RLC resonant circuits
Dielectric resonators - Microwave circuits

At frequencies above gigahertz, passive microwave circuits are made up of elements such as propagation lines, coaxials, microstrips, waveguides, etc., as well as resonant cavities. These elements have dimensions of the same order of magnitude or greater than the length of the electromagnetic wave. The dimensions of dielectric resonators are of the order of the wavelength in the material, or a fraction thereof, which explains why dimensions decrease as permittivity increases, as explained in § 3