Re: Clamp-on RF current meters - a question for Ian and others..

Posted by VK7JB on Nov 07, 2011; 11:34am
URL: http://elecraft.85.s1.nabble.com/Clamp-on-RF-current-meters-a-question-for-Ian-and-others-tp6961533p6970036.html

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