Coaxial cable

For the weapon, see coaxial weapon.

Coaxial Cable

Coaxial cable is an electrical cable consisting of an inner conductor or several uninsulated conductors tightly twisted together, often surrounded by an insulating spacer, surrounded by an outer cylindrical conducting shield (sheath), and usually surrounded by a final insulating layer (jacket). The term coaxial comes from the inner conductor and the outer shield sharing ("co-") the same axis. It is often used as a high-frequency transmission line to carry a high-frequency or broadband signal but may also be used for frequencies as low as audio frequency. The electromagnetic field carrying the signal exists (ideally) only in the space between the inner and outer conductors. The shielding reduces interference from external electromagnetic fields, although coax cable does radiate energy, shielding does somewhat reduce the possibility of a transmitting device causing undesired interference through transmission line leakage.

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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. Some connectors are gold or rhodium plated, while some connectors use nickel or tin plating. Silver is also used due to its excellent conductivity; although silver tends to oxidize rather quickly silver oxide is still conductive, this may pose a cosmetic issue but it does not degrade the performance of the connector.

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 $Z_{0}$.

• Characteristic impedance in ohm (Ω). Neglecting resistance per unit length and conductance per unit length for most coaxial cables, the characteristic impedance is determined from the capacitance per unit length ($C$) and the inductance per unit length ($L$). The simplified expression is ($Z_{0}={\sqrt {L/C}}$). Those parameters are determined from the ratio of the inner (d) and outer (D) diameters and the dielectric constant ($\epsilon _{r}$). The characteristic impedance is given by

$Z_{0}={\frac {1}{2\pi }}{\sqrt {\frac {\mu }{\epsilon }}}\ln {\frac {D}{d}}\approx {\frac {138}{\sqrt {\epsilon _{r}}}}\log _{10}{\frac {D}{d}}$

Assuming the dielectric properties of the material inside the cable do not vary appreciably over the operating range of the cable, this impedance is frequency independent above about 5 times the shield cutoff frequency. For typical coaxial cables, the shield cutoff frequency is 600 (RG-6A) to 2,000 Hz (RG-58C).

• Shunt Capacitance per unit length, in farads per metre.

$C={2\pi \epsilon \over \ln(D/d)}={2\pi \epsilon _{0}\epsilon _{r} \over \ln(D/d)}$

• Series Inductance per unit length, in Henrys per metre.

$L={\mu \over 2\pi }\ln(D/d)={\mu _{0}\mu _{r} \over 2\pi }\ln(D/d)$

• Series Resistance per unit length, in ohms per metre. The resistance per unit length is just the resistance of inner conductor and the shield at low frequencies. At higher frequencies, skin effect increases the effective resistance by confining the conduction to a thin layer of each conductor.
• Shunt Conductance per unit length, in mhos per metre. The shunt conductance is usually very small because insulators with good dielectric properties are used (a very low loss tangent). At high frequencies, a dielectric can have a significant resistive loss.
• 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 outer 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.
• Velocity of propagation, which depends on the dielectric constant and permeability (which is usually 1).

${1 \over {\sqrt {\epsilon \mu }}}={c \over {\sqrt {\epsilon _{r}\mu _{r}}}}$

• Cutoff frequency is determined by the possibility of exciting other propagation modes in the coaxial cable. The average circumference of the insulator is $\pi (D+d)/2$. Make that length equal to 1 wavelength in the dielectric. The TE01 cutoff frequency is therefore

$f_{c}={1 \over \pi ({D+d \over 2}){\sqrt {\mu \epsilon }}}={c \over \pi ({D+d \over 2}){\sqrt {\mu _{r}\epsilon _{r}}}}$.

• Peak Voltage
• Outside diameter, which dictates which connectors must be used to terminate the cable.

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.

Another way of looking at it is to consider skin effect. The RF current in an imperfect conductor decays exponentially with distance beneath the surface, with the depth of penetration being proportional to the square root of the resistivity. This means that in a shield of finite thickness, some small amount of current will still be flowing on the opposite surface of the conductor. With a perfect conductor (i.e., zero resistivity), all of the current would flow at the surface, with no penetration into and through the conductor. Real cables have a shield made of an imperfect, although usually very good, conductor, so there will always be some leakage.

Another mechanism for leakage is that cable shields usually contains some gaps or holes, allowing 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, or within the circuitry between the cable and either the source or the load.

Leakage is bi-directional, in that it can pass RF not only out of the cable, but into it as well. Thus, it is possible to measure small voltages on the inside of the shield caused by normal electromagnetic fields outside the shield. By these means, a typical leakage of 90 dB has been measured.

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:

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			Thicknet (10base5) and amateur radio
RG-9/U	51		PE			0.420	10.7			Thicknet Thicknet (10base5)
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. Generally it has poor shielding but will carry a HQ HD signal or video over short distances. Not legal for use with any CATV or MATV system.
RG-62/U	92		PE			0.242	6.1	single	0.84	used for ARCNET and automotive radio antennas.
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		common for wifi pigtails, more flexible but higher loss than RG58; used with LEMO 00 connectors in NIM electronics.
RG-178/U	50	7x0.1 mm Ag pltd Cu clad Steel	PTFE	0.033	0.84	0.071	1.8	single	0.69
RG-179/U	75	7x0.1 mm Ag pltd Cu	PTFE	0.063	1.6	0.098	2.5	single	0.67	VGA RGBHV
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/U	50	7x0.0296 in	PTFE	0.285	7.2	0.425	10.8	double	0.66
RG-218	50	0.195 in Cu	PE	0.660 (0.680?)	16.76 (17.27?)	0.870	22	single	0.66	large diameter, not very flexible, low loss (2.5dB/100' @ 400MHz), 11kV dielectric withstand.
RG-223	50	2.74mm	FE	.285	7.24	.405	10.29	Double
RG-316/U	50	7×0.0067 in	PTFE	0.060	1.5	0.102	2.6	single		used with LEMO 00 connectors in NIM electronics

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-195	50									low loss drop-in replacement for RG-58
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 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
• Specs for MIL-C-17 Coaxial Cable Q.P.L.
• 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
• Pasternack 2007 Catalog

Significance of impedance

A question that is often asked is what the significance of a 52 or 75 Ω characteristic impedance is. The best coaxial cable impedances to use in high-power, high-voltage, and low-attenuation applications were experimentally determined in 1929 at Bell Laboratories to be 30, 60, and 77 Ω respectively. 30 Ω cable is exceedingly hard to make however, so a compromise between 30 Ω and 60 Ω was reached at 52 Ω, which has persisted; note this also corresponds very closely to the drive impedance of a half wave dipole antenna in real environments, and provides an acceptable match to the drive impedance of quarter wave monopoles as well. 73 Ω is an exact match for a centre fed dipole aerial/antenna in free space (approximated by very high dipoles without ground reflections), so 75 was adopted as a compromise between 73 and 77 ohms.

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

1-5/8" hard line

Hard line, is often confused with Waveguide but the two are not the same. Hard line is used in broadcasting as well as many other forms of radio communication, hard line is a coaxial cable constructed using round copper, silver or gold tubing or a combination of such metals as a shield. Some lower quality hard line may use aluminum shielding, aluminum however is easily oxidized and unlike silver or gold oxide, aluminum oxide drastically loses effective conductivity. Therefore all connections must be air and water tight. The center conductor may consist of solid copper, or copper plated aluminum. Since skin effect is an issue with RF, copper plating provides sufficient surface for an effective conductor. Most varieties of hardline used for external chassis or when exposed to the elements have a PVC jacket; however, some internal applications may omit the insulation jacket. Hard line can be very thick, typically at least a half inch or 13 mm and up to several times that, and has low loss even at high power. These large scale hard lines are almost always used in the connection between a transmitter on the ground and the antenna or aerial on a tower. Hard line may also be known by trademarked names such as Heliax^tm (Andrew), or Cablewave^tm (RFS/Cablewave). Larger varieties of hardline may consist of a center conductor which is constructed from either rigid or corrugated copper tubing. The dielectric in hard line may consist of polyethylene foam, air or a pressurized gas such as Nitrogen or desiccated air (dried air). In gas-charged lines, hard plastics such as nylon are used as spacers to separate the inner and outer conductors. The addition of these gases into the dielectric space reduces moisture contamination, provides a stable dielectric constant, as well as a reduced risk of internal arcing. Gas-filled hardlines are usually used on high powered RF transmitters such as television or radio broadcasting, military transmitters, as well as high powered Amateur radio applications but may also be used on some critical lower powered applications such as those in the microwave bands. Although in the microwave region waveguide is more often used than hard line for transmitter to antenna, or antenna to receiver applications. The various shields used in hardline also differ; some forms use rigid tubing, or pipe, others may use a corrugated tubing which makes bending easier, as well as reduces kinking when the cable is bent to conform. Smaller varieties of hard line may be used internally in some high frequency applications, particularly in equipment within the microwave range, to reduce interference between stages of the device.

Radiax^tm(Andrew) is another form of coaxial cable which is constructed in a similar fashion to hard line, however Radiax^tm 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.

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, externally resembling that of lamp cord, or speaker wire. Biax is used in some proprietary computer networks. Others may be familiar with 75Ω biax which at one time was popular on many cable TV services.

Semi-rigid cable is a coaxial form using a solid copper outer sheath. This type of coax offers superior screening compared to cables with a braided outer conductor, especially at higher frequencies. The major disadvantage is that the cable, as its name implies, is not very flexible, and is not intended to be flexed after initial forming. (See "hard line")

Interference and troubleshooting

Coaxial cable insulation can degrade requiring cable replacement, especially if it has been exposed to the elements on a continuous basis. The shield is normally grounded, and if even a single thread of the braid or filament of foil touches the center conductor, the signal will be shorted causing significant or total signal loss. This most often occurs at improperly installed end connectors and splices. Also, the connector or splice must be properly attached to the shield, as this provides the 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 attaching a connector easier and more reliable. "Quad-shield" cable, using two low-coverage aluminum braid shields and two layers of foil, is often used in situations involving troublesome interference, but is less effective than a single layer of foil and single high-coverage copper braid shield such as is found on broadcast-quality precision video cable.

In the United States and some other countries, cable 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.
• 1894 — Oliver Lodge demonstrates waveguide transmission at the Royal Institution. Nikola Tesla receives U.S. patent 0,514,167, 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 1,835,031.
• 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.
• 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
• Shielded cable

References

• Is There an Ideal Impedance? — Wired for Sound: Steve Lampen