Module 10  Introduction to Wave Propagation, Transmission Lines, and Antennas
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LEAKAGE CURRENT flows between the wires of a transmission line through the dielectric. The dielectric acts as a resistor. An ELECTROMAGNETIC FIELD exists along transmission line when current flows through it. 351
CHARACTERISTIC IMPEDANCE, Z_{0}, is the ratio of E to I at every point along the line. For maximum transfer of electrical power, the characteristic impedance and load impedance must be matched. The VELOCITY at which a wave travels over a given length of transmission line can be found by using the formula: A transmission line that is not terminated in its characteristic impedance is said to be FINITE. When dc is applied to an OPENENDED line, the voltage is reflected back from the open end without any change in polarity, amplitude, or shape. Current is reflected back with the same amplitude and shape but with opposite polarity. 352
When dc is applied to a SHORTCIRCUITED line, the current is reflected back with the same amplitude, and polarity. The voltage is reflected back with the same amplitude but with opposite polarity. When ac is applied to an OPENEND line, voltage is always reflected back in phase with the incident wave and current is reflected back out of phase. When ac is applied to a SHORTCIRCUITED line, voltage is reflected in opposite phase, while current is reflected in phase. 353
A NONRESONANT line has NO STANDING WAVES of current and voltage and is either infinitely long or terminated in its characteristic impedance. A RESONANT line has STANDING WAVES of current and voltage and is of finite length and is NOT terminated in its characteristic impedance. On an openended resonant line, and at all odd 1/4λ points, the voltage is minimum, the current is maximum, and the impedance is minimum. At all even 1/4λ points, the voltage is maximum, the current is minimum and the impedance is maximum. 354
There are a variety of TERMINATIONS for rf lines. Each termination has an effect on the standing waves on the line. 355
A transmission line can be terminated in its characteristic impedance as an open or shortcircuit, or in capacitance or inductance. Whenever the termination on a transmission line is NOT EQUAL TO Z_{0}, there are reflections on the line. The amount of voltage reflected may be found by using the equation: When the termination on a transmission line EQUALS Z_{0}, there is NO reflected voltage. The measurement of standing waves on a transmission line yields information about operating conditions. If there are NO standing waves, the termination for that line is correct and maximum power transfer takes place. The STANDING WAVE RATIO is the measurement of maximum voltage (current) to minimum voltage (current) on a transmission line and measures the perfection of the termination of the line. A ratio of 1:1 describes a line terminated in its characteristic impedance. 356
ANSWERS TO QUESTIONS Q1. THROUGH Q30. A1. Transmission line. A2. Input end, generator end, transmitter end, sending end, and source. A3. Output end, receiving end, load end and sink. A4. Parallel twowire, twisted pair, shielded pair, coaxial line and waveguide. A5. Power lines, rural telephone lines, and telegraph lines. A6. High radiation losses and noise pickup. A7. Twin lead. A8. The conductors are balanced to ground. A9. Air coaxial (rigid) and solid coaxial (flexible). A10. The ability to minimize radiation losses. A11. Expensive to construct, must be kept dry, and high frequency losses limit the practical length of the line. A12. Cylindrical and rectangular. A13. Copper, dielectric, and radiation. A14. Copper losses. A15. Dielectric losses. A16. PHWHUV A17. (1) Type of line used, (2) dielectric in the line, and (3) length of line. A18. Inductance is expressed in microhenrys per unit length, capacitance is expressed in picofarads per unit length, and resistance is expressed in ohms per unit length. A19. The small amount of current that flows through the dielectric between two wires of a transmission line and is expressed in micromhos per unit length. A20. When the characteristic impedance of the transmission line and the load impedance are equal. A21. Z_{0} and it is the ratio of E to I at every point along the line. A22. Between 50 and 600 ohms. A23. Incident waves from generator to load. Reflected waves from load back to generator. A24. 2 and 6 have zero resultant wave and they indicate that the incident and reflected waves are 180 degrees out of phase at all parts. A25. Onefourth the distance from each end of the line. 357
A26. The load impedance of such a line is equal to Z0. A27. Even quarterwave points (1/2λ, 1λ, 3/2λ, etc.). A28. At 1/2 wavelength from the end and at every 1/2 wavelength along the line. A29. Power standingwave ratio (pswr). A30. The existence of voltage variations on a line. 358
NEETS Table of Contents
 Introduction to Matter, Energy,
and Direct Current
 Introduction to Alternating Current and Transformers
 Introduction to Circuit Protection,
Control, and Measurement
 Introduction to Electrical Conductors, Wiring
Techniques, and Schematic Reading
 Introduction to Generators and Motors
 Introduction to Electronic Emission, Tubes,
and Power Supplies
 Introduction to SolidState Devices and
Power Supplies
 Introduction to Amplifiers
 Introduction to WaveGeneration and WaveShaping
Circuits
 Introduction to Wave Propagation, Transmission
Lines, and Antennas
 Microwave Principles
 Modulation Principles
 Introduction to Number Systems and Logic Circuits
 Introduction to Microelectronics
 Principles of Synchros, Servos, and Gyros
 Introduction to Test Equipment
 RadioFrequency Communications Principles
 Radar Principles
 The Technician's Handbook, Master Glossary
 Test Methods and Practices
 Introduction to Digital Computers
 Magnetic Recording
 Introduction to Fiber Optics
