How to Get Started:
Beyond FM: Antennas
By Jim Aguirre, W7DHC
In the previous article, we explored various types of VHF/UHF and microwave equipment; now it's time to take a look at antennas.
Unlike most FM repeater and simplex operations, weak signal VHF/UHF and microwave stations use high-gain directional antennas almost exclusively. They also use horizontally polarized antennas instead of verticals or ground planes. This can sometimes pose a problem for newcomers to the weak signal side as there is a substantial signal loss if both stations are not similarly polarized'sometimes as much as a 30 dB difference!
For the most part, directional antennas used in weak signal work fall into four categories; conventional Yagis, loop Yagis ('loopers'), dishes and feedhorns (or 'horns'). Of these, the most popular is the Yagi, at least on the lower bands.
Yagi antennas used at VHF/UHF and microwave frequencies are essentially the same in concept as those used at HF, but usually have many more elements and are more finely tuned. As you probably know, gain goes up with the number of elements, so weak signal operators tend to go with long Yagis and lots of elements
At the low end of the VHF range'50 MHz (6M)'a three- or four-element Yagi is an excellent choice for starters. They are relatively small in size and can be turned with an inexpensive TV rotor. For serious weak-signal work, you may want to go with something in the six- to eight-element range. A well-designed eight-element Yagi will give an honest 12-13 dB gain. Even larger antennas are available if you have a way to support and turn them. Really serious weak signal operators stack a pair, three or even four large Yagis for even more gain!
Beginning with 144 MHz (2M) and running through 432 MHz (70CM), multi-element Yagis are most commonly used, ranging from 10-elements to more than 20. Gain on these antennas can reach 20 dB. Again, stacked antennas on these bands are quire common.
A 12-element Yagi at 144 mHz is a good performer, as is a 16-element at 222 MHz and a 22-element on 432 MHz. All of these antennas have a boom length of less than 18 feet and have gain in the 13-15 dB range. They are quite easy to build and you can find specifications in the ARRL Antenna Handbook.
At 903 MHz and up, antenna preferences are mixed. Conventional Yagis are sometimes used at 903 and 1296 MHz, but loop Yagis seem to be growing in preference. They are similar to conventional Yagis; however, the elements are circular rather than straight. Loop Yagis are actually highly refined versions of the 'Quad' antenna. 'Loopers' may have from 30 to 100 elements. The 3456 MHz looper I just put together has 76 elements and it's not the biggest available.
Loop Yagis are also easy to build, but require careful measurement as elements are small and dimensions are critical. There are some looper designs published in VHF/UHF and microwave handbooks, and on the Internet.
There is also a growing use of dish antennas at the higher frequencies. Surplus satellite TV antennas can be adapted for use on bands from 432 MHz on up, but are, more commonly found at 1296 MHz and above. Long-used for 'moonbounce' operations, large dish antennas can provide very high gain'sometimes in the 25-30 dB range. It's not uncommon for serious mounbounce operators to construct 30-foot diameter dish antennas. On the other hand, on bands from 2304 MHz to 10 GHz, two- to three-foot dishes can be quite effective.
Feedhorns are used primarily on 3456 and up, and are essentially a conical or trapazoidal reflector surrounding a radiating element. They project a signal in a narrow beam and can be very high in gain. 'Radar guns' used by law enforcement officers are one example of a feedhorn antenna.
I know of no commercial source for this type of antenna, but they can be relatively easily constructed. The tricky part is getting a good feed system to work. VHF/UHF and microwave publications like the ARRL Microwave Experimenters Handbook and the VHF/UHF Project Manuals provide some information on constructing these antennas.
That's a quick overview of VHF/UHF and microwave antennas. Stay tuned for more 'Beyond FM' articles!.