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This is in reply to a posting on the e2k mailing list. The owner of that list does not allow any postings larger than a few lines, since one or two of the list members are technically challenged and try to read messages using a cell phone, so all must suffer.
The Spooks list does allow messages longer than two or three paragraphs, so it can be posted here. Hopefully those from the e2k list who are interested will find the message here to read.
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 �V/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/SMeterBlues.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.
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--- Chris Smolinski Black Cat Systems http://www.blackcatsystems.com ______________________________________________________________ Spooks mailing list Home: http://mailman.qth.net/mailman/listinfo/spooks Help: http://mailman.qth.net/faq.htm Post: mailto:[EMAIL PROTECTED] - Visit http://www.spynumbers.com/ for complete information about Spy Numbers Stations
