Research On Wideband Antennas And Orbital Angular Momentum Antennas

With the rapid development of modern society,wireless communications play an important role in daily life and work,and user's experience is directly affected by the capacity and quality of communications.As an effective method to enhance the channel capacity,wideband technology has draw extensive attention and deep research.Modern wireless communications put forward higher requirements for the operating bandwidth,radiation characteristic,structure and dimension of wideband antennas.In this dissertation,the research on wideband antennas starts from wideband single antenna,then extends to multi input multi output(MIMO)antennas and array antennas.Combining with band notched/multiband/spatial division multiplexing technologies,a series of wideband antennas with small dimensions are developed.Besides,the orbital angular momentum(OAM)antennas using mode-division multiplexing,which become a new hotspot in the research field of wireless communications,are also discussed and studied in detail.The major contributions obtained in these two parts are as follows:1.The compact printed wideband/multiband antennas are studied and three antenna designs for different communication systems are developed.By loading a U-shaped resonator in the half-ring radiator,an additional resonance and an equivalent parallel resonance can be introduced for producing notched bands indirectly and directly,respectively.Hence,an ultrawideband antenna with 5.2/5.8 GHz Wireless Local Area Networks(WLAN)and 3.5/5.5 GHz Worldwide Interoperability for Microwave Access(Wi MAX)dual-band notched applications is proposed.The ring antennas and slot antennas used in portable terminals have narrow resonant bands,and the antennas with ring/slot resonators have disadvantages such as complex structures,high cross-polarizations and less omnidirectional patterns.To address this problem,a planar quasi self-complementary wideband antennas for multi-mode applications is designed.The radiators and ground plane are modified to improve impedance matching and producing additional resonances,a multi-mode wideband antenna with simple structure,good radiation characteristic and ability of covering 2G/3G/4G working bands.Based on similar design idea,a quasi self-complementary antennas for tri-wideband WLAN/Wi MAX applications is presented.2.The compact wideband MIMO antennas with high isolation are investigated,two types of antenna elements are developed and adopted to construct three MIMO antenna with different arrangements.The circular monopole antenna with modified reflective ground and tapered feedline is employed as an antenna element,a dual-element MIMO antenna is formed with horizontal mirror configuration.By etching a T-shaped slot on the ground,the port isolation across the entire UWB bands is improved to better than 20 d B.Based on the same antenna element,a four-element MIMO antenna is constructed with orthogonal arrangement.With etched rectangular slits on the ground,the antenna has port isolation of more than 15/20 d B at the lower/higher band.The tapered monopole antenna with microstrip feedlines is used as another antenna element,a dual-element MIMO antenna is designed with orthogonal arrangement,and good polarization diversity performance can be realized.Without any decoupling structures,the port isolation of the proposed antenna at both lower and higher WLAN bands is greater than 20 d B.3.The tightly coupled antennas are combined with polarization diversity technology,and dual-polarized tightly coupled array antennas are analyzed and designed.The bandwidth limits of ideal tightly coupled arrays with different substrate materials and orders are calculated on basis of the impedance matching equations.The bow-tie dipole is employed as an antenna element,and the coupling capacitors are introduced by overlapping of the dipole arms in two polarizations.Then the original thick superstrate is replaced by a doublelayer metal patch superstrate,and a microstrip feedline to parallel transmission line structure transition as well as shorting metal cylinders are employed for balance feed.Morever,the antenna design is extended from the infinited periodic array to the finite array.A 4×4 dualpolarized array antenna is constructed,and the truncation effect is reduced by prolonging the metal arms of the edge dipoles.In the dual-polarized wideband feeding networks,there is no overlaps between the microstrip feedlines in two polarizations due to the use of right-angle transitions.4.The OAM antennas using mode-division multiplexing technology are studied,and dual polarized multi-mode OAM array antennas are developed with the existing polarization diversity technology.Circular phased array is built to genearte OAM waves,and a quasicross-shaped aperture-coupled microstrip antenna excited by a U-shaped and an M-shaped microstrip feedlines is chosen as an antenna element,which leads to a single-layer feeding structure and two orthogonal polarizations with high isolation.In order to produce distinct OAM modes in the two polarizations,two phase-shifting feeding networks with different phase steps are used to feed the array antenna.Besides,the feeding network can be simplifed by adopting a mirror image arrangement,and finaly +1 and-1 OAM modes can be obtained simultaneously.To increase the mode numbers and improve the mode purity,two eightelement OAM array antennas are designed using mutiple feeding method,and OAM waves with mode ±1 and ±2 can be generated in two polarizations,respectively.On this basis,the three designed antennas have been fabricated and tested.The mode purity of generated OAM waves with different antenna numbers and modes is evaluated and analyzed based on the near-field measured results,and the mode purity enhancement is obtained quantificationally.