Sal M. Onella wrote:

I've heard the same thing, but the reality is that the S-meter usually
follows the AGC and the signal for S9 could be different for every design.
But Flex radio and others define S9 as 50 uV. . .

Adjusting the S-meter to read a specific value for one signal strength
is simple, and there's often an adjustment for doing it. I think it's
fairly common to find S9 to be around 50 uV -- on one band at least --
although I wouldn't be surprised to see a fair amount of variation from
band to band. But that has nothing to do with what the difference is
between S7 and S8, S8 and S9, etc.

Roy Lewallen, W7EL