Ive been chasing a problem with my GNC355 for quite some time. I had it installed last year but unfortunately the installer passed away shortly after the install so I cant really go back to him for a fix. Anyway, when I key the mic to talk for any longer than 2 seconds, the GPS signal drops out and I get a GPS signal lost. It will then reacquire the signal and come back online after a few more seconds. Obviously this is not going to work for any type of IFR flying and it has delayed me from working on any IFR approaches. I thought that maybe it was the fact that I had an old Narco CP136 audio panel but since I had plans ot replace it in the near future, I didnt put too much thought in to it. I recently had the Narco swapped for a PS Engineeering 450b but the issue has now followed to the new audio panel. Any ideas?
Do you have the installation manual? I seem to remember there being a statement on the minimum distance you can have the GPS antenna from the COM antenna. You might want to break out your tape measure. Beyond that, perhaps the COM antenna wiring is old and is leaking signal out to the GPS. Or the GPS antenna wiring is bad and is admitting the COM signal.
I just read an article about a guy with a similar issue but it only happened for him anytime he was near the 121.5 freq. It turns out it was his ELT interfering and he swapped it for a new 406ELT and it fixed it. Our local tower is 120.7 so Im going to try a freq far out on the spectrum and see if it still does it. If not then a new ELT may be in order.
Do you have just the single COM? Does the drop out happen if you use the 2nd COM? If it was an older 2nd radio that was causing this dropout, I'd say you probably need a harmonic filter on the other radio, but it sounds like its the GNC's COM that is causing the dropout? Was the COM antenna wiring installed/replaced with RG-142/RG-400 (i.e. not still using the old RG-58?) Was the GPS antenna installed at least 2 feet from any COM antenna (note, this can be difficult on smaller aircraft, but should have a best effort for maximum separation distance). Is there a decent ground plane around the GPS antenna (Garmin wants at least a 7.5" distance all the way around the antenna). And like already mentioned, harmonic re-radiation from older ELTs is a definite possibility. Can temporarily remove the ELT from the aircraft and see if it goes away. FYI, there is a VHF COM interference check in the install manual that should have been done that would have found this at install time.
Yeah, my avionics shop that installed the new audio panel mentioned the COM interference check but I dont think the original installer ever did that.
Distance between antennas is important. I would give the coax cabling for all the antennas a good inspection before anything else happens however.
I had that happen on my 182. They put filters on the GPS antenna. They said writhing a certain range of frequencies it blocks out the GPS reception. Especially on 121 range. They put the filters on in under an hour, haven’t had the problem since.
Someone I know had to replace their old radio because of this. They put in GTR 225 and the problem went away.
I had this issue with my GNX375 specifically at lower frequencies around 121.xx. It was interference from my old ELT. Filter on the Com eliminated my problem.
So I ran a test and found that my GPS will drop out if I key the mic on and channel from 120.6 thru 121.8 with COM2 on. If I turn COM2 off then it does not drop out on any channel.
