for their discussions about my work. At last I want to thank my parents for their support and encouragement. v TABLE OF CONTENTS LIST OF TABLES .

Wireless sensor and actor networks are becoming more and more popular in the recent years. Each WSAN consists of numerous sensors and a few actors working collaboratively to carry out specific tasks. Unfortunately, actors are prone to failure due to harsh deployment environments and constrained power, which may break network connectivity resulting in disjoint components. Thus, maintaining the connectivity among actors is especially important. This paper proposes hybrid connectivity restoration (HCR), which integrates proactive selection and reactive motion. An actor protectively selects a backup node through its one-hop neighbor table and informs the backup node to supervise its stage. Once it fails, the backup node moves to the best position to restore the connectivity of the failed node's neighbors reactively. This triggers a local recovery process at the backup node, which is repeated until network connectivity is restored. In order to minimize travel distance, HCR selects the backup node which moves the shortest distance to restore connectivity. Furthermore, HCR opts to reduce the number of messages by just informing the failure to its backup node. The correctness and effectiveness of HCR are validated through both theoretical analysis and simulations.

To address the effects of surface roughness on wave propagation in the microwave and millimeter-wave bands, this paper studies electromagnetic wave propagation and focuses on the propagation loss within an inner environment featuring surface roughness of the metallic waveguide structures. The conductivity gradient model is first developed to treat surface roughness with inhomogeneous conductivity, and then the concept of a frequency-dependent effective conductivity is introduced to model the effects of surface roughness in waveguide structures. With the effective conductivity used in the commercial 3D field solver, High Frequency Structure Simulator (HFSS™), a surface impedance boundary condition strategy is applied on ideally smooth surfaces to model the actual behavior of rough metallic waveguide surfaces. Finally, the propagation performance is predicted by comparing the effects of surface roughness in metallic waveguide structures and the results from the models with ideally smooth surfaces, using rectangular waveguide, cylindrical resonator and coupling cavity filters models. Overall, this effective conductive concept can be applied using various commercial field solvers to handle surface roughness effects accurately and efficiently in any work involving conducting microwave structures.

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Low-frequency electromagnetic waves have the characteristics of long propagation distance, strong resistance to electromagnetic pulse interference, and slow attenuation in seawater and other media. However, conventional low-frequency transmitting antennas have problems such as bulkiness, high power consumption, and low efficiency, which are not conducive to the performance of low-frequency electromagnetic waves. The mechanical antenna is a new type of low-frequency transmitting antenna that generates time-varying electromagnetic field radiation through the mechanical movement of electric charges or magnetic dipoles. The new radiation principle enables mechanical antennas to break the constraints on the physical size of electromagnetic waves in the traditional antenna field, thereby achieving low-frequency communication with a smaller size and higher efficiency, providing a subversive solution to scenarios such as submarine communication and through-the-earth communication. In recent years, mechanical antennas have attracted much attention and become a hot research topic in the field of low-frequency communication. In this paper, we briefly review the development history, development direction, and existing problems of traditional large-scale land-based low-frequency transmit antennas and persistent mobile low-frequency transmit antennas; we mention the details of the working principles and recent research progress of different mechanical antenna implementations including electret, permanent magnet and piezoelectric mechanical antennas; we compare and analyze the radiation performance, innovations, advantages and disadvantages of each specific implementation scheme; and we also discuss the characteristics of the existing frequency modulation, amplitude modulation, polarization modulation and other signal modulation methods of mechanical antennas and the application schemes of several signal modulation methods of different types of mechanical antennas; finally, we prospect the research direction of mechanical antennas in the next stage. At present, the feasibility of the mechanical antenna scheme has been verified theoretically and experimentally, but it is limited by the antenna volume, power consumption, driving device and other factors, and the radiation intensity of the mechanical antenna is limited. We believe that the research in the field of mechanical antennas in the next stage will focus on the design of antennas for achieving longer communication distances at the sacrifice of certain small and light weight indicators, and innovative signal loading and modulation methods to improve communication rates will also be worth paying attention to in the field of mechanical antennas.