Coaxial cable
Encyclopedia
Coaxial
In geometry, coaxial means that two or more forms share a common axis; it is the three-dimensional linear analogue of concentric.Coaxial cable, as a common example, has a wire conductor in the centre a circumferential outer conductor and an insulating medium called the dielectric separating...
comes from the inner conductor and the outer shield sharing the same geometric axis. Coaxial cable was invented by English engineer and mathematician Oliver Heaviside
Oliver Heaviside
Oliver Heaviside was a self-taught English electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques to the solution of differential equations , reformulated Maxwell's field equations in terms of electric and...
, who patented the design in 1880.
Coaxial cable is used as a transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
for radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
signals. Its applications include feedlines connecting radio transmitters and receivers with their antennas, computer network (Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
) connections, and distributing cable television
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
signals. One advantage of coax over other types of radio transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
is that in an ideal coaxial cable the electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
carrying the signal exists only in the space between the inner and outer conductor
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
s. 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 external electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
.
Coaxial cable differs from other shielded cable
Shielded cable
A shielded or screened cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer. The shield may be composed of braided strands of copper , a non-braided spiral winding of copper tape, or a layer of conducting polymer. Usually, this shield is covered...
used for carrying lower-frequency signals, such as audio signal
Audio signal
An audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...
s, in that the dimensions of the cable are controlled to give a precise, constant conductor spacing, which is needed for it to function efficiently as a radio frequency transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
.
Description
Leakage
Leakage may refer to:*Leakage *Memory leak, in computer science*Leakage *Leakage *Leakage *Leakage...
outside the shield. On the converse, electric and magnetic fields outside the cable are largely kept from causing interference to signals inside the cable. This property makes coaxial cable a good choice for carrying weak signals that cannot tolerate interference from the environment or for higher electrical signals that must not be allowed to radiate or couple into adjacent structures or circuits.
Common applications of coaxial cable include video and CATV distribution, RF and microwave transmission, and computer and instrumentation data connections.
The characteristic impedance
Characteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
of the cable (
) is determined by the dielectric constantDielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
of the inner insulator and the radiuses of the inner and outer conductors. A controlled cable characteristic impedance is important because the source and load impedance should be matched
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
to ensure maximum power transfer and minimum Standing Wave Ratio
Standing wave ratio
In telecommunications, standing wave ratio is the ratio of the amplitude of a partial standing wave at an antinode to the amplitude at an adjacent node , in an electrical transmission line....
. Other important properties of coaxial cable include attenuation as a function of frequency, voltage handling capability, and shield quality.
Construction
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. 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 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
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
(PE) insulator, used in lower-loss cables. Solid Teflon (PTFE) 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.
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. 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 take sharp bends, as the shield will kink, causing losses in the cable.
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 rippled like a bellows to permit flexibility and the inner conductor is held in position by a plastic spiral to approximate an air dielectric.
Coaxial cables require an internal structure of an insulating (dielectric) material to maintain the spacing between the center conductor and shield. The dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
losses increase in this order: Ideal dielectric (no loss), vacuum, air, Polytetrafluoroethylene
Polytetrafluoroethylene
Polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. PTFE is most well known by the DuPont brand name Teflon....
(PTFE), polyethylene foam, and solid polyethylene. A low relative permittivity allows for higher-frequency usage. An inhomogeneous dielectric needs to be compensated by a non-circular conductor to avoid current hot-spots.
Most cables have a solid dielectric; others have a foam dielectric that contains as much air as possible to reduce the losses. Foam coax will have about 15% less attenuation but can absorb moisture—especially at its many surfaces — in humid environments, increasing the loss. Supports shaped like stars or spokes are even better but more expensive. 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 an 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 lower dielectric constant
Permittivity
In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
of air allows for a greater inner diameter at the same impedance and a greater outer diameter at the same cutoff frequency, lowering ohmic losses. Inner conductors are sometimes silver-plated to smooth the surface and reduce losses due to skin effect
Skin effect
Skin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
. A rough surface prolongs the path for the current and concentrates the current at peaks and, thus, increases ohmic losses.
The insulating jacket can be made from many materials. A common choice is PVC
PVC
Polyvinyl chloride is a plastic.PVC may also refer to:*Param Vir Chakra, India's highest military honor*Peripheral venous catheter, a small, flexible tube placed into a peripheral vein in order to administer medication or fluids...
, but some 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.
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 impedanceCharacteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
, causing reflection of the signal back toward the source. 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
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
and detuning of the line. Coaxial lines 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.
In radio-frequency applications up to a few gigahertz, the wave propagates primarily 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
Cutoff frequency
In physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced rather than passing through.Typically in electronic systems such as filters and...
, transverse electric (TE) and/or transverse magnetic (TM) modes can also propagate, as they do in a waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
. It is usually undesirable to transmit signals above the cutoff frequency, since it may cause multiple modes with different phase velocities
Phase velocity
The phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the speed at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave will appear to travel at the phase velocity...
to propagate, interfering with each other. The outer diameter is roughly inversely proportional to the cutoff frequency
Cutoff frequency
In physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced rather than passing through.Typically in electronic systems such as filters and...
. A propagating surface-wave mode that does not involve or require the outer shield but only a single central conductor
Single-wire transmission line
A single-wire transmission line is a method of supplying electrical power through a single electrical conductor.-History:In 1729, the English physicist Stephen Gray noticed the phenomenon of electrical conductivity...
also exists in coax but this mode is effectively suppressed in coax 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 of waveguide
Waveguide (electromagnetism)
In electromagnetics and communications engineering, the term waveguide may refer to any linear structure that conveys electromagnetic waves between its endpoints. However, the original and most common meaning is a hollow metal pipe used to carry radio waves...
. Power is transmitted through the radial electric field and the circumferential magnetic field in the TEM00 transverse mode
Transverse mode
A transverse mode of a beam of electromagnetic radiation is a particular electromagnetic field pattern of radiation measured in a plane perpendicular to the propagation direction of the beam...
. This is the dominant mode from zero frequency (DC) to an upper limit determined by the electrical dimensions of the cable.
Connectors
The ends of coaxial cables usually terminate with connectors. Coaxial connectors are designed to maintain a coaxial form across the connection and have the same well-defined impedance as the attached cable. Connectors are often plated with high-conductivity metals such as silver or gold. Due to the skin effectSkin effect
Skin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
, the RF signal is only carried by the plating and does not penetrate to the connector body. Although silver oxidizes quickly, the silver oxide that is produced is still conductive. While this may pose a cosmetic issue, it does not degrade performance.
Important parameters
Coaxial cable is a particular kind of transmission lineTransmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
, so the circuit models developed for general transmission lines are appropriate. See Telegrapher's equation.
Physical parameters
In the following section, these symbols are used:- Length of the cable,
. - Outside diameter of inner conductor,
. - Inside diameter of the shield,
. - Dielectric constantDielectric constantThe relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
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 the relative permeability 
referred to the permeability of free space 
: 
. The relative permeability will almost always be 1.
Fundamental electrical parameters
- Shunt capacitanceCapacitanceIn electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
per unit length, in faradFaradThe farad is the SI unit of capacitance. The unit is named after the English physicist Michael Faraday.- Definition :A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt. A coulomb is 1 ampere second...
s per metre.
-
- Series inductanceInductanceIn electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
per unit length, in henrys per metre.
- Series inductance
- Series resistanceElectrical resistanceThe electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
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 effectSkin effectSkin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
increases the effective resistance by confining the conduction to a thin layer of each conductor. - Shunt conductanceConductanceConductance may refer to:* Electrical conductance, the ability for electricity to flow a certain path* Fluid conductance, the ability for fluid to transmit through materials* Thermal conductivity, the ability for temperatures to transmit through materials...
per unit length, in siemensSiemens (unit)The siemens is the SI derived unit of electric conductance and electric admittance. Conductance and admittance are the reciprocals of resistance and impedance respectively, hence one siemens is equal to the reciprocal of one ohm, and is sometimes referred to as the mho. In English, the term...
per metre. The shunt conductance is usually very small because insulators with good dielectric properties are used (a very low loss tangentLoss tangentThe loss tangent is a parameter of a dielectric material that quantifies its inherent dissipation of electromagnetic energy. The term refers to the tangent of the angle in a complex plane between the resistive component of an electromagnetic field and its reactive component.-Electromagnetic...
). At high frequencies, a dielectric can have a significant resistive loss.
- Series resistance
Derived electrical parameters
- Characteristic impedanceCharacteristic impedanceThe characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
in ohms (Ω). Neglecting resistance per unit length for most coaxial cables, the characteristic impedance is determined from the capacitance per unit length (
) and the inductance per unit length (
). The simplified expression is (
). Those parameters are determined from the ratio of the inner (d) and outer (D) diameters and the dielectric constantDielectric constantThe relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
(
). The characteristic impedance is given by
-
- 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 five 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 decibelDecibelThe decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
s 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 propagationVelocity of propagationThe velocity factor , also called wave propagation speed or velocity of propagation , of a transmission medium is the speed at which a wavefront passes through the medium, relative to the...
, in meters per second. The velocity of propagation depends on the dielectric constant and permeability (which is usually 1).
- Single-mode band. In coaxial cable, the dominant mode (the mode with the lowest cutoff frequencyCutoff frequencyIn physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced rather than passing through.Typically in electronic systems such as filters and...
) is the TEM mode, which has a cutoff frequency of zero; it propagates all the way down to d.c. 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
- Single-mode band. In coaxial cable, the dominant mode (the mode with the lowest cutoff frequency
.
- Attenuation (loss) per unit length, in decibel
- Hence, the cable is single-mode from to d.c. up to this frequency, and might in practice be used up to 90% of this frequency.
- Peak Voltage. The peak voltage is set by the breakdown voltage of the insulator. One website gives:
- where
- Smils is the insulator's breakdown voltage in volts per mil
- din is the inner diameter in inches
- The 1150 factor converts inches (diameter) to mils (radius) and log10 to ln.
- The above expression may be rewritten as
- where
- S is the insulator's breakdown voltage in volts per meter
- d is the inner diameter in meters
- The calculated peak voltage is often reduced by a safety factor.
Choice of impedance
The best coaxial cable impedances in high-power, high-voltage, and low-attenuation applications were experimentally determined at Bell Laboratories in 1929 to be 30, 60, and 77 Ω, respectively. 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. When more common dielectrics are considered, the best-loss impedance drops down to a value between 52–64 Ω. Maximum power handling is achieved at 30 Ω.The approximate impedance required to match a centre-fed dipole antenna
Dipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
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 with CATV
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
, although many broadcast TV installations and CATV headends use 300 Ω folded dipole antenna
Dipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
s to receive off-the-air signals, 75 Ω coax makes a convenient 4:1 balun
Balun
A balun is a type of electrical transformer that can convert electrical signals that are balanced about ground to signals that are unbalanced , and the reverse. They are also often used to connect lines of differing impedance...
transformer for these as well as possessing low attenuation.
The arithmetic mean
Arithmetic mean
In mathematics and statistics, the arithmetic mean, often referred to as simply the mean or average when the context is clear, is a method to derive the central tendency of a sample space...
between 30 Ω and 77 Ω is 53.5 Ω; the geometric mean
Geometric mean
The geometric mean, in mathematics, is a type of mean or average, which indicates the central tendency or typical value of a set of numbers. It is similar to the arithmetic mean, except that the numbers are multiplied and then the nth root of the resulting product is taken.For instance, the...
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. 50 Ω also works out well because it corresponds very closely to the drive impedance of a half-wave dipole in real environments, and provides an acceptable match to the drive impedance of a quarter-wave monopole, as well.
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 connect IBM 3270
IBM 3270
The IBM 3270 is a class of block oriented terminals made by IBM since 1972 normally used to communicate with IBM mainframes. As such, it was the successor to the IBM 2260 display terminal. Due to the text colour on the original models, these terminals are informally known as green screen terminals...
terminals to IBM 3274/3174 terminal cluster controllers). Later, some manufacturers of LAN equipment, such as Datapoint for ARCNET
ARCNET
ARCNET is a local area network protocol, similar in purpose to Ethernet or Token Ring. ARCNET was the first widely available networking system for microcomputers and became popular in the 1980s for office automation tasks...
, 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. Capacitance is the enemy of square-wave data transmission (in particular, it slows down edge transitions), and this is a much more important factor for baseband digital data transmission than power handling or attenuation.
All of the components of a coaxial system should have the same impedance to reduce internal reflections at connections between components. Such reflections may cause signal attenuation; multiple reflections may cause the original signal to be followed by one or more echos. In analog video or TV systems, this causes ghosting
Ghosting (television)
In television, a ghost is a replica of the transmitted image, offset in position, that is super-imposed on top of the main image on an analogue broadcast.-Common causes:Common causes of ghosts are:...
in the image. Reflections also introduce standing waves, which cause increased losses and can even result in cable dielectric breakdown with high-power transmission (see Impedance matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
).
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 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 and radio frequency interferenceElectromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
to nearby devices.
For example, in the United States, signal leakage from cable television
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
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. Excessive noise
Electronic noise
Electronic noise is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects...
can overwhelm the signal, making it useless.
An ideal shield would be a perfect conductor with no holes, gaps, or bumps connected to a perfect ground. However, a smooth solid copper shield would be heavy, inflexible, and expensive. Practical cables must make compromises between shield efficacy, flexibility, and cost, such as the corrugated surface of hardline, flexible braid, or foil shields. Since the shields are not perfect conductors, electric fields can exist inside the shield, thus allowing radiating electromagnetic fields to go through the shield.
Consider the skin effect
Skin effect
Skin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
. 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 will 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.
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, 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. See ground loopGround loop (electricity)
In an electrical system, a ground loop usually refers to a current, almost always unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed...
.
Induction
External current sources like switched-mode power suppliesSwitched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...
create a voltage across the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
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 transformerTransformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
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
Mu-metal
Mu-metal is a nickel-iron alloy that is notable for its high magnetic permeability. The high permeability makes mu-metal very effective at screening static or low-frequency magnetic fields, which cannot be attenuated by other methods. The name came from the Greek letter mu which represents...
. 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.
Common mode current and radiation
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.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 helps ladder 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, and would actually cause energy to radiate from the coax itself, making it appear to be part of the antenna, affecting the radiation pattern
Radiation pattern
In the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....
of the antenna and possibly introducing dangerous radio-frequency energy into areas near people, with the risk of radiation burns if the coax is being used for sufficiently high-power transmissions. A properly-placed and -sized balun
Balun
A balun is a type of electrical transformer that can convert electrical signals that are balanced about ground to signals that are unbalanced , and the reverse. They are also often used to connect lines of differing impedance...
can prevent common mode radiation in coax.
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 wave guides or optical fiberOptical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
, 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-6RG-6
RG-6/U is a common type of coaxial cable used in a wide variety of residential and commercial applications. The term "RG-6" itself is quite generic and refers to a wide variety of cable designs, which differ from one another in shielding characteristics, center conductor composition, and...
is the most commonly-used coaxial cable for home use, and the majority of connections outside Europe are by F connector
F connector
The F connector is a type of coaxial RF connector commonly used for "over the air" terrestrial television, cable television and universally for satellite television and cable modems, usually with RG-6/U cable or, in older installations, with RG-59/U cable. It was invented by Eric E...
s.
A series of standard types of coaxial cable were specified for military uses, in the form "RG-#" or "RG-#/U". They date from World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
and were listed in MIL-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 is MIL-SPEC
Defense Standard
A United States defense standard, often called a military standard, "MIL-STD", "MIL-SPEC", or "MilSpecs", is used to help achieve standardization objectives by the U.S...
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 connector
Electrical connector
An electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or...
s that fit the inner conductor, dielectric, and jacket dimensions of the old RG-series cables.
| type | impedance Characteristic impedance The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm... ohms |
core | Dielectric Type | Dielectric VF | Dielectric in | Dielectric mm | OD in | OD mm | shields | comments | max attenuation @ 750 MHz |
|---|---|---|---|---|---|---|---|---|---|---|---|
| RG-6 RG-6 RG-6/U is a common type of coaxial cable used in a wide variety of residential and commercial applications. The term "RG-6" itself is quite generic and refers to a wide variety of cable designs, which differ from one another in shielding characteristics, center conductor composition, and... /U |
75 | 1.0 mm | PF | 0.75 | 0.185 | 4.7 | 0.270 | 6.86 | double | Low loss at high frequency for cable television Cable television Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional... , satellite television Satellite television Satellite television is television programming delivered by the means of communications satellite and received by an outdoor antenna, usually a parabolic mirror generally referred to as a satellite dish, and as far as household usage is concerned, a satellite receiver either in the form of an... and cable modem Cable modem A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a HFC and RFoG infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high... s |
5.65dB/100 ft |
| RG-6 RG-6 RG-6/U is a common type of coaxial cable used in a wide variety of residential and commercial applications. The term "RG-6" itself is quite generic and refers to a wide variety of cable designs, which differ from one another in shielding characteristics, center conductor composition, and... /UQ |
75 | PF | 0.298 | 7.57 | quad | This is "quad shield RG-6". It has four layers of shielding Electromagnetic shielding Electromagnetic shielding is the process of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive and/or magnetic materials. Shielding is typically applied to enclosures to isolate electrical devices from the 'outside world' and to cables to isolate... ; regular RG-6 has only one or two |
5.65dB/100 ft | ||||
| RG-7 | 75 | 1.30 mm | PF | 0.225 | 5.72 | 0.320 | 8.13 | double | Low loss at high frequency for cable television Cable television Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional... , satellite television Satellite television Satellite television is television programming delivered by the means of communications satellite and received by an outdoor antenna, usually a parabolic mirror generally referred to as a satellite dish, and as far as household usage is concerned, a satellite receiver either in the form of an... and cable modem Cable modem A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a HFC and RFoG infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high... s |
4.57dB/100 ft | |
| RG-8/U | 50 | 2.17 mm | PE | 0.285 | 7.2 | 0.405 | 10.3 | Amateur radio Amateur radio Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication... ; Thicknet (10BASE5 10BASE5 10BASE5 was the original commercially available variant of Ethernet.For its physical layer it used cable similar to RG-8/U coaxial cable but with extra braided shielding. This is a stiff, diameter cable with an impedance of 50 ohms , a solid center conductor, a foam insulating filler, a shielding... ) is similar |
5.967 dB/100 ft | ||
| RG-8X | 50 | 1.0 mm | PF | 0.75 | 0.185 | 4.7 | 0.242 | 6.1 | double | A thinner version, with the electrical characteristics of RG-8U in a diameter similar to RG-6. | 10.946 dB/100 ft |
| RG-9/U | 51 | PE | 0.420 | 10.7 | |||||||
| RG-11/U | 75 | 1.63 mm | PE | 0.66 | 0.285 | 7.2 | 0.412 | 10.5 | Triple/Quad | Used for long drops and underground conduit | 3.65dB/100 ft |
| RG-58/U RG-58 RG-58/U is a type of coaxial cable often used for low-power signal and RF connections. The cable has a characteristic impedance of either 50 or 52 Ω. "RG" was originally a unit indicator for bulk RF cable in the U.S. military's Joint Electronics Type Designation System... |
50 | 0.81 mm | PE | 0.66 | 0.116 | 2.9 | 0.195 | 5.0 | single | Used for radiocommunication and amateur radio Amateur radio Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication... , thin Ethernet (10BASE2 10BASE2 10BASE2 is a variant of Ethernet that uses thin coaxial cable , terminated with BNC connectors... ) and NIM NIM The Nuclear Instrumentation Module standard defines mechanical and electrical specifications for electronics modules used in experimental particle and nuclear physics... electronics. Common. |
13.104 dB/100 ft |
| RG-59/U RG-59 RG-59/U is a specific type of coaxial cable, often used for low-power video and RF signal connections. The cable has a characteristic impedance of 75 ohms. The 75 ohm impedance matches a dipole antenna in free space. "RG" was originally a unit indicator for bulk radio frequency cable in the... |
75 | 0.81 mm | PE | 0.66 | 0.146 | 3.7 | 0.242 | 6.1 | single | Used to carry baseband Baseband In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies... video in closed-circuit television Closed-circuit television Closed-circuit television is the use of video cameras to transmit a signal to a specific place, on a limited set of monitors.... , previously used for cable television. In general, it has poor shielding but will carry an HQ HD signal or video over short distances. |
9.708 dB/100 ft |
| 3C-2V | 75 | 0.50 mm | PE | 0.85 | 3.0 | 5.4 | single | Used to carry television, video observation systems, and other. PVC jacket. | |||
| 5C-2V | 75 | 0.80 mm | PE | 0.82 +/-2 | 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. | |
| RG-60/U | 50 | 1.024 mm | PE | 0.425 | 10.8 | single | Used for high-definition cable TV and high-speed cable Internet. | ||||
| RG-62/U | 92 | PF | 0.84 | 0.242 | 6.1 | single | Used for ARCNET ARCNET ARCNET is a local area network protocol, similar in purpose to Ethernet or Token Ring. ARCNET was the first widely available networking system for microcomputers and became popular in the 1980s for office automation tasks... and automotive radio antennas. |
||||
| RG-62A | 93 | ASP | 0.242 | 6.1 | single | Used for NIM NIM The Nuclear Instrumentation Module standard defines mechanical and electrical specifications for electronics modules used in experimental particle and nuclear physics... electronics |
|||||
| RG-174/U | 50 | 7x0.16 mm | PE | 0.66 | 0.059 | 1.5 | 0.100 | 2.55 | single | Common for wifi WIFI WIFI is a radio station broadcasting a brokered format. Licensed to Florence, New Jersey, USA, the station is currently operated by Florence Broadcasting Partners, LLC.This station was previously owned by Real Life Broadcasting... pigtails: more flexible but higher loss than RG58; used with LEMO LEMO LEMO is both the name of an electronic and fibre optic connector manufacturer based in Switzerland, and the name commonly used to refer to push-pull connectors made by that company, which are used in medical, industrial, audio/visual, telecommunications, military, scientific research and... 00 connectors in NIM NIM The Nuclear Instrumentation Module standard defines mechanical and electrical specifications for electronics modules used in experimental particle and nuclear physics... electronics. |
23.565 dB/100 ft |
| RG-178/U | 50 | 7×0.1 mm (Ag-plated Cu-clad Steel) |
PTFE | 0.69 | 0.033 | 0.84 | 0.071 | 1.8 | single | Used for high-frequency signal transmission. | 42.7 dB/100ft @ 900MHz |
| RG-179/U | 75 | 7×0.1 mm (Ag-plated Cu) |
PTFE | 0.67 | 0.063 | 1.6 | 0.098 | 2.5 | single | VGA RGBHV | |
| RG-180B/U | 95 | 0.0120 in (Ag-plated Cu-clad steel) |
PTFE | 0.102 | 2.59 | 0.145 | 3.68 | single Ag covered Cu | VGA RGBHV | ||
| RG-213/U | 50 | 7×0.0296 in Cu | PE | 0.66 | 0.285 | 7.2 | 0.405 | 10.3 | single | For radiocommunication and amateur radio Amateur radio Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication... , EMC test antenna cables. Typically lower loss than RG58. Common. |
5.967 dB/100 ft |
| RG-214/U | 50 | 7×0.0296 in | PE | 0.66 | 0.285 | 7.2 | 0.425 | 10.8 | double | Used for high-frequency signal transmission. | 6.702 dB/100 ft |
| RG-218 | 50 | 0.195 in Cu | PE | 0.66 | 0.660 (0.680?) | 16.76 (17.27?) | 0.870 | 22 | single | Large diameter, not very flexible, low loss (2.5dB/100' @ 400 MHz), 11kV dielectric withstand. | 2.834 dB/100 ft |
| RG-223/U | 50 | 0.88 mm | PE | 0.66 | 0.0815 | 2.07 | 0.212 | 5.4 | double | Silver-plated shields. Sample RG-223 Datasheet | 11.461 dB/100 ft |
| RG-316/U | 50 | 7x0.0067 in | PTFE | 0.695 | 0.060 | 1.5 | 0.098 | 2.6 | single | used with LEMO LEMO LEMO is both the name of an electronic and fibre optic connector manufacturer based in Switzerland, and the name commonly used to refer to push-pull connectors made by that company, which are used in medical, industrial, audio/visual, telecommunications, military, scientific research and... 00 connectors in NIM NIM The Nuclear Instrumentation Module standard defines mechanical and electrical specifications for electronics modules used in experimental particle and nuclear physics... electronics; |
22.452 dB/100 ft |
| RG-400/U | 50 | 19x0.20mm | PTFE | 2.95 | 4.95 | double | 12.566 dB/100 ft | ||||
| H155 | 50 | 19 x 0.28 mm | 0.79 | 5.4 | lower loss at high frequency for radiocommunication and amateur radio Amateur radio Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication... |
||||||
| H500 | 50 | 0.82 | low loss at high frequency for radiocommunication and amateur radio Amateur radio Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication... |
||||||||
| LMR-100 | 50 | 2.79 | low loss communications, 1.36 dB/meter @ 2.4 GHz | 20.725 dB/100 ft | |||||||
| LMR-195 | 50 | low loss drop-in replacement for RG-58 | 10.125 dB/100 ft | ||||||||
| LMR-200 HDF-200 CFD-200 |
50 | 1.12 mm Cu | PF | 0.83 | 0.116 | 2.95 | 0.195 | 4.95 | low loss communications, 0.554 dB/meter @ 2.4 GHz | 9.035 dB/100 ft | |
| LMR-240 | 50 | 1.42 mm Cu | PF | 0.84 | 0.150 | 3.81 | 0.240 | 6.1 | double | Amateur radio Amateur radio Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication... , low loss replacement for RG-8X |
6.877 dB/100 ft |
| LMR-400 HDF-400 CFD-400 |
50 | 2.74 mm (Cu-clad Al) |
PF | 0.85 | 0.285 | 7.24 | 0.405 | 10.29 | low loss communications, 0.223 dB/meter @ 2.4 GHz | 3.544 dB/100 ft | |
| LMR-600 | 50 | 4.47 mm (Cu-clad Al) |
PF | 0.87 | 0.455 | 11.56 | 0.590 | 14.99 | low loss communications, 0.144 dB/meter @ 2.4 GHz | 2.264 dB/100 ft | |
| LMR-900 | 50 | 6.65 mm (BC tube) |
PF | 0.87 | 0.680 | 17.27 | 0.870 | 22.10 | low loss communications, 0.098 dB/meter @ 2.4 GHz | 1.537 dB/100 ft | |
| LMR-1200 | 50 | 8.86 mm (BC tube) |
PF | 0.88 | 0.920 | 23.37 | 1.200 | 30.48 | low loss communications, 0.075 dB/meter @ 2.4 GHz | 1.143 dB/100 ft | |
| LMR-1700 | 50 | 13.39 mm (BC tube) |
PF | 0.89 | 1.350 | 34.29 | 1.670 | 42.42 | low loss communications, 0.056 dB/meter @ 2.4 GHz | 0.844 dB/100 ft | |
| QR-320 | 75 | 1.80 mm | PF | 0.395 | 10.03 | single | Low loss line, which replaced RG-11 in most applications | 3.34dB/100 ft | |||
| QR-540 | 75 | 3.15 mm | PF | 0.610 | 15.49 | single | Low loss hard line | 1.85dB/100 ft | |||
| QR-715 | 75 | 4.22 mm | PF | 0.785 | 19.94 | single | Low loss hard line | 1.49dB/100 ft | |||
| QR-860 | 75 | 5.16 mm | PF | 0.960 | 24.38 | single | Low loss hard line | 1.24dB/100 ft | |||
| QR-1125 | 75 | 6.68 mm | PF | 1.225 | 31.12 | single | Low loss hard line | 1.01dB/100 ft | |||
Dielectric Material Codes
- FPE is foamed polyethylene
- PE is solid polyethylenePolyethylenePolyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
- PF is polyethylene foam
- PTFE is polytetrafluoroethylenePolytetrafluoroethylenePolytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. PTFE is most well known by the DuPont brand name Teflon....
; - ASP is air space polyethylene
VF is the Velocity Factor; it is determined by the effective
and 
- VF for solid PE is about 0.66
- VF for foam PE is about 0.79 to 0.88
- VF for air is about 1.00
- VF for solid PTFE is about 0.70
- VF for foam PTFE is about 0.84
There are also other designation schemes for coaxial cables such as The URM, CT, BT, RA, PSF and WF series.
Uses
Short coaxial cables are commonly used to connect home videoVideo
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
equipment, in ham radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
setups, and in measurement electronics
NIM
The Nuclear Instrumentation Module standard defines mechanical and electrical specifications for electronics modules used in experimental particle and nuclear physics...
. They used to be common for implementing computer network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
s, in particular Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, but twisted pair
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
cables have replaced them in most applications except in the growing consumer cable modem
Cable modem
A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a HFC and RFoG infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high...
market for broadband Internet access
Broadband Internet access
Broadband Internet access, often shortened to just "broadband", is a high data rate, low-latency connection to the Internet— typically contrasted with dial-up access using a 56 kbit/s modem or satellite Internet with inherently high latency....
.
Long distance coaxial cable was used in the 20th century to connect radio network
Radio network
There are two types of radio networks currently in use around the world: the one-to-many broadcast type commonly used for public information and mass media entertainment; and the two-way type used more commonly for public safety and public services such as police, fire, taxicabs, and delivery...
s, television network
Television network
A television network is a telecommunications network for distribution of television program content, whereby a central operation provides programming to many television stations or pay TV providers. Until the mid-1980s, television programming in most countries of the world was dominated by a small...
s, and Long Distance telephone
L-carrier
SystemYearFrequencyCoax per cableDistance between repeatersVoice circuits per coax tubeL-119413 MHz48 miles600L-21942840 kHz416 miles360L-319508 MHz84 miles1,860L-4196717 MHz202 miles3,600L-5197257 MHz221 mile10,800L-5E197566 MHz221 mile13,200...
networks though this has largely been superseded by later methods (fibre optics, T1
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
/E1
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
, satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
). Shorter coaxials still carry cable television
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
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
Two-way radio
A two-way radio is a radio that can both transmit and receive , unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency...
frequency applications (including radio, and telecommunications), although 75 ohms is commonly used for broadcast
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
television and radio.
Hard line
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
as well as many other forms of radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
communication
Communication
Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...
. 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
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
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 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 as nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
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 internal arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
ing. Gas-filled hardlines are usually used on high-power RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
transmitters such as television or radio broadcasting, military transmitters, and high-power amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
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 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, in particular in equipment within the microwave range, to reduce interference between stages of the device.
Radiating
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
RG-6 is available in four 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. "PlenumPlenum cable
Plenum cable is cable that is laid in the plenum spaces of buildings. The plenum is the space that can facilitate air circulation for heating and air conditioning systems, by providing pathways for either heated/conditioned or return airflows...
" wire comes with a special Teflon outer jacket designed for use in ventilation ducts to meet fire codes.
Triaxial cable
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
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
Biaxial cable, biax or Twin-Lead 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 networkComputer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
s. Others may be familiar with 75Ω biax which at one time was popular on many cable TV services.
Semi-rigid
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 withouth the need for specialist tools, similar to standard coaxial cable.
Rigid line
Rigid line is a coaxial formed by two copper tubes supported every other meter using PTFE-supports. Rigid lines are not possible to bend, so they often need elbows. Interconnection with rigid line is done with an inner bullet/inner support and a flange or connection kit. Rigid line is mainly used indoors for interconnection between transmitter and other RF-components, but even more rigid rigid line with flanges is used outdoors in antenna masts etc. With a flange connector it is possible to go from rigid line to hard line. Many broadcasting antennas and antenna splitters uses 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:
| Outer conductor | Inner conductor | |||
|---|---|---|---|---|
| Size | Outer diameter (not flanged) | Inner diameter | Outer diameter | Inner diameter |
| 7/8" | 22.2 mm | 20 mm | 8.7 mm | 7.4 mm |
| 1 5/8" | 41.3 mm | 38.8 mm | 16.9 mm | 15.0 mm |
| 3 1/8" | 79.4 mm | 76.9 mm | 33.4 mm | 42.6 mm |
| 4 1/2" | 106 mm | 103 mm | 44.8 mm | 42.8 mm |
| 6 1/8" | 155.6 mm | 151.9 mm | 66.0 mm | 64.0 mm |
Interference and troubleshooting
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. For cable television
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
, 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
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
and some other countries, cable television
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
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.
History
- 1880 — Coaxial cable patented in EnglandEnglandEngland is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
by Oliver HeavisideOliver HeavisideOliver Heaviside was a self-taught English electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques to the solution of differential equations , reformulated Maxwell's field equations in terms of electric and...
, patent no. 1,407. - 1884 — Siemens & HalskeSiemens & HalskeSiemens & Halske AG was a German electrical engineering company that later became part of Siemens AG.It was founded on 12 October 1847 as Telegraphen-Bauanstalt von Siemens & Halske by Ernst Werner von Siemens and Johann Georg Halske...
patent coaxial cable in GermanyGermanyGermany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
(Patent No. 28,978, 27 March 1884). - 1894 — Oliver Lodge demonstrates waveguide transmission at the Royal InstitutionRoyal InstitutionThe Royal Institution of Great Britain is an organization devoted to scientific education and research, based in London.-Overview:...
. - 1929 — First modern coaxial cable patented by Lloyd EspenschiedLloyd EspenschiedLloyd Espenschied was an American electrical engineer who invented the modern coaxial cable with Herman Andrew Affel.-Biography:He was born in St. Louis, Missouri on April 27, 1889....
and Herman AffelHerman AffelHerman Andrew Affel was an American electrical engineer who invented the modern coaxial cable.-Biography:He was born on August 4, 1893. He attended MIT. He later married Bertha May Plummer....
of AT&T's Bell Telephone LaboratoriesBell LabsBell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
. - 1936 — First closed circuitClosed-circuit televisionClosed-circuit television is the use of video cameras to transmit a signal to a specific place, on a limited set of monitors....
transmission of TVTelevisionTelevision is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
pictures on coaxial cable, from the 1936 Summer Olympics1936 Summer OlympicsThe 1936 Summer Olympics, officially known as the Games of the XI Olympiad, was an international multi-sport event which was held in 1936 in Berlin, Germany. Berlin won the bid to host the Games over Barcelona, Spain on April 26, 1931, at the 29th IOC Session in Barcelona...
in Berlin to LeipzigLeipzigLeipzig Leipzig has always been a trade city, situated during the time of the Holy Roman Empire at the intersection of the Via Regia and Via Imperii, two important trade routes. At one time, Leipzig was one of the major European centres of learning and culture in fields such as music and publishing...
. - 1936 — World's first underwater coaxial cable installed between Apollo Bay, near MelbourneMelbourneMelbourne is the capital and most populous city in the state of Victoria, and the second most populous city in Australia. The Melbourne City Centre is the hub of the greater metropolitan area and the Census statistical division—of which "Melbourne" is the common name. As of June 2009, the greater...
, Australia, and Stanley, TasmaniaStanley, TasmaniaStanley is a town on the north-west coast of Tasmania, Australia. Travelling west, Stanley is the second-last major township on the north-west coast of Tasmania, Smithton being the larger township in the Circular Head municipality...
. The 300 km cable can carry one 8.5-kHz broadcast channel and seven telephone channels. - 1936 — AT&T installs experimental coaxial telephone and television cable between New YorkNew YorkNew York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
and Philadelphia, with automatic booster stations every ten miles. Completed in December, it can transmit 240 telephone calls simultaneously. - 1936 — Coaxial cable laid by the General Post OfficeGeneral Post OfficeGeneral Post Office is the name of the British postal system from 1660 until 1969.General Post Office may also refer to:* General Post Office, Perth* General Post Office, Sydney* General Post Office, Melbourne* General Post Office, Brisbane...
(now BT) between LondonLondonLondon is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
and BirminghamBirminghamBirmingham is a city and metropolitan borough in the West Midlands of England. It is the most populous British city outside the capital London, with a population of 1,036,900 , and lies at the heart of the West Midlands conurbation, the second most populous urban area in the United Kingdom with a...
, providing 40 telephone channels. - 1941 — First commercial use in USA by AT&T, between Minneapolis, Minnesota and Stevens Point, Wisconsin. L1 system with capacity of one TV channel or 480 telephone circuits.
- 1956 — First transatlantic coaxial cable laid, TAT-1TAT-1TAT-1 was the first submarine transatlantic telephone cable system. It was laid between Gallanach Bay, near Oban, Scotland and Clarenville, Newfoundland between 1955 and 1956. It was inaugurated on September 25, 1956, initially carrying 36 telephone channels.-History:The first transatlantic...
.
See also
- transmission lineTransmission lineIn communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
- Radio frequency power transmissionRadio frequency power transmissionRadio frequency power transmission is the transmission of the output power of a transmitter to an antenna. When the antenna is not situated close to the transmitter, special transmission lines are required....
- L-carrierL-carrierSystemYearFrequencyCoax per cableDistance between repeatersVoice circuits per coax tubeL-119413 MHz48 miles600L-21942840 kHz416 miles360L-319508 MHz84 miles1,860L-4196717 MHz202 miles3,600L-5197257 MHz221 mile10,800L-5E197566 MHz221 mile13,200...
- Balanced pair
- Data cableData cableA data cable is any media that allows baseband transmissions from a transmitter to a receiver.Examples Are:*Networking Media**Ethernet Cables **Token Ring Cables **Coaxial cable...
- Shielded cableShielded cableA shielded or screened cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer. The shield may be composed of braided strands of copper , a non-braided spiral winding of copper tape, or a layer of conducting polymer. Usually, this shield is covered...
- CableCableA cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
- Category 5 cableCategory 5 cableCategory 5 cable is a twisted pair cable for carrying signals. This type of cable is used in structured cabling for computer networks such as Ethernet. It is also used to carry other signals such as telephony and video. The cable is commonly connected using punch down blocks and modular connectors...