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If the noise floor was 50 mv/m I’d need a 70mv/m signal to be usable. Yes?
Sorry, but no.
According to the FCC, a 26 dB ratio is needed between desired and undesired co-channel signals for good reception of an AM broadcast station, regardless of the absolute value of the noise voltage at that receive location.
So that value is not fixed to be 20 mV/m greater than whatever noise voltage exists at that receive location.
The useful value needed for the desired, received signal depends on the r-f noise present at that receive location.