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
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