Significant Reduction of Resonant Frequency by Multi-Layered Dielectric Material-Loaded Coaxial Cavity for Microwave Heating

This paper presents a microwave heating method using a cavity whose size is much smaller than the free-space wavelength. The resonant frequency was reduced by inserting multi-layer dielectrics into the cavity, and an appropriate mode was generated in the cavity to heat a specific area inside it. High-permittivity dielectrics were used to make the cavity resonate in the frequency range of a few gigahertz. A formula for the resonant frequency of the multi-layer dielectric material-loaded cylindrical cavity was analytically derived. The frequency reduction by using a dielectric-loaded cylindrical cavity geometry was predicted from the derived formula, from 12.2 GHz to 4.6 GHz, whereas the experiment results showed a reduction from 10.8 GHz to 4.5 GHz. The analytical and the experiment results were compared and analyzed with simulations, which showed good agreement. The heating efficiency at the target in the multi-layered dielectric geometry was analyzed. The electric field inside the target material was measured to prove the temperature response of the microwave heating and was compared with the simulation result. This paper confirms a technical possibility of microwave heating of a smaller-sized cavity with an insertion of low-loss dielectric material in the vicinity of a heating target.