Dielectric Materials Characterization by Nondestructive Perturbation Technique at Ka-band

Abstract

Materials characterization plays a key role in many applications, and there is extensive research in both industrial and academic fields. In antenna and circuit design, the structure performance depends on the accurate acknowledge of relative permittivity and loss tangent. The cavity perturbation technique is an ideal candidate to be employed to characterize dielectrics. As an accurate and simple method, it can gather dielectric material properties, like permittivity and dielectric loss, and is widely adopted. With the development of 5G communication systems, there is a demand to characterize materials at higher frequency ranges, such as at the mm-wave frequency ranges. Currently, characterization of materials by rectangular cavity perturbation is commonly conducted from low frequency up to the microwave frequency range, however it has not been widely used at mm-wave frequencies. A novel characterization technique is proposed to fulfill the gap between current studies and the demand. In this study, dielectrics are successfully measured at the Ka-band (at around 26 GHz and 33 GHz) using a rectangular cavity perturbation technique, and some relevant issues and challenges are solved. First, a reliable method is introduced to determine the coupling aperture radius which can stimulate the cavity resonator efficiently. The excited cavity has an excellent Q-factor (1410) using the aperture determined by the proposed reliable method. Second, the material under test is creatively and directly pushed into the cavity rather than through a commonly required tiny slot which can be difficult to machine at the required dimensions. This nondestructive cavity perturbation technique is economically resulting from saving the waveguide and avoiding the slot cutting. Third, a positioning system is built up with micropositioners, which are accessible for most microwave labs, to precisely control the sample location in the small cavity. Fourth, an analysis of the frequency dependence of the rectangular cavity perturbation technique, and a new frequency-corrected formula to calculate the loss tangent are provided. The novel characterization technique and the relevant proposed method and formula serving for improving its performance are demonstrated both in simulation and experiment. The proposed cavity perturbation technique characterizes five sets of samples. Results are compared with those tested by other methods and good agreements (the uncertainty is lower than 10 %) for both relative permittivity and loss tangent are achieved. Since it has tiny volume and simple shape, there is a potential for this method for samples requiring casting rather than machining.