Index


General Electrical Wiring Information Page

    General information

    In a wire the metal (usually copper) has been drawn into a very long, thin thread or rod. Cable is bundle of insulated wires through which at least a single signal (both components or component+ground) can be passed. A single wire cannot be a cable, nor can a single fiber optic conductor. Technically a cable is not a wire, nevertheless, general usage since the advent of the telegraph / telephone system has been synonymous with cable. In the cable the individual wires inside the cable are insulated usingsome suitable insulation material (also called dielectric). Typical insulation materials are rubber and plastic, but there arealso other possibilities (air, vacuum, paper etc.). The insulation material used and the thickness of insulation of determine the cable capacitance and what voltages the the insulation can withstand. The dielectric constant of cable has effect on the cable capacitance. Here is table with information of dielectric constants of some common insulation materials:

    • PVC = 3-8
    • Polyethylene = 2.25 (66%)
    • Teflon = 2.1 (70%)
    • Chemically foamed polyethylene = 1.64 (78%)
    • Gas Injected Foam Polyethylene = 1.54 (83%)
    • Air = 1
    Voltage rating of the cable insulation determines the maximum voltage.The cable insulation must withstand the normal operation voltageand the higher surge voltages which can be perent in the cablesometimes. Most cable types list the highest allowed operating voltagein their technical specifications.Current rating of the cable is another necessary characteristic in cable selection.The heat loss in the cable determines the maximum current it can safely carry without excessive deterioration. How much heat loss is allowed depends on how hot the cable is llowed to get (varies from insulation material and environment).In many electrical applications the current rating for cable is lower than heat loss figure gives in order to keep the voltage drop within permissible values. The cable materials have effect on cable electrical and mechanical properties. Here is some of the most common material used in current carrying parts of the cable:
    • Copper: This is most commonly used material for the conductors in cables. Copper has very good condictivity. This is the conductor material most often used in electrical cables not matter where it is made for (low voltage, mains voltage, computer cables, RF cables etc.)
    • Steel: This is used for shield for some armoured cables.
    • Iron overed with copper: This is used in some special cables which must withstand lots of mechanical stress (like some hanging telephone wires).
    • Aluminium: This material is used for some mains feeder cables as condictors (becase it is cheaper than copper) and also as the shield layer material in many cable. The problem with aluminium is that it oxidizes easily and that aluminium oxide is an insulator. This means that specific care needs to be taken when making connections with the aluminium cables. This is why aluminium cables are not used much. The conductance of aluminium is lower than copper so thicker wire is needed to carry the same current.
    • Electrically conducting plastic: This material is used as cable shield layer material in some audio cable. The conductance of this type of plastics is so low that is is not useful as actual condictor of current.
    • Silver: This is expensive wiring material which is not used much. This material is used in some very expensive "high end" audio cables. Silver has very good condictivity.
    • Gold: This is very expensive and it not used in cables. Very thin gold wires are generally used inside IC cases to connect the silicon chip to the pins in the case. Gold has very good condictivity.
    There are also varipus different insulation materials. The most commonly used being PVC, thermoplastics, rubber, teflon and paper. Typical single wire caharacterics:
    • Resistance: This is function of wire material resistance, wire thickness and length
    • Length: the longer the wire, the larger the resistance
    • Wire conductor material: Wire is usually composed of fine copper strands, but other conductive materials can be used. The type of material used has an effect on the resistance of the wire
    • Wire gauge: The wire gauge, or size of the wire, also determines how much resistance the wire has. The larger the wire, the less resistance. The smaller the gauge, the larger the wire.
    • Wire insulation: The rating of the insulation on the wire (if there is any). Typical specifications for insulation include the following: insulation material, voltage rating, temperature rating, insulation thickness
    • Conductor type: Solid or stranded
    Both temperature and wire length affect resistance. A wire has a current rating based on it's characteristics, enviroment conditions and how much it is allowed to heat up. If too much current goes through a wire, it can overheat and melt. The amount of current that a wire can handle depends on its length, composition, size and how it is bundledThe cable bundling affects the current carrying capacity. The way a wire is bundled affects how well it can dissipate heat. If the wire is in a bundle with 50 other wires, it can carry a lot less current than if it were the only wire in the bundle. When we take a wel formed wire bundle and put an extra insulation layer over it, we get a cable that has many wires in it. Typical cable characteristics are:
    • Impedance (Ohms) represents the total resistance that the cable presents to the electrical current passing through it. At low frequencies the impedance is largely a function of the conductor size, but at high frequencies, conductor size, insulation material and insulation thickness all affect the cable's impedance. Matching impedance is very important at high frequency systems. The impedance value means the total opposition to the flow of the electrons, offered by a transmission line. In a transmission system at long distance or high frequency, it is very importance that the Ohm value could be constant at the starting point and continuous till the receiver.
    • Attenuation is measured in decibels per unit length (dB/m), and provides an indication of the signal loss as it travels through the cable. Attenuation is very dependent on signal frequency (usually increases when frequency increases)
    • Shielding is normally specified as a cable construction detail. For example, the cable may be unshielded, contain shielded pairs, have an overall aluminum/mylar tape and drain wire or even a double shield. Cable shield acts as a barrier to keep external signal from getting in and internal signals from getting out. In some application cable shield is used be a part of the electrical circuit (usually as the signal ground wire). Shielding effectiveness is very complex to measure and depends on the data frequency within the cable and the precise shield design.
    • Capacitance in cable is usually measured as picofarads per foot (pf/m). Cable capacitance indicates how much charge the cable can store within itself. A cable with a high capacitance slows down signal level chages so that square wave signals fed to cable may come out of the cable looking more like "saw-teeth", rather than square waves.
    • Shielding is a a metal sleeve surrounding wire conductors (coax or triax cable) to lessen interference, interaction, or current leakage. The shield is usually grounded. The amount of shielding is typically expressed as decibels of attenuation at certain frequency.
    Cables used to carry high frequency electrical signals are generally analysed as a form of Transmission Line. The E-fields between the conductors mean that each length of the pair of conductors has a capacitance. The H-fields surrounding them mean they also have inductance. The longer the cable, the larger the resulting values of these might be. The amount of capacitance/metre and inductance/metre depends mainly upon the size and shape of the conductors. The inductance relates the amount of energy stored in the magnetic field around the cable to the current level. The Characteristic Impedance depends upon the ratio of the values of the capacitance per metre and inductance per metre. To understand its meaning, consider a very long run of cable that stretches away towards infinity from a signal source. The source transmits signals down the cable which vanish off into the distance. In order to carry energy, the signal must have both a non-zero current, and a non-zero potential. (i.e. both the E-field and the H-field must exist and propagate along, guided by the cable.) Since the far end is a long way away, the signals transmitted from the source can?t initially be influence by the properties of any destination before they finally arrive. Hence the ratio of the field carried along the cable (and hence the current/voltage ratio) are determined solely by properties of the cable. The result, when the signal power vanishes, never to be seen again, is that the cable behaves like a resistive load of an effective resistance set by the cable itself. This value is called the Characteristic Impedance, of the cable.As a matter of convention, there is a tendency for many co-axial cables to be designed and manufactured to have an impedance of either 75 ohm (used by TV and video industry) or 50 ohm (used by scientists and engineers for instrumentation and communications, also used for many radio transmitters). Engineers use a variety of type and impedances of cables. At RF, the use of 300 ohm twin feed is fairly common, and 600 on is often used at audio frequencies (for example in telecommunications). In modern telecommunications the cables used are typically 100 ohm (modern structured cabling) ohm or 120 ohm (many telephone cables in ground) unshielded twisted pair wiring. Standard values tend to be adopted for convenience in a given application area as this makes it easier for people to build systems from compatible elements.Cables are often divided also their intended use. Some cables are designed to be used inside building, some for outside wiring. Those different places have different needs for cable characteristics. Here are examples of some different conditions and needs made by environment:
    • Wirings inside normal building (fixed wiring): Inside normal buildings is easy place for the materials itself. The building codes define what kind of wires are allowed in the buildings. The fire dafety ratings are very important in all wires installed inside buildings. Preferably the cables should not burn easily, the cable should not be able to burn itself or spread fire and preferably not generate any very dangerous gases when it burns. PVC material is often used insulation material in inside wirings, because it can resist fire quite well.
    • Wiring in plenums: When installing cable in an office building, fire codes often dictate that cables running through the air spaces in the building's walls (called plenums) must have an outer sheath made from a material that does not emit toxic gases when it burns. The PVC sheathing used on most cables does emit these gases, so there is a separate grade of cable, called plenum cables, that have sheathing made of a safer material. Plenum cables are less flexible and more expensive, but if local codes require it, use them.
    • Underground wiring: The cable must be designed in such way that the the water around it cannot get to the cable in any way. Undeground cables generally use special tight outside shealt and quite often have some form of filling material that keeps the mousture out of cable.
    • Outside wiring: The wiring outside is subject to the outside consitions like sunlight, summer heat, winter cold, possibly mechanical stress etc. The cables used outside must be such that they keep the mousture out of the cable and can withstand the UV radiation from the sunlight. If cable outside is hung from place to another it must be mechanically strong enough to withstand the intended use.
    • Wiring on hot places: Special cables are needed in very hot places where normal cables do not survive.
    • Places with chemicals: You must select cable that does not reach to the chemicals.
    • Cables that are moved often: You must select a cable with stranded conductors. Dependign on the applications those cabpes generally have wery flexible PVC or rubber insulation.
    • Cables that are moved on cold conditions: You must select a stranded cable that has insulation that can withstand the low temperatures without breaking. On winter conditions usually the cables with rubber insulation are good (check the cable manufacturers for specifications).
    When cable is used to carry sensitive signals, the system noise coupled to cable needs to be taken into consideration. Noise signals can appear in a cable asthe result of capacitive coupling of nearbyE (electric) fields, inductive couplingof local M (magnetic) fields, EM (electromagnetic)coupling of radio signals inspace, and C (conduction) via intentionalor leakage paths. The coupled signalappears as an additional signal in serieswith the line or lines. Dependingon the cable type, the coupledsignal may appear in normal or common mode. Twisted-pairs equally interceptcoupled signals, so the incident signalsappear only in common mode. Pairs withidentical impedances to the local commonare balanced.

    Technical information

      Cable resistance

      Cable resistance is generally defined by the properties of the cable material resistance. Generally in copper cables the resistance of cable is defined by the cable thickness and length and the nominal resistance of copper. Same applies also to cables made from other materials. The cable resistance is not constanly constant in all conditions. The cable temperature and signal frequency on cable can affect the resistance. Generally in the cables made of copper have their resistance increase somewhat when the cable temperature increases. Skin effect is a physical phenomenon that relates to the limited penetration into a conductor of a high frequency signal according to its frequency. The less of the cable the current uses, the higher the resistance appears to be. This has speficically effect on cables that are thick and carry high frequency signals.

      • DC = Entire cable
      • 1 kHz = 82.5 mils
      • 10 kHz = 26.1 mils
      • 100 kHz = 8.25 mils
      • 1 MHz = .261 mils
      • 10 MHz = .0825 mils
      • 100 MHz = .0261 mils
      • 1 GHz = .00825 mils (8 micro-inches)
      The unit mils means one thousands of inch (0.024 mm).

      Coaxial cables

      A coaxial cable is one that consists of two conductors that share acommon axis. The inner conductor is typically a straight wire, eithersolid or stranded and the outer conductor is typically a shield that might be braided or a foil. Coaxial cable by its very construction doesn't radiate much, and its outer shield makes it fairly impervious to having interference induced in it.

      Coaxial cable is a cable type used to carry radio signals, video signals, measurement signals and data signals. Coaxial cables exists 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 ceter 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 agains 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 are typically characterized with the impedance and cable loss. The length has nothing to do with a 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 (Zo) 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. There are probably lots of stories about how 50 Ohms came to be. Around World War II, impedances were chosen depending on the application. For maximum power handling, somewhere between 30 and 44 Ohms was used. On the other hand, lowest attenuation for an air filled line was around 93 Ohms. In the US, 50 Ohms was chosen as a compromise, and it became MIL specs. Europe chose 60 Ohms at that time. Europeans were forced to change because of the influence of international companies. 75 Ohms is the telecommunications standard, because in a dielectric filled line, somewhere around 77 Ohms gives the lowest loss. RF industry has converged on a limited range of impedances for coaxial cables. According to IEC publication 78 (1967), 75 ohm is a popular coaxial impedance standard because you can easily match it to several popular antenna configurations. It also defines a solid polyethylene-based 50 ohm cable because, given a fixed outer-shield diameter and a fixed dielectric constant of about 2.2 (the value for solid polyethylene), 50 ohm minimizes the skin-effect losses.

      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 (foe example high frequency oscilloscope probe cables) and high frequency digital applications (fe 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 nowdays RF system 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 baseband 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 thsi was used in some digital commication systems (IBM 3270 terminal networks) and some early LAN systems.

      Of the four hundred odd coaxial cables listed in MIL-C-17, over 85 percent are 50 Ohms. 75 Ohm (variously 70 or 72 Ohm) is much less common, but is the standard impedance used for cable TV, video applications and some telecom applications. There are a very limited number of cables at impedances other than 50 or 75 Ohm - 93 Ohm normally is used for pulse transmissions - RG-62/U or RG-71/U being two examples.

      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 chanes somewhat with the frequency. Impedance changes with frequency until resistance is a minor effect and until dielectric dielectric constant is table. Where it levels out is the "characteristic impedance". The freqnency where the impedance matches to the characteristic impedance varies somwehat 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 behaviour 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.). The dielectric of a coaxial cable serves but one purpose - to maintain physical support and a constant spacing between the inner conductor and the outer shield. In terms of efficiency, there is no better dielectric material than air. In most practical cables cable companies use a variety of hydrocarbon-based materials such as polystyrene, polypropylenes, polyolefins and other synthetics to maintain structural integrity.

      Sometimes coaxial cables are used also for carrying low frequency signals, like audio signals or measurement device signals. In audio applications especially the coaxial cable impedance does not matter much (it is a high frequency property of cable). Generally coaxial has a certain amount of capacitance (50 pF/foot is typical) and a certain amount of inductance. But it has very little resistance.

      General characteristics of cables:

      • A typical 50 ohm coax coaxial cable is pretty much 30pf per foot (doesn't apply to miniature cables or big transmitter cables, check a cable catalogue for more details). 50 ohms coaxial cables are used in most radio applications, in coaxial Ethernet and in many instrumentation applications.
      • A typical 75 ohm coaxial cable is about 20 pf per foot (doesn't apply to miniature cables or big transmitter cables, check a cable catalogue for more details). 75 ohms cable is used for all video application (baseband video, monitor cables, antenna networks cable TV, CCTV etc.), for digital audio (S/PDIF, coaxial AES/EBU) and for telecommunication application (for example for E1 coaxial cabling).
      • A typical 93 ohm is around 13 pf per foot (does not apply to special cables). This cable type is ued for some special applications.
      Please note that these are general statements. A specific 75 ohm cable couldbe 20pF/ft. Another 75 ohm cable could be 16pF/ft. There is no exact correlation between characteristic impedance and capacitance. In general, a constant impedance (including connectors)cable, when terminated at both ends with the correct load, represents pure resistive loss. Thus, cable capacitance is immaterial for video and digital applications.

      Typical coaxial cable constructions are:

      • Flexible (Braided) Coaxial Cable is by far the mostcommon type of closed transmission line because of itsflexibility. It is a coaxial cable, meaning that both the signaland the ground conductors are on the same center axis.The outer conductor is made from fine braided wire, hencethe name "braided coaxial cable". This type of cable isused in practically all applications requiring completeshielding of the center conductor. The effectiveness of theshielding depends upon the weave of the braid and thenumber of braid layers. One of the draw-backsof braided cable is that the shielding is not 100% effective, especially at higher frequencies. This is because the braided construction can permit small amounts of shortwavelength (high frequency) energy to radiate. Normally this does not present a problem; however, if a higherdegree of shielding is required, semirigid coaxial cable is recommended. In some high frequency flexible coaxial cablesthe outer shield consists if normal braids and an extra aluminium foil shield to give better high frequency shielding.
      • Semirigid Coaxial Cable uses a solid tubular outer conductor, so that all the RF energy is contained within the cable. The cable construction is such that the cable can be bent somewhat without too much causing problem to it's performance. For applications using frequencies higher than 30 GHz a miniature semirigid cable is recommended.
      • Rigix coaxial cables are constructed using a a solid tubular outer conductor so that all the RF energy is contained within the cable. Some high power coaxial feed lines on high power transmitter sites are built as rigid coaxial cables. Those rigix feedlines are typicaly built by placing two different size copper tubes inside each other, having air or suitable gas as insulator and having something that keeps those different tubes in the right places. The tube diameters are selected so that wanted impedance (typically 50 ohms) line is formed. Many early coaxial lines were actually existing materials consisting of standard rods and water pipes, 51.5 Ohms was quite common
      • Ribbon Coaxial Cable combines the advantages of both ribbon cable and coaxial cable. Ribbon Coaxial Cable consists of many tiny coaxial cables placed physically on the side of each other to form a flat cable. Each individual coaxial cable consists of the signal conductor, dielectric, a foil shield and a drain wire which is in continuous contact with the foil. The entire assembly is then covered with an outer insulating jacket. The major advantage of this cable is the speed and ease with which it can be mass terminated with the insulation displacement technique.
      Often you will hear the term shielded cable. This is verysimilar to coaxial cable except the spacing between centerconductor and shield is not carefully controlled duringmanufacture, resulting in non-constant impedance.If the cable impedance is critical enough to worryabout correctly choosing between 50 and 75 Ohms,then the capacitance will not matter. The reasonthis is so is that the cable will be either loadterminated or source terminated, or both, and thedistributed capacitance of the cable combines withits distributed inductance to form its impedance.A cable with a matched termination resistance atthe other end appears in all respects resistive,no matter whether it is an inch long or a mile.The capacitance is not relevant except insofar asit affects the impedance, already accounted for. In fact, there is noelectrical measurement you could make, at just the end of the cable, that could distinguish a75 Ohm (ideal) cable with a 75 Ohm load on the far endfrom that same load without intervening cable. Given that the line is teminated with a proper 75ohm load (and if it's not, it damn well should be!), the load is 75ohms resistive, and the lumped capacitance of the cable is irrelevant. Same applies to other impedance cables also when terminated to their nominal impedance.

      There exist an effect that characteristicimpedance of a cable if changed with frequency. If this frequency-dependent change in impedance is large enough, the cablewill be impedance-matched to the load and source at some frequencies,and mismatched at others. At high frequencies, typically greater than 1 MHz, the coaxial cable will approach the ??steady state?? value that is referred to as the nominal or typical impedance. This is the value stated by most manufacturers as the characteristic impedance. Traditionally, cable manufacturers have specified this value as typical or nominal. In some cases, the value had a tolerance. In the case of video cable, a typical value was 75 ?? 3 ohms. Therefore, the characteristic value is between the value of 72 and 78 ohms or about ?? 4% Recently, manufacturers have begun to tighten this impedance tolerance. The coaxial cable low freuquency curve starts to kick-in at freuquencies between 10 kHz and 100 kHz. And below 10 kHz is is dominant causing impedance to raise as frequency lowers (can be up to several kohms at 10 Hz and few hundred ohms at 1 kHz). The characteristic impedance value is important because the cable must be as close to source and load impedances (usually 75 ohms or 50 ohms depeding on the application) as possible to minimize reflective losses.

      Input impedance is the term used to describe the impedance at any given specific frequency. The term vector impedance is also sometimes used. Unfortunately, this impedance is not uniform at all locations and all frequencies within the cable. After all, this is the REAL world! Therefore, what is really happening within a cable can be best understood (and measured) by looking at the reflection coefficient. When signal traveling in a coax encounters an impedance mismatch, a portion of the signal will be reflected back to the source. An extreme example of this could be a cable that has an excellent characteristic impedance value, but but has terrible impedance variation within the cable. It is critical that the variation of impedance around the nominal be minimal, because impedance mismatches cause signal reflections. The right characteristic impedance value alone will not guarantee the performance level of the cable, the imepdance must stay constant over the length of the cable (cable needs to be constructed in such way that the construction, materials and measurements stay the same over the length of the cable as much as possible).

      Cable impedance is not the only detail in cable. However there is another effect that can cause loss of detail fast-risetime signals. There is such a thing asfrequency-dependent losses in the cable. There is also a property of controlled impedance cables known as dispersion, where different frequencies travel at slightly different velocities and with slightly different loss.

      The properties of transmission line can be described with with transmission line parameters. Those parameters are divided to primary and secondary parameters. The primary pareters are:

      • series resistance Ri in ohm/km
      • inductance Li in H/km
      • parallel capacitance Ci in F/km
      • parallel resistance Gi in S/km
      he secondary parameters are:
      • characteristic impedance Zc in ohms
      • propagation constant myy
      • phase constant beta in rad/km
      • attenuation constant alpha in N/km

      The dielectric in coaxial cable has considerable effect on the performance of the coaxial cable. The purpose of the dielectric is to separate and insulate the center conductor from the ground shielding. There is a specific relation between the diameter of the conductor (d), the diameter of the dielectric (D) and dielectric characteristics of the dielectric material that must be maintained to ensure the correct cable impedance (usually 50 ohms or 75 ohms). It is helpful to know about some of the standard materials used in the industry so you can make logical decisions. A number of different "standard" materials are used for creating a dielectric barrier. They include PVC, Polyethylene, foamed (gas injected) Polyethylene, Polypropylene, Nylon and Teflon. Each of these materials have their own unique dielectric rating directly related to the materials ability to store energy. A perfect dielectric will store zero energy. The closest to a perfect dielectric is air with a dielectric value of one, but it is not usually practical to use air as a dielectric (you would need some special support to keep the central conductor in place, usually only practical in some high power transmitter feeder lines). The practical solution is to use industry standard materials such as those listed above. Those are listed starting from the best in electrical properties.

      Dielectric Material Dielectric Constant
      Air 1
      Foamed Polyethylene 1.5 to 2.1
      Teflon 2.03
      Polyethylene 2.27 to 2.5
      Polypropylene 2.25
      Nylon 4.0 to 4.6
      PVC 3.8 to 8.0

      With the exception of Nylon and PVC, the materials listed above have closely matched dielectric constants. Usually the selection of the material between them is determined by other properties of the material than the dielectric constant. As seen by dielectric constants Nylon and PVC are not very good materials, but yet they are still used by some cable manufacturers due to their low cost.

      There are also foamed dielectic materials, for example foamed Polyethylene. Air is the best possible dielectric so having air pockets in the material somewhat improves the dielectric characteristics of the material. During manufacturing, nitrogen or other inert gases are injected into the liquid material in order to create bubbles. When the material cools and hardens the pockets and gaps from the bubbles remain resulting in the formation of a foamed material, but the nitrogen itself does not remain within the cable. The dielectric constant for foamed Polyethylene (1.5 to 2.1) is a variable dependant on the manufacturing process. For this reason, it may or may not be as good a dielectric as Teflon. Teflon is a more consistent and uniform material and has a predictable and repeatable dielectric constant.

      In some communications applications (special computer networks, fast differential ECL signals) a pair of 50 ohm coaxial cables are used to transmit a differential signal on two non-interacting pieces of 50-ohm coax. The total voltage between the two coaxialconductors is double the single-ended voltage, butthe net current in each is the same, so thedifferential impedance between two coax cable usedin a differential configuration would be 100 ohms.As long as the signal paths don't interact, thed ifferential impedance is always precisely twice the single-ended impedance of either path.

      Despite being shielded, some interference can occur on coaxial cable lines. This interference can be a considerable problem when low level high frequency signals are transported. For cable television it is important to use the correct type of coaxial cable. RG-59/U should be avoided, and only RG-6/U, or in cases of severe interference, RG-6/UQ (quad-shield) used. Many consumers have purchased the cheaper RG-59/U to use as an extension for cable television, only to find it causes severe interference. The reason for the interference is that 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. If the same frequencies transport different programs, severe cross interferences of two program signals exist. Leakage of cabe TV signals can also cause interference to aircraft communications which operate on the same frequency as several cable channels. This may even be a violation of the law.

      Hard line is a very heavy-duty coaxial cable, where the outside shielding is a rigid or semi-rigid pipe, rather than flexible and braided wire. It is used in broadcasting and other forms of radio communication transmitter antenna connections. 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. Physical separation between the inner conductor and outer shielding is maintained by spacers, usually made out of tough solid plastics like nylon.

      There are several variations from normal coaxial construction. Triaxial cable or triax is coaxial cable with a third layer of shielding, insulation and sheathing. The outer shield, which is earthed, protects the inner shield from electromagnetic interference from outside sources. Triaxial cable is generally used on applications where very good shielding is needed (typically instrumentation applications) and also in video broadcasting applications (for TV camera connections to carry both power and video signals). 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. Twin-axial cable is used on some proprietary computer networks. Biaxial cable or biax is a figure-8 configuration of two 50 ohm coaxial cables. It is used on some proprietary computer networks. Multi-conductor coaxial cable is also sometimes used.

      The current military standard for coaxial cables 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-xx. 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.

      The grounding practices for coaxial cables depend on the application. Coax grounded at no more than one point is pretty good against common mode signals. If you ground at more than one point it is fairly easy to get large currents through the shield (especially at 50hz or 60Hz) with no compensating current in the center conductor. When operating at high frequencies and low frequencies are not of any convern, grounding at the both ends is usually the best option.

      Coax Connnector Information

      RF coax(ial) connectors are a vital link in the system which uses coaxial cables and high frequency signals.Coax connectors are often used to interface two units such as the antenna to a transmission line, a receiver or a transmitter. The proper choice of a coax connector will facilitate this interface. Coax connectors come in many impedances, sizes, shapes and finishings. There are also female and male versions of each. As a consequence, there are thousands of models and variations, each with its advantages and disadvantages. Coax connectors are usually referred to by series designations. Fortunately there are only about a dozen or so groupings or series designations. Each has its own important characteristics, The most popular RF coax connector series not in any particular order are UHF, N, BNC, TNC , SMA, 7-16 DIN and F.Here is quicl introduction to those connector types:

      • "UHF" connector: The "UHF" connector is the old industry standby for frequencies above 50 MHz (during World War II, 100 MHz was considered UHF). The UHF connector is primarily an inexpensive all purpose screw on type that is not truly 50 Ohms. Therefore, it's primarily used below 300 MHz. Power handling of this connector is 500 Watts through 300 MHz. The frequency range is 0-300 MHz.
      • "N" connectors: "N" connectors were developed at Bell Labs soon after World War II so it is one of the oldest high performance coax connectors. It has good VSWR and low loss through 11 GHz. Power handling of this connector is 300 Watts through 1 GHz. The frequency range is 0-11 GHz.
      • "BNC" connctor: "BNC" connectors have a bayonet-lock interface which is suitable for uses where where numerous quick connect/disconnect insertions are required. BNC connector are for exampel used in various laboratory instruments and radio equipment. BNC connector has much lower cutoff frequency and higher loss than the N connector. BNC connectors are commonly available at 50 ohms and 75 ohms versions. Power handling of this connector is 80 Watts at 1 GHz. The frequency range is 0-4 GHz for 50 ohm connector and 0-1 GHz for normal 75 ohm connector (higher frequency 75 ohm connectors could be available). A often asked question is what is the difference of 50 and 75 ohm connector. The connector metal parts are identical in both versions in the contact area. The 50 ohm connectors have additional 'insulation' members, which are actually dielectric to increase the shunt capacitance and thus lower Zo from 75 to 50 ohms. The difference can be different insulation shape and/or different insulation material. The main visible physical difference is that the 75 Ohm plug does not have extended dielectric around its outer spring fingers. The current BNC connector standard is IEC 60169-8. In the distant past, there were also other variations (the centre pins of the 50 and 75 ohms connectors were once of different diameters, either the connectors won't mate or the '75 ohm' pin will not make good contact with the '50 ohm' sleeve).
      • "TNC" connectors are an improved version of the BNC with a threaded interface. Power handling of this connector is 100 Watts at 1 GHz. The frequency range is 0-11 GHz.
      • "SMA" connector: "SMA" or miniature connectors became available in the mid 1960's. They are primarily designed for semi-rigid small diameter (0.141" OD and less) metal jacketed cable. Power handling of this connector is 100 Watts at 1 GHz. The frequency range is 0-18 GHz.
      • "7-16 DIN" connector: "7-16 DIN" connectors are recently developed in Europe. The part number represents the size in metric millimeters and DIN specifications. This quite expensive connector series was primarily designed for high power applications where many devices are co-located (like cellular poles). Power handling of this connector is 2500 Watts at 1 GHz. The frequency range is 0-7.5 GHz.
      • "F" connector: "F" connectors were primarily designed for very low cost high volume 75 Ohm applications much as TV and CATV. In this connector the center wire of the coax becomes the center conductor. High quality F connectors can handle 0-3 GHz frequency range. Technical specifications typically specified at 1 GHz frequency. "F" connector is very simple and low loss connector.
      • "IEC antenna connector": This is a very low-cost high volume 75 ohm connector used for TV and radio antenna connections around Europe. This connector is primarily designed for antenna signal input primarily into consumer audio and video equipment. The typical frequency range this is used is 0-1 GHz. This connector can be found on both equipment and on fixed wiring wall outlets in Europe. The connector is defined in IEC 169-2 standard.
      • RCA connector: Home Theater market has adapted RCA connectors for all video interconnects. Those RCA connectors were originally designed ar very inexpensive inter-module RF conenctors in the beginning, and later they were largely deployded in audio interconnections. Based on their diameters, the internal impedances of RCA connectors are between 35-ohm and 50-ohm. They were never intended to have an internal impedance of 75-ohm. hat being the case, there are no known video cables who have a true 75-ohm RCA connectors, regardless of the handful of suppliers that make this claim ("75-ohm type" RCA is only about 55-ohms or slightly higher). The fact is, RCA connectors are usually adequate for consumer video applications even though they are not true 75-ohms.

      There are also some special connectors and special variations of connectors used for some special applications. For example FCC has required that suppliers of RF LANs (local area networks) have an RF interface that cannot be matched by the present available RF connector series (idea is to prevent connecting higher gain antennas to those devices). As a result, several so called "reverse polarity connectors" have been designed. The reverse polarity TNC is one of the most popular where the threads are left-hand instead of the conventional right-hand type.

      There are many different ways coxial connectors are connected to the cable. The most commonly used technologies are:

      • Screw
      • Crimp
      • Solder
      • Compression

      There are many variations of Crimp type connectors. On some version the crimp applies force to the whole cable (for example many F connectors) or just to the outside part of the cable (for example many BNC etc. connectors). Hex-crimp F-connector creates 6 points where connector applies force to the cable, changing it's shape slightly. This causes a slight impedance mismatch, which is a source of reflections at high frequecies. Compression fittings have impedance matching and usually better pullout strength than hex-crimp.

      The way crimping works on BNC connectors this kind of mismatch is not created. Two main variants of the BNC connector exist in terms of assembly style. The first of these is the compression gland type. In this style the centre pin of the connector is usually a solder pin whilst the braid and sheath of the cable are held by an expanding compression gland fixed by a nut at the rear of the connector. This type of connector by its nature can cope with a (limited) range of cable sizes and requires no specialised tooling to assemble. The second is the crimp connector. In this type the centre pin is usually (but not exclusively) crimped to the centre conductor. This crimped pin is then pushed into position through an inner ferrule which separates the inner insulation sheath and the braid of the cable. An outer ferrule is then crimped over the braid and outer insulation which fixes the cable to the connector. Due to the accuracy required, virtually every cable type requires a corresponding crimp connector variant. Further, assembly requires very accurate crimping tools to optimise the integrity of the connection, and must be correct first time. In both types of connector it is essential that the exact amount of insulation is stripped from each section to ensure accurate assembly. For volume production, the crimp style connection is always preferred.

      Many other coaxial cable connectors are available using same kind of assembly styles as BNC connectors.

      If the BNC plugs and sockets in question, and indeed other types of connectors, are to be used only at a few hundred MHz and below, then the user should be aware that a 75/50 ohm mismatch over a distance considerably less than the length of a connector will cause absolutely negligible measuring errors in such matters as SWR. For example in video applications you can see sometimes 50 ohm BNC connectors used sometimes in otherwise 75 ohm system. Start worrying only when there is a large energy content in the signal above 1 GHz. It is implicit in IEC 169-8 that 75 ohm BNCs made to comply with that standard will mate in a non-destructive manner with the 50 ohm BNC connectors described in IEC 169-8. Good quality BNC connectors manufactured within last 15 years or so should not have mechanical incompatibility between 50 ohm and 75 ohm types, other than extremely rare (and very obvious) manufacturing faults.

      It is necessary only to ensure a good, solid, electrical contact in connectors at DC and the HF properties will look after themselves. Mechanically wobbling, intermittent, connectors are not good at any frequency.

      If the dimensions of pins of mixed 50/75 ohm BNC connectors allow good, solid, DC connections then there's nothing whatever to worry about. The HF properties and performance of single mixed plugs and sockets up to and including VHF will look after themselves quite satisfactorily. There's far too much importance attached to actual impedances below 1 Ghz. The 50/75 issue is important in standard definition serial digital video running at 270 mbits (ccir 601). The impedance variations can cause the serial receivers to lose lock (takes more than 1 connector to mess things up). Given the lack of evidence that 75 ohm connectors are less robust in use than 50 ohm versions, it is suggested that users avoid the potential confusion mentioned above by using 75 ohm connectors for 75 ohm systems and 50 ohm versions for 50 ohm systems.

      Triax cables

      The TRIAX system exploits a triple co-axial construction. Instead of one shield the cable features two concentric shields. Trinax cables are like coaxial cables where the one center conductor is surrounded with two shield layers insulated from each other. You can think a triaxial (or short triax) cable as being a full coaxial cable surrounde by an extra metallic shield layer and outer insulator. Typical triax cable is like a very low loss coaxial cable with very good shielding properties. Triax cables are generally used in some instumentation and RF applications where special shielding is needed. Triax cable is ideal for high cross-talk environments such as antenna-, radar-, and broadcast systems.Triaxial connectors and cable assemblies are used where very low- and high level RF signals are transmitted simultaneously through cables which are bundled or located in high energy fields caused by radar or transmitters. In instrumentation aplications the outer shield is usually used as the earth, while the inner shield is usually fed by its own driver amplifier. Triax cables are used in TV broadcast industry for TV camera interconnections (connecting camera to CCU and supplying power to camera). Triaxial cables are constructed with a solid or stranded center conductor and two isolated shields. The center conductor and the inner isolated shield make up a coaxial cable configuration that functions to carry the video signal. The outer isolated shield can be used for several separate signals by means of multiplexing that may include power feed, teleprompter feeds and control for automation. Triax Cable is designed with two isolated shields to provide multiple functions through one cable to your camera such as power. There are two versions of triax cable commonly used in TV industry: RG59 (3/8") and RG11 (1/2"). Typical triax camera system can send the picture from over a triax cable for up to 500 meters with no degradation. Camera set-ups that can be remotely adjusted though ta cable, as well as usually intercom functions. There are (at least have been) two types of triax systems in use in broadcast industry: analogue triax and digital triax.In conventional Analog Triax the signals (component video, audio, intercom, control etc.) are modulated onto different frequency FM carriers which are carried through the same cable. Digital Triax is Component Digital video (plus other signals) running down the cable.

      Twinax cables

      Twinax, or twinaxial, is a type of communication transmission cable consisting of two center conductors surrounded by an insulating spacer which in turn is surrounded by a tubular outer conductor (usually a braid, foil or both). The entire assembly is then covered with an insulating and protective outer layer. Twinax is constructed much like coaxial cable, execpt it has two center conductors instead of one.Twinax cable is like a very low loss shielded twisted pair cable withbetter transmission characteristics and shieldind than normal twisted pair. Twinax, or twinaxial, is a type of communication transmission cable consisting of two center conductors surrounded by an insulating spacer which in turn is surrounded by a tubular outer conductor (usually a braid, foil or both). The entire assembly is then covered with an insulating and protective outer layer. Twinax is constructed much like coaxial cable, execpt it has two center conductors instead of one. However, it is similar to twisted pair cabling in that it uses differential, or "balanced", transmission. Twinax is used in some special data communication applications(some old IBM terminals and military communications).The use of twinax is quite similar to twisted pair cabling in that it uses differential, or "balanced", transmission. Twinax has better transmission characteristics than twisted pair media. One common twinax cable has 150 ohm impedance. It is the cable used in IBM Type 1 cabling. Another common is 110 ohm twinax cabling used on some IBM terminal systems. Another common type is RG-108/U_A that consists of pair of 20AWG cables. It has impedance of 78 ohms. MIL-STD-1553B Data Bus specifies 70 to 85 ohms at 1 MHz impedance. Twinax type cable with impedance of around 100-120 ohms are used on some industrial automation networks to carry RS-485 signals. Sometimes it is hard to see the difference where twinaxial construction ends and the cable can be considered just as a shielded twisted pair cable (each pair shielded).

      Twisted pair wiring

      Twisted pair cable consists of a pair of insulated wires twisted together. It is a cable type used in telecommunication for very long time. Cable twisting helps to reduce noise pickup from outside sources and crosstalk on multi-pair cables. Twisted pair cable is good for transferring balanced differential signals. The practice of transmitting signals differentially dates back to the early days of telegraph and radio. The advantages of improved signal-to-noise ratio, crosstalk, and ground bounce that balanced signal transmission bring are particularly valuable in wide bandwidth and high fidelity systems. By transmitting signals along with a 180 degree out-of-phase complement, emissions and ground currents are theoretically canceled. This eases the requirements on the ground and shield compared to single ended transmission and results in improved EMI performance.

      The most commonly used form of twisted pair is unshielded twisted pair (UTP). It is just two insulated wires twisted together. any data communication cables and normal telephone cables are this type. Shielded twisted pair(STP) differs from UTP in that it has a foil jacket that helps prevent crosstalk and noise from outside source. In data communications there is a cable type called FTP (foil shielded pairs) which consists of four twisted pair inside one common shield (made of aluminium foil).

      When cable twisted at constant twist rate over the lenght of the cable, a cable with well defined characteristic impedance is formed. Characteristic impedance of twisted pair is determined by the size and spacing of the conductors and the type of dielectric used between them. Balanced pair, or twin lines, have a Zo which depends on the ratio of the wire spacing to wire diameter and the foregoing remarks still apply. For practical lines, Zo at high frequencies is very nearly, but not exactly, a pure resistance. Because the impedance of a cable is actually a function of the spacing of the conductors, so separating the conductors significantly changes the cable impedance at that point. When many twisted pairs are put together to form a multi-pair cable, individual conductors are twisted into pairs with varying twists to minimize crosstalk. Specified color combinations for wire colors are used to provide pair identification.

      Twisted pair impedence varies as a function of wire size, spacing, and the dielectric constant of the insulating medium. The practical impedances on different twisted pair applications are typically in 60-300 ohms range. Twisted pair wire impedance is more or less influenced by the proximity to other conductors or ground. This means that if you put unshielded twisted pair wire near a grounded plane, the impedance of it changes somewhat (how much depends on the distance from the plane and twisted pair construction).

      Nowadays the most commonly used twisted pair cable impedance is 100 ohms. It is widely used for data communications and telecommunications applications in structured cabling systems. In most twisted pair cable applications the cable impedance is between 100 ohms and 150 ohms. When a cable has a long distance between the conductors, higher impedances are possible. Typical wire conductor sizes for cables used in telecommunications 26, 24, 22 or 19 AWG.

      Here are some common impedances related to twisted pair lines:

      • 100 ohms: This impedance is the standardized impedance to be used in the twisted pair wiring used in structured wiring systems standardized EIA/TIA 568 standard. Both unshielded and shielded "CAT5 and better" cables used on this kind of applications have 100 ohms impedance (usually at +-15% or better accuracy). Nowadays the most common LAN standard, Ethernet, is designed for 100 ohms twisted pair cable. Many telecommunication twisted pair cables have impedance of aroudn 100 ohms, and many modern digital communication system are matched to this impedance. Nowadays practically all modern in-building twisted pair wiring for telecom applications has 100 ohms impedance.
      • 110 ohms: 110 ohms shielded twisted pair cable is standardized as the cable type to be used for digital AES/EBU sound interface. It is also seen on some industrial bus applications.
      • 120 ohms: 120 ohms shielded cable is generally used for for RS485 commmunications in indutrial networking. There are many industrial "control and data" cables which have impedance of around 120 ohms. Also some telecom cables (both shielded and unshielded) have impedance of 120 ohms, and there are digital telecpm systems matched to this impedance also (for example some E1 systems).
      • 150 ohms: This was the impedance used in shielded twisted pair wiring IBM cabling system and Token Ring network. There are also many shielded "control and data" cables that has impedance of around 150 ohms in use nowadays. Some modern microphone cabling (shielded twisted pair) has impedance of aroudn 150 ohms at high frequencies and you can sometimes hear 150 ohms impedance mentioned in analogue audio applications (typical dynamic professional microphones have impedance of 150-200 ohms usually).
      • 300 ohms: The twin lead wire used in some antenna applications has impedance of 300 ohms. This is a very low loss antennna cable type. 300 ohms is generally not used for anything else than some antenna applications.
      • 600 ohms: 600 ohms is a standardized impedance used in telephone world. The first long telephone air lines (two wires on the poles separated from each other at some distance) used to have impedance of around 600 ohms. In practice the modern telephone cable do not have impedance of 600 ohms, but for historical reasons this imepdance is spoken often and many telephone equipment are still matched to this impedance. You can sometimes (quite rarely nowadays) hear 600 ohms matching also in audio world.

      Shielded Twisted Pair Cable is used to eliminate inductiveand capacitive coupling. Twisting cancels out inductivecoupling, while the shield eliminates capacitive coupling.Most applications for this cable are between equipment,racks and buildings. Shielding adds usually some attenuation to the cable(compared to unshielded), but usually not becausein the case of balanced transmission, the complementing signals will effectively cancel out any shield currents, so shield current losses are negligible.

      The noise pickup characteritics of twisted pair cable is determined by the following cable characteristics: number of twists per meter (generally more twists per meter gives better performance), uniform cable construction, capacitance balance (less capacitance difference to groud, the better), cable diameter (less are between wires is better) and the amount of shielding (more shielding, the better).

      It is very easy to pick up a common mode noise (the same voltage or current on both wires of a pair) signal with UTP cable. Twisted pair wiring applications typically use balanced/differential signaling forms that get rid of that common mode noise. In most typical applications a transformer at each end keeps that signal away from the rest of the electronics (this applies for example to Ethernet over UTP and telecom equipment).

      The twisting of the pairs keeps out most external differential mode signals. Signals with a wavelength shorter than the twist pitch, and localized so they don't average out over the cable length could get through. With the usual twist, that might be in GHz range, and at fairly high power to couple in enough to interfere with the signal.

      Signal and cable matching

      In many high frequency, telecommunication and high speed data applications it is necessary to match the signal source and destination impedances to cable impedance to minimize signal reflections and signal losses.


<webmaster@epanorama.net>

Back to ePanorama main page ??