| At present,antennas used in wireless communication system are basically constructed from metal conductors.Under the demand of miniaturization,multi-function and multifrequency band for the relevant equipment,the design for antenna faces unprecedented challenges.It's difficult to design an antenna in consideration of all the condition,including miniaturization of the structure,broad bandwidth and good radiation performance.In some communication systems,the traditional antenna can meet the corresponding requirements no longer.As a result,new antennas come into being spontaneously,and one of which is the liquid antenna.According to its composition and concentration,the liquid applied in the design of antennas are mainly divided into two kinds: as the radiation factor or dielectric loading.Liquid antenna has attracted more and more attention because of its reconfigurability,small radar cross section and transparency.However,the high relative permittivity of water brings about the problem of narrow bandwidth.Also,since the dielectric loss of most of the liquid materials is very large,the radiation efficiency of the antennas is too low to put into application.How to improve the radiation efficiency under the premise of ensuring the bandwidth of the antenna is the key to the research.In this paper,pure water or water solution is taken as the liquid material of the antenna.The paper lays emphasis on hybrid structures of the antennas so as to increase the bandwidth of the antenna and improve the radiation efficiency of the antenna at the same time.Moreover,the characteristics of fluidity of water are utilized for reconfigurability to design multifunctional antennas.The reconfigurable functions include frequency,radiation pattern and polarization reconfigurability.The main work is as follows:1.A wideband and high efficiency coaxial dual-tube water monopole is designed.The antenna utilizes the basic structure of monopole antenna and water takes the place of the metal conductor for radiating.With the coaxial nesting structure of two polyvinyl chloride(PVC)tubes,pure water(Permittivity?81,Conductivity?0.0002S/m at room temperature)is inserted into the inner tube while salty water(Permittivity?81,Conductivity?8S/m at room temperature)is inserted into the outer tube.The coaxial dual-tube structure ensures the combined effect of pure water and salty water,and the compact structure is available for the coupling of the modes introduced by the different liquid materials to form a broad bandwidth.Moreover,the loading effect of distilled water and the low dielectric property of Teflon base play a part in increasing the radiation efficiency.The paper represents the designing process,simulation and analysis procedure of the antenna.It can be drawn that resonant frequency can be adjusted by changing the height of the water columns.Finally,the coaxial dual-tube water monopole as well as a same-size single-tube water monopole are constructed and measured.The result shows that,in comparison of the sing-tube structure,the dual-tube water monopole takes effect in improving the bandwidth as well as radiation efficiency.2.A reconfigurable water dense dielectric patch antenna(DDPA)with cross-shape feeding structure is designed.The basic structure of the antenna is the square pure water layer(Permittivity?81,Conductivity?0.0002S/m at room temperature)and the feeding structure consists of a cross-shape metal patch on the upper surface of a substrate and a probe through the center of the substrate.To further optimize the matching performance of the antenna,four small cylindrical metal rods are fixed on the four arms of the cross-shape patch respectively.The metal rods are also used to prop up the water holder and insert an air gap between the substrate and the water layer,which play a part in increasing the radiation efficiency.The antenna utilized the performance of the similar electric wall of the interface between the air gap and the water layer.The bottom surface of the water layer is equivalent to a conventional microstrip patch and the air gap acts as the dielectric substrate of the microstrip antenna.The proposed antenna is excited by the mixed cross-shape feeding structure.The paper expatiates the radiation theory and designing process of the water DDPA.The simulated results shows that the cross-shape feeding structure plays an important role in improving the impedance bandwidth of the antenna and resonant frequency can be adjusted by changing the height of the water layer.3.A wideband polarization-reconfigurable water dielectric resonator antenna(DRA)is designed.The antenna is mainly constructed with a cylindrical plexiglass holder,the interior of which is divided into four sectors averagely.Without changing the structure of the antenna,left hand circular polarization(LHCP),right hand circular polarization(RHCP)and linear polarization(LP)can be switched over by injecting pure water(Permittivity?81,Conductivity?0.0002S/m at room temperature)into different sectors of the holder.Besides,there is an air layer between the water holder and the ground.Except for the performance of DRA,water used in the proposed antenna also acts as a dense dielectric patch antenna(DDPA).The interface between water and air acts as an electric wall and the side faces of the air layer acts as a magnetic wall.The air layer below the water also has the performance as a resonator,similar to the water DRA.In the state of circular polarization(CP),modes for DRA and DDPA are excited to form a broad axial ratio bandwidth,which is almost equal to the impedance bandwidth.Thus,a broad usable bandwidth is achieved.The paper describes the designing process and analyzes the simulated results in detail.Moreover,the proposed water DRA has also been constructed and measured.The measured results shows that frequency range of the usable bandwidth for all of the three states are almost the same,so are the radiation efficiency,which centralizes in 70%.The three proposed antennas utilize the hybrid structure for improving the performance.Moreover,the antennas has the unique characteristic of reconfigurability for various functions.The first coaxial structure is the attempt to combine different kinds of water,while the second and the third one is the mixture of water layer and air layer because the interface between them acts as an electric wall.The three antennas have the characteristic of reconfigurbility.The first and the second antenna can reconfigure its frequency by changing the height of water,while the third one achieves polarization reconfigurability by injecting water into different sectors of the holder. |
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