Miniaturization And Bandwidth Enhancement For Omnidirectional Circularly Polarized Antennas

Omnidirectional circularly polarized antenna is widely used in wireless communication due to its omnidirectional radiation mode and circular polarization characteristics.Because the omnidirectional and circular polarization characteristics of antenna can not only cover a large service area,but also support the free arrangement between receiving antenna and transmitting antenna.In recent years,there have been many studies on omnidirectional circular polarization antenna.In the first chapter,the research background and development status of omnidirectional circularly polarized antenna are introduced.Three ways to realize omnidirectional circular polarization are summarized,and the corresponding research status of each way is given.On the basis of the research status,the development trend of omnidirectional circularly polarized antenna is summarized,which includes miniaturization,broadband and multi-band design.The second chapter mainly introduces some related theories.It can be divided into three parts:1.the definition,parameters and related properties of circular polarization wave;2.the common realization methods of omnidirectional circular polarization antenna.Two realization methods related to this paper,normal mode helical antenna and tilted dipole array,are introduced in detail;3.the related knowledge of electric small antenna,including the definition and calculation related to Q value and parasitic realization of electric smallness by near-field resonant elements.The principle of antenna matching.Chapter 3 focuses on the miniaturization design of omnidirectional circularly polarized antenna.The work of this chapter can be divided into two parts:impedance matching and bandwidth expansion of normal-mode helical antenna in small size,and equal-amplitude in-phase current realization of multi-loop normal-mode helical antenna.For the normal mode helical antenna with small size,the antenna is matched to 50 ohms transmission line by near-field coupling feeding,and then the overall bandwidth of the antenna is expanded by increasing the antenna size.For multi-coil helical antenna,a double-coiled helical antenna with current close to equal amplitude and phase distribution and helix length close to wavelength is designed by using multi-stage coupling theory.Finally,the realization of the optimal impedance matching bandwidth is preliminarily explored.Chapter 4 focuses on the broadband design of omnidirectional circularly polarized antenna.In this chapter,we focus on the design of omnidirectional circularly polarized antenna with tilted dipoles,and do two parts:the optimization of dipole bandwidth and the design of feeder network.Firstly,a broadband omnidirectional circularly polarized antenna is obtained by using a branch dipole and a feeder network consisting of four Barren plus four quarter wavelength impedance transformers.Then,a periodically capacitively loaded broadband dipole is designed,and an omnidirectional circularly polarized antenna with wide axis ratio bandwidth and impedance bandwidth is obtained by using the tilted dipole array.Chapter 5 summarizes the work of this paper,and puts forward many shortcomings of this paper,and also describes the work to be carried out in the future.