This is the Multiple Choice Questions in Chapter 14: Transmission Lines from the book Electronic Communication Systems by Roy Blake
Start Practice Exam Test Questions
Choose the letter of the best answer in each questions.
MULTIPLE CHOICE
1. SWR stands for:
- a. Shorted Wave Radiation
- b. Sine Wave Response
- c. Shorted Wire Region
- d. none of the above
ANS: D
2. TDR stands for:
- a. Total Distance of Reflection
- b. Time-Domain Reflectometer
- c. Time-Domain Response
- d. Transmission Delay Ratio
ANS: B
3. An example of an unbalanced line is:
- a. a coaxial cable
- b. 300-ohm twin-lead TV cable
- c. an open-wire-line cable
- d. all of the above
ANS: A
4. When analyzing a transmission line, its inductance and capacitance are considered to be:
- a. lumped
- b. distributed
- c. equal reactances
- d. ideal elements
ANS: B
5. As frequency increases, the resistance of a wire:
- a. increases
- b. decreases
- c. stays the same
- d. changes periodically
ANS: A
6. The effect of frequency on the resistance of a wire is called:
- a. I2R loss
- b. the Ohmic effect
- c. the skin effect
- d. there is no such effect
ANS: C
7. As frequency increases, the loss in a cable's dielectric:
- a. increases
- b. decreases
- c. stays the same
- 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
- b. higher than Z0
- c. equal to Z0
- d. 50 ohms
ANS: C
10. The characteristic impedance of a cable:
- a. increases with length
- b. increases with frequency
- c. increases with voltage
- d. none of the above
ANS: D
11. The velocity factor of a cable depends mostly on:
- a. the wire resistance
- b. the dielectric constant
- c. the inductance per foot
- 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
- b. one
- c. as large as possible
- d. there is no optimum value
ANS: B
16. A non-optimum value for SWR will cause:
- a. standing waves
- b. loss of power to load
- c. higher voltage peaks on cable
- d. all of the above
ANS: D
17. VSWR stands for:
- a. variable SWR
- b. vacuum SWR
- c. voltage SWR
- d. none of the above
ANS: C
18. The impedance "looking into" a matched line:
- a. is infinite
- b. is zero
- c. is the characteristic impedance
- d. 50 ohms
ANS: C
19. A Smith Chart is used to calculate:
- a. transmission line impedances
- b. propagation velocity
- c. optimum length of a transmission line
- 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
- b. would be more
- c. would be the same
- d. cannot be compared
ANS: B
21. A balanced load can be connected to an unbalanced cable:
- a. directly
- b. by using a filter
- c. by using a "balun"
- d. cannot be connected
ANS: C
22. On a Smith Chart, you "normalize" the impedance by:
- a. assuming it to be zero
- b. dividing it by 2Ï€
- c. multiplying it by 2Ï€
- d. dividing it by Z0
ANS: D
23. The radius of the circle you draw on a Smith Chart represents:
- a. the voltage
- b. the current
- c. the impedance
- d. none of the above
ANS: D
24. The center of the Smith Chart always represents:
- a. zero
- b. one
- c. the characteristic impedance
- 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 ∈r = 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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