Ridged Semi-Cylindrical Cavity for Complex Permittivity Measurement of Fiber Monofilament

The discrepancy issue of the complex permittivity during fiber production adversely affects the overall electrical properties of the fiber-reinforced composites. As known from the reported works of literature, no test fixture is currently available to directly measure the complex permittivity of the fiber monofilament with micrometer-sized diameter in the low-frequency range. In this article, the ridged semi-cylindrical cavity (RSCC) working at multiple microwave frequencies is proposed for the complex permittivity measurement of the fiber monofilament. This cavity consists of semi-cylinder with the Y-shaped slot and the metallic ridge inside. The RSCC working at ${\mathrm {TE}}_{01n}$ mode provides a highly sensitive measurement environment for the fiber monofilament by compressing the most E-field energy in a narrow region in which the fiber monofilament is placed. The complex permittivity of the fiber monofilament is calculated from the shift of the resonant frequency according to the resonant perturbation method. Several fiber monofilaments are measured by the proposed cavity at frequencies (4.6, 8.7, and 12.9 GHz). The combined uncertainties of the dielectric constant and dielectric loss tangent are calculated at ${\mathrm {TE}}_{01n}$ modes for the fiber monofilament measurements.

I. Introduction

Fiber is a basic raw material for preparing fiber-reinforced composites and fabric [1], [2], which are used to design the radome, substrate, wearable devices, and so on. These mentioned materials and devices are usually applied in the microwave frequency range. One of their raw materials is the fiber monofilament. The optimal performance of the fiberreinforced composites and fabrics relies on the accuracy of the fiber dielectric properties. When the dielectric properties are not accurately tested, repeated testing and preparation process adjustments are inevitable. In addition, the discrepancy of the fiber monofilaments may also lead to the substandard production of composite materials or devices. The accurate knowledge of the fiber monofilament permittivity becomes more critical for increasing preparation efficiency and improving material performance [3]. The current methods and devices for the permittivity measurement of the fiber materials are as follows.