Design of compact multi-band antennas for small IoT devices

Abstract

As the Internet of Things (IoT) evolves rapidly, more and more wireless devices are connected to the Internet. The connectivity is provided by different wireless technologies operating in different bands of frequencies. In this thesis, I focus on designing compact multi-band antennas for small IoT devices. Two major types of IoT devices are considered, with or without a display screen. For screen-equipped devices, such as smartphones and smartwatches, I aim at leveraging the screen structure and its effect on metal-frame antennas to reduce the antenna size. A 9-mode multi-band antenna for large-screen smartphones is first designed by making use of both chassis modes (CMs) (including the screen) and frame modes (FMs). We show that the antenna bandwidth can be expanded by selectively exciting and merging the desired CMs and FMs. This concept is then expanded to a smartwatch’s overall fundamental structure. The resulting antenna can cover up to four discrete frequency bands with two short strips on the side of the watch, taking only 36% of the perimeter. For IoT devices without display screens, such as smart wristbands, smart pencils, and smart glasses, they are smaller than the screen-equipped devices and are usually strip- or bar-shaped. Since a large-size ground is not available, dipole antennas are more suitable for this type of device. Conventional dipole antennas achieve more frequency bands by adding more arm/slot pairs, resulting in increasingly bulky size. In this thesis, I propose an algorithm that uses the fewest arms to design a multi-band linear dipole antenna. The key idea is to share arms after the effective ranges of excitation for each dipole mode are determined by characteristic mode analysis. An exemplified linear dipole antenna with 5 bands is designed with only 2.5 pairs of arms, representing a 50% reduction in the number of arms. Besides, this method can be readily applied to ultra-thin dipole antennas. A classical dipole antenna only resonates at odd modes, which limits the free design of a multi-band dipole antenna. To excite both odd and even modes of the dipole antenna, I propose to mount a capacitive coupling element (CCE) port, together with an embedded port, in the middle of a dipole. With this design, I show that the first several modes (both odd and even) can be excited. In order to tune the resonant frequencies of the multiple modes on a dipole antenna, a method for combining indentations and outdentations is proposed. It can sort the desired modes and tune them separately. Then, we further combine the two ports into one port by a novel feeding structure. With this design, I show that all the first 6 modes of a classical dipole antenna can be excited simultaneously.