Study On Circularly Polarized Non-uniform Loop Antenna

Circularly polarized waves can be employed to suppress rain and fog interference and resist multipath reflection in wireless propagation.Circularly polarized antenna can receive arbitrary polarized waves.Now,circularly polarized antenna has been widely used in RFID,satellite navigation,mobile communications and a variety of wireless communication equipment.Various structures have been used to design circularly polarized antennas.Especially,loop structure is often utilized in the design of circularly polarized antennas.The theoretical research of loop antenna can be dated back to the 1930 s.The basic principles,operation modes,circuit properties and radiation behaviors of the loop antennas have been elaborated in many classical works.Based on the classical theory of loop antenna,this thesis analyzes the operation mode of the loop antenna.Then,we propose that a non-uniform traveling-wave loop antenna with perimeter of one wavelength under even-mode excitation is an inherent circularly polarized(CP)radiator.The main content of this thesis is organized as follows:1.Based on the classical theory of loop antenna,this thesis analyzes the even mode of the loop antenna,establishes the equivalent sources model.According to the current distributions in the equivalent sources model,the natural boundary conditions of the loop antenna can be automatically determined and the radiation behavior of the loop antenna under even-mode is evaluated.2.According to the operation principle of the even-mode loop antenna,a simple square loop antenna with perimeter of one wavelength fed on corner is proposed.The antenna model is constructed and simulated by HFSS.The CP performance of the antenna is investigated and the most critical parameter is found.By carrying out parametric studies,an optimized non-uniform loop antenna with perimeter of one wavelength is designed.3.On the basis of the above-mentioned non-uniform circularly polarized loop antenna,we further discuss the design of circularly polarized antennas with directional radiation.We propose two methods.First,we plan to add a metal reflector on the bottom of the antenna and verify the antenna performance by HFSS.The other method is the usage of differential excitation with symmetrical dipole on the diagonal of the loop antenna.Then the prototypes of one kind of the antenna are fabricated and measured by Agilent's 8720 ET vector network analyzer and Satimo Starlab near field antenna measurement systems.The measured results show that the trend of circularly polarization of the antenna is clear and verify the correctness of the theory of even-modeloop antennas.The operating frequency band of the antenna is 2.35-2.55 GHz.The proposed antenna has a unidirectional CP radiation characteristic.