Lecture 49 : Radiation Characteristics of an Antenna
An antenna has dual nature. When seen from the space side, it has properties of electromagnetic waves like the radiation pattern, polarization, Poynting vector etc, and when seen from the terminal side it has circuit like properties like the impedance, bandwidth and so on.
The antenna therefore has to be characterized from circuit as well as the waves perspective. Following parameters are used for characterizing an antenna.
Radiation Pattern
Radiation pattern is one of the important characteristic of an antenna as tells the spatial relative distribution of the electromagnetic wave generated by the antenna.
The radiation pattern is a plot of the magnitude of the radiation field as a function of direction .
The radiation pattern is essentially a 3-D surface.
Since the radiation pattern is supposed to provide relative distribution of the fields, the absolute size of the 3-D surface does not have any significance. In practice therefore the maximum amplitude is normalized to unity.
The radiation pattern for the Hertz dipole is
The above equation when plotted in the spherical coordinate system ( with we get a three dimensional figure as shown in Fig.
The radiation pattern of the Hertz dipole is like an apple.
At times a full 3-D description of the radiation pattern may not be needed. Invariably therefore two principal sections of the 3-D radiation patterns are given as the radiation patterns.
The two principle sections are obtained by planes one containing the electric field vector and the other containing the magnetic field vector. Consequently, the two planar radiation patterns are called the E-plane and H-plane radiation patterns respectively.
For the Hertz dipole any vertical plane passing through the z-axis is the E-plane and a horizontal plane passing through the antenna is the H-plane. The two radiation patterns for the Hertz dipole are shown in Fig.
NOTE:
In a radiation pattern the physical shape and size of the antenna do not get reflected. The antenna is merely a point at the origin of the radiation pattern
. 
with 
we get a three dimensional figure as shown in Fig. 
