[Elecraft] S meters in the USA

[Ron D'Eau Claire]

An S-meter by itself is pretty useless for comparative measurements, but
it's easy to calibrate! 

A simple check with a step attenuator (e.g. Elecraft AT1) in the receive
antenna line will show one how may dB/S-unit the meter indicates. With an
Elecraft XG1 (or equivalent) signal generator it's easy to do the
calibration in dBm or uV. 

For more accurate comparative work, putting the step attenuator in the line
then adjusting the attenuation to match the original meter reading is a good
way to tell another station how much their signal increased or decreased.(1)

The bottom line is that trying to do comparative measurements for another
station within 3 or 6 dB is pretty hard under most conditions. Slow AGC
gives many signals the appearance of being quite stable when in fact they
aren't: you're really seeing an average over time that may disguise a slow
but steady drift upwards or downwards in strength. Fast AGC is needed for
most checks, and then the S-meter often starts to "dance". 

There's a reason why S-units are about 6 dB. It is true that under quiet
laboratory conditions a person with normal hearing can just discern a change
in sound level of one dB, but when listening to a typical on-air signal on
HF or MF, communications engineers long ago agreed that it's almost
impossible to detect a change of less than 3 dB due to background noise,
QSB, etc. That's a 2:1 power difference; under ideal conditions one has to
double one's power or cut it in half to expect the other station to notice
any difference at all! So twice that, 6 dB, seemed a good definition for an
S-unit since S-units are defined by the perception of the person listening,
not an absolute numerical scale. 

As Toby mentioned, that definition has been adopted by the IARU: "At the
IARU Region 1 Conference in Hungary 1978 the need for a harmonised standard
for the 'S-meter scale' was expressed and a proposal was accepted for
publication in society journals. The essential recommendation was 1 S point
is 6 dB. At the Brighton Conference in 1981 the recommendation was formally
adopted as a standard for amateur radio equipment manufacturers." (2)

Only the S-9 level of 50 uV is mentioned in the K3 FAQ's, but I'd not be
surprised to see the meter set up for 6dB/S-unit. Right now the FAQ's are
vague about what happens when the preamp or attenuator are used. The K2, for
example, does not compensate for this so the "S-meter" jumps up and down 10
to 20 dB depending upon the setting of the preamp or attenuator. 

Ron AC7AC

(1) The best way to help out a buddy who wants comparative signal
measurements is to connect a 'scope or sensitive a-c meter to the receiver
audio output, turn the AGC OFF, and use the step attenuator in the RX
antenna line. The audio voltage level is measured for reference. After the
change is made at the far end, the attenuator is adjusted to reproduce the
same audio voltage without touching any receiver controls! The attenuator
will then show the actual change in signal level in dB as accurately as
propagation conditions allows. Trying to do this with AGC on makes the
change harder to see, since the AGC tries to level out the change as the
signal strength varies. Long ago I disabled the AGC in my KX1 (a simple wire
jumper that can be removed) so I could use it with my AT1 step attenuator as
a sensitive RF voltmeter for just this sort of test on 80/40/30/20 meters. 

(2) http://thf.ref-union.org/6a.pdf