| Vacuum-tube high-speed train is a new type of railway transportation technology,which can achieve ultra-high-speed operation in low-pressure pipelines close to vacuum.As the vacuum-tube high-speed train belongs to the emerging field and the research is still in its infancy,many technologies are not yet mature,especially the vehicle-ground wireless communication system.At present,the communication system of vacuum-tube high-speed trains mainly has the following four problems: First,in the case of high-speed trains operation,the traditional wireless free wave access method is no longer applicable,because this method will produce a serious Doppler effect,and will cause serious fading when entering the pipeline and the carriage;Second,the traditional leaky wave system is affected by the open stop band effect and only produces oblique radiation waves.More importantly,the near-field electric field strength of the system decays exponentially along the longitudinal direction of the system(that is,the train running direction),resulting in unstable near-field signal strength;Third,there will be continuous handovers between neighboring cells,which on the one hand poses a threat to the safe driving of trains,on the other hand it will reduce passenger satisfaction;Fourth,the radiation field of the traditional leaky wave system has multiple reflections in the pipeline,which not only affects the stability of the receiving and sending signals,but also causes a waste of signal power,which is not conducive to long-distance radio signal coverage.In order to solve the above problems,this paper proposes a new type of leaky wave system design method,based on the standard model waveguide WR-284 structure,a new leaky wave system that can achieve normal radiation,uniform near-field distribution,and near-field focusing characteristics is designed.It can effectively solve the Doppler effect in the high-speed train operation,realize long-distance stable radio wave coverage,and can concentrate more radiated power in the carriage to improve power utilization.The main research contents and innovations of this paper are as follows:1.A design method of leaky rectangular waveguide that can be used in vacuum tube high-speed trains is proposed,so that the waveguide has a normal radiation field,and the near field has the characteristics of uniform distribution along the longitudinal direction of the waveguide.Specifically,first,assuming that the electric field of the waveguide aperture is uniformly distributed,determine the required attenuation constant distribution along the longitudinal direction of the waveguide;Secondly,extract the corresponding relationship between the attenuation constant and the waveguide slot structure,and set the corresponding slot structure on the waveguide according to the required attenuation constant,thereby generating a uniform near-field distribution;Finally,the impedance matching method is used to eliminate the open stop band effect and realize the normal radiation effect.By adopting this method,the Doppler effect generated in the high-speed train movement can be effectively eliminated,and a stable near-field intensity distribution can be realized over a long transmission distance.2.Three new leaky rectangular waveguides with different lengths(5 m,10 m,20m)were designed as a verification of the proposed design method.A 5 m long leaky rectangular waveguide was processed and tested.At the same time,the near-field electric field and phase of the waveguide showed a uniform distribution in the simulation and test results,indicating that the design method is feasible.Two different length waveguides of 10 m and 20 m are designed and simulated,which further proves that the method is suitable for the design of leaky rectangular waveguides of any length.Studies have shown that using the proposed method to design a leaky waveguide can achieve uniform near-field distribution and normal radiation effects.The electric field intensity fluctuation along the waveguide direction is within 0.2 d BV/m,and the phase fluctuation is within 2°.3.A design method for the near-field focusing and leakage rectangular waveguide array is proposed.Based on the proposed leaky rectangular waveguide structure,two waveguide arrays with different focal lengths and corresponding power splitters are designed and simulated.The simulation results show that the method can converge the electromagnetic waves radiated by the leaky rectangular waveguide into a "focus line",which extends infinitely along the train’s running direction,and the field intensity distribution and phase distribution along the line are uniform and unchanged.It can make the locus of the train roof antenna or roof sunroof be consistent with the "focus line".Therefore,this kind of leaky wave structure with near-field focusing characteristics can make the radiated energy more received by the train antenna or directly incident into the train,improving the energy utilization rate.It can also reduce the interference of the reflected wave in the pipeline to the leaky wave system. |
