AskDefine | Define coax

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Dictionary Definition

coax n : a transmission line for high-frequency signals [syn: coaxial cable, coax cable] v : influence or urge by gentle urging, caressing, or flattering; "He palavered her into going along" [syn: wheedle, cajole, palaver, blarney, sweet-talk, inveigle]

User Contributed Dictionary

English

Etymology 1

originally (1586) in the slang phrase to make a coax of, from earlier noun coax, cox, cokes "fool, simpleton", itself of obscure origin, perhaps related to cock (male bird, pert boy). The modern spelling is from 1706.

Pronunciation

  • (UK): /kəʊks/, /k@Uks/
  • (US): , /koʊks/, /koUks/
Homophones

Verb

  1. (obsolete) to fondle, kid, pet, tease
  2. To wheedle, persuade (a person, organisation, animal etc.) gradually or by use of flattery to do something.
    He coaxed the horse gently into the trailer.
  3. To manipulate carefully into a particular situation or position.
    They coaxed the rope through the pipe.
Synonyms
Translations
persuade gradually
manipulate carefully into position

Etymology 2

Shortened from coaxial

Pronunciation

  • (UK): /ˈkəʊæks/, /"k@U

Extensive Definition

Coaxial cable is an electrical cable consisting of an inner conductor surrounded by an insulating spacer, surrounded by an outer cylindrical conductor. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis. Coaxial cables are often used as a transmission line for radio frequency signals. In a hypothetical ideal coaxial cable the electromagnetic field carrying the signal exists only in the space between the inner and outer conductors. Practical cables achieve this objective to a high degree. A coaxial cable provides protection of signals from external electromagnetic interference, and effectively guides signals with low emission along the length of the cable.

Description

Coaxial cable design choices affect physical size, frequency performance, attenuation, power handling capabilities, flexibility, and cost. The inner conductor might be solid or stranded; stranded is more flexible. To get better high-frequency performance, the inner conductor may be silver plated. Sometimes copper-plated iron wire is used as an inner conductor.
The insulator surrounding the inner conductor may be solid plastic, a foam plastic, or may be air with spacers supporting the inner wire. The properties of dielectric control some electrical properties of the cable. A common choice is a solid polyethylene (PE) insulator. Lower-loss cables will use a polyethylene foam insulator. Solid Teflon is also used as an insulator. Some coaxial lines use air (or some other gas) and have spacers to keep the inner conductor from touching the shield.
There is also a lot of variety in the shield. Conventional coaxial cable had braided copper wire forming the shield. That allowed the cable to be flexible, but it also means there are gaps in the shield layer. It also means the inner dimension of the shield varies slightly because the braid cannot be flat. Sometimes the braid is silver plated. For better shield performance, some cables have a double-layer shield. The shield might be just two braids, but it is more common now to have a thin foil shield covered by a wire braid. Some cables may invest in more than two shield layers. Other shield designs sacrifice flexibility for better performance; some shields are a solid metal tube. Those cables cannot take sharp bends: the shield kinks. Many Cable television (CATV) distribution systems used such cables.
The insulating jacket can be made from many materials. A common choice is PVC, but applications may require fire-resistant materials. Outdoor applications may require the jacket to resist ultraviolet light and oxidation. For internal chassis connections the insulating jacket may be omitted.
Connections to the ends of coaxial cables are usually made with RF connectors.

Signal propagation

Open wire transmission lines have the property that the electromagnetic wave propagating down the line extends into the space surrounding the parallel wires. These lines have low loss, but also have undesirable characteristics. They cannot be bent, twisted or otherwise shaped without changing their characteristic impedance. They also cannot be run along or attached to anything conductive, as the extended fields will induce currents in the nearby conductors causing unwanted radiation and detuning of the line. Coaxial lines solve this problem by confining the electromagnetic wave to the area inside the cable, between the center conductor and the shield. The transmission of energy in the line occurs totally through the dielectric inside the cable between the conductors. Coaxial lines can therefore be bent and moderately twisted 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 only in the transverse electric magnetic (TEM) mode, which means that the electric and magnetic fields are both perpendicular to the direction of propagation. However, above a certain cutoff frequency, transverse electric (TE) and/or transverse magnetic (TM) modes can also propagate, as they do in a waveguide. It is usually undesirable to transmit signals above the cutoff frequency, since it may cause multiple modes with different phase velocities to propagate, interfering with each other. The outer diameter is roughly inversely proportional to the cutoff frequency.
The outer conductor can also be made of (in order of decreasing leakage and in this case degree of balance): double shield, wound foil, woven tape, braid. The ohmic losses in the conductor increase in this order: Ideal conductor (no loss), superconductor, silver, copper. It is further increased by rough surface (in the order of the skin depth, lateral: current hot spots, longitudinal: long current path) for example due to woven braid, multistranded conductors or a corrugated tube as a conductor) and impurities especially oxygen in the metal (due to a lack of a protective coating). Litz wire is used between 1 kHz and 1 MHz to reduce ohmic losses. Coaxial cables require an internal structure of an insulating (dielectric) material to maintain the spacing between the center conductor and shield. The dielectric losses increase in this order: Ideal dielectric (no loss), vacuum, air, Polytetrafluoroethylene (PTFE), polyethylene foam, and solid polyethylene. It is further increased by impurities like water. In typical applications the loss in polyethylene is comparable to the ohmic loss at 1 GHz and the loss in PTFE is comparable to ohmic losses at 10 GHz. A low dielectric constant allows for a greater center conductor: less ohmic losses. An inhomogeneous dielectric needs to be compensated by a noncircular conductor to avoid current hot-spots.

Connectors

Physical Parameters

  • Outside diameter of inner conductor, d.
  • Inside diameter of shield, D.
  • Dielectric constant of the insulator, \epsilon. The dielectric constant is often quoted as the relative dielectric constant \epsilon_r referred to the dielectric constant of free space \epsilon_0: \epsilon = \epsilon_r \epsilon_0. When the insulator is a mixture of different dielectric materials (e.g., polyethylene foam is a mixture of polyethylene and air), then the term effective dielectric constant \epsilon_ is often used.
  • Permeability of the insulator. \mu Permeability is often quoted as the relative permeability \mu_r referred to the permeability of free space \mu_0: \mu = \mu_r \mu_0. The relative permeability will almost always be 1.

Fundamental Electrical Parameters

C = =
L = \ln(D/d)= \ln(D/d)
  • Series Resistance per unit length, in ohms per metre. The resistance per unit length is just the resistance of inner conductor and the shield at low frequencies. At higher frequencies, skin effect increases the effective resistance by confining the conduction to a thin layer of each conductor.
  • Shunt Conductance per unit length, in mhos per metre. The shunt conductance is usually very small because insulators with good dielectric properties are used (a very low loss tangent). At high frequencies, a dielectric can have a significant resistive loss.

Derived Electrical Parameters

  • Characteristic impedance in ohm (Ω). Neglecting resistance per unit length and conductance per unit length for most coaxial cables, the characteristic impedance is determined from the capacitance per unit length (C) and the inductance per unit length (L). The simplified expression is (Z_0 = \sqrt). Those parameters are determined from the ratio of the inner (d) and outer (D) diameters and the dielectric constant (\epsilon). The characteristic impedance is given by
Z_0=\frac\sqrt\ln\frac\approx\frac \log_\frac
Assuming the dielectric properties of the material inside the cable do not vary appreciably over the operating range of the cable, this impedance is frequency independent above about 5 times the shield cutoff frequency. For typical coaxial cables, the shield cutoff frequency is 600 (RG-6A) to 2,000 Hz (RG-58C).
  • Attenuation (loss) per unit length, in decibels per metre. This is dependent on the loss in the dielectric material filling the cable, and resistive losses in the center conductor and outer shield. These losses are frequency dependent, the losses becoming higher as the frequency increases. Skin effect losses in the conductors can be reduced by increasing the diameter of the cable. A cable with twice the diameter will have half the skin effect resistance. Ignoring dielectric and other losses, the larger cable would halve the dB/meter loss. In designing a system, engineers must consider not only the loss in the cable, but also the loss in the connectors.
  • Velocity of propagation, in meters per second. The velocity of propagation depends on the dielectric constant and permeability (which is usually 1).
=
  • Cutoff frequency is determined by the possibility of exciting other propagation modes in the coaxial cable. The average circumference of the insulator is \pi (D + d) / 2. Make that length equal to 1 wavelength in the dielectric. The TE01 cutoff frequency is therefore
f_c = = .
  • Peak Voltage
  • Outside diameter, which dictates which connectors must be used to terminate the cable.

Significance of impedance

A question that is often asked is what the significance of a 52 or 75 Ω characteristic impedance is. The best coaxial cable impedances to use in high-power, high-voltage, and low-attenuation applications were experimentally determined in 1929 at Bell Laboratories to be 30, 60, and 77 Ω respectively. 30 Ω cable is exceedingly hard to make however, so a compromise between 30 Ω and 60 Ω was reached at 52 Ω, which has persisted; note this also corresponds very closely to the drive impedance of a half wave dipole antenna in real environments, and provides an acceptable match to the drive impedance of quarter wave monopoles as well. 73 Ω is an exact match for a centre fed dipole aerial/antenna in free space (approximated by very high dipoles without ground reflections), so 75 was adopted as a compromise between 73 and 77 ohms. Additionally, all of the components of a coaxial system should share the same impedance value to optimize effective transmission. Impedance mismatches result in partial reflection of the radio signal back towards the source. Such reflections increase signal loss and can result in the reflected signal reaching a receiver with a slight delay from the original. The visual effect is commonly referred to as ghosting. (see Impedance matching)

Issues

Signal Leakage

Signal Leakage is the passage of electromagnetic fields through the shield of a cable and occurs in both directions. Ingress is the passage of an outside signal into the cable and can result in disruption of the desired signal. Egress is the passage of signal intended to remain within the cable into the outside world. Signal egress from the networks of CATV operators is regulated by the FCC since they share freqencies with over-the-air broadcasts such as the aeronautical and radionavigation bands. CATV operators may also choose to monitor their networks for leakage to prevent ingress. Ingress is outside signal passing into the coaxial and can cause unwanted noise and picture ghosting. Signal noise is especially of concern because letting the amount of noise exceed the network's operating threshold can create downtime for servers if the provider utilises PacketCable for phone and internet services. An ideal shield would be a perfect conductor with no holes or gaps connected to a perfect ground. Unfortunately, such materials and conditions are not commonly available.

Imperfect conductor

Since the shields are not perfect conductors, electric fields can exist inside the shield, thus allowing radiating electromagnetic fields to go through the shield.
Another way of looking at it is to consider skin effect. The RF current in an imperfect conductor decays exponentially with distance beneath the surface, with the depth of penetration being proportional to the square root of the resistivity. This means that in a shield of finite thickness, some small amount of current will still be flowing on the opposite surface of the conductor. With a perfect conductor (i.e., zero resistivity), all of the current would flow at the surface, with no penetration into and through the conductor. Real cables have a shield made of an imperfect, although usually very good, conductor, so there will always be some leakage.

Gaps and holes

The gaps or holes, allow some of the electromagnetic field to penetrate to the other side. For example, braided shields have many small gaps. The gaps are smaller when using a foil (solid metal) shield, but there is still a seam running the length of the cable. Foil becomes increasingly rigid with increasing thickness, so a thin foil layer is often surrounded by a layer of braided metal, which offers greater flexibility for a given cross-section.
This type of leakage can also occur at locations of poor contact between connectors at either end of the cable.

Ground loops

A continuous current flow, even if small, along the imperfect shield of a coaxial cable can cause visible or audible interference. In CATV systems the potential difference between the coaxial network and the electrical grounding system of a house can cause a visible "hum bar" in the picture. In a 60Hz power system (deployed throughout the United States) this appears as a wide horizontal distortion bar in the picture that scrolls slowly upward. Such differences in potential can be reduced by proper bonding to a common ground at the house. See ground loop.

Induction

External current sources like switched-mode power supplies create a voltage across the inductance of the outer conductor between sender and receiver. The effect is less when there are several parallel cables, as this reduces the inductance and therefore the voltage. Because the outer conductor carries the reference potential for the signal on the inner conductor, the receiving circuit measures the wrong voltage.

Transformer effect

The transformer effect is sometimes used to mitigate the effect of currents induced in the shield. The inner and outer conductors form the primary and secondary winding of the transformer, and the effect is enhanced in some high quality cables that have an outer layer of mu-metal. Because of this 1:1 transformer, the aforementioned voltage across the outer conductor is transformed onto the inner conductor so that the two voltages can be cancelled by the receiver. Many sender and receivers have means to reduce the leakage even further. They increase the transformer effect by passing the whole cable through a ferrite core sometimes several times.

Miscellaneous

Some senders and receivers use only a limited range of frequencies and block all others by means of an isolating transformer. Such a transformer breaks the shield for high frequencies. Still others avoid the transformer effect altogether by using two capacitors. If the capacitor for the outer conductor is implemented as one thin gap in the shield, no leakage at high frequencies occurs. At high frequencies, beyond the limits of coaxial cables, it becomes more efficient to use other types of transmission line such as glass fibers, which offer low leakage (and much lower losses) around 200 THz and good isolation for all other frequencies.

Standards

Most coaxial cables have a characteristic impedance of either 50, 52, 75, or 93 Ω. The RF industry uses standard type-names for coaxial cables. Thanks to television, RG-6 is the most commonly-used coaxial cable for home use, and the majority of connections outside Europe are by F connectors.
A series of standard types of coaxial cable were specified for military uses, in the form "RG-#" or "RG-#/U". They go back to World War II and were listed in MIL-HDBK-216 published in 1962. These designations are now obsolete. The current military standard is MIL-SPEC MIL-C-17. MIL-C-17 numbers, such as "M17/75-RG214," are given for military cables and manufacturer's catalog numbers for civilian applications. However, the RG-series designations were so common for generations that they are still used, although critical users should be aware that since the handbook is withdrawn there is no standard to guarantee the electrical and physical characteristics of a cable described as "RG-# type". The RG designators are mostly used to identify compatible connectors that fit the inner conductor, dielectric, and jacket dimensions of the old RG-series cables.
Table of RG standards: PE is Polyethylene; PTFE is Polytetrafluoroethylene; ASP is Air Space Polyethylene
Commercial designations:
There are also other designation schemes for coaxial cables such as The URM, CT and WF series

References for this section

Uses

Short coaxial cables are commonly used to connect home video equipment, in ham radio setups, and in measurement electronics. They used to be common for implementing computer networks, in particular Ethernet, but twisted pair cables have replaced them in most applications except in the growing consumer cable modem market for broadband Internet access.
Long distance coaxial cable is used to connect radio networks and television networks, though this has largely been superseded by other more high-tech methods (fibre optics, T1/E1, satellite). It still carries cable television signals to the majority of television receivers, and this purpose consumes the majority of coaxial cable production.
Micro coaxial cables are used in a range of consumer devices, military equipment, and also in ultra-sound scanning equipment.
The most common impedances that are widely used are 50 or 52 ohms, and 75 ohms, although other impedances are available for specific applications. The 50 / 52 ohm cables are widely used for industrial and commercial two-way radio frequency applications (including radio, and telecommunications), although 75 ohms is commonly used for broadcast television and radio.

Types

Hard line is often confused with waveguide but the two are not the same. Hard line is used in broadcasting as well as many other forms of radio communication. It is a coaxial cable constructed using round copper, silver or gold tubing or a combination of such metals as a shield. Some lower quality hard line may use aluminum shielding, aluminum however is easily oxidized and unlike silver or gold oxide, aluminum oxide drastically loses effective conductivity. Therefore all connections must be air and water tight. The center conductor may consist of solid copper, or copper plated aluminum. Since skin effect is an issue with RF, copper plating provides sufficient surface for an effective conductor. Most varieties of hardline used for external chassis or when exposed to the elements have a PVC jacket; however, some internal applications may omit the insulation jacket. Hard line can be very thick, typically at least a half inch or 13 mm and up to several times that, and has low loss even at high power. These large scale hard lines are almost always used in the connection between a transmitter on the ground and the antenna or aerial on a tower. Hard line may also be known by trademarked names such as Heliax (Andrew), or Cablewave (RFS/Cablewave). Larger varieties of hardline may consist of a center conductor which is constructed from either rigid or corrugated copper tubing. The dielectric in hard line may consist of polyethylene foam, air or a pressurized gas such as nitrogen or desiccated air (dried air). In gas-charged lines, hard plastics such as nylon are used as spacers to separate the inner and outer conductors. The addition of these gases into the dielectric space reduces moisture contamination, provides a stable dielectric constant, as well as a reduced risk of internal arcing. Gas-filled hardlines are usually used on high powered RF transmitters such as television or radio broadcasting, military transmitters, as well as high powered Amateur radio applications but may also be used on some critical lower powered applications such as those in the microwave bands. Although in the microwave region waveguide is more often used than hard line for transmitter to antenna, or antenna to receiver applications. The various shields used in hardline also differ; some forms use rigid tubing, or pipe, others may use a corrugated tubing which makes bending easier, as well as reduces kinking when the cable is bent to conform. Smaller varieties of hard line may be used internally in some high frequency applications, particularly in equipment within the microwave range, to reduce interference between stages of the device.
Radiating or Leaky Cable is another form of coaxial cable which is constructed in a similar fashion to hard line, however it is constructed with tuned slots cut into the shield. These slots are tuned to the specific RF wavelength of operation or tuned to a specific radio frequency band. This type of cable is to provide a tuned bi-directional "desired" leakage effect between transmitter and receiver. It is often used in elevator shafts, underground, transportation tunnels and in other areas where an antenna is not feasible. One example of this type of cable is Radiax (Andrew).
RG/6 is available in three different types designed for various applications. "Plain" or "house" wire is designed for indoor or external house wiring. "Flooded" cable is infused with heavy waterproofing for use in underground conduit (ideally) or direct burial. "Messenger" may contain some waterproofing but is distinguished by the addition of a steel messenger wire along its length to carry the tension involved in an aerial drop from a utility pole.
Triaxial cable or triax is coaxial cable with a third layer of shielding, insulation and sheathing. The outer shield, which is earthed (grounded), protects the inner shield from electromagnetic interference from outside sources.
Twin-axial cable or twinax is a balanced, twisted pair within a cylindrical shield. It allows a nearly perfect differential signal which is both shielded and balanced to pass through. Multi-conductor coaxial cable is also sometimes used.
Biaxial cable or biax is a figure-8 configuration of two 50 Ω coaxial cables, externally resembling that of lamp cord, or speaker wire. Biax is used in some proprietary computer networks. Others may be familiar with 75Ω biax which at one time was popular on many cable TV services.
Semi-rigid cable is a coaxial form using a solid copper outer sheath. This type of coax offers superior screening compared to cables with a braided outer conductor, especially at higher frequencies. The major disadvantage is that the cable, as its name implies, is not very flexible, and is not intended to be flexed after initial forming. (See "hard line")

Interference and troubleshooting

Coaxial cable insulation can degrade requiring cable replacement, especially if it has been exposed to the elements on a continuous basis. The shield is normally grounded, and if even a single thread of the braid or filament of foil touches the center conductor, the signal will be shorted causing significant or total signal loss. This most often occurs at improperly installed end connectors and splices. Also, the connector or splice must be properly attached to the shield, as this provides the path to ground for the interfering signal.
Despite being shielded, interference can occur on coaxial cable lines. Susceptibility to interference has little relationship to broad cable type designations (e.g. RG-59, RG-6) but is strongly related to the composition and configuration of the cable's shielding. For cable television, with frequencies extending well into the UHF range, a foil shield is normally provided, and will provide total coverage as well as high effectiveness against high-frequency interference. Foil shielding is ordinarily accompanied by a tinned copper or aluminum braid shield, with anywhere from 60 to 95% coverage. The braid is important to shield effectiveness because (1) it is more effective than foil at absorbing low-frequency interference, (2) it provides higher conductivity to ground than foil, and (3) it makes attaching a connector easier and more reliable. "Quad-shield" cable, using two low-coverage aluminum braid shields and two layers of foil, is often used in situations involving troublesome interference, but is less effective than a single layer of foil and single high-coverage copper braid shield such as is found on broadcast-quality precision video cable.
In the United States and some other countries, cable television distribution systems use extensive networks of outdoor coaxial cable, often with in-line distribution amplifiers. Leakage of signals into and out of cable TV systems can cause interference to cable subscribers and to over-the-air radio services using the same frequencies as those of the cable system.

Timeline

See also

References

coax in Bulgarian: Коаксиален кабел
coax in Czech: Koaxiální kabel
coax in Danish: Koaksialkabel
coax in German: Koaxialkabel
coax in Spanish: Cable coaxial
coax in Basque: Kable ardazkide
coax in French: Câble coaxial
coax in Galician: Cabo coaxial
coax in Croatian: Koaksijalni kabel
coax in Italian: Cavo coassiale
coax in Hebrew: כבל קואקסיאלי
coax in Lithuanian: Bendraašis kabelis
coax in Macedonian: Коаксијален кабел
coax in Malay (macrolanguage): Kabel sepaksi
coax in Dutch: Coaxkabel
coax in Japanese: 同軸ケーブル
coax in Norwegian: Koaksialkabel
coax in Polish: Kabel koncentryczny
coax in Portuguese: Cabo coaxial
coax in Russian: Коаксиальный кабель
coax in Simple English: Coaxial cable
coax in Slovak: Koaxiálny kábel
coax in Finnish: Koaksiaalikaapeli
coax in Swedish: Koaxialkabel
coax in Ukrainian: Коаксіальний кабель
coax in Chinese: 同轴电缆

Synonyms, Antonyms and Related Words

advocate, allure, apply pressure, bait, bait the hook, banter, barter, beguile, beset, besiege, blandish, blarney, bug, butter up, buttonhole, cajole, call on, call upon, charm, con, decoy, draw, draw in, draw on, dun, ensnare, entice, exert pressure, exhort, flirt, flirt with, get, give the come-on, high-pressure, importune, induce, insist, insist upon, inveigle, jawbone, jolly, lead on, lobby, lure, manipulate, nag, nag at, offer bait to, persuade, pester, plague, plead with, ply, press, pressure, prevail, push, recommend, rope in, seduce, soft-soap, suck in, sweet-talk, tease, tempt, urge, wheedle, woo, work on
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