I’ve heard the term dBm output power, which refers to radio equipment. Can someone explain what dBm means in layman’s terms?
Power is commonly expressed in decibel values in order to make adjustments due to loses and gains. The logarithmic nature of decibel units translates the multiplication and division associated with gains and losses into addition and subtraction. Once you get used to it, you will never want to return to watts. Units of dBm are decibels relative to 1 mW of power, hence, 0 dBm 1 mW. 1/100 mW is -20 dBm and 100 mW is +20 dBm. Powers less than 1 mW are always negative dBm values, and powers greater than 1 mW are always positive. See the graph below.
For amplifiers, a common reference unit is the dBm, with 0dBm being equal to 1 milliwatt. Thus, an amp with an output of 30dBm puts out 1 Watt.
There's also dBW (Referenced to 1 WATT), but you generally only use those when dealing with “Big Stuff”, as 30dBW is 1000w.
For antennas, a common reference unit is the dBi, which states the gain of an antenna as referenced to an ISOTROPIC source.
What does dBm mean?
This link should help: http://en.wikipedia.org/wiki/DBm