The researchers developed a resonator with ultra-fine coaxial lines to increase the upper limit frequency of a resonator for permittivity measurement by 1.5 times. They also developed an analysis method for deriving the permittivity from the obtained resonance characteristics, and realized more precise evaluation compared to conventional methods.

Developed balanced-type circular-disk resonator (left) and comparison of permittivity analysis methods (right)

Use of millimeter waves that enable high-speed high-capacity wireless communication is expanding rapidly. The permittivity of the circuit board is an essential physical quantity for circuit designs, but the upper limit frequency of conventional measurement methods using resonators with 1 mm coaxial lines is 110 GHz. This called for technology that can precisely measure permittivity at higher frequencies over an ultra-wide band.

The developed technology utilizes a resonator excited by ultra-fine 0.8 mm coaxial lines to measure the permittivity up to 170 GHz. By selectively exciting specific modes in the resonator consisting of a pair of dielectric samples, a thin copper disk, and metallic plates, the permittivity of the samples at each resonant frequency is determined from the resonance characteristics over an ultra-wide band. The improved analysis method for accurately determining the permittivity was also developed to analyze the influence of the copper disk in detail. These developments made it possible to extend the measurement frequency band by 1.5 times compared to the conventional technologies.

The researchers aim to further increase the upper limit frequency of the balanced-type circular-disk resonator method, and will conduct comparisons with other measurement methods in the millimeter-wave band.