In article , Roy Lewallen
writes:

Any "standard" is useless at best and misleading at worst, because it
bears no relation to what S-meters really read.

Roger that, and I've heard all kinds of "599" reports on-air, too. :-)

In the project I have on-going, this receiver's S-Meter (there mainly
for nostalgia purposes and because I've gotten a few old-style
microammeters that I can use) will read the average carrier power
(integrated to a time-constant yet to be settled on) at the AM
detector using a half of a quad op-amp integrator. That same
detector provides the AGC control line with appropriate DC offset
for the MC1349 gain blocks, but with (maybe) different integration
time-constants. As for the RF input to the antenna connector,
that is known within +/- 2 db down to -130 dbm by separate
calibration of my HP 608 or 606 signal generator. When
completed, this particular receiver S-Meter will be as accurate,
with a custom scale plate, as that signal generator output will
allow.

Of course, to fit the "convention" of those gratuitous "599" reports,
I could borrow from the auto industry's cheapie "gas gauge" which
has a single bulb showing "low gas" on the dashboard...using an
LED driven by an op-amp integrator-comparator for high-tech.
The indicator would show "I got signal" or "I got noise" to fit... :-)