I routed my antenna cables (COM and TXPD) through the central console of my CH601XL-B, along with the other electrical wires going to the back fuselage for flaps, elevator trim, etc. Today I realised that when pushing the PTT-button of the COM, all position indications (Ray Allen, flaps and elevator/aileron trim) are going upset (jumping, showing completely erratic positions). When releasing the PTT-button, everything is normal again.

My plane is currently standing in a basement garage (concrete walls and concrete roof). Could this also be a cause for the bad indications (reflection of the COM waves)?

I guess that the COM antenna cable is the culprit. Does it make sense to route it e.g. along the left side of the fuselage to keep it away from the other wires?

How did you route your antenna cables?

Thx and cheers


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Are you sure all your systems are grounded properly?

Is the COM cable actually bundled or tied to wiring? If so, that's not good. How much separation is necessary? I don't know, but more is better!  Also, I used RG-400 cable - it is double-shielded.



Thx Bruce and John. The antenna was not yet grounded properly. I rerouted the cable along the legt fuselage side and grounded the antenna (CI-121) to the Aluminium skin and things are looking much better ;-)

Hi Martin,

If you haven't properly mounted your antenna(s) you run the risk of damaging the transmitter output stage of either or both devices. The SWR (standing wave ratio), the ratio of what goes out through the antenna and what is reflected back into the transmitter, will be sky high if the antenna has no proper ground plane provided by the body of your aircraft.

The transponder cable especially should be kept isolated from everything as the high power pulse transmisson will get into everything and may cause more problems.

Good Luck,


To add to Paul's advice, my transponder's installation instructions advised that while there was a minimum distance the antenna should be placed relative to the transponder (I think it was about 30-36" in my case), otherwise keep the antenna cable as short as practical. That was one advantage of the Sandia 165R - it's a remote unit you can mount anywhere, so that let's you put the antenna not too far from the transponder, but still have flexibility as to antenna placement in the airframe without having to resort to a long cable  ... the antenna doesn't necessarily have to be up front near the panel, since the transponder isn't in the panel! (It's controlled by the MGL Xtreme EFIS in my installation.)



Hi Paul and John

Thank you for your advice about the SWR - I read about this some time ago but wasn't aware, that it could damage COM and antenna (thought only a COM or TXPD unit without antenna might get damaged). I probably also change the transponder wiring to the other side of the fuselage (COM is on the left now, so XPDR could go to the right) to avoid further RF interference.

I have a Garrecht mode-S transponder. They don't say anything in their installation manual about routing of the XPDR cable, just refer to FAA AC42.13-2B. I will follow your advice about keeping the cable as far as possible away from other electrical wiring.

Thx again, Martin


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