Editing Transmission line (section)

==Practical types==
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===Coaxial cable===
{{Main|coaxial cable}}
Coaxial lines confine virtually all of the electromagnetic wave to the area inside the cable. Coaxial lines can therefore be bent and twisted (subject to limits) without negative effects, and they can be strapped to conductive supports without inducing unwanted currents in them.
In radio-frequency applications up to a few gigahertz, the wave propagates in the [[transverse wave|transverse]] electric and magnetic mode (TEM) only, which means that the electric and magnetic fields are both perpendicular to the direction of propagation (the electric field is radial, and the magnetic field is circumferential). However, at frequencies for which the wavelength (in the dielectric) is significantly shorter than the circumference of the cable other [[transverse mode]]s can propagate. These modes are classified into two groups, transverse electric (TE) and transverse magnetic (TM) [[waveguide]] modes. When more than one mode can exist, bends and other irregularities in the cable geometry can cause power to be transferred from one mode to another.

The most common use for coaxial cables is for television and other signals with bandwidth of multiple megahertz. In the middle 20th&nbsp;century they carried [[long distance telephone]] connections.

===Planar lines===
{{main|Planar transmission line}}
Planar transmission lines are transmission lines with [[electrical conductor|conductors]], or in some cases dielectric strips, that are flat, ribbon-shaped lines.  They are used to interconnect components on [[printed circuit]]s and [[integrated circuit]]s working at microwave frequencies because the planar type fits in well with the manufacturing methods for these components. Several forms of planar transmission lines exist.

====Microstrip====
[[Image:Solec Kujawski longwave antenna feeder.jpg|thumb|right|A type of transmission line called a ''cage line'', used for high power, low frequency applications. It functions similarly to a large coaxial cable. This example is the antenna [[feed line]] for a [[longwave]] radio transmitter in [[Poland]], which operates at a frequency of 225&nbsp;kHz and a power of 1200&nbsp;kW.]]
{{Main|microstrip}}
A microstrip circuit uses a thin flat conductor which is [[Parallel (geometry)|parallel]] to a [[ground plane]]. Microstrip can be made by having a strip of copper on one side of a [[printed circuit board]] (PCB) or ceramic substrate while the other side is a continuous ground plane. The width of the strip, the thickness of the insulating layer (PCB or ceramic) and the [[dielectric constant]] of the insulating layer determine the characteristic impedance. Microstrip is an open structure whereas coaxial cable is a closed structure.

====Stripline====
{{main|Stripline}}
A stripline circuit uses a flat strip of metal which is sandwiched between two parallel ground planes. The insulating material of the substrate forms a dielectric. The width of the strip, the thickness of the substrate and the relative permittivity of the substrate determine the characteristic impedance of the strip which is a transmission line.

====Coplanar waveguide====
{{main|Coplanar waveguide}}
A coplanar waveguide consists of a center strip and two adjacent outer conductors, all three of them flat structures that are deposited onto the same insulating substrate and thus are located in the same plane ("coplanar"). The width of the center conductor, the distance between inner and outer conductors, and the relative permittivity of the substrate determine the characteristic impedance of the coplanar transmission line.

===Balanced lines===
{{Main|Balanced line}}
A balanced line is a transmission line consisting of two conductors of the same type, and equal impedance to ground and other circuits.  There are many formats of balanced lines, amongst the most common are twisted pair, star quad and twin-lead.

====Twisted pair====
{{Main|Twisted pair}}
Twisted pairs are commonly used for terrestrial [[telephone]] communications.  In such cables, many pairs are grouped together in a single cable, from two to several thousand.<ref>Syed V. Ahamed, Victor B. Lawrence, ''Design and engineering of intelligent communication systems'', pp.130–131, Springer, 1997 {{ISBN|0-7923-9870-X}}.</ref>  The format is also used for data network distribution inside buildings, but the cable is more expensive because the transmission line parameters are tightly controlled.

====Star quad====
{{Main|Star quad cable}}
Star quad is a four-conductor cable in which all four conductors are twisted together around the cable axis.  It is sometimes used for two circuits, such as [[4-wire]] telephony and other telecommunications applications.  In this configuration each pair uses two non-adjacent conductors.  Other times it is used for a single, [[balanced line]], such as audio applications and [[2-wire]] telephony.  In this configuration two non-adjacent conductors are terminated together at both ends of the cable, and the other two conductors are also terminated together.

When used for two circuits, crosstalk is reduced relative to cables with two separate twisted pairs.

When used for a single, [[balanced line]], magnetic interference picked up by the cable arrives as a virtually perfect common mode signal, which is easily removed by coupling transformers.

The combined benefits of twisting, balanced signalling, and quadrupole pattern give outstanding noise immunity, especially advantageous for low signal level applications such as microphone cables, even when installed very close to a power cable.<ref>''[http://www.canare.com/UploadedDocuments/A%20Technical%20Paper%20-%20Evaluating%20Microphone%20Cable%20Perfrmance%20and%20Specifications.pdf Evaluating Microphone Cable Performance & Specifications] {{Webarchive|url=https://web.archive.org/web/20160509154052/http://www.canare.com/UploadedDocuments/A%20Technical%20Paper%20-%20Evaluating%20Microphone%20Cable%20Perfrmance%20and%20Specifications.pdf |date=2016-05-09 }}''</ref><ref>''[http://www.belden.com/blog/broadcastav/How-Starquad-Works.cfm How Starquad Works] {{Webarchive|url=https://web.archive.org/web/20161112081214/http://www.belden.com/blog/broadcastav/How-Starquad-Works.cfm |date=2016-11-12 }}''</ref> The disadvantage is that star quad, in combining two conductors, typically has double the capacitance of similar two-conductor twisted and shielded audio cable. High capacitance causes increasing distortion and greater loss of high frequencies as distance increases.<ref>{{cite book|last=Lampen|first=Stephen H.|title=Audio/Video Cable Installer's Pocket Guide|year=2002|publisher=McGraw-Hill|isbn=978-0071386210|pages=32, 110, 112}}</ref><ref>{{cite book|last=Rayburn|first=Ray|title=Eargle's The Microphone Book: From Mono to Stereo to Surround – A Guide to Microphone Design and Application|url=https://archive.org/details/earglesmicrophon00rayb|url-access=limited|edition=3|year=2011|publisher=Focal Press|isbn=978-0240820750|pages=[https://archive.org/details/earglesmicrophon00rayb/page/n168 164]–166}}</ref>

====Twin-lead====
{{Main|Twin-lead}}
Twin-lead consists of a pair of conductors held apart by a continuous insulator.  By holding the conductors a known distance apart, the geometry is fixed and the line characteristics are reliably consistent.  It is lower loss than coaxial cable because the characteristic impedance of twin-lead is generally higher than coaxial cable, leading to lower resistive losses due to the reduced current.  However, it is more susceptible to interference.

====Lecher lines====
{{Main|Lecher lines}}
Lecher lines are a form of parallel conductor that can be used at [[Ultra high frequency|UHF]] for creating resonant circuits.  They are a convenient practical format that fills the gap between [[Lumped-element model|lumped]] components (used at [[High frequency|HF]]/[[VHF]]) and [[Resonant cavity|resonant cavities]] (used at [[Ultra high frequency|UHF]]/[[Super high frequency|SHF]]).

===Single-wire line===
[[Unbalanced line]]s were formerly much used for telegraph transmission, but this form of communication has now fallen into disuse.  Cables are similar to twisted pair in that many cores are bundled into the same cable but only one conductor is provided per circuit and there is no twisting.  All the circuits on the same route use a common path for the return current (earth return).  There is a [[Electric power transmission|power transmission]] version of [[single-wire earth return]] in use in many locations.