AntennaGain is the increase in signal power you get from using a directional antenna. If your signal of 20 dB is too weak, for example, you could add an amplifier for 10 dB and use a directional antenna for another 10 dB. Antenna gain helps make up for PathLoss.
Unlike amplifier gain, antenna gain does not actually add any power to the signal. It just concentrates the existing power in the directions which are useful. A 5 mW transmitter will be easier for me to receive if all of its output is directed at me than if it is being radiated isotropically (in all directions).
Many FccRegulations control the effective radiated power, or ERP, not the total radiated power. This means that using a highly directional antenna can put you beyond the FCC power limits even if you do not use a more powerful transmitter.
Antenna gain works both ways. A +12 dB antenna will pick up more RF energy when receiving, but only from one direction. From other directions, it will be less sensitive than an omnidirectional antenna.
The common "5 dBi omni" antenna is not truly omnidirectional/isotropic; it concentrates transmitted power in a horizontal plane, radiating less power up and down.
(The notation "dBi" indicates a gain of that many decibels over a theoretical isotropic antenna.)
manish
