TRUE/FALSE
1. A transmission line is a metallic cable.
ANS: T
2. Coaxial cables are referred to as "unbalanced".
ANS: T
3. "Balanced" means that both conductors are the same size.
ANS: F
4. Twisted-pair telephone wire is a kind of transmission line.
ANS: T
5. Cable resistance does not depend on frequency.
ANS: F
6. Losses occur in the plastic dielectric of a transmission line.
ANS: T
7. Basically, a transmission line looks like an inductor in series with a small resistor.
ANS: F
8. Distributed parameters (inductance and capacitance) are characteristic of transmission lines.
ANS: T
9. Transmission lines are often considered to be "lossless" at higher frequencies.
ANS: T
10. The characteristic impedance of a cable depends mostly on the resistance of the wires.
ANS: F
11. For coaxial cables, there are only a few standard values in common use.
ANS: T
12. The value of the load at the end of a transmission line must be equal to or less than Z0.
ANS: F
13. RF signals travel slower on a transmission line than they would through free space.
ANS: T
14. Energy can "reflect" from a load at the end of a cable and travel back to the source.
ANS: T
15. A "mismatched" line will cause reflections.
ANS: T
16. The typical load on a cable, such as an antenna, has the same value at any frequency.
ANS: F
17. A mismatched line between a transmitter and an antenna could actually damage the transmitter.
ANS: T
18. A matched transmission line will exhibit "standing" waves.
ANS: F
19. The optimum value for SWR is one.
ANS: T
20. "Ghost" images on a cable TV indicate an impedance mismatch.
ANS: T
21. If a cable is shorter than 1/16 of the signal's wavelength, it does not really behave as a transmission line.
ANS: T
22. Shorted stubs radiate more energy than open stubs do.
ANS: F
23. It is often better to measure SWR at the load rather than the source.
ANS: T
24. The only limit to the power that a transmission line can carry is the heat from I2R losses.
ANS: F
25. A length of transmission line can be used to match impedances at very high frequencies.
ANS: T
26. A circular graph called a "Jones" chart is commonly used to analyze transmission lines.
ANS: F
27. There is no practical way to connect a balanced line to an unbalanced line.
ANS: F
28. A 1/4 wavelength transmission line can be used as a transformer.
ANS: T
29. When used, stubs are usually inserted in series with a transmission line.
ANS: F
30. Slotted-lines are only useful at lower frequencies.
ANS: F
MULTIPLE CHOICE
1. SWR stands for:
| a. | Shorted Wave Radiation | c. | Shorted Wire Region |
| b. | Sine Wave Response | d. | none of the above |
ANS: D
2. TDR stands for:
| a. | Total Distance of Reflection | c. | Time-Domain Response |
| b. | Time-Domain Reflectometer | d. | Transmission Delay Ratio |
ANS: B
3. An example of an unbalanced line is:
| a. | a coaxial cable | c. | an open-wire-line cable |
| b. | 300-ohm twin-lead TV cable | d. | all of the above |
ANS: A
4. When analyzing a transmission line, its inductance and capacitance are considered to be:
| a. | lumped | c. | equal reactances |
| b. | distributed | d. | ideal elements |
ANS: B
5. As frequency increases, the resistance of a wire:
| a. | increases | c. | stays the same |
| b. | decreases | d. | changes periodically |
ANS: A
6. The effect of frequency on the resistance of a wire is called:
| a. | I2R loss | c. | the skin effect |
| b. | the Ohmic effect | d. | there is no such effect |
ANS: C
7. As frequency increases, the loss in a cable's dielectric:
| a. | increases | c. | stays the same |
| b. | decreases | d. | there is no loss in a dielectric |
ANS: A
8. The characteristic impedance of a cable depends on:
| a. | the resistance per foot of the wire used |
| b. | the resistance per foot and the inductance per foot |
| c. | the resistance per foot and the capacitance per foot |
| d. | the inductance per foot and the capacitance per foot |
ANS: D
9. For best matching, the load on a cable should be:
| a. | lower than Z0 | c. | equal to Z0 |
| b. | higher than Z0 | d. | 50 ohms |
ANS: C
10. The characteristic impedance of a cable:
| a. | increases with length | c. | increases with voltage |
| b. | increases with frequency | d. | none of the above |
ANS: D
11. The velocity factor of a cable depends mostly on:
| a. | the wire resistance | c. | the inductance per foot |
| b. | the dielectric constant | d. | all of the above |
ANS: B
12. A positive voltage pulse sent down a transmission line terminated in a short-circuit:
| a. | would reflect as a positive pulse |
| b. | would reflect as a negative pulse |
| c. | would reflect as a positive pulse followed by a negative pulse |
| d. | would not reflect at all |
ANS: B
13. A positive voltage pulse sent down a transmission line terminated with its characteristic impedance:
| a. | would reflect as a positive pulse |
| b. | would reflect as a negative pulse |
| c. | would reflect as a positive pulse followed by a negative pulse |
| d. | would not reflect at all |
ANS: D
14. A positive voltage-pulse sent down a transmission line terminated in an open-circuit:
| a. | would reflect as a positive pulse |
| b. | would reflect as a negative pulse |
| c. | would reflect as a positive pulse followed by a negative pulse |
| d. | would not reflect at all |
ANS: A
15. The optimum value for SWR is:
| a. | zero | c. | as large as possible |
| b. | one | d. | there is no optimum value |
ANS: B
16. A non-optimum value for SWR will cause:
| a. | standing waves | c. | higher voltage peaks on cable |
| b. | loss of power to load | d. | all of the above |
ANS: D
17. VSWR stands for:
| a. | variable SWR | c. | voltage SWR |
| b. | vacuum SWR | d. | none of the above |
ANS: C
18. The impedance "looking into" a matched line:
| a. | is infinite | c. | is the characteristic impedance |
| b. | is zero | d. | 50 ohms |
ANS: C
19. A Smith Chart is used to calculate:
| a. | transmission line impedances | c. | optimum length of a transmission line |
| b. | propagation velocity | d. | transmission line losses |
ANS: A
20. Compared to a 300-ohm line, the loss of a 50-ohm cable carrying the same power:
| a. | would be less | c. | would be the same |
| b. | would be more | d. | cannot be compared |
ANS: B
21. A balanced load can be connected to an unbalanced cable:
| a. | directly | c. | by using a "balun" |
| b. | by using a filter | d. | cannot be connected |
ANS: C
22. On a Smith Chart, you "normalize" the impedance by:
| a. | assuming it to be zero | c. | multiplying it by 2p |
| b. | dividing it by 2p | d. | dividing it by Z0 |
ANS: D
23. The radius of the circle you draw on a Smith Chart represents:
| a. | the voltage | c. | the impedance |
| b. | the current | d. | none of the above |
ANS: D
24. The center of the Smith Chart always represents:
| a. | zero | c. | the characteristic impedance |
| b. | one | d. | none of the above |
ANS: C
25. A TDR is commonly used to:
| a. | measure the characteristic impedance of a cable |
| b. | find the position of a defect in a cable |
| c. | replace a slotted-line |
| d. | all of the above |
ANS: B
COMPLETION
1. A cable that lacks symmetry with respect to ground is called ____________________.
ANS: unbalanced
2. Parallel lines are usually operated as ____________________ lines since both wires are symmetrical with respect to ground.
ANS: balanced
3. Normally, a transmission line is terminated with a load equal to its ____________________ impedance.
ANS: characteristic
4. Twisted-pair cables are transmission lines for relatively ____________________ frequencies.
ANS: low
5. To analyze a transmission line, it is necessary to use ____________________ parameters instead of lumped ones.
ANS: distributed
6. The increase of a wire's resistance with frequency is called the ____________________ effect.
ANS: skin
7. The increase of a wire's resistance with frequency is caused by the ____________________ field inside the wire.
ANS: magnetic
8. Dielectrics become more ____________________ as the frequency increases.
ANS: lossy
9. The inductance and capacitance of a cable are given per unit ____________________.
ANS: length
10. Characteristic impedance is sometimes called ____________________ impedance.
ANS: surge
11. A cable that is terminated in its characteristic impedance is called a ____________________ line.
ANS: matched
12. A pulse sent down a cable terminated in a short-circuit will reflect with the ____________________ polarity.
ANS: opposite
13. The apparently stationary pattern of waves on a mismatched cable is called a ____________________ wave.
ANS: standing
14. SWR stands for ____________________-wave ratio.
ANS: standing
15. The ideal value for SWR is ____________________.
ANS: one
16. Transmission line impedances can be found using a ____________________ chart.
ANS: Smith
17. Short transmission-line sections called ____________________ can be used as capacitors or inductors.
ANS: stubs
18. Any cable that radiates energy can also ____________________ energy.
ANS: absorb
19. A ____________________-dB loss in a cable means only half the power sent reaches the load.
ANS: 3
20. It is often best to measure SWR at the ____________________ end of a cable.
ANS: load
21. Besides heat from I2R, the power a cable can carry is limited by the ____________________ voltage of its dielectric.
ANS: breakdown
22. To normalize an impedance on a Smith Chart, you divide it by ____________________.
ANS: Z0
23. The ____________________ of a Smith Chart always represents the characteristic impedance.
ANS: center
24. A ____________________ wavelength transmission line can be used a transformer.
ANS: one-quarter
25. A slotted line is used to make measurements in the ____________________ domain.
ANS: frequency
SHORT ANSWER
1. A transmission line has 2.5 pF of capacitance per foot and 100 nH of inductance per foot. Calculate its characteristic impedance.
ANS:
Z0 = 200 ohms
2. Two wires with air as a dielectric are one inch apart. The diameter of the wire is .04 inch. Calculate, approximately, its characteristic impedance.
ANS:
386 ohms
3. If a coaxial cable uses plastic insulation with a dielectric constant Îr = 2.6 , what is the velocity factor for the cable?
ANS:
0.62
4. If a cable has a velocity factor of 0.8, how long would it take a signal to travel 3000 kilometers along the cable?
ANS:
12.5 ms
5. If a cable has a velocity factor of 0.8, what length of cable is required for a 90° phase shift at 100 MHz?
ANS:
0.6 meters
6. A cable has a VSWR of 10. If the minimum voltage along the cable is 20 volts, what is the maximum voltage along the cable?
ANS:
200 volts
7. A lossless line has a characteristic impedance of 50 ohms, but is terminated with a 75-ohm resistive load. What SWR do you expect to measure?
ANS:
1.5
8. If a cable has an SWR of 1.5, what will be the absolute value of its voltage coefficient of reflection?
ANS:
0.2
9. A generator matched to a line with a voltage coefficient of reflection equal to 0.2 transmits 100 watts into the line. How much power is actually absorbed by the load?
ANS:
96 watts
10. Using a Smith Chart to analyze a 50-ohm cable, what would be the normalized value of an impedance equal to 200 + j50 ohms?
ANS:
4 + j1
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