December 04, 2002
Tactical Radio Communications, installment 5
Gain
In radio terms, gain means multiplication. Gain is used with respect to amplifiers and antennas. Amplifiers obviously have signal gain, as that is their purpose. Antennas can also have gain. They accomplish this by focusing the energy supplied to them into a beam. Antenna gain works in both directions, transmit and receive. Gain is specified in dB units. dB units are a logarithmic measure, which means that the gain increases at an ever-faster rate for a constant change in numbers. Some simple rules to follow if you don’t understand what logarithmic means are as follows:
0dB = 0% change
1dB = 10% change
3dB = 70% change
6dB = 200% (or 2 times)
10dB = 300% (or 3 times)
20dB = 1000% (or 10 times)
40dB = 10,000% (or 100 times)
An antenna with 6dB gain will transmit 2 times the energy as a no-gain antenna. Thus a 5-watt radio connected to a 6-dB gain antenna will transmit a signal with the effective strength of 10 watts. Since antenna gain works in both directions, any received signals will also appear to be twice as strong.
Posted by Eric Cartman at December 4, 2002 11:03 AM | TrackBackI'm afraid your calculations are incorrect. A 3 dB increase in power corresponds to a doubling of power. Conversely, a 3 dB decrease corresponds to half power. A 5 watt transmitter connected to an antenna offering 3 dB gain (and you have to specify what it is being compared to - usually a dipole in free space (dBd), or an isotropic antenna (dBi)) would perform as if the transmitter had 10 watts of power into the reference antenna. A 6 dB antenna would make it appear as if the power were 5 watts x 2 x 2 = 20 watts. Of course, the advantage would also apply to signals received on the antenna.
A 10 dB increase would correspond to a ten-fold increase in power. The the Antenna Book published by the American Radio Relay League is an excellent source of information on antennas and power gain.
Posted by: al on August 5, 2004 08:58 AM