| Based on the comprehensive detection advantages of long distance and high resolution in complex underground environment,impulse radar has become an important tool for exploration and imaging of rare underground resources.Based on the application of ultra-wideband antenna in borehole logging radar,Three aspects of work have been carried out in this thesis.In principle,the pulse source of borehole logging radar,namely gaussian pulse signal,is analyzed in time domain and frequency domain.Then,from the perspective of engineering application,several technical means of improving antenna bandwidth are listed,and the principle of improving antenna bandwidth performance by impedance loading is deduced.Then the influence of electromagnetic environment on radar wave propagation is analyzed.In the aspect of simulation design,according to the principle of single hole logging,an omnidirectional transmitting and directional receiving borehole logging radar transceiver antenna system is optimized and designed.The first is an omnidirectional dipole antenna for transmission.Firstly,the feedpoint of the dipole antenna was designed with gradient structure,and the input impedance of the antenna was adjusted to be near50.In order to improve the high frequency radiation pattern of the antenna,resistance loading was applied to the dipole antenna.The S11parameters of the optimized transmitting antenna are less than-10d B in the range of 150MHz?1GHz.At the same time,the main radiation direction of the antenna is consistent in the half-power spectrum,and the time-domain radiation waveform of the antenna at the far-field probe is very consistent with the second-order Gaussian pulse signal.The second is the design of the receiving antenna,by adding absorbent sponge on the back of the antenna,backward radiation was shield and the orientation of the antenna was achieved.By designing the gradual change structure of the feed point and optimizing the sweep parameter of the resistance load,the working bandwidth of the antenna is improved and the time domain radiation characteristics of the antenna are improved.The S11parameter of the optimized receiving antenna is less than-10d B in the range of 160MHz?900MHz.In the time domain direction diagram,the field intensity ratio of the antenna reaches 10:1,and the half-power beam width of the forward main lobe isą50°.In terms of testing,the S11parameters of the receiving and receiving antennas were measured,and the time domain direction diagram of the receiving antenna was measured,and the measurement results were in good agreement with the simulation results.The simulation and test of receiving direct wave signal in air environment were carried out,and the test results are highly consistent with the simulation results.On the other hand,the direct wave signals received by the transceiver antenna system before and after the glass fiber reinforced plastics tube were compared,and the influence of glass fiber reinforced plastics tube on the transceiver antenna was analyzed.The receiving simulation and test of reflected wave signal in air environment are carried out for the whole radar.The test results are highly consistent with the simulation results after considering the influence of glass fiber reinforced plastics tube on the radiation characteristics of transceiver antenna.At the same time,the radar system in the air environment imaging test was carried out,according to the image of the calculated distance between the target and the radar and the actual distance is very consistent;Finally,the radar system is measured in the downhole environment,and the measured waveform meets the imaging requirements. |
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