Gain
Antennas are passive devices; they do not amplify a transmitter’s signal with any circuitry or external power. Instead, they amplify or add gain to the signal by shaping the RF energy as it is propagated into free space. In other words, the gain of an antenna is a measure of how effectively it can focus RF energy in a certain direction.
Because an isotropic antenna radiates RF energy in all directions equally, it cannot focus the energy in any certain direction. Recall from Chapter 1 that the gain of an antenna in dBi is measured relative to an isotropic antenna. When an isotropic antenna is compared with itself, the result is a gain of 10log(1) or 0 dBi.
Think of a zero gain antenna producing a perfect sphere. If the sphere is made of rubber, ...
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