At HF it's all very simple.

To summarise -

The S-meter is essentially a 50-ohm power or wattmeter.

It is correctly calibrated with a 50-ohm signal generator with its
open-circuit volts set to 100 microvolts. The S-meter should then read S9.

Therefore, at the standard S9, the input to the receiver is 50 microvolts
across 50 ohms, corresponding to a power input of 50 picowatts.

The standard S-unit = 6 dB.

So for each change of one S-unit the input voltage halves (or doubles).

At S-zero the input voltage is 50/512 = 0.1 microvolts, which is roughly
equal to the the internal noise level of a good receiver with a bandwidth of
a few KHz.. Theoretically this is about the noise level you should get when
the antenna is disconnected.

At S9 + 40dB the input voltage is 50*100 microvolts = 5 millivolts.

Some S-meters may indicate as high as S9 + 60dB. The input voltage is then
50 millivolts which is about the overload point of a very good receiver.

So the whole scale is calibrated logarithmically, with S9 being about
half-way along it, and with 54 dB below S9 and 40 or 60dB above S9. The
input voltage range is from from 0.1 microvolts to 5 or 50 millivolts.

==============================

However, all meters have indicating errors. The problem arises because of
the difficulties and great expense in designing and manufacturing receivers
with an agc meter system which can accommodate a signal level range of 54 +
60 = 114 dB. Economics invariably rules the roost.

(It helps to have very high receiver gain and attenuators at or near the
receiver input.)

Fortunately modern receivers all tend to have errors of the same sort and
sign. So amateurs using different manufacturer's receivers will exhange very
similar signal strength reports.

These errors congregate at the very small signal end of the range. Meter
calibration begins to go wrong at around or below S4 or S5. Meter readings
of, say, S1 or S2 may actually be appropriate to power levels of S3 or S4.
A meter reading of S-zero may be appropriate to a power level of S2 or S3.

That is, at very small signal levels the S-meter underestimates signal power
level. But it's not of great consequence. At HF, to which the foregoing
applies, signals are usually below the noise and QRM levels anyway.
---
Reg.

The distortion in S-meter scale-shape at very small signal levels, below
about S5, could be corrected very easily just by changing scale markings.
The meter scale would then end at about S2 instead of S-zero. And the meter
would indicate correctly as a wattmeter over the whole length of its scale.

But S-meter manufacturers are reluctant to do this. They consider it would
spoil the linear appearance of the scale at the low end and so lose
customers. How foolish they are!

Keen radio amateurs can change their S-meters into accurate instruments over
the whole scale just by scraping off existing scale markings and
re-calibrating with a fine-nibbed pen and black indian ink.

But on second thoughts, to save yourself the trouble, when using the meter
it's even easier just to visualise and remember where S3 should be marked on
the meter scale.
---
Reg, G4FGQ

Nice tutorial, Reg I printed it out for ref.

....however.
The 2M rigs I've measured (3) are WAY off. The +20 +40 and +6o are usually
also around 6dB plus or minus 3 or 4 dB. I measured my TS-830s and it was
not very close, and my 706, but don't recall the results and haven't done
the TS 2000 yet.

73, Steve K9DCI

"Reg Edwards" wrote in message
...
At HF it's all very simple.

To summarise -

The S-meter is essentially a 50-ohm power or wattmeter.

It is correctly calibrated with a 50-ohm signal generator with its
open-circuit volts set to 100 microvolts. The S-meter should then read
S9.

Therefore, at the standard S9, the input to the receiver is 50 microvolts
across 50 ohms, corresponding to a power input of 50 picowatts.

The standard S-unit = 6 dB.

So for each change of one S-unit the input voltage halves (or doubles).

At S-zero the input voltage is 50/512 = 0.1 microvolts, which is roughly
equal to the the internal noise level of a good receiver with a bandwidth
of
a few KHz.. Theoretically this is about the noise level you should get
when
the antenna is disconnected.

At S9 + 40dB the input voltage is 50*100 microvolts = 5 millivolts.

Some S-meters may indicate as high as S9 + 60dB. The input voltage is then
50 millivolts which is about the overload point of a very good receiver.

So the whole scale is calibrated logarithmically, with S9 being about
half-way along it, and with 54 dB below S9 and 40 or 60dB above S9. The
input voltage range is from from 0.1 microvolts to 5 or 50 millivolts.

==============================

However, all meters have indicating errors. The problem arises because of
the difficulties and great expense in designing and manufacturing
receivers
with an agc meter system which can accommodate a signal level range of 54
+
60 = 114 dB. Economics invariably rules the roost.

(It helps to have very high receiver gain and attenuators at or near the
receiver input.)

Fortunately modern receivers all tend to have errors of the same sort and
sign. So amateurs using different manufacturer's receivers will exhange
very
similar signal strength reports.

These errors congregate at the very small signal end of the range. Meter
calibration begins to go wrong at around or below S4 or S5. Meter readings
of, say, S1 or S2 may actually be appropriate to power levels of S3 or S4.
A meter reading of S-zero may be appropriate to a power level of S2 or S3.

That is, at very small signal levels the S-meter underestimates signal
power
level. But it's not of great consequence. At HF, to which the foregoing
applies, signals are usually below the noise and QRM levels anyway.
---
Reg.

"Steve Nosko" wrote in message
...
Nice tutorial, Reg I printed it out for ref.

...however.
The 2M rigs I've measured (3) are WAY off. The +20 +40 and +6o are
usually
also around 6dB plus or minus 3 or 4 dB. I measured my TS-830s and it was
not very close, and my 706, but don't recall the results and haven't done
the TS 2000 yet.

73, Steve K9DCI

"Reg Edwards" wrote in message
...
At HF it's all very simple.

To summarise -

The S-meter is essentially a 50-ohm power or wattmeter.

It is correctly calibrated with a 50-ohm signal generator with its
open-circuit volts set to 100 microvolts. The S-meter should then read
S9.


While I will not debate or dispute the origional S-meter meanings, I would
have to say there is almost no rig that follows that calibration. I have
measured a few low band rigs and several vhf/uhf rigs. The low band units
seem to be somewhat more 'calibrated' than the VHF and above rigs but most
are usually way off. From my observations and the write ups in QST the
S-meter and the 50 uV and all the 6 db per S-unit should not even be spoken
of .

We need to kill the idea of the s-unit being 6 db or anything at all. They
are just numbers on an uncalibrated meter on most receivers. You might just
as well mark the meter from 0 to 100 and go by whatever number the meter is
showing. The meters are not linear, log, or any combination that can be
depended on from one brand to another brand. Even changing bands on the
same receiver will usually change the 'calibration' of the s-meter.