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In [[telecommunications]], '''return loss''' is the loss of [[Signalling (telecommunication)|signal]] [[power (physics)|power]] resulting from the reflection caused at a discontinuity in a [[transmission line]] or [[optical fiber]]. This discontinuity can be a mismatch with the terminating load or with a device inserted in the line.  It is usually expressed as a ratio in [[decibel]]s (dB);
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:<math>RL(\mathrm{dB}) = 10 \log_{10} {P_\mathrm i \over P_\mathrm r}</math>
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:where ''RL''(dB) is the return loss in dB, ''P''<sub>i</sub> is the incident power and ''P''<sub>r</sub> is the reflected power.
Return loss is related to both [[standing wave ratio]] (SWR) and [[reflection coefficient]] (Γ).  Increasing return loss corresponds to lower SWR. Return loss is a measure of how well devices or lines are matched. A match is good if the return loss is high.  A high return loss is desirable and results in a lower [[insertion loss]].
 
Return loss is used in modern practice in preference to SWR because it has better resolution for small values of reflected wave.<ref name=Bird/>
 
==Sign==
Properly, loss quantities, when expressed in decibels, should be positive numbers.<ref group=note>Except for cases where an active device succeeds in reflecting back more power than was sent into it.  This is the case, for instance, with the [[negative resistance#Amplifiers|tunnel diode amplifier]].</ref> However, return loss has historically been expressed as a negative number, and this convention is still widely found in the literature.<ref name=Bird>Trevor S. Bird, [http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=5162049 "Definition and Misuse of Return Loss"], ''IEEE Antennas & Propagation Magazine'', '''vol.51''', iss.2, pp.166-167, April 2009.</ref>
 
Taking the ratio of reflected to incident power results in a negative sign for return loss;
 
:<math>RL'(\mathrm{dB}) = 10 \log_{10} {P_\mathrm r \over P_\mathrm i}</math>
:where ''RL'''(dB) is the negative of ''RL''(dB).
Return loss is identical to the magnitude of Γ when expressed in decibels but of opposite sign. That is, return loss with a negative sign is more properly called reflection coefficient.<ref name=Bird/>  The [[S-parameter]] ''S''<sub>11</sub> from [[two-port network]] theory is frequently also called return loss,<ref>Noel G. Barton, Jacques Periaux, "Coupling of fluids, structures, and waves in aeronautics", proceedings of a French-Australian workshop in Melbourne, Australia, 3–6 December 2001, p.187, Springer, 2003 ISBN 3-540-40222-5.</ref> but is actually equal to Γ.
 
Caution is required when discussing increasing or decreasing return loss since these terms strictly have the opposite meaning when return loss is defined as a negative quantity.
 
==Electrical==
In metallic conductor systems, reflections of a signal traveling down a conductor can occur at a discontinuity or [[Electrical impedance|impedance]] mismatch. The ratio of the amplitude of the reflected wave ''V<sub>r</sub>'' to the amplitude of the incident wave ''V<sub>i</sub>'' is known as the [[reflection coefficient]] <math>\Gamma</math>.
 
:<math>\mathit \Gamma = {V_\mathrm r \over V_\mathrm i}</math>
 
When the source and load impedances are known values, the reflection coefficient is given by
 
:<math>\mathit \Gamma = { {Z_\mathrm L - Z_\mathrm S} \over {Z_\mathrm L + Z_\mathrm S} }</math>
 
where ''Z''<sub>S</sub> is the impedance toward the [[voltage source|source]] and ''Z''<sub>L</sub> is the impedance toward the [[Electrical load|load]].
 
Return loss is the negative of the magnitude of the reflection coefficient in dB. Since power is proportional to the square of the voltage, return loss is given by,
 
:<math>RL(\mathrm{dB}) = -20 \log_{10} \left| \mathit \Gamma \right|</math>
 
where  the [[vertical direction|vertical]] bars indicate [[magnitude (mathematics)|magnitude]]. Thus, a large positive return loss indicates the reflected power is small relative to the incident power, which indicates good impedance match from source to load.
 
When the actual transmitted (incident) power and the reflected power are known (i.e., through measurements and/or calculations), then the return loss in dB can be calculated as the difference between the incident power ''P''<sub>i</sub> (in [[dBm]]) and the reflected power ''P''<sub>r</sub> (in dBm),
 
:<math>RL(\mathrm{dB}) = P_\mathrm i(\mathrm{dBm}) - P_\mathrm r(\mathrm{dBm})\,</math>
 
==Optical==
In [[optics]] (particularly in [[optical fiber|fiberoptics]]) a loss that takes place at discontinuities of [[refractive index]], especially at an air-[[glass]] [[wiktionary:interface|interface]] such as a fiber endface. At those interfaces, a fraction of the optical [[Signalling (telecommunication)|signal]] is reflected back toward the source.  This reflection phenomenon is also called "'''[[Fresnel reflection]] loss'''," or simply "'''Fresnel loss'''."
 
Fiber optic transmission systems use [[laser]]s to transmit signals over optical fiber, and a high optical return loss (ORL) can cause the laser to stop transmitting correctly. The measurement of ORL is becoming more important in the characterization of optical networks as the use of [[wavelength-division multiplexing]] increases.  These systems use lasers that have a lower tolerance for ORL, and introduce elements into the network that are located in close proximity to the laser.
 
:<math>\text{ORL}(\mathrm{dB}) = 10 \log_{10} {P_\mathrm i \over P_\mathrm r}</math>
 
where <math>\scriptstyle P_\mathrm r</math> is the reflected power and <math>\scriptstyle P_\mathrm i</math> is the incident, or input, power.
 
==See also==
* [[Hybrid balance]]
* [[Signal reflection]]
* [[Time-domain reflectometer]]
**[[Optical time domain reflectometer]]
* [[Mismatch loss]]
 
==Notes==
{{Reflist|group=note}}
 
==References==
{{Reflist}}
{{Refbegin}}
* [[Federal Standard 1037C]] and from [[MIL-STD-188]]
* [http://documents.exfo.com/appnotes/anote044-ang.pdf Optical Return Loss Testing—Ensuring High-Quality Transmission] EXFO Application note #044
{{Refend}}
{{Use dmy dates|date=September 2010}}
 
{{DEFAULTSORT:Return Loss}}
[[Category:Wave mechanics]]
[[Category:Radio electronics]]
[[Category:Engineering ratios]]
[[Category:Electrical parameters]]
[[Category:Fiber optics]]
 
[[de:Rückflussdämpfung]]
[[pl:Return Loss]]

Revision as of 10:57, 25 February 2014

Installation is merely the initial step to having a completely functioning video and audio system. A very good quality resolution will aid you to notice every detail. Sadly many of them simply weren't up to the job or saw it being an opportunity to increase profits. CCTV systems can replace the requirement for body guards or attack dogs and they might be a powerful deterrent, if the equipment is mounted in an areas which can be easily visible. In case of forced entry the machine triggers compilation of high decibel sounds; to get a passing police vehicle or neighbours. Home security systems these days have become increasingly affordable owing for the advancements in camera technology and manufacturing systems.

To make this happen, type the static IP address into the browser address bar and then login to the page that appears. They also give you the repair of these advanced security equipment. Raytec is further aiming to turn into a global name in CCtv lighting. These range from the 'Rio +20' climate Conference (which is imminent) but additionally the FIFA championships in 2014 and also the Olympics in 2016.

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The automotive locksmiths are specialized in the provision of advanced services for various kinds for vehicles including tandem, cars, jeeps, buses and trucks. a good deal of of the new model CCTV cameras are wireless and may be operate on battery.

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