GNS430W and SL40 echo / reverberation

I have a GNS430W and an SL40 installed in my aircraft. Each com has a separate bellly mounted whip antenna (comant). The antennas are side by side about 20 inches apart. Both Coms managed through a PSE9000EX audio panel.

When I operate the 430W on frequencies close to the frequency on the SL40 I get a significant echo / reverberation. It is driving me mad....

Any comm gurus out there who can comment....?

Is this common and is there an easy fix?

Regards

Jon

This is exactly the issue that I was asking about on this thread:

http://www.vansairforce.com/community/showthread.php?t=75472

Seems most that responded claimed no issues.

There is a xmt interlock that you can connect that reduces the sensitivity of the opposite reciever when you xmt. This is suppose to prevent issues between the two.

The only problem I see with this is that PS Eng. claims that this must be disabled for their split mode to work properly.

Brantel is correct in that PS Eng states not to use interlock if you want to have both comm's transmit at the same time.

The issue with the GNS430 and SL40 is that the GNS430 supports interlock, but the SL40 doesn't. I've been looking into this the last couple of days for myself. I wish it did, because I may run into the same scenario as you.

The next best thing is physical separation between the two comm antenna. The SL40 manual specifically states to keep the comm antenna more than 24" from a gps antenna and doesn't state anything for comm antenna. The GNS430 states to keep the antenna at least 36" from other antenna, 48" away from ADF, 60" from DME antenna.

I suspect you're getting crosstalk with the antenna just 20" apart.

Did not know the SL40 did not have the interlock. I have an A210 and am adding a 430W and both of them do have the interlock. My antennas are both belly mounted under the seats. I am on the fence on what to do with the interlock. I like the idea of split mode working but I may also have cross talk if I do not interlock them....

Thanks for your responses.

Not sure if I have been clear but my issue relates to receiving rather than transmitting.

When I have both the 430W and the SL40 receinvon the same frequency or a close frequency range the other stations (particularly transmissions from ATC on the ground) have a significant echo and reverberation. It sounds like they are transmitting inside a hollow metal drum or tank if that makes sense and it makes there transmissions difficult to understand.

If I shift the frequencies far enough apart on the two units the problem goes away.

For example - My airfield requires an approach clearance on 133.55 and the area frequency is 135.7 - often I have the GNS 430W listening to 135.7 and the SL-40 listening and transmitting on 133.55 - this causes a sgnificant echo when i receive calls from 133.55.

There is a possibility that it is asscociated with the fact under some circumstances the ATC operator manages both 133.55 and 135.7 from the same desk. That is to say that if you transmit on 133.55 the signal is repeated on 135.7 by ATC so the one controller can manage the call.

Is this possibly caaused by a loop condition associated with my two radios receiving the same trannsmission on two different frequencies...???

I'm stumped

Cheers

Jon

An

Resonance

Jon,

It sounds like what you have happening is resonance. When an antenna receives an electromagnetic/electrostatic signal, it induces a small current in the antenna. That current in turn induces a new signal. This is how parasitic arrays work (think old style TV antennas). By controlling the distance between elements you can control directivity and amplification.

I think this is what is happening to you. One antenna is receiving the signal, rebroadcasting it ever so slightly out of phase and it is them received by the other antenna. That also explains why it is more prominent on the ground, closer to a transmitter. I'll bet that if you had someone generate a test signal from nearby, and then were to rotate the aircraft you would notice a change in the "echo" as the phases between the antennas and the source change. Likewise, is it frequency dependent? There is about a 14 inch difference in wavelength between 118 and 137 mhz.

I think your best solution would be to relocate one of the antennas. However, I would double check all you grounds for your radios and your antennas, chassis grounds, shield grounds, etc. Perhaps set up a ground test like I mentioned before. While you hear the "echo", disconnect one of the antennas and see if the problem goes away. If it does, you have a clue. Now, you could try moving one of the antennas by pulling the cable thru the hole, try and maintain a ground plane, and move it around a few inches. See what happens.

Although the SL30 manual mentions ideally placing one antenna on top and one on the bottom of an aircraft, it didn't rerence a minimum distance. It did mention locating it as far away as possible from the ELT antenna, as there were some resonance problems with that with freqs close to 121.5.

It's all magic. You may actually have difficulty duplicating the problem on the ground. It's tough to troubleshoot something like that. Good luck, and let us know what you discover.

Don

Thanks Don

Everything you say makes sense and to be honest the thoughts and theories bouncing around in my head are somewhat aligned with your comments.

One thing I have noticed is that i get better RX and TX from my SL-40 and yet my GNS-430W is 8W rather than the 5W of the SL-40.

This leads me to think it may be a connection or ground problem associated with the GNS-430W...

I will investigate the connections to the best of my ability and will report back soon.

Cheers,

Jon