Research On Selected Topics Of Antenna And Inverse Scattering Problem

The inverse problem of electromagnetic has always been a significant issue in the electro-magnetic theory and engineering. In this dissertation, two kinds of classic inverse problems of electromagnetic, including the synthesis of antenna, especially for the research of beam forming synthesis for circularly symmetric conformal aperture, and the inverse scattering problems, have been studied.Based on the relationship the source and the field of radiator, a novel and a compact co-planar waveguide fed single-layer printed antenna with an ultra-wide band (UWB) rectangular monopole radiator etched with a half elliptical slot is firstly presented. Collaborating with the UWB radiator, two symmetrical open-circuit stubs are extended from the ground plane to jointly achieve an ultra-wide band impedance match with a compact size. Rectangular monopole radiator etched with a half elliptical slot and two symmetrical open-circuit stubs are resonant at high and low frequency, respectively. And we can estimate the size of two radiators by the current distribution at resonant frequencies. Then by using the CST microwave studio and performing a full wave simulation, the final optimized UWB antenna could be obtained. The proposed design is clear, simple and avoided the disorderly and tedious optimization process. The proposed antenna is fabricated and tested in an anechoic chamber, showing an ultra-wide operating frequency range from3.7GHz to10.1GHz. Simulations show that the proposed antenna has a satisfactory IR performance, implying that it can be used in a wide range of portable and mobile UWB applications.Secondly, the beam forming synthesis of antenna arrays for circularly symmetric aperture is investigated in detail. We have mathematically presented theoretical analysis of the radiation formula in order to obtain a rotationally symmetric radiation pattern for both planar aperture and conformal aperture. Based on the analysis of radiation for the circular aperture, a beam forming approach capable of synthesizing versatile, pre-specified beam shapes using concentric excitations satisfying multiple weighted sine or Bessel function distribution is proposed. The proposed ap-proach, based on the continuous aperture, utilizes the multiple weighted sine or Bessel function distribution to synthesize various radiation beams. After an optimal continuous distribution has been achieved, we can implement the physical aperture and fulfill the antenna arrays with con-centric excitations by a discretisation approach. Besides, we have also extended to the circularly symmetric conformal aperture including five kinds of conformal aperture. And various shaped beams, pencil, flat-topped and bimodal beam with uniform distribution can achieved easily by par-tial simple adjustment of some geometric parameters of the aperture. Moreover, we can could further adjust by the different aperture distribution only with two kinds of amplitude on hybrid circularly symmetrical conformal aperture. Therefore, avoiding the difficulty of the expression-s of the far-filed of the conformal arrays in mathematics and increasing flexibility and freedom of the design, the proposed approach can be utilized fast beam forming for circularly symmetric conformal antenna arrays, especially for large conformal arrays.Finally, we combine two techniques together, the fast Fourier transform-twofold SOM (FFTT-SOM) and the multiplicative regularization (MR) to solve inverse scattering problems. In FFT-TSOM, only the most influential bases are used and when applying the MR to the objective function in FFTTSOM, the new method is referred as MR-FFT-TSOM. In MR-FFT-TSOM, a new stable and effective strategy of the regularization has been proposed. MR-FFT-TSOM not only inherits the advantages of the FFT-TSOM, i.e., lower computational complexity than the TSOM, better stability of the inversion procedure and better robustness against noise compared to the SOM, but also the edge-preserving ability from the MR. Besides a more relaxed condition of choosing the number of current bases being used in the optimization can be obtained. Numerical simulations including both inversion of synthetic data and experimental data from Fresnel data set validate the efficacy of the proposed algorithm.