Research On Broadband Circularly Polarized Metasurface Antenna Based On The Theory Of Characteristic Mode

With the rapid development of wireless technology,antennas,as the essential devices for wireless communication,have been undergoing constant innovation and advancement in design technology.Among various antenna types,Metasurface antennas have drawn extensive attention from both industry and academia due to their advantages of low profile,easy fabrication,and low loss.Different from traditional active analysis methods,Characteristic mode analysis（CMA）provides an efficient and rapid approach to intuitively study the characteristic modes and radiation characteristics of radiating bodies without requiring loading of feed.This paper aims to investigate how to achieve flexible control of electromagnetic waves through Metasurface structure pattern design using characteristic mode analysis,thereby realizing the characteristics of wide bandwidth,low profile,and stable radiation direction for antennas.The specific research scheme is as follows:1、Based on the characteristic mode theory（CMT）,a truncated annular aperture-coupled Metasurface circularly polarized antenna is designed.The radiation structure of the antenna consists of a non-uniformly sized 4×4 Metasurface array,while the feeding network is comprised of a truncated annular slot and microstrip lines.The feeding network excites a pair of current modes that are mutually orthogonal,have consistent radiation characteristics,and exhibit broadband features.Additionally,it provides a 90-degree phase compensation for the two modes.As a result,the antenna achieves right-hand circularly polarized（RHCP）radiation directed towards the+z direction.The antenna has a bandwidth of 24.14%（6.2-7.9 GHz）and boasts a gain range of 7.7-9.5 d Bi.Additionally,it has an in-band radiation efficiency reaching up to 94%.Its operating frequency band belongs to the S band of ITU-R and possesses advantages such as low profile,high radiation efficiency,and broadband capability.It can be applied in various fields such as wireless local area networks,the Internet of Things,satellite communication,and radar systems.2、Based on the CMT,this section proposes a quad-mode operation method for loading capacitive cross-slot couples for circularly polarized（CP）Metasurface antennas in order to increase the operating bandwidth.This method involves four different modes,which are generated by two orthogonal characteristic modes provided by the radiating patch,with relatively high resonance frequencies,and two orthogonal magnetic modes provided by the feeding structure,with relatively low resonance frequencies.The four adjacent modes can form three pairs of mode combinations that generate CP radiation waves,effectively expanding the bandwidth of the CP antenna.Based on this method,we designed a quad-mode broadband CP Metasurface antenna with a coupler feed suitable for the Wi Fi 7 frequency band and processed and tested the simulated antenna model.The antenna fabricated has overall dimensions of 40mm×40mm×4mm（0.90λ₀×0.90λ₀×0.09λ₀,whereλ₀ is the wavelength at the center frequency of 6.8GHz）.According the measurement results,the S₁₁<-10d B bandwidth is 5.75-7.80GHz（30.26%）,and the axial ratio（AR）<3d B bandwidth is 5.75-7.80GHz（30.26%）.Within the working bandwidth,the antenna produces good left-hand CP radiation,with the gain varies from 5.7 to8.7d Biand achieving a radiation efficiency of over 85%.Simultaneously,the operating frequency completely covers the 6GHz frequency band（5925MHz-7125MHz）,where the gain and total radiation efficiency reaches 7d Bi and over 95%,respectively.Hence,this antenna is suitable for future Wi Fi 7 scenarios.