On 12/29/2010 7:35 PM, NPAlex@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
> 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.
73, Jim K9YC
TowerTalk mailing list