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

[Edward Mccann]

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 at 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.