It is unlawful (and potentially anti-social) to use devices which exceed the permitted power level.

5Ghz have several bands with different power permitted. 20dBm is the maximum permitted power output of Wireless LAN (2.4Ghz) in the EU in order to reduce interference and congestion with neighbours. 0dBm is equal to approximately 1mW, and 20dBm is approximately 100mW. This is covered in more depth in our guide on aerial gain.ĭBm is actual power output. The narrower the beam (in both X and Y or veritcal and horizontal planes) the more power is radiated into the beam pattern. This increase compares to the isotropic (sphere) model is stated in dBi. The power isn't increased, but it is concentrated in a narrower pattern. In the context of wireless LANs, you will see dBi used as the gain of all WiFi aerials. 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. A true isotropic aerial doesn't actually exist but actually an isotropic pattern isn't necessarily the most useful pattern. 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.Ī 'perfect' antenna would output its signal in a perfect sphere around itself (an 'isotropic' pattern). 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). 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. It is, of course, inconsistent with other SI units, for example kg and kWh.ĭB is the difference (gain or loss) between two power levels, so if the difference is 0dB, then the two power levels are the same. 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.

About the notationĭB is short for decibel and always notated with a lower case 'd' and upper case 'B'. If you don't have time to read the whole of this guide, this is the least you should know:ĭB - Relative measure between two power outputsĭBi - Gain of an antenna relative to an isotropic antenna (one with radiation in a perfect sphere). This guide gives a brief summary of the meaning of each. There's no such thing as a '5dB aerial', for example. 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).

Dbi to db calculator