High Performance Slow-Wave Empty Substrate Integrated Waveguide And Its Applications

With the rapid development of modern microwave communication,the performance requirements of communication systems become increasingly higher.The performance of passive circuits and antennas in the systems also develop toward miniaturization,high quality factor and low loss.As a technology combing the advantages of planar circuit and traditional metal waveguide,Substrate Integrated Waveguide(SIW)has become an important part of modern microwave circuit system due to its characteristics of low cost,easy integration,low profile and high power capacity.However,the dielectric loss in SIW limits its applications in high performance circuits,especially in high frequency millimeter band.In order to improve this performance,replacing the dielectric in SIW with air can significantly reduce the dielectric loss,at the cost of increasing the transverse dimension.Therefore,by studying the theory of slow wave effect and the transmission characteristics of Empty Substrate Integrated Waveguide(ESIW),a high-performance waveguide structure with high quality factor,low loss and miniaturization is studied on the SIW,and its applications in passive microwave circuits and antennas are explored.It has the important research value and long-term scientific significance in many fields such as wireless network,microwave imaging,biomedical environment detection and radar remote sensing.In this paper,the main research contents and contributions of high performance Slow-Wave Empty Substrate Integrated Waveguide(SW-ESIW)are listed as follow:1.A novel SW-ESIW structure is proposed.Based on the effect of slow-wave theory and empty waveguide characteristics,a triple-layer SW-ESIW structure is designed.A rectangular air cavity is excavated in the middle layer to reduce the dielectric loss of the waveguide.Uniform and compact internal metal blind holes arranged in the bottom layer can effectively separate the electric and magnetic fields and achieve slow wave effect in the air cavity.This results in simultaneous reduction in both longitudinal and lateral dimensions.The different height ratios between the layers also change the whole performance of the SW-ESIW,allowing flexible tradeoff between high quality factor and miniaturization to suit various application requirements.In order to verity the performance of waveguide structure,a fourth-order SW-ESIW straight-through filter is designed and measured.Compared with the same type of filters of different waveguide,when the heights of layers are the same,SW-ESIW filter can reduce the dimension by 37%.Compared with SIW and SW-SIW filters,simulation Q factor of SW-ESIW filter is over 5.3 times and measured Q factor is over 3.8 times,which has a significantly improvement in the quality factor.2.The coupling between multilayer resonant modes based on SW-ESIW structure is studied.A multilayer cavity SW-ESIW coupling structure is proposed,and two fifth-layer filters with high order mode rejection are designed and fabricated.According to the extracted topological structure and coupling matrix,three circular metal holes are hollowed in the center of layer 3 to realize the vertical electrical coupling between the resonators,and two rectangular slots are hollowed in both sides of layer 3 to realize the vertical magnetic coupling between the resonators.Compared with the ESIW filter under the same conditions,the transverse and longitudinal dimensions of the SW-ESIW filter are reduced by more than 30%,while maintaining a low insertion loss.This enables the application of the novel waveguide in multilayer cavities,expands the application range of SW-ESIW in highly integrated wireless systems,and improves the deployment capability of SW-ESIW on the substrates.3.In order to further improve the complete system of ESIW,by studying the working principle of mixed electromagnetic coupling structure on ESIW technology,a mixed electromagnetic coupling fourth-order filter is designed.The topological structure and coupling matrix are extracted to generate two transmission zeros at the upper and lower frequency bands of the passband,which greatly improves the frequency selection of the filter.By etching a H-shaped slot on the top metal layer of two adjacent air cavities,while maintaining the self-encapsulation and high integration of ESIW triple-layer structure,the frequency selectivity of the filter is improved and the insertion loss is reduced,which lays a foundation for designing more complex high-performance microwave circuits and systems.Subsequently,the mixed electromagnetic coupling structure is applied to SW-ESIW,which reduces the operating frequency by 20% under the same size,providing important scientific value for the future research of compact and high-performance SW-ESIW coupling devices.4.Taking SIW back cavity slot antenna as representative,a series of schemes are made to optimize the performance of the antenna.In order to improve antenna gain,1×2 compact triangular SIW cavities are arranged.In order to accommodate a multifunctional reconfigurable antenna,a frequency tunable SIW back cavity slot antenna based on Barium Strontium Titanate(BST)is designed,which can achieve a high frequency tuning rate of 29.8% without affecting the gain.In order to further improve the antenna gain,ESIW and SW-ESIW are applied to back cavity slot antenna.Compared with SIW slot antenna,ESIW and SW-ESIW slot antennas achieve enhanced gains of 1.5d B and 1d B respectively.SW-ESIW can reduce the physical size by 30% compared with ESIW.Therefore,SW-ESIW also has a wide application prospect in the high gain and compact antennas.