Zejian Lu

The design and manufacture of a high frequency CATR

We present a design of a Compact Antenna Test Range (CATR) designed to operate from 50 up to 500 GHz, suitable for measuring high gains antennas, such as those required for mm-wave atmospheric remote sensing satellites. Drawing on experience from the QMUL CATR designed a decade ago, the team has electromagnetically and mechanically designed a CATR with a 1 metre spherical dish and two small aspheric feeding mirrors. The CATR will be set up in BUPT after validation in QMUL.

Circularization and collimation of elliptical Gaussian beam based on kinematic and dynamic ray tracing

A dual shaped reflector system based on kinematic and dynamic ray tracing technique is developed to achieve a circular, collimated, and uniform beam from an elliptical Gaussian beam generated by the Quantum Cascade Laser(QCL) applied in terahertz holographic image system. Simulation result shows that the system has achieved ±0.7dB amplitude ripple and ±7 degree phase vibration for the output beam profile.

Theoretical and Experimental Comparison Results of Dual-Channel 3D Quasi-Optical Network System between Frequency Selective Surface and Wire Grid Polarizer

A dual-channel 3D quasi-optical network (QON) system is analyzed and compared by theoretical and experimental results. It is divided into dual-channel signals by frequency selective surface (FSS) or wire grid polarizer (WGP) for transmitting the 324 GHz signal on the top layer while diverting the 183 GHz signal to the bottom layer. The system structure based on the beam radius is traced by the complex beam parameter and system transfer matrix for deciding the positions of cascade mirrors. The design principles and test results of FSS with perforated hexagonal array and WGP printed on the dielectric substrate are discussed together. In order to evaluate channel performance, the output planar near-fields of QON system are simulated and tested by the four reflections and three transmission results, respectively, where both scalar and vector conversion efficiencies are used to evaluate the quality of output beams. For the distortion phenomenon and energy loss in planar field, the aperture modes of corrugated horn feeds are analyzed by spherical wave expansion (SWE) to explain the asymmetrical pattern characteristic. The cut section of the far-field pattern is also directly measured in the compact antenna test range (CATR), comparing with the far-field patterns that are indirectly obtained based on the principle of near-to-far-field transformation and ideal aperture field integral of quiet zone (QZ). The differences between the near- and far-field test in QON have been compared, and discussion that the corresponding experimental results verify the WGP has better work performance in the good pattern symmetry, high cross-polarization isolation, low energy loss, and side-lobe level.

Design and experimental verification of wire grid polarizer on dielectric substrate for quasi-optical network system

The quasi-optical network (QON) system generally requires that it has a feature of multi-frequency and multi-polarization for transmitting or receiving more channels in a small space to reduce the satellite load. The wire grid polarizer (WGP) can be mainly used into separating double different signals with horizontal polarization or vertical polarization. Due to its frequency irrelevant attribute, the most obvious advantage of WGP is that has much greater working bandwidth comparing with the frequency selective surface (FSS) during the millimeter wave (MMW) and sub-millimeter wave (sub-MMW) frequency. A novel WGP with a wide working band from 1MHz to 500GHz is presented and meanwhile measured from 75GHz to 325GHz in the BUPT, which is printed on the dielectric substrate. Its design principle and the analysis process of experimental data by the Time Domain Analysis technology are given simultaneously. The multipath and stray transmission components are analyzed and filtered to obtain the polarization isolation during the whole working bandwidth. Whats more, the WGP is adopted into a dual-channel 3D quasi-optical network system (QON) to separate 183GHz and 324GHz. The experimental near-field planar results in the output position have also been obtained, which further demonstrates the validity of system design.

Virtual array based APC for a tri-reflector CATR

Usually, the stray signals from antenna test environment and compact antenna test range (CATR) system may cause errors in the measured patterns. In order to improve the measurement accuracy of a specific antenna test system, the techniques about antenna pattern correction for mitigating the effects of these stray signals are required. In this paper, a two-unit virtual array method and its extended three-unit version have been used to correct the measured antenna pattern for an existing tri-reflector CATR system, and some simulated results are presented. It is shown that the method performs well in compensating the measurement errors caused by the quiet zone distortions, also the deviations that come from a specific interference source. Considering the sampling accuracy of the actual antenna test system, the effects of different sampling intervals also have been discussed.

The improvement of transmittance in capacitive grid polarizer with dielectric substrates by antireflection coating

This paper presents an approach for the improvement of transmittance in capacitive grid polarizer with dielectric substrates by introducing antireflection(AR) coating. The capacitive grid polarizer using Cu material is printed in the dielectric substrate of PTFE. According to different position relations between grid and substrate, the four structures of the AR coating are verified by the reflection coefficient of TE wave and transmission coefficient of TM wave working at the frequency range from 1GHz to 500GHz. Comparing different AR structures, the periodic strips with back AR have much better performance, where the coefficients of reflection and transmission are more than 0.993 and 0.93 respectively.

Investigations on Shaped Mirror Systems in Quasi-Optical Mode Converters Based on Irradiance Moments Method

A method of transforming high-order Gaussian beams (GBs) mode into circular symmetry fundamental Gaussian beam (FGB) mode with arbitrary waist size is presented using irradiance moments method in quasi-optical (QO) mode converters. The double shaped mirrors correcting amplitude and phase simultaneously are generated by a single incidence irradiance sampling data and known ideal output FGB taking advantage of linear moment matching technique and Fresnel diffraction theory, which can be applied to a wide frequency range especially significant for terahertz band. The numerical coding procedure of creating double correcting mirrors and its fast convergence speed are discussed at 325 GHz. Numerical and experimental comparisons reveal the conclusion that enhancing surface precision and increasing moments order can improve main lobe levels.

Broadband wire grid polarizer application in millimeter wave frequency

This paper presents a wire grid polarizer (WGP) working at the frequency ranges between 1 GHz to 500 GHz. The WGP is extensively used in polarization sensitive elements for separating different polarized signals at the same channel. The spacing pitch-to-width ratio should be chosen seriously for lower cross polarization levels in double polarizers. The influences between single and double layers WGP are also discussed simultaneously. The single layer simulation results of reflectance and transmittance coefficients are more than 0.96 in double polarization directions, which has been fabricated successfully. The double layers WGP can improve obviously the reflection coefficient and make the transmission coefficient oscillated.

A simulation software for 3D quasi-optical system and shaped-mirror reflectors

This paper introduces a simulation software, named Simatrix, for designing millimetre-wave reflector antenna and transmission system based on Diffracted Gaussian Beam Analysis. DGBA is a fast and modularized method for quasi-optical system (QOS) analysis. The Simatrix offers the functionalities of modeling and simulating 3D asymmetrical QOS, designing systems which contain shaped-mirror reflectors. Those functionalities greatly satisfy the complex demands of QOS design and simplify analysis process.

Modeling and simulation of a tri-reflector CATR system at 300GHz

A tri-reflector Compact Antenna Test Range (CATR) system which operates at 300GHz is modeled and verified in this work. An efficient Gaussian Beam (GB) analysis method based on point matching expansion technique is proposed to evaluate the predicted quiet zone performance of the CATR system. Experimental result shows that the amplitude ripple within the quiet zone is less than 1.1dB while the phase vibration is no more than 20 degrees at 300GHz.