Design Of A Wideband System For Measuring Dielectric Properties

Relative permittivity and loss tangent are the two important parameters of microwave dielectric materials. In recent decades, the microwave devices and antennas have become smaller and smaller in many applications. Meanwhile, there are higher requirements on the properties of the dielectric materials, as well as their precise measurements of the dielectric properties.In this paper, we design a wideband system for measuring the dielectric properties aimed at dealing with the problems of high price, complex measurement procedure and low accuracy of the existing measuring instruments. First, we give a short introduction to the existing measurement methods for measuring dielectric properties. Then a new method which is introduced in our system is discussed in detail. The new wideband system includes a UWB directional antenna unit, a wideband RF circuit as well as a data processing unit. The contributions are as follows:1. This wideband measurement system bases on the method that the dielectric properties can be computed by the change of antenna input impedance due to the influence of dielectric samples. It can accurately measure the relative permittivity and loss tangent of the materials of low to medium permittivity as well as medium to high loss across the band from 1 GHz to 3.5 GHz.2. The design of wideband unidirectional antenna:We have designed a planar monopole directional antenna with an ultra-wide bandwidth from 1 GHz to 10.6 GHz. The dimensions of the antenna are 90mm×90mm× 1mm. The measured and simulated results show that the antenna is mainly directional and has reasonable gain values over the operational band of the system. It meets the requirements of the wideband measurement system.3. The design of RF circuit:We have designed a wideband surface mount RF circuit which can be used to measure the antenna input impedance. In order to eliminate the error caused by the circuit for improving the accuracy of the measurement system, we have proposed a calibration method for the RF circuit. Experimental results show that the calibration method is effective.4. The design of data processing unit:This is used to collect the DC signals from the RF circuit by STM32, and finally realizing the measurement of dielectric properties.