I was led to believe that by tie wrapping a rotor cable (along with coax
cables) to one tower leg establishes over the cable length sufficient
enough distributed capacitance that choking or using RFC's in line with
the rotor cable is not required. I also understand that in a shunt fed
tower with a three of four wire cage spaced out 24 inches or more it is
the cage not the tower that does the radiating where it really counts in
the high current portion of the vertical. The top portion including
where the rotor is located and beam(s) providing the top loading in many
cases, is where the transmit voltages are much higher. In the Ham II or
IV rotor housing the only item of concern would be the beam location
slider pot. However this is very robust and should, with the wiper
grounded to the bell, be able to withstand most normal amateur station
power levels.
HerbSchoenbohm, KV4FZ
On 12/5/2014 11:49 AM, Jim Lux wrote:
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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