Planar Microwave Lenses And Their Application In Multibeam Antennas

In applications such as satellite communications,5G millimeter wave communications and radar,antennas are required to have high gain and wide-angle coverage.Multi-beam antennas provide a solution for this demand.As a common multi-beamforming network,lens has outstanding advantages such as low cost,low complexity,fast response,wide bandwidth,and no feed source blocking effect.However,the lens also has the disadvantages of large size,low efficiency for large-angle scanning,and difficult to be integrated with antennas.This paper mainly focuses on planar Rotman lens,Luneburg lens and their applications,aiming to realize high efficiency,miniaturized microwave lens and wide-angle coverage,highly integrated and high gain multibeam antenna.The major contributions of this thesis are summarized as follows:1.To address the problems of large size and low efficiency of the existing Rotman lens,this paper proposes a high-efficiency microstrip Rotman lens without dummy port.By removing the dummy port of the lens,the absorption loss of the dummy port is avoided,and the focal ratio and focal angle of the lens are optimized so that the resonance problem of the port due to the removal of the dummy port does not occur in the operating frequency band.Not only the lens size is effectively reduced but also the efficiency of the lens is improved,at the cost that the output amplitude is nearly uniformly distributed instead of tapered distribution,which is not conducive to reducing the side lobe level of the lens-fed antenna.In order to verify the effectiveness of the above design method,a microstrip Rotman lens is designed.The results show that the simulated lens efficiency is nearly 60% and the measured efficiency is about 55% in the frequency range of 20-28 GHz with a maximum output phase difference of ±140°(±60° scan range of the antenna array).Compared with the work in the literature,the efficiency of the lens is improved while the fluctuation of the efficiency for different feed port is reduced.2.To address the problems of difficulty in achieving gradient refractive index of Luneburg lens and the lack of refractive index fitting accuracy,a planar metasurface Luneburg lens is proposed in this paper.An inverted substrate parallel-plate waveguide with holes is used to achieve gradient refractive index,and the amount of coupling electromagnetic waves to the dielectric is controlled by adjusting the size of the holes to achieve the desired equivalent refractive index.More layers with different equivalent refractive index are achieved by using a metasurface unit with size variable period,which improves the fitting accuracy.Finally,by adding ports to the lens that can be used as input and output port,the lens is used as a power dividing network,which achieves rotational symmetry and effectively reduces the profile of the lens.A metasurface Luneburg lens is designed and fabricated.The results show that the simulated lens efficiency is about 60% and the measured efficiency is about 40% in the frequency range of 6-9 GHz.The lens is capable of transformation between cylinder waves and plane waves.It has outstanding advantages such as high fitting accuracy,low profile and easy processing.3.In order to reduce the gain roll-off of the Rotman lens-fed wide-angle scanning antenna array,two solutions are proposed in this paper: Option 1 uses the previously designed high-efficiency Rotman lens,the efficiency of the lens is greatly improved by removing the dummuy port,and the fluctuation of the efficiency with the feed port is reduced.Therefore,the gain roll-off of the antenna array fed by the Rotman lens without dummy port is greatly reduced.The simulation results show that ±60° beam coverage is achieved in the operating frequency range of 23.5-27.5 GHz,and the radiation efficiency of the Rotman lens-fed antenna array reaches 52.7%-62.3%,with the gain fluctuation no more than 3.3 d B.To further reduce the gain roll-off,Option 2 utilizes the feature that the efficiency of the conventional Rotman lens decreases as the output phase difference increases.Phase gradient transmission lines are introduced between the output of the lens and the antenna feed network,so that the output phase difference corresponding to the beam port with the lowest lens efficiency changes from the maximum to the minimum,while the output phase difference corresponding to the beam port with the highest lens efficiency changes from the minimum to the maximum.Thus,the aperture loss of the antenna array can be partly compensated by the lens efficiency fluctuation so that the gain roll-off of the Rotman lens-fed antenna array is reduced with almost no additional loss.Since the phase difference of the adopted phase gradient transmission lines is realized by microstrip lines of different lengths that is frequency dependent,the gain of the antenna is decreased when the frequency is deviated from the center.Therefore,the proposed method has a certain bandwidth limitation.To verify the feasibility of the proposed method,the phase gradient transmission lines are applied to the Rotman lens-fed multibeam antenna and processed for testing.The experimental results show that ±56° beam coverage is achieved in the operating band of 24.75-27.5GHz,the gain of the antenna is improved from 12.4-16.8d B to 14-15.5d B,the gain roll-off is reduced from 4.4d B to 1.5d B.Therefore,gain equalization is achieved in wide-angle coverage range.4.In order to realize circularly polarized multibeam antenna,sequential rotation technique is introduced to Rotman lens-fed antenna in this paper.By analyzing and summarizing the feed phase shift caused by the rotation of the subarray and the antenna element in the sequentially rotated array,a feed network that can generate sequential phase and allow beam scanning is proposed.Sequential phase can be maintained at the wavefront when the main beam is scanned off broadside thus low axial-ratio can be achieved at the beam scanning direction.Since the Rotman lens has the true time delay property,the beam pointing angle of the antenna fed by the Rotman lens almost does not vary with frequency,which is convenient to verify the circular polarization performance in the operating frequency band.Therefore,single circularly polarized beam-scanning antenna and dual circularly polarized beam-scanning antenna are designed separately using the proposed network,and the Rotman lens is used to generate multiple beams to verify the low-axial-ratio beam-scanning performance of the array.At the designed center frequency of 19.5 GHz,the measured axial-ratio of the single circularly polarized beam-scan antenna is less than 1.8 d B in ±45° scan range,and the simulated left and right hand circularly polarization axial-ratios of the dual circularly polarized beam-scan antenna are less than 0.85 d B and 0.5 d B in ±50° scan range,respectively.5.In order to improve the gain of the antenna,the previously designed Luneburg lens in this paper is used as the feed network of the circular antenna array.Nearly half the antenna elements of the circular antenna array work simultaneously to synthesize a high gain radiation beam.In order to realize 360° range multibeam,switches are connected to the input and output of the Luneburg lens so that the beam is switchable.A circular antenna array containing 48 antenna elements is designed and combined with the metasurface Luneburg lens.The simulation results show that the antenna directivity is improved from 6.9d B to 16.5d B after using Luneburg lens,and 48 high gain beams with no gain fluctuation are achieved with the outstanding advantages of small size,horizontal omnidirectional multibeam and no gain roll-off.The proposed Luneburg lens-fed array is suitable for mobile communication base stations,radar and other applications that require omnidirectional high gain multibeams.