Research On Key Technologies Of Quasi-optical Feed System For Terahertz Radiometer

The new generation of radiometer is developing toward higher frequency bands,more polarizations,and more channels.The quasi-optical network system has the characteristics of low loss,high power,multi-polarization and broadband,which make this technology widely used in the design of space terahertz radiometers.Based on the demonstration work of FY-5 during the“14th Five-Year Plan”period,this dissertation mainly studies the basic theory and key technologies involved in the quasi-optical network of terahertz radiometers.Based on the system requirements of seven-band quasi-optical network,this dissertation conducts research on the network topology,corrugated horns,and channel separation components（Frequency Selective Surface,FSS）.The main work of this dissertation as follows:1.The relationship between far-field radiation parameters and Gaussian beam parameters is derived,and the quasi-optical network design process is summarized.Two topological expansion schemes are proposed based on the requirements:polarization division first plan and polarization division later plan.Polarization division first plan is convenient for the deployment of subsequent topology but has restrictions on polarization distribution;polarization division later plan can flexibly match the polarization characteristics of the channel but it will increase the design difficulty of FSS components.We designed three kinds of quasi-optical networks according to different schemes,and analyzed their respective adaptation conditions by simulation,which provided a certain basis for the selection of specific design schemes for quasi-optical network systems.2.The theory of cross-polarization produced by quadratic mirrors is deeply studied,and a set of optimization methods for cross-polarization elimination by double mirrors is proposed based on the calculation of cross-polarized multimode cascaded scattering matrices.According to the optimization method,a seven-channel low cross-polarization quasi-optical network with channel multiplexing is designed（the cross-polarization characteristic can reach below-35 d B）,and channels are multiplexed for 89 GHz,165/183 GHz and 229 GHz,which can realize the construction of network topology with fewer reflective surfaces.3.The relationship between the Gauss mode and the traditional waveguide T E_1n,TM_1nor corrugated horn HE_1nmode was deduced in detail combined with the mode coupling theory,and the MATLAB GUI interface program was written to calculate the Gaussian coupling efficiency of the horns and the proportional distribution of HE_1nmodes on the aperture surface.The mode excitation and far-field radiation characteristics of sin²/parallel,tanh/linear and other contour curves commonly used in the design of Ultra-Gaussian corrugated horns were studied.Ultra-Gaussian corrugated horn antenna matching the seven-channel low cross-polarization quasi-optical network was designed,and the Gauss mode analysis was carried out.4.The design method flow of the quasi-optical frequency selective surface（FSS）is studied,and FSSs is designed according to the requirements of the seven-channel quasi-optical network.First,focusing on the free-standing FSS,unit shape and arrangement of units are studied.The proposed hexagonal ring unit structure can delay the appearance of high-frequency grating lobes,and the bandwidth can reach7%;the C-shaped ring modeling method is improved for compact arrangement,and the bandwidth is widened to 13%.The 89 GHz silicon-based C-ring unit FSS is sent to the manufacturing process,and measurement can be carried out after the subsequent completion of the measurement system.Finally,the 23.8/31.4 GHz ultra-wide stop-band low-pass FSS is simulated.5.The design of the polarization grid is studied,and a 0-230 GHz polarization grid is verified by simulation.A test method for wire grid isolation is proposed.By introducing a reference wire grid,the cross-polarization isolation of transceiver antennas is eliminated,and the test results are accurate and reliable.6.Take the designed components（corrugated horns,FSSs）into the seven-band low cross-polarization quasi-optical network by GRASP software,and carry out the simulation analysis of the final far-field radiation response.