Page 200 - Satellite Communications, Fourth Edition

180 Chapter Six

of the patch is less than half the free space wavelength because the
phase velocity v of the wave is less than the free space value. Recall that
p
lf v p , where l is the wavelength and f the frequency, and the phase
e
velocity of a wave in a dielectric medium of relative permittivity r is
, where c is the free space velocity of an electromagnetic wave. For
c> 2e r
a microstrip board of thickness 1.59 mm and a relative permittivity of
where l is the
2.32, at a frequency of 10 GHz, the side length is 0.32l 0 0
free space wavelength [see James et al., 1981, (Table 5.3)].
Other geometries are in use, for example circular patches (disc patches),
and coplanar boards and stripline boards are also used. The patch dimen-
sions are usually half or quarter board wavelength. Figure 6.32a shows
a disc element with a balanced coaxial feed. Figure 6.32b shows a copola-
nar waveguide construction. Here the board has ground planes on both
sides, which are bonded together. The antenna element is etched into
one of the ground planes. Figure 6.32c shows a triplate construction,
Here, a stripline, which is etched on the inner layer of one of the boards,
forms a central conductor which passes under the slot in the upper ground
plane, the slot forming an aperture antenna. The two dielectrics are glued
tightly together, and the ground planes are bonded together too.
The basic microstrip patch is a linear polarized antenna, but various
feed arrangements are in use to convert it to a circularly polarized
antenna (see, e.g., James et al., 1981).

6.19 Planar Arrays
The patch antenna is widely used in planar arrays. These are arrays
of basic antenna elements etched on one side of a printed circuit board.
A multilayer board is normally used so that associated connections and
circuitry can be accommodated, as shown in Fig. 6.33. Flat panels are
used, and these may be circular (as shown) or rectangular. The use of
phase shifters to provide the tracking (beam scanning) is a key feature
of planar arrays. The most economical method of beam forming and
scanning is mechanical as described in the earlier sections. The beam
is formed by a shaped reflector and azimuth and elevation motors pro-
vide the scanning. Such motors can also be used with flat panel arrays,
as shown in Fig. 6.33, although the beam is formed by phasing of the
elements, as described in Sec. 6.17, rather than by means of a mechan-
ical reflector. The beam can be made to scan by introducing a progres-
sive phase shift to the driving voltage applied to the various patch
elements.
Figure 6.34 shows two basic configurations. In the active configura-
tion, each antenna element has its own amplifier and phase shifter,
while in the passive arrangement, a single amplifier drives each element