Design Of Wideband Metasurface Antennas Based On Characteristic Mode Theory

As an important component of wireless communication system,antennas also should possess the broadband property.In the past decades,the microstrip patch antennas have developed substantially,resulting a great progress of entire antenna aera.However,due to the inherently narrow bandwidth,patch antennas have difficulty in further development.Recent years,metasurface(MTS)antennas(MAs),which directly employ the MTS as radiator,have made great progress.Apart from the characteristic of low profile,the MAs have more fruitful modes and wider mode bandwidth than patch antennas.Especially,after the theory of characteristic mode theory(TCM)is introduced to guide feeding structure design,MAs have experienced significant development in wideband and other applications.However,these antennas also have shortcomings.On the one hand,the feeding structures of these MAs bring in strong back radiation,complex configuration,multilayer structure and limited bandwidth.On the other hand,it is still not sufficient to exploit and apply the multi-modes of MAs to broaden bandwidth.Summarily,it is challenging but meaningful to investigate the methods of enhancing bandwidth of the MAs,based on TCM,in the ways of feeding design and multiple modes.Therefore,we execute the research in this thesis as follows:1.A rectangular dielectric resonator(DR)fed MA is proposed.Firstly,guided by TCM,the resonant frequency of desired mode of MTS as well as the type and location of wanted feeding source is predicted.Then,the single probe feeding structure is employed to excite theTE^x_?11 mode of DR,serving as a magnetic current source(MCS)to excite the dominant mode of MTS(J₁).This simple feeding method avoids etching slot on ground plane which is a type of MCS,thus reducing the back radiation.Finally,by optimizing structure parameters and adjusting the locations of J₁ andTE^x_?11 mode,a wide working bandwidth with two modes operation is obtained.The simulated results show that our antenna achieves a 30%impedance bandwidth for|S₁₁|<-10 d B(4.67?6.30 GHz)and better than 15 d B front-to-back ratio(FBR)within the working band.2.A differential DR fed wideband MA is presented.Initially,attributed to single probe feeding structure of the pervious design,the asymmetric radiation pattern can be observed in the high frequencies.Thus,we carry out the differential feeding scheme,improving the asymmetry.Secondly,by analyzing the currents distribution under J₉ mode of MTS,we extract the array factor of the currents to analyze the side lobes of radiation patterns under J₉ mode.Thus,we propose a series of arrangements consisting of loading transverse open-ended slots,shortening the periodicity of MTS patches and loading longitudinal slots,gradually restoring the distorted radiation patterns and improving the in-band gain flatness.Finally,by introducing differential feeding and stepped impedance matching network,the J₉ mode of MTS can be added into band,resulting in a substantially enhanced working bandwidth.The simulated results agree well with the measured ones.Our antenna realizes51.4%impedance bandwidth for|S₁₁|<-10 d B(4.38?7.41 GHz),15 d B front-to-back ratio and 5?9 d Bi boresight gain within the working band.3.A single-layer MA fed by half square loop(HSL)is proposed.At the beginning,the desired mode of MTS and suitable feeding source are predicted by using source-free characteristic mode analysis(CMA).After that,the single-layer substrate integrated HSL is presented as the feedings source of MTS.Besides,the feasibility of this feeding method is verified by CMA.Furthermore,with aid of smith charts,the anti-pads are gradually loaded on the terminal and joints of the HSL,leading to a significantly improved impedance.Finally,under the loading effect of MTS,the well matching is obtained while a wideband of two modes operation(the resonant modes of the HSL and MTS)is achieved.The simulated and measured results indicate that a 22.6%impedance bandwidth for|S₁₁|<-10d B(4.7?5.9 GHz)and better than 17 d B front-to-back ratio within the working band are obtained.Meanwhile,the proposed antenna also has the advantages of single-layer structure and low profile.