Radio xmit + trim indicators

Paul Eastham · #1 08-23-2006, 09:44 PM

Hi everyone,
While testing out my new PTT button installation today, I noticed that my trim indicators freak out when I transmit. The one illuminated LED goes out and a few others come on dimly. Anyone seen that before?

These are the stock LED trim indicators that come with the aileron and elevator servos from Van's. Radio is a Garmin 430 and all coax is RG-400 with no splices or any other funny business. The trim wires do share a short run with the antenna line -- was I expecting too much of the RG-400 shielding?

I guess the trim indicators going out isn't too bad since transmitting is short and infrequent...I am a little more concerned about other systems though (like the Lightspeed.) I wonder if I have a bad RF leak somewhere?

Any thoughts appreciated,
Paul

Paul Eastham · #2 08-23-2006, 09:48 PM

BTW this is with a nice coax-connection belly whip antenna.

Upon reading through the search results again I see someone stating that transmitting inside a hangar might cause the problem. This *was* done in the hangar -- which is 100% galvanized steel. Hmm, I sense an excuse to try out my new tow bar

Finley Atherton · #3 08-24-2006, 04:04 PM

Ditto. Icom-A200, inside a steel shed. I have not tried it outside yet.

Fin 9A

hecilopter · #4 08-24-2006, 08:57 PM

Mine does this too, inside or out and you will find it is a common problem. If you have the Van's manifold pressure gauge, it is very common for it to max out when transmitting as well. I just don't look at them when I am talking.

apatti · #5 08-25-2006, 02:30 PM

I am not at this stage yet so I don't know if it will work or not. However, I have cured RFI problems in the past with the addition of a simple ceramic disk .01 or .001 micro farad capacitor. My ham radio rig was bleeding over into my stereo. I placed one of these caps right across the speaker leads physically at the speaker and the problem went away.

Bascially, a capacitor looks like an open circuit to low frequencies (i.e. DC power sources, audio frequencies, etc.) and they look like a short to high frequencies (i.e. radio frequencies (RF)). You can try placing the capacitor with one leg to ground and the other to the wire that you think may be picking up the RF. For instance, if you think the RF is coming in on the power lead, place the capacitor between the plus power lead and ground. If you think the RFI may be coming in on a sensor lead, place the cap between the sensor lead and ground. (NOTE: take care doing this to any sensor lead that is sensing something that changes very fast; I think trim and flap position indicators would be pretty safe). You want to do this as physically close to the affected instrument as possible. Preferrably, attach the cap right on the back of the instrument being affected. Keep the lead lengths as short as possible.

Long wire runs act like antennas and pick up RF. The cap will short the RF to ground.

You can get these caps at Radio Shack or order them online. They should not cost more than a couple of bucks. Don't get electrolytic caps. Just get el-cheapo ceramic disk (non-polarized) caps.

Like I said, I have not tried this with aircraft instrumentation. But it wouldn't be difficult to give it a shot.