Fundamental Parameters of Antennas
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To describe the performance of an antenna, definitions of various parameters are necessary.
Some of the parameters are interrelated and not all of them need be specified for
complete description of the antenna performance. Parameter definitions will be given
inthis chapter. Many of those inquotationmarks are from the IEEE Standard Definitions
of Terms for Antennas (IEEE Std 145-1983).∗ This is a revisionof the IEEE Std
An antenna radiation pattern or antenna pattern is defined as “a mathematical function
or a graphical representation of the radiation properties of the antenna as a function
of space coordinates. Inmost cases, the radiationpatternis determined in the farfield
region and is represented as a function of the directional coordinates. Radiation
properties include power flux density, radiation intensity, field strength, directivity,
phase or polarization.” The radiation property of most concern is the two- or threedimensional
spatial distribution of radiated energy as a function of the observer’s
position along a path or surface of constant radius. A convenient set of coordinates
is shown in Figure 2.1. A trace of the received electric (magnetic) field at a constant
radius is called the amplitude field pattern. Onthe other hand, a graph of the spatial
variation of the power density along a constant radius is called an amplitude power
Radiation Pattern Lobes
Various parts of a radiationpattern are referred to as lobes, which may be subclassified
into major or main, minor, side, an dback lobes.
A radiation lobe is a “portion of the radiation pattern bounded by regions of
relatively weak radiation intensity.” Figure 2.3(a) demonstrates a symmetrical threedimensional
polar pattern with a number of radiation lobes. Some are of greater
radiation intensity than others, but all are classified as lobes.
For a linearly polarized antenna, performance is often described in terms of its principal
E- an dH-plane patterns. The E-plane is defined as “the plane containing the electricfield
vector and the direction of maximum radiation,” and the H-plane as “the plane
containing the magnetic-field vector and the direction of maximum radiation.” Although
it is very difficult to illustrate the principal patterns without considering a specific
example, it is the usual practice to orient most antennas so that at least one of the
principal plane patterns coincide with one of the geometrical principal planes. An
illustrationis showninFigure 2.5. For this example, the x-z plane (elevation plane;
φ = 0) is the principal E-plane and the x-y plane (azimuthal plane; θ = π/2) is the
principal H-plane. Other coordinate orientations can be selected.
Radian and Steradian
The measure of a plane angle is a radian. One radian is defined as the plane angle with
its vertex at the center of a circle of radius r that is subtended by an arc whose length
is r. A graphical illustration is showninFigur e 2.10(a). Since the circumference of a
circle of radius r is C = 2πr, there are 2π rad (2πr/r) ina full circle.
The measure of a solid angle is a steradian. One steradian is defined as the solid
angle with its vertex at the center of a sphere of radius r that is subtended by a spherical
surface area equal to that of a square with each side of length r. A graphical illustration
is showninFigure 2.10(b). Since the area of a sphere of radius r is A = 4πr2, there
are 4π sr (4πr2/r2) ina closed sphere.