Microwave Filters for Communication Systems: Fundamentals, Design and Applications

CHAPTER 11

MICROWAVE RESONATORS

Resonators are the basic building blocks of any bandpass filter. A resonator is an element that is capable of storing both frequency-dependent electric energy and frequency-dependent magnetic energy. A simple example is an LC resonator, where the magnetic energy is stored in the inductance L and the electric energy is stored in the capacitance C. The resonant frequency of a resonator is the frequency at which the energy stored in electric field equals the energy stored in the magnetic field. At microwave frequencies, resonators can take various shapes and forms. The shape of the microwave structure affects the field distribution and hence the stored electric and magnetic energies. Potentially, any microwave structure should be capable of constructing a resonator whose resonant frequency is determined by the structure's physical characteristics and dimensions. In this chapter, we examine various resonator configurations highlighting their main applications and limitations. We also review the available techniques to calculate and measure the resonant frequency and unloaded Q factors of microwave resonators.

11.1 MICROWAVE RESONATOR CONFIGURATIONS

The main design considerations of microwave resonators are the resonator size, unloaded Q, spurious performance, and power handling capability. The unloaded Q represents the inherent losses in the resonator. The higher the losses are, the lower is the Q value. It is therefore desirable to use resonators with ...