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.

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 sheath
B: copper screen
C: inner dielectric insulator
D: copper core

Signal propagation

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

The outer conductor can also be made of (in order of decreasing leakage and in this case degree of balance): double shield, wound foil, woven tape, braid. The ohmic losses in the conductor increase in this order: Ideal conductor (no loss), superconductor, silver, copper. It is further increased by rough surface (in the order of the skin depth, lateral: current hot spots, longitudinal: long current path) for example due to woven braid, multistranded conductors or a corrugated tube as a conductor) and impurities especially oxygen in the metal (due to a lack of a protective coating). Litz wire is used between 1 kHz and 1 MHz to reduce ohmic losses. Coaxial cables require an internal structure of an insulating (dielectric) material to maintain the spacing between the center conductor and shield. The dielectric losses increase in this order: Ideal dielectric (no loss), vacuum, air, 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.

Connectors

Main article: RF connector

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.

Important parameters

Schematic representation of a coaxial transmission line, showing the characteristic impedance Z₀.

• The characteristic impedance in ohms (Ω) is calculated from the ratio of the inner (d) and outer (D) diameters and the dielectric constant (ε_r). The characteristic impedance is given by .^[1] 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

Leakage

Leakage is the passage of electromagnetic fields through the shield of the cable. An ideal shield is a solid metal tube of perfect conductivity, perfectly sealed to the connectors at either end. Since no electric field can exist inside a perfect conductor, and a radiating electromagnetic field cannot exist without its electric component, it follows that no electromagnetic radiation can pass through a perfect conductor.

Real cables have a shield made of an imperfect, although usually very good, conductor that inevitably contains some holes. It is possible to measure small voltages on the inside of the shield caused by normal electromagnetic fields outside the shield, and very high voltages in the extreme case when a nuclear weapon is detonated outside the shield. By these means, a typical leakage of 90 dB has been measured. This leakage occurs at holes in the shield, or in case of poor contact between connectors at either end of the cable, or within the circuitry between the cable and the radio transceiver. The holes are smaller when using a foil (solid metal) shield, but foil becomes inflexible with increasing thickness. Thus a thin foil layer is often surrounded by a layer of braided metal, which offers greater flexibility for a given cross-section.

Although leakage theoretically changes the balance and impedance of a cable, in practice the effect is negligible.

Medium and low-frequency signals can pass through the shield by various means.

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

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

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

External low-frequency current sources such as ground loops cause voltages across the resistance of the outer conductor. This problem can be lessened by adding parallel cables to increase the total conductance. To further reduce the problem, the sender and receiver are matched to the cable (see Impedance matching) to minimise currents and their effects in the shield.

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". 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:

Commercial designations:

There are also other designation schemes for coaxial cables such as The URM, CT and WF series

References for this section

• RF transmission lines and fittings. Military Standardization Handbook MIL-HDBK-216, U.S. Department of Defense, 4 January 1962. [1]
• Withdrawal Notice for MIL-HDBK-216 2001
• Cables, radio frequency, flexible and rigid. Details Specification MIL-DTL-17H, 19 August 2005 (superseding MIL-C-17G, 9 March 1990). [2]
• Radio-frequency cables, International Standard IEC 60096.
• Coaxial communication cables, International Standard IEC 61196.
• Coaxial cables, British Standard BS EN 50117
• H. P. Westman et al, (ed), Reference Data for Radio Engineers, Fifth Edition, 1968, Howard W. Sams and Co., no ISBN, Library of Congress Card No. 43-14665
• http://www.rfcafe.com/references/electrical/coax_chart.htm
• Talley Communications CAxT Cable Assembly
• Times Microwave Systems LMR Wireless Products Catalog
• CFD Cable Specifications
• Specs of RG174/U, RG58C/U etc.
• RG213/8, RG218, CLX1/4", CLX1/2", CLX7/8", CLX1+5/8" Cable Power & Impedance Specs
• Velocity factor of various coaxial cables

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.^{[citation needed]} 30 Ω cable is exceedingly hard to make however, so a compromise between 30 Ω and 60 Ω was reached at 52 Ω, which has persisted; note this also corresponds very closely to the drive impedance of a half wave dipole antenna in real environments, and provides an acceptable match to the drive impedance of quarter wave monopoles as well. 73 Ω is an exact match for a centre fed dipole aerial/antenna in free space (approximated by very high dipoles without ground reflections), so 75 was adopted as a compromise between 73 and 77 ohms.

Uses

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

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

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

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

Types

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.

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.

Timeline

• 1880 — Coaxial cable patented in England by Oliver Heaviside, patent no. 1,407.
• 1884 — Coaxial cable patented in Germany by Ernst Werner von Siemens, but with no known application. ^{[citation needed]}
• 1894 — Oliver Lodge demonstrates waveguide transmission at the Royal Institution. Nikola Tesla receives U.S. Patent  , Electrical Conductor, on February 6.
• 1929 — First modern coaxial cable patented by Lloyd Espenschied and Herman Affel of AT&T's Bell Telephone Laboratories, U.S. Patent  .
• 1936 — First transmission of TV pictures on coaxial cable, from the 1936 Summer Olympics in Berlin to Leipzig.
• 1936 — World's first underwater coaxial cable installed between Apollo Bay, near Melbourne, Australia, and Stanley, Tasmania. The 300-km cable can carry one broadcast channel and seven telephone channels.
• 1936 — AT&T installs experimental coaxial telephone and television cable between New York and Philadelphia, with automatic booster stations every ten miles. Completed in December, it can transmit 240 telephone calls simultaneously.^[2]
• 1936 — Coaxial cable laid by the Post Office (now BT) between London and Birmingham, providing 40 telephone channels. [Source: archives at http://www.bt.com]
• 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-1.

See also

• Radio frequency power transmission
• L-carrier
• Balanced pair

External links

• Coaxial Cable Specifications
• What does "RG-6" mean?
• Coaxial Cable FAQ Wire Guide
• Coaxial Cable Selection Guide

References

• Discussion of using coaxial cable above cutoff, on "Towertalk" mailing list
• Coax: RG is for radio guide, "Towertalk" mailing list
• Is There an Ideal Impedance? — Wired for Sound: Steve Lampen

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