dB, dBm and power

dB is a ratio

For power, G = 10·log₁₀(P₂/P₁). Equal powers give 0 dB, doubling gives about +3.01 dB and halving about −3.01 dB. A factor of ten is 10 dB. This makes large and small ratios manageable and allows successive gains and losses to be added.

For voltage, the factor becomes 20 because power in a resistance is proportional to voltage squared. Using 20·log₁₀(U₂/U₁) to compare power assumes equal resistances. With different loads, calculate each power first.

dBm has a fixed reference

dBmdBm×dBm expresses an absolute power level relative to 1 milliwatt, combining the logarithmic dB scale with a fixed power reference.Example & full explanation → references power to 1 mW. Thus 0 dBm is exactly 1 mW, 10 dBm is 10 mW and 30 dBm is 1 W. Negative values do not mean negative energy: −30 dBm is 1 µW. Without a reference, dB remains only a ratio.

Across a resistive 50 Ω load, 1 mW produces about 0.2236 V RMS. For a sine wave, peak-to-peak voltage is 2√2 times higher. A generator’s open-circuit voltage is a different quantity and must not be confused with loaded voltage.

Estimate a chain

A transmitter delivers 10 W. After roughly 3 dB cable loss about 5 W remains, and after another 3 dB about 2.5 W. Losses add to 6 dB while linear factors multiply. Use logarithms for exact results; rounded 3 dB steps are a convenient mental estimate.