DBm
Logarithmic power unit referenced to one milliwatt, used in communications.
dBm (decibel-milliwatts) is a unit of power level expressed on a logarithmic decibel scale relative to one milliwatt. It is commonly used by radio, microwave, and fiber-optical communication technicians and engineers to measure the power of system transmissions, allowing both very large and very small values to be expressed in a short form. The dBm is dimensionless but absolute, as it compares to a fixed reference value of one milliwatt.
- 0 dbm corresponds to
- 1 milliwatt
- 10 db increase
- ten-fold increase in power
- 3 db increase
- approximately doubles power
- -3 dbm corresponds to
- about 0.5 mW
- 0 dbm in 600 ω load
- approximately 0.775 volts
- 0 dbm in 50 ω load
- approximately 0.224 volts
- First proposed as industry standard
- 1940
Lore & Background
The dBm was first proposed as an industry standard in 1940. It is not part of the International System of Units (SI) and is discouraged from use in documents adhering to SI units, where the corresponding SI unit is the watt. In audio and telephony, dBm is typically referenced relative to the 600-ohm impedance commonly used in telephone voice networks, while in radio-frequency work it is typically referenced relative to a 50-ohm impedance. A power level of 0 dBm corresponds to a power of 1 milliwatt; a 10 dB increase represents a ten-fold increase in power, and a 3 dB increase approximately doubles the power. Conversely, a 3 dB decrease halves the power, so −3 dBm corresponds to about 0.5 mW. The signal intensity (power per unit area) can be converted to received signal power by multiplying by the square of the wavelength and dividing by 4, as described in free-space path loss. In United States Department of Defense practice, unweighted measurement is normally understood, applicable to a certain bandwidth that must be stated or implied. In European practice, psophometric weighting may be used, equivalent to dBm0p. In audio, 0 dBm often corresponds to approximately 0.775 volts, since 0.775 V dissipates 1 mW in a 600 Ω load; the corresponding voltage level is 0 dBu, without the 600 Ω restriction. For RF situations with a 50 Ω load, 0 dBm corresponds to approximately 0.224 volts.
Reader's Guide
The dBm remains a fundamental unit in radio, microwave, and fiber-optical communication, providing a compact logarithmic scale for power levels that span many orders of magnitude. Its significance lies in its absolute reference to one milliwatt, enabling consistent measurement across systems and disciplines. In audio and telephony, the 600-ohm reference standardizes voice network measurements, while in radio-frequency work the 50-ohm reference aligns with common transmission line impedances. The unit's legacy includes its adoption as an industry standard in 1940 and its continued use despite not being part of the SI system. The dBm facilitates conversions between power and voltage, with known relationships for different load impedances. It is used for optical and electrical power measurements, not for other types of power such as thermal. The unit's logarithmic nature allows easy expression of signal-to-noise ratios and link budgets, and its dimensionless but absolute character makes it a practical tool for technicians and engineers. The dBm is distinct from dBW, which is referenced to one watt rather than one milliwatt.
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