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Nav radio loses signal during com transmit

R

romirez

Member
Hi folks,

Hoping you can help me identify the most likely culprit to troubleshoot here.

I have an RV-7 that I recently added a NAV radio to. VOR+ILS both work very well (100+ mile reception), but when I transmit on com1 or com2, the nav radio signal degrades significantly, to the point of losing glideslope on ILS approaches - after stopping the transmission everything recovers.

The hardware:
NAV radio: MGL N16 + Vega control head (grounded to common avionics groundplate), Archer wingtip antenna (installed according to instructions, grounded to the wing)
COM1: GTR 200 (grounded to common groundplate), comant CI-121 on top of fuselage
COM2: GTR 20 (grounded to common groundplate), comant CI-122 on the belly
Audio panel: GMA 245

Cabling is all RG400.

Transmitting on both COM1 and COM2 causes this, with COM1 appearing to be somewhat worse.

Any ideas?
 
BobTurner said:
Are the canbus wires between the control head and receiver shielded?
Click to expand...

Yes - I'm using an MGL-supplied premade harness and the canbus wires are shielded. Is your thinking that com creates interference in the canbus and head and radio stop being able to talk to each other?
 
Just move your nav antenna wires a few feet from the comm transmit wires, got to keep m separated
 
romirez said:
Yes - I'm using an MGL-supplied premade harness and the canbus wires are shielded. Is your thinking that com creates interference in the canbus and head and radio stop being able to talk to each other?
Click to expand...

I’m suggesting that the RF from the com is somehow getting into the nav receiver and overloading it.
Edit. Are both com antennas’ bases/mounting screws electrically connected to the airframe, no corrosion?
 
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Re: COM antennas - I know that COM2 is for sure since I installed it a few months back. Not sure about COM1 since that was installed by the builder. (if the other fixes don't work, I'll remove it and take a look)

Will try to separate the nav/com cables more, check the wiring harness to see if there's anything there that could be picking up interference from the com transmit, and report back. If there are additional ideas of what to look at, please keep them coming!
 
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Ideas

If you have access to a hand-held comm you might try transmitting on that to see if it has any effect on the nav radio. It could help to isolate certain potential installation issues (i.e.proximity of wiring and power supply).

The MGL radios need clean power supply so worth checking size of wiring and where it is connected vs the other comm radios.

There is an MGL forum at mglavionics.co.za and perhaps some more specific assistance is available there from the manufacturer. The MGL company rep no longer participates on VAF, unfortunately.
 
PaulvS said:
If you have access to a hand-held comm you might try transmitting on that to see if it has any effect on the nav radio. It could help to isolate certain potential installation issues (i.e.proximity of wiring and power supply).

The MGL radios need clean power supply so worth checking size of wiring and where it is connected vs the other comm radios.

There is an MGL forum at mglavionics.co.za and perhaps some more specific assistance is available there from the manufacturer. The MGL company rep no longer participates on VAF, unfortunately.
Click to expand...

OK, so I finally got around to testing with a handheld - with every other avionics thing powered off, I'm able to very easily affect NAV reception just by transmitting on the handheld anywhere close to the airplane, so it appears the problem is not due to proximity of com and nav wiring, or shielding of the nav cable itself, but is something else - the VHF transmission is causing some kind of interference somewhere. Any ideas on what else I can try to isolate the issue?
 
I’m out of ideas, unless you know someone who will lend you a known good unit to swap in. May be time to contact the manufacturer. Hope you don’t have to ship back to South Africa.
 
All right, will try to get in touch w/ MGL and Michigan Avionics to see if they have ideas or if I can get a replacement radio. Appreciate the help! (but if there are any other ideas - please keep them coming)
 
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