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Transmission lines

Transmission lines. Outline. Types of transmission lines parallel conductors coaxial cables transmission line wave propagation Losses characteristics impedance incident and reflected wave and impedance matching. transmission media. Guided

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Transmission lines

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  1. Transmission lines

  2. Outline • Types of transmission lines • parallel conductors • coaxial cables • transmission line wave propagation • Losses • characteristics impedance • incident and reflected wave and impedance matching

  3. transmission media • Guided • some form of conductor that provide conduit in which signals are contained • the conductor directs the signal • examples: copper wire, optical fiber • 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

  4. transmission media • 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)

  5. incident and reflected wave • Incident voltage • voltage that propagates from sources toward the load • Reflected wave • Voltage that propagates from the load toward the sources

  6. classifications of transmission lines • Balanced Transmission line • 2 wire balanced line. • both conductors carry current. But only one conductor carry signals.

  7. classifications of transmission lines

  8. classifications of transmission lines • 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

  9. classifications of transmission lines

  10. classifications of transmission lines • Baluns • Balanced transmission lines connected to unbalanced transmission lines • e.g.: coaxial cable to be connected to antenna

  11. Metallic Transmission Lines types • Parallel conductors • Coaxial cable

  12. parallel conductors • consists of two or more metallic conductors (copper) • separated by insulator – air, rubber etc. • Most common • Open Wire • Twin lead • Twisted Pair (UTP & STP)

  13. parallel conductors • Open Wire • two-wire parallel conductors • Closely spaces by air • Non conductive spaces • support • constant distance between conductors (2-6 inches) • Pro – simple construction • Contra – no shielding, high radiation loss, crosstalk • application – standard voice grade telephone

  14. parallel conductors • 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

  15. parallel conductors • 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

  16. parallel conductors • 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

  17. parallel conductors • UTP – Cable Type • Level 1 (Category 1) • ordinary thin cables • for voice grade telephone and low speed data • Level 2 (Category 2) • Better than cat. 1 • For token ring LAN at tx. rate of 4 Mbps • Category 3 • more stringent requirement than level 1 and 2 • more immunity than crosstalk • for token ring (16Mbps), 10Base T Ethernet (10Mbps)

  18. parallel conductors • UTP – Cable Type • Category 4 • upgrade version of cat. 3 • tighter constraints for attenuation and crosstalk • up to 100 Mbps • Category 5 • better attenuation and crosstalk characteristics • used in modern LAN. Data up to 100Mbps • Category 5e • enhanced category 5 • data speed up to 350 Mbps

  19. parallel conductors • UTP – Cable Type • Category 6 • data speed up to 550 Mbps • fabricated with closer tolerances and use more advance connectors

  20. parallel conductors • 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

  21. parallel conductors • STP – Category • Category 5e Feature individually shielded pairs of twisted wire • Category 7 • 4 pairs • surrounded by common metallic foil shield and shielded foil twisted pair • 1Gbps • Foil twisted pair Four pairs of 24-AWG copper wires encapsulated in a common metallic-foil shield with a PVC outer sheath • to minimize EMI susceptibility while maximizing EMI immunity • > 1Gbps • shielded-foil twisted pair Four pairs of 24-AWG copper wires surrounded by a common metallic-foil shield encapsulated in a braided metallic shield • offer superior EMI protection • > 1Gbps

  22. Coaxial cable • used for high data transmission • coaxial – reduce losses and isolate transmission path • basics • center conductor surrounded by insulation • shielded by foil or braid

  23. Metallic transmission lines Coaxial cable Rigid air filled solid flexible

  24. BNC Connectors To connect coaxial cable to devices, it is necessary to use coaxial connectors. The most common type of connector is the Bayone-Neill-Concelman, or BNC, connectors. Types: BNC connector, BNC barrel, BNC T, Type-N, Type-N barrel. Applications include cable TV networks, and some traditional Ethernet LANs like 10Base-2, or 10-Base5. Guided Media – Coaxial Cable

  25. Two-wire parallel transmission lineelectrical equivalent circuit

  26. Characteristic Impedance of a Line • A terminated transmission line that is matched in its characteristic impedance is called a matched line • The characteristic impedance depends upon the electrical properties of the line, according to the formula: • The characteristic impedance can be calculated by using Ohm’s Law: Zo = Eo / Io where Eo is source voltage and Io is transmission line current

  27. Characteristic Impedance • 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: • Zo = the characteristic impedance (ohms) • D = the distance between the centers • r = the radius of the conductor

  28. Coaxial cable • Z0 = the characteristic impedance (ohms) • D = the diameter of the outer conductor • d = the diameter of the inner conductor • = 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 =

  29. Wave propagation on Metallic transmission lines • 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

  30. transmission line wave propagation • rearranged equation • the velocity via tx. line depends on the dielectric constant of insulating material • ϵr = dielectric constant • The velocity along tx. line varies with inductance and capacitance of the cable

  31. transmission line wave propagation • as • velocity x time = distance • therefore • normalized distance to 1 meter • Vp = velocity of propagation • √LC = seconds • L = inductance • C = capacitance

  32. transmission line wave propagation • Question • A coaxial cable with • distributed capacitance C = 96.6 pf/H • Distributed inductance L = 241.56 nH/m • Relative dielectric constant. ϵr = 2.3 • Determine the velocity of propagation and the velocity factor

  33. Losses • Conductor Losses • conductor heating loss - I2R 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

  34. Losses • Coupling Losses • 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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