Date: Sat, 24 Jan 2026 01:52:43 -0800
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Test Fixture for Common Mode Chokes
<BUT -- what I forgot to add that the ENGINEER in me realized that choke
<designs will be used with feedlines in all sorts of lengths, so my
<design goal was to make the resistive component of the choking Z
<sufficiently large that it would dominate the Z, inserting a near open
<into the common mode circuit at that point. That's why using a resistor
<equal to that value as a load is useful as a first approximation. And,
<of course, because it's a parallel resonant circuit, we know that the
<value of that resistance will fall as we move away from resonance. These
<concepts were developed in tutorials I've written about chokes and their
<use.
73, Jim K9YC
## why don't you folks just use a clamp on RF ammeter, and actually
measure the CM current, at several different places along the coax cable
?? The CM current will vary along the length of the coax due to swr on
the outside braid of the coax, even though the ant swr may well be 1:1.
## Mark the coax with tape, where u take the readings from. Then if you
swap to a different CM choke at the feedpoint, or a 2nd, 3rd CM choke
downstream, or 2 x chokes in series, you can compare CM current results
asap.
## The whole idea is to minimize CM current. If you don't actually
measure it, you really don't know what you are dealing with.
## and when taking measurements use the same PO as before. IE: 1 kw CXR.
Jim VE7RF
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