Q. What do you mean by Antenna gain?

Antenna gain depends on both shape and size. Dipole antennas, commonly used at lower radio frequencies, are made up of a rod or wire of length λ/10 to λ/2, and have an antenna-gain range of 1.5 to 1.64 (1.8 to 2.1 dB). Horn antennas and parabolic dishes (so named after their shapes) have much more gain at higher frequencies. A useful, although approximate, expression for gain is given by

where λ is the wavelength being transmitted (or received), and Ae is the effective aperture or effective area of the antenna. Power amplifiers are needed to overcome the radio-transmission loss just as in a transmission-line system. Similar considerations hold for optical radiation, in which the electromagnetic wave takes the form of a coherent light beam.