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

Posted by Jim Brown-10 on Nov 04, 2011; 4:16pm
URL: http://elecraft.85.s1.nabble.com/Clamp-on-RF-current-meters-a-question-for-Ian-and-others-tp6961533p6963254.html

On 11/4/2011 1:31 AM, Ian White GM3SEK wrote:
> Most victim equipment can tolerate 10mA, and you aren't reporting any
> RFI problems..

I generally agree with everything in Ian's excellent post, but I will
disagree with this statement. Around 2003, I did extensive testing of
pro audio gear for RFI susceptibility to common mode current (the the
Pin One Problem), published as an AES Paper that can be downloaded on my
website. I detected quite a bit of RFI with levels of RF current in the
range of 10mA, over a range of frequencies between 100kHz and 500 MHz.

There's another important answer to your question that Ian did not
address -- receive noise.  All passive linear circuits follow
reciprocity -- that is, they work in both directions.  One of the major
reasons for using a common mode choke is to prevent RF noise picked on
the feedline from coupling to the antenna, and from there to our
receiver.  In this application, the choke generally needs to be at the
antenna feedpoint (that is, where the feedline connects to the antenna)
to be effective.

Several researchers, working independently, have established a choking
impedance on the order of 5,000 ohms as a good design goal. W1HIS and I
did so roughly 6-8 years ago, and a US military research group did so
around 1970 in an internal technical applications note someone from the
CIA engineering group forwarded me a year or so ago.  W1HIS was the
first to identify this issue in the ham community, and common mode
chokes as a way to solve it.  As soon as I saw his paper (around 2006) I
added a choke to one of my antennas that was hearing noise,  and the
noise dropped a couple of S-units.

The choice of 5,000 ohms is only a general design goal. Lower impedances
will provide less reduction of common mode current, but may be
sufficient for many situations, and higher impedances may be required
for others.  Successful common mode chokes sold for many years in the US
for RFI to telephone equipment, for example, provide choking impedances
in the range of 20K ohms, and lower values of choking impedance are
insufficient.

My writing is at http://audiosystemsgroup.com/publish.htm

73, Jim Brown K9YC
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