[Elecraft] K3 S-Meter

[Ron D'Eau Claire]

The "S" in RST is what S-meters supposedly indicate although many Hams
rightfully feel they do it very poorly, Hi! 

A little background:

The RST system was adopted by Hams in the early 1920's when the standard HAM
receiver was a regen.  The "S- meter" on a receiver came along with
automatic gain control (AGC) as superhets came into common use in the 1920s.
The AGC varies according to the signal level and controls the gain to
maintain a roughly constant audio output level. It first came into use on
the RCA "Radiola" series A.M. radios that were the "Cadillac" of broadcast
band receivers just as "broadcasting" was starting up. Home listeners were
delighted with a system that kept strong stations from blasting out and that
helped keep the music level constant when there was fading. 

The S-meter measures the AGC voltage and became a popular addition to Ham
receivers. But AGC was used only on A.M. phone stations by most Hams well
into the 1960's. That's because one could not have a BFO operating for CW
(or, later, SSB) reception and have the AGC active. The BFO was, by
definition, on the I.F., offset only by several hundred Hz to produce the
beat note desired. So it triggered the AGC just like any gigantic signal
being received. With the BFO on, the AGC registered maximum signal and
completely desensitized the receiver.

I didn't care about not having an S-meter on CW, and I never met a Ham who
thought it was important back then. But "phone" operators became used to
having them and missed their S-meters as SSB took over from A.M. and they
had to turn on the BFO and turn off the AGC. That seems to have stimulated a
search for a solution. 

In the 50's and 60's a separate I.F. amp strip just for the AGC started
showing up using a different frequency from the BFO so one could have the
AGC active and the BFO on at the same time. Also, there was a lot of
tinkering around with "audio AGC" where the AGC was triggered by the audio
level, not the I.F. signal, but they weren't popular. At audio it takes a
significantly longer for the AGC voltage to build up than at the much higher
frequency I.F., and that delayed the onset of AGC action. (Although it is
still around in limited use. The Elecraft KX1 uses audio AGC.)

When S-meters started showing up on communications receivers in the 1930's,
manufacturers needed to make the S-meter on all of their receivers respond
similarly to the same signal. That meant calibrating the S-meters to
register the same level for a given signal strength. Collins is usually
credited with deciding that 50uV at the antenna should produce a S-9 level
on the meter. That's how all Collins receivers were calibrated, at least for
a number of years. 

Some other manufacturers such as National and Hallicrafters followed suit,
but some manufacturers discovered that many Hams loved what was called, on
the A.M. phone bands, a "lively" or "generous" S-meter that consistently
gave higher readings than other receivers. Of course, to the unsophisticated
Ham that suggested that his receiver was more "sensitive" - a highly prized
attribute. 

Adding a preamplifier ahead of the receiver - a very popular add-on for 14
MHz and above - added gain that made the S-meter more "lively" too, even on
a well-calibrated receiver S-meter. It wasn't unusual to find that those
receivers registered S-9 on a 10 uV or less signal, often to the great
delight of the Ham owner resulting in happy reactions such as "Wow! You're
70 over 9 on my new Bandbuster receiver OM!"

Only in the last decade or so have I seen any particular interest among Hams
in calibrating their S-meters again, I suppose because it is possible to do
so in rigs like the K3. Somewhere along the way a slope of 6 dB/S-Unit came
into use. I'm not sure just when, but 6 dB/S-uint was the "standard" I was
aware of when I built my first homebrew superhet with an s-meter in the
early 1960's. 

Fast forward to 2013 and for me the most useful signal indicator is the P3,
showing either the signal level at the antenna in -dBm or, for
traditionalists, in S-units. BTW, the P3 assumes 6 dB/S-unit too. 

73, Ron AC7AC





-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Tom H Childers
Sent: Tuesday, March 05, 2013 1:46 PM
To: Elecraft at mailman.qth.net
Subject: Re: [Elecraft] K3 S-Meter

RST is not a reflection of S-METER readings at all. It is a means of
describing the quality of a received signal, not just the strength.

Furthermore Amateur radio manufacturers (not Elecraft) are notorious for not
following the 50 microvolts = S9 rule, so S-meter readings are meaningless
as measure of signal quality in most cases.

The link below describes the RST system for signal quality reports.  

http://en.wikipedia.org/wiki/RST_code

The folks who defined the RST system probably didn't have S-Meters on their
receivers.

73,
Tom
Amateur Radio Operator N5GE
ARRL Lifetime Member
QCWA Lifetime Member