How does antenna work?

Antenna is a electrical device that converts radio-frequency into alternating current. It is used to send or receive electromagnetic waves. Radio antennas have two fold functions. The first of this functions is to radiate the radio frequency energy generated in the transmitter and fed to the antenna by a transmission line. In this capacity the antenna acts as an impedance matching device to match the impedance of the transmission line to that of free space. The other function of the antenna is to direct the energy into desired directions and as suppress to the radiation in unwanted directions.

A completely non-directional or omni-directional radiator radiates uniformly in all directions and is known as isotropic radiator. A point source of sound is an example of an isotropic radiator. the radiation pattern of an antenna is a graphical representation of the radiation of the antenna as a function of direction. If the radiation is expressed as field strength per meter the radiation pattern is field strength pattern. If the radiation in a given direction is expressed in terms of power per unit solid angle , the resulting pattern is power pattern. The co-ordinate system generally used in the specification of antenna radiation pattern is the spherical co-ordinate system. The antenna is locked at or near the origin of this system and the field strength is specified at points on the spherical surface or surface radius. The shape of the radiation pattern is independent of surface radius If the surface radius is chosen sufficiently large. When this is true, the magnitude of the field strength in any direction varies inversely with surface radius and so needs to be stated for only one value of surface radius.for example, in broadcast antenna works, it is customary to state the field strength at a radius of one mile or one kilometre. Often only the relative radiation pattern is used. This gives the relative field strengths in various directions, usually referred to unity in the direction of maximum radiation.