[PPRAANet] S-Unit Table

[W0rw@aol.com]

One S-unit is a change in signal strength POWER of 6dB, or a 3dB
change in VOLTAGE, as measured at the receiver antenna terminals.

          VOLT    POWER
------------------------
 S9+20dB 500uV = - 53dBm
 S9+6dB  100uV = - 67
 S9         50uV = - 73
 S8         25uV = - 79
 S7        12.5uV = - 85
 S6         6.2uV = - 91
 S5         3.1uV = - 97
 S4         1.6uV = -103
 S3       .77uV = -109
 S2      .39uV = -115
 S1      .19uV = -121dBm
--------------------

An S-METER is calibrated by connecting a signal generator to the
antenna terminal and setting the output power to 50uV, or -73dBm,
and adjusting the S-meter calibration pot for a reading of S-9.
Since the S-meter is usually derived from the receiver AGC line, it
*is* relatively linear from about S3-S4 and upward (since a good
AGC usually "kicks in" around -100 to -105dBm).  This linearity is
also due to the diodes used for the AGC detector, once they are 
conducting in the linear region (again, around S3-S4).  Statements
that "S-meters are totally worthless" or "a change in 2 S-units
means nothing" are thus actually quite incorrect.  An S-meter *is* a
fairly good RELATIVE power indicator for received signal strengths
and noise levels.

SO WHAT-THE-HECK IS AN S-METER GOOD FOR?

The purpose of an S-meter is not to provide any absolute indication
of power or voltage, but a RELATIVE indication between received
signal strengths ... such as between two different signals, or between
a signal and the "noise floor" of the band. 

Example: On 40M, typically the "noise" will be S4, or about -103dBm.  If
your receiver has an MDS (minimum detectable signal) of -133dBm, it means
you're loosing 30dB of your dynamic range to the noise!  (133-103=30dB).
In this case, the S-meter is more-or-less giving you an absolute power
DIFFERENCE between it's MDS and the noise floor, in dB.

Example: A station claims his beam antenna has 12dB gain over his dipole.
So he switches between the two and asks you for an "A-B" comparison.  His
signal goes from S7 to S8 ...a 6dB change.  That ain't 12dB!  12dB should
have shown 2 S-units of change.  (I'm assuming his beam antenna *was*
properly pointed at you -hi). 

Likewise, YOU are comparing two antennas at your shack.  You are LISTENING
to a QSO in progress, switching between the two antennas.  One antenna
causes the S-meter to rise about 1/2 S unit.  Well, that's 3dB, and
that's not bad for most wire antennas.  Or ... you are switching between
two antennas and notice that the noise seems to be much less on one,
in fact, the S-meter drops from S4 to S3.  You have a problem with the
antenna with the higher noise.  If the noise drops 2 S-units, you have a
BIG problem with that antenna!  Obviously, you want to use the antenna
with the lowest noise, because an S5 signal will be an S5 signal on the
same receiver.  The difference, is if one antenna has an S4 to S5 noise,
you'll be digging that S5 signal "out of the mud."  With an antenna at
S3 noise level, that S5 signal now has a 2 S-unit (12dB) improvement in
signal-to-noise, and will obviously be much easier to work.

An S-meter also makes it convenient to make internal tuning adjustments
to your receiver, such as peaking any IF cans, filters, etc.  You can
tune to a carrier or QSO in the S8 range, then tune above and below and
mark the frequency where the S-meter drops 1 S-unit (6dB), 2 S-units
(12dB), etc. to make a rough graph of your overall selectivity/filtering
of your receiver.  If your receiver claims the RF amplifier, when kicked
in, provides 12dB of gain, well, you should clearly see about a 2 S-unit
change.  Or if the 3dB filter BW is 300Hz, then you should clearly see a
1 S-unit change over about twice that, huh?  You can do the same with a
DVM on your audio output, but an S-meter sure makes it more convenient,
and quite easy to verify some of the specs and claims the rig/kit
vendor is claiming.  Or to check for a change in performance later on
for troubleshooting purposes.  It is ALWAYS beneficial to do some of
these basic measurements when you put a new rig on-line, so you have a
baseline to check performance later on if troubles begin.  A simple 
S-meter is all you need to record some of these important specs.

WHAT ABOUT THIS QRO vs QRP THING?

You have to QUADRUPLE (X4) your signal to DOUBLE your signal strength at
the receiver end.  Likewise, if you drop your power by one-fourth, your
received signal strength will be one-half less, or 1 S-unit.  You are
working a station running 100W and he is S8.  If he drops his power to
1/4th, or 25W, his signal strength should drop about 1 S-unit, or to S7.
If he drops another 1/4th, to about 6W, he should drop another S unit, or
to about S6.  Therefore, the difference between 100W and 5W QRP is about 2
S-units.  Big deal.  Dropping to 1W is about another S-unit, then to
250mW another S-unit, etc.  OK, now you're getting down into the S4
noise level on 40M.  Now you're hoping the guy on the other end has only
a S3 noise level on his end :-)

Hopefully this answers some of the questions raised about S-meters and
how to use them.

72, Paul NA5N
via w0rw