Study On Key Techniques Of Wideband Directional Antennas For Special Ground Penetrating Radar Systems

Ground Penetrating Radar(GPR)is one of the most important techniques in near-surface geophysics.Special GPRs such as borehole radar,air-borne GPR,or forward-looking GPR are powerful tools applied for particular environments,in which conventional land surface GPR is incapable or inadequate.Antenna,as one of the most crucial components in GPR system,can transmit and receive electromagnetic waves at required frequencies,completing the energy exchange between environments and radar system.The performance of the antenna usually determines the performance of the whole GPR system.Horn antennas,dipole antennas,bow-tie antennas and Vivaldi antennas are frequently used types of antennas in conventional GPR systems.When it comes to special GPR systems,because of their particular working environments such as deep subsurface or high altitude,antennas in these systems are easier to be effected and restricted by the surroundings,which makes the antenna design in these systems more complex and difficult.As special GPR systems always require antennas with wide working bandwidth,high directivity and compact structure,this dissertation mainly focuses on the research and design of wideband directional antennas for special GPR systems.The main contents are summarized as follows:1.The research and design of directional receiving antennas in borehole radar.First of all,by calculating the surface current,port impedance and electric field of a dipole antenna in borehole environment,the basic theory of borehole radar antenna is introduced.Then,two wideband directional receiving antennas for borehole radar are proposed.One is a dipole antenna with RC load,and the other is a Vivaldi antenna.Both antennas are embedded in a cylindrical fiber-reinforced plastic(FRP)with metal reflector mounted on the back side to improve the direction of the antenna.And then,the proposed directional dipole antenna is fabricated and measured.The measured results show that the proposed directional dipole antenna has a very wide operation bandwidth from 190 MHz to 350 MHz.The front-to-back ratio of the time-domain radiation pattern of the proposed antenna is more than 10d B,and the trailing of the receiving signal is small and short,which indicates the proposed directional dipole antenna with a good directional radiation performance.Finally,the proposed directional dipole antenna is installed into a directional borehole radar system.The borehole experiment results show that the directional borehole radar is able to provide3D-information of the cracks around it.2.The research and design of a wideband directional antenna and a cooperative target in UAV-borne GPR.Firstly,a basic theory of air-borne GPR system is introduced.Then,an optimized ultrawide band antipodal Vivaldi antenna(AVA)with modified radiation pattern and compact size is proposed.With exponential tapered slots and slot edges,the proposed AVA demonstrates a wide operating band from 300 MHz to more than 2 GHz.The fabricated prototype of this antenna with a compact size of 450 mm×600 mm×1.5 mm has been measured in anechoic chamber.Both simulation and experimental results show that good radiation patterns with gain 4.4-11.5 d Bi in the end-fire direction could be achieved in the whole working band,which indicates that the proposed antenna is suitable for UAV-borne GPR application.And then,a Surface Acoustic Wave(SAW)sensor based cooperative target is proposed to restrain the environment clutters of the UAV-borne GPR signal.Because of the micro-second delays of the SAW sensor,the signal echoes of the cooperative target can be distinguished from the surrounding reflections,which helps for locating the real target.And as the YXl/128°lithium niobate piezoelectric substrate of the used SAW sensor has high sensitivity to temperature and insensitivity to moisture and pressure,a simple concrete temperature monitoring system is set up by combining the proposed cooperative target with a stepped frequency continuous wave(SFCW)radar.The experimental results show that the proposed method can measure the temperature change of concrete continuously.3.The research and design of an ultrathin-broadband-absorber-based directional antenna for forward-looking GPR system.Firstly,by using transmission line equivalent circuit theory,the basic principle of the ultrathin broadband absorber consisting of frequency selective surface(FSS)and magnetic materials is introduced.Then,a 4-step design method of the ultrathin broadband absorber is proposed.And then a V-shaped-FSS-based ultrathin broadband absorber is also proposed by using the 4-step design method.The proposed absorber achieved a broad absorption band of 50?400MHz with-10 d B reflectivity,and its thickness is smaller0.01?₀.Finally,a wideband directional antenna for forward-looking GPR system is designed and fabricated by utilizing the proposed absorber.The measured results got in an anechoic chamber show that the proposed antenna has a very good directional radiation performance within a wide operation band from 130 MHz to 275 MHz.In addition,because of the ultrathin absorber,the proposed antenna is extremely compact with the whole size of0.52?_c?(15)(13)(24)?_c?0.06?_c.Experimental results also demonstrate the proposed antenna with a very good time-domain radiation performance and can work well in a pulse forward-looking GPR system.