# DBm

DBm

"For other uses, see DBM (disambiguation)"

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:

:$x = 10 log_\left\{10\right\}\left(P/1mW\right)$

and

:$P = 10^\left\{\left(x/10\right)\right\}$

with "P" power in mW and "x" power ratio in dBm. Below is a table summarizing useful cases:

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. [ [http://physics.nist.gov/Pubs/pdf.html Taylor 1995, Guide for the Use of the International System of Units (SI), NIST Special Publication SP811] ]

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.

* dBW
* Decibel
* Transmission level point
* Orders of magnitude (power)

References

Wikimedia Foundation. 2010.

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