The RST system was devised when many, if not most, receivers did not
have an S meter. The S part of RST is not directly related to an S meter
reading,although it may be "somewhat related". The S stands for anything
from "Faint signals, barely perceptable" for S=1, to "Extremely strong
signals" for S=9. When the noise level on the band is really low, a 599
report could accurately and honestly be given to a signal that does not
move the S meter above S1 or S2. On a noisy day, a signal with the same
signal power at the receiver antenna input, might accurately be reported
as 339. And of course, in a contest, you might hear something like "RST
599 PSE AGN UR CL ES QTH" which means your RST is something like 339.
Even if you could calibrate your S meter for exactly 50uV at S9, and
make the circuitry (or firmware) keep each S unit exactly 6dB and each
dB over S9 mark exactly correct, there would likely be some gain drift
with ambient temperature in your receiver. Even the best commercial or
communications receivers are not laboratory grade signal strength
measuring instruments, and they were never intended to be. Instruments
which are intended to accurately measure signal levels almost always
include a built in calibration source, which is used to check and/or
adjust the instrument calibration, before making an accurate
measurement. I have never seen an amplitude calibrator built into a
communications receiver (although I have seen many frequency calibrators
built into receivers) and so would never expect a super accurate signal
strength measurement from a receiver.
DE N6KB
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