IEEE TRANSACTIONS ON BROADCASTING, VOL. 34, NO. 1 , MARCH 1988 63

A NEW AND SYSTEMS ORIENTED METHOD OF DIRECTIONAL ANTENNA TUNING

BY Frank S. Colligan

Senior Member, I.E.E.E.

5111 Westpath Court Bethesda,

Abstract

directional antenna system tuning has been developed. It takes into account and absorbs stray inductances and capacitances, mutual coupling between components and with the results of very high initial adjustment accuracy. No impedance bridges are needed and therefore the equipments that are used add up to only 14 pounds of weight. The desired results can be readily accomplished with greatly increased accuracy, convenience, speed and much more objectivity compared to the prior art. Tower base matching networks (L.T.U.s) can be adjusted with exceptional accuracy and rapidity. Stray self and mutual reactances are automatically absorbed and compensated for with little or no need to be concerned about their actual values. Bandwidth problems are easily detected and located.

The most profound feature of this new methodology is that no coordination of antenna monitor data and phasor trimming is needed while L.T.U. adjustments are being made. No antenna monitor readings or phasor adjustments are needed during L.T.U. network adjustments, a complete departure from the prior art! An entire system can be initially set up, with precision matched transmission lines, in exceptionally short time spans and by one man working alone. safe low power, a few watts, is used and hence adjustments may be made without nearly incessant and irritating on-off operation of the transmitter, running at substantial or high power. It has well proved itself in eleven field tune-up projects including three fifty kilowatt, two ten kilowatt directional antenna systems. Another consisted of a three tower in-line array with only sixty degree spacing between adjacent towers!

Equipment

New and special equipments and their usage are the key t the new procedures to be discussed and they will be described before the procedures are discussed. They must be used on an easily adhered to but very rigorous schedule of sequential priorities.

Any station will already have on board an antenna monitor and a field strength meter. Additional equipment consists.of a high precision reflectometer, a spring loaded and side handled split torodial clamp-on ammeter and a Hewlett-Packard HP-15 programmable calculator for llout- of-shirt-pocket" data processing.

A radically new and unique method of A.M.

Maryland 20816

The reflectometer is a special high precision unit with an exceptional 120 dB linear dynamic range. Combined with its high resolution it can quite readily resolve the difference between 50 and 51 ohms, a V.S.W.R. of 1.02 to 1. As such, it also functions as a power meter with an accuracy exceeding most state of the art power meters in general. Because of its very small size its series insertion inductance and shunt insertion capacitance is insignificantly small and many times less than prior art instruments.

The clamp-on ammeter features a wide dynamic range along with the capability of measuring the small currents (milliamperes) involved with very low power tuning. It reads only the current flowing in a conductor passinq throuqh its torodial center. Mechanically it resembles the typical electrician's 60 cycle instrument. Electrically it is designed and constructed specifically for R.F. use however.

A pocket size digital capacitance/ inductance meter is used to measure transmission line lengths and to keep track of capacitance values of vacuum variable capacitors on occasion.

It is important to note that the series/parallel insertion effects of the instrumentation described thus far, are greatly reduced ant practically nil in comparison to prior art conventional equipment.

The Hewlett-Packard HP-15 is a programmable shirt pocket calculator with nonvolatile program and data memory. It is programmed with a few routines used to analyze measured raw data. These programs are written to determine power distribution, transmission line matching conditions and phase shifts.

Initial Assumptions

It is assumed at the outset that the phasing system has been well designed with component adjustment ranges that cover all reasonable estimates and possibilities of tower driving point impedances. Estimates based on experience are equally as important as calculated values. Both are essential and the design of L.T.U. networks should feature adjustment ranges to deal with the highest and lowest extremes of driving point resistances and reactances at a given phase shift. The new method to be described drastically reduces the attention one must otherwise qive to driving point impedances during the actual tune-up. If necessary they can be

0018-9316/88/03OO-OO63$01 .OO O 1988 IEEE

64

checked after the array has been tuned. In fact that is the only time tower driving point impedances are really ascertainable.

Long experience my many in the industry indicates that despite nebulous results of base driving point impedance calculations and estimates, actual power distribution prediction using theoretical self and mutual loop impedances.

A long standing rule is that the power dividing and phase control system, in the transmitter building, must first be set up to theoretical values. Thereafter all further adjustments are to be confined to the tower base L.T.U.s, during which time phasor adjustments are kept to a minimum, preferably none.

standing rule it to make adjustments, one tower at a time, in descending order of power division. This keeps adjustment routines progressive while minimizing, ahd ofttimes eliminating, regressive ones.

Another long

The preceding foutr paragraphs form the basis for adjusting any directional antenna array by any method. This new method allows one to adhere to those rules evermore firmly. Among other features previously mentioned, one can easily accomplish the desired results with much greater accuracy, proqressiveness, obj ect ivity and rapidity, using very small and light weight instrumentation.

Stray Inductance and Capacitances

Phasing system components are usually large and may of them are installed in large cabinet work. A two tower phasor design is shown in figure 1. Its real world existence is shown in figure 2. Incidental or stray self and mutual reactances are shown in bold line. Self stray lead series inductive reactance can be reliably assumed to be about 4 ohms/foot/MHz. Stray shunt capacitive reactances can be assumed to be about 6 Pfds/foot. Stray mutual coupling between components and from components to cabinet walls is equally serious and attempts to analyze them in any detail would border on guess work. As small as such stray impedances may seem, they are nevertheless the source of serious, multiple and regressively cumulative errors when setting up networks with an impedance bridge. Ofttimes the mere presence of a large and heavy bridge can produce false results after the instrument is removed from close proximity to the component being measured. By themselves, impedance bridges may be quite accurate but they cannot be expected to produce accurate overall results when being used in the surroundings and circumstances described above.

One example consists of setting the reactance of a component that ultimately is to operate with both ends above ground potential, such as in the case of the series arms of a IITII network. One side of the bridge demands a ground and hence one end of the component must be temporarily grounded for measurement. In final, operation of such a component will not produce quite the same value as a bridge would lead us to believe. Further, if the component is high above the cabinet floor, one can even question the qround contact point! The new techniques to be described inherently minimize the number of different qround points one must refer to. In addition, the few grounds needed with these techniques are much closer to true ground than in the prior art.

The New Procedure

The new procedure starts with setting up the phasor itself, and by itself, as an isolated subsystem. It is set to theoretical values-of power division and phase shifting. Together with the phase controllers, the phasor is set to produce the desired voltage ratios and phase shifts up to but not beyond the transmission line input ports. The antenna monitor is used, serving as an excellent vector voltmeter. In addition it provides very accurate 50 ohm terminations for the power divider taps and transmission line ports. They already exist as an integral part of the monitor, at its inputs. Figure 3 shows the test points for the power dividers alone. They are set first. Equal lengths of RG-223 or RG-58 small coaxial test cables are routed from the antenna monitor to the power divider taps. With a few watts of power applied, the power dividers drive the antenna monitor directly and the taps are set to the desired voltage ratios. The resulting small phase shifts incidental to the power divider are recorded. The taps are of course inherently terminated in 50 ohm load resistances provided by the monitor. This presents the ideal time at which to set the common point input impedance to 50 + j0 ohms with the reflectometer mentioned earlier, the reflectometer no larger than a bar of soap.

Next the design phase shift are reviewed in light of prior transmission line length measurements and the incidental phase shifts resulting from the previous power divider adjustments. Slight redesigns of the phase controllers are usually necessary to compensate for slightly different transmission line lengths, resulting from installation trimming as well as the small shifts incidental to the power divider.

65

Figure 4 shows the phase controllers backed up to the power dividers. The test cables from the antenna monitor are transferred to the phasor output ports. The reference tower port feeds the reference input to the antenna monitor. No further adjustments are to be made to the power divider and the phase controllers can now be set to produce the same precalculated port-to-put voltage ratios, previously established, but at the desired total phase shifts. The output ports, later to feed the transmission lines, are terminated in the 50 ohm resistances provided by the antenna monitor. Judicious tap adjustments are made to the phase controllers to produce the same voltages previously set at their respective power dividers but with the desired ultimate phase shifts. By process of elimination, the input impedances of the 1:l phase controllers will automatically becomes 50 ohms resistive. This completes the entire phasor adjustment, with very high accuracy and to an extent not possible with a bridge. The stray lead self and mutual reactance effects are automatically absorbed and therefore ignorable!

At this point a word is necessary in regard to negative resistance towers. In setting up the phasor as described above, this poses no problems but the following must be kept in mind. As the phasor is set up there will of course be no negative resistance load terminating an output port assigned to a negative resistance tower. A certain power divider incidental phase shift has been measured as well as the transmission line lenqth. A phase shift value has been assigned to the L.T.U. at the tower base. This will call for a specific value of phase shift for the phase controller, all with due regard to a future negative resistance load (actually a source). The taraet value of outnut Dort Dhase shift a

is simply assumption resistance

obtainea on* the- temporary of a future positive

load. The kev point lies in the fact that be the load- positive or negative, the phase controller remains a bilateral passive network. The same is true of the negative resistance towerls power divider. The only difference will occur when the array is in final tune and a negative resistance load actually does show up. The common combining bus impedance will change and with a notable increase in resistance. The common point is simply reset as previously described.

L. T .U. Adjustment

L.T.U. adjustments at the tower bases can be set next but only after the phasor has been very accurately set up as previously described. This having been so easily accomplished, the antenna monitor test lead cables are removed from the phasor output ports. The J-plug blades are then set in place so that the phasor will now feed the transmission 1 ines .

Adjustments are made to each L.T.U. in turn and in strict accordance with descending order of power distribution. The required equipment consists of the reflectometer, the clamp-on ammeter and the I i P ~ 1 5 calculator. Common point input power is still a few watts and therefore the L.T.U. taps can be handled safely with power on. Starting with the highest power tower, the L.T.U. network is adjusted for a perfect 1:l V.S.W.R impedance match to the transmission line. This is readily accomplished with the reflectometer. The input, output and shunt currents are subsequently measured and keyed into the HP-15 calculator. The calculator's program will deliver tne phase shift. The tarqet value might be- 90 degrees and initially it could actually be -60 degrees. Next a turn is added to the input arm to "stretch out1' the phase shift. This is followed by adjusting only and strictly the output and shunt arms to reestablish a perfect match. If there is any doubt concerning the algebraic sign of the phase shift, a turn can be temporarily added to the input. If the slight increase in input arm inductance increases the phase shift, then the phase value is negative or lagginq. If the opposite occurs then the value is positive or leading. This check is rarely necessary since one knows at a glance whether the network is constructed to be a lagging or leading section.

Proceeding to the next lower power tower, the above outlined procedures are repeated successively until the required results are realized. Proceeding further and likewise with the remaining towers, in descending order of power distribution, a return visit is made to the first, highest power tower. With occasional exceptions, the measurements will show matching and phase shift to have wandered somewhat off of the desired design centers. They are simply retrimmed back to those centers. Another identical sequence of visits to each tower base is made and usually small offsets are trimmed out to bring the networks back to their design centers. Statistics, gathered thus far, indicate that the number of visits to each individual tower base L.T.U. will equal the total number of towers.

In the adjustment of directional antenna arrays, negative resistance towers provide numerous advantages. In this systems method of adjustments, the L.T.U at the base of a neqative resistance tower need only be adgusted for correct phase shift. Line matching will automatically %ikiftln into place as the remainder of the system is progressively adjusted. It must also be borne carefully in min that as one typically measures input, output and shunt currents, and in that order, the input is on the tower side of the network while the out ut is on the line side, in the case o+ negative resistance tower L.T.U.

66

As the sequence of visits to the tower bases continues, the design center offsets will inherently decrease until, on a final sequence of visits, all L.T.U.s will be found to be in an excellent state of adjustment. The phasor, in the transmitter building will be operating in the same correct way it was when feeding the dummy loads provided by the antenna monitor. Almost effortless precision line matching at the L.T.U.s serves to assure likewise matching at the phasor output ports when the accurately preadjusted phasor feeds all lines instead of 50 ohm dummy load resistors. In addition the close retention of design center phase shifts is virtually automatic. The rule of setting the phasor up to design values and leaving it alone during subsequent L.T.U. adjustments is much more easily and accurately conformed to my using this new technology.

The transportation of only fourteen pounds of test equipment is required as one makes the rounds of the tower bases. No assistance from the transmitter building is required. Trimminq the phasor while adjusting the L.T.U.s is not only unnecessary but, in fact, quite undesirable. No information is needed from the antenna monitor and sampling during L.T.U. adjustment!

With careful application of this technique, one can readily expect the first observation of data from the antenna monitor and sampling system to be within a few degrees of design center phasings and few percent of likewise loop current ratios. Again, no sampling system data is observed until after the total system adjustments are cE@iZted, the reverse of and a major improvement over ths prior art. On first observation they are typically found to be within F.C.C. tolerance specifications!

Component Safety Tests

Before applying full power, components are thoroughly checked to see that they will be operating well within voltage/current ratings. The primary focus of attention is give to the capacitors. their reactance values are derived from their labelled values. In cases of doubt or vacuum variable capacitors, the pocket size digital capacitance meter is used to verify the value. Knowing the resulting reactance, the voltage is obtained by measuring the current through it. With the common point at its desired value of resistance, it becomes a standard reference point. The relative current throuqh a capacitor, or any other component, is simply ratio compared to the common point current, for a constant low input power. What the current will be at full power can obviously be scaled up from the data taken under low power conditions. Future voltages follow in like fashion as noted above. Currents are routinely measured throughout the system and scaled up to full power values. Likewise upscaled voltages are derived therefrom. Knowing the power distribution, driving point resistances can be determined at any

point in the system including the tower base itself, by obvious means. Phase controllers can be checked by the same three current method used to set the L.T.U. phase shifts. This large amount of valuable data is readily obtained using the one pound clamp-on ammeter alone!

Full Power Testinq

Next the antenna monitor is connected to the sampling system and with transmitter power applied, the monitor parameters are observed (for the fjrst time!). This is followed by very minor trimminq of the phasor controls, in descending order of power distribution, to set the system precisely to the theoretical ratio/phase parameters to produce the desired radiation pattern.

Far Field Testinq and Readjustment

Directional antennas simply do not exist in pure environments free of a long list of potential parasite reradiators and assorted uneven and topographic effects. It is an extreme rarity to find the design values of ratio/phase parameters producing an acceptable far field radiation pattern let alone the design pattern itself. It has happened but statistically the odds are overwhelmingly against it. Compensating or counterdistorting adjustments must me made to arrive at an alternative set of ratios and phasings that do not bring the far field pattern into acceptable shape. This is done by a number of long established and well known procedures.

When a new set of parameters is determined to produce an acceptable far field pattern, the systemls networks will have to be reviewed. Power distribution will have shifted and line V.S.W.R. values will have increased to one extent or another.

The precision reflectometer, used in the watt meter mode, is used to measure the new and final values of power distribution. Using these values, the entire syste? 1s gone over from the beginning using all of the procedures discussed previously. The retrimming of both the phasor and all of the L.T.U.s will almost always be found to be quite minor and even more progressive.

Low Power Sources

Low power sources, roughly two watts per tower, are needed to drive the common point during the adjustments. One must be careful to %ring power up slowly11. With a Potomac Instruments Model m-19 is 20 volts and care must be exercised not to exceed that value. In cases where other monitors are being used, consult the instruction manual to determine the input voltage limits.

For driving the common point, the Potomac Instruments Model SD-31 Signal Generator will usually suffice for two and ofttimes three tower arrays. When higher power is needed the transmitter can be switched to feed a dummy load. Several watts may be available from the transmitter's modulation monitor port.

61

Broadbandinq

The location of bandwidth problems is easily accomplished with the measurement equipments used in these new techniques. All of the measurements previously discussed are simply repeated with the signal generator set to say, 10 kHz low. The resulting two sets of data are then compared to that of the carrier frequency. An assistant may be posted at the transmitter building to shift the signal generator at required times. Hence all three sets of data may be obtained in only one tour of the L.T.U. tower base networks.

Conclusions

This new methodology has been well proven in actual field practice and often to a surprising extent. By strictly gnd rigorously following the priority of sequential adjustments, in the order discussed above, small light weight equipment may be used to quickly and easily adjust a directional antenna system. By treating the networks from a systems point of view, adjustments can be made more directly and in terms of target values, goals and objectives. Stray impedances throughout the system are absorbed thus eliminating otherwise seriously misleading errors that result from impedance bridge usage. Myriad unnecessary and piecemeal steps involved in the prior art eliminated. Ultimate performance is obtained with greatly increased and consistent accuracy.

Appendix

Figure 5 shows a typical IITII network with current measuring points A,B, and C . The phase shift through this network is obtained by measuring the three currents, relative or absolute, and solving f o r the angle between the input and output currents using the law of cosines. The routine is easily stored in the program memory of an HP-15 or any other comparable calculator.

Fiqure 6 shows a I*Pil@ network which is occasionally used as a 1:1 phase controller. It is quite useful in cases where a modest phase shift is needed in the range of between 20 and 60 degrees. For phase measuring purposes it must be viewed as a pair of Vfl sections where the left arm of the first section and right arm of the second section are of zero reactance. Current measurements A , B & c are measured for the left side and phase angle. Currents D,E & F are measured for the right side angle. The two angles are then simply added together to obtain the total.

Best accuracy is obtained my measuring the currents strictly at points shown. Shunt currents in both the V.ll and the llPipl section cases should be measured on their ground sides. In the *lPi*l section case currents B and D are theoretically equal. In practice they can be somewhat different owing to the distributed, and usually unsymmetrical, capacitance of the inductor.

Figure 7 shows the current measuring points for an llL" section. An IIL" section can be properly viewed as a "T" section with one arm at zero reactance.

The table below shows a recently made comparison between the new and the prior art. A four tower phasor was set up by individual, one at a time, reactor adjustments with a bridge. Subsequently the antenna monitor was used to check the final results at the phasor output ports.

Port No. Desired Results Bridqe Results

1 . 4 4 4 (-166 .319 /-I40

2 1.000 e 1.000 & 3 . 7 2 2 /+21 .641 /+32 4 .714 /+52 .867 /+86

The errors were substantial and typical. The phasor was then adjusted to produce the desired results using the antenna monitor in the manner described earlier. The final results, after complete L.T.U. adjustments, were such that no further adjustment of the phasor was necessary except for a minor trim on tower No. 4 .

-0- I 5

I 1 T

$ FIG. 1 - T W O TOWER PHASOR UNDER IDEAL CONDIT IONS

M = MUTUALS

# PATH LENCTII’S SHOULD NEVER BE ALLOWED T O EXCEED 6“

F IG. 2 - REAL WORLD OF STRAY PARASITE IMPEDANCES

IDEAI.

I rf &

FIG. 2 A - IDEAL VS. REAL WORLD INDIVIDUAL COMPONENTS

LOW POWER I N -.

ANTENNA MONITOR

SET COMMON POINT 2 IUMkDIATELY AFTER POILP. DIVIDER SETTING EQUAL LENGTHS OF R G - 5 8 OR RC-123

klONlTOR PROVIDES 50 OHM LOADS ON P.D. TAPS

F I G . 3 - I N I T I A L S E T T I N G OF POWER DIVIDERS

ANTENNA MONITOR

NOTE: SET L1 TO MAKE EA LA = EB

F IG. 4 - FOR SUBSEQUENT SETTING OF PHASE CONTROLLERS

FIG. 6 - P I SECTION FIG. 5 - T S E C T I O N

- - - -7- - -- -

FIG. 7 - L SECTION