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{{For|other uses|DBM (disambiguation){{!}}DBM}}
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{{lowercase|title=dBm}}


[[File:Relationship between dBu and dBm.svg|thumb|A schematic showing the relationship between [[dBu]] (the [[voltage source]]) and dBm (the power dissipated as [[heat]] by the 600 Ω [[resistor]])]]
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'''dBm''' (sometimes '''dBmW''') is an abbreviation for the power ratio in [[decibel]]s (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 (physics)|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&nbsp;mW).
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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]].
 
In audio and telephony, dBm is typically referenced relative to a 600&nbsp;ohm impedance,<ref>{{cite book|last=Bigelow|first=Stephen|title=Understanding Telephone Electronics|publisher=Newnes|isbn=978-0750671750|pages=16}}</ref> while in radio frequency work dBm is typically referenced relative to a 50&nbsp;ohm impedance.<ref>{{cite book|last=Carr|first=Joseph|title=RF Components and Circuits|year=2002|publisher=Newnes|isbn=978-0750648448|pages=45–46}}</ref>
 
==Unit conversions==
0&nbsp;dBm equals 1 milliwatt. A 3&nbsp;dB increase represents roughly doubling the power, which means that 3&nbsp;dBm equals roughly 2&nbsp;mW. For a 3&nbsp;dB decrease, the power is reduced by about one half, making −3&nbsp;dBm equal to about 0.5 milliwatt or 500 microwatts.
 
To express an arbitrary power ''P'' in watts as ''x'' in dBm, or vice versa, the following equivalent expressions may be used:
 
:<math>\begin{align}
  x &= 10 \log_{10} \frac{P}{ 1 \mathrm{mW}} \\
  x &= 30+10 \log_{10} \frac{P}{ 1 \mathrm{W}}
\end{align}</math>
 
and
 
:<math>\begin{align}
  P &= 1 \text{mW} \cdot 10^{\frac{x}{10}}\\
  P &= 1 \text{W} \cdot 10^{ \frac{ x-30 }{10} }
\end{align}</math>
 
where ''P'' is the power in W and ''x'' is the power ratio in dBm. Below is a table summarizing useful cases:
 
{{Main|Orders of magnitude (power)}}
{| class="wikitable"
! dBm level !! Power !! Notes
|-
| 80 dBm || 100&nbsp;kW || Typical [[Effective radiated power|transmission power]] of [[FM radio]] station with {{convert|adj=on|50|km|mi}} range
|-
| 62 dBm || 1.588&nbsp;kW = 1,588 W ||1500 W is the maximum legal power output of a U.S. [[ham radio]] station.<ref>{{cite web|url=http://www.arrl.org/part-97-amateur-radio |title=Part 97 - Amateur Radio|publisher=ARRL|accessdate=2012-09-21}}</ref>
|-
| 60 dBm || 1&nbsp;kW = 1,000 W || Typical combined radiated RF power of [[microwave oven]] elements
|-
| 50 dBm || 100 W || Typical [[Black-body_radiation#Human body emission|thermal radiation emitted by a human body]]
Typical maximum output RF power from a ham radio [[High frequency|HF]] transceiver
|-
| 40 dBm || 10 W || Typical PLC ([[Power Line Carrier]]) transmit power
|-
| 37 dBm || 5 W || Typical maximum output RF power from a handheld ham radio VHF/UHF transceiver
|-
| 36 dBm || 4 W || Typical maximum output power for a [[Citizens' band radio]] station (27&nbsp;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 = 1,000&nbsp;[[Milliwatt|mW]] || Typical RF leakage from a microwave oven{{fact|date=December 2013}}
DCS or GSM 1,800/1,900&nbsp;MHz mobile phone.
[[Equivalent isotropically radiated power|EIRP]] IEEE 802.11a (20&nbsp;MHz-wide channels) in either 5&nbsp;GHz Subband 2 (5,470–5,725&nbsp;MHz) provided that transmitters are also IEEE 802.11h-compliant, ''or'' [[U-NII]]-3 (5,725–5,825&nbsp;MHz). The former is EU only, the latter is US only.
|-
| 29 dBm || 794&nbsp;mW ||
|-
| 28 dBm || 631&nbsp;mW ||
|-
| 27 dBm || 500&nbsp;mW || Typical [[cellular phone]] transmission power
Maximum output from a UMTS/3G mobile phone (Power class 2 mobiles)
|-
| 26 dBm || 400&nbsp;mW ||
|-
| 25 dBm || 316&nbsp;mW
|-
| 24 dBm || 251&nbsp;mW || Maximum output from a UMTS/3G mobile phone (Power class 3 mobiles)
1,880–1,900&nbsp;MHz [[DECT]] (250&nbsp;mW per 1,728&nbsp;kHz channel).
[[Equivalent isotropically radiated power|EIRP]] for Wireless LAN IEEE 802.11a (20&nbsp;MHz-wide channels) in either the 5&nbsp;GHz Subband 1 (5,180–5,320&nbsp;MHz) or [[U-NII]]-2 & -W ranges (5,250–5,350&nbsp;MHz & 5,470–5,725&nbsp;MHz respectively). The former is EU only, the latter is US only.
|-
| 23 dBm || 200&nbsp;mW || [[Equivalent isotropically radiated power|EIRP]] for IEEE 802.11n Wireless LAN 40&nbsp;MHz-wide (5&nbsp;mW/MHz) channels in 5&nbsp;GHz subband 4 (5,735–5,835&nbsp;MHz, US only) or 5&nbsp;GHz subband 2 (5,470–5,725&nbsp;MHz, EU only). Also applies to 20&nbsp;MHz-wide (10&nbsp;mW/MHz) IEEE 802.11a Wireless LAN in 5&nbsp;GHz Subband 1 (5,180–5,320&nbsp;MHz) ''if'' also IEEE 802.11h compliant (otherwise only 3&nbsp;mW/MHz → 60&nbsp;mW when unable to dynamically adjust transmission power, and only 1.5&nbsp;mW/MHz → 30&nbsp;mW when a transmitter also cannot [[Dynamic Frequency Selection|dynamically select frequency]]).
|-
| 22 dBm || 158&nbsp;mW
|-
| 21 dBm || 125&nbsp;mW || Maximum output from a UMTS/3G mobile phone (Power class 4 mobiles)
|-
| 20 dBm || 100&nbsp;mW || [[Equivalent isotropically radiated power|EIRP]] for IEEE 802.11b/g Wireless LAN 20&nbsp;MHz-wide channels in the 2.4&nbsp;GHz [[ISM band]] (5&nbsp;mW/MHz).
[[Bluetooth]] Class 1 radio.
Maximum output power from unlicensed [[AM transmitter]] per U.S. [[Federal Communications Commission]] (FCC) rules 15.219.<ref>[http://www.hallikainen.org/FCC/FccRules/CiteFind/015219.htm FCC Web Documents citing 15.219]</ref>  
|-
| 19 dBm || 79&nbsp;mW ||
|-
| 18 dBm || 63&nbsp;mW ||
|-
| 17 dBm || 50&nbsp;mW ||
|-
| 15 dBm || 32&nbsp;mW || Typical [[Wireless LAN]] transmission power in laptops.
|-
| 10 dBm || 10&nbsp;mW
|-
| 6 dBm || 4.0&nbsp;mW
|-
| 5 dBm || 3.2&nbsp;mW
|-
| 4 dBm || 2.5&nbsp;mW || Bluetooth Class 2 radio, 10 m range
|-
| 3 dBm || 2.0&nbsp;mW ||  More precisely (to 8 [[decimal places]])  1.9952623&nbsp;mW
|-
| 2 dBm || 1.6&nbsp;mW
|-
| 1 dBm || 1.3&nbsp;mW
|-
| 0 dBm || 1.0&nbsp;mW = 1,000&nbsp;µW || Bluetooth standard (Class 3) radio, 1&nbsp;m range
|-
| −1 dBm || 794&nbsp;µW
|-
| −3 dBm || 501&nbsp;µW
|-
| −5 dBm || 316&nbsp;µW
|-
| −10 dBm || 100&nbsp;µW ||Typical maximum received signal power  (−10 to −30&nbsp;dBm) of [[wireless network]]
|-
| −20 dBm || 10&nbsp;µW
|-
| −30 dBm || 1.0&nbsp;µW = 1,000&nbsp;[[Milliwatt#Multiples|nW]]
|-
| −40 dBm || 100&nbsp;[[Milliwatt#Multiples|nW]]
|-
| −50 dBm || 10&nbsp;nW
|-
| −60 dBm || 1.0&nbsp;nW = 1,000 [[Milliwatt|pW]] || The [[Earth]] receives one nanowatt per square metre from a [[Apparent magnitude|magnitude]] +3.5 [[star]]<ref>[http://webhome.cs.uvic.ca/~pearson/files/radiant_flux.html Radiant Flux of a Magnitude +3.5 Star]</ref>
|-
| −70 dBm || 100 [[Milliwatt#Multiples|pW]]
|-
| −73 dBm || 50.12 pW || "S9" signal strength, a strong signal, on the [[S meter|S-meter]] of a typical [[Amateur radio|ham]] or [[shortwave]] radio receiver
|-
| −80 dBm || 10 pW || Typical range (−70 to −90&nbsp;dBm) of wireless received signal power over a network (802.11 variants)
|-
| −100 dBm || 0.1 pW
|-
| −111 dBm || 0.008 pW = 8 [[Milliwatt#Multiples|fW]] || [[Johnson–Nyquist noise|Thermal noise floor]] for commercial [[GPS]] single channel signal bandwidth (2&nbsp;MHz)
|-
| −127.5 dBm || 0.178 fW = 178 [[Milliwatt#Multiples|aW]] || Typical received signal power from a [[GPS satellite]]
|-
| −174 dBm  || 0.004 aW = 4 [[Milliwatt#Multiples|zW]]|| Thermal noise floor for 1&nbsp;Hz bandwidth at room temperature (20&nbsp;°C)
|-
| −192.5 dBm || 0.056 [[Milliwatt#Multiples|zW]] = 56 [[Milliwatt#Multiples|yW]] || Thermal noise floor for 1&nbsp;Hz bandwidth in outer space (4 [[kelvin]]s)
|-
| −∞ 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, [[Weighting filter|unweighted]] measurement is normally understood, applicable to a certain [[Bandwidth (signal processing)|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 Volts dissipates 1&nbsp;mW in a 600 Ω load.<ref name=srh /> [[Decibel#Voltage|dBu]] measures against this reference voltage without the 600 Ω restriction.
 
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.<ref>[http://physics.nist.gov/cuu/pdf/sp811.pdf Thompson and Taylor 2008, Guide for the Use of the International System of Units (SI), NIST Special Publication SP811]</ref>
 
Expression in dBm is typically used for optical and electrical power measurements, not for other types of power (such as thermal). A [[Orders of magnitude (power)|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<ref name=srh>{{cite book|last=Davis|first=Gary|title=The Sound Reinforcement Handbook|year=1988|publisher=Yamaha|isbn=0881889008|pages=22}}</ref>  in the paper "A New Standard Volume Indicator and Reference Level".<ref>{{cite journal|last=Chinn|first=H.A.|coauthors=D.K. Gannett, R.M.Moris|title=A New Standard Volume Indicator and Reference Level|journal=Proceedings of the Institute of Radio Engineers|date=January 1940|volume=28|issue=1|pages=1–17|doi=10.1109/JRPROC.1940.228815|url=http://www.aes.org/aeshc/pdf/chinn_a-new-svi.pdf}}</ref>
 
== See also ==
*[[dBW]]
*[[Decibel]]
 
== References ==
{{FS1037C MS188}}
{{reflist}}
 
== External links ==
*[http://www.sengpielaudio.com/calculator-volt.htm The dBm calculator for impedance matching]
*[http://cgi.www.telestrian.co.uk/cgi-bin/www.telestrian.co.uk/dBm.pl Convert dBm to watts]
{{Decibel}}
 
{{DEFAULTSORT:Dbm}}
[[Category:Units of measurement]]
[[Category:Radio frequency propagation]]

Latest revision as of 07:04, 13 November 2014

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