MULTIPLE CHOICES

QUESTIONS IN

Communications Engineering

By: S. S. Cuervo & K. L. N. Suello

400 Solved Problems

Table of Contents

Transmission Lines 65 problems solved

Radio Wave Propagation 39 problems solved

Antenna 68 problems solved

Noise 71 problems solved

Amplitude Modulation 101 problems solved

Frequency Modulation 56 problems solved

________________

Total = 400 problems solved

TRANSMISSION LINES

1. A parallel wire line spaced at 1.27 cm has a diameter of 0.21 cm. what is the
characteristic impedance?

a. 300 ohms
b. 400 ohms
c. 500 ohms
d. 600 ohms
2. In a coaxial line if the inner diameter is 0.51 cm and the center conductor has a diameter
of 0.22 cm. what is the characteristic impedance?

a. 50.1 ohms
b. 50.2 ohms
c. 50.3 ohms
d. 50.4 ohms
3. A strip line is formed using a multilayer board (dielectric constant = 2). The center track
is 0.15 inch wide and 0.005 inch thick and the PC board first layer thickness is 0.05 inch
thick, with an overall board thickness of twice the single layer. What is the characteristic
impedance?

a. 14.7 ohms
b. 14.8 ohms
c. 14.9 ohms
d. 15.0 ohms
4. In an improperly loaded transmission line, determine the power reflected from the load if
the incident power at the load is 500 W and the reflection coefficient is 0.71.

a. 252.04 W
b. 252.05 W
c. 252.06 W
d. 252.07 W
5. In wire communications system, calculate the return loss in dB, if the load impedance is
600 ohms and the characteristic impedance of the line is 300 ohms.

a. 9.52 dB
b. 9.53 dB
c. 9.54 dB
d. 9.55 dB
6. The VSWR on a loss free line of 50 ohm characteristic impedance is 4.2. Determine the
value of the purely resistive load impedance which is known to be larger than 50 ohms.

a. 210 ohms
b. 211 ohms
c. 212 ohms
d. 213 ohms
7. A micro strip line is formed using a 0.095 inch thick PC board, with a bottom ground
plane and a single 0.15 inch wide, 0.008 inch thick track on the top. What is its
characteristic impedance?

a. 72.1 ohms
b. 72.2 ohms
c. 72.3 ohms
d. 72.4 ohms
8. Determine the impedance of a balanced 2 wire with unequal diameters d1 = 12 mm and
d2 = 10 mm. the spacing between wires is 30mm.

a. 35.35 M ohms
b. 35.36 M ohms
c. 35.37 M ohms
d. 35.38 M ohms
9. For a parallel wire line, determine the conductance if the conductivity is 200 x 10 -6 S/m
and the conductor diameter spacing is 10mm.

a. 1.23mS/m
b. 1.24mS/m
c. 1.25mS/m
d. 1.26mS/m
10. What is the characteristic impedance of an open line with conductors 4 mm in diameter
separated by 15 mm?

a. 241.52 ohms
b. 241.53 ohms
c. 241.54 ohms
d. 241.55 ohms
11. The forward power in a transmission line is 150 W and the reverse power is 20 W.
Calculate the SWR on the line.

a. 2.13
 b. 2.14
c. 2.15
d. 2.16
12. What is the characteristic impedance of a coaxial cable using a solid polyethylene
dielectric having a relative permeability of 2.3, an inner conductor of 1 mm diameter and
an outer conductor of diameter 5mm?

a. 63.4 ohms
b. 63.5 ohms
c. 63.6 ohms
d. 63.7 ohms
13. Calculate the velocity factor of a coaxial cable used as a transmission line, with the
characteristic impedance of 50 ohms; capacitance is 40 pF/m, and inductance equal to
50microH/m.

a. 0.0745 ft
b. 0.0746 ft
c. 0.0747 ft
d. 0.0748 ft
14. What is the actual length in feet of a one quarter wavelength of a coaxial cable with a
velocity of 0.69 at 40 MHz?

a. 4.23 ft
b. 4.24 ft
c. 4.25 ft
d. 4.26 ft
15. What is the actual length in feet of one half wavelengths of a coax with velocity factor of
0.63 at 28 MHz?

a. 11.07 ft
b. 11.08 ft
c. 11.09 ft
d. 11.10 ft
16. What is the actual length in feet of one quarter wavelength of a coax with a velocity
factor of 0.68 at 39 MHz?

a. 5.577 ft
b. 5.576 ft
c. 5.575 ft
d. 5.574 ft
17. What is the actual length in feet of a one half wavelength of a coax with a velocity factor
of 0.61 at 27 MHz?

a. 11.10 ft
b. 11.11 ft
c. 11.12 ft
d. 11.13 ft
18. What is the actual length in feet of a one half wavelength of a coax with a velocity factor
of 0.59 at 26 MHz?

a. 11.16 ft
b. 11.17 ft
c. 11.18 ft
d. 11.19 ft
19. What is the actual length in feet of a one quarter wavelength of a coax with a velocity
factor of 0.695 at 42 MHz?

a. 4.05 ft
b. 4.06 ft
c. 4.07 ft
d. 4.08 ft
20. An amplifier with a 20 dB gain is connected to another with 10 dB gains by means of a
transmission line with a loss of 4 dB; if a signal with a power level of 14 dBm were
applied to the system, calculate the power output.

a. 11dBm
b. 12dBm
c. 13dBm
d. 14dBm
21. An attenuator has a loss of 26 dB. If a power of 3 W is applied to the attenuator, find the
output power.

a. 7.54mW
b. 7.55mW
c. 7.56mW
d. 7.57mW
22. What would be the approximate series impedance of a quarter wave matching line used to
match a 600 ohms feed to 70 ohms antenna?

a. 204. 94 ohms
 b. 204.95 ohms
c. 204.96 ohms
d. 204.97 ohms
23. What is the impedance of a balance 4 wire with a diameter of 0.25 cm and spaced 2.5 cm
apart using an insulator with a dielectric constant of 2.56

a. 98 ohms
b. 99 ohms
c. 100 ohms
d. 101 ohms
24. What is the characteristic impedance of a single wire with diameter d = 0.25 mm placed
at the center between grounded parallel planes separated by 1 mm apart? The wire is held
by a material with a velocity factor of 0.75.

a. 71 ohms
b. 72 ohms
c. 73 ohms
d. 75 ohms
25. What is the phase coefficient of a transmission line at the frequency of 10 MHz if the
velocity of propagation is 2.5 x 108m/s?

a. 0.25rad/m
b. 0.26rad/m
c. 0.27rad/m
d. 0.28rad/m
26. Determine the conductance of a two wire open line with the following parameters; D = 4
in; d = 0.1 in. and p = 2.6 x108 ohms-m.

a. 2.75 x 10-9 S/m

b. 2.76 x 10-9 S/m
c. 2.77 x 10-9 S/m
d. 2.78 x 10-9 S/m
27. What is the input impedance of a transmission line if its characteristic impedance is 300
ohms and the load is 600 ohms? Assume a quarter wavelength sections only.

a. 149 ohms
b. 150 ohms
c. 151 ohms
d. 152 ohms
28. Determine the standing wave ratio of a 300 ohms line whose load is 400 + j150 ohms.
 a. 1.67
b. 1.68
c. 1.69
d. 1.70
29. What proportion of the incident power is reflected back for the load for a 75 ohms line
terminated with ZL = 50 – j25 ohms.

a. 7.82%
b. 7.83%
c. 7.84%
d. 7.85%
30. What is the attenuation of a 50 ohms coaxial cable if it has a resistance of 0.121 ohms/m?

a. 1.21 x10-3Np/m
b. 1.22 x10-3Np/m
c. 1.23 x10-3Np/m
d. 1.24 x10-3Np/m
31. Find the inductance of a 5 meter length transmission line if its characteristic impedance is
52 ohms and the capacitance is 75pF/m.

a. 1.013uH
b. 1.014uH
c. 1.015uH
d. 1.016uH
32. An open wire transmission line with a 2 mm diameter each separated by 12 mm. calculate
the characteristic impedance.

a. 297 ohms
b. 298 ohms
c. 299 ohms
d. 300 ohms
33. A coaxial cable with inner diameter of 3 mm and an outer diameter of 9 mm. determine
the characteristic impedance if the dielectric is polyethylene having ER = 2.3.

a. 43.2 ohms
b. 43.3 ohms
c. 43.4 ohms
d. 43.5 ohms
34. A coaxial transmission line uses Teflon as dielectric. What is the propagation velocity if
the signal?

a. 207 x 106 m/s

b. 208 x 106 m/s
c. 209 x 106 m/s
d. 210 x 106 m/s
35. A 530 MHz signal is propagating along a RG 8/u coaxial cable with a velocity factor
0.66. If a 75 degree phase shift is needed, what is the length if the transmission line?

a. 7.6 cm
b. 7.7 cm
c. 7.8 cm
d. 7.9 cm
36. Calculate the SWR of the line if the characteristic impedance of the line is 75 ohms and is
terminated by 50 ohms.

a. 1.4
b. 1.5
c. 1.6
d. 1.7
37. A transmitter sends a 5 W of power to a 75 ohms line. Suppose the transmitter and the
line is matched but the load is not. Determine the power absorbed by the reflection is
0.75.

a. 2.1821 W
b. 2.1822 W
c. 2.1823 W
d. 2.1825 W
38. A generator sends a 250 W of power down a line. If the SWR of the line is 2.5, determine
the power absorbed by the load.

a. 204.0 W
b. 204.1 W
c. 204.2 W
d. 204.3 W
39. A 75 ohm characteristics impedance transmission line is terminated in by a load
impedance of 120 ohms. Determine the input impedance looking into the line 1 meter
long, if the line has a velocity factor of 0.9 and operates at a frequency of 50 MHz.
 a. 34.4 – j12.2 ohms
b. 354 – j12.2 ohms
c. 36. 4 – j12.2 ohms
d. 37. 4 – j12.2 ohms
40. A parallel tuned circuit is to be constructed from a shorted section of parallel wire lie. If
the operating frequency is 2.4 GHz and dielectric used is polyethylene, what is the length
of the shorted section?

a. 2.0622 cm
b. 2.0623 cm
c. 2.0624 cm
d. 2.0625 cm
41. The transmitter has an output power of 150 W. suppose the line is 50 meters long and
perfectly matched with a loss of 3 dB/100m, how much power is received in the antenna?

a. 106.1 w
b. 106.2 w
c. 106.3 w
d. 106.4 w
42. TDR equipment receives the signal 250 ns after the test signal had been transmitted. If
the discontinuity of the line is found 35 meter from the test point, what is the velocity
factor of the line?

a. 0.933
b. 0.656
c. 0.588
d. 0.2685
43. The line has 97% velocity factor. If the two minima are located 30 cm apart, determine
the frequency.

a. 485 MHz
b. 488 MHz
c. 478 MHz
d. 456 MHz
44. The power incident in the line is 20 W. assuming the reflected power is 15 W what is the
SWR of the line?

a. 1.71
b. 1.75
c. 1.25
 d. 1.26
45. A transmission line 90 meter long has a characteristic impedance of 75 ohms. The line is
connected through a 15 V dc source and 75 ohms source resistance at time t = 0. What is
the voltage across the input immediately after t = 0?

a. 0 and 7.5 V
b. 1 and 2.3 V
c. 45 and 29 V
d. 69 and 56V

46. A transmission line 90 meter long has a characteristic impedance of 75 ohms. The line is
connected through a 15 V dc source and 75 ohms source resistance at time = 0. The
voltage at the input end changes to 10 V, with the same polarity as before, at time t = 1.5
micro seconds. What is the resistance that terminates the lines?

a. 465 ohms
b. 896 ohms
c. 563 ohms
d. 150 ohms
47. A parallel wire transmission line has an inductance of 120 nH/m and a characteristic
impedance of 320 ohms. Calculate the capacitance of the line.

a. 2.25Pf/m
b. 1.17pF/m
c. 26.25pF/m
d. 15.89pF/m
48. A radio frequency 50 ohms transmission line is connected to a coil with internal
resistance of 20 ohms and inductive reactance of 5 ohms at a frequency of 10 MHz. the
line is to be matched to the load by means of a quarter wave line and a stub that are
connected across the load. Calculate the characteristic impedance of the quarter wave
transformer.

a. 56.2 ohms
b. 32.5 ohms
c. 32.6 ohms
d. 25.3 ohms
49. An open wire line has a 3 mm diameter separated 6 mm from each other by an air
dielectric. Calculate the characteristic impedance of the line.

a. 555 ohms
b. 166 ohms
c. 584 ohms
d. 126 ohms