| The base-station antenna is an indispensable part of the mobile communication systems.Miniaturization and light weight are necessary for the development of base station antennas,of which low profile is an important aspect.Nowadays,a variety of communication standards with different working frequencies and protocols have been adopted in mobile communications,such as 2G,3G,4G,and 5G,which leads to immediate needs for multi-band base station antennas.In addition,the development of base stations is expensive.Due to limited installation spaces,there are tight size constraints on antenna deployments.Antennas for different bands have to be integrated in the same footprint to form an aperture-shared multi-band antenna.Based on metasurfaces,the investigations on a low-profile broadband antenna,an aperture-shared dualband antenna and an aperture-shared dual-band subarray have been carried out in this dissertation,which are shown as follows.1.Based on a metasurface,a broadband low-profile antenna and its array are developed.When the height of a dipole antenna is lowered,though the radiation performances of the dipole may remain good,it is difficult to achieve impedance matching.By introducing a metasurface above the dipole antenna with a reduced height,the impedance matching of the dipole antenna has been realized over a broad bandwidth.The height of the low-profile antenna is reduced from 0.3?2.2GHz to 0.18?2.2GHz.The low-profile antenna has an impedance bandwidth(|S11|/|S22|<-15 d B)of 1.7-2.7 GHz(45.4%),an isolation of 30 d B,stable radiation patterns,and gain of about 10.2 d Bi.A four-element linear array of the low-profile antenna has also been developed.Compared to the array of the original antennas with the same element space,the grating lobes of low-profile array have been reduced,the front-toback ratio and cross-polarization have been improved.The bandwidth of the low-profile antenna array also covers 1.7-2.7 GHz.The radiation patterns are stable and antenna gain is about 14.8 d Bi.2.An aperture-shared dual-band antenna based on a low-pass partially reflective surface(PRS)is proposed.By placing the high-band antenna(HBA)under the low-band antenna (LBA),a fully aperture-shared dual-band antenna is formed.The mutual coupling for the fully aperture-shared dual-band antenna causes severe shielding effect on the HBA while no influence on the LBA.To alleviate the mutual coupling,a long-strip low-pass PRS is integrated on the substrate of the LBA.The long-strip low-pass PRS is partially reflective in the high band.A Fabry-Perot resonant cavity for the high band is created between the long-strip low-pass PRS and the reflector to deal with the shielding effect.Because the longstrip low-pass PRS is highly transmissive in the low band,it has little influence on the LBA.Experimental results show that the aperture-shared dual-band antenna has a bandwidth of 32.7%(690-960 MHz)for the low band and 45.4%(1.7-2.7 GHz)for the high band.Stable radiation patterns have been obtained with average gains of 7.8 d Bi and 8.6 d Bi for the low and high bands,respectively.3.An aperture-shared dual-band subarray based on two different low-pass PRSs is presented.The subarray is developed by adding two more HBAs to the aperture-shared dual-band antenna.The HBA and the LBA is either fully aperture-shared or partially overlapped in the subarray.The mutual coupling for the fully aperture-shared HBA and LBA is dealt with long-strip low-pass PRS.A ring-shaped low-pass PRS has been introduced above the HBA to alliveivate the mutual coupling between the partially overlapped LBA and HBA.By combining the long-strip low-pass PRS and the ring-shaped low-pass PRS into the subarray,the HBAs and a LBA are coaxially arrayed in different element spacing to form an aperture-shared dual-band array.The experimental results show that the subarray achieves impedance bandwidth(-|S11|/-|S22| >15 d B)of 690-960 MHz in the low band and 1.7-2.7 GHz in the high band.Stable radiation patterns and gains are also obtained over both bands. |
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