[TowerTalk] Choking on chokes

[Jim Lux]

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.