Tools · Reference
dB Converter
Watts, dBm, dBW, S-units, and gain chains — the decibel arithmetic your brain refuses to do at 11 PM.
Watts
=
dBm
=
dBW
Power ratio ×
=
dB
S-meter
dBm (HF)
µV @ 50Ω
Transmitter power (W)
At the antenna (ERP-ish)
—
Memorize three numbers and you can do everything in your head:
3 dB = 2×
,
6 dB = 4×
,
10 dB = 10×
.
One S-unit is 6 dB — a 4× power change. Going from 100 W to 25 W costs exactly one S-unit at the far end. Nobody will notice.
S9 on HF is defined as −73 dBm (50 µV into 50 Ω). VHF S9 is 20 dB weaker (−93 dBm) by convention.
In the gain chain, enter losses as negative (coax −1.5) and gains as positive (yagi +7).