Replies to This Discussion

Permalink Reply by Bruce Cobo on April 18, 2013 at 4:47pm

Are you sure all your systems are grounded properly?

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Permalink Reply by John Austin on April 18, 2013 at 7:41pm

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.

John

N750A

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Permalink Reply by Martin Pohl on April 19, 2013 at 6:00am

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 ;-)

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Permalink Reply by Paul Toone on April 19, 2013 at 8:37am

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,

Paul

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Permalink Reply by John Austin on April 19, 2013 at 8:59am

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.)

John

N750A

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Permalink Reply by Martin Pohl on April 19, 2013 at 10:44am

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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