Have a question for an avionics tech or engineer that knows radio wave interference and filters.

[jimmyjack]

Looks like my Narco MK12D is a noisy little **** and, upon PTT, shoots RF interference all over my 430W, killing the signal.

That's what my tech assumed after some back and forth phone calls. And it was solidified when he asked me to try the entire 121.xx and 131.xx bands. Those are the only freqs that kill the GPS.

After Googling, it appears this is pretty common and it's the 12th and 13th harmonics from those frequencies interfering with the GPS frequency of 1575.42.

Anyhow, with the blessing of my tech, I found and purchased two of these:

http://www.aircraftspruce.com/catalog/avpages/tednotch.php?clickkey=1242064

I currently have them daisy chained immediately behind the comm.

The results aren't perfect, but they're a ton better, certainly enough to believe I'm on the right track.

I phoned in my results to the tech, who told me better not ever daisy chain them. Instead, one behind the comm, the other on the antenna.

I called the manufacturer who said the same thing, but in both instances the phone calls were rushed and I couldn't get the chance to ask this question:

Why can't I daisy chain them? Why does there need to be (a certain?) distance between the filters?

 

[denverpilot]

Paging @weirdjim because I don't feel like explaining quarter-wave nulls in transmission line transformer theory right at the moment...

 

[kgruber]

Much better to install a radio that was designed to coexist with GPS.

 

[bnt83]

Sounds like someone in the past didn't read very far when installing the 430w http://static.garmin.com/pumac/GNS430W_InstallationManual_190-00356-02_.pdf

 

[Capt. Geoffrey Thorpe]

Why can't I daisy chain them? Why does there need to be (a certain?) distance between the filters?

Because the angry pixies that work inside the notch filters don't like each other so it's best to keep them separated because otherwise each will attempt to undo the other's work.

 

[jimmyjack]

Issue has been resolved.

Back to the hangar today. Per my avionics shop, I pulled one filter off the COM and put it on the ELT. The other stayed on the COM.

Zero loss of signal on the GPS. Upon PTT, on a few of the 121 and 132 frequencies I get very mild degradation on a couple satellites under the satellite page, but it's less than 5% than what it was before, even when the two filters were on the COM.

After reading several forum posts on similar issues, it's likely not the COM that's the primary culprit, but instead the ancient 121.5 ELT. I could buy a 406 ELT I guess, but the situation is resolved now w/o any additional expense.