Page 223 - Antennas for Base Stations in Wireless Communications
P. 223
196 Chapter Five
900−1900 MHz. The narrowest bandwidth occurs at 800 MHz because
of the strong coupling between the slot antenna and parasitic cylin-
der. However, this antenna can achieve omnidirectional radiation as
a dielectric substrate of 30-mm wide (0.09l at 900 MHz) is used. The
bandwidth reaches about 15% although the width is electrically large.
5.3.4 Booster Antenna
In mobile communication networks, booster systems are used to prevent
possible oscillation caused by the interference waves between transmit-
ting and receiving antennas. To enable the mobility of communications,
a frequency offset booster has been proposed in order to realize the rera-
diation system in a blocked area, such as in the shadow of a mountain
or building. Such a system has a back-to-back antenna arrangement,
where one antenna or array points toward the base station whereas
the other one points to mobile terminals. It is necessary to identify an
absolute value for the mutual coupling between the antennas such that
it is greater than amplifier gain. 22
Figure 5.23 shows the structures of four- and sixteen-element choke-
loaded patch antenna arrays, where the chocks are around dielectric
substrates. The antennas are used to increase the front-to-back (FB)
radiation ratio to reduce the mutual coupling in the back-to-back
antenna arrangement. Figure 5.23a shows the configuration of a four-
element chock-loaded patch antenna array, and Figure 5.23b shows
a sixteen-element chock-loaded patch antenna array. Both antenna
arrays are etched onto 1.2-mm thick dielectric substrates.
The radiation patterns in both E- and H-planes for both four- and
sixteen-element chock-loaded patch antenna arrays are shown in
Figure 5.24. Table 5.2 tabulates the measured HPBW and FB ratio.
The FB ratios of the four- and sixteen-element choke-loaded patch array
antennas are 37 dB/40 dB in E-planes and 32.5 dB/40 dB in H-planes,
respectively. The HPBW of two four- and sixteen-element antennas
are 39.8°/25.7° in E-planes and 36.4°/21.5° in H-planes, respectively.
When the separation between the transmitting and receiving antennas
increases to 4l, the measured mutual coupling between the front and
back arrays in the four-element choke-loaded patch array is less than
−84 dB, as shown in Figure 5.25.
5.3.5 Control of Vertical Radiation Pattern
Whereas the HPBW of the radiation patterns in the horizontal planes
mainly determines the coverage of base station antennas, the vertical
radiation patterns determine the number of vertical array elements
that are needed to obtain the desired gain in coverage and to design the
required HPBW. Employing a shaped-beam pattern for a base station