Analysis of Coupling Mechanism in Mobile Handset Antennas

Abstract

Modern mobile handsets have become versatile handheld devices even though it has been smaller and thinner than before because of consumers’ demand. In spite of size reduction of modern mobile handsets, it is obvious that the number of frequency bands supported by a mobile handset is constantly increasing because of the evolution of wireless communication technology. New wireless systems for guarantee of high-speed data communication, high quality voice communication have been intensively studied and developed. From two years ago, long-term evolution (LTE) service begun to be provided by mobile operators worldwide, which are expected to rapidly expand its market share in the near future. In order to be a leader of the future wireless service technology, researches on future wireless communication system called 5G warranting better wireless performances are actively progressed as well. Thus, mobile antennas must have much better radiation performances. However, the dimension reserved for antenna design in mobile handsets is also decreasing. The laws of physics state that radiation performance such as the efficiency and bandwidth of electrically small antenna are critically limited and degraded. As a result, antenna design for modern mobile handsets is very challengeable. This doctoral thesis contributes to enhancement of impedance bandwidth regarded as one of challenges arising from the theoretical and practical limitations of electrically small handset antennas. Mobile handset antennas are composed of a system ground plane, antenna element and feed structure including a source. This thesis is focusing on analyzing the coupling among the source, the feed structure and the designed antenna structure including ground plane in mobile handsets. By using suitable feed structures, internal mobile handset antennas in this thesis can attain enough wide impedance bandwidth to cover all of the desired bands. For more insight into the operating mechanism of mobile handset antennas, a parallel and series resonator are analyzed and discussed. The coupling between a source and resonator in a feed structure is demonstrated by using impedance matrix of two-port network. Reaction concepts are applied to explain the coupling between designed antenna element and ground plane. This approach makes antenna designers convenient because it is possible to design wideband antennas without significant modification of pattern of antenna element.