Variation of Impedance along a line

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Variation of Impedance along a line

• A matched line presents its characteristic
  impedance Zo.
• If it is not matched Impedance varies along the
  line.
• Why? At points where V is high and current
  is low, Z will be higher than the opposite
  condition of V and I.

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Impedance along a line contd.

• Phase Angle- At some points on the line it
  appears inductive with other points appearing
  capacitive while some points present themselves
  purely resistive.
• Near the load the impedance looking into the line
  is close to ZL.
• Impedance varies in a similar fashion as SWR
  does.
• At the load and at multiples of ½ wavelength back
  from the load Z ZL

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Note
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Transmission Line Losses

• (copper losses)
  
• dielectric of insulators
• open wire lines
• Why is it better to take SWR readings from the
  load?
• - ratio of incident to reflected signals is
  greater at source than at load due to line
  losses.
• Why is a transmission line able to handle more
  power with a matched line?

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Power Rating

• Factors limiting Pmax applied to a transmission
  line
• power dissipation on the line (heating,
  temperature)
• exceeding a voltage max can breakdown dielectric
• Why is a transmission line able to handle more
  power with a matched line?
• As SWR increases so does power dissipation and
  voltage.
• What impact does increasing Zo have on losses?
• 1) Voltage increases therefore dielectric
  may suffer.
• 2) Current reduces therefore
  losses are reduced.

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Tuned transmission line circuit

• Ex. A shorted section of air dielectric coaxial
  cable is to be implemented as a parallel tuned
  circuit at 1 GHz. What length will be used?

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Impedance Matching

• LUMPED inductors, capacitors and resistors
• Wound transformer
• DISTRIBUTED Transmission Line Sections
  
• a.) quarter
  wavelength transformer
• b.) half
  wavelength transformer
• c.) open stubs
• d.) shorted
  stubs

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Inductive Reactance or Capacitive susceptance
R (real) or G (conductance)
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Capacitive Reactance or Inductive susceptance
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