Design And Implementation Of High Resolution Antenna For Urban Ground Penetrating Radar

With the rapid progress of domestic urbanization,disasters,and accidents caused by the failure to identify the underground space promptly occur from time to time.There is an urgent need for a high-resolution rapid detection technology to guarantee the safety of urban underground space.Ground penetrating radar（GPR）,as an efficient and non-destructive geophysical exploration technology,has been widely used in urban underground space exploration.However,with the development of urban underground space,the existing detection technology cannot meet the practical detection needs,and the problem exists that the resolution and detection depth cannot be achieved simultaneously.As an essential part of the GPR system,the antenna system critically impacts system performance.Thus,developing an antenna system that can have high resolution and decent detection depth is significant and has practical value for urban underground disaster early warning regarding identifying underground pipelines and cavities.Stepped-frequency continuous-wave（SFCW）GPR is widely used as a frequency domain GPR.Furthermore,it has a broader working bandwidth and controllable working frequency band than impulse pulse ground penetrating radar.It is feasible to realize the consideration of detection depth and resolution.Therefore,this paper adopts the SFCW GPR system based on the vector network analyzer,combined with the techniques of array forming and ultra-wideband（UWB）,and focuses on the design of the GPR antenna,which can be used for urban underground high-resolution detection with the depth of 0～5m.During this research,the underground space is divided into two areas: 0 ～ 1.5m and 1.5 ～ 5m.According to the different detection requirements,two UWB antennas are designed,and the SFCW GPR system is built.Experimental examples are conducted to verify the effectiveness and practicability of the designed antennas.The main research contents of this paper are as follows:（1）The relationship between the detection depth and resolution of GPR and the antenna’s operating frequency,bandwidth,and beam width is studied.Through theory and forward simulation,it is theoretically explained that the characteristics of the UWB antenna,the array’s narrow beam,and the SFCW radar’s flexible frequency band control ability can improve the detection depth and resolution of the GPR system.It provides a theoretical basis for high-resolution detection of the urban underground with a depth of 0～5m using GPR.（2）The high-resolution electromagnetic wave focusing array technique for urban underground complex targets at a depth of 0～1.5m is studied.Aiming at the problem that the broad radiation beam of the traditional bow-tie antenna leads to insufficient angular resolution,which makes it challenging to distinguish complex shape targets accurately,a synchronous transmission array based on the high-frequency UWB Vivaldi antenna is developed.Based on the theory of the side fire array with equal amplitude,the appropriate number and spacing of array elements are selected to achieve the effect of electromagnetic wave focusing and suppress the interference of diffracted waves.Through gpr Max forward modeling,the high-resolution advantage of the synchronous radiation array for complex geology in underground space is demonstrated.The designed four-element synchronous transmitting array has a 3d B beamwidth smaller than 30 ° and a maximum gain of more than 15 dbi.It works in the frequency band of 600 MHz ～ 2.5 GHz,with an absolute bandwidth of 1.9 GHz.In the medium with a relative dielectric constant of 10,its theoretical longitudinal resolution is 2.49 cm.（3）A low-frequency UWB bow-tie antenna for urban underground targets at a depth of 1.5 ～ 5 m is proposed.Aiming at the problem that the current mainstream commercial time-domain GPR system cannot take care of the detection depth and resolution simultaneously,a low-frequency UWB bow-tie antenna based on the loading technology is investigated.The working bandwidth of the antenna is enhanced,and geometry is miniaturized by truncating the antenna arm.The basic structure parameters and arm loading parameters of the traditional bow-tie antenna are optimized based on the genetic algorithm,and the resistance loading is carried out to improve the surface current distribution of the antenna.The improved antenna uses the finite element simulation software HFSS to verify the ability to detect the pipeline.The tested results show that the size of the designed low-frequency UWB bow-tie antenna is 35 cm * 22 cm.It works in the frequency band of 350 MHz ～ 1.77 GHz.The absolute bandwidth is 1.4 GHz.In the medium with a relative dielectric constant of 10,its theoretical longitudinal resolution is 3.39 cmFinally,the construction of the SFCW GPR system based on the vector network analyzer is finalized.The system can operate at 300 MHz ～ 3GHz,and the theoretical vertical resolution comes to the centimeter level.Several comparative experiments are carried out between the designed and commercial GPR systems,zond-12 e.The experimental results show that the antenna’s narrow beamwidth and UWB design can effectively improve the resolution,and the designed SFCW GPR system can realize the high-resolution detection pipelines and cavities with a depth of 0～5m in urban areas underground space.The research results of this paper build a good experimental platform for subsequent scientific research and application,moreover,it has guiding significance for urban underground 0 ～ 5 m high-resolution detection.