A New Approach to Beam Antennas |
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This article describes a method for tuning beam antennas from the ground rather than needing to experiment with matching capacitors and/or inductors at the antenna end of the transmission line. The scheme uses a separate length of transmission line attached to the reflector element at a length an integer number of half-wavelengths as required to reach the ground. That way, tuning elements can be installed and adjusted for one or more bands without needing to climb up to the antenna. The method can be used for any number of elements, but of course it could get a bit messy with more than a couple. The author, Major Charles E. Spitz (W7JHS), describes using a field strength meter to verify tuning correctness. Having read many QST magazine articles on antennas over the past decade or so, I don't recall ever seeing this technique presented. A New Approach to Beam Antennas
Obtain optimum performance over the entire band by tuning your beam to frequency from the ground. The amateur who erects a beam antenna is usually prepared for a lot of cut and try, and it takes it. Perhaps he copies or buys a commercial beam. The service technician buys a commercial beam for that FM or TV installation simply because he doesn't have time to fuss with it. The purpose of this article is not to deride the manufactured products. There are good assemblies available, but to illustrate the difficult nature of beam antenna installations and the fact that the process of tuning can be readily solved by the use of a simple method.
Fig. 1 - "Lazy H" antenna - feedlines go to X.
Fig. 2 - (A) and (B) are conventional beams. Tuning method described is shown in (C). After experimenting for some time with the described system, the author has achieved results that more than came up to expectations, and possibilities seem to be unlimited. Consider first a simple dipole and reflector. The radiator is usually cut for frequency and left at that length. Not so the reflector, however, as considerable adjusting must be done for either maximum forward gain, or front-to-back ratio at a particular frequency. Many surplus telephones have been pressed into service as a necessary aid. It's a two-man job to adjust a receiving beam, and a three-man job for transmitting - someone has to carry the field-strength meter! If the antenna is out of reach when finally installed, as many are, a derrick is a mighty handy accessory. Most people tune on the ground and then just hope it stays that way when the antenna gets into position. Look at Fig. 2. The classical method of adjustment is to slide small tubing ends into a larger diameter main piece, as in A. In B, tuning is accomplished by a small stub of a few inches and is the usual method for wire. It will be seen that the method in C is essentially that of B, in that the stub L can be tuned remotely (in effect) if the reflector feedline is tuned from a position any number of half-waves longer. Tuning a reflector with this system of parasitic excitation means there are standing waves on the reflector feedline; however, several experimental feedline lengths of fifty feet and beyond did not indicate excessive losses by virtue of increased lengths. The antenna used in the tests, the results of which are shown in the graphs, consists of a radiator composed of four half-waves in-phase, two elements stacked above two, and is popularly known as the "Lazy H." A similar curtain was placed one quarter-wave behind. It will be noted that the feedlines between the upper and lower elements are not transposed, being phased by center feeding, and present a load impedance of about 70 ohms. The resultant broadband characteristic is easily seen in the graphs. For those who may be unfamiliar with the antenna, it is shown in Fig. 1. Another advantage of remote reflector tuning is the fact that it may be tuned to any frequency, within reasonable limits, by simply turning a condenser dial at the equipment. This should be a major aid to the e.c.o. enthusiast, as well as valuable in conjunction with FM or TV channel selection. Feedlines used in the tests were RG-11/U and RG-8/U coaxial cable with little differences noted. A field-strength meter and a folded dipole were set up 300 feet from the antenna, and with a remote meter at the operating position and the reflector tuned for maximum forward signal, the meter was set so that zero on the graph indicated a maximum signal with 100 watts of r.f. power. A Model MM2 "Micromatch" was used to monitor power level and to keep a check on standing-wave ratios. A condenser dial reading of 100 indicated 570 μμfd. (two sections of 285 in parallel) and zero for minimum capacity.
Table 1 - Performance characteristics of the antenna described, measured at 28.5 mc.
Fig. 3 - Condenser settings for reflector tuning to give maximum front-to-back ratio.
Fig. 4 - Measurements of front-to-back ratio for three different frequencies, as the reflector tuning condenser is varied through its range. A tuned 55 foot coaxial feeder was used on the reflector for these tests. Due to limitations in the measuring equipment the performance figures below minus 26 db. were calcu lated on the basis of over-the-air reports. The graphs A, B, and C of Fig. 4 illustrate the field-strength and front-to-back ratios at all positions of condenser tuning at the frequencies of 28.5, 29 and 29.5 megacycles with a 55-foot coaxial feedline from the reflector. Accurate measurements were not possible beyond -26 db., and the -30 db. measurements were filled in by over-the-air reports. Fig. 3 shows an example of the utility of reflector tuning. A check was made for the point of maximum front-to-back ratio every 100 kc. from 29.7 to 28.4 mc., and with the aid of the graph, tuning could be set at any time at the best position for the operating frequency in use. As may be surmised, tuning the reflector affects the standing-wave ratio of the radiator feedline by changing the antenna impedance, and the effect is noted in Table 1; however, readings of forward gain indicated little field-strength loss for ratio orders of 2 to 1. Coverages of the frequencies 27,150 to 28,500 kc. was accomplished by use of a fifty-foot feedline. Actually a fifty-foot feedline is used with a five-foot extension plugged in for the 28.4 to 29.7 mc. range, since it was felt that introducing such variables as tuned coils would unnecessarily complicate the system. You may note that all discussions have referred to field-strength readings and wonder about receiving capabilities. It was noted that receiving checks in the amateur band were difficult to make due to fading, which would not occur in local FM or TV areas for such reception. An interesting application came to light, however, when the author was requested by KZ5AZ, who was visiting, to hook up with a Canal Zone station, so that he could talk to his wife. The antenna was swung in that general direction on a crowded weekend. W6 and W7 signals were pouring through by the hundreds, so as each signal was tuned in, the condenser was turned until the signal dipped or rose to indicate a southeastern signal. In that manner of DF-ing, five signals were quickly selected, three turning out to be Puerto Ricans, and, two in the Canal Zone, one of which, KZ5CJ (a few houses from KZ5AZ's home QTH), was quickly raised and all in a matter of a few minutes! The coaxial feedline to the balanced antenna proved poor in receiving discrimination. However a two-wire coaxial cable such as RG-22/U should improve receiving performance considerably, and in some installations molded parallel lines, provided impedances are matched, might be suitable. The business of trimming the reflector feedline for a desired frequency and band may bother you; however, it is suggested that whatever feedline length seems readily available be tried. If the results are not satisfactory, then the feedline may be added to until the proper frequency is reached within tuning range. This is not so difficult as it might seem, as at high frequencies the transmission-line velocity factor is taken into account. With a factor of 0.66 for coaxial cable, on ten meters a half-wave becomes about ten feet, and any tuning range within that band could be covered within any portion of it as an added length. A larger tuning condenser than that used, in order to obtain a greater bandwidth, could not be employed, as at maximum capacity the impedance becomes so low as to be the equivalent of a short circuit. Double the capacity was tried and was of no value, simply making tuning at minimum capacity critical. The value of reflector tuning may be questioned by some who wonder about the small forward gain changes indicated. It was mentioned that a broadband antenna was used in these checks which the curves bear out. A close spaced parasitic or Yagi antenna, being of a much higher Q should exhibit a distinct curve about the forward line. Where a broadband antenna is in use, tuning the reflector should be of value in reducing interference, BCI, or TVI, and for all reception, short-wave, amateur, FM, and TV it should increase external signal-to-noise ratios. Some thought was given to the use of a phasing section between two elements or curtains of an array, and tests were made. The results were poor, however, since at the reflector the direct wave, when off frequency, exhibited a phase difference to the radiated wave, and unless the spacing between the elements was also variable, the bandwidth would be limited. This effect would be aggravated by a low transmission line velocity factor. What about tuning all parasitic elements, such as directors, etc.? What about arrangements to steer radiation angles? The author has given some thought to these problems and is conducting further experiments. The sky seems to be the limit in tuning parasitic elements, and it surely seems that the manually-tuned beam is on the way out. Acknowledgement must be made here of the patience and valued assistance of Lynn Mutrix, W5O1X; Captain Bascom E. Tillotson, W5PDW; Chief Warrant Officer Chester B. Harmon, WOJG; Reynold B. Champagne, W4KQW; and the many others whose observations aided the work.
Posted February 20, 2020 |
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