Design and Analysis of Multiband and Dual-Band Dual-Polarized Antennas using Characteristic Mode Analysis

Abstract

Miniaturized communication systems require single antenna element that covers frequency bands for different applications and also offers dual-polarizations, maximum broadside and similar radiations. Therefore, dual and multiband antennas offering multiple functionalities are vital to meet the requirements of miniaturization in modern communication systems. Theory of characteristic modes provides a convenient way to resolve complicated problems of modern antenna systems by providing complete information about radiating modes of a structure in the range of frequencies under consideration. It is an excellent antenna analysis tool to identify radiating modes inherent to the structure, and aids designer to tune antenna response according to the required application. In this thesis, two antenna design problems are addressed, by proposing systematic antenna design procedures using characteristic mode analysis (CMA). In the first design, a method is proposed to design dual-band dual-polarized antenna system having low frequency ratio, similar broadside radiations and linear-orthogonal polarization. While, in the second design, a systematic procedure to design multiband antenna with usable radiations in all bands is proposed. In the first design procedure, fundamental degenerate modes (DMs) of patch are utilized to design dual-band dual-polarized antenna. Fundamental degenerate modes have similar broadside radiations, orthogonal-polarizations but they are resonant at the same frequency and cannot be used for the dual-band operation. A systematic way of slotting is introduced, which exploits the current symmetries of fundamental degenerate modes of patch antenna to achieve dual-band dual-polarized antenna operation. An equilateral triangular slot is inserted along one of the edges to separate resonant frequencies of degenerate modes and simultaneously exciting them at different resonant frequencies. Location of the slot is chosen according to the current symmetries of the horizontal mode to elongate its current paths and moving it to a lower resonant frequency, while keeping vertical mode at its fundamental resonant frequency. Mathematical formulation for the proposed slot is also provided, and it is shown that the frequency tuning provided by the mathematical model is in close correspondence to the tuning provided by the CMA. Prototype of proposed single probe-fed antenna is fabricated and tested. The designed antenna exhibits low frequency ratio and broadside radiation patterns with gains of 4.96dBi and 6.10dBi in the first and second band respectively. Low cross-polarization levels and linear-orthogonal-polarization are achieved in both bands. Proposed technique is also applied to the square geometry, and it is shown that the model formulated for the modified square shape is following similar trend in terms of frequency ratio, radiations, current symmetries and orthogonal-polarization. Moreover, degenerate modes of Partial Koch fractal, octagon and a modified-octagon are separated, showing that the proposed technique is also applicable to the irregular shaped geometries. The second design procedure is based on the excitation of lower order modes in the resonant frequencies of the higher order modes to provide enhanced antenna radiations. A procedure is proposed to design multiband patch antenna that provides radiations without nulls in all bands of interest. Design procedure is used to identify dominant modes, lower order modes and the modes with the nulls in radiations. After identification of the dominant modes with nulls, superposition of the currents of lower order modes and higher order mode is carried out to provide current perturbation for null removal. Radiating capability of the lower order modes within band v is enhanced, and their superposition with the dominant modes is carried out to provide enhanced total antenna radiations in the broadside direction. Radiating capability of the lower order modes beyond their resonant frequencies is increased by incorporating an air gap between the patch and ground plane. Using the proposed technique, a multiband Partial Koch fractal antenna with desired radiations in all bands is designed by removing nulls in the radiations in the second and the third band. An improvement of 10dBi in the broadside direction of antenna in the second band and 20dBi in the third band is achieved after superposition of two modes in the second band and three modes in the third band. The proposed technique is applied to a square patch for dual-band antenna design with desirable radiations in both bands. Using superposition of two modes in the second band, enhancement of 18dBi is achieved in the antenna broadside radiations.