VHF/UHF S-meter

IARU Region 1 recommendation defines S9 for VHF/UHF to be a receiver input power of -93 dBm. 

This is the equivalent of 5 µV in 50 Ω. Again, one S-unit corresponds to a difference of 6 dB,
equivalent to a voltage ratio of two, or a power ratio of four.
 

Conversion between power and VHF/UHF S-units

S-reading

Pin @50Ω

Vin @50Ω

[dB above1µV]Vout[1µV] @50Ω

S9 + 40 dB

-53 dBm

0.50 mV

54 dBµV

S9 + 30 dB

-63 dBm

0.16 mV

44 dBµV

S9 + 20 dB

-73 dBm

50 µV

34 dBµV

S9 + 10 dB

-83 dBm

16 µV

24 dBµV

S9

-93 dBm

5.0 µV

14 dBµV

S8

-99 dBm

2.5 µV

8 dBµV

S7

-105 dBm

1.26 µV

2 dBµV

S6

-111 dBm

630 nV

-4 dBµV

S5

-117 dBm

320 nV

-10 dBµV

S4

-123 dBm

160 nV

-16 dBµV

S3

-129 dBm

80 nV

-22 dBµV

S2

-135 dBm

40 nV

-28 dBµV

S1

-141 dBm

20 nV

-34 dBµV

 

HF S-meter

Many amateur radio and shortwave broadcast receivers feature a signal strength meter (S‑meter).4 
In 1981, the International Amateur Radio Union (IARU) Region 1 agreed on a technical
recommendation for S‑meter calibration of HF and VHF/UHF transceivers.5,6

IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input
power of -73 dBm.
 This is a level of 50 µV at the receiver’s antenna input assuming the input impedance
of the receiver is 50 Ω.

The recommendation defines a difference of one S-unit corresponds to a difference of 6 dB, 
equivalent to a voltage ratio of two, or a power ratio of four. Signals stronger than S9 are given with
an additional dB rating, thus “S9 + 20 dB”, or, verbally, “20 decibel over S9”, or simply “20 over 9”
or even the simpler “20 over.”

Well-designed S-meter on the DRS WJ-8711A HF transceiver. Source: N9EWO

Conversion between power and HF S-units

S-reading

Pout @50Ω

Vin @50Ω

[dB above1µV]Vout[1µV] @50Ω

S9 + 40 dB

-33 dBm

5.0 mV

74 dBµV

S9 + 30 dB

-43 dBm

1.6 mV

64 dBµV

S9 + 20 dB

-53 dBm

0.50 mV

54 dBµV

S9 + 10 dB

-63 dBm

0.16 mV

44 dBµV

S9

-73 dBm

50 µV

34 dBµV

S8

-79 dBm

25 µV

28 dBµV

S7

-85 dBm

12.6 µV

22 dBµV

S6

-91 dBm

6.3 µV

16 dBµV

S5

-97 dBm

3.2 µV

10 dBµV

S4

-103 dBm

1.6 µV

4 dBµV

S3

-109 dBm

800 nV

-2 dBµV

S2

-115 dBm

400 nV

-8 dBµV

S1

-121 dBm

200 nV

-14 dBµV

source: https://hamwaves.com/decibel/en/