Dispersion engineering of periodic structures and its applications to antenna designs

Abstract

In the history of antenna development, periodic structures have been extensively studied and applied. Typically, through the dispersion analysis, the propagation and radiation performances of periodic structures can be predicted. There is no doubt that dispersion relation plays a vital role in periodic structures based antenna designs.

In this thesis, a systematic study on dispersion engineering of periodic structures and its applications to antenna designs is presented. The unifying goal of this study is to change the antenna properties by manipulating the dispersion relations to meet various design requirements and address the bottleneck problems. In addition, several novel antenna designs with excellent performances are proposed and presented in this thesis.

Firstly, a novel one dimensional multiple periodic (MP) structure is proposed. Different from the conventional periodic structures that have single periodicity, the proposed MP structure increases the periodicity by enclosing different unit cells (UCs) into each periodic element. Through the dispersion analysis, it is found that some new features can be offered by the MP structures. With the increase of the periodicity, the separation distance between the space harmonics are reduced and several new stopbands are opened up. The former feature will lead to excitations of multiple space harmonic modes in a certain frequency range. These new features are proved to be universal for both dispersive and non-dispersive materials. Regarding to each material, the analyses are performed from media and lumped circuit perspectives. The MP structures provide room to manipulate the dispersions. By proper adjusting the geometric parameters, the dispersion relations can be engineered. Furthermore, a general dispersion relation and a general Bragg condition for MP structures are also derived.

Secondly, the proposed theory of MP structures is extended to two new designs of supercell (SC) based dual-beam leaky-wave antennas. By proper engineering the dispersion relations, two space harmonic modes (m= -1 and m= -2) are excited to form two radiation beams. The two designs improve the link quality of the communication system and address the problems that are dominated by the intrinsic characteristics of the links, such as the multipath effect and the mutual interference. The first antenna is based on the microstrip lines and operates in the microwave region. It validates the feasibility of the proposed theory. On the other hand, another antenna is realized by the dielectric grating and works in the millimeter-wave (MMW) region (60-GHz). It employs a symmetrically configured SC and have better radiation performance. Both two designs can realize beam steering in the same clockwise or anti-clockwise direction against the frequency, which is rarely discussed in the previous literatures. They are of very simple structures that are easy for manufactures.

Thirdly, a new collimated surface-wave (SW) excited high impedance surface (HIS) leaky-wave antenna is proposed. This antenna realizes a transformation from TM0 SW mode to m= -1 leaky-wave mode. A novel substrate-integrated waveguide (SIW) based planar parabolic reflector is designed as the SW launcher (SWL). Different from the previous literatures that the SWs propagate in a cylindric wave fashion, the proposed SWL can efficiently excite collimated SWs. On the other hand, the dispersion relation is engineered by introducing an additional secondary slit on the HIS to solve the bottleneck problem, open-stopband effect. Through proper design, the open-stopband effect is suppressed successfully in the design and implementation.

Finally, two antenna designs that can operate in the near-field region by using the inductive coupling principle are proposed for radio frequency identification (RFID) applications. The first antenna works in dual frequency bands (HF/UHF bands) that have nearly 70 times frequency difference. The design is challenged by the impaired UHF bandwidth due to existence of the spiral coil that operates in the HF band. A novel meander lines based diagonal symmetric structure is designed to support multiple resonance modes to expand the impedance bandwidth under the large inductive circumstance of the coil. On the other hand, the second design starts from a segmented loop antenna that behaves as a magnetic dipole and has better performance than coil. In order to meet the requirement of compactness of the RFID tag antenna, the dispersion relation is engineered by adopting multiple identical radiators. The zeroth-order resonance frequency is reduced with same dimension by this method. As a result, the physical dimension of the antenna can be further reduced at the desired frequency. Through the dispersion engineering, the antenna miniaturization is realized.

In summary, this thesis contributes to 1) the development of novel periodic structures and their relevant theory for the new features, 2) addressing practical issues by engineering the dispersion relations and 3) providing a new procedure of using the dispersion engineering for antenna designs.