Garmin SL40 Transmit Problem

[matt.hocker]

I have a Garmin SL40 which works great for frequencies above about 120.00 but below that, I get a nasty buzzing sound when I try to transmit. The recipient hears nothing.

Receiving works fine. I took the radio in for service and it bench tested perfectly. The radio person advised me to install a small aluminum plate inside the aircraft, under the antenna because the composite structure that I have the antenna attached to doesn't provide any grounding.

I'm having trouble understanding why this would be. For example, my handheld radio has no such grounding plate and it works fine at any frequency. Also, why would this affect just low frequencies?

If this is required, how big should this plate be? Is there a pre-made one commercially available?

Thanks for your help.

 

[Mike S]

Google antenna ground plane---------will explain it way better than I can.

Yes, you need it, and yes you can make it pretty easily.

http://www.rst-engr.com/index.html

 

[BobTurner]

technical explanation of sorts

A common antenna is a half wave dipole, in free space (far from anything else), which consists of two 1/4 wave length pieces of wire, end to end, with a small gap between them. The feedline is connected at the gap. Now imagine an infinite plane, thru the feed point gap, perpendicular to the wires. From the symmetry one can see (at least I can see!) that everywhere on this plane the potential (voltage) is the same, a so-called equipotential surface. So you can place a real equipotential surface (metal conductors are very near ideal) in that plane, and eliminate one of the 1/4 wavelength wires, and have the same radiation pattern except it is only on the remaining wire side; it is zero where there's no wire.

So the "ground plane" is an important part of the typical 1/4 wave vertical antenna. It doesn't have to be a plate; aluminum foil imbedded in the fiberglass is fine. It can also be an array of wires, like the spokes on a bicycle wheel. The more wires the better, but even if you just have four that will be pretty good. In any event (plate, foil, wires) the ground plane must be electrically connected to the coax shield near the antenna attachment. It need not be connected to the aircraft ground. Bigger is better, but at a minimum the radius should be a quarter wavelength, or about two feet.

Your handheld has a completely different type of antenna which is designed to work without a proper ground plane (note how small it is compared to your whip).

 

[BobTurner]

more

Without the ground plane you will have a poor impedance match between the coax and the antenna, resulting in significant power being reflected back into the transmitter. The exact mismatch will be frequency dependent. In the old, early transistor days, you would have destroyed your transmitter. Modern designs have fault detection circuits. My guess is that below 120 MHz the transmitter is shutting down to protect itself; above that frequency it is (barely) tolerating the situation.
If you think the antenna is okay now, you'll be amazed at how much better it is with a ground plane!