"Reg Edwards" wrote in
message ...

wrote
Only by coincidence and only in a 50 ohm system
elsewhere
S9 is 50 uV (microVolts) equals 50pW.

==========================================
Allison, I'm afraid you are not quite correct about
coincidences.

The beauty of calibrating an S-meter in watts is that the
result is
independent of receiver impedance.

Of course, there must be a conjugate match between antenna
and
receiver. And there usually is. This is taken care of in
the
calibration process.

The received signal STRENGTH is indicated in watts, which
is all one
wants to know. When reading the meter, who cares about
what impedance
the measuring instrument happens to be?

To sum up : There is a transmitter of given power output.
There is a
radio path which is an attenuator, And there is a
received signal
signal strength meter which indicates watts.

The overall loss between transmitter and power meter may
be deduced in
terms of decibels. Professional radio engineers do it all
the time.

It can be misleading to think in terms of S9 = 50 uV when
one doesn't
know what the receiver input impedance is.
----
Reg, G4FGQ

Tradionally S-9 has been defined as 50uV, but there
really is not a standard. The S numbers are taken from the
old R-S-T system of reporting signals (Readability,
Strength, Tone). While the meter can be calibrated using a
calibrated signal generator the reading for 50uV will vary
with the gain of the receiver so it will not be the same on
all bands or even from one end of a band to the other.
Some receivers have meters calibrated in something other
than S units. For instance, the old Hammarlund SP-600. The
meters on these are usually calibrated for RF in db above 1
uv. But, the same thing applies, the receiver gain varies
with band and frequency, the AGC is not linear, and the
reading only an indication of relative strength rather than
any actual value.



--
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Richard Knoppow
Los Angeles, CA, USA