[Elecraft] Clamp-on RF current meters - a question for Ian and others..

[VK7JB]

Hello Ian, Jim & group,

Thanks to everyone for your advice and thoughts on this matter.  It's led to
a very interesting weekend on a common mode clean up.

I'd always thought I'd had a "clean shack" from a stray RF/common mode
current perspective.  I'd had a few "minor" issues with RF getting into a
hi-fi speaker, but no other RF problems I'd ever noticed.  But after the
recent discussions, I got out my MFJ 854 clamp on meter and spent the
weekend sniffing out CMC in my setup.  The results were revealing.  

One rig had about 15-20mA of common mode current on the coax at the
connection with the rig, but hunting around, I found about 40mA on the AC
cable feeding the power supply connected to that rig.  There was no CMC on
the coax leading from the ATU attached to that rig to the DX engineering 1:1
balun what I use to transition to open wire line to feed my horizontal loop. 
10 turns of the power supply's AC cable through a 240-43 toroid removed all
detectable CMC from that set up.  

The second rig set up ( switchable to the same loop antenna but operating
from a different power supply) was more of a puzzle.  Here, all the coax
cables and inter-connections had no detectable CMC but  I found about 30mA
of current on the AC cabling supplying my Ameritron AL 811 amplifier.  This
has an itegrated linear power supply, which I plug into a power distribution
board.  This power board also has the powersupply running my 2nd rig plugged
in to it.  With the amp turned off,  but plugged in to the board, there was
35mA of CMC on its AC line and about 15mA on the rest of the AC cabling
supplying that rig.  If I unplugged the Amp from the distribution board, the
CMC fell to undetectable.  The fix was another large toroid wound around the
AC cord of the amp and I put one on the distribution board too. It seemed
that the choke on the amplifier AC cord was what did the trick.

None of my audio or DC power supply cables had any CMC detectable.  But
clearly, CMC was present on the AC power cabling supplying the power
supplies.  As predicted, simplw clip on ferrites were useless.  To reduce
the measured CMC to undetectable, I needed 8-10 turns of the cabling through
a large diameter 43 mix toroid.

I'm still not sure why the amplifier cable was so important in carrying CMC
into my set up, but it was.  

This has been a very interesting exercise.  Thanks to all for your comments
on and off the list.

73,
John
VK7JB




--
View this message in context: http://elecraft.365791.n2.nabble.com/Clamp-on-RF-current-meters-a-question-for-Ian-and-others-tp6961533p6970036.html
Sent from the Elecraft mailing list archive at Nabble.com.