| By using the characteristic of vertical-polarized electromagnetic waves which can propagate along the surface of conductive oceans,High-Frequency Oceanographic Radar(HFOR)can over-the-horizon detect the real-time surface dynamic parameters of large sea areas in all weather.Because of this distinguished feature,HFOR takes an important position in “The National Ocean Observing Network”.Since the wavelength is equivalent to the hard targets,such as low-altitude aircraft,ships,and icebergs,HFOR has also been widely studied and applied in the field of target detection.The antenna system is the most differentiated part among HFORs.According to the forms of receiving arrays,the existing HFORs can be divided into the phased radar and the compact radar.The phased HFORs employ phased-array system,whose large aperture is as long as hundred meters.The compact radar employs crossedloop/monopole antenna(CLM)to achieve small array aperture as small as one single pole.This antenna system can adapt to the intricate coastlines and reducing the cost.By applying super-resolution algorithm,the compact HFORs can achieve a comparable sea current observation performance as the phased systems,while the phased HFORs are superior to the wave observation and target detection.With the development of electronic technology,dual/multi-frequency operation mode is the main development direction of HFORs,either in ocean environment monitoring or hard target detection.Dual/multi-frequency operation improve the HFORs' performance of in the following three aspects: 1)Adapt to different sea conditions.The lower frequency can detect further,and it is1)Adapt to different sea conditions.The lower frequency can detect further,and it is suitable for high waves.So that it can adapt to extremely high sea states,such as typhoons and tsunamis.While the higher frequency can finely measure low waves under low and normal sea conditions.2)Improve performance of targets detection.Different wavelength would resonance with different target dimension,which would reduce the missed detection rate.Multi-frequency observation can also retrieve targets submerged by sea clutter,and improve the detection probability.3)Improve the ability of anti-interference.Multi-frequency observation can avoid the background noise,the radio-frequency interference and the ionospheric clutter.Current compact HFOR is limited by the antenna system of narrow-band,which can only operate at a fixed single frequency.So far,dual /multi-frequency observation of compact systems is mostly accomplished by deploying multiple sets of radar.This rough manner cannot meet the original purpose of the compact radar.This paper designs and implements a dual-frequency compact antenna system,which inherits the advantages of compact antenna system,including the extremely small aperture,compact structure and low cost.The proposed antenna system also can expand the frequency selection range,rid the ground electrical characteristics dependence,and improve the performance of beamforming.Therefore,the main research work of this paper includes the following works:1)Design and implement a single-pole dual-frequency center-fed transmitting antenna,which has broad bandwidth and the ground-independent radiation property in both band.The paper details the design method,the mathematical model,and the prototype measurements.Firstly,the lumped loading wave trap technology is used to realize dual-band operation,and an external sleeve is added to broad the bandwidth.Also the design of the modules in the root is proposed to achieve ground-independent radiation property.Secondly,the Fourier series method is applied to analyze the electrical characteristics.Finally,experimental measurements of several antenna configurations are presented to confirm the whole design scheme and the numerical analysis.The simulation and the measured results show that the VSWR<2 bandwidth range of the proposed antenna can cover 6-8MHz and 11-13 MHz bands,while the corresponding bandwidth are 30% and 24%,respectively.Meanwhile,the grounding system is not required.2)Design and implement a dual-frequency vector sensor(VS),including the actual noise figure analysis,electrical characteristics study,radiation characteristics study,and prototype measurements.Firstly,the actual noise figure of the receiving antenna at high-frequency band is studied,which is the base of antenna miniaturization.Secondly,the electrical characteristics of the active loop antenna are fully studied.Symmetric PIN diodes scheme is proposed,so that the active loop antenna is able to achieve time-shared dual-frequency operation.Finally,a new type of "mouth-type" crossed-loop is proposed.The measured results show that this proposed layout can improve antenna gain,crossed-loops' orthogonality and the anti-interference ability for near-field environment disturbance.3)In order to verify the directional characteristics of the dual-frequency VS in the actual environment,the “indoor-playground-shore” three-level pattern-measuring flow was established.In the first two sections,with the help of pattern calibrate algorithm,the actual directional characteristics of various design schemes are deeply studied.The measurement results show that the propoed layout has the best directional characteristics.At the shore-based radar station,the dual-frequency patterns of the proposed layout are measured to verify the proposed VS has good directional characteristics,and its performance can satisfy the application requirements of compact HFOR.Finally,the dual-frequency ocean current field inversion are completed.The maximum detection distance are 200 km for 7.7MHz and 100 km for 13 MHz.The radial current velocity correlation coefficients between radar and the buoy data are 0.89 and 0.91,and the RMSE are 16.97 cm/s and 15.68 cm/s,respectively.These experiments results can prove the validity of the dualfrequency antenna system design.4)Propose a robust adaptive beamforming algorithm of two-element dual-frequency VS array in order to refine the wave field inversion.Firstly,the characteristics of both the VS and the two-element VS array(2-VSA)are analyzed in detail.The 2-VSA can be arrayed when the spacing exceeding half-wavelength,this is the theoretical base for the dual-frequency 2-VSA.Regarding the signal processing feature of compact HFOR,a robust adaptive beamforming algorithm for dualfrequency 2-VSA is proposed.The proposed algorithm firstly adopts covariance matrix reconstruction algorithm to break through the limitation of array aperture,and then uses modified steering vector estimation algorithm to overcome the serious pattern mismatch.Finally,a 7.8/13 MHz 2-VSA model with a spacing of 16 m is constructed based on the measured pattern.The simulation results demonstrate that the proposed algorithm can efficiently reduce the mainlobe width and depress the sidelobe power.The proposed algorithm can prominently avoid the sidelobe blending in the mainlobe when steering angle deviated away from the normal direction.Besides,the 2-VSA pattern can retain an identical beam width in(-100°,100°)for 7.8 MHz and(-50°,50°)for 13 MHz,which is equivalent to that of normal direction.In other range of angle,the proposed algorithm still has an advantage over classic beamforming. |
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