What It Looks Like

Given the range of avionics available in general aviation airplanes today - dual navigation receivers and communications transceivers, ILS glideslope receiver, transponder, marker beacon, GPS and/or loran, ADF, DME, lightning detector, collision avoidance device, radar altimeter, weather radar, and a mandatory ELT - it's no wonder antenna sticks and blades and wires and bumps seem to sprout from every available patch of real estate on the airplane. How do you tell which tree in that forest does what?

The basic answer to the antenna identity question is shape and size and, to a lesser extent, location. The antenna for each avionics receiver, transmitter, or transceiver has a unique size and shape based on the frequency range in which it operates.

As a general rule, the length of the antenna determines its operating frequency; the lower the frequency, the longer the radiating element. A VHF navigation antenna, which has a frequency range, or bandwidth, of 108 to 118 MHz, has a radiating element that is about 22 inches long.

Voice communications radios cover a slightly higher VHF spectrum - from 118 to 136 MHz. At about 20 inches long, a com antenna is slightly shorter than a nav antenna.

The radiating element on a nav or com antenna may be a stainless steel bent whip rod or 20 to 22 inches of copper wire coiled inside a 12-inch-long blade antenna. Most modern nav and com antennas intended for use on light general aviation airplanes today are straight fiberglass-coated rods that extend at a slight angle from the base back toward the tail of the aircraft.

The rear antenna in the photo appears to be slightly longer, so it's probably a VHF nav antenna. The other likely is a VHF com antenna.