Dummies Guide to Aircraft Antennas - by Michael Coats of XCOM Avionics
I offer a wide variety of antenna types for the many different aviation communication applications.
Transponder and Radio Antenna Cables
I offer transponder antenna cables made by Larry Goddard in several lengths. They include a TNC connector for connection to the transponder and a BNC connector for connection to a transponder antenna. They are available in 3 m (9.84 feet) and 5 m (16.4 feet) lengths. I also offer radio antenna cables from Larry Goddard in several lengths. They include BNC connectors on both ends, or a BNC connector at the radio end and bare leads (NC for "No Connector") at the antenna end.
Several transponder antenna options are listed on the Antennas page. The RAMI rod style and "shark fin" styles are the most popular. I recommend the shark fin style because I recall from my fluid dynamics courses that the two worst types of bodies in regard to drag are spheres and cylinders. For installations inside fiberglass, wood and fabric gliders, or in the vertical fin in carbon fiber gliders (with the fin made of fiberglass) you may want to consider the L2 antenna from AAE.
Aviation Radio Communication Band: 118.000 to 136.975 MHz
Aviation Radio Navigation Band: 108.000 to 117.975 MHz
Weather: 161.650 to 163.275 MHz
ELTs (standard): 121.5 and 243.0 MHz
ELTs (new): 121.5 and 406.028 MHz
DME: 950 to 1220 MHz
Transponders: 1030 and 1090 MHz
Aircraft radio antennas come in several length styles: 1/4 wave, 1/2 wave and 5/8 wave are common. To calculate the length of antenna required for a specific frequency you can use the formula below. However, the calculations below are approximate only. Other factor such as the distance to the ground plane, length of the antenna cable and antenna end effects make it impossible to accurately calculate the length of any antenna. The easiest way to test an antenna is to use an VSWR meter or SWR analyzer such as the MFJ-259B from MFJ Enterprises, Incorporated. I use the MFJ-259B to accurately test and set the length of all the adjustable length antennas I sell. Note: When testing the antenna length it is important that the testing be done with the same or similar ground plane that it will be used with. The antenna cable length is also very important. All tests done by Cumulus soaring are done using a specified cable length and using a good ground plane. Magnet base antennas are tested on the roof of my sedan.
- Antenna Length (meters) = 300 / Freq (in MHz) * length fraction (1/4 wave, 1/2 wave, 5/8 wave, etc.)
- Antenna Length (inches) = 300 / Freq (in MHz) * 3.281 ft/m * 12 in/ft * length fraction (1/4 wave, 1/2 wave, 5/8 wave, etc.)
- Example: 300 / 123.4 MHz *3.281 *12 * 1/4 = 23.93 inches
Since every frequency has a unique optimal antenna length you will need to find a compromise on the antenna length. It will work well for the frequencies near the frequency it is tuned to.
Tips for Ground Planes for Aircraft Radio Antennas
Most radio antennas require a ground plane. The AV-534 is a good example of an antenna that performs best with a ground plane. A ground plane is a metal surface that is connected to the antennas ground (common). There is no need for a ground plane when mounting an antenna on a metal aircraft, but it can be tough to accommodate in a fiberglass aircraft such as a sailplane.
Note: Radio antennas can be mounted inside a fiberglass aircraft, but they would not work well if mounted inside a carbon fiber aircraft.
Below is a note from RAMI about ground planes for aircraft radio antennas.
A proper ground plane is one that extends 1/4 wavelength (at the operating frequency) outward in all directions from the feed point or base of the antenna. 1/4 wavelength at aircraft com frequencies is approximately 24 inches. So the ground plane would be approximately 4 ft. X 4 ft. (or 4 ft. diameter) conductive surface with the antenna mounted in the center. You can also accomplish this with (2) four foot ( 1.22 m) long conductive wires running perpendicular to each other and electrically connected to the shield of the transmission line at the base of the antenna at their mid points. Similar to a base station antenna seen at an airport terminal building. The wires can be run inside the fuselage forward and aft and down the inside wall.
Engineering Manager / GAA Product Line Manager
Tips for Ground Planes for Transponder Antennas
I talked to Ben Ennenga at RAMI about transponder antenna ground planes. Some interesting notes from that conversation are below. Thank you Ben for the friendly and helpful support!
- A ground plane is required (of course) with both rod style and blade style transponder antennas.
- The smallest recommended ground plane would be 12 inches in diameter (6 inches radius).
- A ground plane works about equally well regardless of whether it is made from a solid plate (such as aluminum) or instead made using 2 wires in a "+" arrangement. In either case, the radius should be at least 6 inches.
- When working with carbon fiber glider fuselages, the antenna must be mounted on the outside of the fuselage, but it is OK to mount the ground plane on the inside.
- The mounting screws for the RAMI AV-74 antenna are connected to the antenna ground. Therefore, you can connect the ground plane to either or both of the mounting screws.
- It is OK for the ground plane wires ("+" configuration) to curve and follow the inside of the glider fuselage.