Page 210 - Antennas for Base Stations in Wireless Communications

Antenna Issues and Technologies for Enhancing System Capacity 183

a W-CDMA network is an important design parameter. For instance, the
HPBW of the antennas in both downlink and uplink significantly affects
the power and interference distribution in the network, as mentioned pre-
viously. As a result, the optimization of the HPBW and sidelobe levels are
the most important considerations in base station antennas design in W-
CDMA networks, although gain is another crucial parameter as well. 8,9
Furthermore, radiation patterns in the horizontal and vertical planes
have different requirements. In the horizontal planes, usually the
HPBW plays the important role in determining coverage and controlling
the possible interference among adjacent cells or sectors. Sidelobes are
hardly the problem because only single elements or an array with few,
for instance, two or four, elements are used in the horizontal plane. In
the vertical plane, the number of vertical array elements is determined
by the required gain of the base station antenna. Moreover, controlling
sidelobe levels becomes an antenna design consideration because the
sidelobe levels will affect the inference performance.
In addition to the electrical specifications, which are determined by
system requirements, design considerations for base station antennas
primarily include
■ Mechanical strength
■ Compact size for limited installation space
■ Low construction cost
■ Light weight
■ Less environment influence
■ High water resistance/weather proofing
■ Low material and manufacturing cost

In addition, base station antennas are always installed outdoors in
high places like on the roofs of buildings or on antenna towers where
wind pressure affects the mechanical strength of antenna installation. 10
The wind pressure can be calculated as in Eq. 5.1:

P = Q × C × A (5.1)
where P is the wind pressure, Q the dynamic wind pressure, C the pres-
sure coefficient, and A the exposed surface area of antenna to the wind
direction. For example, C is 1.4 for flat plates and 0.9 for cylinders at
the base station antenna. The cylinder is 0.64(= 0.9/1.4) of the flat plate
value. 11,12 Cylindrical radomes are, therefore, more commonly used.
In base station antenna engineering, slim antennas reduce wind pres-
sure. In Japan in particular, some places experience extremely power-
ful winds produced by typhoons. The record of the most powerful wind