Page 211 - Antennas for Base Stations in Wireless Communications

184 Chapter Five

pressure is up to 85.3 m/s from a typhoon. Base station antennas must
be windproof and waterproof against typhoons.
In the early days of the 3G network, system capacity was sacrificed
to using slim antennas that had the same coverage area as the 2G net-
10
work. However, the HPBW, which is narrower than those in the 2G
7,8
network, was used due to the increased number of subscribers. Due to
these reasons and the effects of HPBW on system capacity, as shown in
Figure 5.4 and Figure 5.5, the ratio of the HPBW to the sector division
angle of around 0.75 was used to increase system capacity and reduce
the areas without low-power levels. Thus, the HPBW of 90° and 45° are
for the three- and six-sector per cell structures, respectively. 13
Moreover, multiband antennas have been used to reduce the space
required for antenna installation. The limited space available for antenna
installation, especially in metropolitan areas, has been a big problem in
Japan. A compact design for antennas is strongly desired.

5.3 Case Studies

Based on the design considerations just mentioned, five typical antenna
designs have been selected for study. Techniques for controlling the
HPBW for optimal system capacity and miniaturizing the width of the
antenna are introduced in the design case studies.

5.3.1 Slim Antenna

5.3.1.1 Reduction in Width of Reflector Usually, a base station antenna is
designed with a reflector behind the radiators to suppress possible back
radiation for avoiding possible interference among antennas or sectors.
Figure 5.6 shows three 120° beam antennas with reflectors behind the
14
reflectors and radomes that protect the antennas. The reflectors are
installed in parallel to the arms of dipoles with a spacing of l/4 (l is the
wavelength at operating frequency) to keep high gain. The diameter of
the antenna of the flat reflector is 0.53l, as shown in Figure 5.6a. The
width of a corner reflector antenna can be shrunk but with the same
HPBW as the antenna with a flat reflector. For example, the diameter of
the corner reflector antenna with the same HPBW of 120° as the antenna
with a flat reflector is reduced to 0.3l from 0.53l, as shown in Figure 5.6b.
A semi-cylindrical reflector can be used to reduce the diameter of the
antenna reflector further. Such a diameter can be reduced to only 0.267l,
as depicted in Figure 5.6c, so the slimmest design can be achieved.
Figure 5.7 compares the radiation patterns for antennas with a flat
reflector and a semi-cylindrical reflector. The radiation patterns are
roughly identical and the latter pattern has an approximately 3-dB
improvement in back radiation. Therefore, the diameter of the antenna
can be reduced by about 50% using a semi-cylindrical reflector, as shown