| Multi-beam antennas have more and more applications in current communication systems,and they have shown great promising for future mobile communication.As modern communication and radar systems have higher requirements for high-gain multi-beam antennas,the research on millimeter-wave multi-beam antennas has also made great progress.Th e metasurface is a two-dimensional form of metamaterials,which can effectively control the polarization and wavefront phase of electromagnetic waves.Compared with metamaterials,metasurfaces have the advantages of low profile,low loss,low cost,and eas y processing.The advantages make their applications to antennas widely concerned by scholars.This article mainly studies how to use metasurfaces to design multi-beam transmitarray antennas.First,a high-gain multi-beam folded transmitarray antenna is developed by using the matasurface to control the phase of the wavefront.The antenna consists of a main reflectarray,a Febry-Perot transmitarray and 7 source antennas（2×2 microstrip antenna array）.The reflectarray（RA）is composed of reflected units which has ability to realize 90°polarization conversion.The 7planar feed sources are embedded in the RA to improve the integration of the antenna system.Experimental results show that the multi-beam folded transmitarray antenna can generate 7 high-gain beams with different directions,and cover a scanning range of±21°.For the 0°beam,the sidelobe level in the E-plane is lower than-20d B at 12.5GHz,the one in the H plane is lower than-15d B,the peak gain is 22.4d Bi,and the aperture efficiency is 15.2%.Then the electromagnetic metasurface is used to change the electromagnetic wave（EM）polarization,according to the principle of linear-circular polarization conversion,a stacked transmitted unit with broadband circular polarization（CP）characteristics is designed.Base on this CP transmitted unit cell,a high-gain low-profile multi-beam CP transmitarray antenna is designed and experimented.Experimental results show that the profile of the antenna is only 1.8λ,which can generate 5 high-gain CP beams with different directions,covering a scanning of±10°.The common bandwidth of the axial ratio less than 3d B and S11<-10d B is from 12.1GHz to 13.7GHz.Within this range,the isolation between adjacent ports is less than-20d B,the peak gain is 22d Bi,and the aperture efficiency is 20.8%.Both the multi-feed excitation theory and folded solution can be used to reduce the height of the array antenna.In this paper,these two methods are combined innovatively to design a quad-feed high-gain folded transimtarray antenna with extremely low-profile based on metasurface unit.Four planar U-shaped slot microstrip patch antennas are designed as the feed sources.A 1:4wilkinson power splitter is designed to excite four sources at the same time.Therefore,the overall height of the antenna has not increased,and still maintaining its high integration performance.According to the experimental results:the quad-feed folded transmitarray antenna operates at 12.5GHz has a profile of 1.4λwhich is nearly 76.7%times lower than the 6λheight of the traditional single-feed transmitarray antenna.The bandwidth of S 11<-10d B is12-13.8GHz,the peak gain is 23.1d Bi,and the aperture efficiency is around11.7%. |
