[TowerTalk] 1/4w matching, Common mode chokes and BAL-UN

[Jim Brown]

On 12/29/2010 7:35 PM, NPAlex at aol.com wrote:
> I think a more accurate description of several coiled turns of coaxial
> cable would be a RF Choke, serving to limit common mode currents on the outside
>   of the coax cable.

That's an Alice In Wonderland view of things. Yes, the coil of coax is 
an inductance in the common mode circuit, but the common mode circuit 
also includes the series impedance of the transmission line. If that 
line is less than 1/4 wave, it is a CAPACITIVE REACTANCE, so the 
inductive reactance CANCELS all or part of it and the common mode 
current INCREASES rather than decreases.  The same thing happens if the 
line is between 1/2 and 3/4 wavelengths. Further, the electrical length 
is based on a Vf of 0.98, NOT the Vf of the coax (because the common 
mode current flows on the outside of the shield).  The 0.98 factor 
accounts for the slight shortening effect of the outer jacket.

The only effective common mode choke is a very lossy parallel resonant 
circuit, which is what you get if you wind multiple turns of coax around 
a lossy ferrite core (like #31 or #43). All you have to do is use the 
number of turns that puts the very broad resonance where it covers the 
bands you work.

Study http://audiosystemsgroup.com/RFI-Ham.pdf

73, Jim K9YC