Development of Microwave Caliper for Sheet Materials

A microwave caliper used for performing noncontact measurements of the thickness of dry sheet materials is developed using a microwave coaxial cavity resonator. Since the coaxial cavity is open-ended on one side, even a thick sheet material can be measured. High resolution of thickness is obtained due to a strong electric field on the open circuit plane of the cavity. The fringing electric field distribution of the open-ended coaxial cavity, which has been difficult to observe, is monitored as a thermal distribution by a thermography. The frequency shift of a resonance peak is found to be closely related to the dielectric constant.