Theory Of Frequency Selective Metasurfaces And Its Application In Aperture Imaging System

Electromagnetic metasurfaces can modulate the frequency,polarization,amplitude and phase of electromagnetic waves.A variety of metasurfaces play important roles in different application fields.This dissertation emphatically conducts research on frequency selective metasurfaces.Specifically,we discuss frequency selective surface which modulates filter characteristics of electromagnetic waves and frequency diverse metasurface which regulates radiation feature of electromagnetic waves.Frequency selective surface is a kind of spatial filter which is widely applied for the design of radar radome.It becomes an indispensable assembly unit of radar.With rapid development of communication and radar systems,multifunctional frequency selective radomes with broadband,frequency suppression and harmonic suppression features are becoming more and more popular.However,how to design frequency selective surfaces with multi specific properties efficiently and quickly is still a hot area of research.On the basis of analytical method and researches of frequency selective surface,this dissertation proposed a kind of multi-resonant equivalent circuit model for arbitrary frequency selective structures.The multi-resonant circuit model could be used to accurately analyze frequency selective surface in a wide range of frequency band.On this basis,the circuit model is utilized to design a transplantable frequency selective element for high-order harmonic suppression which could be loaded on arbitrary band pass frequency selective spatial filters to suppress the harmonic resonance.Moreover,this dissertation utilizes the transmit array antenna which is developed from the multi-layer frequency selective surface to design a near-field non-diffracting transmitting Bessel vortex beam.The Bessel vortex beam launcher could be applied to near-field non-diffracting vortex electromagnetic communication systems in the future.Recently,metamaterial aperture antenna which is composed of frequency diverse metasurfaces is widely applied to microwave imaging field.On account of the weak radiation feature of metamaterial aperture antenna(MAA),this dissertation proposes a new method to improve the radiation efficiency of metamaterial aperture antenna.And a co-aperture antenna system which integrates Ka band metamaterial aperture antenna for imaging in the near-field distance with X band one dimensional beam scanning phased array antenna for detection in the far-field distance.This co-aperture antenna system could be used for ambient intelligence in the future.The specific research content of this dissertation includes the following aspects:1.Design and application of equivalent circuit for frequency selective surfaces.This dissertation proposes a kind of multi-resonant band pass and band stop LC circuit model to extract the equivalent circuit of arbitrary frequency selective structures.Firstly,the frequency selective surfaces are divided into two categories: band pass type and band stop type and the respective equivalent LC circuit models are established which could be used to accurately analyze the transmission feature of the FSS.Moreover,multi-layer FSSs can be designed and analyzed by cascading the single-layer equivalent circuit.Simulated and measured results show that multi-resonant circuit model could be used to efficiently design the low-profile broadband frequency selective surface with out-of-band rejections.2.Design of a transplantable frequency selective surface for high-order harmonic suppression.On the basis of multi-resonant LC equivalent circuit model,a transplantable frequency selective radome for high-order harmonic suppression is designed.The transplantable frequency selective surface could be added onto arbitrary band pass FSSs without deteriorating the pass band feature of original band pass FSSs.This design makes the integration of harmonic suppression FSS with traditional band pass FSS much easier.3.Applications of transmit array antenna in generating orbital angular momentum(OAM)vortex electromagnetic wave and high-order Bessel vortex beam.Transmit array antenna is developed from multi-layer frequency selective surface.By modulating the wave front phase of the feed,transmit array antenna could be utilized to generate arbitrary beams.Orbital angular momentum vortex electromagnetic wave has attracted a lot of attentions recently,which could be used for high-capacity communication systems in the next generation.However,OAM vortex beam owns severe diffraction problems while propagating in the space.This makes the design of receiving antenna for communication system difficult.In this context,amplitude and phase modulated transmit array antenna is utilized to generate the non-diffracting transmitting Bessel vortex beam in the near-field distance.The Bessel vortex beam launcher could be applied to near-field communication systems of non-diffracting vortex beam.4.Improvement of radiation efficiency for metamaterial aperture antenna,and a co-aperture antenna system which integrate the MAA with X band one dimensional beam scanning phased array antenna.Metamaterial aperture imaging(MAI)system is firstly proposed by the researchers at Duke University.Firstly,the MAA which is composed of frequency diverse metasurfaces generates frequency agile radiation patterns,and the target scene is scanned by frequency.Finally,the scene is reconstructed using theory of compressive sensing.This dissertation proposed a new method to improve the radiation efficiency of the MAA by introducing proportional unit cells which own high radiation efficiency in the antenna.The simulated and measured results show that the new method could efficiently improve the radiation efficiency of MAA.Since the metamaterial aperture imaging system is limited to near-field applications,this dissertation proposed a co-aperture antenna system which integrates Ka band metamaterial aperture antenna for imaging in the near-field distance with X band one dimensional beam scanning phased array antenna for detection in the far-field distance.The co-aperture antenna system could be used for ambient intelligence in the future.