Study On The Miniaturization And Optimization Of Frequency Selective Surfaces

AFrequency Selective Surface (FSS) is basically composed of identical patch oraperture elements periodically arranged in two-dimension infinite array. The uniquespatial filtering characteristic of FSS to incident electromagnetic waves with differentpolarizations and angles of incidence activates its extensive use in electromagneticdomain. The study of this dissertation is concentrated on the realization of sizereduction and profile decrease for FSSs, and the application of Differential Evolutionin the optimization of FSSs. Investigations on the miniaturization and optimization ofFSSs have been carried out, and the main contributions of this thesis can besummarized as follows:(1) Started with the investigations on several FSSs with basic shapes, the effectsof parameters variations and changes of array configuration on the performances ofthese FSSs are analyzed. The transmission performances of some single-screenaperture FSSs with convoluted and interdigitated elements are studied, and theeffects of parameters variations on the resonant frequency and bandwidth of theseFSSs are also analyzed, with an emphasis on the number of convolution orinterdigitation. Asingle-screen aperture FSS with interdigitated haxagnal unit cells isdesigned, with its periodicity smaller than λ/30(where λ represents the wavelength infree space at resonance), a fractional bandwidth of63.4%for-3dB bandwidth atnormal incidence, and a shift of frequency less than1%for the incident angleranging from0to60degrees.(2) Comparision study has been carried out between two approaches of sizereduction of FSSs, i.e. element convolution and interdigitation, with respet to theextent of miniaturization and bandwidth. A conclusion has been drawn that theinterdigitated FSSs possess inherently the higher extent of miniaturization and widerbandwidth compared with the convoluted ones, and the physical interpretation ofimprovement has been presented from the viewpoint of equivalent magnetic currentswithin the slots. Prototypes of double-layered aperture FSSs with convoluted andinterdigitated hexagonal loops were fabricated. The transmission coefficients weretested in microwave chamber, and the measured results manifested the conclusionaforementioned.(3) Based on the design method of FSS composed of antenna-filter-antennaarrays (AFA-FSS), three AFA-FSSs have been designed, which exhibit transmission response of the second-order, the third-order and the third-order with transmission azero. Effects of parameter variation on the transmission reponse have beenconsidered. The AFA-FSS of the second order is selected as the building block todesign high-order AFA-FSSs with high roll-off characterisitic and low profile bydirectly cascading the modules.(4) The fractal technique is introduced into the design of AFA-FSS so as toachieve size reduction for the conventional AFA-FSS. The influences of iterationorder and iteration factors on the transmission performance are investigated. AfractalAFA-FSS of second order is designed, which adopts the Minkowski island andfractal cross as the patches and coupling slot, respectively. This proposed AFA-FSSexhibits a considerable size reduction and yields a two-pole frequency response inthe pass-band, while keeping a low profile. A prototype of second-order AFA-FSS isfabricated and measured using waveguide testing system, and the well agreementbetween measured and simulated results validate the effectness of design approach.(5) The spectral domain approach (SDA), which can be used for the analysis ofinfinite sigle-layered and multi-layered FSSs based on multi-layered isotropic media,is deduced. On the basis of Floquet theorem, the calculation region is restrictedwithin only one unit cell, and then the Fourier transformation is adopted to build theelectric (or magnetic) field integral equation (EFIE or MFIE) in the spectral domain.The EFIE (or MFIE) is solved by using the method of moments so that the unkownelecreic current on the pathes (or the fictious magnetic current within the apertures)can be computed, where the spectral dyadic Green’s function is obtained by means ofthe spectral domain immitance approach and the unkown electric (or magnetic)current is as the combination of the roof-top basis function. The omputed electric (ormagnetic) current yields the scattering fields in space and finally leads to thecalculation of transmission or reflection coefficients.(6) The differential evolution (DE) algorithm is combined with the SDA tooptimize FSSs’ performance. A sigle-layered band-stop FSS with sinlge square loopelement is designed by optimizing the geometry parameters via DE and SDA. Asingle-layered tri-band FSS with doube square loops is further designed by means ofDE optimization. At last, the Boolean DE coupled with SDA is utilized to synthesizea tri-band FSS with unit cell of arbitrary shape.