| With the development of wireless communication technology,more and more complicated tasks should be dealt with in the communication systems thus which should have large volume,high rate,low loss,etc.As the important components in the systems,frequency selective surfaces?FSSs?and linear to circular polarization converters?LCPCs?can protect the systems from interference of the out-of-band signal and alter the polarization of the in-band signal for different applications.Both two types of periodic surfaces,have been rapidly developed since presented.Many different types of them,especially based on the miniaturized elements which have the advantages of high incident angle stability and good suppression of grating lobes,are widely paid attention to by researchers.Several high-performance FSSs and LCPCs with dual-band,wideband and wide-angle capabilities,based on the miniaturized structures,are investigated by using full-wave simulation method and equivalent circuit model method in this dissertation.The main contents are listed as follows:1)As a support substrate for the periodic surfaces,electrical parameters of a printed circuit board?PCB?affect the performances of the periodic surfaces directly,which is very significant to be measured accurately.To address the narrow band and low efficiency for PCB characterization based on a conventional resonant cavity,broadband PCB characterization using a multimode substrate integrated waveguide?SIW?resonator is proposed in this dissertation.The proposed method is that the SIW cavity is fed by two closed rectangular waveguides through two coupling slots positioned at top and bottom metallic layers of the cavity.A series of TE10k resonant modes with large mode indexes can be excited sequentially in the cavity.This design is able to perform an effective control of the frequency interval??f?between two neighboring modes.Besides,?f approaches to be a constant with a large mode index k.For a given operation band,the detailed design process is introduced to synthesize the dimensions of the resonator and the feeding structure for the purpose of testing dielectric materials at small and approximately uniform frequency intervals.As an example,the Taconic TLY-5 substrate is firstly measured at Ka-band and two types of samples with different thicknesses are tested to calibrate the conductor loss.The measured results have a good agreement with those by using the microstrip resonant loop,validating the accuracy of this method.After that,this SIW multimode measurement is employed to accurately test the same substrates over the whole W-band with approximately uniform frequency intervals.2)The FSSs based on the SIW cavities have the advantages of low insertion loss,flat passband,high sideband selectivity etc.,but show low incident angle stability due to large element spacing.To address the problem,a dual-band and wide-angle FSS with miniaturized elements based on a quarter-mode SIW?QMSIW?cavity is proposed in this dissertation.The periodic element includes the QMSIW cavity and two“L”-type slots etched on the top and bottom copper claddings of the cavity.The slot not only performs as the equivalent magnetic wall of the QMSIW cavity,but also provides a transmission pole near the resonant frequency of the fundamental mode in the QMSIW cavity to improve the passband characteristics of the FSS.Compared to the conventional full-mode SIW FSS,the element size can be reduced by 65%and the frequency interval between the passband and the first parasitic passband can increase by2.7 times at the same operation frequency while keeping a similar performance.Finally,an X-band QMSIW FSS is designed,fabricated and measured.As a result,it is insensitive to the incident angle and polarization of the incident wave.Such a miniaturized FSS element can also be employed in the design of a multiband FSS with controllable transmission zeroes.3)To address the narrow band of the SIW FSS caused by the mutual coupling between the cavity and slot resonant modes,a wideband and wide-angle FSS based on QMSIW cavities and loop slots is proposed in this dissertation.The periodic element consists of a QMSIW cavity surrounded by two identical loop slots on both sides of the cavity.Two resonances and one transmission zero are caused by the fundamental mode of the QMSIW cavity,the slot mode and the coupling between them,respectively.By precisely tuning the dual-mode configuration,the proposed FSS shows a wide passband with a good transmission efficiency and a rapid roll-off at the high side of the operation band due to the transmission zero.With the miniaturized elements,the FSS is also insensitive to a large angle of incidence.Thanks to the novel structure,several higher-order modes close to the target band are suppressed,making the parasitic passband far away from both sides of the designed band.To verify the advantages,a prototype operated at 10GHz is fabricated and tested.The measured results show a wide passband performance with the 1/3dB relative bandwidths of 34%/49%for normal incidence,which agree well with the simulated results.Besides,the 3dB bandwidth still remains 23%for the incident angle of 60°.4)The conventional dual-band linear-to-circular polarization converter?LCPC?based on resonant structures of split loops,antenna-filter-antenna shows narrow band due to strong coupling between orthogonal structures.A complex center-connected element with high isolation is used to address the problem.In this case,a dual-band and wideband LCPC based on a single-layer dielectric substrate is proposed in this dissertation.The element of the converter consists of two metallic surfaces separated by the substrate.The two metallic layers are identical,with a combination of a connected Jerusalem cross and an“I”-type dipole for each layer.The proposed converter is designed by using an equivalent circuit model.Left-handed circularly-polarized?LHCP?and right-handed circularly-polarized?RHCP?beams can be respectively generated at K-and Ka-band excited by a linearly-polarized?LP?wave tilted 45°relative to the x and y directions of the converter.Besides,the converter covers two operation bands of K-/Ka-band satellite communications with high conversion efficiency and high polarization extinction ratio?PER?.The measured results show a good agreement with the simulated ones.The measured relative 3dB AR bandwidths are 29%and 12%in dual bands for normal incidence.Besides,the measured AR remains stable in the lower operation band and a slight fluctuation in the higher band for the incident angle of 20°.5)To address the low incident angle stability for the dual-band and wideband LCPC due to the large element size,a dual-band,wideband and wide-angle LCPC based on miniaturized elements is proposed in this dissertation by placing different polarization structures on different metallic layers.The periodic element consists of three metallic layers separated by two substrates.The top metallic layer contains two metal strips and an“I”-type strip with four stubs for y-polarization,which is identical with the bottom metallic layer.The middle metallic layer comprises an infinitely long metal strip and two“I”-type strips for x-polarization.The LCPC performs a wideband response in both operation bands due to wide passbands caused by the orthogonal“I”-type strips.By virtue of orthogonal structures placed at different layers,the element size and the cross-polarization level are decreased by 60%and 10dB respectively,compared to the previous work.Based on the miniaturized elements,the LCPC shows high incident angle stability.To verify the proposed design,a K-/Ka-band prototype is fabricated and measured.The simulated/measured AR remains below 3dB over the frequency range of 15-20.8GHz?32.5%?/15-21.2GHz?34.3%?for the lower band and26.9-30.2GHz?11.6%?/27.4-29.8GHz?8.4%?for the higher band under normal incidence,respectively.The measured AR still possesses the 3dB bandwidth of30%/11.7%in both bands for the incident angle of 45°.6)To address the problem of how to achieve both wide band and high incident angle stability for the single-band LCPC,a wideband and wide-angle LCPC based on a single-layer dielectric substrate is proposed in this dissertation by using high polarization isolation structures of subwavelength center-connected elements and narrow strips.The converter element consists of a metal cross strip backed by a strip horizontally and centrically located on the other metallic layer.The vertical arm of the cross strip across the whole surface performs as a high-pass filter,generating a wide passband above a low cut-off frequency.Meanwhile,the horizontal strip resonates at a high out-of-band frequency and offers a wideband response below the resonant frequency.Another two short strips vertically placed on both sides of the crosses increase the phase shift caused by the crosses and improve both the passbands.Using equivalent circuit models and ANSOFT HFSS,an example of LCPC is designed with the overall element size of only 0.11?0×0.21?0 for a 90°phase difference and wide-angle stability over the wide operation band.The prototype shows the simulated/measured axial ratios?AR?remain below 3dB over the bandwidth of 69%/74%for a normal incidence wave and 54%for the oblique incident angle of 55°,respectively.With the insertion loss of less than 3 and 2dB,the measured 3dB AR bandwidth keeps 70%and55%,respectively. |
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