[TowerTalk] Measuring Common Mode Choke with Anritsu VNA

Avery Davis wb4rtp at mindspring.com
Mon Jan 1 21:10:10 EST 2018

On 12/31/2017 7:30 PM,Jim Brown <jim at audiosystemsgroup.com> said:

"OK.  BUT -- it's NOT an inductor.  See below. Also study
k9yc.com/RFI-Ham.pdf  Or study it in the ARRL Handbook. It was added in
2011. "

I advised a setup for the measurement like that for a simple inductor,
not that a choke IS a simple inductor.  Of course, it is a complex
impedance, but the primary effect, isolation, is mainly achieved through
inductive impedance, as the charts in <k9yc.com/RFI-Ham.pdf>  clearly
demonstrate.  I apologize for not being more clear, but the point of the
exercise is not to measure the complete complex impedance to within 10%,
although you are right that a simple S11 reflection measurement has a
limited impedance range depending on the instrument, but to  assess the
suitability for use in a certain frequency range, and the S11
measurement on a Smith chart will clearly demonstrate resonance effects
and the effective frequency range.  
I do want to thank you for your comments as it prompted me to do more
research and found a paper which discusses increasing the range of
impedance measurements possible with a VNA using shunt and series S21
measurements.  See slides 63-69:

Avery, WB4RTP

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