VK3QI said;
All cables, beams, rotator, switching relays etc. run down the centre of the
> tower and exit at the bottom and into a large box which contains all cable
> terminations etc. as well as the tuning unit for 160mx.
>
> I am currently replacing all this with a SteppIR antenna on top. So
far,
> I have run shielded control cable for the SteppIR underground (about 90
> feet) to the terminating box where it is terminated, and intended running
> the shielded cable up the tower centre.
>
> What I am wondering, is did you take any extra precautions other than the
> cable shielding to reduce any chance of RF pickup on the SteppIR motors,
> whilst operating shunt feed on 160mx.?
Firstly, I haven't had chance to get it all finalised yet! What I have done
is to bond all the coax outers together and to the tower at the top, and
again at the bottom. The Stepper leads have ferrite chokes on each motor
box - about 5 sleeves, each an inch or so long. Then there's more ferrite
sleeves at the bottom of the tower, and the screen is bonded to the tower
ground at the bottom of the tower. There's more ferrite at the Steppir
control box.
The Steppir people didn't think there'd be a problem, and having the
ferrite loaidng in place probably wasn't necessary. One point I did find
with the 205BA as top loading was that the RF current that flows through
that capacity hat means there's a reasonably big RF current up the stub mast
and into the beam. This managed to burn up the position indicator pot on the
T2X. After that, I bypassed both sides ot the pot to the wiper, and fitted a
strap around the stub mast and clamped that back to the tower head unit to
provide a low impedance path for the current. When bypassing in the rotator,
use amply rated capacitors - I found 50v DC working ceramics failed pretty
spectacularly.
73
Peter G3RZP
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