| Tunable devices are needed to accomplish requirements of modern wireless systems.The nematic liquid crystal(NLC)has been employed as a tuning element in these devices according to its low loss properties at frequency band above 10 GHz.This dissertation is devoted for potential applications of tunable phase shifter based on nematic liquid crystal in microwave antenna structures,liquid crystal behaviour,liquid crystal characteristics,tunable structure design,simulation results and evaluation of tunable devices.An interesting issue is to design an efficient tunable device that has ability and flexibility to enrol easily into a microwave antenna for achieving phased-shift.This dissertation comprises 3 main parts.As a first part,the theory of liquid crystal(LC)is investigated including its electromagnetic fields,derivation of generalized orientation angle equation for LC molecules,the numerical method of solutions for differential equation of the orientation angle and response time for LC.The orientation angle is derived and solved firstly to the electric field using Finite-Difference Schemes,the solution for the differential equation of the orientation angle is obtained which is more accurate than one approximation constants(K11=K22=K33).In the second part,this research started on design and model a novel electrically inverted microstrip line(IMSL)phase shifter device based on NLC with using a standard thickness of Rogers RT5880 dielectric material.The orientation angle of NLC molecules is controlled by applying AC voltage.The NLC molecules changed their positional order toward the electric field directions when increased the applied voltage through the IMSL.As a result,When the AC bias voltage increased from 0 V to 20 V,the difference phase shift of the device tuned up to 45° with low insertion loss from 0.56 d B to 1.06 d B.Meanwhile,the Fo M value is obtained up to 76.15°/d B.The tunable phase shifter can be a suitable device and easy to enrol it in microwave antennas and wireless communication systems for achieving phased-shifter.For the third part,the phased shift antennas are important concepts for realization an electrical beamforming or beam scanning at a fixed frequency for modern wireless communication applications.In the move application,the beam of microwave antennas needs to be scanned for the transmitter or receiver.The electrical beam steering antennas offer high tunability,fast tuning,low profile and low voltage controlling.For the tunable single-beam antenna,A tunable half-mode integrated substrate leaky wave antenna(HMSIW-LWA)fed rectangular patches based on NLC is proposed.Rectangular patches are used as radiators for the opening sidewall of the waveguide and matched section part for a unit-cell.The transition structure is added for enhancing the efficiency of HMSIW-LWA due to convert most input power to leaky mode.The novel IMSL phase shifter is used for investigating the tunable dielectric characteristics of NLC by applying an electric field to the LC cell,which is controlled the orientations angle of the LC molecules.As a result,the HMSIW-LWA can be tuning up to ±26° for a fixed frequency by tuning the NLC with applied voltage from 0 V to 20 V.In addition,the realized gain changed from 6 to 9.4 d B for a fixed tuned frequency,and 46° steerable for rest main beams range of the HMSIW-LWA in both forward and backward directions.For the tunable dual-beam antenna,a novel tunable dual-beam for (?)-shaped microstrip leaky-wave antenna(MLWA)based on NLC is presented.A novel (?)-shaped microstrip(Berbers alphabet: pronounced |z|)is designed to achieve dual-beam radiation pattern.A tunable phase shifter is obtained with NLC which controlled via IMSL.The proposed MLWA is meandered configuration to reduce its length up to 55%.This antenna has symmetric farfield beams at pointing angle of ±38° in YZ-plane.In addition,it has phased shift for the dual-beam radiation pattern of ±25° at a fixed frequency in XZ-plane of forward and backward directions,the phase shifter is controlled by 0 V to 20 V applied AC voltage.The antenna's realized gain is more than 6 d B,and it is scanning with dualbeam farfield angles range of 48° in XZ-plane.The dual-beam MLWA can be a candidate for satellite communications and radar systems.For the tunable frequency antenna,a new tunable frequency for (?)-shaped microstrip patch array based on the NLC featuring dual-band dual-beam is presented.A single (?)-shaped microstrip(Berbers alphabet;pronounced |z|)is proposed to obtain dual-beam farfield radiation pattern.the dual-band is adjusted by increasing the applied AC voltage on the IMSL which is controlled the liquid crystal's molecules.The first and the second frequency bands are centred at 9.955 GHz and 12.23 GHz,due to apply AC voltage from 0 V to 20 V,the frequency bands are tuned up to 1.085 GHz and 1.43 GHz respectively.At the frequency band 9.955 GHz,the dual-beam radiation pattern is obtained with symmetric beam angles at ±39° in YZ-plane,48.5° half power beam width(-3 d B)at each beam.At the frequency band 12.23 GHz,the single-beam radiation pattern is achieved with maximum with 53.7° half power beam width(-3 d B).In addition,the realized gain of the antenna array is achieved larger than 7.18 d B.This antenna also is suitable for satellite communications and radar systems. |
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