Radio Interference - Last Resort

[techie49]

This may be of some help to anyone struggling to cure residual radio interference on an aircraft. I'm at the 'about to fly' stage in my build but was not happy about the ignition interference on the radio, which is a Flightline 760, very similar to a Microair. I've got suppressed plug leads and even went to the extent of building a filtered input socket ( 15 way D type) for the radio. Still no joy. The antenna is well earthed to the aluminium fuselage and the connections are good on both ends of the coax cable. What I had forgotten to check was where the interference was getting into the radio ! Yes blindingly obvious I know, from someone who is supposed to be very experienced with radio from HF to Microwave !

 

I quickly established that that filters aside, the interference was getting in via the antenna. I was getting into band pass filters on the antenna, etc, when the thought struck me. The cowling is fibre glass and thus transparent to RF. Imagine a car bonnet - metal (usually anyway ).

 

So what to do? I had similar screening problems years ago on noisy switched mode power units in plastic cases and had cured it by the use of nickel screening spray.

 

I carefully sprayed the upper cowling and used copper flashing to create a connection to the frame of the aircraft via the cowling fixing screws.

 

The result - silence apart from normal radio traffic.

 

The moral of the story - don't be too clever. Look for the glaringly obvious.

 

Paul

 

 

[facthunter]

Don't use suppressed plug leads on the ignition system unless approved for it. On magneto's it will cause failure. Originally they were "shielded" with the centre wire still copper, (not a carbon high resistance thread). Special (expensive) plugs as well. Nev

 

 

[techie49]

I perhaps should have added, my engine is an auto conversion, a Subaru EA 81, so no magnetos involved. Suppressed leads aside, the principles are the same. Look up Occam's Razor..........

 

Paul

 

 

[facthunter]

Doesn't hurt to re-inforce the point. lot's of people read this forum. I wasn't intending to assert you would be doing the wrong thing, but many just don't know of these things. Magneto's are becoming a thing of the past, but still have their place in the world. Nev

 

 

[dutchroll]

Interesting. I've had a long term interference problem (radiating directly through the com antenna) from electronic ignition noise. Initially we suspected a coil problem. Narrowing it down from one of 18 to one of 9 is simple depending on which ignition it is, but it gets hard after that, especially because it's quite intermittent. However several coil replacements have not fixed it.

 

I had some com reception problems recently and after more troubleshooting we found some poor crimps on the coax (not mine - it was factory built) and a badly installed connector. Continuity testing showed it was very poor. I'm hoping this might address the problem, though the plane is down for other work so I haven't been able to test fly it yet.

 

Who would be an avionics technician?

 

 

[kasper]

Similar to the initial post getting good radio and Mode S installs on the trikes is always fun - if you look at the trike in my avatar there is not much between the engine/electrical noise generator and the rest of the plane.

 

My solutions were to install the radio and transponder units inside the dash combing that had - like the initial post here - been metal lined to restrict radio interference into the boxes and connectors and to put the radio aerial on the nose in front of the combing (use the combing as a radio blocker to the aerial) and the mode S blade aerial went under the pod with metal ground plane inside the pod to again shield it ... but that was mark II ground plane - initial crossed copper X plane was interfered with by engine on the transponder so had to go to full round ground plane to 'protect' the aerial from the engine.

 

Final result was an install that was clear and well received.

 

It takes a bit of knowledge (I have next to none on electronics ... they are metal boxes with needs per their manuals that I try to understand and keep to) and planning - but as Techie49 says you can be surprised.

 

 

[601man]

I have wired up my com radio and everything works except when I connect the mic socket to the metal dash I get a hum( grounding feedback issue) ?

 

If I hold the socket in the air, nothing. If it touches the metal fuse then the hum. Somone told me to isolate the socket by heat shrinking the threaded part and fiber washers on each side of the nuts..

 

Never heard of this before.. Is that the normal installation or will it be just hiding an issue.

 

Chris..

 

 

[nomadpete]

It's a good idea to isolate the mic socket from the aircraft ground. As has been suggested.