To measure the total inductance, or more importantly, the +jX of the
windings: Short both conductors together at both ends so the choke forming
a single conductor consisting of two paralleled conductors on the core.
This is for test purposes, only. Then connect one end to the S11 center
pin and the other to the S11 backshell. Then, on the Smith Chart using the
cursors, you can measure the total inductance and +jX of the windings on
the core. To provide adequate choking, the total +jX should be at least 5
times the system impedance (some state 10X). For a 50-ohm system
impedance, this, of course would be +j250, ideally at the lowest frequency
of interest. If you don't want 160-meters, that requirement becomes much
easier to reach.
With my doublet fed with open wire transmission line, I measure (from 1.8
through 29.5 MHz) : real: 25 to 5 ohms ¡ÀjX: -200 to +80
So I have read data to address with my choke designs. You might make the
same measurements of your antenna to properly design your CM choke as I
have. No single solution fits all applications. Fortunately, with the
advent of the NANOVNA's, we now have the tools to measure and to design to
what we have installed as an antenna.
Dave - W?LEV
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On Fri, Nov 6, 2020 at 6:03 PM AG6CX <edwmccann@...> wrote:
Martin:
Re:
If you wish to examine the common mode impedance of chokes like this,
connect the VNA across the windings between both wires of the Balun
connected together at the input and output of the choke.
Could be a bit more specific about the connections,? Can¡¯t quite figure
out your suggested wiring.
Thanks.
Ed McCann
AG6CX
--
*Dave - W?LEV*
*Just Let Darwin Work*
