dBm – a unit of absolute level of reference to the power of 1 mW (milliwatt); a dBmW expression ought to be used according to the rule but the expression of dBm is exceptionally used here.
This unit is commonly used in telecommunications (e.g. an output power of an access point is provided) or it is used in designing WLAN network. A unit expressed in dB informs us how much this power is lower than power expressed in 1 mW. The level 0 dB corresponds to 1 mW (milliwatt).
A commonly used power formula is used in converting mW into dBm.
P(ref) – a referential value, which is a reference measure, which in this case is 1 mW (milliwatt)
A power formula in dBm which is more straightforward visually:
Example Let us convert 10 mW into dBm:
If we want to convert dBm into mW, we use the following formula:
Example Let us convert 20 dBm into mW (milliwatts).
Tab. 1. Conversion of dBm into mW - wide range
Tab. 2. Conversion of dBm into mW - detailed range
We can also convert power to voltage very easily: dBm into dBµV and dBmV. In order to convert voltage from power we have to know internal impedance. In terrestrial, cable and industrial television systems, impedance of 75 ohm is used. In radio systems it is 50 ohm.
The formulas for these two environments are provided below: