# Microsoft Word AM Antenna Systems Paper.doc

## Transcript

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

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

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

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

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.

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

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

### Microsoft Word Antenna.doc

A-INFO A-INFO A-INFO A-INFO A-INFO Microwave A-INFO A-INFO A-INFO A-INFO Antenna Products 英联微波 WeChat ID: Horn Antenna Spiral Antenna Scan the QR code to follow us on Wechat: Microstrip Antenna A-INFO...

### 601

Approved by OMB COMMISSION FCC FEDERAL COMMUNICATIONS 601 Form 3060 – 0798 Main and Instructions Est. Burden Avg. Information Response: Per 1.25 hours Application for Radio Service Authorization: FCC ...

### SARA R4/N4 series

R4/N4 series SARA - System Integration Manual System Integration Manual - SARA R4/N4 Abstract This document - R4/N4 series describes the features and the integration of the size - optimized SARA cellu...

### APX Mobile O2, O3, O5, O7 & O9 Control Head Installation Manual

APX™ TWO-WAY RADIOS APX MOBILES O2, O3, O5, O7 & O9 CONTROL HEAD INSTALLATION MANUAL

### GSG 5/6 Series GNSS Simulator User Manual

GSG-5/6 Series GNSS Simulator User Manual with SCPI Guide Spectracom Part No.: 4031-600-54001 Revision: 26 Date: 16-Jan-2018 spectracom.com

### GOES RSeriesDataBook

GOES R Series - Data Book Prepared for National Aeronautics and Space Administration GOES - R Series Program Office Goddard Space Flight Center Greenbelt, Maryland 20771 Pursuant to Contract NNG09HR00...

### Programming Guide for ZPL II, ZBI 2, Set Get Do, Mirror, WML (en)

Programming Guide ZPL II ZBI 2 Set-Get-Do Mirror WML

### Microsoft Word Rep 6327 2008 0568 Issue 3 .doc

Page 1 of 87 ERA Technology Ltd ERA Business Unit: Report Title: RF Measurement Assessment of Potential Wind Farm Interference to Fixed Links and Scanning Telemetry Devices B S Randhawa (ERA), R Rudd ...

### UL White Book

GUIDE INFORMATION FOR ELECTRICAL EQUIPMENT THE WHITE BOOK 2015-16 UL PRODUCT CATEGORIES CORRELATED TO THE 2011 AND 2014 NATIONAL ELECTRICAL CODE® UL’s General Guide Information is updated daily. To co...

### SES02158.pdf

PUBLIC NOTICE FEDERAL COMMUNICATIONS COMMISSION 445 12th STREET S.W. WASHINGTON D.C. 20554 News media information 202-418-0500 Internet: http://www.fcc.gov (or ftp.fcc.gov) TTY (202) 418-2555 Wednesda...

### Microsoft Word ChoosingTheCorrectBalun 22Oct09.doc

Baluns: Choosing the Correct Balun By Tom, W8JI General Info on Baluns is an acronym for BAL anced to Balun balanced, which describes cert ain circuit behavior in a UN transmission line, source or loa...

### Non SCE Antennas Placed on SCE Poles

SOUTHERN CALIFORNIA EDISON (SCE) Non-SCE Antennas Placed on SCE Poles External Manual January 2019 Non-SCE Antennas Placed on Wood Distribution Poles

### catalog 2019

2019 ® HARLEY-DAVIDSON GENUINE MOTOR PARTS & ACCESSORIES

### RECOMMENDATION ITU R M.1177 4* Techniques for measurement of unwanted emissions of radar systems

Recommendation ITU-R M.1177-4 (04/2011) Techniques for measurement of unwanted emissions of radar systems M Series Mobile, radiodetermination, amateur and related satellite services

### Last Call for SATCOM Security

Research-fueled Security Services \ WHITE PAPER \ Last Call for SATCOM Security Ruben Santamarta August 2018

### zetacast whitespaces technical new.indd

Cambridge White Spaces Consortium Cambridge TV White Spaces Trial A Summary of the Technical Findings © 2012 Cambridge White Spaces Consortium

### AN627

AN627 PA M X 6 X AND EZR32 L OW -P OWER Si4 ATCHING 1. Introduction This application note provides a description of the matching techniques applied to the low-power Si4060 TX, Si4460/61/67, and EZR 32...

### ADN W AM Antenna Module

ADN-W AM Antenna Module Instruction manual

### Microsoft Word TCL Roku TV User Guide 3750 3800 series.docx

• Roku TV TCL User Guide M odel s : 28S3750, 32S3750, 40FS3750, 48FS3750, 55FS3750, 8 32S3 8 0 0, 32S3850 (A/B/P ) , 4 0 FS3 00, 5 0 FS3 8 00 2 Version 6. English Illustrations in this guide are provi...