Research On The Theory And Applications Of Time-domain Antenna In GPR

Principles and key techniques of time-domain antennas for pulse ground penetrating radar(GPR)without carrier are thoroughly studied in this paper.Some important concepts of time-domain antennas are clarified,and difference between time-domain and frequency-domain antennas in idea of design and methods for analyzing is pointed out.New concepts of waveform fidelity and independence matching for time-domain antennas are presented.Concrete demands from GPR without carrier for its antenna are clarified based on the above.After comparing of GPR antennas both inside and outside our country,we developed a new type of ridged TEM pulse horn antenna,with resistor loaded.Computer simulation and experiments on representative targets are carried out with this antenna.Both theoretical analysis and practical application prove that the developed time-domain antennas have virtue of good shape of radiation waveform,high efficiency of radiation,little radiation in back and side,high isolation of transmitting and receiving,and so on.At the same time,the antenna can meet the demand of detecting for target underground in a not big depth,helped by its simple structure,small size as well as light weight.Contents of the paper can be divided into two parts.One is to clarify important concepts and rules for designing and analyzing in theory;the other is to solve key technical problems that will meet in application of the antenna.Theoretically,concept of waveform fidelity is studied thoroughly first,followed by the idea that waveform fidelity is fairly blurry,and waveform fidelity in absolute meaning does not in exist.Waveform relation between an antenna used in condition of transmitting and receiving is derived from reciprocity theorem in frequency domain.Difference between time-domain antenna and ultra-wide band antenna is marked out.After discussion of waveform fidelity,general demand of pulse GPR without carrier for time-domain antenna is summed up for the first time,based on experience of our designing.Generally,the five aspects are ultra-wide band,waveform fidelity,low band of microwave,requirement for valid area of radiation,and request for reduction of direct coupled wave.Dissimilarity of analyzing methods and designing rules for antenna in time-domain and frequency-domain is pointed out.False idea of analyzing and designing antenna by impedance matching in frequency domain is corrected.Based on comparison of radiating features of three horn antennas,the new concept of impedance matching in time-domain is put forward,and concept of time sequence together with theory of wave mode are used to provide a reasonable explanation.Important difference between performance of loaded antennas in time-domain and frequency-domain was revealed by numerical calculation.We clarify clearly that loading is totally different for transient and stable radiation.For application,aimed at balance transformation for coaxial cable to parallel twin cables, two new time-domain baluns are designed.For different structures in the baluns,diverse feeding analysis models are used.Concepts of time-domain are first used to discuss impact of baluns on impedance matching,balance of current and radiating waveform.Thorough study is carried out on simulation and experiments of loaded planar dipole butterfly and TEM horn antennas.A resistor loaded dielectric ridge TEM horn antenna is developed,who avoids unbalanced feeding in structure and can attain good unilateral radiating without extra baluns. Another important feature is that a wedge dielectric ridge is loaded between the metal bottom board and the slant board.The ridge has function of aggregation and guidance for electromagnetic field,making the field go forward in the wedge while reducing the coupling field in side directions.At the same time,overlapped footprints of the receiving and transmitting antennas can be enlarged,thus,signal from the target will boost.Experiments have been done on typical target underground----point target,pipelines and surface target.The results indicate that the antenna has ultra-wide band,good radiation waveform,high radiating efficiency and small radiation in back and side as well as high isolation between receiving and transmitting antennas,can be applied on the ground penetrating radar and meet the demand of detecting shallow targets underground.