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Re: [TowerTalk] Balun Recommendation

Subject: Re: [TowerTalk] Balun Recommendation
From: Steve Hunt <>
Date: Fri, 20 Apr 2012 15:38:17 +0100
List-post: <">>

Your reasoning is correct - you can have zero CM current (perfect 
balance) and high SWR, and you can also have high CM current in a 
perfectly matched system (SWR=1:1).

However, in an unbalanced system the amount of CM current that flows 
depends - amongst other things - on the load impedance, and the load 
impedance determines the SWR on the feedline. So you cannot say there is 
no linkage between the choke dissipation and the SWR.

Steve G3TXQ

On 20/04/2012 15:26, Tod Olson wrote:
> Jim:
> I read with some interest the following section from one of your emails on
> this subject.
>> S(...  A second major reason for 5K ohms is power handling. 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 feed line lengthS(..
> I read that you are saying it is possible to have 1:1 SWR on the feed line
> and simultaneously have significant common mode current on that feed line.
> For me that is an idea that I had not had before.
> Do you suppose that implies that the feed line could have very high SWR
> but no common mode current? I suppose so since I can imagine open wire
> line that might have high SWR but is balanced with respect to the voltage
> on each side.
> If I consider that same open wire line I can imagine a situation where the
> feed point impedance is exactly equal to the line impedance [off center
> fed] but the line has common mode currents.
> Where there is no common mode current and there is high SWR a common mode
> choke would be expected to have very little heat created I suppose. Where
> there is a very low SWR with a high common mode current one might expect
> substantial heat.
> In all of these 'thought experiments' the choke does not cause SWR [nor
> cure it]. I suppose the choke may affect the accuracy of the instruments
> measuring the SWR.
> There must be someone reading this who can set me straight if I am wrong
> in the way I am reasoning.
> Tod, K0TO

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