The circuit you show is not an S-meter, but a VU-meter, it is proportional to the quadratic law of the diodes used, quite similar to the logarithmic law of dBV (mostly referred to an impedance of a few hundred ¦¸) to l output from circuit BF, responding poorly to RF levels received in the antenna.
An S-meter is a substantially different circuit, it is proportional to precise RF levels (defined by the IARU as in the table below for the HF) at the antenna input and varies in steps of 6 dBm referred to the ?V levels on an impedance of 50¦¸.
If you want to see a needle that moves it's fine, if you want to give reports that make sense it's useless!
Regards, Loris IW4AJR.
