[TowerTalk] Balun Recommendation

Tod Olson tod at k0to.us
Fri Apr 20 07:26:02 PDT 2012


I read with some interest the following section from one of your emails on
this subject.

>Š...  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 lengthŠ..

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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