The difference between dB, dBM and dBi

The terms dB, dBM and dBi are often used interchangeably and hence incorrectly because they each mean different things (although from the context it's often obvious what someone means). There's no such thing as a "5dB aerial", for example. This guide gives a brief summary of the meaning of each.

Very Quick Summary

If you don't have time to read the whole of this guide, this is the least you should know:

dB - Relative measure between two power outputs

dBm - Power Output (alternative to mW)

dBi - Gain of an antenna relative to an isotropic antenna (one with radiation in a perfect sphere).

About the notation

dB is short for decibel and always notated with a lower case 'd' and upper case 'B'. This is because the unit of measurement is the 'Bell' hence upper case B and the d is for 'deci' which is just a smaller unit of the Bell. It is, of course, inconsistent with other SI units, for example kg and kWh.

dB is the difference (gain or loss) between two power levels, so if the difference is 0dB, then the two power levels are the same. A 3dB gain is a doubling in power but the scale is logarithmic (as opposed to linear) so that a 6dB gain is a 4x multiple, 12dB would be 16x and so on. Given two powers, the gain/loss in dB can be calcuated by the formula 10log x/y (where x and y are the two outputs). dB is useful when measuring the loss of wireless cabling or connectors (Wireless LAN signals are quite lossy even on the best cable so if you use a cable to extend the position of an aerial, consider the loss of the cable - even a metre can have significant loss.

dBi

A 'perfect' antenna would output its signal in a perfect sphere around itself (an 'isotropic' pattern). A true isotropic aerial doesn't actually exist but actually an isotropic pattern isn't necessarily the most useful pattern. In a single level office, for example, signal going straight up and down is wasted (there's noone above or below you needing access) so it would be better if the signal radiates mostly around the aerial horizontally.

In the context of wireless LANs, you will see dBi used as the gain of all WiFi aerials. The power isn't increased, but it is concentrated in a narrower pattern. This increase compares to the isotropic (sphere) model is stated in dBi. The narrower the beam (in both X and Y or veritcal and horizontal planes) the more power is radiated into the beam pattern. This is covered in more depth in our guide on aerial gain.

dBm

dBm is actual power output. 0dBm is equal to approximately 1mW, and 20dBm is approximately 100mW. 20dBm is the maximum permitted power output of Wireless LAN (2.4Ghz) in the EU in order to reduce interference and congestion with neighbours. 5Ghz have several bands with different power permitted. It is unlawful (and potentially anti-social) to use devices which exceed the permitted power level. Having a wireless access point with an output higher than 20dBm would be of limited use anyway assuming that your wireless devices (laptops, tablets and phones) are limited to 20dBm.

Comparing the power output specification of devices also isn't generally useful because the efficiency of the design of the actual access point or router will vary.

You may also be interested in our Antenna Theory, Gain & Bands article.

Note : We'll be happy to receive corrections and suggestions on this or any other guide, but if you are a wireless engineer, we appreciate that this guide is not applicable for all given scenarios. The above applies to common usage in Wireless LANs and for the purposes such users are likely to need them for. The usage varies by industry and application.