[BARC-List] S-meter reliability

[djmalloy at mwisp.net]

Hello to all,
   This message came up on a numbers station info list, and I thought it
would be informative to many in the club.
DE dan, Agent RDZ, in search of Pussy Galore! ;-)

Date: Tue, 27 Jul 2004 09:46:41 -0400 
 From: Chris Smolinski <csmolinski at blackcatsystems.com> 
 Subject: [Spooks] Signal strengths measurements reliability 
 To: spooks at mailman.qth.net 
 Message-ID: <p06020408bd2c0cb7f1f9@[192.168.0.18]> 
 Content-Type: text/plain; charset="iso-8859-1" ;
 format="flowed" 

 
 >Hello group, 
 > 
 >Whilst preparing the HTML edition of E2K NL 23, a question
 came to my mind 
 >about S-metres reliability, and what trust we can give to
 them. 

 None. See below ;-) 

 >It is clear that with the same RX at the same QTH and the same
 aerials, 
 >S_metre readings give an accurate idea of signal strength
 evolution over 
 >time. 

 Fair enough. 

 >Now, if in the middle of a transmission, at the same QTH, we
 switch-off the 
 >RX and replace it by a different model of a different brand,
 will the 
 >S-metre give the same value ? Will S5 stay S5 and not become
 S3 or S7 ? 
 >S-metre values are linked to the electric field generated by
 the signal 
 >(usually in the µV/m order) but how caring are manufacturers
 to respect the 
 >equivalence ? Can we trust S-metres values at different QTHs
 in order to 
 >deduce what direction (if any) a numbers station is beaming
 its signal to ? 

 Short answer - no you cannot trust an S-meter. 

 Long answer: 

 The S unit system is a logarithmic based system. 
 Each S unit is supposed to be 6 dB apart. That 
 is, S9 is 6 dB more than S8, etc. This is rarely 
 the case in most radios. Not only is it usually 
 not 6 dB, it often varies as the signal varies 
 from S1 to S9 and beyond. 

 S9 corresponds to 50 microvolts across the 
 antenna input terminals of the radio. But some 
 radio manufacturers use different standards for 
 S9. 

 >From this, you can deduce the input levels for 
 any given signal strength in S units, and 
 vice-versa. 

 Above S9, of course, we generally refer to a signal as
 "S9+10dB", etc. 

 >Any eperiment made ? 

 Yes, take a look at 
 http://www.seed-solutions.com/gregordy/Amateur%20Radio/Experimentation/SMet
erBlues.htm


 Short answer - S meters are good for *relative* 
 signal strength measurements. Directly comparing 
 readings between two different radios, even if 
 using the same antenna, is risky at best. The S 
 meter apparently is something added onto the 
 radio at the last moment by the manufacturer. 
 Most work by measuring the AGC voltage in the 
 radio, not by directly measuring the input signal 
 level. 

 Long answer - you could calibrate your S meter, 
 much as the author of that web page did. Also 
 note from his tests that on a given radio, the s 
 meter sensitivity varies from band to band. This 
 is to be expected, since the sensitivity of most 
 radios is not constant across all of HF. 

 Now, you can pull out your RF signal generator 
 (calibrated against a NIST reference I hope!) and 
 calibrate your radio's S meter on all bands, for 
 all signal levels. Then do the same for any other 
 radio you have. 

 But also bear in mind that the RF voltage at the 
 antenna input terminal of the radio is also a 
 function of the impedance of the antenna 
 connected to the radio, not to mention the actual 
 efficiency of the antenna itself. 

 There are a large number of variables at work 
 here. Hence, an S meter is best used as a 
 relative measurement of the signal strength, not 
 an absolute one. 


 -- 

 --- 
 Chris Smolinski 
 Black Cat Systems 
 http://www.blackcatsystems.com 


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