Researches On Special Wideband Base Station Antennas

With the rapid development of mobile communication technology,wideband base-station antennas are required to cover multiple communication systems such as 2G,3G and 4G systems.Conventional sector base-station antennas with horizontal 3-dB beamwidth of around 65° have been widely employed to serve 120° sector cell.However,special antennas are in urgent need to meet the application requirements of base stations in special areas.For instance in some applications such as microbase stations and base stations in sparsely populated areas,omnidirectional antennas with 360o full coverage are more appropriate due to limited service size and cost saving.In various indoor environments such as elevator and stadium,narrow-beam antennas are desirable to cover their narrow communication areas.In densely populated areas,wideband multi-beam antennas are in urgent need to increase the channel capacity,thus preventing dropped calls.Therefore,the design methods of special base-station antennas are studied and several special base-station antennas are presented in this dissertation.Specially,the main research contents are as follows:(1)A dual-polarized wideband base-station antenna element and array covering the frequency band of 1.42-2.7 GHz are proposed.To generate ±45° dual-polarized radiation pattern for the element,four folded dipoles are connected together by coplanar striplines and excited simultaneously by two integrated baluns.To enhance the impedance matching over the operating band of 1.42-2.7 GHz,a parasitic patch is employed and placed above the folded dipoles.To stabilize the horizontal radiation pattern with 3-dB beamwidth of 70o ± 2.5o,a reflector is employed and designed in a convex shape.In order to further improve the horizontal 3-dB beamwidth within the range of 65.7o ± 3.2o,a two-element subarray is proposed.It consists of two antenna elements which are misaligned in the horizontal plane.Based on the misaligned subarray,a six-element antenna array with a high gain is proposed.(2)A wideband horizontally polarized omnidirectional antenna unit and a six-unit array covering the frequency band of 1.69-2.7 GHz are proposed.In order to generate HP omnidirectional radiation pattern and obtain good impedance matching over a wide frequency band for each unit,three new wideband planar dipoles are employed and excited by a wideband three-way power divider with equal magnitude and phase,conducting as a loop antenna.To further reduce the gain variation from 2.5 dBi to 1.2 dBi,a method by using a skewed two-element subarray is employed.Based on this method,a six-unit array with low gain variation is proposed for practical base-station applications.(3)A wideband dual-polarized omnidirectional antenna unit and a ten-unit array covering the frequency band of 1.69-2.7 GHz are proposed.In order to generate ±45o dual-polarized omnidirectional radiation pattern and obtain good impedance matching over a wide frequency band for each unit,three wideband crossed-dipole elements are employed and excited by two wideband three-way power dividers with equal magnitude and phase,conducting as a dual-polarized loop antenna.Based on the design of the unit,a ten-unit array with a high gain is proposed for practical base-station applications.The antenna array has a high gain of 10.3 ± 0.9 dBi and low gain variation of less than 2.5 dBi.(4)A wideband narrow-beam rhombic antenna array covering the frequency band of 1.69-2.7 GHz and a dual-wideband narrow-beam rhombic antenna array covering the frequency bands of 0.69-0.96 GHz and 1.69-2.7 GHz are proposed.To obtain narrow-beam radiation with low side lobe level in the horizontal and vertical planes over a wide frequency band for the wideband antenna array,four wideband crossed-dipole elements are uniformly arranged in a rhombic shape and excited by two wideband 1-to-4 power dividers with equal magnitude and phase.Based on the design of the single-wideband rhombic antenna array,a compact dual-band antenna array is designed by inserting four upper-band dipole elements inside four lower-band rhombic-arranged dipole elements.The rhombic antenna arrays has good impedance matching,high port-to-port isolation better than 28 dB,stable 3-dB beamwidth of around 33o and low sid-lobe level of less than-22 dB over the operating bands.(5)A wideband dual-polarized two-beam 4 × 6 antenna array covering the frequency band of 1.69-2.7 GHz is proposed.It consists of three 4 × 2 subarrays.For each 4 × 2 subarray,the lower and upper 4 × 1 subarrays are misaligned in the horizontal plane.By using this novel scheme,the 4 × 2 subarray is equivalent to a 8 × 1 subarray with a half adjacent-element spacing,resulting in good grating-lobe suppression.To achieve stable two-beam radiation with low side lobe over a wide frequency band,specific wideband beam-forming networks with little magnitude and phase imbalances are designed.Moreover,the adjacent-element spacing is optimized to obtain stable 10-dB beamwidth around 120°,thus satisfying the coverage requirement of 120° in the horizontal plane for base-station applications.The array has obtained desired two-beam radiation performance across a wide frequency band of 1.69-2.7 GHz: the cross levels around-10 dB between two beams,stable 10-dB beamwidth around 120° in the horizontal plane,and low side-lobe and grating-lobe levels below-18 dB of the two-beam radiation in the horizontal and vertical planes.