After spending much time frustration and money beating on the problem of static in our radio transmission (which get worse in direct proportion to the engine RPM) we're at this point close to certain it's RFI from the ignition system over the air (not over any aircraft wiring.)
(Won't bother documenting the reasons it's narrowed down to that at this point,
except to say that an avionics guy standing next to our running plane
transmitting with a handheld gets the same RFI proportional to our
RPM that we do with our microair 760.)

Anyway, we have unshielded ignition wiring.
So a couple of questions along that line:

1) Anyone know of sources for sheilded ignition wiring that could be suitable?

2) Curious if anyone's using sheilded ignition wiring on their 912?
2b) Curious if anyone had, or knows of, an RFI radio noise issue that was resolved by
installing shielded ignition wiring on the 912?

3) Anyone know anything about sparks plugs with RFI suppressors built in on the 912?
3b)Anyone know anything about the availability of and the pros and cons of
resistive type and inductive type RFI suppressor sparkplugs -- particularly on the 912?

4) Finally:
I used to be in the computer industry and we used to use RFI absorbing paint to confine radio interference inside the computer cases to pass FCC regs.
Was wondering if painting the inside of the fiberglass engine cowling with this stuff
might work?
Any thoughts on that?

All help appreciated, as clear radio transmission is obviously of importance to our and others' safety.


Footnotes, of possible relevance.
#We're certainly not opposed to using factory approved or recommended parts
or proceedured but because we're an ELSA are not limited to those if
there's reason to do otherwise.
# Our 912 has about 500 hours on it.
#Our aircraft is a Skyranger and, again just FWIW, the entire engine cowl is fiberglass so any RFI coming out of the engine, unlike if the cowling were metal, radiates out pretty much freely.
# It's a ragwing, so similarly any airborn RFI from almost anywhere in the plane will reach the antenna.
  • Re: Sources for shielded ignition wires for 912UL? Etc

    by » 12 years ago

    Most people don't need to shield on a 912. Just keep the wires away from the antenna wires and there are adjustments in many radio settings to get rid of this. If you have a digital radio that would be the first thing I did and then you may need to re-route the antenna wires.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

  • Re: Sources for shielded ignition wires for 912UL? Etc

    by » 12 years ago

    Thanks Roger,

    We've pretty much already covered all the ground (routing and shielding wires, antenna is at about 8+ feet from engine, etc, and have other reasons to be certain we're getting IFR over the airwaves from engine compartment.

    You say "most people don't need to shield on a 921." We may be some of the other people. Still would like to hear of sources for sheilded ignition wire and any reports and experiences with it.


  • Re: Sources for shielded ignition wires for 912UL? Etc

    by » 12 years ago

    Hi Al

    It sounds like you have done a lot of fault tracing on this problem and you seem certain that you know what is causing the interference, BUT I think you are looking in the wrong place. Do you have an electronic display as part of your instruments.

    If you think of all the other aircraft that are flying with the radio within a foot of the engine ignition system and they don't have a problem, why should your engine be different.

    I would be looking somewhere else.

    Cheers Mark

  • Re: Sources for shielded ignition wires for 912UL? Etc

    by » 12 years ago

    Is this radio a new installation? I had the same problem with the microair in a Kitfox with the 912UL. This radio is not the best in the world but after chasing the RFI around I was finally able to make it "usable" when I directly grounded the radio box from the back to a common ground. I rewired all mic jacks with shielded wire to the same common ground and made sure the jacks were not grounded to the airframe. Make sure that you have no ground loops. If your cockpit engine noise is high, try a good quality electret mic with lots of foam around it and check the mic gain which is adjusted inside the radio box. My issues turned out to be both cockpit noise from the engine and RFI. My microair works ok now but again, it's not the best radio in the world.

    Good Luck!!!

    Oh yea, one last thing, if you are using this unit for an intercom, when you key the mic to talk on the radio, BOTH mics are hot and ATC hears all the noise from the cockpit via two microphones....I doubt that the engine ignition is going to be that much of a problem.

  • Re: Sources for shielded ignition wires for 912UL? Etc

    by » 12 years ago


    Two sources I can think of for shielded wire are California Power Systems and Aircraft spruce. I know CPS has the woven shielding you can slip over your existing wiring and the metal spark plug caps. The plugs that Rotax recommends already have the built in resistor to reduce RFI. I did what Roger recommended and moved my antenna and wiring. It took a lot of moving and a lot of experimentation but I finally got my problem down to barely noticeable levels. If your radio has a sidetone adjustment, turning it off will help. Also keep the squelch as low as possible. Ferrite chokes on the power supply to the radio sometimes help. Shielding on your mag kill switches sometimes help. Only drawback to spark plug wire shielding is that your plug wires must be in good condition or you can lose some spark efficiency.


    Thank you said by: Al C

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