectric to application. These properties are published foam. It does ionize more easily than foam, in charts and graphs provided by the however, and this results in the lower peak manufacturers of such lines. Every engineer power ratings. Because air does not heat up should keep several transmission line as foam does with power applied, air catalogs handy. In addition to line ratings, dielectric lines can handle higher average they contain all kinds of other useful power than foam lines of the same size. engineering information. Copper heating becomes the primary average power limiting factor in air 9.31 Impedance dielectric lines. Impedance is one of the most important properties of a transmission line. The value of a transmission line's impedance 9.22 Rigid Lines Rigid lines are available in sizes is determined by the size and spacing of the from 7/8" to 9-3/16". They have inherent inner and outer conductors as well as the low losses and high power handling dielectric constant of the dielectric material capability. Rigid lines are normally made in between the conductors. Most lines in use 20-foot lengths with flanges on each end. today are rated at 50 Ω , although I have Inner conductors are made of high- measured the impedance of such lines to conductivity oxygen-free copper that are is a Ω ±10% of the nominal rated value. 75 supported inside the outer conductor by peg common impedance value in older systems, or disk insulators with a low dielectric particularly AM systems. Occasionally, constant (usually ceramic or Teflon). transmission lines with characteristic If you have ever experienced a , 63 and even Ω Ω , 52 Ω impedances of 51½ catastrophic failure of a transmission line, odder values can be found in older you will appreciate the repairability of rigid installations. For all practical purposes, line. Often, when a burnout occurs, it is unless you are replacing an older line and confined to a relatively small area. The there is some compelling reason to use an affected section(s) can be easily replaced odd-impedance line, 50 Ω is likely to be the and the remainder of the line cleaned desired impedance of any line purchased thoroughly to remove soot particles. For a today. fraction of the cost of replacing the entire transmission line run, a station can often have its rigid line repaired and be back on 9.32 Power Rating the air in a very short period of time. Many The power handling capability of a stations using rigid lines keep a spare transmission line is absolutely critical to its section or two on hand along with spare proper selection and safe use. It is limited by connectors, flanges and hardware for just either the maximum peak power

29 AM Antenna Systems Alexander/29 (determined by the electric field strength a number of other factors. In choosing a and dielectric constant) or the maximum transmission line, always allow for 2:1 average power (determined by the allowed VSWR. This will provide an adequate safety temperature rise of the inner conductor). margin in most cases. Using the manufacturer's supplied The formula for derating a ratings, you can tell at a glance what the transmission line for VSWR and modulation average and peak power capabilities of a is as follows: given line are. What takes more than a glance to determine is whether a given line P PK = P D 2 size is suitable for a given application. A ) 1 M VSWR ( + look at just about any manufacturer's power rating graph will also show that the peak and = Transmission line derated Where: P D average power ratings tend to converge at power lower frequencies so that at AM frequencies, where skin effect is minimal, they are the = Transmission line rated peak P PK same. power 9.33 Derating M = Modulation percentage as a Consider an AM station that needs to decimal replace the transmission line to one of its directional array elements. Let's assume that Using this formula, you can compute there is normally 10 kW of power flowing to that our 7/8" foam line, rated at 44 kW peak that particular element. According to the power, is only good for a little over 4 kW! manufacturer's published ratings, 7/8" foam Take my word for it; at a station here in dielectric line is capable of safely handling Dallas, this very scenario played out with of safety 44 kW. That should be plenty the result being a burned up, brand new margin, shouldn't it? Let's look closer. transmission line. If I had my way, If we're going to modulate the power manufacturers would overlay this formula in being fed to the DA element 100% positive, red on their power rating tables and graphs! the peak power will be equal to 40 kW. That's getting pretty close to our 44 kW 9.34 Attenuation rating. If we allow for 125% positive Attenuation is another important line modulation, our peak power is over 50 kW, characteristic. It is caused by a combination well above the peak power handling 2 R losses of the copper and the of the I capability of the transmission line. dielectric losses of the dielectric material. Another variable that we need to The losses in the dielectric material tend to allow for is VSWR. In any real world be directly proportional to the frequency. situation, even in the best matched system, Conductor losses are related to the there will be times when the VSWR on a dimensions, permeability and conductivity transmission line will be higher than 1.0:1. of the material and tend to vary with the This can be due to ice, changing ground square root of the frequency. conductivity, defective ATU components or While the rated attenuation of a

30 AM Antenna Systems Alexander/30 transmission line is very significant at FM types of fitting are available. and TV frequencies, it is seldom significant When choosing the fitting to use on a at AM frequencies. Line losses are so low in transmission line, the connection provided a typical AM system that they can be by the equipment manufacturer should be ignored altogether, provided that the considered. Obviously, the two have to transmission lines are otherwise adequately match. In high altitude situations, the end rated. terminal adaptor may be a better choice than the EIA flange because of the larger spacing between the stud at the end of the terminal 9.4 Fittings and the body of the connector. EIA flanged Many types of fittings and connections are prone to arc over in high terminations are available to allow us to power situations at high altitude, where the make the transition between the coaxial air ionized more easily. transmission line and RF sources and is in end terminal adaptor terminations. The common use at AM frequencies. This device 10.0 RF Ammeters To maintain proper operating creates an airtight seal to the transmission parameters in an antenna system, it is line and provides a brass stud connection for necessary to have a means of accurately the center conductor. The body of the fitting measuring the antenna current. We have is equipped with threads and a nut for the several tools available to us to measure this ground connection. On air-dielectric lines, a parameter, and in this part of our AM gas port is provided on the fitting to antennas series, we will examine the facilitate pressurization or purging. By far, different types and look at their advantages this is one of the easiest means of and limitations. connecting the tubing in a phasor or antenna tuning unit to the transmission line. LC-type connectors were once 10.1 The Thermocouple RF Ammeter popular at lower power levels for lines up to The most popular RF current 1" in diameter. This is a coaxial screw-on measuring device has long been the fitting very similar to the PL-259 connectors thermocouple ammeter. This device consists used on RG-8 cable. of a DC meter movement connected to a EIA flange connections are very thermocouple through which RF current popular means of line termination. The outer flows. When the thermocouple is heated, a conductor of the flange bolts onto the proportional current flows through the meter chassis of the equipment, and the center winding, causing deflection of the meter. conductor is attached with a “bullet”, which Thermocouple RF ammeters have is a spring-loaded expansion connector that the advantage of simplicity. They are fits inside the center conductor. EIA flange typically manufactured with stud-type terminations for air dielectric lines provide a connections by which they are connected gas port. None is needed on foam line into the RF circuit under test. To prevent terminations. Rigid transmission lines damage due to lightning currents or static almost always terminate in EIA flanges, discharges, a make-before-break switch is although adaptors to end terminals and other typically provided in permanent metering

31 AM Antenna Systems Alexander/31 circuits to allow the meter to be placed into base current ratios, it may be desirable the circuit for measurement and removed during the construction and tuneup process from the circuit for normal operation, all to calibrate the RF ammeters against one without disturbing the continuity of the RF another. If one meter in the set is at current path. significant variance with the others, it can Some metering circuits are then be replaced with one that tracks the manufactured with a “hot jack,” which is a other meters or a calibration chart can be make-before-break receptacle into which a made. Likewise, if the base currents in an removable meter is plugged. The meter itself array are found to be out of tolerance but all is usually mounted on a phenolic handle to the other array parameters and monitor point permit it to be safely plugged into the RF field strengths are normal, it is a good idea circuit without causing RF burns to the user. to check the calibration of the thermocouple The advantage of this plug-in type of meter RF ammeters before doing anything else. is that the thermocouple meter itself is If the inaccuracy of a thermocouple stored in the transmitter building, out of the meter is believed to be caused by a elements that can affect its calibration. magnetized meter movement, a degaussing Thermocouple RF ammeters have coil can be used to demagnetize it. In most the advantages of relatively low cost, cases, the meter’s accuracy will return when simplicity, accuracy and broad frequency the residual magnetism has been removed. range. The disadvantages are susceptibility to damage. Lightning currents and static 10.2 The Toroidal RF Ammeter discharges through a thermocouple meter The other type of RF ammeter that is are usually fatal. Nearby lightning becoming very popular is the toroidal-type, discharges can cause magnetization of the which consists of a shielded, toroidal current meter movement, resulting in inaccurate transformer connected to a rectifier/filter readings. This type of damage is particularly that in turn drives a meter. This type of RF troublesome, since it may not be readily ammeter employs a separate, shielded apparent, as a catastrophic failure would be. current transformer which produces a fixed It is easy to check the calibration of a voltage per ampere of RF current flowing in thermocouple RF ammeter, because of its the circuit being measured. The meter is wide frequency range. Simply connect the housed in a separate unit and contains the meter being checked in series with a rectifier, filter and a switch that removes the calibrated AC ammeter across the secondary meter itself from the circuit. A relay that of a variac. By adjusting the amount of allows remote activation of the meter is current flowing with the variac, the optional on some units. The current indications of the two ammeters can be transformer is connected to the meter unit compared. If it is discovered that the RF with a piece of coaxial cable. At the factory, ammeter is in error, repairs can be attempted the meter, transformer and cable are or a calibration chart can be made to calibrated as a system. In the field, using a validate the meter’s indications. different coaxial cable or switching metering In a directional array, where base units between transformers will result in current RF ammeters are used to maintain inaccurate current readings.

32 AM Antenna Systems Alexander/32 One of the biggest advantages of this calibration. type of RF ammeter is that there is no direct About the only disadvantage to the connection to the circuit under test. This has toroidal RF ammeter is initial cost. They are a twofold effect: one, it does not introduce a more expensive than their thermocouple new component into the circuit’s counterparts. Long term, however, their cost impedance; and two, the propensity for is lower than that of thermocouple meters. damage by lightning strikes and static Their better durability, stability and discharges is decreased by a large factor. accuracy will more than compensate for Another advantage is that with a their higher initial cost. In some cases where single pickup transformer, a meter with multiple scales are needed to accommodate multiple, switchable scales can be used. different patterns or current values, a single Many radio stations operate with switchable-scale toroidal meter may be less significantly different power day and night. expensive than two thermocouple meters Some of those that operate with similar with all the requisite switching hardware. powers operate different directional patterns where the current in a particular element 11.0 Sampling Systems may be large in one configuration and very At the heart of every directional small in another. It is difficult to measure antenna is a sampling system that provides such diverse values of current with a meter indication of the relative currents and phases using a single scale. Using a multiple, in the elements of the array. The key to a switchable scale toroidal RF ammeter, a successful sampling system is stability. A current of 25 amperes can be accurately stable sampling system will make true measured on one scale while a current of changes in array operating parameters less than 5 amperes can be measured on evident so that they can be compensated for another. Optional relay switching within the and the array kept in adjustment. A ammeter allows remote base current sampling system that is not stable will metering on the appropriate scale. This can provide false indications of array drift that be tied into the pattern switching logic of the the engineer will then “chase” with phasor array and the correct scale will always be controls, unknowingly cranking the array selected for the current mode of operation. out of proper adjustment with no clear way One manufacturer is now offering a back! toroidal RF ammeter package on an A directional antenna sampling insulated carry-around, plug-in frame. This system is made up of three elements: a package offers the same advantages as the sample loop or sample transformer for each plug-in thermocouple RF ammeter — it element, an antenna monitor that measures allows the meter to be stored in the the relative amplitude and phase of each controlled environment of the transmitter sample, and transmission lines that connect building, out of harms way. Another the samples to the antenna monitor. advantage is that base current ratios can be read much more accurately using a single 11.1 Sample Loops meter than they can using multiple meters A sample loop is just what its name with varying amounts of error in their implies — a one-turn metallic loop that is

33 Alexander/33 AM Antenna Systems permanently attached to the tower. A typical 11.2 Sample Transformers loop is made of galvanized or stainless steel Sample transformers consist of a angle iron. Some loops are made of large- shielded toroidal loop through which a diameter copper tubing, but these are not as conductor carrying RF current to the tower durable as steel loops. As a rule, a sample is passed. Such transformers are essentially loop must be mounted 10 - 15 feet above the the same as those used in toroidal RF ground, except on tall towers, where the ammeters. A given RF voltage is produced loop is positioned 90 electrical degrees per RF ampere flowing through the below the top of the tower. If the towers in conductor being sampled, e.g. one or two the array are all the same height, the sample volts per amp. loops should all be located at the same Sample transformers are typically height above the base insulator. mounted inside the tuning house or ATU Sample loops can be either insulated enclosure and as such, have the advantage of from the tower and kept at ground potential, being out of the elements. Such pickups are or operated at tower potential. The latter very stable and sampling systems that method is more common. In such cases, the employ them seldom exhibit drift. In sample line itself is wound into an isolation addition, no iso-coil is needed. The coil that presents a high impedance at the disadvantage of sample transformers is that carrier frequency to carry the current sample they sample the current below the base across the base insulator without insulator as opposed to the actual radiation significantly affecting the impedance of the current. The sample taken at that point will tower or disturbing the sample itself. This include a small component that flows to iso-coil provides a convenient static drain ground through the base insulator for the tower, and a capacitor can be used capacitance. In addition, sample across part of the winding to achieve a transformers become unreliable is the parallel resonance and “float” the tower for element being driven is more than 130 or so modes where that element is not used (such electrical degrees tall. as non-directional operation). When ordering a set of sample Sample loops have the advantage of transformers, the manufacturer can provide being mounted above the base insulator, on a set that is matched as closely as possible. the radiating element itself. As such, they Voltage output within 1% and phase tend to provide a superior sample and give a tracking within 0.5 ° are attainable, and when relatively good indication of the current and using sample transformers to set up an array, phase at the loop location. The disadvantage it is best to be as accurate as possible. is that they are exposed to the elements all By way of comparison, it is usually the time and subject to deterioration and easier to set up a new directional antenna damage. When the antenna monitor using sample loops than using sample suddenly shows array parameters at transformers. From an operational variance, particularly when only one standpoint, sample transformers are better to element seems to be affected, check the work with because of their stability and sample loops first. reliability. Ideally, one would use sample loops to set up an array and sample

34 AM Antenna Systems Alexander/34 transformers to monitor it. The expense of between the two lines would be expected. using both kinds of sample is too great, This is unacceptable in a directional antenna however, so this is seldom (if ever) done. sampling system. In choosing a sampling system transmission line, there is both normal and 11.3 Sample Lines phase-stabilized line. Phase-stabilized The transmission lines used in a transmission line has been temperature sampling system are a critical element. In a treated to achieve a repetitive phase- typical system, all lines are of the same temperature characteristic for reliable electrical length, exhibiting the same tracking. This is achieved through heat _ amount of phase shift. Typically, " or ½ " cycling at the factory that relieves the foam coaxial cable is used for sample lines. mechanical stresses that are part of the When installing the system, the line to the manufacturing process. Phase-stabilized farthest element in the array is laid out and transmission line is considerably more cut. Then, the remaining lines are cut to expensive than normal line, and its value is match the longest line. To achieve equal questionable. After a few weeks or months phase shifts in all the lines, it is then in service, a normal transmission line necessary to measure the electrical length of becomes naturally phase-stabilized, and if the lines, either with a time domain the lines are all installed in the same reflectometer (TDR) or by measuring the environmental conditions, the phase shifts of resonant frequency of the transmission line. all the sample lines in the system will Through careful measurement and trimming, change by the same amount with it should be possible to achieve phase shifts temperature variations anyway. from line to line. ° within 0.1 When installing sampling 11.4 Antenna Monitors transmission lines, it is important to install The final element in a directional all of each of the lines in similar antenna sampling system is the antenna environmental conditions. For example, if monitor. This device terminates the sample one line is buried underground, all the lines lines and electronically compares the should be buried underground. If the last 20 relative currents and phases of the samples. feet of one line is above ground in the One tower for each mode of operation is transmitter building, the last 20 feet of all selected as the reference tower, or the tower the lines should be similarly installed. to which the other tower samples are Excess lengths of line, such as that of lines referenced. The indicated phase of the that run to the nearer towers in the array, reference tower is usually zero, and the should be stored in the same conditions as current ratio is 1.000. the rest of the lines. Typically, if the sample The phases of the other towers in the lines are buried underground, the excess line array are displayed as leading or lagging is also buried. The purpose of this is to that of the reference tower. For example, if maintain a uniform phase shift from line to ° on the one tower in the array indicates -80 line. Were all of one line buried and another antenna monitor, that means that the current line half buried and half in direct sunlight, in that sample is lagging 80 ° behind the considerable variation in phase shift

35 AM Antenna Systems Alexander/35 current in the reference tower sample. calibrated. With modern moment method The currents in the other towers in computer modeling, however, it is possible the array are displayed as a ratio, or a to closely predict what the actual driving percentage of the current in the reference point ratios and phases should be. With an tower. For example, if one tower in the array accurate sampling system and this computer indicates a ratio of 0.650 on the antenna model in hand, it is much easier to set up an monitor, the current in that element is 65% array. of that in the reference tower. More important than accuracy, Older antenna monitors could only however, is stability. Since a directional select and monitor one tower at a time. The array is set up and the proper indicated loop reference, or calibration of the phases and ratios are determined by field reference tower current sample, had to be strength measurements, it is more important adjusted each time before the remaining that a sampling system indicate changes ratios were read in order to insure accuracy. from the properly adjusted indications than This type of antenna monitor was difficult it is that it accurately resemble the actual (but not impossible) to accommodate with a currents and phases in the system. remote control system. Modern antenna monitors 12.0 Control Systems simultaneously monitor all tower samples at An important part of most directional once. The loop reference is continuously and antenna systems is the control circuitry. The automatically set. An analog voltage sample majority of stations operating with a representing each tower’s ratio and phase is directional antenna have more than one continuously available, making remote mode of operation. Some stations operate monitoring of the antenna monitor easy. non-directional daytime and directional at These modern antenna monitors also have night. There are a few that operate just the an amplitude mode, where the actual RMS opposite — directional day and ND at night. voltage of the sample is displayed. If the Others operate directional day and night, but loss of the transmission line and the volts with different patterns. Some stations have per amp output of the sample transformer (if yet a third mode or pattern for critical hours. used) are known, this display can be used to Some means of switching between patterns remotely monitor base currents, a very and modes is needed. handy feature indeed! 12.1 Why switch? A station that operates non- 11.5 Accuracy Because of distortions in the directional part of the time and directional sinusoidal current distribution in the towers during other times must have a means of of a directional array that are caused by detuning the unused towers. The same is mutual coupling between the elements, it is true of multiple pattern directional arrays just about impossible to obtain a sample that where some towers in the array are not used resembles the theoretical phases and ratios in some modes. The method used to detune of the properly adjusted array — no matter the unused towers differs with the situation, how carefully the sampling system is and some engineers prefer one method over

36 AM Antenna Systems Alexander/36 others. Whatever method is used, however, Another type of RF switch that is to implement the detuning it is necessary to sometimes seen in situations where lower switch out that tower’s directional network power levels are involved is the vacuum and switch in a detuning network at the relay. Depending upon the impedance of the tower. RF circuit where the relay is inserted, this When operating with different type of switch can be a very economical directional modes, it is often necessary to alternative. Vacuum relay coils draw very switch in different components in the little current, their operation is very fast, and antenna tuning unit (ATU) for the different there is generally only one moving part. At modes. For example, the daytime directional higher power levels or in circuits where RF pattern may require a 90-degree lagging voltages or currents are high, a vacuum network at one tower, while the nighttime relay may not work. pattern may require an 80-degree leading While RF contactors are generally network. Two separate tee-networks would ruggedly designed, they can easily be be required, and a means of switching damaged by lightning, arcing contacts and between them would be needed. the like. Lightning, if not properly shunted, may jump from the RF conductors over to the motor or solenoid as it seeks ground 12.2 RF Contactors through the AC neutral. The results are Switching of components and predictable — a burned out solenoid or networks in an AM antenna system is motor. Limit switches are frequent usually accomplished using different casualties of lightning as well. variations of the RF contactor. This device RF arcs can occur if the excitation is uses sturdy, plated finger-stock and plated not completely removed before the contactor shorting bars to create a high-current goes into motion. An improperly designed capacity switch. Several individual SPST or operating control system may allow this switches are typically arranged on an condition to occur. When it does happen, insulated frame with a rocker-type armature arcs develop between the finger stock and to create SPDT and DPDT switch the shorting bar. The plating is destroyed at arrangements. 110/220-volt AC solenoids or this point, and eventually the shorting bar electric motors are used to actuate the may become spot-welded to the finger stock armature. Solenoid-driven switches are very or the supporting frame, preventing the fast and provide almost instantaneous switch from moving. Another effect is that transition between switch states, but the the points which have arced tend to be poor current draw of the solenoids is high and the connections and heat builds up there as rather violent mechanical action produces current flows. This heat can destroy the significant wear and tear. Motorized RF switch in a short period of time. contactors are much gentler and they typically require very little current to operate, but they are mechanically more 12.3 A Proper Control System At the heart of any multi-mode AM complex and are quite slow to operate, antenna system is a control system to taking a full second or more to transition manage the operation of all the RF switches between states.

37 AM Antenna Systems Alexander/37 and transmitters. The job of this system is to 50 kW daytime transmitter be allowed to properly sequence all the events that must come on feeding a system with one ATU take place as a mode change occurs and to still configured for the 1 kW nighttime prevent excitation of the system without all pattern, in all likelihood the capacitors in the the switches in the proper position. ATU will be destroyed, and the transmission As an example, let’s consider a line is at serious risk of being damaged as multi-tower directional array that must well. switch between DA-day and DA-night modes. When the command is received from 12.4 Safety Functions the local pushbutton or the remote control to Operator safety functions should also switch patterns, first the controller must be incorporated into an antenna control mute the excitation. This is achieved either system. In the systems I design, I always by opening an interlock, disabling the drive insist in fully interlocked phasor cabinet or inhibiting the plate circuit of the doors so that opening any door or cover transmitter in use at the time. After a preset plate on the phasor will remove the RF amount of time has passed (usually a tenth excitation from the system. I also to a quarter of a second or so to allow the incorporate a key-switch operated interlock transmitter excitation to shut down bypass circuit which will allow operation of completely), a command to move all the RF either transmitter into the dummy load while switches in each of the antenna tuning units the main is on the air. We have all been in and phasor is initiated. Once a tally is situations, some stressful, where we get in a received from all the switches showing that hurry and perhaps forget to turn the they have successfully moved to the new transmitter off before opening an equipment position and another preset amount of time access door. It is important that we protect has passed, the transmitter excitation is ourselves and other workers from these reenabled. From pattern selection to situations with safety features. successful completion of the switch, if everything is properly adjusted, should be 12.5 Relays vs. Logic less than a half second — well within the Over the years, antenna control recovery time of most AM receiver AGCs. systems have traditionally been designed In a properly designed, properly adjusted using relay logic. Throughout my career, I system, listeners will notice little more than have often thought that these same functions a pop when the switch is made. could be achieved easily and more If something goes awry during the economically using logic circuits instead of switch, say a contactor does not move, the mechanically latching relays and time-delay control system should sense this and prevent relays. That is true, but with the control the excitation from coming back on. Phasor system indirectly connected to all that steel and ATU components, including sticking up into the air, the mean time transmission lines, can be seriously between failures could be measured as the damaged or destroyed if excitation is time span between the last repair and the allowed to come on feeding an incorrectly next thunderstorm! Relays are fairly configured system. Should, for example, a immune to the large surges that are bound to

38 Alexander/38 AM Antenna Systems come in when lightning strikes, and they can that the system power supply must be on handle large amounts of current. and all the relays/switches in the proper Only recently, however, one position for the selected mode before the manufacturer has come out with a PCL transmitter will be allowed to come on. (Programmable Logic Controller) based Redundant power supplies are common, antenna switching controller. This controller since the failure of the controller supply will is completely programmable to operate a set prevent the transmitter from coming on, of RF switches and transmitter interfaces in even if all the RF circuitry is correctly any combination that can be imagined. The configured! interface to the outside world has been Because the control system is so ruggedized to give the system good central to the transmitter site’s operation, surge/lightning immunity. periodic checks of the power supply voltage Whether relay controlled or logic are important. Also, it has been said that controlled, the fact remains that either cleanliness is next to godliness. This is motors or solenoids that run on either 110 or especially true when it comes to relay-based 220 volts AC must be actuated by the control systems. A dirty relay here can keep control system. There are two ways to do the station off the air. this: One involves running a constant AC The limit and travel switches on power feed to each tower and using low contactor motors or solenoids are especially voltage slave relays to actually apply the AC prone to give trouble. Because of the rather power to the solenoids or motors. The violent action of solenoid operated advantage with this method is that a single contactors, these switches really get run of AC power cable can be run to each slammed open and shut. Proper adjustment tower and other AC powered loads — tower of limit switches and actuating hardware is lights, ATU lighting, utility outlets, etc. — critical. can be operated from that same source. The For some reason, in those areas that down side is the added components (relays, are plagued with them, fire ants seem to be sockets and wiring) necessary. attracted to AC power (you guys in the The other method involves switching north, get ready — they’re coming!).I have the AC current to the motors or solenoids seen fire ants completely pack solenoid directly from the controller. If solenoids are mechanisms to the point where the contactor used, this involves running a rather large could not move. Periodic cleaning out of (#10 or better) conductor to each side of these areas with a vacuum or high pressure each solenoid plus a neutral. If motors are air is necessary to keep the mechanisms used, smaller conductors can usually be moving freely. Also, a few moth balls in a used. A separate source of AC power must cup left in the ATU will go a long way then be run to each tower for the ancillary toward keeping ants, wasps and other pests loads. away. Finally, it is important to periodically test all the interlock and safety 12.6 Care and Feeding circuits in the system. If the transmitter Most modern antenna control excitation is not being completely killed systems are built to be fail safe. That means

39 AM Antenna Systems Alexander/39 before any contactor starts to move, arcing of the mutual coupling between the array and arc damage will occur. An improperly elements. working interlock can get you killed. Test You have probably noticed that if these features at least once a year. you crank the phase of one tower out a The antenna control system should couple of degrees and touch nothing else, be invisible if it is working properly. By there is a change in all the other phases, starting with a good design and continuing ratios and base currents in the system in with good maintenance, you can achieve addition to a change in the common point trouble-free operation for many years. resistance/reactance. The same thing generally holds true no matter what the cause of the original change, be it a phasor 13.0 Troubleshooting control being adjusted or a component From time to time, things go wrong changing value. with all antenna systems. Things are bad When an engineer calls me saying enough when it happens with a simple non- that something is wrong with his array, I directional antenna. When a multi-element always start by asking him if the common directional array goes haywire, it can be point impedance has changed. If it hasn't, we maddening! start with the antenna monitor. Rule number one in this situation is A good next step is to check the The tendency is to grab DON'T PANIC! antenna monitor by swapping its inputs phasor controls and try to correct the around. A common failure mode in some situation immediately; this is the wrong antenna monitors is a stuck or open relay. until you thing to do! Don't touch anything The mercury-wetted relays in these units have enough information to make an should be trouble-free and long-lived but educated decision of what to do. they tend to wear out over time. I have replaced scores of them over the years. 13.1 Incorrect Antenna Monitor When a relay sticks, it may cause all the Parameters tower readings other than the reference Let's assume that one or more of the tower readings to be incorrect. To check for parameters as shown on the antenna monitor this, disconnect all inputs but the reference are at variance with their proper values. tower, then connect one of the other tower Start by making a note of the proper values sample lines to each of the other inputs in on a piece of paper with the values as read turn. You should see the normal indication on the monitor alongside. In this way you of phase and ratio for the sample line being can quickly see what parameters are at used as it is moved from input to input. variance. If only one or two values are out When you come to a channel where you do of whack while all the others are normal, not get the correct readings, that is the one there is a good chance that the problem is with the bad relay. If there is a stuck relay, it with the sampling system or monitor and the tends to load the other channels so while the array is functioning normally. As a rule, phase readings may be normal during this when something changes in an array due to procedure, the ratio readings will often be a component malfunction, all the parameters the channel with the defective low for all but are affected to some degree. This is because

40 AM Antenna Systems Alexander/40 relay. Sample loops can cause trouble, with Occasionally, antenna monitor welds and insulators breaking. Sometimes sample line terminating resistors can high winds can blow loops around so that become damaged by arcs or lightning they are no longer properly oriented. A strikes. The symptom will be a very high sample loop should be positioned so that it ratio on one of the antenna monitor is perpendicular to the tower face behind it. channels. Check these resistors with an A good way to check loop alignment is to ohmmeter while the sample lines are stand at the tower base and look up at the disconnected. They should all be very close loop — if positioned properly, the loop to the same value. should line up with the guy wire. Inspect the The detector diode in your antenna loops up close, looking for corrosion, loose monitor is one of those "future failure connections and hardware, etc. Most loops components" to watch out for. Usually this attach to the sample line with an N- or UHF- diode is a germanium type, very prone to connector of some sort. Check these damage from lightning. If this happens, connectors for water, corrosion, etc. There hopefully it will open completely. I have may be a copper strap or braid used to seen them become non-linear, however, jumper from the connector to the open end giving incorrect readings on all towers. The of the loop. This strap can easily break loose symptom of this condition will be a from the loop. Be sure that it is well bonded significantly changed loop reference setting to the metal of the loop. on the reference tower. If you have to crank Toroidal transformers can, from time that control more than half a turn to get to time, cause problems when used in a 100% on the loop meter, suspect the sampling system. Some are prone to arc detector diode. Another failure mode occurs internally when their output is unloaded. when a resistance develops in the detector While sample lines should always be diode. The symptom in this case is excessive terminated in the load resistors in the ratio meter wiggle with modulation, and all antenna monitor, it is possible that enough the ratios tend to be incorrect. voltage could develop at the tower end of a Another possibility is the faulty long sample line to allow an arc to occur. If sample line. You can check your sample a transformer is suspect, swap it with one lines by running an open circuit/short circuit from another tower (any but the reference impedance test on them. This will give you tower) and see if the problem disappears. the characteristic impedance and approximate electrical length of each of the 13.2 Incorrect Base Currents lines. You can also bridge the sample lines The most suspect indicating open-circuit at an odd quarter-wavelength instrument in any radio station is the base resonant frequency to determine exactly the current ammeter. They lie like a slick electrical length. The best way to check politician. From the moment they leave the sample lines is with a time-domain factory their calibration becomes suspect. reflectometer (TDR). These devices are Vibration, magnetic anomalies, temperature, generally available for rent, and many tower humidity, insects, moisture — everything — riggers now have TDRs in their shops. affects their accuracy. Toroidal current

41 AM Antenna Systems Alexander/41 meters can also lie, but they are more failure component is the mica capacitor. I reliable than thermocouple meters. If always suspect them first, but they aren't everything else is okay (antenna monitor always the problem. parameters and monitor points), suspect one A symptom common to component of the meters. The very best way to keep failure is heat. Shut the system down and feel of all the but carefully base current meters accurate is to use one immediately components in the phasor and ATUs. Some meter at all the towers, carrying it to the components may be warm but none should towers and plugging it in when needed and run hot. Suspect any hot component. Look at storing it in the controlled environment of all the coils for discoloration caused by the transmitter building when not in use. heating. Loose hardware can get red hot This can be done with either type of meter. under current, causing oxidation and an intermittent connection. Look for leaking 13.3 High Monitor Point capacitors as well. A likely cause of a high monitor If you have an RF bridge on hand, point reading is an anomaly at or in the use it to measure the reactance of any vicinity of the point itself. Reradiators and suspect components. If you aren't fortunate other factors beyond the station's control can enough to have a bridge at your disposal, influence the field strength at a monitor you can use a capacitor checker. Many point. If you find a point high, don't adjust stations have the old-style "magic eye" anything. Measure five or six points on the capacitor checkers lying about. Use a radial and see how they compare to the last capacitor of known value to test the checker full or partial proof. If they are in, you can before testing the suspect cap. Take lead assume the array is in adjustment and the inductance into consideration when reading monitor point itself has become unusable. capacitor value on the checker. If you don't §73.158 specifies the procedure for have a bridge or capacitor checker, many of changing the monitor point on a radial. the better digital multimeters (some in the If the entire radial is high, the array under-$100 range) have a built-in capacitor may be out of adjustment, even though the check function. It might be worthwhile to array parameters are all within tolerance. invest in one. This can easily occur in arrays with very There are many things that can go tight nulls. Before you start cranking, wrong with a directional array and there is though, it is a good idea to put the array in usually a ready fix for each type of problem. the non-directional mode and look at five or Seldom, however, will cranking on the six points along the radial both ND and DA. phasor repair a problem. You may be Compare the ratios with those in the last full chasing a bad antenna monitor or sample proof. It could be that a conductivity change line, or compensating for a component with is responsible for the high readings and the a changing value. When trouble comes, stop, array is in adjustment. and troubleshoot the problem through think thorough investigation and logical thinking. 13.4 Faulty Component Crank on the array only after the problem From time to time, parts do fail in has been diagnosed and fixed. directional antennas. The most common

42 AM Antenna Systems Alexander/42 authority as to the proper input power), so be sure you use the common point current 14.0 Regulatory Requirements value specified there rather than one The FCC regulates broadcast calculated for the station’s nominal power. stations, and in the past, there were many §73.1215 lists the requirements for and more specific regulations than there are indicating instruments, including common now. In the age of deregulation, the rules are point and base current ammeters. Their full of the seemingly harmless phrase, “as accuracy must be within 2% of the full scale often as necessary to insure compliance”. reading, and the full scale value must not be This phrase took the place of specific more than five times the nominal reading. intervals for various readings, calibrations Further, the meter must not read off-scale and the like that were once in the rules. during modulation. There are certain other Now, although we have the freedom to make requirements pertaining to scale divisions these measurements on our own schedule, and the like contained in that section. If you the monkey is clearly on our backs to keep are employing a meter manufactured or all the parameters within the terms of our supplied by one of the mainstream broadcast station licenses. If you are caught with a equipment manufacturers, you can be parameter out of tolerance, you are likely to reasonably sure that these minor not only receive a Notice of Apparent requirements have been met. Liability (NAL) for the parameter violation You should be sure, however, that but for not having measured it as often as the normal reading is greater than one-fifth necessary to insure that it is within full scale, and that the meter does not exceed compliance as well! That may not seem fair, full scale under modulation. Many times, but it goes to show that deregulation may following a lightning strike or some other not be such a good thing after all. incident, an RF ammeter is replaced with We will begin in the transmitter whatever is currently available. Particularly building and take a walk through the in the case of low power operation, regulatory requirements pertaining to every sometimes the replacement meter will not be part of an AM antenna system. of the proper scale. If you find yourself in this position 14.1 Antenna Current and you are using a toroidal RF ammeter An often misunderstood provision of (such as those manufactured by Delta the FCC’s rules pertaining to operating Electronics), you can increase the sensitivity power in directional antenna systems says of the meter by running multiple turns of the that the authorized input power shall exceed RF feed through the toroid pickup. The the nominal power by 8% for stations reading will be proportional to the number authorized 5 kW or less, and by 5.3% for of turns through the donut (i.e. two turns stations authorized more than 5 kW (see will double the reading, three turns will §73.51). That means a 5 kW station triple it, etc.). For example, suppose your employing a directional antenna will employ low power nighttime operation has a an antenna input power of 5.4 kW. The licensed common point current of 0.2 station license should reflect the correct amperes and the lowest scale meter you can input power (and is, in any case, the final

43 Alexander/43 AM Antenna Systems find is five amperes. By wrapping five turns water in the soil. Similar (and sometimes through the toroid, the 0.2 ampere reading more dramatic) changes can occur when the will appear as a 1.0 amp indication, which is ground freezes. If you have a common point in compliance with the rules. If you do this, bridge built into your phasor, you are all set however, you will need to file a form 302 to make a resistance measurement every (license application) with the FCC to modify time you visit the site. Otherwise you will the station license to show the new indicated have to use an operating impedance bridge current. to make the measurement from time to time The requirements for remote reading as required. If the resistance has changed ammeters are contained in §73.57. Remote more than 2% from the value specified in reading ammeters must be accurate to within the license, you will need to file an 2% of the reading on the regular (local) RF application for modification of license. ammeter. The sensors or pickups for remote reading RF ammeters must be located on the 14.3 Sample Systems transmitter side of the local RF ammeter. If In directional systems, next to RF your site employs remote reading RF ammeters, the sampling system is the most ammeters, it should be part of your regular likely element to produce false indications. routine to compare the readings between the The FCC rules contain specifications for local and remote meters. sampling systems, and specifically contain RF ammeters, particularly the requirements for “approved” sampling thermocouple meters, are the most likely systems. The basic requirement for an indicating instruments in an AM antenna “approved” sampling system is that the system to give incorrect readings or trouble. transmission lines exhibit uniform phase It is an excellent idea to keep a new, factory- difference from the shift (less than 0.5 ° calibrated meter (preferably a plug-in type shortest line to the longest line with normal meter) in the transmitter building. From time temperature variations). to time (at least annually), all the meters in Should some component in the the system can be calibrated against this sample system become damaged or reference meter. malfunction, it is permissible to operate for up to 120 days without notifying the FCC as 14.2 Antenna Resistance long as all other parameters (base current Periodic checks should be made of ratios, monitor points and common point the resistance at the common point or non- current) are maintained. Should the outage directional antenna base. The frequency of exceed 120 days, the FCC must be notified this resistance check will vary from station and special temporary authority requested. If to station, and will probably be determined portions of the system above the base by the nature of the environment around the insulator are replaced, a partial proof of antenna. Areas with poor ground performance will be required. Sample conductivity or sites with poor or system rules are contained in §73.68. deteriorated ground systems may see a considerable shift in base or common point 14.4 Monitor Points resistance with changes in the amount of Field strength measurements at the

44 AM Antenna Systems Alexander/44 monitor points (MPs) of directional stations §73.62. must be made “as often as necessary to ensure that the field at those points does not exceed the values specified in the station 14.6 Fencing authorization”(§73.61). This is one of those §73.49 requires AM sites using instances where it doesn’t matter if you read series-fed, folded unipole and insulated base the MPs yesterday and they were correct, if antennas to enclose the base of each tower the FCC inspector finds one or more of them within a locked fence. The size of the higher than the licensed limit today – you enclosing fence is determined by the E- and are likely to get dinged both for the high MP H-field present in the vicinity of each tower and for not measuring the points often base. The safe thing to do is fence each enough. If you do not have an approved element in the array at the radius specified sample system, the same requirement in the FCC’s OET Bulletin No. 65 for the applies, but the interval between nominal power of the station. By doing this, measurements must not exceed 120 days. you can be assured that if something should go wrong and all the transmitter power is fed to any one element, no person can enter 14.5 Directional Operating Parameters On the antenna monitor, the into a field that exceeds the ANSI limit. Of indicated ratios must be maintained within course, you can fence the entire perimeter of 5% of the values specified in the station the property and forego fencing of the license. Phases must likewise be maintained individual elements, but in that case you must otherwise delineate the areas where the within 3 ° of their licensed values. At the RF radiation exceeds the ANSI limit. tower bases, base current ratios must be maintained within 5% of their licensed 14.7 Proof Documents values. Note that it is not the base current Finally, there is a requirement in values themselves that are licensed, it is the §73.154 that the results of the most recent ratios. set of directional partial proof of Should an instance arise where the performance measurements be kept in the parameters cannot be maintained within the station records and available to the FCC for prescribed values, you must measure the inspection. Many stations, however, do not field strength at each of the MPs. If they are have a copy of their last proof on hand. If all below the licensed values, you may you find that you do not have this document, continue to operate at full power for up to 30 you can order it from one of the copy days without further authority from the FCC services in Washington. Not only will while the problem is being corrected. If one having this document on hand comply with or more of the MP field strengths exceeds the rules, it will provide you with a the licensed maximum, you must reduce benchmark for your directional system. power to a level that brings the high MP Present readings can be compared to those field strength below the maximum. The in the proof to determine if a perceived rules pertaining to directional antenna problem actually exists. system tolerances and procedures for operating at variance are contained in

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