A coaxial cable is one that consists of two conductors that share a common axis. The inner conductor is typically a straight wire, either solid or stranded and the outer conductor is typically a shield that might be braided or a foil.
Coaxial cable is a cable type used to carry radio signals, video signals, measurement signals and data signals. Coaxial cables exist because we can't run open-wire line near metallic objects (such as ducting) or bury it. We trade signal loss for convenience and flexibility. Coaxial cable consists of an insulated center conductor which is covered with a shield. The signal is carried between the cable shield and the center conductor. This arrangement give quite good shielding against noise from outside cable, keeps the signal well inside the cable and keeps cable characteristics stable.
Coaxial cables and systems connected to them are not ideal. There is always some signal radiating from coaxial cable. Hence, the outer conductor also functions as a shield to reduce coupling of the signal into adjacent wiring. More shield coverage means less radiation of energy (but it does not necessarily mean less signal attenuation).
Coaxial cable is typically characterized with the impedance and cable loss. The length has nothing to do with coaxial cable impedance. Characteristic impedance is determined by the size and spacing of the conductors and the type of dielectric used between them. For ordinary coaxial cable used at reasonable frequency, the characteristic impedance depends on the dimensions of the inner and outer conductors. The characteristic impedance of a cable is determined by the formula 138 log b/a, where b represents the inside diameter of the outer conductor (read: shield or braid), and a represents the outside diameter of the inner conductor.
Most common coaxial cable impedances in use in various applications are 50 ohms and 75 ohms. 50 ohms cable is used in radio transmitter antenna connections, many measurement devices and in data communications (Ethernet). 75 ohms coaxial cable is used to carry video signals, TV antenna signals and digital audio signals. There are also other impedances in use in some special applications (for example 93 ohms). It is possible to build cables at other impedances, but those mentioned earlier are the standard ones that are easy to get. It is usually no point in trying to get something very little different for some marginal benefit, because standard cables are easy to get, cheap and generally very good. Different impedances have different characteristics. For maximum power handling, somewhere between 30 and 44 Ohms is the optimum. Impedance somewhere around 77 Ohms gives the lowest loss in a dielectric filled line. 93 Ohms cable gives low capacitance per foot. It is practically very hard to find any coaxial cables with impedance much higher than that.
Here is a quick overview of common coaxial cable impedances and their main uses:
50 ohms: 50 ohms coaxial cable is very widely used with radio transmitter applications. It is used here because it matches nicely to many common transmitter antenna types, can quite easily handle high transmitter power and is traditionally used in this type of applications (transmitters are generally matched to 50 ohms impedance). In addition to this 50 ohm coaxial cable can be found on coaxial Ethernet networks, electronics laboratory interconnection (for example high frequency oscilloscope probe cables) and high frequency digital applications (for example ECL and PECL logic matches nicely to 50 ohms cable). Commonly used 50 Ohm constructions include RG-8 and RG-58.
60 Ohms: Europe chose 60 ohms for radio applications around 1950s. It was used in both transmitting applications and antenna networks. The use of this cable has been pretty much phased out, and nowadays RF systems in Europe use either 50 ohms or 75 ohms cable depending on the application.
75 ohms: The characteristic impedance 75 ohms is an international standard, based on optimizing the design of long distance coaxial cables. 75 ohms video cable is the coaxial cable type widely used in video, audio and telecommunications applications. Generally all base band video applications that use coaxial cable (both analogue and digital) are matched for 75 ohm impedance cable. Also RF video signal systems like antenna signal distribution networks in houses and cable TV systems are built from 75 ohms coaxial cable (those applications use very low loss cable types). In audio world digital audio (S/PDIF and coaxial AES/EBU) uses 75 ohms coaxial cable, as well as radio receiver connections at home and in car. In addition to this some telecom applications (for example some E1 links) use 75 ohms coaxial cable. 75 Ohms is the telecommunications standard, because in a dielectric filled line, somewhere around 77 Ohms gives the lowest loss. For 75 Ohm use common cables are RG-6, RG-11 and RG-59.
93 Ohms: This is not much used nowadays. 93 ohms was once used for short runs such as the connection between computers and their monitors because of low capacitance per foot which would reduce the loading on circuits and allow longer cable runs. In addition this was used in some digital communication systems (IBM 3270 terminal networks) and some early LAN systems.
The characteristic impedance of a coaxial cable is determined by the relation of outer conductor diameter to inner conductor diameter and by the dielectric constant of the insulation. The impedance of the coaxial cable changes somewhat with the frequency. Impedance changes with frequency until resistance is a minor effect and until dielectric constant is table. Where it levels out is the "characteristic impedance". The frequency where the impedance matches to the characteristic impedance varies somewhat between different cables, but this generally happens at frequency range of around 100 kHz (can vary).
Essential properties of coaxial cables are their characteristic impedance and its regularity, their attenuation as well as their behavior concerning the electrical separation of cable and environment, i.e. their screening efficiency. In applications where the cable is used to supply voltage for active components in the cabling system, the DC resistance has significance. Also the cable velocity information is needed on some applications. The coaxial cable velocity of propagation is defined by the velocity of the dielectric. It is expressed in percents of speed of light. Here is some data of come common coaxial cable insulation materials and their velocities: Polyethylene (PE) 66% Teflon 70% Foam 78..86%
Return loss is one number which shows cable performance meaning how well it matches the nominal impedance. Poor cable return loss can show cable manufacturing defects and installation defects (cable damaged on installation). With a good quality coaxial cable in good condition you generally get better than -30 dB return loss, and you should generally not got much worse than -20 dB. Return loss is same thing as VSWR term used in radio world, only expressed differently (15 dB return loss = 1.43:1 VSWR, 23 dB return loss = 1.15:1 VSWR etc.).
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