There has been a de-facto HF standard for 60 years.

The USA military first used it in specifications of radio equipment when
placing contracts with manufacturers around the end of WW2. There may have
been some restrictions on publicity at the time.

The Standard is 6 dB per S-unit and 50 micro-volts into 50 ohms at S=9.

Therefore an S-meter is essentially a power or wattmeter.

The Standard is quite logically derived.

The 6 dB fits in very nicely between a typical receiver's internal noise
level (S=0) and a typical receiver's signal overload point (S=9+30 or 40
dB).

S=9 is about half way up the scale which is linear in dB's, or S-units, from
one end to the other.

There's nothing wrong with the standard. If your S-meter reads incorrectly
then don't blame the standard - re-calibrate the meter. If you can't
re-calibrate it blame the poor quality of the meter.

I have two relatively modern commercial transceivers plus two home-brewed
transceivers. Their S-meters are accurate enough for the intended purpose.
What more should I expect?
----
Reg, G4FGQ

Most people appear to be fatally attempting to use, in reverse, their
favourite S-meter as a means of calibrating the Standard. No wonder we have
so many different standards around.

The difficulties in making an accurate power meter lie solely in the very
wide range of power levels encountered.

0 to S-9 corresponds to 9 times 6 dB = 54 dB.

S-9 to +40 dB corresponds to 40 dB.

Making a very high total range of 94 dB for a power meter.

That explains why S-9 usually appears just over half way up the scale.

For a range of 94 dB it is not beyond modern technology to make a linear dB
scale out of it. The limitation is manufacturing cost. But who wants to pay
an extra hundred dollars to replace a receiver they are already happy with.

Of course, if you MUST have an accurate S-meter, the cheap way is to obtain
a blank meter scale, a fine-nibbed pen, a bottle of black ink, a signal
generator, and a 0-100 dB switched attenuator. You will proudly end up with
a work of art and a beautifully cramped scale at the bottom end. But when
done it's as accurate as you like!

Hint: There's no need to obtain a new blank scale if the existing scale can
just be turned over to its white side. Such small divergencies can make
restful breaks in between investigations of skin depths at 1 to 10 Hz of the
ocean bottom of transatlantic submarine cables.
----
Reg

Just to tidy up.

And I've been through this before but I'm just a bloody foreigner who
favours French wine.

The S-meter is a power meter.

The standard receiver input impedance is 50 ohms.

That's why you get a conjugal match when you switch the transmit tuner from
transmit to receive but you don't get such a match when you switch back.

The standard, HF, 50 microvolts at S-9 into 50 ohms corresponds to 50
pico-watts which is an inconvenient quantity to refer to in signal strength
reports. Hence the popular S-units.

S-9 requires a standard 50-ohm signal generator, set to a standard
open-circuit 100 micro-volts, to be connected to the receiver. Receiver
manufacturers in their maintenance manuals usually prescibe this at the
non-descript frequency of 7 MHz.

The internal thermal and other noise level of a typical receiver with an
input stage consisting of a balanced modulator (the first frequency
changer), referred to the receiver's input terminals, with a receiver SSB
bandwidth of 4 KHz, is of the order of 60 dB below S-9. That is a little
less than S-zero on the meter. A signal level of the same order as the noise
takes the meter to S-zero.

A signal level of S-9 plus 40 dB, or 40 dB above 50 pico-watts, corresponds
to a signal input voltage of 50 micro-volts times 100 which equals 5
milli-volts. At which point a good receiver begins to overload and suffers
from non-linear intermod products.

Hence we have a meter range of 54 + 40 = 94 dB as displayed on a typical
meter.

All this fits in very nicely with the recognised S-meter Calibration
Standard.

(I do hope I have not made an arithmetical error. But I'm sure you Americans
get the general idea nevertheless.)
----
Reg, G4FGQ

"Reg Edwards" wrote in message
...

Most people appear to be fatally attempting to use, in reverse, their
favourite S-meter as a means of calibrating the Standard. No wonder we
have
so many different standards around.

We have seen a dozen or so posters indicate that S9=50uv

Nowadays, very few rigs have analog meters. Instead, they have a computer
that can take that AGC voltage and display it however the computer decides.

In the Icom calibration procedure for the 706, there are three calibration
points known by the computer ... S0 (0 uv), S9 (50 uv), and S9+60 (50 mv).
The calibration procedure sets these three with known inputs, and presumably
the computer interpolates from there. That really isn't a bad calibration,
and I'd be surprised if other modern rigs were much different.

Now it is quite likely that there is some dependence on the AGC voltage with
frequency, but again, in a modern rig that should be manageable.

What isn't so constant, of course, is the antenna. The voltage at the
receiver could range over several orders of magnitude for the same signal
depending on the antenna. So in terms of providing input to the sender, the
S meter reading is still of limited usefulness, even if we all had
calibrated radios and agreed that S9=50uv.

...

The 756 pro is "properly" calibrated, each S-Point is individually
calibrated.

Sam
"xpyttl" wrote in message
...
"Reg Edwards" wrote in message
...

Most people appear to be fatally attempting to use, in reverse, their
favourite S-meter as a means of calibrating the Standard. No wonder we
have
so many different standards around.

We have seen a dozen or so posters indicate that S9=50uv

Nowadays, very few rigs have analog meters. Instead, they have a computer
that can take that AGC voltage and display it however the computer
decides.

In the Icom calibration procedure for the 706, there are three calibration
points known by the computer ... S0 (0 uv), S9 (50 uv), and S9+60 (50 mv).
The calibration procedure sets these three with known inputs, and
presumably
the computer interpolates from there. That really isn't a bad
calibration,
and I'd be surprised if other modern rigs were much different.

Now it is quite likely that there is some dependence on the AGC voltage
with
frequency, but again, in a modern rig that should be manageable.

What isn't so constant, of course, is the antenna. The voltage at the
receiver could range over several orders of magnitude for the same signal
depending on the antenna. So in terms of providing input to the sender,
the
S meter reading is still of limited usefulness, even if we all had
calibrated radios and agreed that S9=50uv.

..




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There has been a de-facto HF standard for 60 years.
The USA military first used it in specifications of radio equipment when
placing contracts with manufacturers around the end of WW2. There may have
been some restrictions on publicity at the time.
The Standard is 6 dB per S-unit and 50 micro-volts into 50 ohms at S=9.
=======================
I have read somewhere that it was Art Collins , of Collins Radio fame ,who
first mooted/established the above standard for up to 30 MHz. Much later
,among radio amateurs, the S9 signal level for freqs above 30 MHz was set
at 5 microvolts into 50 Ohms .

Frank GM0CSZ / KN6WH