MGL Avionics User Forum

Hi!

I have an RV-7, to which I recently added an N16+Vega, along with an Archer wingtip antenna, connected via RG400 cable. N16/Vega are grounded on my avionics grounding plate behind the panel, antenna grounded to a metal rib on the wing.

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 com transmission everything recovers. I can generate the same effect on the ground using a handheld VHF transceiver, with all avionics other than N16/Vega powered down, so I don't believe the issue is with interference from other plane systems.

Any ideas what could be causing this issue, and how I can diagnose / fix it?

Thanks!
Roman

COM is very close to Localizer in the frequency band and if you transmit on COM it is possible that your localizer antenna is getting a very strong signal that is larger than what the N16 input can handle - the N16 reacts to this by reducing the gain of a pre-amplifier and if that still is too much it then actively reduces the signal at the antenna input by means of sending a current through a pin diode - this way it can survive very large signals without damage. All of these measures reduce the signal available on other frequencies. But at least the signals are not clamped - if these measures are not in place the strong signal at a single frequency would block the input (signal blocker) and the receiver cannot work at all (this is the way electronic warfare systems work that you point to drones - it swamps the receivers including GPS).

So you need to reduce the coupling between your VHF COM antenna and the Localizer antenna (Glideslope is also affected if the gain is reduced as it's the same input).
Distance is one measure - a second is a filter to squash the frequencies above 117.9 Mhz at the N16 input.

Combination antennas used for both COM/LOC and GS are difficult here - it now depends on how good the line to the nav radio is isolated from the huge signal going the other way when you press the PTT. I have never seen a combo antenna that does not nails the navigation receiver during transmit so I don't really like them - I prefer a separate COM and combination LOC/GS antenna.

Thanks - given that I'm getting the interference even when I test with just a handheld transceiver from basically anywhere around the airplane (tested by transmitting on 123.45), I'm not sure distancing the com antenna from the VOR antenna is going to solve the problem. The archer antenna in the wingtip is a very common setup for RVs that generally works well according to all reports (although I haven't found folks who've done it with the MGL radios specifically) so I'm trying to figure out if this is possibly something specific to the N16, or there's some installation issue with the antenna/wiring that I'm missing...

On filtering - is there a best practice on how to accomplish that? (would I need to buy something like https://www.minicircuits.com/WebStore/d ... PF-A113%2B - ~$120 for the board+filter is a bit pricy but if I don't find other solutions I'll give it a shot)

The filter is not very steep so the effect likely is limited unless from the higher COM frequencies.

The N16 should not really be affected much by nearby COM transmitters - I'm thinking there is something else going on there...

Can you give us a better idea on what the COM and NAV installation looks like (locations, cables and routing, antennas etc) - anything that could throw a light so to speak. Also focus on power supplies - it's quite possible the RF is injected to the N16 via grounds or supply (ferrite should eliminate that).

I'm getting the same interference simply by transmitting with a handheld VHF transceiver, so I don't think the issue is due to com radio proximity / wiring. Here is a video testing on the ground, tuned to a local VOR - https://photos.app.goo.gl/HX83xy5eMohDvsBE7

https://docs.google.com/drawings/d/1y5n ... BmjAE/edit - this is the diagram of how everything is wired, with a picture of how the antenna is connected (in the photo it's upside down). Power is connected to the avionics bus and then to the battery - I can try testing with a standalone 12v power supply to see if that fixes it.

Really appreciate any ideas!