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Re: Topband: Testing for common mode noise

Subject: Re: Topband: Testing for common mode noise
From: Don Kirk <>
Date: Mon, 22 Aug 2011 10:50:36 -0400
List-post: <">>
 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.
Don (wd8dsb) 




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