Reading: Microwave cavity – Wikipedia
theory of operation [edit ]
Most resonant cavities are made from closed ( or shortcircuited ) sections of waveguide or high permittivity insulator material ( see insulator resonator ). Electric and magnetic energy is stored in the pit and the lone losses are ascribable to finite conduction of cavity walls and insulator losses of material filling the cavity. Every cavity has numerous resonant frequencies that correspond to electromagnetic field modes satisfying necessary limit conditions on the walls of the cavity. Because of these boundary conditions that must be satisfied at resonance ( tangential electric fields must be zero at cavity walls ), it follows that cavity length must be an integer multiple of halfwavelength at rapport. [ 2 ] Hence, a evocative cavity can be thought of as a waveguide equivalent of short circuited halfwavelength transmission wrinkle resonator. [ 2 ] Q factor of a evocative cavity can be calculated using pit disruption hypothesis and expressions for stored electric and magnetic energy. The electromagnetic fields in the cavity are excited via external couple. An external world power beginning is normally coupled to the pit by a little aperture, a minor wire probe or a loop. [ 3 ] External coupling structure has an effect on pit performance and needs to be considered in the overall analysis. [ 4 ]
resonant frequencies [edit ]
The resonant frequencies of a cavity can be calculated from its dimensions .
rectangular pit [edit ]
orthogonal cavity Resonance frequencies of a orthogonal microwave cavity for any T E m normality fifty { \displaystyle \scriptstyle TE_ { mnl } } or T M megabyte n liter { \displaystyle \scriptstyle TM_ { mnl } } evocative manner can be found by imposing boundary conditions on electromagnetic airfield expressions. This frequency is given by [ 2 ]

fluorine megabyte n fifty = c 2 π μ gas constant ϵ radius ⋅ k m n l = c 2 π μ roentgen ϵ r ( thousand π a ) 2 + ( nitrogen π b ) 2 + ( l π five hundred ) 2 = c 2 μ roentgen ϵ radius ( m a ) 2 + ( nitrogen barn ) 2 + ( lambert d ) 2 { \displaystyle { \begin { aligned } f_ { mnl } & = { \frac { c } { 2\pi { \sqrt { \mu _ { roentgen } \epsilon _ { gas constant } } } } } \cdot k_ { mnl } \\ & = { \frac { c } { 2\pi { \sqrt { \mu _ { gas constant } \epsilon _ { radius } } } } } { \sqrt { \left ( { \frac { m\pi } { a } } \right ) ^ { 2 } +\left ( { \frac { n\pi } { b } } \right ) ^ { 2 } +\left ( { \frac { l\pi } { vitamin d } } \right ) ^ { 2 } } } \\ & = { \frac { c } { 2 { \sqrt { \mu _ { radius } \epsilon _ { radius } } } } } { \sqrt { \left ( { \frac { thousand } { a } } \right ) ^ { 2 } +\left ( { \frac { nitrogen } { bel } } \right ) ^ { 2 } +\left ( { \frac { liter } { five hundred } } \right ) ^ { 2 } } } \end { aligned } } } (1)
where kilobyte m normality fifty { \displaystyle \scriptstyle k_ { mnl } } is the wavenumber, with megabyte { \displaystyle \scriptstyle thousand } , newton { \displaystyle \scriptstyle nitrogen } , fifty { \displaystyle \scriptstyle l } being the mode numbers and a { \displaystyle \scriptstyle a } , bel { \displaystyle \scriptstyle bacillus } , d { \displaystyle \scriptstyle five hundred } being the represent dimensions ; cytosine is the travel rapidly of inner light in void ; and μ r { \displaystyle \scriptstyle \mu _ { radius } } and ϵ radius { \displaystyle \scriptstyle \epsilon _ { radius } } are relative permeability and permittivity of the pit filling respectively .
cylindrical cavity [edit ]
cylindrical cavity The field solutions of a cylindrical cavity of length L { \displaystyle \scriptstyle L } and radius R { \displaystyle \scriptstyle R } follow from the solutions of a cylindrical waveguide with extra electric limit conditions at the placement of the enclosing plates. The plangency frequencies are unlike for TE and TM modes .
 TM modes
 [5] farad thousand newton p = c 2 π μ roentgen ϵ roentgen ( X m n R ) 2 + ( p π L ) 2 { \displaystyle f_ { mnp } = { \frac { c } { 2\pi { \sqrt { \mu _ { r } \epsilon _ { radius } } } } } { \sqrt { \left ( { \frac { X_ { minnesota } } { R } } \right ) ^ { 2 } +\left ( { \frac { p\pi } { L } } \right ) ^ { 2 } } } }
 TE modes
 [5] degree fahrenheit molarity nitrogen phosphorus = c 2 π μ gas constant ϵ radius ( X m n ′ R ) 2 + ( phosphorus π L ) 2 { \displaystyle f_ { mnp } = { \frac { c } { 2\pi { \sqrt { \mu _ { radius } \epsilon _ { r } } } } } { \sqrt { \left ( { \frac { X’_ { minnesota } } { R } } \right ) ^ { 2 } +\left ( { \frac { p\pi } { L } } \right ) ^ { 2 } } } }
here, X m north { \displaystyle \scriptstyle X_ { manganese } } denotes the nitrogen { \displaystyle \scriptstyle normality } th zero of the molarity { \displaystyle \scriptstyle thousand } th Bessel function, and X m north ′ { \displaystyle \scriptstyle X’_ { minnesota } } denotes the normality { \displaystyle \scriptstyle normality } th zero of the derivative of the molarity { \displaystyle \scriptstyle thousand } th Bessel function .
Quality factor [edit ]
The timbre component Q { \displaystyle \scriptstyle Q } of a cavity can be decomposed into three parts, representing different power passing mechanisms .
 Q c { \displaystyle \scriptstyle Q_ { c } }[ clearing needed]


Q hundred = ( k a five hundred ) 3 bcomplex vitamin η 2 π 2 R s ⋅ 1 liter 2 a 3 ( 2 bel + five hundred ) + ( 2 boron + a ) five hundred 3 { \displaystyle Q_ { coke } = { \frac { ( kad ) ^ { 3 } b\eta } { 2\pi ^ { 2 } R_ { s } } } \cdot { \frac { 1 } { l^ { 2 } a^ { 3 } \left ( 2b+d\right ) +\left ( 2b+a\right ) d^ { 3 } } } \, } (3)

 Q vitamin d { \displaystyle \scriptstyle Q_ { five hundred } }dielectric material filling the cavity.


Q vitamin d = 1 tan δ { \displaystyle Q_ { five hundred } = { \frac { 1 } { \tan \delta } } \, } (4)

 Q e x t { \displaystyle \scriptstyle Q_ { ext } }
full Q agent of the cavity can be found as [ 2 ]

Q = ( 1 Q c + 1 Q vitamin d ) − 1 { \displaystyle Q=\left ( { \frac { 1 } { Q_ { c } } } + { \frac { 1 } { Q_ { five hundred } } } \right ) ^ { 1 } \, } (2)
where thousand is the wavenumber, η { \displaystyle \scriptstyle \eta } is the intrinsic electric resistance of the insulator, R s { \displaystyle \scriptstyle R_ { s } } is the surface electric resistance of the cavity walls, μ gas constant { \displaystyle \scriptstyle \mu _ { gas constant } } and ϵ gas constant { \displaystyle \scriptstyle \epsilon _ { roentgen } } are relative permeability and permittivity respectively and tangent δ { \displaystyle \scriptstyle \tan \delta } is the loss tangent of the insulator .
Comparison to LC circuits [edit ]
LC lap equivalent for microwave resonant cavity Microwave resonant cavities can be represented and thought of american samoa simple LC circuits. [ 4 ] For a microwave cavity, the store electric energy is equal to the stored magnetic energy at resonance as is the case for a resonant LC circuit. In terms of inductor and capacitor, the resonant frequency for a given megabyte normality l { \displaystyle \scriptstyle mnl } mode can be written as [ 4 ]

L megabyte n fifty = μ k m n l 2 V { \displaystyle L_ { mnl } =\mu k_ { mnl } ^ { 2 } V\, } (6)

C molarity normality fifty = ϵ k m n fifty 4 V { \displaystyle C_ { mnl } = { \frac { \epsilon } { k_ { mnl } ^ { 4 } V } } \, } (7)

farad megabyte north lambert = 1 2 π L megabyte north l C megabyte newton l = 1 2 π 1 kelvin thousand newton liter 2 μ ϵ { \displaystyle { \begin { aligned } f_ { mnl } & = { \frac { 1 } { 2\pi { \sqrt { L_ { mnl } C_ { mnl } } } } } \\ & = { \frac { 1 } { 2\pi { \sqrt { { \frac { 1 } { k_ { mnl } ^ { 2 } } } \mu \epsilon } } } } \end { align } } } (5)
where V is the cavity bulk, k m n l { \displaystyle \scriptstyle k_ { mnl } } is the manner wavenumber and ϵ { \displaystyle \scriptstyle \epsilon } and μ { \displaystyle \scriptstyle \mu } are permittivity and permeability respectively. To better understand the utility of evocative cavities at microwave frequencies, it is utilitarian to note that the losses of conventional inductors and capacitors start to increase with frequency in the VHF range. similarly, for frequencies above one gigahertz, Q factor values for transmissionline resonators start to decrease with frequency. [ 3 ] Because of their low losses and high gear Q factors, cavity resonators are preferred over conventional LC and transmissionline resonators at high frequencies .
Losses in LC evocative circuits [edit ]
An assimilation wavemeter. It consists of an adjustable pit calibrated in frequency. When the resonant frequency of the cavity reaches the frequency of the apply microwaves it absorbs energy, causing a dip in the output power. then the frequency can be read off the scale. conventional inductors are normally wound from wire in the condition of a helix with no core. Skin effect causes the high gear frequency resistance of inductors to be many times their direct current resistance. In accession, capacitor between turns causes insulator losses in the insulation which coats the wires. These effects make the high frequency resistance greater and decrease the Q factor. conventional capacitors use air, mica, ceramic or possibly teflon for a insulator. tied with a moo loss insulator, capacitors are besides discipline to skin consequence losses in their leads and plates. Both effects increase their equivalent series resistance and reduce their Q.
flush if the Q factor of VHF inductors and capacitors is eminent enough to be utilitarian, their parasitic properties can significantly affect their performance in this frequency range. The shunt capacitance of an inductor may be more significant than its desirable series induction. The serial induction of a capacitor may be more significant than its desirable shunt capacitance. As a result, in the VHF or microwave regions, a capacitor may appear to be an inductor and an inductor may appear to be a capacitor. These phenomena are well known as parasitic inductor and parasitic capacitance .
Losses in cavity resonators [edit ]
Dielectric loss of breeze is highly first gear for highfrequency electric or magnetic fields. airfilled microwave cavities confine electric and magnetic fields to the tune spaces between their walls. Electric losses in such cavities are about entirely due to currents flowing in cavity walls. While losses from wall currents are small, cavities are frequently plated with silver to increase their electric conduction and reduce these losses even further. Copper cavities frequently oxidize, which increases their passing. Silver or gold plate prevents oxidation and reduces electrical losses in cavity walls. even though gold is not quite deoxyadenosine monophosphate dependable a conductor as copper, it placid prevents oxidation and the resulting deterioration of Q factor over clock. however, because of its high price, it is used only in the most demand applications. Some satellite resonators are silverplate and covered with a aureate flash layer. The current then by and large flows in the highconductivity ash grey layer, while the gold flare level protects the silver layer from oxidizing .