The Decibel
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:-
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 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.