Introduction:

In wireless communication systems, signals are radiated in space as an electromagnetic wave by using a transmitting antenna and a fraction of this radiated power is intercepted by using a receiving antenna. Thus, an antenna is a device used for radiating or receiveing radio waves. An antenna can also be thought of as a transitional structure between free space and a guiding device (such as transmission line or waveguide). Usually antennas are metallic structures, but dielectric antennas are also used now a days. In our discussion we shall consider only metallic antennas. Here we shall restrict our discussion to some very commonly used antenna structures. Some of the most commonly used antenna structures are shown in fig 7.1.

Fundamentals of Radiation:

Radiation is the process of emitting energy from a source. Electromagnetic radiation can be at all frequencies except zero (DC), but radiation at various frequencies may take different forms. At relatively lower frequencies it is in the form of electromagnetic waves, in the visible domain the emission is in the form of light and at still higher frequencies it may be in the form of ultra violet or X-ray radiation. The energy associated with the radiation depends on the frequency.

Time varying currents radiate electromagnetic waves. A time varying current generates time varying electric and magnetic fields. When such fields exist, power is generated and propagated. Although, theoritically any structure carrying time varying current can radiate electromagnetic waves, all structures are not equally efficient in doing that. While in many applications we try to reduce the radiation, when radiation is intended, launching of waves into space is accomplished with the aid of specially designed structures called antennas. If the time varying current density established on an antenna structure is known, the radiated fields can be calculated without great difficulty. A more difficult problem is determination of current density on an antenna such that the resultant field will satisfy the required boundary conditions on the antenna surface.