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"The rg-400 coax is better that the rg-58, but both work if properly
installed. It's best to separate the coax from other wires, both power and
signal."
Amen. I recently resolved a problem on a Legacy
FG where the engine temp readings would go erratic when transmitting on the com
radios. This is an e-glass FG and uses a duplexer (not the same as a
diplexer) to share an antenna between the two com radios. All connections
are RG-400. What I found was a com coax nearly touching the wire bundle going
from the engine monitor to the engine thermocouples. I separated the coax
and wire bundle by about 2" and the problem went away. Given the fairly
robust shielding of RG-400 I was initially surprised by this. But
when you think about it the shield of a coax carries RF currents, just like the
center conductor. Granted the shield is connected to ground but ground is
simply a reference point and at high frequencies the AC voltage along the
shield of a coax will not be zero volts with respect to ground. There
won't be much voltage wrt to ground, but when the coax is very close to a
sensitive signal, like a thermocouple wire, it doesn't take much to cause
interference.
Tom Gourley
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