dBm
dBm (sometimes dBmW) is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form. Compare dBW, which is referenced to one watt (1000 mW).
Since it is referenced to the watt, it is an absolute unit, used when measuring absolute power. By comparison, the decibel (dB) is a dimensionless unit, used for quantifying the ratio between two values, such as signal-to-noise ratio.
Unit conversions
Zero dBm equals one milliwatt. A 3 dB increase represents roughly doubling the power, which means that 3 dBm equals roughly 2 mW. For a 3 dB decrease, the power is reduced by about one half, making −3 dBm equal to about 0.5 milliwatt. To express an arbitrary power P as x dBm, or go in the other direction, the following equations may be used:
- or,
and
- or,
where P is the power in W and x is the power ratio in dBm. Below is a table summarizing useful cases:
| dBm level | Power | Notes |
|---|---|---|
| 80 dBm | 100 kW | Typical transmission power of FM radio station with 50 km range |
| 60 dBm | 1 kW = 1000 W | Typical combined radiated RF power of microwave oven elements
Maximum allowed output RF power from a ham radio transceiver ("rig") without special permissions |
| 50 dBm | 100 W | Typical thermal radiation emitted by a human body
Typical maximum output RF power from a ham radio transceiver ("rig") |
| 40 dBm | 10 W | Typical PLC (Power Line Carrier) Transmit Power |
| 37 dBm | 5 W | Typical maximum output RF power from a hand held ham radio transceiver ("rig") |
| 36 dBm | 4 W | Typical maximum output power for a Citizens' band radio station (27 MHz) in many countries |
| 33 dBm | 2 W | Maximum output from a UMTS/3G mobile phone (Power class 1 mobiles)
Maximum output from a GSM850/900 mobile phone |
| 30 dBm | 1 W = 1000 mW | Typical RF leakage from a microwave oven - Maximum output power for DCS 1800 MHz mobile phone
Maximum output from a GSM1800/1900 mobile phone |
| 27 dBm | 500 mW | Typical cellular phone transmission power
Maximum output from a UMTS/3G mobile phone (Power class 2 mobiles) |
| 26 dBm | 400 mW | Access point for Wireless networking |
| 25 dBm | 316 mW | |
| 24 dBm | 250 mW | Maximum output from a UMTS/3G mobile phone (Power class 3 mobiles) |
| 23 dBm | 200 mW | |
| 22 dBm | 160 mW | |
| 21 dBm | 125 mW | Maximum output from a UMTS/3G mobile phone (Power class 4 mobiles) |
| 20 dBm | 100 mW | Bluetooth Class 1 radio, 100 m range
Maximum output power from unlicensed FM transmitter. Typical wireless router transmission power. |
| 15 dBm | 32 mW | Typical WiFi transmission power in laptops. |
| 10 dBm | 10 mW | |
| 6 dBm | 4.0 mW | |
| 5 dBm | 3.2 mW | |
| 4 dBm | 2.5 mW | Bluetooth Class 2 radio, 10 m range |
| 3 dBm | 2.0 mW | More precisely (to 8 decimal places) 1.9952623 mW |
| 2 dBm | 1.6 mW | |
| 1 dBm | 1.3 mW | |
| 0 dBm | 1.0 mW = 1000 µW | Bluetooth standard (Class 3) radio, 1 m range |
| −1 dBm | 794 µW | |
| −3 dBm | 501 µW | |
| −5 dBm | 316 µW | |
| −10 dBm | 100 µW | Typical maximum received signal power (−10 to −30 dBm) of wireless network |
| −20 dBm | 10 µW | |
| −30 dBm | 1.0 µW = 1000 nW | |
| −40 dBm | 100 nW | |
| −50 dBm | 10 nW | |
| −60 dBm | 1.0 nW = 1000 pW | The Earth receives one nanowatt per square metre from a magnitude +3.5 star[1] |
| −70 dBm | 100 pW | Typical range (−60 to −80 dBm) of wireless received signal power over a network (802.11 variants) |
| −80 dBm | 10 pW | |
| −100 dBm | 0.1 pW | |
| −111 dBm | 0.008 pW = 8 fW | Thermal noise floor for commercial GPS single channel signal bandwidth (2 MHz) |
| −127.5 dBm | 0.178 fW = 178 aW | Typical received signal power from a GPS satellite |
| −174 dBm | 0.004 aW = 4 zW | Thermal noise floor for 1 Hz bandwidth at room temperature (20 °C) |
| −192.5 dBm | 0.056 zW = 56 yW | Thermal noise floor for 1 Hz bandwidth in outer space (4 kelvins) |
| −∞ dBm | 0 W | Zero power is not well-expressed in dBm (value is negative infinity) |
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 4π (see Free-space path loss).
In United States Department of Defense practice, unweighted measurement is normally understood, applicable to a certain bandwidth, which must be stated or implied.
In European practice, psophometric weighting may be, as indicated by context, equivalent to dBm0p, which is preferred.
The dBm is not a part of the International System of Units and therefore is discouraged from use in documents or systems that adhere to SI units (the corresponding SI unit is the watt). However the straight decibel (dB), being a unitless ratio of two numbers, is perfectly acceptable.[2]
Expression in dBm is typically used for optical and electrical power measurements, not for other types of power (such as thermal). A listing by power levels in watts is available that includes a variety of examples not necessarily related to electrical or optical power.
See also
References
This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).