Antenna Issues

Common issues with com radio antennas include poor range, garbled signals, and directionality. As antennas are simply a calibrated length of wire, unless it is physically damaged, problems are usually due to installation issues. To be installed correctly, the antenna needs the correct polarization, a good ground plane, and a clean attachment to the ground plane.

Polarization simply means that the antenna is pointed in the correct direction to maximize signal output. Com antennas are 'vertically polarized', meaning they need to be pointed up or down. So as long as a standard com antenna is mounted on the belly or on the cabin roof, it should be positioned reasonably well.

All aviation rod type antennas need a ground plane to project a signal with any distance. On a metal skin aircraft, this means that the mounting surface also functions as the ground plane. However, if you mount an antenna on a non conductive (composite) panel or on a fabric covered aircraft, you will need to fabricate the ground plane with metal screen or tape attached to the inside of the non conductive surface. Then the antenna is mounted in the center of the fabricated ground plane. Round ground planes work best and lopsided ground planes give unequal transmitting distances, i.e. if the antenna is mounted close to one edge of the ground plane, it will have shorter range on the 'short side' of the ground plane. Remember, a ground plane is an area. It needs square inches of surface in order to function. Just attaching a ground wire to a mounting screw is NOT a ground plane.

Anything that prevents an antenna from physically and electrically contacting the ground plane needs to be avoided. Gaskets should not be used. Paint on a metal panel needs to be removed from around each mounting screw hole to insure good contact between the antenna and its ground plane. If you are using a fabricated ground plane, ALL mounting hardware needs to contact the antenna and the ground plane.

Example; A customer had issues with a com radio in an ag aircraft (new to him). After I replaced the control com head, the pilot reported that other stations reported him as scratchy/garbled. After I removed the antenna, this is what I found. One pic shows the antenna base and the cork gasket that was attached to it. The other pic shows the metal panel where the antenna was mounted. Notice that the primer was not removed before the antenna was installed.

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I also found heavy paint layers peeling from the inside of the panel at the mounting position. I scraped the loose paint away from this panel. On the outside of the panel I used a bonding brush in a drill motor to clean the paint away from each mounting screw hole. I didn't get a pic of that so this pic of the gray panel is just an example of what it looks like when done.

To reinstall this antenna, first clean the antenna base and the panel with alcohol or similar. If you are in a salt water environment you can the apply a small amount of anti corrosion stuff. Set the antenna on the holes and drop your mounting screws. I have been using a mounting plate of my own design to install antennas so all I had to do was get each screw started in it's nut plate. Here are pics of the mount plate with the antenna, the plate after installation, and the antenna after installation. There is a tip if you have multiple antennas; use a Sharpie and write on the antenna base, what that antenna is connected to, com 1, com 2, FM, etc. Makes troubleshooting easier in the future.

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And finally, I installed a new coax cable fabricated from RG 400 and crimped on connectors. If you do ANY radio work and you find the old, black RG 58 coax, just remove it and install new tan, RG 400. Much better quality cable and better electrical characteristics.

After testing, the screw heads were sealed with clear RTV and a thin bead of RTV was run around the antenna base.

The pilot reports much clearer transmission and better range.

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Thanks for taking the time to write up that info!

After the bonding brush, it’s a good idea to use alumaprep and Aldine to help prevent corrosion in the future.

Web, any experience with the Advanced Aircraft Electronics high gain antennas? I’ve been thinking of using one in the fuselage of my Clipper so I don’t have an external antenna. Aircraft Spruce sells them, they are 43” long, so I’m guessing 1/2 wave rather that the more typical 1/4 wave antenna.


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I don't use them a lot. They work best in a wood structure instead of metal and need to be set vertically to work well. But I do like the idea.

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In the Clipper, it will be diagonal behind the cabin (vertical dimension is only about 39”). I’ll mount it to a piece of wood going from a lower longeron to the opposite upper longeron. Their literature says being in a parallel plane with the prop is more important than being exactly vertical.


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In that installation I'll be curious how the steel tubing affects the range. It will decrease range but I'm not sure by how much for that particular airframe.

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It will likely be a year before I’m done. When finished I’ll let you know.


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