Page 83 - Antennas for Base Stations in Wireless Communications

56 Chapter Two

Materials used successfully have included glass-reinforced plastic
(grp/fiberglass), polystyrene, ABS, ASA, and UPVC. For outdoor
installation, the material must be stabilized against ultraviolet
deterioration. In the case of grp, the resin used must be chosen with
care. A typical resin absorbs a sufficient amount of water to change
its permittivity between a dried-out condition under prolonged hot
sunshine and equilibrium water content in wet weather. In an experi-
ment by the author, a pultruded polyester resin grp radome 1.2-m
long released a wineglass-full of water (125 g) after being dried out
following a 24-hour period of exposure to water on its external sur-
faces. The result of water absorption was a substantial increase in
the input VSWR of the antenna.
There are two contributions to the loss caused by a radome. Loss
caused by wave propagation through the thickness of the radome is
inevitable, but close placement of the radome to the radiating ele-
ments results in further loss caused by stored-energy fields, local to
the element, intersecting the radome.
k. The net gain is equal to the directivity of the array (f) minus the sum
of all losses (g + h + i + j).

2.3.3.1 Elevation Pattern Shaping Because the user specification relates
only to the amplitude and not the phase of the signal radiated at dif-
ferent elevation angles, there is no unique set of complex currents that
will deliver the specified performance. The antenna designer can choose
to constrain a number of variables in order to arrive at a solution that
meets the requirements of particular construction methods. A general
solution will require both the amplitudes and phases of the radiating
currents to be different for each element in the array. Among others,
there are classes of solution for which the element currents are equal
in magnitude and vary only in phase, solutions in which pairs of adja-
cent elements have equal complex currents, and solutions in which the
required power dividers have a limited number of simple ratios, suitable
for construction from coaxial cables.

3.3.3.2 Mutual Impedance A current flowing in one element of an array
induces currents in other elements, particularly in the adjacent ele-
ments. The feed impedance of each element is a function of the current
that it carries itself and also of the currents in its neighbors. To take a
typical example of the third element in a long array:

I I I I
Z = I 1 3 Z + I 2 3 Z + Z + I 4 3 Z + I 5 3 Z + ...
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43
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3
13
23