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dielectric resonator
A dielectric resonator is a piece of dielectric (nonconductive) 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. At certain frequencies, the resonant frequencies, the microwaves form standing waves in the resonator, oscillating with large amplitudes. Dielectric resonators generally consist of a "puck" of ceramic that has a large dielectric constant and a low dissipation factor. The resonant frequency is determined by the overall physical dimensions of the resonator and the dielectric constant of the material.
Dielectric resonators function similarly to cavity resonators, hollow metal boxes that are also widely used as resonators at microwave frequencies, except that the radio waves are reflected by the large change in permittivity rather than by the conductivity of metal. At millimeter wave frequencies, metal surfaces become lossy reflectors, so dielectric resonators are used at these frequencies. Dielectric resonators' main use is in millimeter-wave electronic oscillators (dielectric resonator oscillator, DRO) to control the frequency of the radio waves generated. They are also used as bandpass filters as well as antennas.
==Historical overview==
In the late 19th century, Lord Rayleigh demonstrated that an infinitely long cylindrical rod made up of dielectric material could serve as a waveguide.〔Lord Rayleigh, “On the Passage of Waves Through Tubes, or the Vibration of Dielectric Cylinders”, Philosophical Magazine, Vol. 43, pp. 125-132, February 1897.〕 Additional theoretical 〔D. Hondros, “Ueber elektromagnetische Drahtwelle,” Annalen der Physik, Vol. 30, pp. 905-949, 1909.〕 and experimental 〔H. Zahn, “Ueber den Nachweis elektromagnetischer Wellen an dielektrischen Draehten,”, Annalen der Physik, vol. 37, pp. 907-933, 1916.〕 work done in Germany in early 20th century, offered further insight into the behavior of electromagnetic waves in dielectic rod waveguides. Since a dielectric resonator can be thought of as a truncated dielectric rod waveguide, this research was essential for scientific understanding of electromagnetic phenomena in dielectric resonators. In 1939 Robert D. Richtmyer published a study 〔R.D. Richtmyer, “Dielectric Resonators”, J.Appl. Phys., Vol. 10, pp. 391-398, June 1939.〕 in which he showed that dielectric structures can act just as metallic cavity resonators. He appropriately named these structures ''dielectric resonators''. Richtmyer also demonstrated that, if exposed to free space, dielectic resonators must radiate because of the boundary conditions at the dielectric-to-air interface. These results were later used in development of DRA (Dielectric Resonator Antenna). 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. They offered a size-reducing design alternative to bulky waveguide filters and lower-cost alternatives for electronic oscillator,〔Darko Kajfez and Piere Guillon, Dielectric Resonators, Artech House, Dedham, MA, 1986.〕 frequency selective limiter 〔Marian W. Pospieszalski, “Cylindrical Dielectric Resonators and Their Applications in TEM Line Microwave Circuits”, IEEE Trans. Microwave Theory Tech., Vol. MTT-27, pp. 233-238, March 1979.〕 and slow-wave 〔Marian W. Pospieszalski, “Cylindrical Dielectric Resonators and Their Applications in TEM Line Microwave Circuits”, IEEE Trans. Microwave Theory Tech., Vol. MTT-27, pp. 233-238, March 1979.〕 circuits. 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.
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