As the name wireless already indicates ,this communication mode involves gettin rid of wires and transmitting signals through spaces without guidance . We do not need any medium for transport of electromagnetic waves .Somehow ,we have to couple the energy from transmitters to the outsider and in reverse from outside world to the receiver .This is exactly that antenna do.Antennas couple electromagnetic energy to and from space and a wire or co-axial cable(or any other appropriate conductor)
A theoritical referance antenna is the istotropic radiator , a point in space radiating equal power in all directions ,i.e all the points with equal poer are located on a sphere with antenna as its center .The radiation pattern is symmetric in all directions (a three dimesnional in real sense)
Howeaver ,such antenna doesnt exist in reality .Real antennas all inhibit directive effects , the intensity of radiation is not the same in all directions from antenna .The simplest real antenna is a thin ,center fed dipole, also called Hertzian dipole.The dipole consist of two non-collinear conductors of length ,seperated by a small feeding gap.The lenght of dipole is not arbitary ,but for example half the wave-length of the signal to transmit results in an efficent way ,radiation of energy. If mounted in the roof of car, the length of lanbda /4 .This is known as macroni- antenna.
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