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:-

10 Log 10
Power Out dB gain
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Power  In
Enter values for "Power Out" + "Power In" in red Then 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:-
Antilog dBW Watts
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10
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:-
20 Log10
V Out dB gain
V In

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