Simultaneous Evaluation of Permeability and Permittivity Using a Flexible Microstrip Line-Type Probe up to 67 GHz

Complex relative permeability and permittivity were evaluated simultaneously in the band of 100 MHz–67 GHz using a flexible microstrip line-type probe. In principle, the probe can evaluate the permeability and permittivity of electromagnetic materials regardless of size. The measured permeability and permittivity were obtained from the transmission coefficient ( $S_{21}$ ). Since $S_{21}$ under the infinite magnetic field can be estimated by changing the dc magnetic field stepwise, the magnetic and dielectric signals were separated accurately. The complex permeability and permittivity of a NiZn ferrite sheet (10 mm $\times10$ mm, 0.1 mm thick) and polytetrafluoroethylene (PTFE) (25 mm $\times25$ mm, 0.79 mm thick) were evaluated accurately.

I. Introduction

The development of various devices and systems for 5G, beyond 5G, etc. is underway, and research and development of materials for electromagnetic wave absorbers, noise suppressors, and electromagnetic shields covering several tens of GHz bands is also advancing. Demand for high-frequency permeability and permittivity evaluation of electromagnetic materials is increasing because high-frequency electromagnetic materials have both permeability and permittivity. There are several methods to evaluate the permeability and permittivity of electromagnetic materials simultaneously. In the coaxial probe method, the sample must be processed into a solenoid shape and narrow bandwidth [1]. In the Nicolson–Ross–Weir method, problems such as narrow frequency band and sample size limitation arise [2]. Furthermore, in the harmonic resonance cavity perturbation method, it is necessary to process the sample into a special shape such as a needle shape and measurement frequency is single or restrictive [3]. In measurement by transmission in free space, the method has the disadvantages of requiring a large sheet as the sample and low sensitivity [4].