| As a bridge between devices on the body and other wireless transceivers,wearable antennas are one of the most important components in body area network(BAN).Due to the fact that the wearable antennas operate in the proximity of the human body,some issues,such as wearing comfort,radiating safety,and polarizing mismatch caused by human body movement,must be considered in the initial stage of antennas design.Moreover,the wearable antennas with flexibility,broadband and circular polarization(CP)should be robust in the harsh electromagnetic environment.For the integration,multiple functions can be implemented in the wearable devices by employing multiband and multimode antennas.Therefore,the focus of this dissertation is on the wearable antennas’ flexibility,CP,broadband,multiband and multimode.This research was supported in part by the Science and Technology Planning Projects of Guangdong Province(2015B010101006,2014A010103014,2017xm057),and in part by the National Natural Science Foundation of China(61372008).The innovative contributions of the dissertation are summarized as follows:(1)With the research requirements of wearable antennas in human body loading,a flexible CP antenna with axial ratio(AR)bandwidth enhancement is proposed.Based on the principle of microstrip patch antenna loading probe to generate perturbation,the CP radiation is achieved.The 3-d B AR bandwidth is doubled by adding two inverted L-shaped parasitic stubs on both sides of the antenna,which addresses the issue that the AR bandwidth partially covers the 5.8-GHz Industrial,Scientific and Medical(ISM)band.Also,due to the adoption of lightweight polyimide as substrate and flexible foam as support,the proposed antenna achieves wide bandwidth,and can totally conform to the curve-shaped human body.The measured results show that the impedance bandwidth of the antenna is 6.6%(5.67–6.05 GHz),the AR bandwidth is 3.85%(5.730–5.955 GHz),and the peak gain reaches 7.2 d Bic.Due to the movement of human body,the flexible antennas will be deformed,resulting in the deviation of antenna resonant frequency.To address the issue of frequency offset,it is essential to investigate the broadband methods of wearable CP antennas.(2)A flexible CP textile antenna loaded with metasurface and its broadband conformal array are investigated.By loading a modified metasurface above the rectangular microstrip patch antenna,the impedance and AR bandwidths of more than 17.5%(5.02–5.98 GHz)are achieved,which can fully cover the WBAN band of 5.15–5.825 GHz.Notably,characteristic mode analysis is introduced to clarify operating mechanism,providing clear physical insight into each mode at various frequencies.Consequently,each mode can be tuned independently.In addition,considering the real wearing scenarios of wearable antennas,the human body loading,bending,crumpling and humidity variation of the antenna are studied in detail.Furthermore,the proposed CP textile antenna works as element,constituting the broadband conformal antenna array.Hence,the omnidirectional radiation pattern is achieved for on-/offbody armband device communications.However,the two antennas designed above are single-band CP antennas,which can only be used in single-link transmission.To realize the multiband applications of wearable CP antennas for data transmission or health monitoring,the methods of realizing multiband in wearable CP antennas need to be further investigated.(3)A design method of loading polarization rotation artificial magnetic conductor(PRAMC)to realize dual-band and dual-CP radiation is presented.The compact dual-CP radiation of the proposed antenna is built by clinging to a layer of PRAMC under the dual-band monopole.The left-handed CP at 3.5 GHz is achieved for data transmission,and the righthanded CP at 5.8 GHz is implemented for health monitoring applications.In addition,the effects of human body and clothing material loading,as well as structural deformation on antenna performance are deeply studied.And the prototype of the antenna was fabricated by two types of manufacturing process(i.e.,laser cutting and screen printing).The measured results show that the –10-d B reflection coefficient bandwidths of 11.7% and 9.1%,the 3-d B AR bandwidths of 2.0% and 8.2%,and the peak gains of 6.6 and 7.2 d Bic,are obtained in dual bands,respectively.The above dual-band wearable CP antenna realizes the single-mode radiation,which can only be used for off-body communication.To satisfy both on-/off-body communications,it is also necessary to study the radiation mode diversity technologies of dual-band wearable CP antennas.(4)A method of radiation mode diversity based on microstrip antenna loading probe is suggested.According to this method,two low profile dual-band wearable antennas with dual radiation modes are designed.First,a dual-band textile antenna with polarization and radiation mode diversity is studied.By loading shorting pins on a single-layer microstrip patch antenna instead of integrating two radiators,a dual-band and dual-polarized antenna with simple structure,low profile,and radiation mode diversity is realized.Second,based on the technology of stubs extension and rotating L-shaped stubs loading,a compact dual-band CP textile antenna with radiation mode diversity is designed.The proposed antenna has the structure of low profile,single layer and single port,and can realize unidirectional and omnidirectional CP radiation at the same time.Moreover,due to the adoption of textile(i.e.,wool felt)as substrate and nylon conductive fabric as conductor,the proposed two antennas are flexible,totally conforming to the human body.Finally,the measured results show that the two antennas have the advantages of low profile,flexibility,dual bands,CP,and dual radiation modes,which can meet the requirements of on-/off-body communications,simultaneously.Through the above four aspects of research,the technical difficulties and limitations in the research of wearable CP antennas,such as flexibility,broadband,dual bands and dual modes,are comprehensively studied,which enriches the design theory of wearable antennas and provides instructive guideline for relevant researchers. |
