Date: Wed, 18 Dec 2019 23:46:56 +0000
From: Chuck Hutton <charlesh3@msn.com>
To: "topband@contesting.com" <topband@contesting.com>
Subject: Topband: Measuring Common Mode Chokes
<In the past, I have simply used my N2PK VNA to measure impedance of the choke
by connecting the shield of the coax to the VNA ports.
<Recently I've been discussing common mode chokes with others who have a
different methodology.
<They prefer to do a transmission test through the choke and report the "common
mode rejection".
<This is done by placing a crossover cable between the VNA output and the
choke. The choke output is connected in a normal fashion (center to center,
shield to shield) to the VNA input.
<This does not seem ideal to me.
<First, the choke is being driven in differential mode rather than common mode.
<Second, the measurement depends on (varying) isolation between the coax center
and shield. So it's not truly common mode rejection.
<Am I on thr right track?
<A handful of Googles has not netted me any clear summary of test methodology
for reporting CMRR. I find a small number of tests reporting impedance.
<Chuck
## I measure the actual common mode current with a MFJ-854 clamp on RF
ammeter. Good from less than 1ma to 3A. Just clamps around the 213-U
coax.
Then when swapping CM chokes, any changes or improvements, or worsening is
readily apparent at a glance. I measure In several places on the
coax run, and all are well marked.
https://www.mfjenterprises.com/Product.php?productid=MFJ-854 Switchable In
5 ranges.
Jim VE7RF
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