Transmission lines Outline l Types of transmission lines
- Slides: 34
Transmission lines
Outline l Types of transmission lines l parallel conductors l coaxial cables l transmission line wave propagation l Losses l characteristics impedance l incident and reflected wave and impedance matching
transmission media l Guided ¡ some form of conductor that provide conduit in which signals are contained ¡ the conductor directs the signal ¡ examples: copper wire, optical fiber l Unguided ¡ wireless systems – without physical conductor ¡ signals are radiated through air or vacuum ¡ direction – depends on which direction the signal is emitted ¡ examples: air, free space
transmission media l Cable transmission media guided transmission medium and can be any physical facility used to propagate EM signals between two locations ¡ e. g. : metallic cables (open wire, twisted pair), optical cables (plastic, glass core) ¡
incident and reflected wave Incident voltage ¡ voltage that propagates from sources toward the load l Reflected wave ¡ Voltage that propagates from the load toward the sources l
classifications of transmission lines l Balanced Transmission line 2 wire balanced line. ¡ both conductors carry current. But only one conductor carry signals. ¡
classifications of transmission lines l
classifications of transmission lines l Unbalanced Transmission line ¡ ¡ One wire is at ground potential the other wire is at signal potential advantages – only one wire for each signal disadvantages –reduced immunity to noises
classifications of transmission lines l
classifications of transmission lines l Baluns Balanced transmission lines connected to unbalanced transmission lines ¡ e. g. : coaxial cable to be connected to antenna ¡
Metallic Transmission Lines types Parallel conductors l Coaxial cable l
parallel conductors consists of two or more metallic conductors (copper) l separated by insulator – air, rubber etc. l Most common l ¡ ¡ ¡ Open Wire Twin lead Twisted Pair (UTP & STP)
parallel conductors l Open Wire two-wire parallel conductors ¡ Closely spaces by air ¡ Non conductive spaces ¡ l l ¡ ¡ ¡ support constant distance between conductors (2 -6 inches) Pro – simple construction Contra – no shielding, high radiation loss, crosstalk application – standard voice grade telephone
parallel conductors l Twin lead spacers between the two conductor are replaced with continuous dielectric – uniform spacing ¡ application – to connect TV to rooftop antennas ¡ material used for dielectric – Teflon, polyethylene ¡
parallel conductors l Twisted pair formed by twisting two insulated conductors around each other ¡ Neighboring pairs is twisted each other to reduce EMI and RFI from external sources ¡ reduce crosstalk between cable pairs ¡
parallel conductors l Unshielded Twisted Pair two copper wire encapsulated in PVC ¡ twisted to reduce crosstalk and interference ¡ improve the bandwidth significantly ¡ Used for telephone systems and local area network ¡
parallel conductors l UTP – Cable Type ¡ Level 1 (Category 1) l l ¡ Level 2 (Category 2) l l ¡ ordinary thin cables for voice grade telephone and low speed data Better than cat. 1 For token ring LAN at tx. rate of 4 Mbps Category 3 l l l more stringent requirement than level 1 and 2 more immunity than crosstalk for token ring (16 Mbps), 10 Base T Ethernet (10 Mbps)
parallel conductors l UTP – Cable Type ¡ Category 4 l l l ¡ Category 5 l l ¡ upgrade version of cat. 3 tighter constraints for attenuation and crosstalk up to 100 Mbps better attenuation and crosstalk characteristics used in modern LAN. Data up to 100 Mbps Category 5 e l l enhanced category 5 data speed up to 350 Mbps
parallel conductors l UTP – Cable Type ¡ Category 6 data speed up to 550 Mbps l fabricated with closer tolerances and use more advance connectors l
parallel conductors l Shielded Twisted Pair (STP) wires and dielectric are enclosed in a conductive metal sleeve called foil or mesh called braid ¡ the sleeve connected to ground acts as shield – prevent the signal radiating beyond the boundaries ¡
parallel conductors l STP – Category ¡ Category 5 e Feature individually shielded pairs of twisted wire ¡ Category 7 l 4 pairs l surrounded by common metallic foil shield and shielded foil twisted pair l 1 Gbps ¡ Foil twisted pair Four pairs of 24 -AWG copper wires encapsulated in a common metallic-foil shield with a PVC outer sheath l to minimize EMI susceptibility while maximizing EMI immunity l > 1 Gbps ¡ shielded-foil twisted pair Four pairs of 24 -AWG copper wires surrounded by a common metallic-foil shield encapsulated in a braided metallic shield l offer superior EMI protection l > 1 Gbps
Coaxial cable used for high data transmission l coaxial – reduce losses and isolate transmission path l basics l ¡ ¡ center conductor surrounded by insulation shielded by foil or braid
Metallic transmission lines Coaxial cable Rigid air filled solid flexible
Guided Media – Coaxial Cable BNC Connectors l To connect coaxial cable to devices, it is necessary to use coaxial connectors. l The most common type of connector is the Bayone-Neill-Concelman, or BNC, connectors. l Types: BNC connector, BNC barrel, BNC T, Type-N barrel. l Applications include cable TV networks, and some traditional Ethernet LANs like 10 Base-2, or 10 -Base 5.
Two-wire parallel transmission line electrical equivalent circuit
Characteristic Impedance of a Line A terminated transmission line that is matched in its characteristic impedance is called a matched line l The characteristic impedance depends upon the electrical properties of the line, according to the formula: q The characteristic impedance can be calculated by using Ohm’s Law: Zo = Eo / I o l where Eo is source voltage and Io is transmission line current
Characteristic Impedance l The characteristic impedance for any type of transmission line can be calculated by calculating the inductance and impedance per unit length ¡ For a parallel line with an air the dielectric impedance is: l l l Zo = the characteristic impedance (ohms) D = the distance between the centers r = the radius of the conductor
Coaxial cable Z 0 = the characteristic impedance (ohms) D = the diameter of the outer conductor d = the diameter of the inner conductor e = the permittivity of the material r = the relative permittivity or dielectric constant of the medium 0 = the permeability of free space For extremely high frequencies, characteristic impedance can be given by Zo =
Wave propagation on Metallic transmission lines l Velocity factor ¡ The ratio of the actual velocity of propagation of EM wave through a given medium to the velocity of propagation through vacuum ¡ ¡ Vf = velocity factor Vp = actual velocity of propagation c = velocity of propagation in vacuum
transmission line wave propagation rearranged equation l the velocity via tx. line depends on the dielectric constant of insulating material l ¡ l ϵr = dielectric constant The velocity along tx. line varies with inductance and capacitance of the cable
transmission line wave propagation l as ¡ velocity x time = distance ¡ therefore ¡ normalized distance to 1 meter l l Vp = velocity of propagation √LC = seconds L = inductance C = capacitance
transmission line wave propagation l Question ¡ A coaxial cable with l l l ¡ distributed capacitance C = 96. 6 pf/H Distributed inductance L = 241. 56 n. H/m Relative dielectric constant. ϵr = 2. 3 Determine the velocity of propagation and the velocity factor
Losses l l l Conductor Losses ¡ conductor heating loss - I 2 R power loss ¡ the loss varies depends on the length of the tx. line Dielectric Heating Losses ¡ difference of potential between two conductors of a metallic tx lines ¡ Negligible for air dielectric ¡ increase with frequency for solid core tx line Radiation Losses ¡ the energy of electrostatic and EM field radiated from the wire and transfer to the nearby conductive material ¡ Reduced by shielding the cable
Losses l Coupling Losses ¡ ¡ ¡ l whenever connection is made between two tx line discontinuities due to mechanical connection where dissimilar material meets tend to heat up, radiate energy and dissipate power Corona ¡ ¡ luminous discharge that occurs between two conductors of transmission line when the difference of potential between lines exceeds the breakdown voltage of dielectric insulator
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