Ground Radiation Techniques for Electrically Small Antennas

Abstract

This dissertation introduced novel ground radiation techniques for electrically small antennas. In ground radiation techniques, small antennas operate as coupling elements to the larger platforms, utilizing the platforms for far-field radiation, so that these antenna elements can then be considered as electrically large ones, and their radiation performances are determined by the mutual coupling with the platform and by the characteristics of the platform. To investigate this technique, the antenna impedance calculated from the combination of the antenna element and the ground plane (platform) is discussed based on theory of characteristic mode as well as the two-port network analysis. It has been demonstrated that the radiation resistance is attributed to the summation of the mutual impedance between the antenna element and each radiating characteristic mode in the ground plane, while the coupling with all the non-radiating modes contributes to the reactive components. More specifically, the total current distributions over the ground plane, which is generated by exciting all the radiating ground modes, produce the far-field radiation, while the ones over the antenna element only contribute to near-field radiation. Accordingly, antenna performance can be optimized by controlling the mutual coupling between the antenna element and the radiating ground modes, and by manipulating the impedance characteristics of the ground modes. Next, this dissertation introduced two examples to show how ground radiation techniques are utilized to design antennas with circular polarization (CP) performance, and the objective lies in accomplishing CP antennas at an arbitrary location of an arbitrary ground plane. In the first study case, a single antenna element utilizing both electric coupling and magnetic coupling is introduced to satisfy the magnitude conditions, while ground-mode tuning (GMT) is adopted to meet the phase conditions. In the second study case, the antenna is composed of two independent elements, where magnitude conditions are determined by their mutual coupling, and the phase conditions can be guaranteed by the resonance difference between the two elements. Moreover, multiple-input multiple-output (MIMO) antennas are also investigated for antennas utilizing ground radiation techniques. A novel mode-based decoupling technique that is especially suitable when antennas are utilizing the same ground mode is investigated by considering the ground mode effect, so that additional decoupling structures not only can provide high isolation between antennas but also can maintain the high radiation performance of the antennas. In addition, CP MIMO antennas are also investigated, which presents polarization diversity and high isolation performances. It can be concluded that ground radiation techniques are also interesting candidates for MIMO applications with novel characteristics in achieving decoupling and diversity.