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.
On 20/04/2012 15:26, Tod Olson wrote:
> 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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