On Tue, 23 May 2006 16:26:43 +0100, "Reg Edwards"
wrote:


"Paul Keinanen" wrote in message
.. .
On Tue, 23 May 2006 12:16:27 +0100, "Reg Edwards"
wrote:

Replace the meter scale with a white card and calibrate it yourself
with pen and ink. You will also need a signal generator and an
attenuator.

If you derive the S-meter reading from the a.g.c. control voltage, be
prepared to perform the calibration at various temperatures, since the
gain of simple IF-strips vary with temperature (and thus need a
different amount of a.g.c.), unless the gain is controlled by passive
components only.

With various front end filter for various frequency bands and
attenuation depending of the frequency, you would have to perform the
calibration on several frequencies.

Instead of a pen and ink, it would be more practical to use a micro
controller with sufficient non-volatile storage for the conversion
tables and use analog inputs for a.g.c. voltage and temperature and
some kind of input for the frequency being received. With these inputs
and the conversion table, a voltage proportional of the log of the
antenna terminal power could be generated.

Paul OH3LWR
===========================================

I don't doubt that the problem is capable of being solved.
Just throw enough electronic hardware and software at it.
The cost can be estimated at 100,000 Euros.
The size and weight of the receiver will be quadrupled.
No amateur would be able to afford buying one.
American tax-payers would prevent NASA from doing the research and
development work. Even Bush would object.

While NASA might have needed such sums in the 1960's, the
functionality suggested above should be doable these days with any
micro controller with a few kilobytes of non-volatile storage.

While the frequency, power level and temperature calibration points
might require a three dimensional array, in practice, it should be
acceptable to get a few single dimensional vectors for each amateur
band. One vector for a specific band might contain the front end
frequency response for 5-10 frequencies within and around a band.

Getting the a.g.c. setting for different power levels for a fixed
station at three different temperatures (say +10, +20 and +30 C) or
five for a mobile station (with additional values for, say -50 and +70
C) should be enough. A few kilobytes should be more than enough to get
the sample points for interpolation.

As for me and most other amateurs, I'm quite happy with an S-meter
reading which puts signal strength reports in the right ball-park.
Very strong, very weak, or indifferent.

Exchanging RS(T) reports is so stupid, so I try to avoid it.

The only meaningful exchange would be to exchange the field strength
(V/m) or the power density (W/m²) in order to study the propagation
conditions. Even if we could reliably measure the antenna terminal
voltage (e.g. in dBuV) or power (dBm), we still would have to measure
the antenna efficiency and capture area to get any meaningful
readings.

Paul OH3LWR

"Reg Edwards" wrote in
message ...

wrote
Only by coincidence and only in a 50 ohm system
elsewhere
S9 is 50 uV (microVolts) equals 50pW.

==========================================
Allison, I'm afraid you are not quite correct about
coincidences.

The beauty of calibrating an S-meter in watts is that the
result is
independent of receiver impedance.

Of course, there must be a conjugate match between antenna
and
receiver. And there usually is. This is taken care of in
the
calibration process.

The received signal STRENGTH is indicated in watts, which
is all one
wants to know. When reading the meter, who cares about
what impedance
the measuring instrument happens to be?

To sum up : There is a transmitter of given power output.
There is a
radio path which is an attenuator, And there is a
received signal
signal strength meter which indicates watts.

The overall loss between transmitter and power meter may
be deduced in
terms of decibels. Professional radio engineers do it all
the time.

It can be misleading to think in terms of S9 = 50 uV when
one doesn't
know what the receiver input impedance is.
----
Reg, G4FGQ

Tradionally S-9 has been defined as 50uV, but there
really is not a standard. The S numbers are taken from the
old R-S-T system of reporting signals (Readability,
Strength, Tone). While the meter can be calibrated using a
calibrated signal generator the reading for 50uV will vary
with the gain of the receiver so it will not be the same on
all bands or even from one end of a band to the other.
Some receivers have meters calibrated in something other
than S units. For instance, the old Hammarlund SP-600. The
meters on these are usually calibrated for RF in db above 1
uv. But, the same thing applies, the receiver gain varies
with band and frequency, the AGC is not linear, and the
reading only an indication of relative strength rather than
any actual value.



--
---
Richard Knoppow
Los Angeles, CA, USA