On 4/20/2012 10:26 AM, Tod Olson wrote:
> 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.
My sloping, center fed, half wave, fan dipole had a very low swr of
about 1.2-1.3:1 yet it had a common mode voltage high enough to get back
to the shack and light just about every LED to full brilliance at around
1KW out. I had one choke at the feed point, (IIRC 6T of BuryFlex on 6,
2.4" cores). I added a second choke where the coax comes back to the
tower. With the addition of the second choke the RF in the shack is no
longer measurable. As the one end of that dipole is about 8' off the
ground and the other is at 90 feet it has to be a long way from being
I would think you could have a high SWR caused by mismatched, resistive
load with no, or very little common mode voltage if the antenna is balanced.
> 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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