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((Short description|Device or series of devices that receive, then transmit (relay) a signal))
((More citations needed|date=December 2011))
A [[communications satellite]]'s [[transponder]] is the series of interconnected units that form a [[communications channel]] between the receiving and the transmitting antennas.<ref>((cite book |last=Roddi |first=Dennis |title=Satellite Communications |edition=3rd |year=2001 |publisher=McGraw Hill |location=New York |isbn=0-07-138285-2|pages=587))</ref> 
It is mainly used in satellite communication to transfer the received signals.

A transponder is typically composed of:
* an input band-limiting device (an input [[band-pass filter]]),
* an input [[low-noise amplifier]] (LNA), designed to amplify the signals received from the Earth station (normally very weak, because of the large distances involved),
* a frequency translator (normally composed of an [[oscillator]] and a [[frequency mixer]]) used to convert the frequency of the received signal to the frequency required for the transmitted signal,
* an output [[band-pass filter]],
* a  power amplifier (this can be a [[traveling-wave tube]] or a solid-state amplifier).
 
Most communication satellites are [[radio relay]] stations in orbit and carry dozens of transponders, each with a [[Bandwidth (signal processing)|bandwidth]] of tens of megahertz.  Most transponders operate on a '''((visible anchor|bent pipe))''' (i.e., [[u-bend]]) principle, sending back to Earth what goes into the conduit with only amplification and a shift from [[Telecommunications link|uplink]] to downlink frequency. However, some modern satellites use on-board processing, where the signal is demodulated, decoded, re-encoded and modulated aboard the satellite. This type, called a "regenerative" transponder, is more complex, but has many advantages, such as improving the signal to noise ratio as the signal is regenerated from the digital domain, and also permits selective processing of the data in the digital domain.

With [[data compression]] and [[multiplexing]], several [[video]] (including  [[digital video]]) and [[sound reproduction|audio]] channels may travel through a single transponder on a single [[wideband]] [[carrier wave|carrier]].

Original [[analog video]] only had one channel per transponder, with [[subcarrier]]s for audio and automatic transmission-identification service [[Automatic Transmitter Identification System (television)|ATIS]]. Non-multiplexed [[radio station]]s can also travel in [[single channel per carrier]] (SCPC) mode, with multiple carriers (analog or digital) per transponder. This allows each station to transmit directly to the satellite, rather than paying for a whole transponder or using [[landline]]s to send it to an [[Earth station (communications)|Earth station]] for [[multiplexing]] with other stations.

NASA distinguishes between a "[[transceiver]]" and "transponder". A transceiver has an independent transmitter and receiver packaged in the same unit.  In a transponder the transmit [[carrier frequency]] is derived from the received signal.  The frequency linkage allows an interrogating ground station to recover the Doppler shift and thus infer range and speed from a communication signal without allocating power to a separate ranging signal.<ref>[http://esc.gsfc.nasa.gov/assets/files/450-SNUG.pdf Space Network Users Guide] ((Webarchive|url=https://web.archive.org/web/20160305123252/http://esc.gsfc.nasa.gov/assets/files/450-SNUG.pdf |date=2016-03-05 )).</ref>

==Transponder equivalent==

A '''transponder equivalent''' ('''TPE''') is a normalized way to refer to transponder bandwidth. It simply means how many transponders would be used if the same total bandwidths used only 36&nbsp;MHz transponders.<ref name=frost-20130703 /><ref name=satmagazine-201102fulton /><ref name=jsati-transponder /> So, for example, the [[ARSAT-1]] has 24 IEEE [[Ku band|K<sub>u</sub> band]] transponders: 12 with a bandwidth of 36&nbsp;MHz, 8 with 54&nbsp;MHz, and 4 with 72&nbsp;MHz, which totals to 1152&nbsp; MHz, or 32&nbsp;TPE (i.e., 1152&nbsp;MHz divided by 36&nbsp;MHz).<ref name=aerospacetech-arsat1 /><ref name=nsf-20141006arsat1 />

== References ==
((Reflist|30em|refs=
<ref name=aerospacetech-arsat1>((cite web|title=ARSAT-1 Telecommunication Satellite, Argentina|url=http://www.aerospace-technology.com/projects/arsat-1-telecommunication-satellite/|publisher=Aerospace Technology|access-date=2015-08-31))</ref>((Unreliable source?|reason=domain on WP:BLACKLIST|date=July 2016))
<ref name=frost-20130703>((cite web |title=Commercial Geostationary Satellite Transponder Markets for Latin America |url=http://www.frost.com/prod/servlet/report-brochure.pag?id=A397-01-00-00-00 |publisher=Frost & Sullivan Research Service |access-date=2015-08-31|date=2003-07-03))</ref>
<ref name=jsati-transponder>((cite web |title=Transponder |url=http://www.jsati.com/why-satellite-what-Transponder.asp |publisher=JSat International |access-date=2015-08-31))</ref>
<ref name=nsf-20141006arsat1>((cite web |url=http://www.nasaspaceflight.com/2014/10/arsat-1-argentina-communicate-heightened-space-ambitions/ |title=ArSat-1: Argentina to communicate its heightened space ambitions |last=Belluscio |first=Alejandro G. |publisher=NASA SpaceFlight |date=2014-10-06 |access-date=2015-08-25))</ref>
<ref name=satmagazine-201102fulton>((cite web |author1=Jacbo Gullish, Futron |title=The North American Satellite Market New Methodology Required... |url=http://www.satmagazine.com/story.php?number=1585073081 |website=www.satmagazine.com |publisher=SatMagazine |access-date=2015-08-31 |date=February 2011))</ref>
))

((Satcomm))

[[Category:Communications satellites]]
[[Category:Satellite broadcasting]]
[[Category:Radio electronics]]

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