What dBm means
dBm expresses a power level in decibels relative to one milliwatt. It is the standard unit in radio, telecom, audio interfaces and fiber optics, because it turns huge ranges of power into small, additive numbers.
P (dBm) = 10 × log10(P ÷ 1 mW) and P (mW) = 10^(dBm ÷ 10)
Useful landmarks
- 0 dBm = 1 mW
- 10 dBm = 10 mW
- 20 dBm = 100 mW (typical Wi-Fi transmitter)
- 30 dBm = 1 W
- −30 dBm = 1 µW
- −90 dBm: a weak but usable cell or Wi-Fi signal
Every +3 dB roughly doubles power, and every +10 dB multiplies it by ten. dBW is the same idea relative to one watt: dBW = dBm − 30.
From power to voltage
Across a known impedance, power and voltage are linked by P = V² ÷ R. So V (RMS) = √(P × R). For a sine wave, the peak-to-peak voltage is 2√2 times the RMS value.
Radio and EMC work also uses dBµV, decibels relative to one microvolt: dBµV = 20 × log10(V ÷ 1 µV). In a 50 Ω system, dBµV ≈ dBm + 107.
Example
20 dBm in a 50 Ω system:
- power: 10^(20 ÷ 10) = 100 mW, or 0.1 W and −10 dBW;
- RMS voltage: √(0.1 × 50) ≈ 2.24 V;
- peak-to-peak: about 6.32 V;
- level: about 127 dBµV.
Link budgets
Because decibels add, a radio link is easy to estimate: transmit power in dBm, plus antenna gains in dBi, minus cable losses and path loss in dB, gives the received power in dBm. Compare it with the receiver sensitivity to see the margin.
Common mistakes
- dB alone is a ratio, not a power. Only dBm, dBW or dBµV are absolute levels.
- Use 10 × log for power ratios and 20 × log for voltage ratios.
- Voltage readings depend on the impedance: 50 Ω for RF, 75 Ω for video and TV, 600 Ω for old telephone and audio gear.