The most basic properties of an antenna are its radiation pattern, gain, impedance, and polarization. These properties are identical for linear passive antennas used either as a transmitter or receiver by virtue of the reciprocity theorem . A complete definition of the terms used for antennas can be found in .
The radiation pattern is defined in  as the spatial distribution of a quantity that characterizes the electromagnetic field generated by an antenna. The field intensity of the propagating wave decreases by 1/R with distance R from the source.
To understand how an antenna radiates, consider a pulse of electric charge moving along a straight conductor. A static electric charge or a charge moving with a uniform velocity does not radiate. However, when charges are accelerated along a conductor and are decelerated upon reflection from its end, radiated fields are produced along the wire and at each end. A detailed explanation of how an antenna radiates is given in [3-5].
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