The 5G has been commercialized in Japan from March 2020, which will enable ultra-high-speed wireless communication at up to 10 Gbps by utilizing higher frequency bands. In Japan, sub-6 GHz and 28 GHz bands have already been allocated for 5G, and international agreements have been reached for allocation up to 71 GHz. While 5G is currently being commercialized as communication infrastructure, the research and development of 6G has garnered significant attention worldwide. 6G, which is expected to be introduced around 2030, will use millimeter waves at frequencies above 100 GHz to realize high-speed and large-capacity communication with performances far exceeding those of 5G. The development of advanced materials for lower power consumption is crucial for realizing 6G because the transmission loss of a planar circuit generally increases as the frequency increases.

The researchers devised an ultra-wide band conductivity measurement technique up to over 100 GHz for metal materials used in high-frequency planar circuits.

In general, overall transmission loss of a circuit is determined by dielectric and conductor losses, which are determined by the complex permittivity of a dielectric substrate and conductivity of metal lines, respectively. Millimeter-wave planar circuits have a problem of a reduction in effective conductivity due to the roughening treatment on the dielectric surface for maintaining the adhesion with metal lines, leading to a significant increase in the conductor loss. Meanwhile, conventional conductivity measurement requires a resonator consisting of a tiny dielectric rod and provides measurements only at single frequency determined by the rod size. Therefore, no practical technique had been established for measurement of metal conductivity in frequency bands above 100 GHz. In the developed technique, a metal foil under test is sandwiched between dielectric substrates to form a dielectric resonator. The researchers developed an algorithm based on rigorous electromagnetic field analysis to determine the conductivity of metal foil from the measured resonant sharpness of the higher-order mode excitations. The developed technique is practical in that it does not require precise machining of dielectrics and provides measurements of metal conductivity over ultra-wideband frequencies of 10 - 100 GHz and higher with the same accuracy as conventional techniques. The developed technique holds potential in accelerating development of advanced materials for lower power consumption in 5G and 6G communication.