Hi Ian,
> The stray currents always originate on the outer surface of the coax
> shield, at the feedpoint... but what happens then can depend on:
>
> * What else the shield is connected to up there. If it's connected to
> the boom (as in a gamma match) then some of the shield current will flow
> into the boom, mast and tower, depending on what impedance those offer
> at the connection point.
I agree a gamma match isn't good. I've actually had feedlines arc to the
tower legs when using a Gamma match on a Wilson 20m Yagi.
But in that case, the problem is the boom and the entire tower become part
of the radiating (and receiving) system. The feedline and feed system is
still the problem.
> * What else the coax is taped closely to. Over the whole height of the
> tower, there can be enough capacitance through insulated jackets to
> cause significant currents in other conductors.
I agree, but the unwanted signal gets into the receiver ONLY through the
inside of the feedline, assuming connections in the house up to the antenna
are properly done with no high resistance shield connections. The only
significant ingress point is where the cable center conductor and shield are
no longer overlapped at the antenna, and the rotor is still much more remote
from that point (even if connected through the mast) than the coax shield
outside wall is to the coax inside!!!
If you have a problem with noise ingress into the feedline (or feedline
radiation) it is pointless to decouple rotor cables or anything else.
Better to fix the real problem! Without a good feedpoint, you'll just chase
your tail. If you have common mode noise, fix the feedpoint. If you still
have noise, fix the noise source.
73 Tom
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