By Jonathan Imberi
Both vertical and beam antennas are suitable for most VHF and UHF base-station operation. Vertical polarization is almost always used for VHF and UHF FM and repeater operation.
The electrical-field characteristics of a radio wave define polarization. Polarization is very simple: it can be thought of as how the antenna is positioned. A dipole antenna, or an antenna that is parallel to the Earth's surface, will produce horizontally polarized radio waves. On the other hand, an antenna that is perpendicular to the Earth's surface, like a quarter-wave vertical antenna, will produce vertically polarized waves.
Polarization plays its biggest role when installing antennas for operation on VHF or UHF because propagation at these frequencies is mainly line-of-sight. The polarization of a transmitted signal, which is terrestrial (on the ground rather than in space), does not change from the transmitting antenna to the receiving antenna. Signal reception is best when both the transmitting and receiving stations use the same polarization. Because the polarization of a HF signal can change many times as it passes through the Earth's atmosphere, antenna polarization is not as important on the HF bands.
Almost all amateurs use a vertical antenna of some kind of their mobiles, because placing a beam antenna on a vehicle is very impractical. There are many different types of mobile antennas, ranging from those that mount on a car roof or trunk lid, to those that mount on a glass window.
VHF/UHF FM and data communications are almost always done with vertically polarized antennas. These types of antennas are very popular for repeaters because of their widespread use on mobiles. Another reason vertical antennas are very popular for repeaters and base stations is because they are not directional. Distance communication on FM is also done best with a vertical antenna. CW and SSB operation on UHF/VHF, however, is done almost always with a horizontally polarized antenna. For those amateurs wishing to work both VHF weak-signals and FM repeaters, it will probably be best to have separate antennas for each. This is mainly because at the VHF and UHF frequencies signal strength is affected drastically if your antenna has a different polarization than the station you are trying to communicate with.
Yagi beam antennas can have both horizontal or vertical polarization based on how they are situated. If the antenna's elements are vertical, the antenna will produce vertically polarized waves. If the antenna elements are horizontal, the antenna will produce horizontally polarized waves. In either case, the boom of the antenna will remain horizontal to the ground.
One of the most popular HF antennas is the quarter-wavelength vertical, because it provides low angle radiation when a beam or dipole cannot be placed far enough above the ground. Low angle radiation refers to those signals that travel closest to the horizon. This type of radiation is best when you are trying to contact distant stations. Vertical antennas, no matter what length, will always radiate vertically polarized waves.
Since most man-made noise is vertically polarized, using a horizontally polarized antenna will minimize the reception of this noise.
There is one last type of polarization to cover: circular. Orbiting satellites produce signals that are circularly polarized, so it helps if ground-based antennas receiving these signals are also circularly polarized. Just imagine that the Yagi beam antenna we talked about earlier had both vertical and horizontal elements, this would produce circularly polarized waves.