dB Is a Ratio
Use 10 log for a power ratio and 20 log for a voltage ratio only when the impedances are the same.
dB = 10 log10(P2/P1)
dB = 20 log10(V2/V1)
A power increase of 10 times is +10 dB. A power reduction to one tenth is -10 dB.
dBm Is Absolute Power
Zero dBm equals 1 mW. Positive and negative dBm values are both normal.
dBm = 10 log10(PmW)
P(W) = 10(dBm - 30)/10
Examples: 30 dBm = 1 W, 0 dBm = 1 mW, and -30 dBm = 1 µW.
Why Log Units Help
RF systems often contain large ranges of power and several gains and losses. In decibel form, multiplication becomes addition: transmitter power plus antenna gain minus cable and path losses.
Common Mistakes
- Adding two dBm power levels as if dBm were a ratio.
- Using 20 log for power.
- Using a voltage ratio without confirming equal impedances.
- Confusing dBi antenna gain with dBm power.