Characteristic Impedance Contnd.

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# Characteristic Impedance Contnd. - PowerPoint PPT Presentation

Characteristic Impedance Contnd. Air Dielectric Parallel Line Coaxial Cable. Where: D = spacings between centres of the conductors r = conductor radius. Velocity Factor. The speed at which an energy is propagated along a transmission line is always less than the speed of light.

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Presentation Transcript
Characteristic Impedance Contnd.
• Air Dielectric Parallel Line
• Coaxial Cable

Where: D = spacings between centres of the conductors

Velocity Factor
• The speed at which an energy is propagated along a transmission line is always less than the speed of light.
• Almost entirely dependant upon the dielectric constant
• Propagation velocity of signal can vary from 66% (coax with polyethylene dielectric) to 95%(air).
Response of Line
• CONDITIONS
• Step Impulses
• Assume lossless line and infinite length with Zo equal to characteristic impedance of the line
• Discuss:

-Reflections along a line of finite length that is:

a.) Open at point of termination (end of line)

b.) Shorted at point of termination

c.) Matched load at point of termination

Open Circuited Line
• Switch is closed and followed by a surge down line.
• How much of the source voltage appears across the source? (V/2)
• What is the state of voltage and current at the end of the line?
• For what time frame do the initial conditions exist? (2T)
• What is the relative direction of incident and reflected current?(opposite)
Short Circuit Line
• What is the state of voltage at the source prior to 2T? (V/2)
• What is the state of voltage and current when the surge reaches the load? (V=0 and I depends on system characteristics)
• What is the direction of incident and reflected current? (same)
Pulse Input To Transmission Line
• With a matched line the load absorbs energy and there is no reflection
• Open circuit has positive reflections
• Short Circuit has negative reflections
• REFLECTION COEFFICIENT(Gamma)

- Open circuit line > gamma = 1

- Matched line > gamma = 0

- Short circuit line > gamma = -1

Traveling Waves Along A Line
• Assume a matched line and a sinusoidal signal source.
• Traveling wave
• After initial conditions a steady state situation exists.
• Signal will appear the same as the source at any point on the line except for time delay.
• Time delay causes a phase shift ( one period = 360 degrees)
Standing Waves
• Assume a transmission line with an open termination, a reasonably long line and a sinusoidal source
• After initial reflection the instantaneous values of incident and reflected voltage add algebraically to give a total voltage
• Resultant amplitude will vary greatly due to constructive and destructive interference between incident and reflected waves
Standing Waves contnd.
• Reminder: A sine wave applied to a matched line develops an identical sine wave except for phase.
• If the line is unmatched there will be a reflected wave.
• The interaction of the two travelling waves (vr and vi) result in a standing wave.
• SWR = Vmax/Vmin
Sample question
• What length of RG-8/U (vf = .66) would be required to obtain a 30 degree phase shift at 100 Mhz?