At 10:39 12 June 2018, Chris Wedgwood wrote:

It would be helpful to detail what you've done, what equipment is
there,
etc

I generally try to modify wiring to comply with John DeRosa's excellent
presentations. I haven't found much on his or any other website about
radio interference but would be pleased to be corrected.

http://aviation.derosaweb.net/presentations/#wiring

I have checked for poor batteries, undersized cables, loose or voltage
drops on connectors, fuses, circuit breakers, switches, terminal blocks
and earthing.
I've separated (physically and electrically) as best as reasonably
possible, the wiring of the radio from other wiring.
I've placed solid ferrites on power wiring to the radio and each
instrument.
I've put split ferrites on data, display, memory card and radio mic/spr/
ptt cables.
I took the ferrites back off the memory card cables as I seemed to have
more trouble with the cards while the ferrites were present (but this
may have been caused by people corrupting the cards in a mac pc).

The ' primary culprit' often seems to be the Flarm, especially during the
first minute after the master switch is turned on when everything is
initializing. A hand held radio seems to confirm this. The interference
seems to be worse at higher frequencies with 122.7Mhz being a bit
better than above 130Mhz.

I haven't tried "a 12v interference suppression capacitor" across the
radio leads, will order some. Also ordering some copper foil to wrap
around the flarms (or to make a protective hat)

I'm not sure if its a good idea to clip a ferrite onto the flarm GPS
antenna lead or the radio antenna lead, suggestions are welcome.