Coaxial Cables and Applications

Coaxial Cables and ApplicationsContributed By:Martin J. Van Der BurgtSenior Product Engineering Project Manager,Belden Electronics DivisionOverview: Coaxial cables are perhaps themost common, basic, and easy to understandcables of common transmission line designs.Basically, they are used to transmit electricalenergy, or signals, from one location to another:to connect a source to a load, such as atransmitter to an antenna. This is accomplishedwith a transverse electrometic (TEM) wave fielddistribution propagation within the transmissionline, or coaxial cable, and is governed bytransmission line theory.Coaxial Cable (coax) is typically identified orclassified according to its impedance or RG-type.For example, a 50-ohm coax or an RG-8 typecoax. This chapter will examine flexible coaxcable in more detail and explain the basicconstruction, electrical, and applicationcharacteristics.A coax cable consists of two conductorsseparated by a dielectric material. The centerconductor and the outer conductor, or shield, areconfigured in such a way that they formconcentric cylinders with a common axis. Hencethe term and name co-axial.copper clad materials may be used in higherfrequency applications ( > 50 MHz) to improvetensile strength and reduce weight and cost.(Skin-effect is the result of higher frequencysignals propagating along the outermost surface,or skin, of the conductor.)The insulation, or dielectric material, is used toprovide separation between the conductors. It isdesirable that the material has stable electricalcharacteristics (dielectric constant anddissipation factor) across a broad frequencyrange. The most common materials used arepolyethylene (PE), polypropylene (PP),fluorinated ethylene propylene (FEP), andpolytetrafluoroethylene (PTFE). PE and PP aredesirable in lower cost, power, and temperaturerange applications (PE is 85C, PP is 105C) 1 . FEPand PTFE are for higher power and temperaturerange applications (200C) 1 , and offer someadditional environmental resistance properties.However, they are also much higher in cost. Thematerials may be used in their natural form(solid), or injected with air bubbles (foam orcellular) to improve the dielectric constant andelectrical properties of the material and cable.Some designs also incorporate a mono-filamentor disc designs sometimes referred to as air gap.1 Temperature range of the cable is often limited by thechoice of jacket material.Construction: The center conductor may bemade of various materials and constructions.Most common constructions are solid or sevenstrandconductors. Solid conductors are used inpermanent, infrequently handled or low flexapplications and stranded conductors are used inflexible cable applications. Common materialsinclude copper, tinned or silver plated copper,copper clad steel and copper clad aluminum.Plated copper is used to aid in solderability ofconnectors or to minimize corrosion effects.Because of a phenomena known as skin-effect,The outer conductor is typically made from anumber of smaller aluminum or copperconductors combined together. These conductorsare woven together to form a braid around thedielectric core. For higher frequencyapplications, a second braid or aluminum foiltapes are often added to improve attenuation andshield effectiveness.The jacket material serves as a protectivecovering from the environment and may alsoserve to add in the overall flame retardantproperties of the cable. Typical materials includepolyvinyl chloride (PVC), PE, FEP, andpolyvinylidene fluoride (PVDF).2003 Page 1 of 5 Belden Electronics Division

permanent install and flex radius is for flexing.Flex radius should not be misunderstood to meanthat the cable is designed for continuous flexing.Pulling Tension is the maximum load bearingweight for the cable. It is typically a safe valuewell below the break strength of the cable.Staying below this maximum assures that theconductor will not be stretched resulting inelectrical performance problems in the cable.Applications: One of the broadest uses ofcoaxial cable is for video distribution. FromCATV signals around the neighborhood toprecision digital signals in a post-productionstudio, these signals are routed on 75 ohmcoaxial cable. Broadband CATV (or MATV)signals typically use a series 59 (RG-59) orseries 6 (RG-6) type coaxial cable, or drop cable.These cables use copper covered steelconductors and aluminum foil/braid combinationshields (outer conductors). They are designed foruse above 50 MHz, with high strength, lowweight and low cost the primary factors. Thesecables are all typically designed to a SCTEstandard and are a commodity-type item.Electrical performance is good and clearlydefined in the industry standards. Cables used inoutdoor environments typically utilize apolyethylene jacket material. The distributiontrunk cable is series 11 (RG-11) type or larger –typically “hard line” cable utilizing a corrugatedcopper outer conductor. These cables aredesigned for minimum attenuation and goodpower handling capability.Another type of 75 ohm video cable is standardvideo cable. These cables are based uponstandard RG-type designs, utilizing coppercovered steel, copper, or tinned copperconductors and bare or tinned copper braid onlyshields. This cable is used primarily for CCTV,security, surveillance, and other “non-critical”video applications – video for consumption thatwill not be manipulated, saved, or used for otherpurposes. These cables are not typicallyassociated with an industry standard, so there ismuch variation and differentiation amongavailable designs. Typical cables specifynominal electrical values only.The high end of video cables is precision videocable. These cables are loosely based upon thestandard RG-type designs. They have solid barecopper conductors, foam dielectric, combinationfoil and 95% tinned copper braid shields. Theyare designed for maximum bandwidth, minimumreturn loss, and minimum attenuation loss. Thecables have very tight requirements on allelectrical attributes and are used for both analogand digital video in broadcast, post-production,and other critical video applications.There are also many specialty types of videocables, such as S-VHS, RGB, DBS, and Triax toname a few.Cables used for two-way communication, RFand microwave transmission, data transmissionand instrumentation/control are typically 50 ohmcoaxial cables. These cables are based uponMIL-Spec designs and are most often referred toby their RG type number. The cables fall into afew categories: Transmission and computercables that are based upon RG type designs, butnot per the current MIL-Spec; MIL-C-17 QPLcables that are in compliance with the currentMIL-Spec; and low-loss cables based looselyupon RG type designs. The RG type cables arethe most used products since the majority ofcable applications do not require QPL cables.Furthermore, most manufacturers have a limitedavailability of QPL cables for this reason. Themajor differences between the cables aretypically the type of PVC jacket material usedand less stringent electrical and physicalrequirements. For wireless and antennaapplications, the low-loss cables are gainingpopularity because of their size, weight, cost, andperformance advantages over the traditionaldesigns. These cables typically utilize a foameddielectric material and combination foil/braidshields.Other impedance cables are used for datatransmission and instrumentation/control. Theseare typically 93 ohm or 125 ohm and are not ascommon as they once were. As with thetransmission and computer cables, the majorityof cables used are RG type cables with minimaluse of QPL product.Specialty coaxial cables include low-noise andmusical interconnects cables. These arespecially designed to minimize piezoelectric andtriboelectric noise in cables and are not for RFuse. The cables may be loosely based upon RGtype designs, but utilize semi-conductivematerial layers within the cable.2003 Page 4 of 5 Belden Electronics Division

Conclusion:Hopefully this chapter has given you a briefoverview of coaxial cable and a better insight tothe relevant design properties. When choosing acable for your application, work backwards fromthe end-need requirements. What is the specificapplication? Do you know what impedance isrequired? What is the frequency of interest?What is the loss budget (how much cableattenuation loss is allowed)? How long is thecable run length? Indoor or outdoor? Are thereagency requirements (UL, NEC, AWM, MIL-Spec, etc.)? What connector type will be used?These few questions should help you easilyidentify an appropriate coaxial cable.Martin J. Van Der BurgtMarty is a Senior Product Engineering ProjectManager for Belden Electronics Division. Hisexperience encompasses project managementand product development positions. Hecurrently has responsibility for all design anddevelopment efforts, and manages the day-to-dayactivity of product engineers, for Belden’sentertainment, service provider, and industrialproduct market areas, and has recently filed anapplication for his first patent. Marty receivedhis Bachelor of Science Degree in ElectricalEngineering (BSEE) from Marquette University,Milwaukee, WI in 1992 and has been withBelden since that time. He is a member ofseveral professional organizations and standardsbodies, and has had several articles onAudio/Video and RF topics published in trademagazines and journals. Most recently, coauthoredtutorials on “High-Definition Cablingand Return Loss” in the January 2001 SMPTEJournal and “Impedance” in the 2002 NABBroadcast Engineering Conference Proceedings.Marty also holds a FCC Amateur Extra Classamateur radio license. He and his wife Beth livein Richmond, IN.Marty Van Der Burgt2003 Page 5 of 5 Belden Electronics Division