Editing Transmission line (section)

==Overview==
Ordinary electrical cables suffice to carry low frequency [[alternating current]] (AC), such as [[mains power]], which reverses direction 100 to 120 times per second, and [[audio signal]]s.  However, they are not generally used to carry currents in the [[radio frequency]] range,<ref name="Jackman">{{cite book
  | last = Jackman
  | first = Shawn M.
  |author2=Matt Swartz |author3=Marcus Burton |author4=Thomas W. Head
  | title = CWDP Certified Wireless Design Professional Official Study Guide: Exam PW0-250
  | publisher = John Wiley & Sons
  | year = 2011
  | pages = Ch. 7
  | url = https://books.google.com/books?id=AQ8WJGshLBEC&pg=PT300
  | isbn = 978-1118041611}}</ref> above about 30&nbsp;kHz, because the energy tends to radiate off the cable as [[radio wave]]s, causing power losses.  Radio frequency currents also tend to reflect from discontinuities in the cable such as [[electrical connector|connectors]] and joints, and travel back down the cable toward the source.<ref name="Jackman" /><ref name="Oklobdzija">{{cite book
  | last = Oklobdzija
  | first = Vojin G.
  | author2=Ram K. Krishnamurthy
  | title = High-Performance Energy-Efficient Microprocessor Design
  | publisher = Springer Science & Business Media
  | year = 2006
  | pages = 297
  | url = https://books.google.com/books?id=LmfHof1p3qUC&pg=PA297
  | isbn = 978-0387340470}}</ref>  These reflections act as bottlenecks, preventing the signal power from reaching the destination.  Transmission lines use specialized construction, and [[impedance matching]], to carry electromagnetic signals with minimal reflections and power losses.  The distinguishing feature of most transmission lines is that they have uniform cross sectional dimensions along their length, giving them a uniform [[Electrical impedance|impedance]], called the ''[[characteristic impedance]]'',<ref name="Oklobdzija" /><ref name="Guru">{{cite book
  | last = Guru
  | first = Bhag Singh
  |author2=Hüseyin R. Hızıroğlu 
  | title = Electromagnetic Field Theory Fundamentals, 2nd Ed.
  | publisher = Cambridge Univ. Press
  | year = 2004
  | pages = 422–423
  | url = https://books.google.com/books?id=qzNdDtZUPXMC&pg=PA422
  | isbn = 978-1139451925}}</ref><ref name="Schmitt">{{cite book
  | last = Schmitt
  | first = Ron Schmitt
  | title = Electromagnetics Explained: A Handbook for Wireless/ RF, EMC, and High-Speed Electronics
  | publisher = Newnes
  | year = 2002
  | pages = [https://archive.org/details/electromagnetics0000schm/page/153 153]
  | url = https://archive.org/details/electromagnetics0000schm
  | url-access = registration
  | isbn = 978-0080505237}}</ref> to prevent reflections.  Types of transmission line include parallel line ([[ladder line]], [[twisted pair]]), [[coaxial cable]], and [[planar transmission line]]s such as [[stripline]] and [[microstrip]].<ref name="Carr">{{cite book
  | last = Carr
  | first = Joseph J.
  | title = Microwave & Wireless Communications Technology
  | publisher = Newnes
  | year = 1997
  | location = USA
  | pages = 46–47
  | url = https://books.google.com/books?id=1j1E541LKVoC&pg=PA46
  | isbn = 978-0750697071}}</ref><ref name="Raisanen">{{cite book
  | last = Raisanen
  | first = Antti V.
  |author2=Arto Lehto
  | title = Radio Engineering for Wireless Communication and Sensor Applications
  | publisher = Artech House
  | year = 2003
  | pages = 35–37
  | url = https://books.google.com/books?id=m8Dgkvf84xoC&pg=PA35
  | isbn = 978-1580536691}}</ref> The higher the frequency of electromagnetic waves moving through a given cable or medium, the shorter the [[wavelength]] of the waves.  Transmission lines become necessary when the transmitted frequency's wavelength is sufficiently short that the length of the cable becomes a significant part of a wavelength.

At frequencies of [[microwave]] and higher, power losses in transmission lines become excessive, and [[waveguide]]s are used instead,<ref name="Jackman" /> which function as "pipes" to confine and guide the electromagnetic waves.<ref name="Raisanen" />  Some sources define waveguides as a type of transmission line;<ref name="Raisanen" /> however, this article will not include them.