[TowerTalk] Choking on chokes

Patrick Greenlee patrick_g at windstream.net
Fri Dec 5 08:13:29 EST 2014

Might not the rotor control cable act as an antenna and pick up RF in the 
near field of the transmitting antenna?  Depending on the extant conditions 
might this not be enough to cause some problem?  Proper routing should 
minimize this but not all installations end up optimal.

Patrick    NJ5G

-----Original Message----- 
From: Ian White
Sent: Friday, December 05, 2014 2:44 AM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Choking on chokes

>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 

[Afterthought: another potential cause of common-mode current on the rotator 
cable could be a complete break in the shield of the coax feedline, forcing 
RF currents to find a return path outside of the coax. But once again the 
rotator cable would only be a symptom, not the root cause.]

I guess the real point is that we can work out quite a lot by standing still 
and taking a moment to think.

73 from Ian GM3SEK


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