| A scheme of transmitting and receiving antenna system of borehole radar is demonstrated, designed and fabricated in this paper, in which omni-directional antenna is used as transmitter while directional antenna array is used as receiver. Then the key parameters of the antenna system are measured through experiment.The characteristic parameters of time-domain antenna are described systematically, and then based on the research indexes of project they are demonstrated and given by time-domain electromagnetic theory. The omni-directional antenna fed by quasi-monocycle pulse with peak-peak pulse width of3ns and peak-peak voltage of1kV is optimized by CST according to the method of equidistant loading resistor, then it is designed and fabricated. Its radiation waveform is quasi-second order Gauss pulse, the ratio of amplitude between peak and tailing signal is greater than10:1, while the duration of tailing signal is less than10ns, the E-plane time-domain radiation pattern at half-power of electromagnetic pulse beam is±45°and the H-plane radiation pattern is omni-directional. The scheme of dipole with equidistant loading resistor is fabricated which embedded into ferrite substrate material RH2100. Considering the absorbing characteristic of ferrite and the matching with the environment of borehole surrounded by limestone, then the receiving antennas with1×3array are optimized by CST and made into actual antenna. When the omni-directional antenna is used as transmitter and directional antenna array as receiver, the receiving electromagnetic pulse is quasi-second order Gauss pulse waveform, the peak amplitude ratio of receiving signal and tailing signal is greater than10:1, the duration of tailing signal is less than10ns, front-to-back ratio is2.9:1, the H-plane r time-domain radiation pattern at half-power is±60. The simulated results based on the array way of making the receiving antenna array equidistant in line with center distance120cm show that the coupling between receiving element in working band is less than40dB. The key parameters of omni-directional transmitting and directional antennas are measured. The measured results agree well with simulation results within the allowable range of experimental error. If the HE11mode in borehole transmits along the material near borehole and in the metal wire of the radar system, the image quality of radar system will be seriously affected. The transmitting conditions and attenuation of HE11are systematically simulated by CST, which can provide reference data for the design of whole radar system.Aim at the results of the antenna design and the optimization results of HE11transmitting characteristic, the electromagnetic pulse parameters of the omni-directional antenna, the directional receiving antenna element and the1×3array are tested in simulative single and cross borehole, respectively. The experimental results show that measured results agree well with simulated results, but measured waveform disorder may be due to the influence of the surrounding environment. |
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