Please i need know why we using dBm unit in power ?

Question

what mean the minas sign befor the unit of for example -32dBm ?

Answer 1

The minus sign means that the power is less than 1 mW (the reference power 'm' in dBm).

-32 dBm = 10 log(P / 1mW)

P = 0.000631 mW = 631 nanoWatts

IF it was +32 dBm, then the power would be 1.58 Watts

dBm is a convenient way of expressing power levels over a vast range  without resorting to using exponents, or scientific notation.

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Answer 2

dBm (sometimes dBmW) is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one Milli watt (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.

[The Decibel watt or dBW is a unit for the measurement of the strength of a signal expressed in decibels relative to one watt. It is used because of its capability to express both very large and very small values of power in a short range of number, eg 1 watt = 0 dBW, 10 watts = 10 dBW, 100 watts = 20 dBW and 1,000,000 W = 60 dBW.]

**P = 10^(x / 10)

with P power in mW and x power ratio in dBm

P=6.30957 e-4 mW

P=0.000630957 mW

x = 10* log [to the base 10] (P)