MPPalmer(at)aol.com
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 Posted: Thu Jun 19, 2008 2:40 pm Post subject: More RFI tricks |
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As I mentioned before, I've been in a fog lately. (Literally as well as figuratively. I heard a report that dementia has been correlated with poor vision. I can verify that!) As I think about it more, I remember now that, on that intercom RFI problem from years ago, the solution wasn't to go with triax but rather with 100% shielded wire. That is, foil wrapped audio cables.
The numbers vary, but IIRC, the shield on ordinary braided coax is only about 60%. So a lot of energy leaks out. (This is why new house TV cable installers recommend "quad shielded coax.") That doesn't sound like Tom's problem, since, when he tried the dummy load, most of his RFI went away. (Sounds like he must have some good coax (or triax) in his installation.) But for others, the usual trick to identify the culprit is to wrap a continuos piece of aluminum foil along the coax (or any other suspected wire). Then take a bare wire and spiral wrap that (about a turn per half foot is fine) along the length of your foil. That's called a "drain wire." Ground that at one end or the other (usually near your common ground point) and see if that reduces your problem. It's a little hokey, but it works.
In our Glasair, I made the mistake of ordering the shorter of the two ADF antenna cables that King offered. The original plan was to have all receiver cables go down the pilot side of the fuselage and all transmit cables go down the center. (Strobe wiring to the tail and potential autopilot altitude servo on the right side.) But, due to the shortness of the wire, I had to run the ADF cable near the transponder cable down the center. Wrapping both in aluminum foil and doing the drain ground reduced the "rrapp pop" sound from the transponder in the ADF to the point that it doesn't interfere with ADF operation. (ADF? What's that?)
Hope this helps,
Mike Palmer <><
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