Study Of Stub Loaded Helix Antenna In Ground Penetration Radar

Antennas are designed to radiate or receive electromagnetic wave in the part of transmitting or receiving system. They are transducers which transform electric energy into waves so as to radiate radio waves, that's the primary purpose of antenna.The GPR direction radiates high frequency electromagnetic wave (1MHz-1 GHz) underground with the pulse, using transmitting antenna. When radar wave is transmitted underground, electromagnetic wave will be reflected if coming on stratum or object with different conductivity, and it will be received by antenna. Based on analyzing and processing radar wave by antenna, spatial position, structure, electrical properties and geometry of the objects underground can be concluded accord with received wave's shape, strength and time etc. Consequently, stratum or object underground could be explored.Due to Ground Penetration Radar be used in lower frequencies, the size of the antenna becomes large and carrying discommodious. A new reduced size axial mode helical antenna, named the Stub Loaded Helix, was introduced in this dissertation. Utilizing the characteristics of helix antenna operating in the axial mode including high directivity, broad bandwidth and circular polarization in Ground Penetration Radar is detailed described in this thesis.First the historical development of the helical antenna was outlined along with a discussion of how the helical structure supports traveling waves. In last years, the GPR becomes the main method that explores underground objects for the shallow layer, the application is more. As the main part of GPR, the studies based on antenna becomes significantly. For acquiring an improvement of the exploration depth and differentiate rate, the antenna must radiate the directive beam and have a more broad bandwidth. We explored the operation of the conventional axial mode helix as developed by Kraus. The theory of operation of the axial mode helix was expounded by Kraus and others were presented. Results of simulations using NEC were presented toexplain the operation of the conventional axial mode helix in terms of current phase along the helix and how it relates to the Hansen-Woodyard condition for the endfire operation. Also we discussed some of the variations on the basic axial mode helix. When helix antenna works at a low frequency, the size is very big, matching section can become physically large and awkward. Any antenna which reduces the size will mean the test convenience for the system of GPR, economical cost. As a transformation of helix antenna, the SLH has the basic attributes of the helix antenna, which reduce size of the antenna by the stub. Most involve the use of dielectric materials with moderate or high permittivity to create a slow wave structure on which traveling wave will propagate and radiate. While some of these approaches have been successful they usually suffer from two deficiencies: size reduction is achieved through the use of higher permittivity dielectrics which results in higher losses and lower radiation efficiency, and the cost of the dielectric materials are usually prohibitively high. The Stub Loaded Helix achieves its size reduction solely through the use of a unique and innovative geometry of the helix winding and does not require costly materials that limit performance. So the SLH antenna reduce the size of the antenna by utilizing itself structures, therefore SLH Antenna becomes the first choice for the studies of GPR antenna, For making various electricity performance of the SLH antenna according with the request of GPR, we adopt the HFSS 3D simulation software developed by Ansoft Company to modeling antenna for high frequency electromagnetic field. The HFSS simulation software is based on the finite element method. In the simulation of structures, the limits is pimping, as long as the users can be accurate to draw structures, and set the attribute of the structure material , the boundary condition, the fed method , can compute a significant results. We know that a key step of the finite element method is small matrix generated by the dispersing element. For 3D finite elements, choosing the rectangle piece, the tetrahedron and hexahedrons respectively ,as the general discrete element , but the different dispersed elements need compute accuracies, speeds, the memory respectively, because they are different. The Ansoft HFSS adopted tetrahedron as basic dispersed elements, using adaptive iterative arithmetic to compute the mesh .For making the result of the simulation more accurately, we use the HFSS software to set up model for the antenna, that required accurately setting the boundary condition, especially radiator must be away fromboundaries 1/4 wave length.Using the vector network analyzer tested S parameter of the antenna in the experiment, which fit together perfectly with simulation result, the result explained that the HFSS simulate the antenna was credible. Therefore the HFSS simulated the patterns, input impedance, gains, axial rate, and current distributions of the port respectively. The patterns of SLH antenna indicated endfire performance at frequency range1.1GHz-1.8GHz, the maximal gain 13.6dB. The axial ratio less than 3dB at 1.1GHz-1.6GHz, approximately circle polarization which was fit for detecting circles, and avoided the interferences from the horizontal and vertical polarization. We made use of HFSS proceed emulation about surface current distribution of antennae port, observe port's current distribution from 0 to 180 degree and the current value change balance gentleness at 1.5GHz using command Animate. SLH antennae adopt the feeding classification like helices at input impedance analytically, one port feed with concentric line internal conductor and the other port join with concentric line outer conductor, and generally we adjoin a metallic Mach disc at that terminal for eliminating current on concentric line shuck and acting helices electric loop. The reactance of SLH antenna hardly changes between frequency 1.1 GHz and 1.7 GHz and only float around zero. Therefore, the input resistance of antenna are approximate to pure resistance and so that the input impedance and output impedance can be matched. To test the capability of SLH antenna, we adopt SLH antenna and Vivaldi antenna to explore a cylindrical iron. The result shows the electrical capability of the SLH antenna can satisfy the request of GPR. Through the above analyses, these electrical properties of satisfy GPR 'requires, and the effects are ideal. Therefore, Ansoft HFSS is a very excellent electromagnetism computing software, for comparatively complex three-dimensional, if only the modeling is right, you can receive perfect result, so the applying of Ansoft software can save time and cost for the research of the GPR antenna, and greatly improve work efficiency, it is a suit of effective methods for engineering.The future development trend of the antenna is reducing size, so the next step, we concentrate on changing the structure of the antenna, at the same time we study match feed, and using loaded high permittivity can reduce size of the SLH antenna. For making the properties of the SLH antenna to optimization, we will continue to study the new function of HFSS, and then apply that to the research of theantenna.