Topband: Testing for common mode noise

[Don Kirk]

 I received numerous responses regarding my proposed common mode noise test, and here is a summary of the responses.

1) The majority of respondents supported my proposed test.

2)  There was really only one respondent that I thought provided a thumbs down to my proposed test by saying "first test is much less likely to produce equivocal results", but a second respondent might have been hinting at this too.

3) One person said I could "determine the common mode current by simply terminating the antenna end of the feed line in it's characteristic impedance and then listen to the rig", and I thought I should mention in this summary that this technique has long been considered an invalid test. 

4) Guy (K2AV) felt small S-meter changes would be hard to detect when listening to the noise, and he suggested the use of an AC voltmeter to measure the receiver audio with the AGC off in order to detect small changes.  I did some preliminary testing with my high end AC voltmeter which also provides long term averaging, and this method appears to be very beneficial in detecting small changes (versus using the S-meter).

Thanks to everyone that responded.
73's
Don (wd8dsb)