Basics of Very High Frequency and Ultra High Frequency Antennas for Ham Radio - dummies

Basics of Very High Frequency and Ultra High Frequency Antennas for Ham Radio

By H. Ward Silver

Most ham radio antennas used above 50 MHz at fixed stations are either short whips (thin steel or aluminum rods) or beams. Whips mounted so that they’re vertical are used for local FM operation, whereas beams — antennas that transmit and receive in a preferred direction — are used for VHF/UHF DXing (contacting a distant station) on SSB and CW.

The most common type of beam antenna is called a Yagi, after Japanese scientists Yagi and Uda, who invented the antenna in the 1920s. A Yagi has several straight rods or tubes (called elements) mounted on a long supporting tube called a boom. Log-periodics that look like large TV antennas with lots of elements are also types of beam antennas used by hams.

Polarization — the orientation with respect to the ground of the antenna and the radio waves from it — is most important on the VHF and UHF bands, where signals usually arrive with their polarization largely intact. If the radio wave and the antenna are oriented differently, the antenna won’t receive the radio wave very effectively.

FM operating is done with vertically polarized antennas because vertical antennas on vehicles are much simpler to construct and install. Vertical antennas are also omnidirectional, meaning that they transmit and receive equally well in all directions. These characteristics are important for mobile operation — the first widespread use of FM. To prevent cross-polarization, the base antennas are vertical. This convention is universal.

A popular and inexpensive vertical antenna is the simple quarter-wave whip, or ground-plane, antenna. Many hams build a short ground-plane antenna as a first antenna project.

Operators chasing long-distance VHF and UHF contacts use beam antennas that are horizontally polarized. Many of the long-distance VHF and UHF propagation mechanisms respond best to horizontally polarized waves. If you have an all-mode radio and want to use it for both FM and SSB/CW/digital operating, you’ll need both vertically and horizontally polarized antennas.

If you choose to use a beam on VHF and UHF bands, it’s a good idea to use 3 to 5 elements on 6 meters and 5 to 12 elements at higher frequencies. These antennas are small enough to mount and turn with heavy-duty TV antenna hardware.