Transmission Lines

1. 12 Transmission Lines

2. Types of Transmission Lines • Two-Wire Open Line • Transmission line between antenna and transmitter or antenna and receiver. • Simple construction. • Twisted Pair • Two insulated wires twisted to form flexible line without use of spacers.

3. Types of Transmission Lines • Unshielded Twisted Pair (UTP) • Used for computer networking; CAT6 and CAT5e. • See Table 12-1: T568A/T568B Wiring

4. Table 12-1 T568A/T568B Wiring

5. Types of Transmission Lines • Unshielded Twisted Pair (UTP) • Used for computer networking; CAT6 and CAT5e. • See Table 12-2: Different Categories for Twisted-Pair Cable

6. Table 12-2 Different Categories for Twisted-Pair Cabl3

7. Types of Transmission Lines • Shielded Pair • Parallel conductors separated from each other, and surrounded by solid dielectric.

8. Types of Transmission Lines • Coaxial Lines • Rigid or air; flexible or solid. • Balanced/Unbalanced Lines • Unbalanced • Amplitude of electrical signal by center conductor in coaxial line measured with respect to grounded outer conductor. • Balanced • Same current flows in each wire but 180° out of phase.

9. Electrical Characteristics of Transmission Lines • Two-Wire Transmission Line • Electrical characteristics • Construction of line. • Characteristic Impedance • Impedance measured at any point on line would be the same.

10. Electrical Characteristics of Transmission Lines • Transmission Line Losses • Copper • Dielectric • Radiation or induction

11. Propagation of DCVoltage Down a Line • Physical Explanation of Propagation • To understand characteristics of transmission line with ac voltage applied, infinitely long transmission line analyzed with dc voltage applied. • Velocity of Propagation • Current moving down the line, its electric and magnetic fields are propagated down the line.

12. Propagation of DCVoltage Down a Line • Delay Line • Delay signal by some specific amount of time. • Wavelength • Distance traveled by wave during a period of one cycle.

13. Nonresonant Line • Traveling DC Waves • Nonresonant line • Line of infinite length or as one terminated with resistive load equal in ohmic value to characteristic impedance of line. • Traveling waves • Voltage and current waves; move in phase with one another from source to load.

14. Nonresonant Line • Traveling AC Waves • Little difference between charging of line when ac voltage applied to it and when dc voltage applied.

15. Resonant Transmission • DC Applied to an Open-Circuited Line • Resonant line • Transmission line terminated with impedance not equal to characteristic impedance. • Because impedances are equal, applied voltage divided equally between source and line.

16. Resonant Transmission • Incident and Reflected Waves • Voltage on open-circuited wire equal to source voltage, and current is zero.

17. Resonant Transmission • DC Applied to a Short-Circuited Line • Voltage reflection from open circuit is in phase, while from short circuit it is out of phase. • Current reflection from open circuit is out of phase, while from short circuit it is in phase.

18. Resonant Transmission • Standing Waves • Open Line • Mismatch • Incident and reflected waves interact. • Standing wave • Remains in one position, varying only in amplitude.

19. Resonant Transmission • Standing Waves • Shorted Line • Out-of-phase reflection that occurs for current on open line and voltage on shorted line. • Impedance • Voltage divided by current.

20. Resonant Transmission • Quarter-Wavelength Sections • Phase inversion of voltage and current every quarter wavelength. • Cavity filter or selective cavity extends idea of quarter-wavelength coaxial stub filter to high-power operation.

21. Standing Wave Ratio • Standing Wave • Instantaneous vector addition of incident-wave amplitude with that of reflected wave as result of impedance mismatch between transmission line and load. • Reflections • Occur when impedance mismatch.

22. Standing Wave Ratio • Degree of Mismatch • Reflection coefficient, voltage standing-wave ratio, return loss.

23. Standing Wave Ratio • Voltage standing wave ratio (VSWR) • Ratio of maximum voltage to minimum of standing wave on a line. • Standing wave ratio (SWR) • Equal to ratio of maximum current to minimum current.

24. Standing Wave Ratio • Effect of Mismatch • Flat line • Perfect condition of no reflection when load purely resistive and equal to Z0. • Higher the VSWR, the greater is mismatch on line.

25. Standing Wave Ratio • Quarter-Wavelength Transformer • Not physically a transformer; offers property of impedance transformation. • Electrical Length • Line can be miles long physically and electrically short at low frequencies.

26. The Smith Chart • Transmission Line Impedance • Impedance • Constantly changing along line and equal to ratio of voltage to current at given point. • Smith chart presents solution to impedance-matching problems posed by complex sources and complex loads.

27. The Smith Chart • Transmission Line Impedance • Smith chart impedance-matching tool for transmission lines. • Graphical Smith chart used to represent network analyzer solutions.

28. The Smith Chart • Smith Chart • Normalizing • Dividing all impedances by characteristic impedance of the line. • Reciprocal of impedance defined as admittance and reciprocal of reactance is called susceptance.

29. The Smith Chart • Smith Chart • Greatest utility as impedance-matching calculator. • Allows for simple conversion of impedance to admittance, and vice versa.

30. The Smith Chart • Smith Chart • Spiral • True standing wave representation on Smith chart. • Many calculations with transmission lines pertain to matching load to line and keeping VSWR as low as possible. • Use of short-circuited stubs prevalent in matching problems.

31. Transmission Line Applications • Discrete Circuit Simulation • Transmission line sections: used to simulate inductance, capacitance, LC resonance. • Baluns • Unbalanced-to-balanced transformer.

32. Transmission Line Applications • Transmission Lines as Filters • Quarter-wave section of transmission line used as efficient filter or suppressor of even harmonics.

33. Transmission Line Applications • Slotted Lines • Section of coaxial line with lengthwise slot cut in outer conductor; pickup probe inserted into slot. • VSWR, generator frequency, unknown load impedance.

34. Transmission Line Applications • Time-Domain Reflectometry • Short-duration pulse transmitted into a line. • Monitored with oscilloscope.

35. Impedance Matching and Network Analysis • Network analyzer • Test equipment to characterize linear impedance characteristics of devices under test (DUTs). • Scalar network analyzer • Measures magnitude of DUT impedance.

36. Impedance Matching and Network Analysis • Vector network analyzer (VNA) • Determines impedance magnitude and phase characteristics of DUTs. • Scattering or S-parameters • Linear transmission and reflection characteristics of signals applied to DUTs.

37. Impedance Matching and Network Analysis • Scattering matrix • Describes all voltages incident to and reflected from all ports. • Impedance analysis not confined to microwave systems.

38. Impedance Matching and Network Analysis • Vector network analyzer • System of three receivers and two-port “test set” that separates forward and reflected components of applied signals through directional couplers.