Research Of Novel Antennas Based On 3-D Printing Technology And Fluid Dielectric Reconfigurable Antennas

3-D printing technology,which is also referred as to additive manufacturing technique,is a recently growing processing tool to fabricate 3-D objects.It allows 3-D objects with arbitrary geometry to be formed up with layer-by-layer printing from bottom to top,thus reducing the processing difficulty of complex structure.Besides,compared with conventional cutting craft,3-D printing possesses advantages of saving materials,no waste and low power dissipation,which makes it widely used in various industries.In radio frequency(RF)spectrum,numerous kinds of RF and microwave components have been fabricated by this technology,such as waveguide,antenna,filter,frequency selective surface and so on.Among them,3-D printed antenna has a promising furture due to its incomparable fast prototyping and low cost.However,the reported 3-D printed antennas are mostly repeated designs of conventional antenna structures,ignoring the degree of freedom in advanced design by using 3-D printing.To address this problem,several antennas with novel structures and improved functionalities are here proposed in virtue of the concept of 3-D printing.The main innovations and contributions from this dissertation are summarized as follows:1.3-D printed inhomogeneous substrate for circularly polarized patch antenna:3-D printed inhomogeneous substrate is prpopsed to demonstrate that introducing perturbation in substarte can also realize circularly polarized radiation just like altering the shape of patch conductor.In consideration of the fact that in 3D printing,the dimensional tolerance for dielectric is less than the one for metal,the presented approach has more superiority than the conventional circularly polarized methods.In addition,three antenna structures based on this approach are proposed to improve antenna performances.(1)Stacked patch structure where the lower intact patch exactly serves as the ground of the upper one is designed to achieve dual-band dual circularly polarized property.The independent operating frequency bands of dual circular polarizations are utilized to achieve small frequency ratio(1.16).(2)Air-filled mesh-grid structure is proposed to reduce the permittivity of substrate.By controlling the value of its permittivity,a wide 3-dB axial-ratio beamwidth(120°)can be achieved.(3)Metallic screws are introduced to assemble the whole antenna structure,while they work as electrically shorting pins to enhance the radiation gain of about 2.07 dB.2.Dielectric helical antenna based on 3-D printed zirconia ceramic:A dielectric helical antenna structure based on 3-D printed zirconia ceramic is proposed to verify that dielectric helical antenna can also radiate circularly polarized waves like the metallic one even although their radiation principles are different.When a dielectric waveguide operates at the cut-off state of higher mode,electromagnetic waves cannot propagate in waveguide but leak to air,leading to radiation.At the moment,original slow-wave transimission system truns into fast-wave radiative state.The proposed structure utilizes the radiation of dielectric wavegude and realizes circularly polarized radiation by folding a straight waveguide into helical structure.Besides,the effect of folding process on propagation constant of electromagnetic waves in waveguide is further investigated.The accurate value of propagation constant can be extracted by using commercial simulation software to demonstrate that circularly polarized radiation is related to the folding process.Compared with conventional metal helical antenna,the proposed dielectric one poccesses lower radar cross section,which improves the invisibility of antenna.3.3-D all-metal circular patch antenna with improved functionality:A 3-D all-metal circular patch structure is proposed to improve the impedance matching of TM₀₂ mode.It is known that the electric field at center point of a circular patch under TM₀₂ mode is strongest,which results in maximal input resistance and difficulty of impedance matching.In this context,a 3-D structure with concave at its center is proposed.Since the vertical surrounding of concave playing as electric wall weakens the original strong electric field,the input resistance at center point can be reduced,aiming at good impedance matching.Besides,based on the proposed concave structure,two rings of inverted-L stubs are introduced on the ground plane and patch respectively to achieve dual-band dual circularly polarized conical beam radiation.Different from conventional methods of dual-band dual circularly polarized designs where two pairs of degenerate modes(four modes)are usually needed,the proposed approach only makes use of three modes,where a wide patch mode provides E_?components and E_?components are caused by another two narrow parasitic modes.Meanwhile,the phases of E_?produced by these two different modes lead and lag the phase of E_?respectively,leading to opposite CP rotations.In other words,a shared mode is here used to realize left-hand and right-hand circular polarizations in two frequency bands respectively.Small frequency ratio(1.15)can be easily obtained by using the proposed approach where a shared mode works.4.Fluid dielectric reconfigurable antenna with 3-D printed container:Based on 3-D printed container,three structures of liquid reconfigurable antennas are proposed by using water and low-loss transformer oil.(1)Pure-water dielectric resonator antenna is presented to achieve with frequency reconfigurability.By altering the height of water,the working frequency of TE₁₁₁ mode can be shifted.(2)Sea-water Yagi-Uda antenna is designed for pattern reconfigurability.The parasitic water monopole can work as a director or reflector by controlling its height,thus altering the beam direction.(3)Frequency-reconfigurable patch antenna is proposed by using low-loss transformer oil.Since the layer of transformer oil is here served as the substrate of patch,the operating frequency of patch antenna can be changed with the volume of liquid.Besides,benifitied from the low loss of transformer oil,the proposed antenna maintains high radiation efficiency in the whole tuning range.