Reg Edwards wrote:
"Will" wrote
I just wonder how field strength meters are designed, especially the
new
solid state ones. Spectrum analyzers too have a pretty flat
response. I
want to build this ability into my homebrew receiver much the same
way
a selective level meter works. Maybe someone has done it.

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

The accuracy of S-meters varies from one band to another because the
receiver input impedance varies from one band to another. Even from
one end of a band to the other, especially if the input stage is
tuned.

Also, the stage-gain of an RF stage can vary very much from one band
to another.

To ensure a consistent gain it is necessary to convert to the
intermediate frequency (IF) at the very first stage of a receiver.
Preferably using a high-level, balanced diode-bridge, modulator.

The IF stages can be assumed to have a constant, wide-band gain,
except, of course, for the automatic gain control (AGC) action.

Then ensure that the receiver input impedance is constant (usually 50
ohms) over the whole HF range of the receiver. Remember the S-meter
is a power or watt-meter.

It so happens that with dual-gate FET's, and 3 or 4 IF amplifier
stages, a meter which responds to AGC volts will fairly accurately
indicate S-units and dB above S9 in a linear fashion.

However, below about S4 the linearity of S-meter readings begins to
fail. And you will have to make and calibrate your own meter scale.
There's no way of avoiding it! That is unless you can mentally
visualise what the scale ought to look like as you use it.

Accurate commercial field strength meters use antennas dedicated to
the job, conjugate-matched to the receiver, are insensitive to weak
signals, are narrow band and are very expensive.

To obtain an S-meter scale to be proud of, you will have to use pen
and ink with a signal generator and 100-dB stepped attenuator.
Attempts to calibrate the scale with clever, highly complicated
electronics will get you nowhere in a long time. And will cost you
more than the remainder of the receiver.

Sorry to be so despondent.
----
Reg, G4FGQ


Thanks Reg for the summary.

I think the Flex radio and the Winradio have the ability to be
calibrated accurately via a look up table. I always admired shortwave
receivers like the R&S EK 2000 which had a accurate meter calibrated
in the DbUv scale. I think i will just have to abandon my super
accurate S meter project.

Will