[TowerTalk] Balun Recommendation

[Jim Lux]

On 4/20/12 7:26 AM, Tod Olson wrote:
> Jim:
>
> 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.

a trivial example.. 50 ohm load at the end of the feedline, with a big 
wire hanging off.  Now, in this case, the common mode current isn't even 
from the source at the source end of the feedline, it's flowing in the 
loop around chassis, feedline, big wire CM interference source chassis

>
> 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.
>

Yes, that would be an example.  What about an open circuit or short at 
the load end.


> 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.

yes.. that's a flavor of my trivial example, except that instead of a 50 
ohm load, you have a radiating antenna.  So you have the circuit of the 
differential mode (the intended path with the antenna) and the circuit 
of the common mode (the feedline acting as an antenna picking up the 
radiated signal, because it's asymmetric to the antenna)

>
> 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.

Yes.
In an ideal case, the SWR instrument is immune to common mode signals, 
since it measures, by definition, the differential mode current and voltage.
In real life, perhaps not.