dBm

dBm (sometimes dBmW or decibel-milliwatts) is unit of level used to indicate that a power ratio is expressed in decibels (dB) with reference to one milliwatt (mW). It is used in radio, microwave and fiber-optical communication 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 compared to 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. The dBm is also dimensionless but since it compares to a fixed reference value the dBm rating is an absolute one.

In audio and telephony, dBm is typically referenced relative to a 600-ohm impedance,[1] while in radio-frequency work dBm is typically referenced relative to a 50-ohm impedance.[2]

Unit conversions

A power level of 0 dBm corresponds to a power of 1 milliwatt. A 10 dB increase in level is equivalent to a 10-fold increase in power. A 3 dB increase in level is approximately equivalent to doubling the power, which means that a level of 3 dBm corresponds roughly to a power of 2 mW. Similarly, for each 3 dB decrease in level, the power is reduced by about one half, making −3 dBm correspond to a power of about 0.5 mW.

To express an arbitrary power P in mW as x in dBm, or vice versa, the following equivalent expressions may be used:

Alternatively, using 1 W as the reference value instead of 1 mW

Below is a table summarizing useful cases:

Power levelPowerNotes
80 dBm100 kWTypical transmission power of FM radio station with 50-kilometre (31 mi) range
62 dBm1.588 kW = 1,588 W1,500 W is the maximal legal power output of a U.S. ham radio station.[3]
60 dBm1 kW = 1,000 WTypical combined radiated RF power of microwave oven elements
55 dBm~300 WTypical single-channel RF output power of a Ku-band geostationary satellite
50 dBm100 WTypical total thermal radiation emitted by a human body, peak at 31.5 THz (9.5 μm)

Typical maximal output RF power from a ham radio HF transceiver

40 dBm10 WTypical PLC transmission power
37 dBm5 WTypical maximal output RF power from a handheld ham radio VHF/UHF transceiver
36 dBm4 WTypical maximal output power for a citizens band radio station (27 MHz) in many countries
33 dBm2 WMaximal output from a UMTS/3G mobile phone (Power class 1 mobiles)

Maximal output from a GSM850/900 mobile phone

30 dBm1 W = 1000 mW

DCS or GSM 1,800/1,900 MHz mobile phone. EIRP IEEE 802.11a (20 MHz-wide channels) in either 5 GHz subband 2 (5,470–5,725 MHz) provided that transmitters are also IEEE 802.11h-compliant, or U-NII-3 (5,725–5,825 MHz). The former is EU only, the latter is US only. Also, maximal power allowed by the FCC for American amateur radio licensees to fly radio-controlled aircraft or operate RC models of any other type on the amateur radio bands in the US.[4]

29 dBm794 mW
28 dBm631 mW
27 dBm500 mWTypical cellular phone transmission power

Maximal output from a UMTS/3G mobile phone (Power class 2 mobiles)

26 dBm400 mW
25 dBm316 mW
24 dBm251 mWMaximal output from a UMTS/3G mobile phone (Power class 3 mobiles)

1,880–1,900 MHz DECT (250 mW per 1,728 kHz channel). EIRP for wireless LAN IEEE 802.11a (20 MHz-wide channels) in either the 5 GHz subband 1 (5,180–5,320 MHz) or U-NII-2 & -W ranges (5,250–5,350 MHz & 5,470–5,725 MHz respectively). The former is EU only, the latter is US only.

23 dBm200 mWEIRP for IEEE 802.11n wireless LAN 40 MHz-wide (5 mW/MHz) channels in 5 GHz subband 4 (5,735–5,835 MHz, US only) or 5 GHz subband 2 (5,470–5,725 MHz, EU only). Also applies to 20 MHz-wide (10 mW/MHz) IEEE 802.11a wireless LAN in 5 GHz subband 1 (5,180–5,320 MHz) if also IEEE 802.11h-compliant (otherwise only 3 mW/MHz → 60 mW when unable to dynamically adjust transmission power, and only 1.5 mW/MHz → 30 mW when a transmitter also cannot dynamically select frequency).
22 dBm158 mW
21 dBm125 mWMaximal output from a UMTS/3G mobile phone (Power class 4 mobiles)
20 dBm100 mWEIRP for IEEE 802.11b/g wireless LAN 20 MHz-wide channels in the 2.4 GHz Wi-Fi/ISM band (5 mW/MHz).

Bluetooth Class 1 radio. Maximal output power from unlicensed AM transmitter per U.S. FCC rules 15.219[5]

19 dBm79 mW
18 dBm63 mW
17 dBm50 mW
15 dBm32 mWTypical wireless LAN transmission power in laptops
10 dBm10 mW
7 dBm5.0 mWCommon power level required to test the automatic gain control circuitry in an AM receiver
6 dBm4.0 mW
5 dBm3.2 mW
4 dBm2.5 mWBluetooth Class 2 radio, 10 m range
3 dBm2.0 mW
2 dBm1.6 mW
1 dBm1.3 mW
0 dBm1.0 mW = 1000 μWBluetooth standard (Class 3) radio, 1 m range
−1 dBm794 μW
−3 dBm501 μW
−5 dBm316 μW
−10 dBm100 μWMaximal received signal power of wireless network (802.11 variants)
−13 dBm50.12 μWDial Tone for the Precise Tone Plan found on public switched telephone networks in North America
−20 dBm10 μW
−30 dBm1.0 μW = 1000 nW
−40 dBm100 nW
−50 dBm10 nW
−60 dBm1.0 nW = 1000 pWThe Earth receives one nanowatt per square metre from a magnitude +3.5 star[6]
−70 dBm100 pW
−73 dBm50.12 pW"S9" signal strength, a strong signal, on the S meter of a typical ham or shortwave radio receiver
−80 dBm10 pW
−100 dBm0.1 pWMinimal received signal power of wireless network (802.11 variants)
−111 dBm0.008 pW = 8 fWThermal noise floor for commercial GPS single-channel signal bandwidth (2 MHz)
−127.5 dBm0.178 fW = 178 aWTypical received signal power from a GPS satellite
−174 dBm0.004 aW = 4 zWThermal noise floor for 1 Hz bandwidth at room temperature (20 °C)
−192.5 dBm0.056 zW = 56 yWThermal noise floor for 1 Hz bandwidth in outer space (4 kelvins)
−∞ dBm0 WZero 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.

In audio, 0 dBm often corresponds to approximately 0.775 volts, since 0.775 V dissipates 1 mW in a 600 Ω load.[7] The corresponding voltage level is 0 dBu, without the 600 Ω restriction. Conversely, for RF situations with a 50 Ω load, 0 dBm corresponds to approximately 0.224 volts, since 0.224 V dissipates 1 mW in a 50 Ω load.

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 unit decibel (dB), without the 'm' suffix, is accepted for use alongside SI units.[8]

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.

The dBm was first proposed as an industry standard[7] in the paper "A New Standard Volume Indicator and Reference Level".[9]

See also

References

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

  1. Bigelow, Stephen. Understanding Telephone Electronics. Newnes. p. 16. ISBN 978-0750671750.
  2. Carr, Joseph (2002). RF Components and Circuits. Newnes. pp. 45–46. ISBN 978-0750648448.
  3. "Part 97 - Amateur Radio". ARRL. Archived from the original on 2012-10-09. Retrieved 2012-09-21.
  4. Archived 2016-12-22 at the Wayback Machine FCC Part 97 Amateur Radio Service - Rule 97.215, Telecommand of model craft, section (c).
  5. FCC Web Documents citing 15.219 Archived 2011-11-06 at the Wayback Machine.
  6. "Radiant Flux of a Magnitude +3.5 Star". Archived from the original on 2012-06-30. Retrieved 2009-07-22.
  7. Davis, Gary (1988). The Sound Reinforcement Handbook. Yamaha. p. 22. ISBN 0881889008.
  8. Thompson and Taylor 2008, Guide for the Use of the International System of Units (SI), NIST Special Publication SP811 Archived 2016-06-03 at the Wayback Machine.
  9. Chinn, H. A.; D. K. Gannett; R. M. Moris (January 1940). "A New Standard Volume Indicator and Reference Level" (PDF). Proceedings of the Institute of Radio Engineers. 28 (1): 1–17. doi:10.1109/JRPROC.1940.228815. Archived (PDF) from the original on 2012-02-13. Retrieved 2012-08-04.
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