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The Dreaded dB

I see it all the time, it's important and I have no idea what it is!!

This is a quick and superficial overview of the deciBel (or colloqually the dB) (see the side bar links for the real 'stuff').

The deciBel is a unitless relative measure of power between two values and is most commonly used to describe gain or loss in wireless (and other) communications systems. The standard 'power ratio' equation for dB is written as:

dB = 10 log 10 ( P 2 / P 1)

In the above equation P1 is the reference value and in most wireless systems this is 1 milliWatt (mW), so dB strictly speaking should be written as dBm to indicate the reference value being used. Finally, to precisely define the value a resistance is also assumed which for most wireless systems is 50 ohms so the final form should be dBm(50). You may also see other forms of db the most common of which are defined in this table.

You can calculate the value you want by substituting the P2 for the power of your radio and P1 for 1 mW, use our conversion calculator, one of the excellent dB calculators referenced in the links section, or you can read the value from the table below!

Remember: The dB is a log not a number, so a difference of 10 dB is a FACTOR of 10, 3 dB is almost twice the power, and so on.

Negative dB

Negative dBm (for example, -87 dBm) just says that the power is less than 1 milliwatt and is most frequently used to describe Receiver Sensitivity. Typical receivers will operate in the range -80 to -110 dBm. With typical free space loss being over a 100 dBm for just sending down the corridor - you need it.

dBm to Watts Conversion

dBm Watts Notes dBm Watts Notes
0 1.0 mW 19 79 mW
1 1.3 mW 20 100 mW
2 1.5 mW 21 126 mW
3 2.0 mW 22 158 mW
4 2.5 mW 23 200 mW
5 3.2 mW 24 250 mW
6 4.0 mW 25 316 mW
7 5.0 mW 26 398 mW
8 6.3 mW 27 500 mW
9 8.0 mW 28 630 mW
10 10.0 mW 29 800 mW
11 12.5 mW 30 1.0 W Maximum radio power for FHSS in 2.4 GHz ISM band
12 15.8 mW 31 1.3 W
13 20 mW 32 1.6 W
14 25 mW 33 2.0 W
15 32 mW 34 2.5 W
16 40 mW 35 3.2 W
17 1.0mW 36 4.0 W Maximum EIRP for FHSS in 2.4 GHz ISM band.
18 63 mW 37 5.0 W

Common dB Terms

Term Meaning
dB Completely meaningless since it defines no base (it should be something like dBm or dBW, see table below) but depending on context usually means the 'most common measure', for example, in low power wireless it is normally the power ratio relative to 1 milliWatt and therefore should be written as dBm.
dBi Some times used to describe antenna gain the 'i' stands for 'isotropic'
dBm The power ratio relative to 1 milliWatt
dBW The power ration relative to 1 Watt


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Page modified: September 22 2013.

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