Antenna Fundamentals
Introduction
Antennas are device designed to radiate electromagnetic energy efficiently in a prescribed manner. It is the current distributions on the antennas that produce the radiation. Usually these current distributions are excited by transmission lines or waveguides.
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Antenna Parameters
Poynting Vector and Power Density
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Note that Poynting vector is a real vector. Its magnitude gives the instantaneous or average power density of the electromagnetic wave. Its direction gives the direction of the power flow at that particular point
Power Intensity
sr = steradian, unit for measuring the solid angle.
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Radiated Power
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Note that the integration is over a closed surface with the antenna inside and the surface is sufficiently far from the antenna (far field conditions).
Radiation Pattern
A radiation pattern (or field pattern) is a graph that describes the relative far field value, E or H, with direction at a fixed distance from the antenna. A field pattern includes an magnitude pattern |E| or |H| and a phase pattern ∠E or ∠H.
A power pattern is a graph that describes the relative (average) radiated power density |Pav| of the far-field with direction at a fixed distance from the antenna. By the reciprocity theorem, the radiation patterns of an antenna in the transmitting mode is same as the those for the antenna in the receiving mode.
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A radiation pattern shows only the relative values but not the absolute values of the field or power quantity. Hence the values are usually normalized (i.e., divided) by the maximum value.
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Polarization
The polarization of an antenna in a given direction is defined as the polarization of the plane wave transmitted by the antenna in that direction. The polarization of a plane wave is the figure the tip of the instantaneous electric-field vector E traces out with time at a fixed observation point. There are three types of typical antenna polarizations: the linear, circular, and elliptical polarizations, corresponding to the same three types of typical plane wave polarizations.
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Polarization of Plane Waves
(a) Linear polarization
A plane wave is linearly polarized at a fixed observation point if the tip of the electric-field vector at that point moves along the same straight line at every instant of time.
(b) Circular Polarization
A plane wave is circularly polarized at a a fixed observation point if the tip of the electric-field vector at that point traces out a circle as a function of time.Circular polarization can be either right-handed or left-handed corresponding to the electric-field vector rotating clockwise (right-handed) or anticlockwise (left-handed).
(c) Elliptical Polarization
A plane wave is elliptically polarized at a a fixed observation point if the tip of the electric-field vector at that point traces out an ellipse as a function of time. Elliptically polarization can be either right-handed or left handed corresponding to the electric-field vector rotating clockwise (right-handed) or anti-clockwise (left-handed).
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