| As an important means of stealth technology,stealth coating is widely used in the military field because of its convenient construction and being free from surface shape limitation.The radar wave absorption performance of the stealth coating is mainly characterized by the microwave reflectivity.In the laboratory environment,there are many standard test methods,such as arch method,RCS(Radar-Cross Section)method and sealed transmission line method,to test the coating samples.However,when the stealth coating is applied in practice,it is necessary to conduct the in-situ measurement to evaluate whether the performance of the coating meet the application requirements because of the following reasons:(1)the coating raw material may be chemically deteriorated during the production and spraying equipment may be poor in spraying process,which lead to the deviation between the actual performance and design index of the coating;(2)the chemical properties of the coated coating may change due to various factors such as the external forces and the life during using,which leads to the deterioration of the absorbing properties of the coating;(3)after renewing and repairing the damaged coating,the absorption properties of the repaired coating may be substandard because of the poor repair technology.The in-situ measurement involved in this dissertation includes two aspects:(1)for the coating raw material and the spraying equipment,the coating sample is made by the spraying equipment and its reflectivity is measured on-site;(2)for the coating sprayed on the surface of equipment,its reflectivity is measured and tracked.In both cases,the test equipment is required to adapt to the complex site environment and be able to carry out rapid measurement in portable mode.Based on the theory of the free-space reflection measurement,this dissertation studies the in-situ measurement technology of the microwave reflectivity of the stealth coating from three aspects of theoretical analysis,test modeling,and system development.The dissertation focuses on solving the problems of characterization method,antenna design,data processing and error analysis in the reflectivity in-situ measurement.By analyzing the interaction mechanism between the material and the near-field beam of the antenna,a new method for the reflectivity measurement under the near-field beam is proposed.Based on the theories of the plane wave spectrum and the plane-wave scattering-matrix,the conversion model between the reflection coefficients of the Gaussian beam and the plane wave is established,and the characterization relationship of the reflectivity of material under the near-field beam is obtained.By establishing the single reflection test error model,the change law of the system directivity is studied,which provides design criteria for the antenna performance under the different transceiver configurations.Based on the error model,a new error analysis method of separating complex vector signals is proposed,which solves the problem of the quantitative evaluation of the single error term.For the data processing of the test signal,the time-domain gating algorithm is deduced to suppress the interference signals effectively.For the phase acquirement of the reflection coefficient under the scalar measurement,a new method for the vector derivation is proposed by introducing a standard sample network,and the solution of the complex reflection coefficient is realized by the amplitude value.According to the requirement for the antenna performance in the in-situ measurement,three kinds of test antennas are developed.The desktop and hand-held reflectivity in-situ measurement systems are set up,and the test software is written.The main research work and innovation of this dissertation are as follows:1.A new method for the reflectivity in-situ measurement based on the antenna near-field beam is proposed.According to the theory of the antenna near-field radiation,the propagation characteristics and performance parameters of two typical near-field beams are analyzed.Based on the distance shortening and focusing technique in the near-field measurement,the research idea of using the near-field beam as the test signal to act on the material is proposed.2.Study on the interaction mechanism between the antenna near-field beam and material.Taking the plane-wave beam and Gaussian beam as the research objects,the field distribution characteristics of two kinds of incident beams reflected by the planar material are analyzed.The reflection coefficients of material under the plane-wave beam and Gaussian beam are deduced,and the conversion model between the reflection coefficients of two kinds of beams is established,which provides theoretical guidance for the near-field test modeling of the reflectivity and design guidance for the radiation performance of the near-field antenna.3.Modeling and error analysis of the reflectivity in-situ measurement.According to the theory of the electromagnetic wave transmission network,a single reflection test model based on the single antenna monostatic configuration and dual antennas bistatic configuration is constructed.The error sources and the calibration method of the model are analyzed and a simplified calibration technique using the additional signal processing algorithm is proposed.For the error term analysis in the in-situ measurement,a method of separating complex interference signals into different single error terms is proposed.The errors introduced by the multipath reflection,transceiver coupling and edge effect are quantitatively evaluated.4.Data processing of the reflectivity in-situ measurement.For the suppression of the interference signals,the optimum correction process of the time-domain gating is deduced for the original measurement data,and the time-domain gate setting and its influence on the test results are analyzed.The effective extraction of the reflection signal and the effective suppression of the interference signal are realized.For the problem of the phase acquirement of the reflection coefficient under the scalar measurement,a new method for the vector derivation is proposed by introducing a standard sample network.5.Development and experimental verification of the reflectivity in-situ measurement systems.According to the test physical model and the design requirements of the antenna in the engineering applications,the spot-focusing lens antenna,the tightly coupled Vivaldi antenna array and the double-antipodal Vivaldi antenna are developed.Based on the above antennas,desktop and hand-held reflectivity in-situ test systems are built,and the performance of the systems is tested.Through the comparison and analysis of the test data,the overall performance of the proposed system and the evaluation of the test error are completed.Through the above systematic research,the proposed microwave reflectivity in-situ measurement system achieves the following technical indexes:Test frequency:2~18 GHz.Test range of the reflectivity(typical value)Desktop system:0~-12 dB(2~4 GHz);0~-20 dB(4~18 GHz).Hand-held system:0~-6 dB(2~4 GHz);0~-20 dB(4~18 GHz).Test error,desktop system:≤13.1%;hand-held system:≤15.6%.Size,desktop system:60×60×50 cm~3;hand-held system:24×15×39 cm~3.Weight,desktop system:8 kg;hand-held system:3.5 kg. |
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