transmission lines

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transmission lines
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Outline

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

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Outline

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

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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

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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)

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Types of transmission lines

• Balanced Transmission line
• 2 wire balanced line.
• both conductors carry current. But only one
  conductor carry signals.

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Types of transmission lines

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Types 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

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Types of transmission lines

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Types of transmission lines

• Baluns
• Balanced transmission lines connected to
  unbalanced transmission lines
• e.g. coaxial cable to be connected to antenna

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Metallic Transmission Lines

• Parallel conductors
• Coaxial cable

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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)

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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

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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

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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

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parallel conductors

• Unsheilded 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

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parallel conductors

• UTP Cable Type
• Category 1
• ordinary thin cables
• for voice grade telephone and low speed data
• 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)

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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

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parallel conductors

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

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parallel conductors

• Sheilded 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 sheild
  prevent the signal radiating beyond the boundaries

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parallel conductors

• STP Category
• Category 7
• 4 pairs
• surrounded by common metallic foil shield and
  shielded foil twisted pair
• 1Gbps
• Foil twisted pair
• gt 1Gbps
• shielded-foil twisted pair
• gt 1Gbps

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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

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transmission line wave propagation

• 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

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transmission line wave propagation

• rearranged equation
• the velocity via tx. line depends on the
  dielectric constant of insulating material
• ?r dielectric constant
• The velocity along tx. line varies with
  inductance and capacitance of the cable

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transmission line wave propagation

• as
• velocity x time distance
• therefore
• normalized distance to 1 meter
• Vp velocity of propagation
• vLC seconds
• L inductance
• C capacitance

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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. ?r 2.3
• Determine the velocity of propagation and the
  velocity factor

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Losses

• Conductor Losses
• conductor heating loss - I2R power loss
• the loss varies depends on the length if 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

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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 tx line
• when the difference of potential between lines
  exceeds the breakdown voltage of dielectric
  insulator

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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

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incident and reflected wave

• Resonant and non resonant tx line
• Flat _at_ non-resonant line
• Tx line with no reflected power
• Infinite length tx line
• terminated with a resistive load equal in ohmic
  value to the characteristic impedance of tx line
• Resonant tx line
• When the load is not equal to the char.
  impedance of the tx line, some incident power
  reflected back toward the source
• energy present on the line would reflected back
  and forth (oscillate) between the source and load

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incident and reflected wave

• reflection coefficient
• vector quantity that represents the ratio of
  reflected voltage to incident voltage (or
  current)
• G reflected coefficient
• Ei/Ii incident voltage/current
• Er/Ir reflected voltage/current