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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 Conductor s, 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 Braid ed 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 Connector s. 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). 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 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 Phone s. IMPORTANT PARAMETERS
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 Loop s 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 Connector s. 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 Connector s 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
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 Network s, 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 Network s and Television Network s, 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 Temperature s generated by thousands of Watt s 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 Plastic s 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 Network s. 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 Splitter s 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
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