dBm
dBm is the logarithmic expression of power relative to a 1 mW reference and, unlike dB, it is an absolute value. 0 dBm = 1 mW, 10 dBm = 10 mW, 14 dBm = 25 mW, 20 dBm = 100 mW, 27 dBm ≈ 500 mW. Transmitter output power, receiver sensitivity and received signal level are all written in this unit.
dBm and dB must not be confused: dBm is a power level, while dB is the difference between two levels. dB is added to or subtracted from a dBm value and the result is again dBm. A transmitter with a 14 dBm output delivers 12 dBm to the antenna after 2 dB of cable loss.
- Negative dBm is normal: the level reaching the receiver is typically between −40 and −110 dBm.
- A 3 dBm difference is twice the power; 17 dBm is twice 14 dBm.
- Sensitivity is also in dBm: a receiver with −120 dBm sensitivity has a 10 dB advantage over one with −110 dBm, that is roughly a factor of 3 in distance.
The limits frequently quoted for European short range device bands are also expressed in this unit; for example in the 2.4 GHz ISM band the upper limit under the ETSI framework is 20 dBm EIRP. These values belong to the band and are not attributed to individual products; the actual output power of a set is stated in its declaration of conformity and technical file.
Context in which this term is used: Wireless remote control installation: pairing, relay, safety