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Standing wave ratio
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==Practical implications of SWR== [[File:VSWR Return Loss.jpg|thumb|Example of estimated bandwidth of antenna according to the schedule VSWR by the help of the [[Ansys HFSS]]<ref> {{cite conference |author1=Sliusar, I. |author2=Slyusar, V. |author3=Voloshko, S. |author4=Zinchenko, A. |author5=Utkin, Y. |title=Synthesis of a broadband ring antenna of a two-tape design |conference=12th International Conference on Antenna Theory and Techniques (ICATT-2020) |date=22β27 June 2020 |place=Kharkiv, Ukraine |url=https://slyusar.kiev.ua/Slyusar_ICATT-2020.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://slyusar.kiev.ua/Slyusar_ICATT-2020.pdf |archive-date=2022-10-09 |url-status=live }} </ref>]] The most common case for measuring and examining SWR is when installing and tuning transmitting [[Antenna (radio)|antenna]]s. When a transmitter is connected to an antenna by a [[feed line]], the [[Antenna (radio)#Impedance|driving point impedance]] of the antenna must match the characteristic impedance of the feed line in order for the transmitter to see the impedance it was designed for (the impedance of the feed line, usually 50 or 75 ohms). The impedance of a particular antenna design can vary due to a number of factors that cannot always be clearly identified. This includes the transmitter frequency (as compared to the antenna's design or [[Antenna (radio)#Resonant frequency|resonant]] frequency), the antenna's height above and quality of the ground, proximity to large metal structures, and variations in the exact size of the conductors used to construct the antenna.<ref name=ARRL-2001-Hdbk> {{cite book | editor-last = Hutchinson | editor-first = Chuck | year = 2000 | title = The ARRL Handbook for Radio Amateurs 2001 | publisher = [[American Radio Relay League]] | location = Newington, CT | isbn = 978-0-87259-186-8 | pages= 19.4β19.6, 19.13, 20.2 <!-- cumulative --> }} </ref>{{rp|style=ama|p=20.2}} When an antenna and feed line do not have matching impedances, the transmitter sees an unexpected impedance, where it might not be able to produce its full power, and can even damage the transmitter in some cases.<ref name=ARRL-2001-Hdbk/>{{rp|style=ama|pp=19.4β19.6}} The reflected power in the transmission line increases the average current and therefore losses in the transmission line compared to power actually delivered to the load.<ref name="QST"> {{citation | last = Ford | first = Steve | date=April 1994 | title = The SWR obsession | magazine = [[QST]] Magazine | volume = 78 | issue = 4 | pages = 70β72 | publisher = [[American Radio Relay League]] | location = Newington, CT | url = http://www.qsl.net/4/4z4tl//pub/swr_obsession.pdf | access-date = 2014-11-04 }} </ref> It is the interaction of these reflected waves with forward waves which causes standing wave patterns,<ref name=ARRL-2001-Hdbk/>{{rp|style=ama|pp=19.4β19.6}} with the negative repercussions we have noted.<ref name=ARRL-2001-Hdbk/>{{rp|style=ama|p=19.13}} Matching the impedance of the antenna to the impedance of the feed line can sometimes be accomplished through adjusting the antenna itself, but otherwise is possible using an [[antenna tuner]], an impedance matching device. Installing the tuner between the feed line and the antenna allows for the feed line to see a load close to its characteristic impedance, while sending most of the transmitter's power (a small amount may be dissipated within the tuner) to be radiated by the antenna despite its otherwise unacceptable feed point impedance. Installing a tuner in between the transmitter and the feed line can also transform the impedance seen at the transmitter end of the feed line to one preferred by the transmitter. However, in the latter case, the feed line still has a high SWR present, with the resulting increased feed line losses unmitigated. The magnitude of those losses are dependent on the type of transmission line, and its length. They always increase with frequency. For example, a certain antenna used well away from its resonant frequency may have an SWR of 6:1. For a frequency of 3.5 MHz, with that antenna fed through 75 meters of RG-8A coax, the loss due to standing waves would be 2.2 dB. However the same 6:1 mismatch through 75 meters of RG-8A coax would incur 10.8 dB of loss at 146 MHz.<ref name=ARRL-2001-Hdbk/>{{rp|style=ama|pp=19.4β19.6}} Thus, a better match of the antenna to the feed line, that is, a lower SWR, becomes increasingly important with increasing frequency, even if the transmitter is able to accommodate the impedance seen (or an antenna tuner is used between the transmitter and feed line). Certain types of transmissions can suffer other negative effects from reflected waves on a transmission line. Analog TV can experience "ghosts" from delayed signals bouncing back and forth on a long line. FM stereo can also be affected and digital signals can experience delayed pulses leading to bit errors. Whenever the delay times for a signal going back down and then again up the line are comparable to the modulation time constants, effects occur. For this reason, these types of transmissions require a low SWR on the feedline, even if SWR induced loss might be acceptable and matching is done at the transmitter.
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