0195136047.pdf

15.1 WAVES, TRANSMISSION LINES, WAVEGUIDES, AND ANTENNA FUNDAMENTALS 681

Transmission

line

Transmission line

Active element

(regular or folded dipole)

Throat

Aperture

Horn

Reflector throat

(a)

(d) (e)

(g) (h)

(f)

Directors

(b) (c)

z

a

b

d

d

a

s

b

P

d

S

z

L

N turns

L

2

λ

= 4

L

2

λ

4

Transmission

line Transmission

line

λ

2

−−=

y

x

z

L

z

z

y

f'

z

y x

A

B

y

D z

f

Focus

Focus f

Hyperbola

Feeding (subreflector)

line

Parabola

a

b

θ

φ

Figure 15.1.7(a)Half-wave dipole.(b)Folded half-wave dipole.(c)Helical-beam antenna.(d)YAGI-UDA

array antenna.(e)Pyramidal horn.(f)Conical horn.(g)Paraboloidal antenna.(h)Cassegrain antenna (section).

thebeamwidth,which is designated asθBorφBin the principal-plane patterns. Another useful,

although approximate, expression for gain in terms of the beamwidthsθBandφBof the pattern’s

main lobe is given by

G=

4 π

θBφB

(15.1.26)

when beamwidths are expressed in radians, or

G=

41. 3 × 103

θBφB

(15.1.27)

when beamwidths are expressed in degrees.

Theantenna impedance, looking into the feed-point terminals of an antenna, will in general

have both resistive and reactive components. Theantenna resistanceis the combination of the

radiation resistance (accounting for the power radiated by the antenna) and theloss resistance