[TowerTalk] Feedline (choke) question

[David Gilbert]

I can't claim all of the following perspective to be based upon much 
practical experience, but from a theoretical point of view I lean toward 
the following as an ideal case:

1.  A common mode choke at the antenna to keep RF that has been picked 
up by the coax from getting into the antenna and affecting the pattern 
or the match.

2.  A common mode choke at the output of the rig to keep RF that has 
been picked up by the coax from getting into the rig.  Choking common 
mode current at the antenna doesn't necessarily keep the coax run to the 
rig from picking up unwanted RF.  The coax shield is an antenna and 
making it look like an open at one end doesn't keep it from being an 
antenna.

3.  A common mode choke, like any other inductor, isn't going to have 
much effect at a current minimum, so placing common mode chokes at 
current maximum(s) along the coax seems like it would be good additional 
protection ... and they aren't necessarily located at either end of the 
coax.  Note that this would be the current maximums for the common mode 
current, not necessarily for the differential current.  It would 
probably require a field probe to find those points if they exist.

Thoughts?

Dave   AB7E


On 9/25/2025 7:55 PM, Tim Duffy wrote:
> I agree with Jim , K9YC recommendations.
>
> I also have had success installing high current mode chokes on the coax to
> the shack right at the amplifier output. Also sometimes a high common mode
> choke from the radio output before the amplifier input can be effective.
> Always at the antenna - always at the phasing/matching box. These must be
> high CMI chokes in excess of 4000 ohms at the frequencies of interest. Good
> quality ferrites and good quality connectors.
>
> 73
> Tim K3LR
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Jim
> Brown
> Sent: Thursday, September 25, 2025 10:24 PM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Feedline (choke) question
>
> On 9/25/2025 4:01 PM, Gary Smith wrote:
>> Just connect the coax to the phasing bars directly or do as suggested by
> byMosley which is make a six turn choke near the phasing bars or go without?
>
> My detailed recommendations, based on nearly 20 years of research, are
> here. http://k9yc.com/2018Cookbook.pdf
>
> In general, I recommend the matching system or "balun" recommended by
> the mfr be used, with a choke based on my designs added between that and
> the feedline.
>
> 73, Jim K9YC
>
>
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