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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
