Research On Broadband Antenna Technology For Wireless Communication

With the rapid development of wireless communication,high data rate and high speed rate have become an important trend of modern wireless communication systems,which puts forward high demand on the antenna bandwidth.In order to cover multiple frequency bands or a very wide band,multiple antennas can be used,but they would increase the cost and complexity of systems.Broadband antennas can cover multiple bands at the same time,decreasing the cost and complexity,which has become an important research topic.In this dissertation,broadband antennas,including broadband microstrip omnidirectional antennas,circularly polarized antennas with bidirectional pattern,broadband circularly polarized antennas with directional patterns and broadband circularly polarized antenna arrays have been studied.The main research contents are summarized as follows:1.Broadband microstrip monopole antennas with omnidirectional patterns are studied.Periodical patch array of Artificial Magnetic Conductor(AMC structured metasurface has the property of zero phase reflection and can support surface waves.With these characteristics,two low profile surface wave antennas which can generate omnidirectional patterns are proposed.Firstly,a broadband surface-wave antenna using uniform metasurface is proposed.By loading a truncated metasurface ring in the feeding layer,the antenna with a low profile of 0.07?₀?achieves a wide impedance bandwidth of 19.4%(4.69?5.7 GHz and an average gain of 7.32 d Bi.Additionally,the antenna generates a monopole-like radiation pattern with a very low cross polarization level.Secondly,a broadband surface wave using non-uniform metasuface is proposed.The non-uniform metasurface with a tapered distribution can simultaneously excite two resonate modes of a main mode TM₀₁ and a higher order mode TM₀₂,significantly enhancing the bandwidth.Further,by reducing the size of ground plane,the TM₀₂ mode is improved and the surface-wave fields are enhanced,which simultaneously increases the antenna bandwidth and gain.The measured results show that the antenna with a low profile of 0.07?₀?achieves a 33.1%(4.56?6.37 GHz impedance bandwidth and a good omnidirectional radiation pattern with very low cross polarization.2.Broadband circularly polarized printed antennas with bidirectional patterns are studied.Firstly,a simple circularly polarized(CP monopole slot antenna is proposed.By using a C-shape radiator and loading a wide circular slot on the ground plane,the antenna achieves a very wide impedance bandwidth of 120%(3.2?12.8 GHz and an axial ratio(AR bandwidth of 60.7%(3.1?5.8 GHz.In addition,it generates bidirectional radiation patterns with a left-hand circular polarized(LHCP radiation pattern in the+z direction and a right-hand circular polarized(RHCP radiation pattern in the–z direction.Then,a broadband circular polarized monopole antenna with parasitic loops and defective ground plane is proposed.By loading a pair of asymmetric square open loops on the both sides of the monopole antenna,the loop antenna mode is excited to generate three circularly polarized resonate points for achieving broadband CP radiation.The defective ground plane with a vertical slit and a horizontal slit can excite the monopole mode to generate additional resonate points in the upper band.The measured results show that the antenna has a wide impedance bandwidth of 89.7%(2.55?6.7GHz,an AR bandwidth of 84.6%(2.59?6.39 GHz and bidirectional circularly polarized radiation patterns in the+z and-z directions,respectively.Finally,a broadband circularly polarized printed crossed dipole antenna is proposed.Two orthogonal flower shaped dipoles are utilized to generate CP radiation with wide impedance bandwidth.By loading two half circular rings that connected by metal vias at the both sides of substrate,AR bandwidth is significantly enhanced.In addition,four parasitic patches are introduced into the corners of substrate to further improve the AR bandwidth.The measured results show that the antenna achieves a wide impedance bandwidth of 121%(1.66?6.74 GHz and an AR bandwidth of114.8%(1.8?6.38 GHz.Over the CP passband,stable bidirectional CP radiation patterns in the?z directions are generated.3.Broadband circularly polarized crossed dipole antennas with directional radiation patterns are studied.Firstly,a broadband circularly polarized crossed dipole antenna with an asymmetrical cross loop is proposed.By controlling the circumference of the loop and properly adjusting the length of the loop along the x and y directions,fundamental mode and high order mode of loop antenna are excited to significantly enhance the impedance and AR bandwidths.The measured results show that the antenna achieves an impedance bandwidth of 67.5%(3.14?6.34 GHz,an AR bandwidth of 53.4%(3.83-6.62 GHz and a maximum gain of 10.62 d Bi.Secondly,a broadband circularly polarized crossed bowtie dipole antenna with parasitic elements is proposed.Based on the sequential rotated phase technique,three groups of four-element parasitic radiators,including of unequal cross slots,triangular patches and rectangular patches are introduced to excite three additional resonant points for significantly enhancing the AR bandwidth.The measured results show that the antenna achieves a wide impedance bandwidth of 93.1%(2.21?6.06 GHz,an AR bandwidth of 90.9%(2.4?6.4 GHz and a maximum gain of 9.1 d Bi.And then,a low-profile broadband circularly polarized crossed fractal dipole antenna is proposed.To achieve broad bandwidth,two bowtie shaped dipoles are cutted by three slits into fractal triangular patches.Four parasitic triangular patches are loaded at the bottom of substrate and four inverted L shaped metal patches are loaded on the ground plane,not only enhancing the AR bandwidth but also reducing the antenna profile.The measured results show that the antenna with a low profile of 0.126?₀ achieves a wide impedance bandwidth of 92.9%(2.65?7.25 GHz,an AR bandwidth of 81.4%(2.9?6.88%and a maximum gain of 7.96 d Bi.Finally,a broadband circularly polarized crossed stepped dipole antenna is proposed.Two step shaped dipoles are used to improve the amplitude ratio of two orthogonal modes for achieving a broadband CP radiation.By introducing the cross slot into the antenna,the cross slot is excited to generate magnetic fields,which can be equivalent to a magnetic dipole.The crossed dipole and cross slot are combined to realize a pair of complementary dipoles,significantly increasing the AR bandwidth and improving the radiation pattern.The measured results show that the antenna achieves a wide impedance bandwidth of 75.3%(3.45?7.62 GHz,an AR bandwidth of 61.2%(3.86-7.26 GHz and a maximum gain of 11.9 d Bi.In addition,the antenna generates RHCP radiation patterns with very symmetrical patterns and low cross polarization in the E and H planes.4.Broadband circularly polarized antenna array is studied.Firstly,the operation of circular polarization for the sequential rotated phase array is discussed,and the equivalent circuit of the ring shaped sequential phase feeding network is analyzed.Secondly,a broadband CP crossed stepped dipole antenna is utilized to analyze a 2×1,2×2 and 4×4 antenna array based on the sequential rotated phase array technology.Finally,appropriate feeding networks are proposed to design these antenna arrays.The measured results show that the AR bandwidths of the antenna arrays are 52.9%,73.1%and 83%respectively.It indicates that the design of a circularly polarized antenna array by using sequential rotated phase array technology can effectively enhance the AR bandwidth and gain by increasing the element of antenna array.5.Broadband circularly polarized multi-beam switched antenna array is studied.First,without using any shifter and cross junctions,a simple four-input port feeding network composed of four double box branch-line couplers is designed.Then broadband circularly polarized crossed dipole antennas are used to design a 2×2 antenna array.With the suitable feeding excitation mechanism the 2×2 circularly polarized antenna array can generate circularly polarized beams in nine directions.The measured results demonstrate that the proposed antenna array has a very wide impedance bandwidth of?68.9%and an AR bandwidth of?44%in every beam direction.Additionally,it achieves stable radiation patterns with a maximum gain of?11.7 dBi.