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 Log10( Power Out ) dB gain
Power In
Enter values for 'Power Out' and 'Power In' and press the ' = ' button.


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:-

dBW ) Watts

Antenna Gain

The gain of an antenna is relative to either a theoretical, isotropic aerial (dBi) or relative to a half-wave dipole (dBd). For any given antenna, the isotropic gain is 2.1 dB greater than the dBd figure so...
     dBd = dBi - 2.1 or...
     dBi = dBd + 2.1

Voltage Gain

The dB gain of one voltage relative to another can be calculated with the formula:-

20 Log10(
V Out ) dBv gain
V In
However, the result is only correct when the input and output impedances of the circuit are the same.
73 from John G4VWL @GB7DEE email: [email protected]
This page was found on Packet Radio. If you find it useful send thanks to John (G4VWL)