Dielectric resonators - Microwave circuits

Dielectric resonators are objects made of insulating materials, typically oxides, in the form of bare or metallized ceramics, such as discs, cylinders, cubes, spheres, etc. They serve as resonators (components) in the construction of filters, oscillators, and antennas at microwave frequencies (ranging from a few tens of megahertz (10 ⁶ Hz) to a few tens of gigahertz (10 ⁹ Hz)).

The dielectric resonator replaces empty resonant cavities (air-only) and, like them, confines electromagnetic energy thanks to its high permittivity (between 10 and 100). It exhibits natural resonance frequencies (modes), offering the following advantages over resonant cavities:

• reduction in the component's size proportional to the square root of the permittivity;

• thermal stability coefficient of the resonant frequency, which is adjustable and approximately 10, ^–6, /°C, allowing it to adapt to its environment;

• ease of implementation.

Depending on the material composition and the environment, the quality factor may be higher than that of a metallic cavity, with the exception of superconducting cavities.

Although dielectric resonators are easy to use in practice, theoretically determining the resonant frequency or quality factor of the component is more complex than in the case of a metallic cavity. The use of numerical calculation techniques helps to overcome this difficulty.

This article explains the concept of the resonator in the transverse electric (TE) mode _01δ , as well as its characterization by materials manufacturers. It describes how to couple it to the external circuit using various elements (loop, microstrip line). Characterizing the resonator as a filter element allows for the determination of the material’s dielectric losses. Materials for dielectric resonators are also used in different modes (hybrid, transverse magnetic TM, transverse electromagnetic TEM, etc.). These modes are relevant for filters (spatial, mobile radio): the goal is generally to achieve a compromise between the highest possible quality factor and the minimum volume or mass of the resonator.