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First off, planes don't have GROUND, unless you carried
some aboard on your shoes. GROUND is what you stand on before climbing into your
plane. Every circuit requires two conductors, source and return. If your problem
is with a 10 segment LED, the device that I am familiar with is driven from
an IC which lights the digits based on an analog input voltage. This IC
also has a control for brightness. If rf energy from transmitting gets into
that circuit, it can effect the digits or the brighness. One way this energy
gets into a lot of our circuitry is on the OUTSIDE of the coax; this is known as
an antenna current. Anytime your coax goes away from the antenna element such
that the coax is illuminated stronger from one half of the antenna that from the
other, you have antenna currents flowing on the outside of the coax. Those
currents will walk right up into your instrument panel where they will get into
many susceptible circuits. That's the price we pay for having a non-metallic
airplane that allows the radiation to enter the plane. One of the things these
antenna currents cause is squealing feedback into the mike circuit at
certain Comm frequencies, usually in the higher portion in the 130s. What to do?
Most often, putting a clamp-on ferrite at the antenna end of the coax and one
near the rx will stop it. You can get them at Radio Shack, where they charge you
a bundle, or order them from Jameco, www.jameco.com, PN 218173, 0.2" ID,
$0.99, 217841, 0.34" ID, $1.45, or 318705, 0.45" ID, $1.19. They call them
EMI Suppression Cores. 'Don't believe it? Ask Larry Kruchten, who just put some
on his Thorp T-18 to get rid of squeal. The other good thing to do, if
you've put a lot of money in your stack, is to throw out any RG 58 or even the
RG 400, and replace it with Andrew FSJ1-50 coax; there's no better on the
market. You'll tx an rx much farther, with lots less nulls in your
pattern, because of its low loss and solid outer conductor with essentially NO
leakage! Same weight and bend radius as the leaky braided coax. But hey!
why spend lots of bucks on good coax, when there's such cheap s**t available!
$15k on the Garmin '03s but $10 on RG58.
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