Coaxial cable, orcoax (pronounced/ˈkoʊ.æks/), is a type ofelectrical cable consisting of an innerconductor surrounded by a concentric conductingshield, with the two separated by adielectric (insulating material); many coaxial cables also have a protective outer sheath or jacket. The termcoaxial refers to the inner conductor and the outer shield sharing a geometric axis.
In his 1880 British patent,Oliver Heaviside showed how coaxial cable could eliminate signal interference between parallel cables.
Coaxial cable was used in the first (1858) and followingtransatlantic cable installations, but its theory was not described until 1880 by English physicist, engineer, and mathematicianOliver Heaviside, who patented the design in that year (British patent No. 1,407).[1]
Coaxial cable is used as atransmission line for radio frequency signals. Its applications includefeedlines connectingradio transmitters andreceivers to their antennas, computer network (e.g.,Ethernet) connections,digital audio (S/PDIF), and distribution ofcable television signals. One advantage of coaxial over other types of radiotransmission line is that in an ideal coaxial cable theelectromagnetic field carrying the signal exists only in the space between the inner and outerconductors. This allows coaxial cable runs to be installed next to metal objects such as gutters without the power losses that occur in other types of transmission lines. Coaxial cable also provides protection of the signal from externalelectromagnetic interference.
Coaxial cable conducts electrical signals using an inner conductor (usually a solid copper, stranded copper or copper-plated steel wire) surrounded by an insulating layer and all enclosed by a shield, typically one to four layers of woven metallic braid and metallic tape.[2] The cable is protected by an outer insulating jacket. Normally, the outside of the shield is kept at ground potential and a signal carrying voltage is applied to the center conductor. When usingdifferential signaling, coaxial cable provides an advantage of equal push-pull currents on the inner conductor and inside of the outer conductor that restrict the signal's electric and magnetic fields to thedielectric, with littleleakage outside the shield.[citation needed] Further, electric and magnetic fields outside the cable are largely kept from interfering with signals inside the cable, if unequal currents are filtered out at the receiving end of the line. This property makes coaxial cable a good choice both for carrying weak signals that cannot tolerate interference from the environment, and for stronger electrical signals that must not be allowed to radiate or couple into adjacent structures or circuits.[3] Larger diameter cables and cables with multiple shields have less leakage.
Common applications of coaxial cable include video andCATV distribution, RF and microwave transmission, and computer and instrumentation data connections.[4]
Thecharacteristic impedance of the cable (Z0) is determined by thedielectric constant of the inner insulator and the radii of the inner and outer conductors. In radio frequency systems, where the cable length is comparable to the wavelength of the signals transmitted, a uniform cable characteristic impedance is important to minimize loss. Thesource and load impedances are chosen to match the impedance of the cable to ensuremaximum power transfer and minimumstanding wave ratio. Other important properties of coaxial cable include attenuation as a function of frequency, voltage handling capability, and shield quality.[3]
Coaxial cable design choices affect physical size, frequency performance, attenuation, power handling capabilities, flexibility, strength, 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. Copper-plated steel wire is often used as an inner conductor for cable used in the cable TV industry.[5]
The insulator surrounding the inner conductor may be solid plastic, a foam plastic, or air with spacers supporting the inner wire. The properties of the dielectric insulator determine some of the electrical properties of the cable. A common choice is a solidpolyethylene (PE) insulator, used in lower-loss cables. SolidTeflon (PTFE) is also used as an insulator, and exclusively inplenum-rated cables.[citation needed] Some coaxial lines use air (or some other gas) and have spacers to keep the inner conductor from touching the shield.
Many conventional coaxial cables use braided copper wire forming the shield. This allows the cable to be flexible, but it also means there are gaps in the shield layer, and 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.[5] 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, such as "quad-shield", which uses four alternating layers of foil and braid. Other shield designs sacrifice flexibility for better performance; some shields are a solid metal tube. Those cables cannot be bent sharply, as the shield will kink, causing losses in the cable. When a foil shield is used a small wire conductor incorporated into the foil makes soldering the shield termination easier.[examples needed]
For high-power radio-frequency transmission up to about 1 GHz, coaxial cable with a solid copper outer conductor is available in sizes of 0.25 inch upward. The outer conductor is corrugated like abellows to permit flexibility and the inner conductor is held in position by a plastic spiral to approximate an air dielectric.[5] One brand name for such cable isHeliax.[6]
Coaxial cables require an internal structure of an insulating (dielectric) material to maintain the spacing between the center conductor and shield. Thedielectric losses increase in this order: Ideal dielectric (no loss), vacuum, air,polytetrafluoroethylene (PTFE), polyethylene foam, and solid polyethylene. An inhomogeneous dielectric needs to be compensated by a non-circular conductor to avoid current hot-spots.
While many cables have a solid dielectric, many others have a foam dielectric that contains as much air or other gas as possible to reduce the losses by allowing the use of a larger diameter center conductor. Foam coax will have about 15% less attenuation but some types of foam dielectric can absorb moisture—especially at its many surfaces—in humid environments, significantly increasing the loss. Supports shaped like stars or spokes are even better but more expensive and very susceptible to moisture infiltration. Still more expensive were the air-spaced coaxials used for some inter-city communications in the mid-20th century. The center conductor was suspended by polyethylene discs every few centimeters. In some low-loss coaxial cables such as the RG-62 type, the inner conductor is supported by a spiral strand of polyethylene, so that an air space exists between most of the conductor and the inside of the jacket. The lowerdielectric constant of air allows for a greater inner diameter at the same impedance and a greater outer diameter at the same cutoff frequency, loweringohmic losses. Inner conductors are sometimes silver-plated to smooth the surface and reduce losses due toskin effect.[5] A rough surface extends the current path and concentrates the current at peaks, thus increasing ohmic loss.
The insulating jacket can be made from many materials. A common choice isPVC, but some applications may require fire-resistant materials. Outdoor applications may require the jacket to resistultraviolet light,oxidation, rodent damage, ordirect burial. Flooded coaxial cables use a water-blocking gel to protect the cable from water infiltration through minor cuts in the jacket. For internal chassis connections the insulating jacket may be omitted.[examples needed]
Twin-lead transmission lines have the property that theelectromagnetic 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, tightly twisted, or otherwise shaped without changing theircharacteristic impedance, causing reflection of the signal back toward the source. They also cannot be buried or run along or attached to anythingconductive, as the extended fields will induce currents in the nearby conductors causing unwantedradiation and detuning of the line.Standoff insulators are used to keep them away from parallel metal surfaces. Coaxial lines largely solve this problem by confining virtually all of the electromagnetic wave to the area inside the cable. 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, so long as provisions are made to ensure differential signalling push-pull currents in the cable.
In radio-frequency applications up to a fewgigahertz, the wave propagates primarily in thetransverse electric magnetic (TEM) mode, which means that the electric and magnetic fields are both perpendicular to the direction of propagation. However, above a certaincutoff frequency, transverse electric (TE) or transverse magnetic (TM) modes can also propagate, as they do in a hollowwaveguide. It is usually undesirable to transmit signals above the cutoff frequency, since it may cause multiple modes with differentphase velocities to propagate,interfering with each other. The outer diameter is roughly inversely proportional to thecutoff frequency. A propagating surface-wave mode that only involves thecentral conductor also exists, but is effectively suppressed in coaxial cable of conventional geometry and common impedance. Electric field lines for this TM mode have a longitudinal component and require line lengths of a half-wavelength or longer.
Coaxial cable may be viewed as a type ofwaveguide. Power is transmitted through the radial electric field and the circumferential magnetic field in the TEM mode. This is the dominant mode from zero frequency (DC) to an upper limit determined by the electrical dimensions of the cable.[7]
Coaxial connectors are designed to maintain a coaxial form across the connection and have the same impedance as the attached cable.[5] Connectors are usually plated with high-conductivity metals such as silver or tarnish-resistant gold. Due to theskin effect, the RF signal is only carried by the plating at higher frequencies and does not penetrate to the connector body. Silver however tarnishes quickly and thesilver sulfide that is produced is poorly conductive, degrading connector performance, making silver a poor choice for this application.[8]
Coaxial cable is a particular kind oftransmission line, so the circuit models developed for general transmission lines are appropriate. SeeTelegrapher's equation.
Schematic representation of the elementary components of a transmission lineSchematic representation of a coaxial transmission line, showing the characteristic impedance
Dielectric constant of the insulator, The dielectric constant is often quoted as the relative dielectric constant referred to the dielectric constant of free space 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 is often used.
Magnetic permeability of the insulator, Permeability is often quoted as therelative permeability referred to the permeability of free space The relative permeability will almost always be1.
Seriesinductance per unit length, inhenries per metre, considering the central conductor to be a thin hollow cylinder (due toskin effect).
Seriesresistance 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.
Shuntconductance per unit length, insiemens per metre. The shunt conductance is usually very small because insulators with good dielectric properties are used (a very lowloss tangent). At high frequencies, a dielectric can have a significant resistive loss.
Characteristic impedance in ohms (Ω). The complex impedanceZo of an infinite length of transmission line is:
WhereR is the resistance per unit length,L is the inductance per unit length,G is the conductance per unit length of the dielectric,C is the capacitance per unit length, ands =jω =j2πf is the frequency. The "per unit length" dimensions cancel out in the impedance formula.
AtDC the two reactive terms are zero, so the impedance is real-valued, and is extremely high. It looks like
With increasing frequency, the reactive components take effect and the impedance of the line is complex-valued. At very low frequencies (audio range, of interest to telephone systems)G is typically much smaller thansC, so the impedance at low frequencies is
which has a phase value of -45 degrees.
At higher frequencies, the reactive terms usually dominateR andG, and the cable impedance again becomes real-valued. That value isZo, thecharacteristic impedance of the cable:
Assuming the dielectric properties of the material inside the cable do not vary appreciably over the operating range of the cable, the characteristic impedance is frequency independent above about five times theshield cutoff frequency. For typical coaxial cables, the shield cutoff frequency is 600 Hz (for RG-6A) to 2,000 Hz (for RG-58C).[10]
The parametersL andC are determined from the ratio of the inner (d) and outer (D) diameters and thedielectric constant (ε). The characteristic impedance is given by[11]
Attenuation (loss) per unit length, indecibels per meter. 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 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 usually1).
Single-mode band. In coaxial cable, the dominant mode (the mode with the lowestcutoff frequency) is the TEM mode,[7] which has a cutoff frequency of zero; it propagates all the way down to DC. The mode with the next lowest cutoff is the TE11 mode. This mode has one 'wave' (two reversals of polarity) in going around the circumference of the cable. To a good approximation, the condition for the TE11 mode to propagate is that the wavelength in the dielectric is no longer than the average circumference of the insulator; that is that the frequency is at least
Hence, the cable is single-mode from DC up to this frequency, and might in practice be used up to 90%[12] of this frequency.
Peak Voltage. The peak voltage is set by the breakdown voltage of the insulator.:[13]
where
Vp is the peak voltage
Ed is the insulator's breakdown voltage in volts per meter
d is the inner diameter in meters
D is the outer diameter in meters
The calculated peak voltage is often reduced by a safety factor.
The best coaxial cable impedances were experimentally determined atBell Laboratories in 1929 to be 77 Ω for low-attenuation, 60 Ω for high-voltage, and 30 Ω for high-power. For a coaxial cable with air dielectric and a shield of a given inner diameter, the attenuation is minimized by choosing the diameter of the inner conductor to give a characteristic impedance of 76.7 Ω.[14] When more common dielectrics are considered, the lowestinsertion loss impedance drops down to a value between 52 and 64 Ω. Maximum power handling is achieved at 30 Ω.[15]
The approximate impedance required to match a centre-feddipole antenna in free space (i.e., a dipole without ground reflections) is 73 Ω, so 75 Ω coax was commonly used for connecting shortwave antennas to receivers. These typically involve such low levels of RF power that power-handling and high-voltage breakdown characteristics are unimportant when compared to attenuation. Likewise withCATV, although many broadcast TV installations and CATV headends use 300 Ω foldeddipole antennas to receive off-the-air signals, 75 Ω coax makes a convenient 4:1balun transformer for these as well as possessing low attenuation.
Thearithmetic mean between 30 Ω and 77 Ω is 53.5 Ω; thegeometric mean is 48 Ω. The selection of 50 Ω as a compromise between power-handling capability and attenuation is in general cited as the reason for the number.[14] 50 Ω also works out tolerably well because it corresponds approximately to the feedpoint impedance of a half-wave dipole, mounted approximately a half-wave above "normal" ground (ideally 73 Ω, but reduced for low-hanging horizontal wires).
RG-62 is a 93 Ω coaxial cable originally used in mainframe computer networks in the 1970s and early 1980s (it was the cable used to connectIBM 3270 terminals to IBM 3274/3174 terminal cluster controllers). Later, some manufacturers of LAN equipment, such as Datapoint forARCNET, adopted RG-62 as their coaxial cable standard. The cable has the lowest capacitance per unit-length when compared to other coaxial cables of similar size.
All of the components of a coaxial system should have the same impedance to avoid internal reflections at connections between components (seeImpedance matching). Such reflections may cause signal attenuation. They introduce standing waves, which increase losses and can even result in cable dielectric breakdown with high-power transmission. In analog video or TV systems, reflections causeghosting in the image; multiple reflections may cause the original signal to be followed by more than one echo. If a coaxial cable is open (not connected at the end), the termination has nearly infinite resistance, which causes reflections. If the coaxial cable is short-circuited, the termination resistance is nearly zero, which causes reflections with the opposite polarity. Reflections will be nearly eliminated if the coaxial cable is terminated in a pure resistance equal to its impedance.
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 noise and disruption of the desired signal. Egress is the passage of signal intended to remain within the cable into the outside world and can result in a weaker signal at the end of the cable andradio frequency interference to nearby devices. Severe leakage usually results from improperly installed connectors or faults in the cable shield.
For example, in the United States, signal leakage fromcable television systems is regulated by the FCC, since cable signals use the same frequencies as aeronautical and radionavigation bands. CATV operators may also choose to monitor their networks for leakage to prevent ingress. Outside signals entering the cable can cause unwanted noise and picture ghosting. Excessivenoise can overwhelm the signal, making it useless. In-channel ingress can be digitally removed byingress cancellation.
An ideal shield would be a perfect conductor with no holes, gaps, or bumps connected to a perfect ground. However, a smooth solid highly conductive shield would be heavy, inflexible, and expensive. Such coax is used for straight-line feeds to commercial radio broadcast towers. More economical cables must make compromises between shield efficacy, flexibility, and cost, such as the corrugated surface of flexible hardline, flexible braid, or foil shields. Since shields cannot be perfect conductors, current flowing on the inside of the shield produces an electromagnetic field on the outer surface of the shield.
Consider theskin effect. The magnitude of an alternating current in a 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 must always be some leakage.
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.
Signal leakage can be severe if there is poor contact at the interface to connectors at either end of the cable or if there is a break in the shield.
To greatly reduce signal leakage into or out of the cable, by a factor of 1000, or even 10,000, superscreened cables are often used in critical applications, such as forneutron flux counters innuclear reactors.
Superscreened cables for nuclear use are defined in IEC 96-4-1, 1990, however as there have been long gaps in the construction of nuclear power stations in Europe, many existing installations are using superscreened cables to the UK standard AESS(TRG) 71181[16] which is referenced in IEC 61917.[17]
A continuous current, even if small, along the imperfect shield of a coaxial cable can cause visible or audible interference. In CATV systems distributing analog signals the potential difference between the coaxial network and the electrical grounding system of a house can cause a visible "hum bar" in the picture. 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. Seeground loop.
External fields create a voltage across theinductance of the outside 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.
Thetransformer 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 ofmu-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 senders and receivers have means to reduce the leakage even further. They increase the transformer effect by passing the whole cable through a ferrite core one or more times.
Common mode current occurs when stray currents in the shield flow in the same direction as the current in the center conductor, causing the coax to radiate. They are the opposite of the desired "push-pull" differential signalling currents, where the signal currents on the inner and outer conductor are equal and opposite.
Most of the shield effect in coax results from opposing currents in the center conductor and shield creating opposite magnetic fields that cancel, and thus do not radiate. The same effect helpsladder line. However, ladder line is extremely sensitive to surrounding metal objects, which can enter the fields before they completely cancel. Coax does not have this problem, since the field is enclosed in the shield. However, it is still possible for a field to form between the shield and other connected objects, such as the antenna the coax feeds. The current formed by the field between the antenna and the coax shield would flow in the same direction as the current in the center conductor, and thus not be canceled. Energy would radiate from the coax itself, affecting theradiation pattern of the antenna. With sufficient power, this could be a hazard to people near the cable. A properly placed and properly sizedbalun can prevent common-mode radiation in coax. An isolating transformer or blockingcapacitor can be used to couple a coaxial cable to equipment, where it is desirable to pass radio-frequency signals but to block direct current or low-frequency power.
The characteristic impedance formula above is a good approximation atradio frequencies however for frequencies below 100 kHz (such asaudio) it becomes important to use the completetelegrapher's equation:
Applying this formula to typical 75 ohm coax we find the measured impedance across the audio spectrum will range from ~150 ohms to ~5K ohms, much higher than nominal. Thevelocity of propagation also slows considerably. Thus we can expect coax cable impedances to be consistent at RF frequencies but variable across audio frequencies. This effect was manifested when trying to send a plain voice signal across thetransatlantic telegraph cable, with poor results.[18][19]
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 byF connectors.
A series of standard types of coaxial cable were specified formilitary uses, in the form "RG-#" or "RG-#/U". They date fromWorld War II and were listed inMIL-HDBK-216 published in 1962. These designations are now obsolete. The RG designation stands for Radio Guide; the U designation stands for Universal. The current military standard isMIL-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 compatibleconnectors that fit the inner conductor, dielectric, and jacket dimensions of the old RG-series cables.
Low loss at high frequency for cable and satellite television. Used for long drops and underground conduit, similar to RG7 but generally lower loss.[23][24]
Used to carrybaseband video inclosed-circuit television, previously used for cable television. In general, it has poor shielding but will carry an HQ HD signal or video over short distances.[26]
Used to carry television, video observation systems, and other. PVC jacket.
5C-2V
75
0.80
PE
0.82±0.02
0.181
4.6
0.256
6.5
Double
Used for interior lines for monitoring system, CCTV feeder lines, wiring between the camera and control unit and video signal transmission. PVC jacket.
Shorter coaxials still carrycable television signals to the majority of television receivers, and this purpose consumes the majority of coaxial cable production. In 1980s and early 1990s coaxial cable was also used incomputer networking, most prominently inEthernet networks, where it was later in late 1990s to early 2000s replaced byUTP cables in North America andSTP cables in Western Europe, both with 8P8C modular connectors.
Micro coaxial cables are used in a range of consumer devices, military equipment, and also in ultrasound 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 commercialtwo-way radio frequency applications (including radio, and telecommunications), although 75 ohms is commonly used for broadcast television and radio.
Coaxial cable is often used to carry signals from anantenna to a receiver. In many cases, the same cable carries power toward the antenna, to power apreamplifier. In some cases, a single cable carries unidirectional power and bidirectional data/signals, as inDiSEqC.
1+5⁄8 in (41 mm) flexible line with (mostly) air dielectric1+5⁄8 in (41 mm) Heliax coaxial cable with FPE foamed polyethylene dielectric
Larger varieties of hardline may have a center conductor that 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 asnitrogen 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, and provides a reduced risk of internalarcing. Gas-filled hardlines are usually used on high-powerRF transmitters such as television or radio broadcasting, military transmitters, and high-poweramateur radio applications but may also be used on some critical lower-power applications such as those in the microwave bands. However, 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 hard line also differ; some forms use rigid tubing, or pipe, while 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, in particular in equipment within the microwave range, to reduce interference between stages of the device.
Radiating orleaky 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, US Navy Ships, underground transportation tunnels and in other areas where an antenna is not feasible. One example of this type of cable is Radiax (CommScope).[43]
RG-6 is available in four different types designed for various applications. In addition, the core may be copper clad steel (CCS) or bare solid copper (BC). "Plain" or "house" RG-6 is designed for indoor or external house wiring. "Flooded" cable is infused with water-blocking gel for use in underground conduit or direct burial. "Messenger" may contain some waterproofing but is distinguished by the addition of a steelmessenger wire along its length to carry the tension involved in an aerial drop from a utility pole. "Plenum" cabling is expensive and comes with a special Teflon-based outer jacket designed for use in ventilation ducts to meet fire codes. It was developed since the plastics used as the outer jacket and inner insulation in many "Plain" or "house" cabling gives off poisonous gas when burned.
Triaxial cable ortriax 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.
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)
Conformable cable is a flexible reformable alternative to semi-rigid coaxial cable used where flexibility is required. Conformable cable can be stripped and formed by hand without the need for specialized tools, similar to standard coaxial cable.
Rigid line is a coaxial line formed by two copper tubes maintained concentric every other meter using PTFE-supports. Rigid lines cannot be bent, so they often need elbows. Interconnection with rigid line is done with an inner bullet/inner support and a flange or connection kit. Typically, rigid lines are connected using standardisedEIA RF Connectors whose bullet and flange sizes match the standard line diameters. For each outer diameter, either 75 or 50 ohm inner tubes can be obtained.Rigid line is commonly used indoors for interconnection between high-power transmitters and other RF-components, but more rugged rigid line with weatherproof flanges is used outdoors on antenna masts, etc. In the interests of saving weight and costs, on masts and similar structures the outer line is often aluminium, and special care must be taken to prevent corrosion.With a flange connector, it is also possible to go from rigid line to hard line. Many broadcasting antennas and antenna splitters use the flanged rigid line interface even when connecting to flexible coaxial cables and hard line.Rigid line is produced in a number of different sizes:
Coaxial cable insulation may degrade, requiring replacement of the cable, 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. Forcable 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 preventing 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 theUnited 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.
Early coaxial antenna feedline of50 kW radio stationWNBC, New York, 1930sAT&T coaxial cable trunkline installed between East Coast and Midwest in 1948. Each of the 8 coaxial subcables could carry 480 telephone calls or one television channel.
1936 — Underwater coaxial cable installed betweenApollo Bay, nearMelbourne, Australia, andStanley, Tasmania. The 300 km (190 mi) cable can carry one 8.5-kHz broadcast channel and seven telephone channels.[49]
1936 — AT&T installs experimental coaxial telephone and television cable betweenNew York andPhiladelphia, with automatic booster stations every ten miles (16 km). Completed in December, it can transmit 240 telephone calls simultaneously.[50][51]
1941 — First commercial use in US by AT&T, betweenMinneapolis, Minnesota and Stevens Point, Wisconsin. L1 system with capacity of one TV channel or 480 telephone circuits.
1949 — On January 11, eight stations on the US East Coast and seven Midwestern stations are linked via a long-distance coaxial cable.[54]
1956 — First transatlantic telephone coaxial cable laid,TAT-1.[55][56]
1962 — 960 km (600 mi)Sydney–Melbourne co-axial cable commissioned, carrying 3 x 1,260 simultaneous telephone connections, and-or simultaneous inter-city television transmission.[57][58]
^Nahin, Paul J. (2002).Oliver Heaviside: The Life, Work, and Times of an Electrical Genius of the Victorian Age. JHU Press.ISBN0-8018-6909-9.
^"Article 100 Definitions".NFPA 70 National Electrical Code. Quincy, Massachusetts: National Fire Protection Association. 2017. RetrievedOctober 9, 2023.Coaxial cable: a cylindrical assembly composed of a conductor center inside a metallic tube or shield, separated by a dielectric material and usually covered by an insulating jacket.
^abcde"5: Transmission Media".The ARRL UHF/Microwave Experimenter's Manual. Newington, CT: American Radio Relay League. 1990. pp. 5.19 –5.21.ISBN0-87259-312-6.
^Silver, H. Ward; Ford, Steven R.; Wilson, Mark J., eds. (2015). "23.1.2".ARRL Antenna Book for Radio Communications (23rd ed.). Netwon, CT: ARRL.ISBN978-1-62595-044-4.
