A Research On Low-Profile Antenna With End-Fire Radiation Based On Yagi-uda Antenna

In recent years,there has been a great desire for the development of low-profile antennas with end-fire radiation because their broad applications in airborne,missile and in-vehicle communication systems.As a form of antenna that can achieve endfire,the Yagi-Uda antenna is widely used because of its advantages of simple structure,low cost and easy realization of low profile.With the development of the above-mentioned communication systems,more standards are put forward for the end-fire antenna,such as miniaturization,large beam angles,multiple bands,high gain,switchable beams and so on.Based on the aboved background the basic theory of the Yagi antenna,the profile of the Yagi antenna is reduced by two forms of microstrip and top-hat loading.Meanwhile,the working characteristics of the Yagi antenna at different modes are deeply analyzed,and a variety of planar Yagi antennas with miniaturization,large beam angle,multi band,high gain,beam switchable and other functions are designed,which will provide some experiences for the design of planar antennas.The main work and innovation of the paper are as follows:1.Through a four-element microstrip Yagi antenna with a slot-loaded reflector and two pin-loaded directors,the characteristics of the microstrip Yagi antenna working at TM₀₁mode and the method to reduce the size and enlarge the beam angle of the microstrip Yagi antenna is studied.Firstly,the size of the reflector is reduced by etching a slot on it so that the size of the reflector is consistent with the size of the driven element,thereby reducing the area of the entire radiation patches and achieving miniaturization.Secondly,through the simulation results,the method of expanding the beam angle under the antenna structure is studied,and the antenna pattern is calculated by the equivalent magnetic currents to give a theoretical explanation.Finally,a four-element antenna with a size of 1.66λ₀×0.83λ₀is fabricated and measured.The measured results show that the maximum tilt angle of the beam reaches approximately 56°from the broadside direction.2.The method for the microstrip Yagi antenna to realize the endfire radiation on the infinite floor is researched,and the microstrip Yagi patch antenna working at the high-order mode TM₀₂mode is proposed for the first time.It is widely known that the magnetic currents around the two radiating edges of a driven patch are out of phase at TM₀₂mode.As such,the far-field radiation pattern generated by these two magnetic currents will cancel in the broadside direction but superpose in the end-fire direction.As a result,an exact end-fire radiation can be intuitively generated for the antenna on an infinite ground plane if the parasitic elements are properly formed and allocated.The radiation characteristic of the microstrip Yagi antenna working at the high-order mode is thoroughly studied and a four-element microstrip Yagi antenna is finally designed.The simulated results show that the antenna can achieve exact end-fire radiation if the ground plane is infinite and the measured gain of the antenna in the entire frequency bands is 7.2d Bi to 8.9d Bi.In order to further increase the gain of the antenna and reduce the side lobe of the antenna,a five-element high-gain quasi-microstrip Yagi antenna based on TM₀₂mode is designed.The antenna uses two parasitic elements of the same size as a group instead of the original single parasitic element.This arrangement makes the aperture of H plane is enlarged,so that the beamwidth of the H plane will be narrowed and the beam will be more directed to the director.Thereby the gain of the antenna will be increased and the side lobe of the antenna will be reduced.The designed antenna has a measured gain of8.8d Bi to 11.3d Bi in the entire frequency band,and the sidelobe level in the entire frequency bands is below-9d B3.The method for a microstrip Yagi antenna to realize dual band by using TM₀₁mode and TM₀₂modeis studied.In order to achieve dual-band quasi-endfire radiation on a single-layer substrate,four slots are etched on each patch to adjust the resonant frequency of the TM₀₁mode and TM₀₂mode,so that the electrical spacing between the two patches can meet the requirements for achieving quasi-endfire radiation at the both modes.A five-element microstrip Yagi antenna is fabricated and measured.The measured results reveal that the dual operating bands of 2.76～2.88 GHz and 4.88～5.03GHz for|S₁₁|<-10 d B are satisfactorily achieved.4.The design method of low-profile Yagi antennas to realize beam switching is researched and three quasi-Yagi antennas with beam switching function are proposed.Firstly,a dual-port microstrip Yagi antenna with beam swiching ability is designed.By switching between TM₀₁mode and TM₀₂mode of the antenna,two states can be obtained.The operating bands of the dual modes are reallocated in close proximity to be overlapped with each other by introducing the shorting pins and narrow slots on each patch.Two feeding ports are utilized herein to switch and isolate these two modes instead of a single port.The proposed antenna has only three radiating patches,so it has a compact size.Meanwhile,there are fewer switches introduced in the design,which will simplify the structure of the antenna.A single-port microstrip Yagi antenna with beam swiching ability is then designed.By placing two small-sized rectangular patches on the non-radiating sides of the driven patch,the antenna realizes beam switching.The center of each rectangular patch is loaded with a shorting pin,and the working state of the antenna can be changed by controlling the shorting pin to be connected to or isolated with to the ground plane.Compared to the other beam-switching microstrip Yagi antennas,the proposed antenna has a simple structure and a compact size.Finally,a top-hat monopole Yagi antenna is designed.First,a metal hat is loaded on the monopole to reduce the profile of the antenna.Second,the six top-hat monopoles are placed around the driven monopole as parasitic elements.When the parasitic element is connected to the ground floor,it can be used as a reflector.Therefore,the state of the antenna can be controlled by control the parasitic monopole to be connected to or isolated with to the ground plane.The designed antenna has three working states and the beam scanning in the horizontal plane over a 360°range can be achieved.