Ground mode control for compact circular polarization antenna and noise suppression

Abstract

This dissertation is a study on a ground radiation antenna and ground mode control for enhancement of the antenna performance. The ground radiation antenna is receiving attention for utilizing the radiation of the ground body to achieve the miniaturization of the antenna size. Practical mobile antennas are different from the classical theory that assuming the infinite ground, and the limited dimension of the ground substantially affects the radiation performance. The radiation of the ground becomes dominant as the electrical size of the ground is reduced to a half-wavelength. The inverted F antenna (IFA) and quarter-wavelength monopole antenna can be modeled as the radiation of the ground induced by a current source located on the maximum electric field of the ground mode. The ground radiation antenna facilitates an equivalent magnetic source located on the maximum current of the ground mode. The loop structure consists of a radiating loop that controls the resonant frequency and a feeding loop that controls the input impedance. The principle of ground radiation antenna is analyzed based on the theory of characteristic modes (TCM). The structure of rectangular shape exhibits wide bandwidth and high radiation performance because most of the characteristic modes of it have a very low quality factor. The equivalent circuit of the ground radiation antenna can be modeled as a combination of two loop-type resonances (a feeding loop and a radiating loop) and a ground mode. The feeding loop is designed for impedance matching while the radiating loop is for the resonant frequency control. The input impedance of the ground radiation antenna is interpreted using two-port network analysis. The ground mode control technique is proposed to further improve the radiation performance of the ground body. When the electrical size of the ground is about a quarter-wavelength, the lowest characteristic mode of the ground resonates about twice higher than the operating frequency. In this case, it is impossible to utilize the ground mode due to a large difference between the operating frequency and the resonant frequency of the ground mode. The ground mode control solves this problem by efficiently lowering the resonant frequency of ground mode. The ground mode control is realized by two different structures, a closed-loop and a T-strip conductor. The Chapters 3, 4, and 5 are advanced researches for each topics of mobile antenns, and they commonly based on ground radiation technique and ground mode control. In Chapter 3, a single-fed circularly polarized ground radiation antenna is proposed for 2.4 GHz mobile applications. The ground mode control is realized by closed-loop structures, and the TCM analysis is presented to interpret operation mechanism. The proposed design enables to control the degenerated modes of ground independently, so the optimization of axial ratio bandwidth is achieved. Moreover, very compact sizes of both ground and antenna element demonstartes superiority compared to other referenced researches. In chapter 4, a single-fed circularly polarized slot antenna is proposed for mobile GPS applications. For the validation of ground mode control, reference antenna is also designed and experimented as well as the proposed antenna. As a result, ground mode control achieved 2.5 times wider axial ratio bandwidth than no ground mode control case. The optimization process of design parameters for the proposed antenna is also given in the context. In Chapter 5, the antenna sensitivity is analyzed depending on the configurations of the shield can. The shield can is intended for improving the antenna sensitivity in many practical cases, it should be noted that discontinuous soldering of the shield can even degrade the antenna sensitivity. The shield can is analyzed as a modified ground structure, and spurious resonance induced by the shield degraded the antenna sensitivity.