# The Decibel

The *decibel (dB)* is the unit of relative power. If one signalis twice as
loud as another, it has a gain of 3dB. One advantage ofexpressing power in decibels
is that, in a multi-stage amplifier,the power gains of the stages can simply be added together.The general expression for calculating the decibel gain of a circuit
(or an antenna) is:-

*Enter values for 'Power Out' and 'Power In' and press the ' = ' button.*

##
dBW

The UK amateur radio licence expresses the maximum permitted output power for each band in
dBW. This is theoutput power, in dB, relative to 1 Watt. To express this in Watts,the following formula can be used:-

##
Antenna Gain

The gain of an antenna is relative to either a theoretical, *isotropic* aerial (dBi) or relative to a half-wave dipole (dBd).

##
Voltage Gain

The dB gain of one *voltage* relative to another can becalculated with the formula:-

However, the result is only correct when the input and output impedancesof the circuit are
the same.

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