[TowerTalk] Best coaxial to make Chokes

[Jim Brown]

On 11/12/2013 5:28 PM, Peter Voelpel wrote:
> Any choke is inductive.
> If not it is no choke at all.

WRONG.  A proper common mode choke inductively couples loss from the 
ferrite core. A parallel resonance is formed by the inductance of the 
winding (the wire passing through the core) and the stray capacitance 
between the winding. The coupled resistance is what forms the common 
mode choke, and because the core material is very lossy, the circuit Q 
is quite low (typically on the order of 0.5), so the resonance is quite 
broad.

A single turn through a ferrite core resonates in the 150 MHz range. 
Like any coil, winding multiple turns multiplies L by the square of the 
turns, capacitance increases approximately linearly with the number of 
turns, the resistance, because it is coupled inductively is multiplied 
by the square of the turns. This moves the resonance to the part of the 
HF spectrum where choking action is needed, and it is the RESISTANCE 
that provides the most reliable choking action.  A proper choke wound on 
a suitable core material can provide resistive choking Z in the range of 
3K - 8K Ohms over a 2 octave frequency range (because the circuit Q is 
so low).

Again, any choke that is not strongly resistive at frequencies of 
interest will resonate with the line (which behaves as an antenna) at 
frequencies which depend on the length of the feedline and the reactance 
of the choke.

For a thorough development of these concepts, see my RFI tutorial, cited 
in my prior email.

73, Jim K9YC