[TowerTalk] Measured S-meter db/S-unit TS-450

[Bob Duckworth]

Brian and you guys,

Here is my take on the S meter stuff....

Assumption that Brians pad is db as power ratio
in accordance with the usual db=10log(P2/P1)
and applying 10dbP2/P1=20dbV2/V1 cause
the S meter db is referenced to voltage ratios.


| S-METER READING      DB ABOVE 0	      db=20logV2/V1	delta
| 0                        0					 0	
| 1                        4					12		 12
| 2                        9					18	 	  6	
| 3                       13					26	 	  8
| 4                       16					32	 	  6
| 5                       19					38   	 	  6
| 6                       20					40	   	  2	
| 7                       22					44         	  4
| 8                       25					50         	  6
| 9                       29					58	  	  8
| 9+10                    36					72         	 14
| 9+20                    46					92         	 10
| 9+30                    64				           128          	 36


Looks like roughly 6db/S unit to me.
Meter is reading volts and is pretty coarse (LED bar graph)
For power ratio it's square root
or after taking log, 1/2 times to get power ratio.

Taking the Hallicrafters meter mentioned in an earlier message, 
the manual stated S9 as approximately equivilent to 50 microvolts


S		db		microvolts
9		0		50
8		-6		25
7		-12		12.5
6		-18		  6.25
5		-24		  3.13
4		-30		  1.56
3		-36		   0.78
2		-42		   0.4
1		-48	  	   0.2
0		-54		   0.1

How does this fit reality? Most HF RX have published sensitivity of
0.03 to 0.05 microvolt or another 6 to 12db  down from S0 for some
10db s+n/n or so. My ear and the signals that don't move the meter
tell me this is about right.

Bottom line.
1 S unit or 6db on the Hallicrafters (and I suspect all S meters) is a 
measure of relative voltage levels. To convert to power for checking 
antenna gain, divide by two.

I suspect a pad is going to be more accurate than the S meter.
A good volt meter on the AGC line and staying in a linear region is
going to be easier to read. Dig out that old VTVM and pad.
Find linear range of AGC. then use the S meter or VTVM to 
compare signals. Take 20log(V2/V1)=10log(P2/P1) cause you are
measuring relative voltage and antenna is specified in relative
power. 
With your VTVM or other voltmeter to get db, just take the square
root of the voltage ratios to get the power ratios and then by every
definition I can find for db as a measure of relative power your
gain or loss :-) in db=10log(P2/P1) although it's might sound more
impressive to
say, "Running 12kW ERP" than "running 1500W to phased array with 9db gain"


-bob
wb4mnf



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