[TowerTalk] Common-mode choke

[Jim Brown]

On 5/19/2014 2:08 PM, Jim Lux wrote:
> I would guess that most antenna applications are more the "broadband 
> choke" than the "narrowband notch".

In ALL cases where ferrite chokes are used for suppression, it is the 
resistive component of the impedance that is always useful. A choke that 
depends only on inductive reactance will resonate with the cable that it 
is choking, which increases the current rather than suppressing it.

The RFI-Ham.pdf tutorial shows the only trustworthy measurement setup I 
know of other than the dedicated HP Impedance instrument. Both that 
tutorial, and the Power Point pdf on Coax Chokes show several chokes 
with their measurements, then computes circuit values for their parallel 
equivalent circuits.

Note that my measurements are scalar (magnitude only), so those circuit 
values are developed by empirical means. Rp is the Z at resonance, a 
first approximation for L is determined from the shape of the curve well 
below resonance, C is computed from the resonance equation. Then L and C 
values are tweaked manually until the plot of the resonance equation 
most closely matches the measured data.

Also note that because the equivalent circuit for #31 chokes with lots 
of turns includes two resonance, stagger-tuned, the above method must be 
considered a first approximation. See the tutorial for a discussion of 
the second resonance.

73, Jim K9YC