Design Of Broadband Periodic Absorbing Structure Based On Electromagnetic Resonances

The aim of the radar absorbing material, a class of materials used to effectivelyabsorb incident electromagnetic waves, is to obtain the highest absorption within thewidest operating waveband in the least thickness. However, any traditional absorbingmaterial with a certain thickness operates only in a limited frequency band. Thetraditional methods to expand the absorption bandwidth, such as increasing materialpermeability and carrying out multilayer design, have no longer met the challenge ofmilitary and civil application. As a result, to expand the absorption bandwidth andimprove the absorption at low frequency becomes fatal problems in the field ofelectromagnetic wave absorption.Different from traditional radar absorbing material, periodic absorbing structures(PAS) utilize the ability of controlling incident electromagnetic fields to introduce highabsorption. Due to the special ability, PAS are promising broadband absorbing material.In this thesis, the design of PAS consisting of resistive film, magnetic coating materialor magnetic film is studied. Based on the electromagnetic field and surface currentdistribution, an equivalent circuit model is built to analyze the relationships ofabsorption, equivalent circuit parameters and structure parameters. The aim is to expandabsorption bandwidth, especially the absorption bandwidth in the low frequency band.1. According to the equivalent circuit theory, constraint equations of broadbandabsorption are built, and defects of PAS with common pattern are presented.(1) Based on the equivalent circuit theory, the constraint equations of broadbandabsorption are obtained by the inverse calculation method. The equations reveal therelationship between absorption and equivalent parameters. The equations also calculatethe ideal equivalent resistance for high absorption.(2) By analyzing the absorption characteristics of resistive-type periodic absorbingstructures with rectangle, square and square ring pattern, defects of sole resonantabsorption in low frequency band are found. The reason turns out to be thenon-dispersive surface current distribution.2. According to the constraint equations, multi-resonances are designed to improvethe absorption of resistive-type PAS in low frequency band.(1) A resistive-type PAS with trapezoids-coupling pattern is proposed to deal with the constraint condition between absorption and equivalent parameters. Theoreticalanalysis, mathematical calculation and experiment show that the couplings between andwithin unit cells realize the dispersive distribution of surface current to excite doubleresonances in the structure. The designed structure effectively improves absorption inlow frequency band.(2) A resistive-type PAS with non-homogeneous sheet resistances is proposed todeal with the constraint condition of equivalent resistance. The structure excites threeresonances by introducing relative near-field interaction and relative far-field interaction.These resonances effectively expand the absorption bandwidth in low frequency band.3. By loading periodic structures, multi-resonances are designed to improve theabsorption of magnetic-type PAS in low frequency band.(1) Through partly adjusting resonant characteristics in the magnetic coatingmaterial of dispersive electromagnetic parameters, magnetic-type PAS with needle-pointpattern is proposed to realize double-resonance absorption in low frequency band.Different from traditional non-magnetic PAS, the magnetic-type PAS achievesbroadband absorption by both dielectric and magnetic loss.(2) Through exciting dispersive electromagnetic responses, metal array which isembedded in the magnetic layer can further expand the absorption bandwidth. The arrayimprove the usual interference absorption in high frequency band while introducedimensional resonant absorption in low frequency band.3. Some unique PASs consisting of metal magnetic film arrays are innovativelyproposed.(1) Combining the PAS design theory and the electromagnetic characteristic ofmagnetic film, magnetic-field-controlling structure is proposed. It is found that themagnetic field is re-oriented by the film array, and then absorbed through the highpermeability of the film. The design solves the impedance-mismatching problemcausing by the high conductivity.(2) Combining the different electromagnetic characteristic and re-oriented effect ofdifferent films, a multi-resonance structure is proposed. In the design, theelectromagnetic responses can be determined by the conductivity. The film of highconductivity re-orients the direction of magnetic field while the film of low conductivity.The design provides new method for design of multiband absorber.