| Dielectric materials are widely used in the production and development of microwave circuits in the field of microwave and millimeter wave recently. The complex permittivity of different dielectric materials is different, even the dielectric properties of these material samples will change with their different preparation methods, different purity and different porosity rate. It is necessary to know the complex permittivity of dielectric materials at the beginning of design since the dielectric parameters affect the performance of microwave circuits directly. In the microwave and millimeter wave frequency range, application of electromagnetic open resonator technique breaks through the boundaries of closed cavity. The quasi-optical cavity with an open plane-concave structure has some specific characteristics, such as a sparse mode, a higher quality factor, etc. In another aspect, the baseline level of the s parameters is title when the quasi-optical cavity test system combined with portable network analyzer is built to measure the complex permittivity of the material under test. It is important to emphasize that a great error will be introduced to the measurement using traditional measurement method while this problem is solved very well because of the introduction of the perturbation device.In this paper, a W-band quasi-optical cavity measurement system with a high precision based on perturbation theory is developed to measure the dielectric permittivity of dielectric material, works are completed as follows:First of all, starting from the basic parameters of the quasi-optical resonator, such as the curvature factor and the Fresnel number, the beam waist radius, the resonant frequency and the quality factor, etc., their effects on the performance of the quasi-optical cavity have been analyzed in this paper. According to the analysis results, the quality factor is increased to improve the test precision. Meanwhile, in order to reduce the influence of the spherical mirror reflection on the wave front of Gaussian beam, the fitting degree between the two is further improved by increasing the mirror separation. At the same time, the field distributions and energy distributions of Gaussian beam are simulated numerically by Matlab. The fundamental modes TEM00 q are chosen as the operating modes after analyzing the characteristics of fundamental mode Gaussian beam and higher-order mode Gaussian beam.Secondly, the plane-concave cavity is designed to achieve the goal of high quality factor and low waist radius. The relative size of plane-concave cavity is determined by means of analysis and design, and the corresponding mechanical configuration is designed according to the fixed length method. Mechanical structure also has additional features as follows: set the measured sample in center, cover the cavity with the dustproof hand rod, hide perturbation device. Taking into account the adverse effects caused by a series of discontinuous networks in the process of measurement, the perturbation device is introduced to improve the measurement precision and the design of the circuit is carried out.Finally, an automatic test system with quasi-optical cavity is built with portable network analyzer. According to the complex permittivity measurement method and related formula, the entire measurement process is carried out and the MFC dialog box program is prepared to control portable network analyzer and perturbation circuit to realize the automatic measurement. |
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