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decibels relative to one milliwatt (dBm)

Contributor(s): Johan Sjoquist

The expression dBm is used to define signal strength in wires and cables at RF and AF frequencies. The symbol is an abbreviation for "decibels relative to one milliwatt," where one milliwatt (1 mW) equals 1/1000 of a watt (0.001 W or 10 -3 W). This unit is commonly used in test laboratories.

The dBm increment is based on the decibel , a logarithm ic measure of relative power . Suppose a signal has a power level of P mW. Then the signal strength in dBm, symbolized S dBm , is:

S dBm = 10 log 10 P

A 1-mW signal has a level of 0 dBm. Signals weaker than 1 mW have negative dBm values; signals stronger than 1 mW have positive dBm values.

This was last updated in March 2011

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