Design And Simulation Of A Novel Borehole Radar Antenna

Borehole pulse radar has been extensively studied in recent decades because of its high resolution, far detection distance, simple structure, etc. It has been widely used in the areas such as petroleum exploration, geological surveying, and cavity detection.One of the most critical hardware components for the borehole pulse radar system is the antenna. And the main purpose of antennas is to transmit and receive electromagnetic waves in the borehole. Therefore, the antenna performance is play an important role in a borehole radar system. And the antennas applied in the borehole radar system which makes design and related research is an important component in the entire borehole radar system study. Borehole radar antenna is worked in the downhole environment, and the antenna will covered in a radome (a structural, waterproof enclosure that protects a microwave antenna), the diameter of the antenna is limited by the borehole aperture, so the antennas’operating frequency, operating bandwidth, gain, directivity, and radiation waveform will be affected, then it is difficult to achieve the excellent characteristics of all aspects. In this paper, the characteristics of the borehole radar antenna, using a new type of ultra-wideband antenna loading technique, namely Wu-King loading methods, and on this basis, a cavity-backed dipole antenna with ferrite (an absorbing material) substrate is adopted to improve the antennas’directivity and broadband performance.The design of borehole radar antenna in this paper is ultra wideband dipole antenna which load profiles, the resistive loading based on Wu-King resistive loading profile. According to a special way which is logarithmically spaced to set multiple lumped resistive elements symmetrically loaded into the cylindrical dipole antenna on the arms. Ideally, the Wu-King profile makes the current on the antenna arms travel without internal or open-end reflections, and stabilizes the input impedance over a wide bandwidth at expense of the radiation efficiency. When the current travel along the antenna arms and at the same time the antenna radiate energy to the surrounding space, then the loading profiles absorb part of the electromagnetic energy. And that energy reflected from the end of the antenna is significantly reduced, so that the antenna currents near in the traveling wave, thereby improving the performance and impedance wideband antenna performance and reduces the length of the antenna. In addition, the resistor-loaded dipole is embedded in the ferrite substrate material, and the substrate material is coated by a metal back cavity that can further optimize the performance of the antenna, and the metal back cavity can also play a role in some of the radome. When we choice the filler material, taking into account the absorbing properties of the ferrite material and matching relationship with the surrounding medium, such as limestone and other drilling medium, then we choose ferrite absorbing materials as the substrate material in the antenna design of this paper.There are two type of dipole antenna which based on Wu-King resistive loading profiles and embedded in the ferrite substrate material which is coated by a metal back cavity were designed. One is the rectangular back cavity, in this case, optimizing the design by the CST, we can see the S-parameters in this model have two intervals below-10dB bandwidth is 29MHz～38MHz and 74MHz～600MHz respectively, half-power time-domain direction of the H-plane is±50.4°. The other one is the cylindrical back cavity, optimizing the design by the CST, S-parameters below-lOdB bandwidth is 34MHz-156MHz, up to 5-octave bandwidth, and the half-power time-domain direction of the H-plane is±135°. By comparing two different cavity-backed antennas loaded, we can see that the cylindrical cavity-backed loaded dipole antenna have some advantages than the rectangular cavity-backed loaded dipole antenna just like the operating frequency is lower, the detection distance is farther.