On 12/5/14, 12:44 AM, Ian White wrote:
Just had a thought. Do I need to choke my rotator control cable too? 73,
David, AA9G
You certainly shouldn't need to do that.
Let's think about this, because RF current on a rotator control cable can't
just appear out of nowhere. The defining feature of current is that it must
have come *from* somewhere and be flowing *to* somewhere else... so where would
this unwanted current be coming from?
The only possible source of RF current is at the Yagi feedpoint, where a design
error or a bad choice of balun might cause unwanted common-mode current to flow
along the feedline, the boom and the mast... so the feedpoint is also where the
solution must be found. Choking the rotator cable would only be treating a
symptom, while ignoring the much bigger root cause.
I would actually think that "cable acting as antenna" might be a bigger
source of common mode current. Of course, putting a choke at the top
wouldn't change that.
I would think that the internal capacitance of the rotor is large enough
that from an RF standpoint, the top end of the rotor cable is
essentially shorted to the tower. It might even have internal bypass
capacitors. And even if not, probably at least one of the wires is
connected to the case, and the enormous parallel C between the wires in
the cable means that for all intents, the cable is connected to the
tower at the top.
Where you might want a choke on the rotor cable is at the shack end.. to
keep the RF out of the shack. The pattern disruption from current
along the cable is probably no different than the pattern disruption
from the tower or guy wires or whatever else is around.
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