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| {{Antennas|Components of Antenna Systems}}
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| In [[microwave]] and [[radio-frequency]] engineering, a '''stub''' is a length of [[transmission line]] or [[waveguide (electromagnetism)|waveguide]] that is connected at one end only. The free end of the stub is either left open-circuit or (especially in the case of waveguides) short-circuited. Neglecting transmission line losses, the [[input impedance]] of the stub is purely [[reactance (electronics)|reactive]]; either [[capacitive]] or [[inductance|inductive]], depending on the [[electrical length]] of the stub, and on whether it is open or short circuit. Stubs may thus be considered to be frequency-dependent capacitors and frequency-dependent inductors.
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| Because stubs take on [[Reactance (electronics)|reactive]] properties as a function of their electrical length, stubs are most common in [[UHF]] or [[microwave]] circuits where the line lengths are more manageable. Stubs are commonly used in [[antenna (radio)|antenna]] [[impedance matching]] circuits and frequency selective [[electronic filter|filters]].
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| [[Smith chart]]s can also be used to determine what length line to use to obtain a desired reactance.
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| ==Short circuited stub==
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| The [[Reflections of signals on conducting lines#Input impedance|input impedance]] of a lossless short circuited line is,
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| ::<math>
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| Z_\mathrm{SC} = j Z_0 \tan(\beta l)\,\!
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| </math>
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| where j is the [[imaginary unit]], <math>Z_0</math> is the [[characteristic impedance]] of the line, <math>\beta</math> is the [[phase constant]] of the line, and <math>l</math> is the physical length of the line.
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| Thus, depending on whether <math>\tan(\beta l)</math> is positive or negative, the stub will be inductive or capacitive, respectively.
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| The Length of a stub to act as a capacitor ''C'' at an [[angular frequency]] of <math>\omega</math> is then given by:
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| ::<math>
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| l = \frac{1}{\beta} \left[(n+1)\pi - \arctan \left(\frac{1}{\omega C Z_0}\right) \right]
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| </math>
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| The length of a stub to act as an inductor ''L'' at the same frequency is given by:
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| ::<math>
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| l = \frac{1}{\beta} \left[ n \pi + \arctan\left(\frac{\omega L}{Z_0}\right) \right]
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| </math>
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| ==Open circuited stub==
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| The input impedance of a lossless open circuit stub is given by
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| ::<math>
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| Z_\mathrm{OC} = -j Z_0 \cot (\beta l) \,\!
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| </math>
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| It follows that whether <math>\cot(\beta l)</math> is positive or negative, the stub will be capacitive or inductive, respectively.
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| The length of an open circuit stub to act as an Inductor ''L'' at an angular frequency of <math>\omega</math> is:
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| ::<math>
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| l = \frac{1}{\beta} \left[(n+1)\pi - \arccot\left(\frac{\omega L}{Z_0}\right) \right]
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| </math>
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| The length of an open circuit stub to act as a capacitor ''C'' at the same frequency is:
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| ::<math>
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| l = \frac{1}{\beta} \left[n \pi + \arccot\left(\frac{1}{\omega C Z_0}\right) \right]
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| </math>
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| ==Resonant stub==
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| Stubs are often used as resonant circuits in [[distributed element filter]]s. An open circuit stub of length <math>\scriptstyle l</math> will have a capacitive impedance at low frequency when <math>\scriptstyle \beta l < \pi /2</math>. Above this frequency the impedance is inductive. At precisely <math>\scriptstyle \beta l = \pi /2</math> the stub presents a short circuit. This is qualitatively the same behaviour as a series resonant circuit. For a lossless line the phase change constant is proportional to frequency,
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| ::<math>\beta = {\omega \over v}</math>
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| where ''v'' is the velocity of propagation and is constant with frequency for a lossless line. For such a case the resonant frequency is given by,
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| ::<math>\omega_0 = \frac {\pi v}{2 l}</math>
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| While this is qualitatively like a [[lumped element model|lumped element]] resonant circuit the impedance function is not precisely the same quantitatively. In particular, the impedance will not continue to rise [[monotonic function|monotonically]] with frequency after resonance. It will rise until the point where <math>\scriptstyle \beta l = \pi </math> at which point it will be open circuit. After this point (which is actually an [[anti-resonance]] point) the impedance will again become capacitive and start to fall. It will continue to fall until at <math>\scriptstyle \beta l = 3 \pi /2</math> it again presents a short circuit. At this point the filtering action of the stub has totally failed. This response of the stub continues to repeat with increasing frequency alternating between resonance and anti-resonance. It is not only a characteristic of stubs, but of all distributed element filters, that there is some frequency beyond which the filter fails and multiple unwanted [[passband]]s are produced.<ref name=Gupta>Ganesh Prasad Srivastava, Vijay Laxmi Gupta, ''Microwave Devices and Circuit Design'', pp.29-31, PHI Learning, 2006 ISBN 81-203-2195-2.</ref>
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| Similarly, a short circuit stub is an anti-resonator at <math>\scriptstyle \pi /2</math>, that is, it behaves as a parallel resonant circuit, but again fails as <math>\scriptstyle 3 \pi /2</math> is approached.<ref name=Gupta/>
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| ==Stub matching==
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| [[File:Stripline stub matching.svg|thumb|300px|In a stripline circuit, a stub may be placed just before an output connector to compensate for small mismatches due to the device's output load or the connector itself.]]<!-- to be uploaded when complete - don't delete -->
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| Stubs can be used to match a load impedance to the transmission line characteristic impedance. The stub is positioned a distance from the load. This distance is chosen so that at that point the resistive part of the load impedance is made equal to the resistive part of the characteristic impedance by [[quarter wave impedance transformer|impedance transformer action]] of the length of the main line. The length of the stub is chosen so that it exactly cancels the reactive part of the presented impedance. That is, the stub is made capacitive or inductive according to whether the main line is presenting an inductive or capacitive impedance respectively. This is not the same as the actual impedance of the load since the reactive part of the load impedance will be subject to impedance transformer action as well as the resistive part. Matching stubs can be made adjustable so that matching can be corrected on test.<ref>F.R. Connor, ''Wave Transmission'', pp.32-34, Edward Arnold Ltd., 1972 ISBN 0-7131-3278-7.</ref>
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| A single stub will only achieve a perfect match at one specific frequency. For wideband matching several stubs may be used spaced along the main transmission line. The resulting structure is filter-like and filter design techniques are applied. For instance, the matching network may be designed as a [[Chebyshev filter]] but is optimised for impedance matching instead of passband transmission. The resulting transmission function of the network has a passband [[Ripple (electrical)#Frequency-domain ripple|ripple]] like the Chebyshev filter, but the ripples never reach 0dB insertion loss at any point in the passband, as they would do for the standard filter.<ref>Matthaei, G.; Young, L.; Jones, E. M. T., ''Microwave Filters, Impedance-Matching Networks, and Coupling Structures'', pp.681-713, McGraw-Hill 1964.</ref>
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| ==References==
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| {{reflist}}
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| ==See also==
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| {{commons category|Stubs}}
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| *[[Quarter wave impedance transformer]]
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| [[Category:Antennas (radio)]]
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| [[Category:Distributed element circuits]]
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| [[Category:Telecommunications equipment]]
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| [[Category:Radio electronics]]
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The writer is called Irwin. Hiring is his profession. To gather cash is a thing that I'm completely addicted to. Minnesota has usually been his home but his spouse wants them to move.
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