I'm fairly certain the problem is a composite airplane, meaning
everything metal inside is bathed in transmitted RF, with "floating"
electrical grounds at RF, and getting into the Navaid. I suspect the
RF is getting in through the its ground, as this wire seemed the most
sensitive also to "AC hum" pickup, which sent the thing a grinding.
In bench-checking the Navaid A/P recently, several other things were
observed, using TX from a handheld. Using shielded for 3-wire cable
to the servo did help, and the shield need not be (and possibly best
not) grounded. Shielding either the panel unit or the servo unit
didn't help. It didn't appear to be the green servo signal cable
either, as it's a nominal 130 Hz square wave, which appears immune to
about 200mV of higher-frequency junk superimposed on it when the servo
motor is working.
Interesting was adding a radiating element (wire) to the servo arm -
it's at ground potential, and it was then more sensitive to
transmitted RF, depending upon length/orientation of the wire. This
suggests the servo's mechanical connection to the aileron torque tube
should be nonconductive (and sufficiently high capacitance). Which is
what I shall do, in addition to shielded wire.
Anyway, when something is surprisingly sensitive to mere AC hum from
the human body whilst inside the house, high power RF can easily cause
problems. However, there may be no universal solution, as it's all
theoretically dependent upon the lengths and polarization of offending
wire or metal elements vs. VHF comm frequencies. So there's many
possible solutions to try!
Regards,
Fred F.
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