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

[Jim Brown]

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