The *decibel (dB)* is the unit of
relative power. If one signal is twice as loud as another, it has a gain of 3dB.
One advantage of expressing 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:-

dBW

The UK amateur radio licence expresses
the maximum permitted output power for each band in
dBW
.
This is the output 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 be calculated with the formula:-
However, the result is only correct when the input and output impedances of the circuit are the same.