Research And Implementation Of The Radiation System In Ground Penetrating Radar

The Ground Penetrating Radar (GPR) is a detection equipment using super high frequency electromagnetic wave. GPR has many advantages such as rapid detecting speed, high resolution, convenient and flexible operation, non-destructive detection and low cost. It is widely used for detecting of hydrological and engineering geology, and is playing a more and more important role in recent years. The transmission system of GPR is a key part of GPR, the system mainly include high-voltage nansecond impulse source and radiation antenna. The performance of the system directly determines the effectiveness of radar detection. For ground-penetrating radar characteristics, this paper mainly discusses some problems such as analysising, designning and implementation of the ground penetrating radar transmitting system. The main contents are described as follows:1.The transmission system is the key part of GPR tranmitting electromagnetic waves, the system is working under control of GPR main machine, realizing the transmission of probing pulse. At first, based on the principle of GPR, the thesis describe the basic construction of GPR and whole thought of design, and then analyses the relationship between the detection characteristics of GPR and the tranmission system.2.According to the theory of impulse GPR, antenna is the key part of GPR, it realizes the electromagnetic wave transimitting and receiving. Baed on the condition of GPR working, it simplely reviews the development of antenna and related theory, analysis the request of GPR antenna and gives some wideband antenna which can be used for GPR system.3.Bow-tie antenna is widely used wideband antennas, and are also the first choice of the impulse GPR, the ideal bow-tie antenna impedance is mainly determined by its flare angle. However, in general speaking, the bow-tie antennas of GPR are close to the ground, its impedance is also often affected by the underground medium, in order to match between antenna and emitting circuit and eliminate the influence that the underground medium impacts the antenna impedance, we hope that the flare angle of bow-tie antenna can be changed for achieving the same impedance of the antenna. Because the angle of common bow-tie is fixed and can’t be changed when the antenna is done, the paper changes the structure of bow-tie antenna and introduces a bow-tie antenna is made up of many line antenna. For improving the performance of antenna, the paper improves on the linear bow-tie using some ways such as loading the antenna and shielding the antenna. Using numerical analysis method, the paper analysis the radiation feature and impedance feature of the linear bow-tie antenna, and we draw some conclusion as follows:(1) Using the CST simulation tool, the paper analysis the radiation field of antenna. According the simulation result, it shows that in H plane the maximal direction of radiation is not below the antenna, but splits to the two sides of the antenna. With the increase of permittivity of medium underground, the radiation energy coupled into ground become stronger. At the same time, the underground medium is a dissipative medium, the penetrating depth of electromagnetic wave is decreaseing in the medium and there has been lagging.(2) For the influence of underground medium, when the antenna press close to the ground, the imput impedance of bow-tie will undulate with the different detection environment. By further analysing, the undulation of the antennas’input impedance with the frequency diminishes, the closer the antennas come to ground, the more prominent this effect is. When the GPR is used, the closer the antenna come to ground, the better effect of detection is. In order to match between antenna and emmiting circuit, we can change the flare angle of bow-tie antenna to maintain the impedance of antenna is a fixed value.(3) Comparing the loading antenna with unloading antenna, the performance of loading antenna is superior to non-loading antenna. The loading way is conductive to absorb the reflected wave, reducing the ring, the current distribution of loaded antenna is traveling wave. After loading of antenna, it is relatively small that the underground medium influence on the feature of antenna, the reason is the current distribution of loaded antenna is traveling wave in the free space or mediun half-space, the influence of medium is relatively small.(4) The way of resistance loading can improved the feature of antenna and wide the antenna’s bandwidth. However, the way of resistance loading can deduce the radiation efficiency of antenna. In order to solve the problem, we introduced a uncontinuous dot in the end of antenna, the dot can become the second radiation source of antenna. It is conducive to increase radiation efficiency of antenna.(5) The shielding measure can improve the performance of the antenna, Because of using shield meaure, the antenna can radiate more energy into the underground, the anti-jamming ability of antenna can be enhanced, it is conductive to increase detecting depth of radar and improve the resolution of the radar.4. As a key part of GPR, the designning a short pulse generator is very important, the amplitude of pulse directly impact the radiation power of antenna and the detection depth of GPR. The thesis analyses the theory and feature of several the pulse generating circuit, such as the pulse generator composing from avalanche transistor, step recovery diode (SRD), the Max-bank circuit and so on, it provides the necessary theoretic foundation for designning and realizing nanosecond and high voltage pulse generating circuit,Through comparing with these different kinds of circuit simulation and actual test results of the comparison, and we obtain some useful conclusions. As follows, the pulse generating circuit composing from SRD and avalanche transitor are of the characteristics of high repeat frequency; The pulse circuit composed of avalanche transistor is based on the mechanism of impact ionization avalanche breakdown, it has bigger phase noise.