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.
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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.
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There are a variety of TERMINATIONS for RF lines. Each termination has an effect on the
standing waves on the line.
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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.
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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.
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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.
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Matter, Energy,
and Direct Current 
 
Alternating Current and Transformers 
 
Circuit Protection, Control, and Measurement 
 
Electrical Conductors, Wiring Techniques,
and Schematic Reading 
 
Generators and Motors 
 
Electronic Emission, Tubes, and Power Supplies 
 
SolidState Devices and Power Supplies 
 
Amplifiers 
 
WaveGeneration and WaveShaping Circuits 
 
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 
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