Dielectric resonator
A dielectric resonator is a piece of dielectric (nonconductive but polarizable) material, usually ceramic, that is designed to function as a resonator for radio waves, generally in the microwave and millimeter wave bands.
The microwaves are confined inside the resonator material by the abrupt change in permittivity at the surface, and bounce back and forth between the sides.
Dielectric resonators' main use is in millimeter-wave electronic oscillators (dielectric resonator oscillator, DRO) to control the frequency of the radio waves generated.
In the late 19th century, Lord Rayleigh demonstrated that an infinitely long cylindrical rod made up of dielectric material could serve as a waveguide.
[1] Additional theoretical [2] and experimental [3] work done in Germany in early 20th century, offered further insight into the behavior of electromagnetic waves in dielectric rod waveguides.
In 1939 Robert D. Richtmyer published a study [4] in which he showed that dielectric structures can act just as metallic cavity resonators.
Richtmyer also demonstrated that, if exposed to free space, dielectric resonators must radiate because of the boundary conditions at the dielectric-to-air interface.
Due to World War II, lack of advanced materials and adequate manufacturing techniques, dielectric resonators fell in relative obscurity for another two decades after Richtmyer's study was published.
However, in the 1960s, as high-frequency electronics and modern communications industry started to take off, dielectric resonators gained in significance.
In addition to cost and size, other advantages that dielectric resonators have over conventional metal cavity resonators are lower weight, material availability, and ease of manufacturing.
There is a vast availability of different dielectric resonators on the market today with unloaded Q factor on the order of 10000s.
Although dielectric resonators display many similarities to resonant metal cavities, there is one important difference between the two: while the electric and magnetic fields are zero outside the walls of the metal cavity (i.e. open circuit boundary conditions are fully satisfied), these fields are not zero outside the dielectric walls of the resonator (i.e. open circuit boundary conditions are approximately satisfied).
Even so, electric and magnetic fields decay from their maximum values considerably when they are away from the resonator walls.
Dielectric resonators can exhibit an extremely high Q factor that is comparable to a metal walled cavity.
[7] There are three types of resonant modes that can be excited in dielectric resonators: transverse electric (TE), transverse magnetic (TM) or hybrid electromagnetic (HEM) modes.
The size and type of the material encapsulating the cavity can drastically impact the performance of the resonant circuit.
[8] Even though recent improvements in materials science and manufacturing mitigated some of these issues, compensating techniques still may be required to stabilize the circuit performance over temperature and frequency.
