Coaxial cable

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Coaxial Cable
Coaxial Cable

Coaxial cable is an electrical cable consisting of a round conducting wire, surrounded by an insulating spacer, surrounded by a cylindrical conducting sheath, usually surrounded by a final insulating layer (jacket). It is used as a high-frequency transmission line to carry a high-frequency or broadband signal. Because the electromagnetic field carrying the signal exists (ideally) only in the space between the inner and outer conductors, it cannot interfere with or suffer interference from external electromagnetic fields.

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[edit] Description

Coaxial cables may be rigid or flexible. Rigid types have a solid sheath, while flexible types have a braided sheath, usually of thin copper wire. The inner insulator, also called the dielectric, has a significant effect on the cable's properties, such as its characteristic impedance and its attenuation. The dielectric may be solid or perforated with air spaces. Connections to the ends of coaxial cables are usually made with RF connectors.

Radio-grade flexible coaxial cable.A: outer plastic sheathB: copper screenC: inner dielectric insulatorD: copper core
Radio-grade flexible coaxial cable.
A: outer plastic sheath
B: copper screen
C: inner dielectric insulator
D: copper core

[edit] Signal propagation

Open wire transmission lines have the property that the electromagnetic wave propagating down the line extends into the space surrounding the parallel wires. These lines have low loss, but also have undesirable characteristics. They cannot be bent, twisted or otherwise shaped without changing their characteristic impedance. They also cannot be run along or attached to anything conductive, as the extended fields will induce currents in the nearby conductors causing unwanted radiation and detuning of the line. Coaxial lines solve this problem by confining the electromagnetic wave to the area inside the cable, between the center conductor and the shield. The transmission of energy in the line occurs totally through the dielectric inside the cable between the conductors. Coaxial lines can therefore be bent and moderately twisted without negative effects, and they can be strapped to conductive supports without inducing unwanted currents in them. In radio-frequency applications up to a few gigahertz, the wave propagates only in the transverse electric magnetic (TEM) mode, which means that the electric and magnetic fields are both perpendicular to the direction of propagation. However, above a certain cutoff frequency, transverse electric (TE) and/or transverse magnetic (TM) modes can also propagate, as they do in a waveguide. It is usually undesirable to transmit signals above the cutoff frequency, since it may cause multiple modes with different phase velocities to propagate, interfering with each other. The outer diameter is roughly inversely proportional to the cutoff frequency.

The outer conductor can also be made of (in order of increasing leakage and in this case degree of balance): double shield, wound foil, woven tape, braid, and finally the balanced open wire transmission line. A balun can improve transition between cables of different degree of balance. Shielded twisted pair may be better if mains hum is a problem, though pair conductors are not commercially available in the same precision. The ohmic losses in the conductor increase in this order: Ideal conductor (no loss), superconductor, silver, copper. It is further increased by rough surface (in the order of the skin depth, lateral: current hot spots, longitudinal: long current path) for example due to woven braid, multistranded conductors or a corrugated tube as a conductor) and impurities especially oxygen in the metal (due to a lack of a protective coating). Litz wire is used between 1 kHz and 1 MHz to reduce ohmic losses. Coaxial cables require an internal structure of an insulating (dielectric) material to maintain the spacing between the center conductor and shield. The dielectric losses increase in this order: Ideal dielectric (no loss), vacuum, air, PTFE-foam, PTFE, polyethylene. It is further increased by impurities like water. In typical applications the loss in polyethylene is comparable to the ohmic loss at 1 GHz and the loss in PTFE is comparable to ohmic losses at 10 GHz. A low dielectric constant allows for a greater center conductor: Less ohmic losses. An inhomogeneous dielectric needs to be compensated by a noncircular conductor to avoid current hot-spots.

[edit] Connectors

Main article: RF connector

Coaxial connectors
Coaxial connectors

From the signal point of view, a connector can be viewed as a short, rigid cable. The connector usually has the same impedance as the related cable and probably has a similar cutoff frequency although its dielectric may be different. High-quality connectors are usually gold or rhodium plated, with lower-quality connectors using nickel or tin plating. Silver is occasionally used in some high-end connectors due to its excellent conductivity, but it usually requires extra plating of another metal since silver readily oxidizes in the presence of air.

One increasing development has been the wider adoption of micro-miniature coaxial cable in the consumer electronics sector in recent years. Wire and cable companies such as Tyco, Sumitomo Electric, Hitachi Cable, Fujikura and LS Cable all manufacture these cables, which can be used in mobile phones.

[edit] Important parameters

  • The characteristic impedance in ohms (Ω) is calculated from the ratio of the inner and outer diameters and the dielectric constant. 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.
  • Capacitance, in farads per metre.
  • Resistance, in ohms per metre.
  • Attenuation or loss, in decibels per metre. This is dependent on the loss in the dielectric material filling the cable, and resistive losses in the center conductor and shield. These losses are frequency dependent, the losses becoming higher as the frequency increases. In designing a system, engineers must consider not only the loss in the actual cable itself, but also the insertion loss in the connectors.
  • Outside diameter, which dictates which connectors must be used to terminate the cable.
  • Velocity of propagation, which depends on the type of dielectric.
  • Cutoff frequency

[edit] Standards

Most coaxial cables have a characteristic impedance of either 50, 52, 75, or 93 Ω. The RF industry uses standard type-names for coaxial cables. Thanks to television, RG-6 is the most commonly-used coaxial cable, and the majority of connections outside Europe are by F connectors.

A series of standard types of coaxial cable were specified for military uses, in the form "RG-#" or "RG-#/U" (RG from Radio Grade, /U indicates multiple uses). They go back to World War II and were listed in MIL-HDBK-216 published in 1962. These designations are now obsolete. The current military standard is MIL-SPEC MIL-C-17. MIL-C-17 numbers, such as "M17/75-RG214," are given for military cables and manufacturer's catalog numbers for civilian applications. However, the RG-series designations were so common for generations that they are still used, although critical users should be aware that since the handbook is withdrawn there is no standard to guarantee the electrical and physical characteristics of a cable described as "RG-# type". The RG designators are mostly used to identify compatible connectors that fit the inner conductor, dielectric, and jacket dimensions of the old RG-series cables.

Table of RG standards:

type approx. imped. [Ω] core dielectric overall diameter braid velocity factor comments
type [in] [mm] [in] [mm]
RG-6/U 75 1.0 mm PE 0.185 4.7 0.332 8.4 double low loss at high frequency for cable television, satellite television and cable modems
RG-6/UQ 75 PE 0.298 7.62 quad This is "quad shield RG-6". It has four layers of shielding, regular RG-6 only has one or two
RG-8/U 50 2.17 mm PE 0.285 7.2 0.405 10.3 used for thick Ethernet (10base5) and amateur radio
RG-9/U 51 PE 0.420 10.7
RG-11/U 75 1.63 mm PE 0.285 7.2 0.412 10.5 0.66 Used for long drops and underground
RG-58/U 50 0.9 mm PE 0.116 2.9 0.195 5.0 single 0.66 used for radiocommunication and amateur radio, thin Ethernet (10base2) and NIM electronics. Common.
RG-59/U 75 0.81 mm PE 0.146 3.7 0.242 6.1 single 0.66 used to carry baseband video in closed-circuit television, previously used for cable television
RG-62/U 92 PE 0.242 6.1 single 0.84 used for ARCNET
RG-62A 93 ASP 0.242 6.1 single used for NIM electronics
RG-174/U 50 0.48 mm PE 0.100 2.5 0.100 2.55 single [1] Common for wifi pigtails, more flexible but higher loss than RG58.
RG-178/U 50 0.079 2.0 single
RG-179/U 75 0.094 2.4 single
RG-213/U 50 7×0.0296 in Cu PE 0.285 7.2 0.405 10.3 single 0.66 for radiocommunication and amateur radio, EMC test antenna cables. Typically lower loss than RG58. Common.
RG-214 50 0.406 10.8
RG-223 50 0.203 5.4
RG-316/U 50 7×0.0067 in PTFE 0.060 1.5 0.102 2.6 single

Commercial designations:

type approx. imped. [Ω] core dielectric overall diameter braid velocity factor comments
type [in] [mm] [in] [mm]
H155 50 0.79 lower loss at high frequency for radiocommunication and amateur radio
H500 50 0.82 low loss at high frequency for radiocommunication and amateur radio
LMR-200 HDF-200 CFD-200 50 1.12 mm Cu PF CF 0.116 2.95 0.195 4.95 0.83 low loss communications, 0.554 dB/meter @ 2.4 GHz
LMR-400 HDF-400 CFD-400 50 2.74 mm Cu clad Al PF CF 0.285 7.24 0.405 10.29 0.85 low loss communications, 0.223 dB/meter @ 2.4 GHz
LMR-600 50 4.47 mm Cu clad Al PF 0.455 11.56 0.590 14.99 0.87 low loss communications, 0.144 dB/meter @ 2.4 GHz
LMR-900 50 6.65 mm BC tube PF 0.680 17.27 0.870 22.10 0.87 low loss communications, 0.098 dB/meter @ 2.4 GHz
LMR-1200 50 8.86 mm BC tube PF 0.920 23.37 1.200 30.48 0.88 low loss communications, 0.075 dB/meter @ 2.4 GHz
LMR-1700 50 13.39 mm BC tube PF 1.350 34.29 1.670 42.42 0.89 low loss communications, 0.056 dB/meter @ 2.4 GHz

There are also other designation schemes for coaxial cables such as URM and CT

[edit] References for this section

[edit] Significance of impedance

A question that is often asked is what the significance of a 52 or 75 Ω characteristic impedance is. The best coaxial cable impedances to use in high-power, high-voltage, and low-attenuation applications were experimentally determined in 1929 at Bell Laboratories to be 30, 60, and 77 Ω respectively. 30 Ω cable is exceedingly hard to make however, so a compromise between 30 Ω and 60 Ω was reached at 52 Ω, which has persisted. 73 Ω is an exact match for a centre fed dipole aerial/antenna so 75 was adopted as a compromise between 73 and 77 ohms

[edit] Uses

Short coaxial cables are commonly used to connect home video equipment, in ham radio setups, and in measurement electronics. They used to be common for implementing computer networks, in particular Ethernet, but twisted pair cables have replaced them in most applications except in the growing consumer cable modem market for broadband Internet access.

Long distance coaxial cable is used to connect radio networks and television networks, though this has largely been superseded by other more high-tech methods (fibre optics, T1/E1, satellite). It still carries cable television signals to the majority of television receivers, and this purpose consumes the majority of coaxial cable production.

Micro coaxial cables are used in a range of consumer devices, military equipment, and also in ultra-sound scanning equipment.

The most common impedances that are widely used are 50 or 52 ohms, and 75 ohms, although other impedances are available for specific applications. The 50 / 52 ohm cables are widely used for industrial and commercial radio frequency applications (including radio, and telecommunications), although 75 ohms is commonly used for domestic television and radio.

[edit] Types

In broadcasting and other forms of radio communication, hard line (also known as hard pipe) is a very heavy-duty coaxial cable, where the outside shielding is a rigid or semi-rigid pipe, rather than flexible and braided wire. Hard line is 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. It is almost always used in the connection between a transmitter on the ground and the antenna or aerial on the tower. Hard lines are often made to be pressurised with nitrogen or desiccated air, which provide an excellent dielectric even at the high temperatures generated by thousands of watts of RF power, especially during intense summer heat and sunshine. Physical separation between the inner conductor and outer shielding is maintained by spacers, usually made out of tough solid plastics like nylon.

RG/6 is available in three different types designed for various applications. "Plain" or "house" wire is designed for indoor or external house wiring. "Flooded" cable is infused with heavy waterproofing for use in underground conduit. "Messenger" contains some waterproofing but is distinguished by the addition of a steel messenger wire along its length to carry the tension involved in an aerial drop from a utility pole.

Triaxial cable or triax is coaxial cable with a third layer of shielding, insulation and sheathing. The outer shield, which is earthed (grounded), protects the inner shield from electromagnetic interference from outside sources.

Twin-axial cable or twinax is a balanced, twisted pair within a cylindrical shield. It allows a nearly perfect differential signal which is both shielded and balanced to pass through. Multi-conductor coaxial cable is also sometimes used.

Biaxial cable or biax is a figure-8 configuration of two 50 Ω coaxial cables, used in some proprietary computer networks.

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.

[edit] Interference and troubleshooting

Coaxial cable insulation can degrade requiring cable replacement, especially if it has been exposed to the elements on a continuous basis. The shield is normally grounded, and if even a single thread of the braid or filament of foil touches the center conductor, the signal will be shorted causing significant or total signal loss. This most often occurs at improperly installed end connectors and splices. Also, the connector or splice must be properly attached to the shield, as this provides the return electrical path for the signal.

Despite being shielded, interference can occur on coaxial cable lines. Susceptibility to interference has little relationship to broad cable type designations (e.g. RG-59, RG-6) but is strongly related to the composition and configuration of the cable's shielding. For cable television, with frequencies extending well into the UHF range, a foil shield is normally provided, and will provide total coverage as well as high effectiveness against high-frequency interference. Foil shielding is ordinarily accompanied by a tinned copper or aluminum braid shield, with anywhere from 60 to 95% coverage. The braid is important to shield effectiveness because (1) it is more effective than foil at absorbing low-frequency interference, (2) it provides higher conductivity to ground than foil, and (3) it makes connectorization easier and more reliable. "Quad-shield" cable, using two low-coverage aluminum braid shields and two layers of foil, is often used in situations involving troublesome interference, but is less effective than a single layer of foil and single high-coverage copper braid shield such as is found on broadcast-quality precision video cable.

In the United States and some other countries, cable channels 2-13 share the same frequency as those from television broadcast towers. If the cable consumer is too close to a television tower and the cable company provides the same station on the like channel, interference and 'ghosting' may result. One solution is to make sure the cable signal is at the maximum allowed strength (especially if splitters are used for multiple TV sets), as this will increase the signal-to-noise ratio (the "noise" being the pickup of the broadcast tower). Choosing coaxial cable with high shield effectiveness, and ensuring that connections are sound and tight, can also help reduce interference. Only industrial-quality cable TV amplifiers (generally not available at retail) should be used to amplify weak signals. Cheaper ones, sold at consumer electronics stores, often cause more problems than they solve.

[edit] Timeline

[edit] See also

[edit] External links

[edit] References

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