Mobile Antenna Design by Ground Impedance Decomposition

Abstract

Recently, the market size of the Internet of Things(IoT) is rapidly increasing, and the demand for mobile devices is also increasing rapidly. IoT refers to physical devices that are all connected to a network and that can exchange information between each other. High performance antennas are essential for connecting IoT devices to one another, which is expected to provide advanced connectivity. Mobile devices have many components for performing various functions, so the area allocated for the antenna is reduced. In general, as the antenna size smaller, the antenna radiation performance is worsened due to the high quality factor. To overcome this problem, the ground should be utilized as a radiator. The ground plane, which is the largest conductor in the mobile devices, has very low quality factor, thus it has wide bandwidth and high radiation resistance. For high radiation performance, it is important to optimize the characteristics of the ground and the coupling between the antenna and the ground.

Ground impedance decomposition method in this dissertation is an analysis method that considers the overall ground as a combination of several decomposed parts, and then designs the antenna optimized for the overall ground. To investigate this technique, the input impedance of the antenna which represents the antenna characteristic is discussed based on the two-port network analysis and the Theory of Characteristic Mode (TCM). The mobile device is represented by an equivalent circuit, and the antenna input impedance is derived through two-port network analysis. TCM is a theory that analyzes the characteristics of an arbitrary shaped conductor structure through numerical analysis. In general, the ground of the mobile device has a rectangular shape and has a half-wavelength dipole current distribution along the long axis of the ground at the first resonant frequency. The radiation performance is determined by changing the characteristic mode excited through feed configuration. In other words, the coupling between the antenna and the ground characteristic mode changes according to the shape and position of the antenna, which affects the radiation performance of the antenna.

This dissertation presents a method of designing a high performance mobile antenna by the ground impedance decomposition method, which optimize the characteristics of the ground and the coupling between the antenna and the ground. Chapter 1 introduces the general development of wireless communication technology, and discusses current mobile device antenna technology. Chapter 2 discusses the basic theory of antennas, the theory of characteristic mode, the coupling between the ground and the antenna, and two-port network analysis, and introduces the ground impedance decomposition method. Chapter 3 discusses the input impedance of antenna through a 2-port network analysis by dividing the overall ground plane into the PCB ground and the FPCB. Chapter 4 discussed techniques to improve the isolation and reduce the correlation in MIMO antenna system by exciting different characteristic modes that are orthogonal to each other.