| The Ground Penetrating Radar (GPR) is a new type and effective detecting equipment for object which is under shallow ground. It is widely used for detecting of hydrological and engineering geology, and is playing a more and more important role. The antenna system of GPR is a key part of GPR. The system is consists of a generator of high voltage and short pulses, a pulse radiation antenna, a receiving antenna, a broadband and low-noise RF amplifier, sampling transform circuits and MF amplifiers. In order to match for different detecting environment and objects, different frequency of the antenna system must be chosen. The central frequency of the system is changed from 10MHz to 2000MHz. The performance of the antenna system decides directly the detecting effects of GPR. According to the characteristics of GPR, this thesis researches mainly the problems in analysis, design and realization of GPR antenna system. The main contents of the thesis are described briefly as follow:1. Antenna system is important part of GPR for electromagnetic (EM) wave transmission and receiving, the system is working under control of GPR main machine, realizing the transmission of probing pulse and receiving of echoes. At first, based on the principle of GPR, the thesis describes the basic construction of GPR and whole thought of design, and then analyses the relationship between the detection characteristics of GPR and the construction of antenna system.2. According to the principle of impulse GPR, the short pulse generator is an important basic circuit of GPR, it is an important part of radiation excitation and sampling circuit. The thesis analyses the principles and characteristics of the pulse generator composing from avalanche transistor and step recovery diode (SRD), the analysis provide the necessary theoretic bases for design and realization nanosecond and sub-nanosecond pulse circuit.Through analyzing and comparing the pulse circuit composed from avalanche transistor and SRD, we can obtain some useful conclusions as follows: (1) Both two circuits are of the characteristics of high repeat frequency of pulses, but the circuit composed of SRD is of higher repeat frequency which may high to tens MHz. (2) The pulse circuit composed of SRD can adopt low voltage power supply and produce high voltage pulse under the aid of switch circuit, but the pulse circuit composed of avalanche transistor needs high voltage power supply to cause avalanche breakdown. (3) Rise (or Fall) time of pulse of the circuit composed of avalanche transistor is generally in a few nanoseconds or above Ins, but the rise (or fall) time of the pulse of the circuit composed of SRD can reach tens p-seconds. (4) The pulse circuit composed of avalanche transistor is based on the mechanism of impact ionization avalanche breakdown, compared with the circuit composed of SRD, it has bigger phase noise, leading to jitter of pulse period.IV3. Bow-tie antennas are widely used broadband antennas, and are also the first choice of the impulse GPR. The present thesis paper uses the finite difference time domain (FDTD) method to analyze the pulse radiation characters of Bow-tie antennas and scatter echo characters under several typical conditions. These theory analyses have certain guidance purport to the design of GPR antennas and processing of echo signal.In general, the maximal detection depth of GPR is from a few wavelengths to more than ten of wavelengths, although it is not appropriate to analyze the antennas of GPR with the far field radiation character of antennas. According to the analysis to direction character of bow-tie antennas with FDTD, 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 decrease of permittivity of medium underground, the radiation energy coupled into ground becomes stronger.Because of reflection coming from antenna's end, the excitation pulse acted on GPR will form attenuation vibration on the antenna. After the end of excitation volta...
|
PDF Full Text Request