Research On The Design Of The Novel Microstrip Higher Order Mode Leaky-wave Antenna

As the modern wireless communication systems are developing towards planarization,integration,and functional complexity,the design of leaky wave antennas is also gradually found in the direction of high frequency,miniaturization,and integration.As the design basis of the leaky wave antenna,the leaky wave structure and the leaky wave mode is one of the most important factors that affecting the performance of the leaky wave antenna.When addressing the new needs that arise in the new stage of development,the comprehensive design that takes into account multiple aspects is becoming more and more important.The microstrip line structure,as the most commonly used transmission structure in communication systems,can well meet the requirements of the system for miniaturization and integration.At the same time,the higher-order mode in the microstrip line works in high frequency and it has variegated field distribution,these characteristics make the higher order mode can well meet the design requirements of high frequency and diversified functions of the system.Therefore,the research on how to obtain high-order modes and what are the advantages of high-order modes has important scientific research value and huge application value for the design of leaky wave antennas.In addition,in order to meet the requirements of radar and other systems for the beam scanning range,the design of a leaky wave antenna with forward and backward continuous beam scanning has always been one of the hot spots of concern.This dissertation takes the development direction of leaky wave antennas in modern communication systems as the background and takes the systematic design method of microstrip line high-order mode leaky wave antennas as the main line of research.It mainly focuses on the excitation method of high-order mode,high-order mode with periodic loading and periodic structure design,a systematic theoretical study and experimental verification was carried out.The main contributions of this dissertation are concluded as follows:1.The broadband high-efficiency excitation method of the high-order mode of the microstrip line has been studied.By designing the feeding method,the excitation method of the high-purity second-order high-order mode is obtained,which solves the disadvantages of the impure mode in the traditional feeding method.This dissertation proposes a low-order mode suppression method based on the orthogonality of the different modes and gives the calculation formula of the different mode amplitudes from the excitation,which solves the problem of the lack of theory in the traditional power feeding.The relationship between the amplitude/phase of the excitation voltage at the port and the amplitude of the excited modes is discussed.A broadband power divider with a broadband phase shifter is designed to achieve a broadband high-efficiency excitation of the second-order high-order mode.By extracting the propagation constant of the transmission mode,it verifies the validity of the proposed design method.In addition,combined with an impedance matching circuit,a broadband second-order high-order mode leaky wave antenna is designed.2.The radiation characteristics of second order higher mode loading with different forms of slot have been studied.Based on the current distribution characteristics of second order higher order mode,two types of slot are loaded and studied.By introducing a transversally loaded slot array along the microstrip line,the cross-polarization of the antenna can be largely enhanced,while a longitudinally loaded slot array interrupts the current flow and thus effectively reduces the original two radiation beams.Subsequently,an array of I-shaped slots,including transversal-and longitudinal-slot portions,is etched on the strip conductor and its unique characteristics are then analyzed to obtain a single main radiation beam in the center plane.The design realizes the reconstruction of the second-order high-order mode radiation characteristics,transforms dual-beam radiation characteristics into single-beam radiation,and solves the limitation of dual-beam radiation in the application of radar systems.Compared with the substrate integrated waveguide with similar radiation characteristics,the leaky wave antenna based on the second-order high-order mode radiates higher power within a shorter length in terms of guided wavelength.The radiated power is increased by 40%,the gain is increased by 4d Bi andthe antenna length is reduced by half.3.The design method of high order mode periodic leaky wave antenna has been studied.By extracted the equivalent impedance of different slots in the equivalent circuit model,the corresponding relationship between the structural model and the circuit model is established.By establishing the relationship between the lumped parameter circuit and the distributed circuit model,a navel balanced loading condition in the distribution circuit model is proposed for suppression open-stop band.Based on the balanced loading condition,the longitudinal slots and transversal slots are compatibly introduced for open-stop band suppression.Different from the traditional method,the balanced loading condition is conducted to prescribe and synthesize the loading.It solves the difficulty in the traditional open-stopband suppression design method which cannot design the loading value quantitatively.And it provides an efficient and accurate design formula for a periodic structure.Finally,two periodic leaky-wave antennas are designed based on the proposed balanced loading conditions,first higher order mode or second higher order mode periodic leaky-wave antenna with single or dual beams scanning from backward to forward is designed.4.The balanced loading condition in the asymmetric periodic unit for open stopband suppression has been studied.The introduction of asymmetry is an important solution to achieve Q balance and prevent gain attenuation at the center frequency.However,a very limited study has addressed the design methods of the asymmetric periodic structure.By introducing complex resonant frequency,the frequency balance and Q value balance in open-stopband suppression are transformed into equivalent conditions in numerical problems.And then it is expanded as a balanced loading condition in distributed circuit models.The proposed method solves the difficulty that the loading value cannot be designed quantitatively in the asymmetric periodic element,and gives an efficient and accurate comprehensive design method.Finally,two circuit models are proposed,one is based on the double series loading periodic unit and the other circuit model is with series and shunt loading.Based on the proposed balanced loading condition,each parameter in the circuit model can be calculated from the given formulas,which can efficiently solve the open stopband and realize the design of the leaky wave antenna with backward-to-forward continuous beam scanning.