Page 179 - Satellite Communications, Fourth Edition

Antennas 159

b
E

a

(a)

z

b

a y

Figure 6.14 The pyramidal horn.
x
(b)

For any of the aperture antennas discussed, the isotropic gain can
be found in terms of the area of the physical aperture by using the
relationships given in Eqs. (6.14) and (6.15). For accurate gain deter-
minations, the difficulties lie in determining the illumination effi-
ciency , which can range from 35 to 80 percent for horns and from
I
50 to 80 percent for circular reflectors (Balanis, 1982, p. 475). Circular
reflectors are discussed in Sec. 6.13.

6.13 The Parabolic Reflector
Parabolic reflectors are widely used in satellite communications systems
to enhance the gain of antennas. The reflector provides a focusing mech-
anism which concentrates the energy in a given direction. The most