Resonators

It is found that any distributed transmission line of length λ_g/2 will resonate, if it is shorted or open at one end. This is the principle of resonators.
A resonator is actually a device or a system that exhibits resonance or resonant behaviour. It means it naturally oscillates at some frequencies with greater amplitude than at other frequencies.
The frequencies at which the device oscillate are known as resonant frequencies.
The oscillations produced in a resonator can be either electromagnetic resonance or mechanical resonance.
The resonators are used to generate either waves of specific frequencies or to select specific frequencies from a signal.
The musical instruments use acoustic resonators and it produce sound waves of specific tones.

The resonance is a phenomenon that consists of a given system, which is driven by another vibrating system or by external forces to oscillate with greater amplitude at some particular preferential frequencies.
The resonance occurs, when a system is able to store and easily transfer energy between two or more different storage modes.

Cavity Resonator

The cavity resonators are devices, which is closely related to waveguides and have the practical importance.
If the waveguide is limited by closing the ends of a finite length of the tube in conducting surfaces then it is known as cavity resonator.
The cavity resonators produced by placing end faces on finite length of cylindrical tube waveguides are known as cylindrical cavity resonators.
There are two types of cylindrical cavity resonator, one is rectangular and another is circular. It depends on the cross section of the cavity resonator.

The end faces of cylindrical cavity resonators are plane, and perpendicular to the axis of cavity.
The walls of cylindrical cavity resonators are perfectly conducting.
The transverse cross sectional shape and size of it are uniform.
The cavity of cylindrical cavity resonator is filled with loss-less non-conducting medium.

In cylindrical cavity resonator, electromagnetic wave propagates along the axis of the cavity and they reflects back and forth by the plane end faces.
Both waves i.e., incident wave and reflected wave of cavity resonators superpose, and form a system of standing waves.
It is found that the certain standing wave field distribution can be set up in the cavity corresponds to different cavity modes.
The possible mode of frequencies of cylindrical cavity resonators are known as resonant frequency of the cavity.
It is also found that the cavity resonator show the properties like an electronic resonant circuit i.e.,

They can store energy in oscillating electric and magnetic fields.
They can dissipate a fraction of the stored energy in each cycle of oscillation.

The fraction of the stored energy dissipated per cycle of cavity resonator is about 1/20 the fraction dissipated per cycle in L-C circuit.
The resonant frequencies of cavity resonator ranges from few megacycles. This range is impossible in LC circuit.

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