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Fundamental Parameters of Antennas

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INTRODUCTION

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

145-1973.

RADIATION PATTERN

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

pattern.

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.

Principal Patterns

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.