[Elecraft] S-meters

[James R. Duffey]

Anybody who expects a ham receiver with an S-meter to approximate a
calibrated RF voltmeter is wishing for more than can be delivered for the
price of a K-2. At best, S-meters are OK for relative measurements. I
suspect that most of the readers on this list are also readers of QRP-L, but
for those that aren't, my recent contributions to the S-meter thread there
follows:

First COntribution:

I am somewhat surprised at the amount of interest generated by S-meters. I
have always considered them to be an unnecessary accessory. They are not
"calibrated instruments" for measurements.

The S9 = 50 microVolt/6dB/S unit standard is not universally observed by
receiver manufacturers in practice.

One of my favorite Ham books, now well used, is the British [publication
"The Buyers Guide to Amateur Radio" by Angus McKenzie, G3OSS. My copy is
from 1986. I don't know if a later edition has been published, but I have
not seen one. If t here is one I would like to have it. GQRPers?

Angus tests and reports the S meter calibration for a number of receivers. I
have summarized his results below.

Signal strength (in dBm, or decibels referenced to a millivolt) required for
given S-meter reading
 
Receiver         S1      S9     S9+20   S9+40  (dB/Sunit 1 to 9)
 
Collins         -107    -73     -49     -27     3.8
KWM-380

Drake 2B        -117    -104    -95     -77     1.5

Drake R-4C      -105    -75     -57     -36     3.3

Drake TR-7      -107    -80     -67     -42     3

Icom 730        -98     -82     -72     -57     1.8

Icom 735        -100    -76     -56     -40     2.7

Icom 745        -105    -95     -92     -86     2.2  (FM)
                -109    -75     -64     -49     3.8  (SSB)

Icom 751        -109    -96     -88     -78     1.3  (FM)
                -105    -86     -70     -54     2.1  (SSB)

Kenwood 440S    -112    -96     -92     -88     1.7  (FM)
                -104    -73     -55     -33     3.7  (SSB)

Kenwood 530SP   -110    -75     -51     -30     3.9

Kenwood 830S    -104    -72     -48     -26     3.6

Kenwood 930     -107    -79     -59     -39     3.1

Kenwood 940     -103    -75     -55     -35     3.1

Ten-Tec         -120    -93     -64     -38     3
Corsair

Yaesu FT-1      -101    -86     -71     -57     1.7   (FM)
                -103    -86     -71     -56     1.8   (SSB)

Yaesu FT-200    -83     -74     -70     -64     1.0

Recall Paul's table for the theoretical "S9 = 50 microVolt/6dB/S unit"
S-meter:

Theoretical     -121    -73     -53     -33     6.0

    
What does this tell us? Well, none of these rigs come close to 6 dB/S unit.
Not even Collins whom several claimed invented this standard. Several rigs
do come close to the S-9=50 microvolt/-73dBm standard though. S-meter
calibrations are not even consistent between different rigs from the same
manufacturer! In some of the rigs, the S-meter calibration is not even the
same between FM and SSB!! This is a clear indication that the S-meter is
measuring something other than signal strength at the antenna input. In this
case it is AGC voltage, and since the AGC characteristics for FM and SSB are
different the S-meter readings are different. There are some patholgical
cases, the FT-200  for example. Many receivers do not get the 20 dB and 40
dB intervals correctly.

This one reason I do not take S-meter readings too seriously.

Newer rigs may be better, but I doubt it.

Second Contribution

A quick search of my 2001 Handbook revealed nothing that described the
relationship of S-meter readings to received signal strength. The only
references to S-meters were in the trouble shooting section and a
description of an optional S-meter to the Sierra.

But my 1993 handbook does describe the origin of what many of us consider to
be a "standard":

"An attempt was made by at least one receiver manufacturer in the early
1940s to establish some significant numbers for S-meters. S9 was to be
equivalent to 50 microVolts, and each S-unit would have been equal to 6 dB.
The scale readings above S9 were given in dB. The system never took hold in
the manufacturing world, probably for the reasons given earlier."

The "reasons given earlier" have been hashed over here. My 1979 Handbook
says much the same thing.

The QRPers bible, "Solid State Design for the Radio Amateur" (deMaw and
Hayward), makes no mention of the S-meter in its receiver discussion,
although one is included in the AGC schematic of the Competition Grade
Receiver discussed there.

One of my favorite ARRL publications was "Understanding Amateur Radio". I
say was, because it is no longer published. It was a great introduction into
building and using amateur radio gear. This publication is not kind to
S-meters. The discussion starts;

"This is no doubt the most used and abused adornment on a receiver. An
S-meter face plate contains markings in S-units and decibels, neither of
which can be relied upon when issuing signal strength reports..."

It then goes on to discuss the information above, and continues;

"... an S-meter is primarily a gimmick for the operator to use as a topic of
discussion in a QSO. Its usefulness can be utilized, however, while making
comparative readings (relative readings) when some station operator switches
antennas or power levels...But to rely on what the meter face indicates is
pure folly."

I could say more, but this is too long already. - Dr. Megacycle KK6MC/5
-- 
James R. Duffey KK6MC/5
Cedar Crest, NM   DM65