# Coaxial cable **Coaxial cable**, often shortened to coax, is an electrical transmission line built from a central conductor, a surrounding dielectric insulator, and a concentric conducting shield sharing the same axis as the inner conductor, usually finished with an outer protective jacket. Because the shield encloses the inner conductor completely, the two form an unbalanced line whose fields stay confined between them rather than spreading into the surroundings, which is what lets a well-built coaxial cable carry a radio-frequency signal for a useful distance with comparatively little loss and almost no interference either escaping outward or getting in from outside. A three.js sketch further down this page applies the mismatch idea at the centre of this design to a 75-ohm cable feeding a 50-ohm load. Coaxial cable carries [[Telecommunications|telecommunications]] traffic between telephone exchanges, distributes signals through cable-television networks and broadband internet connections, and links a transmitter or a [[Radio_receiver|radio receiver]] to its [[Antenna_(radio)|antenna]]. What sets it apart from an ordinary shielded wire is dimensional control: because the ratio of the two conductors' diameters is held constant along the cable's whole length, it behaves as a well-defined [[Transmission_line|transmission line]] with a predictable characteristic impedance, rather than merely a wire with a wrapper around it. ## Applications Beyond telephone trunk lines and antenna feeders, coaxial cable historically carried high-speed computer data buses before twisted-pair and fibre took over most of that role, and it remains the standard link between a broadcast studio and its transmitter, between test equipment and the device under test, and inside [[Radar|radar]] and other radio-frequency systems wherever a signal must travel more than a few centimetres between stages. ## Description A coaxial cable's four layers each do one job. The inner conductor, solid or stranded copper (sometimes silver-plated or copper-clad steel for strength), carries the signal current. Around it sits a dielectric insulator, commonly solid or foamed polyethylene or, in the lowest-loss cables, a mostly-air space held apart by a spiral spacer or beads, which sets the cable's capacitance per unit length and much of its loss. Outside that, a conducting shield, most often a woven copper braid, a solid or corrugated tube, or a metal foil backed by a thinner braid, carries the return current and, just as importantly, encloses the inner conductor's field so that little of it reaches the world outside the cable. An outer jacket, usually a plastic such as PVC or a weatherproof polyethylene, protects the whole assembly from moisture and physical damage but plays no electrical role. Because the shield is at the same potential as the receiver or source's ground reference over the cable's whole length, coaxial cable is an unbalanced line, unlike a twisted pair, where the two conductors share the signal symmetrically and neither is grounded along its length. ## Construction Manufacturers trade the same handful of choices against each other in every coaxial design. A solid inner conductor gives lower loss for a given diameter but is stiffer and more likely to fatigue and break if it is repeatedly flexed, so stranded conductors are preferred wherever a cable will be moved or coiled often. A foam or air dielectric loses less signal to heat than solid polyethylene of the same diameter, at the cost of being more easily crushed and more sensitive to moisture finding its way in. A single braid shield is flexible and inexpensive but lets a small amount of signal leak both in and out through the gaps between the woven strands; a second braid, or a continuous foil layer under the braid, closes most of those gaps and is standard wherever shielding effectiveness matters more than flexibility or cost. ## Signal propagation A signal does not travel down a coaxial cable at the speed of light in a vacuum; it travels at a fraction of that speed set by the dielectric's relative permittivity, `v = c / sqrt(epsilon_r)`, called the cable's velocity factor. For solid polyethylene, with a relative permittivity of about 2.3, this works out to a velocity factor of about 0.66, meaning a signal covers a given length of cable in about half again the time light would take in free space; foam and air dielectrics, with less material in the way of the field, reach velocity factors from roughly 0.80 up to about 0.90 for the least lossy air-spaced designs. Loss grows with frequency for two separate reasons: skin effect crowds the signal current into an ever-thinner layer near each conductor's surface as frequency rises, raising the effective resistance in proportion to the square root of frequency, while the dielectric itself absorbs a small, frequency-dependent share of the field as heat. Cable data sheets quote both effects together as an attenuation figure in [[Decibel|decibels]] per hundred metres at a stated frequency, since neither number alone predicts how a given cable will perform at a given frequency. ## Connectors A connector is itself a short length of transmission line and must hold the same characteristic impedance as the cable it joins, or it becomes a small discontinuity and a reflection point of its own. BNC connectors, quick to connect and disconnect, are standard in test equipment and video; N-type connectors, larger and weatherproof, join a [[Radio_receiver|receiver]] or transmitter to its cable into the low gigahertz range outdoors; SMA connectors, small and precise, extend usable performance into the [[Microwave|microwave]] bands; and the F connector, cheap and adequate at the frequencies involved, is the standard fitting for cable and satellite television. ## Important parameters ### Physical parameters The inner conductor's diameter d and the shield's inner diameter D, together with the dielectric's relative permittivity, set essentially every electrical property of the [[Transmission_line|line]] discussed below; a cable's physical size is chosen from the required power handling, flexibility, and loss rather than from the impedance alone. ### Fundamental electrical parameters The characteristic impedance follows directly from that geometry, `Z0 = (138 / sqrt(epsilon_r)) * log10(D/d)` ohms, alongside a capacitance and an inductance per unit length that combine to give the same result, `Z0 = sqrt(L/C)` for a lossless line. ### Derived electrical parameters Attenuation, velocity factor and power handling, covered above and below, all follow from the same physical and fundamental parameters rather than being independent choices a designer makes directly. ### Choice of impedance Two impedances dominate practice, and both are compromises rather than arbitrary conventions. For a given outer-conductor diameter, an air-dielectric coaxial cable has its lowest attenuation at an impedance of roughly 77 ohms and its highest power-handling capacity at roughly 30 ohms; 50 ohms sits between the two, close enough to minimum loss while leaving useful power-handling margin, which is why it is the standard choice for transmitting equipment and antenna feed lines. Where a line only ever receives, as in cable and broadcast television distribution and long telephone trunk routes, power handling is irrelevant and minimum attenuation wins outright, which is why those systems settled on 75 ohms instead; a transmitting [[Antenna_(radio)|antenna]]'s own feed line, by contrast, is built to the 50-ohm compromise because it must carry real power, not merely receive a weak signal. ## Issues ### Signal leakage An imperfect shield, whether from the small gaps in a single braid or from damage and worn connectors, lets a little signal energy escape outward and lets a little outside energy in; in a cable-television system carrying dozens of channels at once, even a small leak can radiate on a frequency shared with an aviation or public-safety radio band, which is why cable operators are required to patrol their plant for leakage. ### Ground loops Grounding a cable's shield at both ends of a long run, where the two ends sit on buildings whose local grounds differ by even a fraction of a volt, lets a slow current circulate along the shield and induce hum onto the signal; the fix is usually to ground the shield firmly at one end only, or to break the loop with an isolating transformer. ### Noise #### Transformer effect Because the shield has some resistance and is coupled to the inner conductor by mutual inductance rather than being a perfect equipotential surface, any current forced to flow along the shield, whether from a ground loop or from external [[Noise_(electronics)|noise]], induces a small unwanted voltage on the inner conductor exactly as a transformer's primary induces a voltage in its secondary; the effect grows with cable length and matters most at the lower end of the audio range. ### Common mode current and radiation A coaxial cable feeding a naturally balanced load, such as a [[Dipole_antenna|dipole]], drives an unwanted current onto the outside of the shield unless something forces the two halves of the load to see the same impedance to ground; that outer-surface current turns the cable itself into an unintended radiator or receiver, detuning the antenna it feeds and coupling noise into anything nearby. ### Higher impedance at audio frequencies The transmission-line treatment used everywhere else on this page assumes the cable is long compared with a wavelength; at audio frequencies a typical cable run is nothing of the sort, and the cable instead behaves mostly as a lumped capacitance loading whatever source drives it, an effect that becomes noticeable on long microphone or instrument cable runs long before it becomes a travelling-wave problem. ## Standards National and international standards bodies fix a cable's dimensions, markings, and the environmental and mechanical tests it must survive before it can be sold under a given type designation, so that a cable bought from one manufacturer under a standard designation performs interchangeably with the same designation from another.[^cn-standards] ## Uses Beyond the general applications above, particular coaxial types are matched to particular jobs: thin, flexible cable serves patch leads and portable test setups where loss over a short run barely matters; heavier, lower-loss cable feeds a base-station or broadcast antenna where every extra decibel of loss between the transmitter and the antenna is power wasted as heat instead of radiated; and the least lossy hard line and rigid types are reserved for the highest-power transmitter feeds, from [[AM_broadcasting|AM]] and [[Frequency_modulation|FM]] radio stations to [[Radar|radar]] installations, where their cost and inflexibility are a small price for the loss they save. ## Types ### Hard line Hard line uses a rigid or semi-flexible tubular outer conductor rather than a woven braid, trading flexibility for lower loss and higher power handling, and is standard for feeding cellular base-station [[Antenna_(radio)|antennas]] and broadcast transmitters over runs of many metres. ### Radiating A radiating, or "leaky," cable is deliberately built with regular gaps in its shield so that it radiates and receives along its entire length instead of only at its ends, giving usable radio coverage inside tunnels, mines and long corridors where a conventional antenna cannot reach. ### RG-6 RG-6 is a common 75-ohm cable, thicker and lower-loss than the older RG-59, used throughout cable and satellite television installations and for the video and data runs found in most homes. ### Triaxial cable Triaxial cable adds a second, insulated outer shield around the first, giving substantially better rejection of external interference than an ordinary coaxial cable at the cost of extra bulk, and is used in broadcast camera cabling and sensitive instrumentation. ### Semi-rigid Semi-rigid cable uses a solid, seamless metal tube as its outer conductor, bent once into shape during assembly rather than flexed repeatedly in service, and its stability and near-total shielding make it standard inside precision radio-frequency test equipment. ### Rigid line Rigid line, the largest and lowest-loss type in ordinary use, carries the highest transmitter powers over the shortest possible runs, sometimes with dry air or nitrogen held under slight pressure inside it to keep moisture from ever reaching the dielectric. ## Interference and troubleshooting A damaged shield, a corroded connector or water that has worked its way into a cable's dielectric each show up electrically as a local discontinuity: a small, unwanted step in [[Standing_wave_ratio|impedance]] at that point along the line. A [[Reflection_coefficient|reflection]] from that step travels back toward the source exactly as it would from a deliberate mismatch, and a time-domain reflectometer finds the fault by timing how long the echo takes to return, converting that delay into a distance with the cable's own velocity factor. ## History Long submarine telegraph cables predate the coaxial cable's formal theory: the first transatlantic cable, laid in 1858, already used a single insulated conductor with the surrounding sea itself serving as the return path, an unbalanced arrangement in the same family the coaxial cable would later formalise.[^cn-1858] The English physicist Oliver Heaviside gave the concentric conductor arrangement its first patented description in 1880, setting out an inner conductor and a surrounding conducting sheath sharing one axis as a way of confining a transmission line's field entirely between its own two conductors.[^heaviside1880] The design remained a largely theoretical curiosity for telegraphy and telephony until Lloyd Espenschied and Herman Affel, working at [[Bell_Labs|Bell Telephone Laboratories]], patented the practical modern coaxial cable in 1929, engineered for the far higher frequencies that [[Radio_wave|radio]] and eventually television would demand.[^espenschied1929] AT&T's first coaxial carrier route, built to that design, entered service in 1936 between New York and Philadelphia, carrying either 240 simultaneous telephone conversations or a single television signal over a line with amplifying repeaters spaced roughly every 10.5 miles, and it served as the working prototype for the long-distance coaxial telephone and television network that followed it across the country.[^att1936] ## Microsims A three.js companion elsewhere on this page carries a 75-ohm cable into a deliberately mismatched 50-ohm load, applying the reflection idea above to the specific pairing of impedances that television and telephone practice on one side, and radio-frequency transmitting practice on the other, have each settled on. The fuller, adjustable version of the same underlying model already lives on the [[Transmission_line]] page, whose primary sketch drives a line of chosen characteristic impedance into any complex load the reader sets and plots the resulting reflection as a point on a Smith chart. *Try:* on that page, set the line's electrical length to match a real cable run and compare a matched load against a badly mismatched one; watch how much more of the standing-wave pattern appears on the mismatched line for the same source power. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Coaxial_cable) : [Wikitube](https://en.wikitube.io/wiki/Coaxial_cable) Skeleton mirrored at revision 1374883089. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Transmission_line]] - [[Reflection_coefficient]] - [[Standing_wave_ratio]] - [[Antenna_(radio)]] - [[Radio_receiver]] ## Notelist Characteristic-impedance figures quoted here, 50 ohms, 75 ohms and the rest, are nominal design values; a real cable's impedance drifts by a percent or two along its length with manufacturing tolerance and varies slightly with frequency, so a precise measurement of an installed cable can differ a little from the number printed on its jacket. ## References [^heaviside1880]: Heaviside, O. British Patent No. 1,407 (accepted April 6, 1880), describing a concentric-conductor transmission line. Summary: https://www.waveform.com/blogs/main/146807175-oliver-heaviside-and-the-coaxial-cable . [^espenschied1929]: Espenschied, L.; Affel, H. A. U.S. Patent 1,835,031 (Bell Telephone Laboratories, filed 1929). National Inventors Hall of Fame record: https://www.invent.org/inductees/herman-affel . [^att1936]: American Telephone and Telegraph Company's first intercity coaxial carrier system, New York-Philadelphia, 1936: "Coaxial Goes to War," *Cybernetic Forests* (computer.rip), December 13, 2021: https://computer.rip/2021-12-13-coaxial-goes-to-war.html . [^cn-1858]: Citation needed: a primary engineering source treating the 1858 transatlantic telegraph cable's single-conductor, sea-return construction as a direct technical antecedent of the later coaxial cable, rather than a coincidentally similar unbalanced arrangement. [^cn-standards]: Citation needed: the specific national and international standards (document numbers and years) that fix coaxial cable dimensions, markings and environmental test requirements has not been pinned down in this pass. ## External links - "Coaxial Goes to War," *Cybernetic Forests* (computer.rip), December 13, 2021: https://computer.rip/2021-12-13-coaxial-goes-to-war.html <!-- Hubs: Signal_processing. Portals: PORTAL_Radio. 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