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Re: [TowerTalk] Common Mode RF choke on a 50 MHz yagi

To: k4to@arrl.net
Subject: Re: [TowerTalk] Common Mode RF choke on a 50 MHz yagi
From: Edward Mccann via TowerTalk <towertalk@contesting.com>
Reply-to: Edward Mccann <edwmccann@yahoo.com>
Date: Thu, 2 Jul 2020 09:29:06 -0700
List-post: <mailto:towertalk@contesting.com>
Re:

>> “But the current loop/probe has to be
>> looped around either
>> the  center conductor.....or the braid, but not both.  Same as clamping
>> on a 60 hz  circuit,
>> only one conductor passes through the probe.   Otherwise the  opposing
>> fields will cancel.”


You have just described how to measure the differential current out and back to 
the antenna. Two measurements described above givesyou two currents, which if 
truly balanced (often unattainable, even with twin lead and hopefully balanced 
antenna)

When you run the intact coax thru the sensor, you are reading the longitudinal, 
or Common Mode Current, the source of feedline radiation.

Google Owen Duffy, couple of great articles, and Lloyd Butler VK5BR, whose 2009 
design reflected only current balance on each leg of feedline; and after course 
correction suggested by Duffy revised his device to measure each of 
differential current and longitudinal (Common-Mode) currents.

Note Butler’s device works on either coax or twin-lead, as circuit floats in 
plastic box.

https://owenduffy.net/module/icm/

https://owenduffy.net/blog/?p=458

http://users.tpg.com.au/ldbutler/Line_Bal_Test_Meter.htm

73,
Ed McCann
AG6CX
Sausalito, CA



Sent from my iPhone

On Jul 2, 2020, at 8:29 AM, Dave Sublette <k4to.dave@gmail.com> wrote:

>>> But the current loop/probe has to be
>>> looped around either
>>> the  center conductor.....or the braid, but not both.  Same as clamping
>>> on a 60 hz  circuit,
>>> only one conductor passes through the probe.   Otherwise the  opposing
>>> fields will cancel.
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