I was wondering how commercial equipment go about calibrating the S
meter and whats the best way of building a calibrated S meter that would
be reliable as a professional field strength meter?

Since most S meters just read AGC voltage and it would be easy to build
some interface with a CPU to read voltages and calibrate this way. The
question is this, is a S meter calibrated this way actually reading in
a accurate way whats occurring at the antennas terminal. Since most
antenna inputs are not 50 ohms J0, how would you design a system like a
spectrum analyzer that measures signal voltage at the antenna terminal?.
I would be interested in some ideas, i am homebrewing a shortwave
receiver however i wanted a calibrated S meter in Dbuv, Dbm, S units and
Millivolts. I also want to use a calibrated antenna for a Antenna
factor input to have a meaningful long term view of propagation signal
strength.

Thanks
Will