[TowerTalk] ferrite clip-ons & OCF dipoles

[Jim Brown]

On Sun, 7 Aug 2005 11:31:32 -0400, Jim Jarvis wrote:

>Your thoughts?  Please restrict comments to the ferrite choke topic.

It's difficult to do that without considering how the antenna works, or 
is intended to work. When you place (an effective) choke balun on the 
coax, you are essentially insuring that the coax does not become part 
of the antenna system. By grounding the coax and putting downstream of 
the grounding point, you FORCE the coax to be part of the antenna 
system, including all of the resonances associated with its length. In 
fact, in this configuration, it's hard to understand why you have the 
chokes there at all, except possibly to make it more likely that the 
antenna current flow to your ground rod.  

It's also important to understand that most ground rods are a VERY 
imperfect ground at RF, even in good soil. 

If you WANT to take the coax out of the antenna equation, and do so at 
all frequencies, the only good place for those cores is at the 
feedpoint. I would use as many as practical (to maximize the choking Z 
and minimize the dissipation in each core), and use those that are as 
large as possible (to maximize the total dissipation rating).  An 
argument could also be made for using those that have the greatest 
practical value of R at the frequencies where you will be using the 
antenna (you want R rather than X to minimize the interaction of the 
choke with the length of the line). 

Jim Brown  K9YC