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Transmission Line Theory in Electronics and Communications

Understand the concepts of return loss, standing wave ratio, input impedance, and special cases of lossless terminated lines in transmission line theory. Explore the implications of short-circuited and open-circuited lines, as well as the matching of multiple lines and reflection and transmission between connected lines.

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Transmission Line Theory in Electronics and Communications

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  1. Transmission Line Theory 4thyear Electronics & Communications, Electrical Engineering Dept., Minia University. Lect. 5 Lecturer: Dr. Dr. Emad Emad Tammam Tammam

  2. Return loss • The "loss" due to the mismatching is called return loss (RL), and is defined (in dB) as

  3. Standing wave ratio • the voltage magnitude oscillates with position z along the line.

  4. Standing wave ratio, cont. • Standing wave ratio (SWR) is a measure of the mismatch of a line and can be defined as The distance between two successive voltage maxima (or minima) is The distance between a maximum and a minimum is

  5. Input impedance • The real power flow on the line is a constant but that the voltage amplitude, at least for a mismat- ched line, is oscillatory with position on the line. • The impedance seen looking into the line must vary with position.

  6. Special Cases of Lossless Terminated Lines • Short-circuited line Standing wave ratio is infinite. V = 0 at the load, while the current is a maximum there

  7. Voltage and current along S.C.T.L

  8. • The impedance is periodic in L, repeating for multiples of λ/2.

  9. Open-circuited line Standing wave ratio is infinite. I = 0 at the load, while the voltage is a maximum,

  10. Open-circuited line, cont. • A half-wavelength line (or any multiple of λ/2) does not alter or transform the load impedance, regardless of the characteristic impedance.

  11. Voltage and current along O.C.T.L

  12. Matching of multiple lines • Consider impedance Z0feeding a line of different characteristic impedance, Z1. If the load line is infinitely long, or if it is terminated in its own characteristic impedance, so that there are no reflections from its end, then the input impedance seen by the feed line is Z1, so that the reflection coefficient is a transmission line of characteristic

  13. Reflection and transmission between two connected lines • Not all of the incident wave is reflected; some of it is transmitted onto the second line. The voltage on the line is given as: The transmission coefficient between two points in a circuit is often expressed in dB as the insertion loss, IL,

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