Interference between the two coax by proximity is no problem,
especially between VHF and transponder where's there's too many
harmonics separating the two freqs, each 3 db down. But only in
perfect world where the antenna is a perfect match, and the RF is a
true sine wave (no harmonics), etc. But in a plastic airplane, the
cables are likely exposed to stronger RF from the actual antennas than
will result from coax proximity due to the less perfect world.
Separate if you prefer.
What is the problem in a composite airframe is that there's no real
ground at the antenna end, making the shields more conducive to pickup
---From noisy, harmnonic-rich wires, particularly strobes and motors,
which induces currents in the center wire. Keep these away from the
coax. In fact, that's how a coaxial balun on a V-type nav antenna
works -- the center conductor of the coax is connected to nothing!
RG58/U, BTW, is solid core, which is not what they specify (RG58/AU,
stranded core). It will work, but needs to be run without sharp bends
and secured to prevent movement.
If you have the xponder antenna in the rear fuselage, it important to
have line of sight to ATC's antenna. The worst case is where ATC is
on the nose, say 35 miles out at 2,500 AGL. That's only a few degrees
of angle, and you don't want the transmitted signal passing through
carbon-based life forms, the engine, and a spinning prop. The first
two distort the radiating pattern, and the latter is similar to what
you might hear in comm transmissions from a helicopter as a tremolo.
Still readable, but close only counts in horseshoes when it comes to
the transponder. ATC has to receive a clean pulse train to be able to
decode it.
Regards,
Fred F.
Alfred Buess wrote:
>
> Hi Europa-cracks,
>
> The cable ducts from the rear fuselage to the instrument panel will be one of
the next steps of my building activity. Before I start to do this, I'd like to
know if there is a problem when I put the coax cables into the same cable duct.
I use RG-400U for the transponder and the comm antennas and RG-58U for the
VOR antenna. Do these coax cables interfere with each other when they are in
a bundle? Any experiences and technical explanations from the electronic
knowledge
blessed among us?
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