VHF/UHF ANTENNAS

Improving an antenna system is one of the most productive moves open to the VHF enthusiast. It can increase transmitting range, improve reception, reduce interference problems and bring other practical benefits. The work itself is by no means the least attractive part of the job. Even with high-gain antennas, experimentation is greatly simplified at VHF and UHF because an array is a workable size, and much can be learned about the nature and adjustment of antennas. No large investment in test equipment is necessary.

 

Whether we buy or build our antennas, we soon find that there is no one best design for all purposes. Selecting the antenna best suited to our needs involves much more than scanning gain figures and prices in a manufacturer’s catalog. The first step should be to establish priorities for the antenna system as a whole. Once the objectives have been sorted out in a general way, we face decisions on specific design features, such as polarization, length and type of transmission line, matching methods, and mechanical design.

 

As has been discussed previously, shaping the pattern of an antenna to concentrate radiated energy, or received signal pickup, in some directions at the expense of others is the only possible way to develop gain. Radiation patterns can be controlled in various ways. One is to use two or more driven elements, fed in phase. Such arrays provide gain without markedly sharpening the frequency response, compared to that of a single element. More gain per element, but with some sacrifice in frequency coverage, is obtained by placing parasitic elements into a Yagi array.

 

Antenna radiation can be made omnidirectional, bidirectional, practically unidirectional, or anything between these conditions. A VHF net operator may find an omnidirectional system almost a necessity but it may be a poor choice otherwise. Noise pickup and other interference problems tend to be greater with omnidirectional antennas. Maximum gain and low radiation angle are usually prime interests of the weak-signal DX aspirant. A clean pattern, with lowest possible pickup and radiation off the sides and back, may be important in high-activity areas, where the noise level is high, or for challenging modes like EME (Earth-Moon-Earth).

 

In general, the higher a VHF antenna is installed, the better will be the results. If raising the antenna clears its view over nearby obstructions, it may make dramatic improvements in coverage. Within reason, greater height is almost always worth its cost, but height gain must be balanced against increased transmission line loss. Line losses can be considerable at VHF and above, and they increase with frequency. The best available line may be none too good, if the run is long in terms of wavelength. Consider line losses in any antenna planning.

 

A given antenna design for 432 MHz, say a 5-element Yagi on a 1-λ boom, will have the same gain as one for 144 MHz, but being only one-third the size it will intercept only one-ninth as much energy in receiving. Thus, to be equal in communication effectiveness, the 432-MHz array should be at least equal in physical size to the 144-MHz one, requiring roughly three times the number of elements. With all the extra difficulties involved in going higher in frequency, it is well to be on the big side in building an antenna for the UHF bands.

 

Whether to position the antenna elements vertically or horizontally has been a question since early VHF operation. Originally, VHF communication was mostly vertically polarized, but horizontal gained favor when directional arrays became widely used. Tests of signal strength and range with different polarizations show little evidence on which to set up a uniform polarization policy. On long paths there is no consistent advantage, either way. Shorter paths tend to yield higher signal levels with horizontal in some kinds of terrain. Man-made noise, especially ignition interference, tends to be lower with horizontal polarization. Vertically polarized antennas, however, are markedly simpler to use in omnidirectional systems and in mobile work, resulting in a standardization on vertical polarization for mobile and repeater operation on FM and for digital communications. Horizontal polarization is the standard for weak signal VHF and UHF operation. (Circular polarization is preferred for satellite work as described below.) A loss in signal strength of 20 dB or more can be expected with cross-polarization so it is important to use antennas with the same polarization as the stations with which you expect to communicate.