0195136047.pdf

682 COMMUNICATION SYSTEMS

x

z

y

A

(a) (b)

Side

lobes

Main

lobe

Direction of maximum

To

R

R

θ

φ

x

A

z

y

Figure 15.1.8(a)Spherical coordinates centered on the antenna atA.(b)Typical radiation-intensity pattern

with main and side lobes (with all possible values ofθandφconsidered).

(accounting for the power dissipated in the antenna itself as losses). Ideally, the antenna resistance

should be equal (matched) to the characteristic impedance of the feeding line or guide to prevent

reflected power, and the antenna reactance should be zero.

Most antennas transmit only onepolarizationof electromagnetic wave. That is to say, the

electric field of the propagating wave is oriented with respect to the antenna in only one direction.

The main lobe in most antennas is directed normal to the plane of the aperture. For phased-array

antennas, however, the main lobe is electronically steered to other angles away from the so-called

broadside. Figure 15.1.10 illustratesvertical, horizontal,andarbitrarily linear polarizationsof

the electric field. In Figure 15.1.10(a), with the electric field lying in the vertical plane, the

radiation is said to be vertically polarized. With the electric field being in the horizontal plane,

as in Figure 15.1.10(b), the radiation is said to be horizontally polarized. Since both vertical and

horizontal polarizations are simply special cases of linear polarization, the electric field, having

both horizontal and vertical components (that are in time phase), can still be in a plane, as shown

in Figure 15.1.10(c).

Some systems transmit simultaneously two linear orthogonal polarizations that are not in

time phase.Elliptical polarizationresults when the two linear components have arbitrary relative

amplitudes and arbitrary time phase.Circular polarization(probably the most useful type) is a

special case in which the horizontal and vertical electric fields are 90° out of time phase and have

equal magnitude.Left-handcircular polarization results when the horizontal radiation component

lags the vertical one by 90° and the resultant field appears to rotate counterclockwise in thexy-

plane with time, as one located at the antenna views the wave leaving the antenna. If the horizontal

component leads the vertical one by 90°,right-handcircular polarization is said to take place.

Referring to Figure 15.1.7, the half-wave dipole is a relatively narrow-band antenna with

its radiated wave linearly polarized. The dipole can be driven by a transmission line of 75-

characteristic impedance. The folded half-wave dipole (a variation of the half-wave dipole) is

used in television, broadcast FM, and other applications. This antenna is well suited for use with

300-television cable. The helical antenna of Figure 15.1.7(c) yields apencil-beam patternin

the axis of the helix with circular wave polarization.

The YAGI-UDA array of Figure 15.1.7(d) is commonly used for television reception. It is

usually seen with 3 to 12 elements, although even 40 elements are sometimes employed. Design

frequencies from 100 to 1000 MHz are typical. A half-wave or folded half-wave dipole is the

active element. The array consists of parallel dipoles, all lying in the same plane. Thereflector,

which reflects waves back toward the active element, enhances radiation in the axis of the array