Ground Radiation Antenna Using Loop-Type Coupling Structures

Abstract

The main purpose of this thesis is to show how the ground plane of mobile devices can be optimally coupled through loop-type coupling structures such that it can efficiently radiate for wireless communications. This is an innovative radiation technique and is named "Ground Radiation Antenna". Unlike the conventional mobile antennas, such as a quarter wavelength planar inverted-F antenna (PIFA) and half wavelength or one wavelength loop antennas, the ground radiation antenna is formed by a very small ground clearance with some lumped capacitors, which can be thought of as a non-radiating coupling structure. Through several examples verified by the simulation and experimental results, it will be demonstrated that the ground radiation antenna produces an even better radiation performance than the conventional mobile antennas, even though the coupling structure is electrically small, the size of which is typically less than about one-tenth of a wavelength. The ground radiation antenna is analyzed based on three resonant modes: feed structure resonant mode, antenna (coupling structure) resonant mode, and ground resonant mode. The ground resonant modes, also known as the characteristic modes, only depend on the shape and size of the ground plane and have significant contribution on radiation behavior. The antenna resonant mode is a loop-shaped current mode formed in the ground plane and plays two important roles in exciting the ground resonant mode and controlling the frequency of the combined resonant modes of the antenna and ground plane. The feed structure resonant mode is used for impedance matching mechanism, which controls the input impedance of the antenna. Good radiation performance in terms of wide impedance bandwidth and high radiation efficiency can be achieved by enhancing the coupling level between the antenna and the ground plane, which is determined by the ground plane size, antenna size, and the antenna position in the ground plane. Thus, the main goal of this thesis is to show how the coupling level can be maximized by designing an optimal coupling structure with a compact size and how the ground resonant mode can be selectively excited by using an active approach to the coupling structure. This will be useful to achieve a desired radiation performance for mobile device industry. In addition, since the ground radiation antenna produces the loop-type current modes formed in the ground plane, the radiation performance can significantly deteriorate in a special case. If a liquid crystal display (LCD) or other conducting plane, which is usually installed beneath the ground plane in a real mobile terminal, fully overlaps the antenna area, the magnetic flux produced by the current loops in the ground plane can then be decreased due to the anti-phase currents induced in the conducting planes, reducing the antenna bandwidth and efficiency. This issue is also analyzed and several solutions are presented in this thesis.