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MPPalmer(at)aol.com
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 Posted: Tue Oct 09, 2007 1:06 am Post subject: Comm RFI, EMI |
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I don't have the time to go into detail, but basically RFI can infect a unit in one of two ways. It can be "conducted" or "radiated."
Conducted means it comes through cabling. The easiest way to test for conducted RFI is to put a good shielded dummy load on your COMM, AT the transmitter. (It has to be a good shielded dummy load.) Then key up.
If your other units go wacky, then the problem is RF leaking out the COMM itself on cabling and getting onto, most likely, power leads. Most modern TSO'd radios shouldn't have this problem. The solution is RF chokes, ferrite beads, etc., on the power leads of the COMM to kill the conducted RF getting out.
More likely in a Glasair, is radiated emissions. The RF from either the antenna itself or the coax is getting into your other boxes. Check for a balun on your comm antenna. No balun means large shield currents on the coax. If the problem has recently started, it could be a corroded BNC connector somewhere in the line. Our ELT started falsing when transmitting on COMM 2 and it was a cheapo Radio Shack solderless BNC in the tail cone that had corroded. Replaced with a real solder BNC and problem gone.
Consider triax cable. Or Paul Lipps wrote about an Andrew FSJ1-50A Heliax cable in the August 07 Kitplanes (thanks, Jack) that has no leakage. (www.andrew.com)
Aluminum foil as a last resort, spiral wrapped with a drain wire grounded at the radio stack. (That end only.)
Ferrite Beads, chokes, on the cabling INTO the offended boxes to try to kill the RF getting into them.
Sometimes a simple change in cable routing can make all the difference, since radiated power varies as the square of distance. We killed most of our autopilot interference from Comm 2 by splitting the bundle. (But not all. Still get a slight uncommanded roll. The boxes are just too close to each other.)
Last thing... since your engine monitor is affected, it may be that you don't have a centralized or a very good ground. Most of these new EM's are impervious to RFI, unless they're not grounded well. You need one central ground point where ALL your avionics meet for ground (so no ground loops). This ground should be connected by a 1/2" braid direct to battery ground. Ground loops on schematics seem innocuous, but in real life, they are loop antennas.
Okay, gotta get back to work. Hope this helps.
Mike Palmer <><
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