TRANSMISSION LINES

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TRANSMISSION LINES

• Channel
• Two conductors separated by some dielectric
• TYPES
• Coaxial (unbalanced)
• Parallel Line (balanced)
• View Samples

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Various Roles

• Path for signal
• Phase Delays
• Time Delays
• Impedance Matching
• Filter
• Tuned Circuit

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Electrical Model of Transmission Line

• Distributed vs. lumped components
• As frequency increases resistance______.
• Magnetic field causes current to flow closer to
  surface at higher frequencies.
• Therefore cross sectional area reduces and
  resistance _______.
• Skin effect

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Electrical Model contind.

• Conductance of dielectric small at low
  frequencies
• Dielectric becomes lossy as frequency increases
• Inductance has minimal effect at d.c. and low
  frequencies
• As frequency increases inductive reactance_____
• What happens to capacitive reactance?
• - decreases as frequency increases.

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Electrical Model contnd.

• At low frequencies Xc is high therefore the shunt
  capacitance has minimal effect.
• As frequency increases parallel Xc decreases
• SUMMARY
• -As frequency increases inductance and
  capacitance will have greater effect
• -At higher frequencies resistive elements can be
  removed and considered lossless
• What is dominant at low and high frequencies?
• R and G at low and XL and XC at high.

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Characteristic Impedance

• Ratio of Voltage and Current along a line
• OPEN CIRCUIT
• Line has a discharge capacitance at start up
• therefore current could be high
• Inductance in line will limit current
• Therefore a voltage to current ratio develops

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