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[AMPS] True Or Not True #2

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Subject: [AMPS] True Or Not True #2
From: (Tom Rauch)
Date: Fri, 7 Sep 2001 07:10:09 -0400
> The Radio Amateur's Handbook established a standard which has been
> used for years - so I continue to use it.  If I find a receiver that

The Radio Amateurs Handbook can not establish an industry  
"standard". Even if they tried, it would only be a standard if 
everyone used or attempted to use it.

> has s-units equal to 1 db, I reject the receiver as a meaningful
> qualitative test instrument.  My Collins KWM-1  is calibrated to 6 db

Most receiver use 3-5dB as the target.

The Collins 75S1 and KWM2 use 30dB from S-zero to S-9 on the 
scale. That is about 3.3 dB per S unit as the calibration if done 
perfectly. Of course due to AGC non-linearity it is about 1dB at the 
low end of the scale even though they planned for 3.3dB. Throw 
your Collins S1's and KWM2's away!

Drake used 5dB per S unit on the meter scale, but it winds up less 
at the low end of the scale in the 2B and R4A and R4C.

My ICOM 751A's use about 4dB if the meter is set per ICOM 
specs, although the scale is obviously plotted so it is 5dB. At the 
low end of the scale they are 1.4dB per S unit if calibrated per 
factory instructions.

My FT1000D's are about 1.7dB at the low end of the scale, and the 
scale is obviously plotted at 5dB per S unit. They are about 5dB 
between S7 and S9 when calibrated to factory specs.

Factually, without remarking the scales, there is no practical way 
to make them 6dB.

> per S unit on 20 meters.  My Icom equipment - gee, I do not have any
> Icom low band equipment.
> So, now when I check the f/b ratio of my 75 meter beam and I get 3 S
> units - I will have to assume that as 3 db - ouch!

Anyone who depends on an S meter without checking it with ALL 
the settings being used for the measurement has no clue at all 
what he is measuring. Obviously most people have never invested 
in a step attenuator (let alone checked a receiver), so they have no 
idea what the meter actually reads on different bands, bandwidths, 
pre-amp setting, and modes. They blind accept that an S unit is 
like an ohm, or a volt, and means something.    

Collins, on most meters, included a dB scale so it is easy to see 
what they planned. My 75S1, for example, starts the dB scale at 
10dB above S-0, and goes to 30dB just above S-9. There are nine 
places between S-0 and S-9. 30/9=3.3333333 dB per S unit.

Drake used 5dB, get a ruler out and measure the scale.

It is a myth that an S unit is 6dB as some sort of  standard. The 
most common goal was 5dB, very few receivers even attempted to 
use six, and virtually none are linear over the scale.
73, Tom W8JI 

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