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Re: [TowerTalk] Balun Recommendation

Subject: Re: [TowerTalk] Balun Recommendation
From: Steve Hunt <>
Date: Wed, 18 Apr 2012 20:16:56 +0100
List-post: <">>

Sorry to disagree again, but the CM choke dissipation DOES also depend 
on the SWR!

Say we had a perfectly balanced 50 Ohm antenna being fed 1kW - the 
differential voltage at the feedpoint would be 224v rms, or 112v rms 
from each leg to "ground". If we feed the antenna with coax, and include 
a CM choke at the feedpoint, that 112v leg voltage appears across the 
choke and CM path in series and is what is driving the CM current.

Now assume the antenna impedance is 75 Ohms instead of 50 Ohms 
(SWR=1.5:1). For the same power, the differential mode voltage at the 
feedpoint increases by a factor 1.225 and all other voltages scale by 
the same factor, including the CM voltage across the choke.

The 1.5:1 SWR increases the dissipation in the choke by 50% just as 
surely as if we'd simply increased the power into the 50 Ohm antenna 
from 1kW to 1.5kW.

Even if the antenna isn't balanced, or if you change what is considered 
to be the "ground" reference, a change in SWR changes the feedpoint 
differential voltage and alters all voltages around the system by the 
same factor - including the CM voltage across the choke.

Steve G3TXQ

On 18/04/2012 17:29, Jim Brown wrote:
> But SWR has NOTHING to do with dissipation in a common mode choke. 
> What matters is the common mode voltage, which is directly related to 
> IMBALANCE in the system, and also to feedline length. 

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