Coaxial cablesDecember 7, 2009
Introduction to coaxial cables
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 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. 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.
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 impednage of the coaxial cable chanes soemwhat with the frequency. Impedance changes with frequency until resitance 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.).
Often used coaxial cable types
General data on some commonly used coaxial cables compared (most data from http://dct.draka.com.sg/coaxial_cables.htm, http://www.drakausa.com/pdfsDSC/pCOAX.pdf and http://users.viawest.net/~aloomis/coaxdat.htm):
Cable type RG-6 RG-59 B/U RG-11 RG-11 A/U RG-12 A/U RG-58 C/U RG-213U RG-62 A/U Impedance (ohms) 75 75 75 75 75 50 50 93 Conductor material Bare Copper Bare Tinned Tinned Tinned Bare Copper Copper Planted Copper Copper Copper Copper Copper Planted Steel Steel Conductor strands 1 1 1 7 7 19 7 1 Conductor area (mm2) 0.95 0.58 1.63 0.40 0.40 0.18 0.75 0.64 Conductor diameter 0.028" 0.023" 0.048" 0.035" 0.089" 0.025" 21AWG 23AWG 18AWG 20AWG 13AWG 22AWG Insulation material Foam PE PE Foam PE PE PE PE Pe PE (semi-solid) Insulation diameter 4.6 mm 3.7 mm 7.24 mm 7.25 mm 9.25 mm 2.95 7.25 3.7 mm Outer conductor Aluminium Bare Aluminium Bare Base Tinned Bare Bare polyester copper polyester copper copper copper copper copper tape and wire tape and wire wire wire wire wire tin copper braid tin copper braid braid braid braid braid braid braid Coverage Foil 100% 95 % Foil 100% 95% 95% 95% 97% 95% braid 61% Braid 61% Outer sheath PVC PVC PVC PVC PE PVC PVC PVC Outside diameter 6.90 mm 6.15 mm 10.3 mm 10.3 mm 14.1 mm 4.95 mm 10.3 6.15 mm Capacitance per meter 67 pF 67 pF 57 pF 67 pF 67 pF 100 pF 100 pF Capacitance per feet 18.6 20.5 16.9 20.6 20.6 pF 28.3 pF 30.8 13.5 pF Velocity 78% 66% 78% 66% 66% 66% 66% 83% Weight (g/m) 59 56 108 140 220 38 Attenuation db/100m 50 MHz 5.3 8 3.3 4.6 4.6 6.3 100 MHz 8.5 12 4.9 7 7 16 7 10 200 MHz 10 18 7.2 10 10 23 9 13 400 MHz 12.5 24 10.5 14 14 33 14 17 500 MHz 16.2 27.5 12.1 16 16 20 900 MHz 21 39.5 17.1 24 24 28.5
NOTE: The comparision table above is for information only. There is no guarantee of correctness of data presented. When selecting cable for a certain application, check the cable data supplied by the cable manifacturer. There can be some differences on the performance and specifications of different cables from different manufacturers. For example the insulation rating of cables vary. Many PE insulated coax cables can handle several kilovots voltage, while some foam insulated coax cables cna handle only 200 volts or so.
NOTE: Several of cables mentioned above are available with foam insulation material. This changes the capacitances to somewhat lower value and gives higher velocity (typically around 0.80).
Cable type RG-6 RG-59 B/U RG-11 RG-11 A/U RG-12 A/U TELLU 13 Tasker RGB-75 Impedance (ohms) 75 75 75 75 75 75 75 Impedance accuracy +-2 ohms +-3 ohms +-2 ohms +-3% Conductor material Bare Copper Bare Tinned Tinned Bare Bare Copper Planted Copper Copper Copper Copper Copper Steel Conductor strands 1 1 1 7 7 1 10 Conductor strand(mm2) 0.95 0.58 1.63 0.40 0.40 1mm diameter 0.10mm diameter Resistance (ohm/km) 44 159 21 21 22 210 Insulation material Foam PE PE Foam PE PE PE Foam PE Insulation diameter 4.6 mm 3.7 mm 7.24 mm 7.25 mm 9.25 mm Outer conductor Aluminium Bare Aluminium Bare Base Copper Tinned polyester copper polyester copper copper foil under copper tape and wire tape and wire wire bare copper tin copper braid tin copper braid braid braid braid braid Coverage Foil 100% 95 % Foil 100% 95% 95% Foil ~95% braid 61% Braid 61% Braid 66% Resistance (ohm/km) 6.5 8.5 4 4 12 ~40 Outer sheath PVC PVC PVC PVC PE PVC (white) PVC Outside diameter 6.90 mm 6.15 mm 10.3 mm 10.3 mm 14.1 mm 7.0 mm 2.8 mm Capacitance per meter 67 pF 67 pF 57 pF 67 pF 67 pF 55 pF ~85 pF Capacitance per feet 18.6 20.5 16.9 20.6 20.6 pF Velocity 78% 66% 78% 66% 66% 80% 66% Screening factor 80 dB Typical voltage (max) 2000V 5000V 1500V Weight (g/m) 59 56 108 140 220 58 Attenuation db/100m 5 MHz 2.5 1.5 50 MHz 5.3 8 3.3 4.6 4.6 4.7 19.5 100 MHz 8.5 12 4.9 7 7 6.2 28.5 200 MHz 10 18 7.2 10 10 8.6 35.6 400 MHz 12.5 24 10.5 14 14 12.6 60.0 500 MHz 16.2 27.5 12.1 16 16 ~14 ~70 900 MHz 21 39.5 17.1 24 24 19.2 90.0 2150 MHz 31.6 3000 MHz 37.4
NOTE: The numbers with ~ mark in front of them are approximations calculated and/or measured from cables or cable data. Those numbera are not from manufacturer literature. NOTE2: Several of cables mentioned above are available in sepcial versionswith foam insulation material. This changes the capacitances to somewhat lower value and gives higher velocity (typically around 0.80).
General coaxial cable details
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 could be 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, cale capacitance is immaterial for video and digital applications.
Typical coaxial cable constructions are:
- Flexible (Braided) Coaxial Cable is by far the most common type of closed transmission line because of its flexibility. It is a coaxial cable, meaning that both the signal and the ground conductors are on the same center axis. The outer conductor is made from fine braided wire, hence the name “braided coaxial cable”. This type of cable is used in practically all applications requiring complete shielding of the center conductor. The effectiveness of the shielding depends upon the weave of the braid and the number of braid layers. One of the draw-backs of 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 short wavelength (high frequency) energy to radiate. Normally this does not present a problem; however, if a higher degree of shielding is required, semirigid coaxial cable is recommended. In some high frequency flexible coaxial cables the 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. For applications using frequencies higher than 30 GHz a miniature semirigid cable is recommended.
- 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 very similar to coaxial cable except the spacing between center conductor and shield is not carefully controlled during manufacture, resulting in non-constant impedance.
If the cable impedance is critical enough to worry about correctly choosing between 50 and 75 Ohms, then the capacitance will not matter. The reason this is so is that the cable will be either load terminated or source terminated, or both, and the distributed capacitance of the cable combines with its distributed inductance to form its impedance.
A cable with a matched termination resistance at the 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 as it affects the impedance, already accounted for. In fact, there is no electrical measurement you could make, at just the end of the cable, that could distinguish a 75 Ohm (ideal) cable with a 75 Ohm load on the far end from that same load without intervening cable. Given that the line is teminated with a proper 75 ohm load (and if it’s not, it damn well should be!), the load is 75 ohms 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 characteristic impedance of a cable if changed with frequency. If this frequency-dependent change in impedance is large enough, the cable will be impedance-matched to the load and source at some frequencies, and mismatched at others. Characteristic 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 as frequency-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.
In some communications applications 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 coaxial conductors is double the single-ended voltage, but the net current in each is the same, so the differential impedance between two coax cable used in a differential configuration would be 100 ohms. As long as the signal paths don’t interact, the differential impedance is always precisely twice the single-ended impedance of either path.
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.
- “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.
- “IEC antenna connector”: This is a very low-cost high volume 75 ohm connector used for TV and radio antenna connections around Europe.
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.