That's why for the HF bands S9 is defined as a receiver input signal of
50 microvolts at 50 Ohms input impedance , translated as -73dBm
For frequencies higher than 30 MHz S9 is defined as 5 microvolts at 50
Ohms input impedance , translated as -93dBm.

The S9 signal strength was first set by Art Collins of Collins Radio
fame during the 1940s and was later accepted by IARU.
This organisation also adopted the 5 microvolt level for a S9 signal for
VHF and higher frequency bands.

Frank GM0CSZ / KN6WH

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It can be misleading to think in terms of S9 = 50 uV when one doesn't
know what the receiver input impedance is.
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Reg, G4FGQ