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New Unidirectional Antennas for Various Wireless Base Stations        209

                  b = 8 mm) for achieving the best impedance matching. The performances
                  of the two prototypes are measured by an HP8510-C Network Analyzer, a
                  compact range with ORBIT/FR MiDAS Far-Field Antenna Measurement
                  and Analysis System. As for the gain measurement, a NARDA-643 stan-
                  dard gain horn is used.
                    Figure 6.2 shows comparisons of the measured results of gain and
                  SWR of both prototypes. As seen from the SWR curves, both antennas
                  have a wide impedance bandwidth of 25% (SWR ≤ 1.5) from 4.42 to
                  5.7 GHz. From the gain curves, obviously the proposed antenna has a
                  better gain of 10 dBi, which is stable across the operating bandwidth
                  with a 1-dB-gain bandwidth of 26%. Compared with the maximum gain
                  of the single-L-probe coupled patch of 8 dBi, there is an improvement
                  of about 2 dB. The improvement is much more significant at the upper
                  region, 5–5.7 GHz, of the operating band, about 3- to 6-dB difference
                  in gain is observed. The increase in gain is mainly due to the effective
                  suppression of cross-polar radiation.
                    Figure 6.3 shows the measured radiation pattern at 5.0 GHz for both
                  antennas. The cross-polar level in the H-plane of the single L-probe cou-
                  pled patch is relatively higher than the twin L-probe case. This higher
                  cross-polar radiation is due to a stronger high-order mode radiation
                  from the patch as well as unwanted radiation from the vertical arm.
                  However, there is significant suppression in the cross-polar radiation
                  when the twin L-probe fed technique is employed. The cross-polarization
                  can be suppressed to less than –20 dB. For both antennas, the broadside
                  patterns are stable across the operating band, and the average beam-
                  width in the H-plane is around 56°, which is slightly narrower than that
                  in the E-plane of around 60°.



                                                                              11
                     3.5                                                      10
                                                        Gain                  9
                      3                                                       8
                                                                 Twin-L-probe  7
                    SWR  2.5                                     Single-L-probe  6 5  Gain/dBi

                      2                                                       4
                                                                              3
                     1.5                                                      2
                                          SWR                                 1
                      1                                                       0
                       4    4.2   4.4  4.6  4.8   5    5.2   5.4  5.6  5.8   6
                                             Frequency/GHz
                                                                                 29
                  Figure 6.2  Measured results of SWR and gain of both single and twin L-probe cases
                  (© 2005 IEEE)
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