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ELECTRONIC COMMUNICATION SYSTEMS 4 Ed BY G. Kennedy & B.Davis MULTIPLE CHOICE QUESTIONS:
1. Indicate which one of the following terms applies to troposcatter propagation? a. SIDs
c. Atmospheric Storms
b. Fading
d. Faraday rotation
2. VLF waves are used for some types of services because a. of the low powers required c. they are very reliable b. the transmitting antennas are of convenient
d. they penetrate the ionosphere easily
size
3. Indicate which of the following frequencies cannot be used for reliable beyond-the-horizon terrestrial communications without repeaters: a. 20 kHz
c. 900 MHz
b. 15 MHz
d. 12 GHz
4. High-frequency waves are a. absorbed by the F 2 layer b. reflected by the D layer
c. capable of use for long-distance communications on the moon d. affected by the solar cycle
5. Distances near the skip distance should be used for sky-wave propagation a. to avoid tilting
c. to avoid the Faraday effect
b. to prevent sky-wave and upper ray
d. so as not to exceed the critical frequency
interference
6. A ship-to-ship communication system is plagued by fading. The best solution seems to be the use of a. a more directional antenna c. frequency diversity b. a broadband antenna
d. space diversity
7. A range of microwave frequencies more easily passed by the atmosphere than are the others is called a a. window b. critical frequency
c. gyro frequency range d. resonance in the atmosphere
8. Frequencies in the UHF range normally propagate by means of a. ground waves
c. surface waves
b. sky waves
d. space waves
9. Tropospheric scatter is used with frequencies in the following range: a. HF b. VHF
c. UHF d. VLF
10. The ground wave eventually disappears, as one moves away from t he transmitter, because of a. interference from the sky wave
c. maximum single-hop distance limitation
b. loss of line-of-sight propagation
d. tilting
11. In electromagnetic waves, polarization a. is caused by reflection b. is due to the transverse nature of the waves
c. results from the longitudinal nature of the waves d. is always vertical in an isotropic medium
12. As electromagnetic waves travel in free space, only one of the t he following can happen to them: a. absorption
c. refraction
b. attenuation
d. reflection
13. The absorption of radio waves by the atmosphere depends on a. frequency b. their distance from the transmitter
c. the polarization of the waves d. the polarization of the atmosphere
14. Electromagnetic waves are refracted when they a. pass into a medium of different dielectric
c. encounter a perfectly conducting surface
constant b. are polarized by right angles to the direction
d. pass through a small slot in a conducting plane
of the propagation
15. Diffraction of electromagnetic waves a. is caused by reflections from the ground
d. may occur around the edge of a sharp
b. arises only with spherical waveforms c. will occur when the waves pass through a
obstacle
large slot
16. When microwave signals follow the curvature of the earth, this is known as a. the Faraday effect
c. tropospheric scatter
b. ducting
d. ionospheric reflection
17. Helical antenna are often used for satellite tracking at VHF because of a. troposcatter c. ionospheric refraction b. superrefraction
d. the Faraday effect
REVIEW PROBLEMS: 2
1) At 20 km in free space from a point source, the power density is 200 μW/m . What is the power density 25 km away from this source?
2) Calculate the power density (a) (a) 500 m from a 500-W source and (b) 36,000 km from a 3-kW source. Both are assumed to be omnidirectional point sources. The second value is, incidentally, the equivalent power density on the ground from a communication satellite in orbit.
3) A deep-space high-gain antenna and receiver system have a noise figure such that a m inimum -18
received power of 3.7 × 10 is required for satisfactory communication. What must be the transmitting power from a Jupiter probe, situated 800 million km from the earth? Assume the transmitting antenna is 2 isotropic, and the equivalent area of the receiving antenna has an area of 8400 m .
4) A wave travelling in free space undergoes refraction after entering a denser medium, such that the original 30° angle of incidence at the boundary between the two media is changed 20°. What is the velocity of electromagnetic waves in the second medium?
5) A 150-m antenna transmitting at 1.2 MHz (and therefore by ground wave), has an antenna current of 8 A. What voltage is received by a receiving antenna 40 km away, with a height of 2 m? Note that this is a typical MF broadcasting situation.
6) Two points on earth are 1500 1 500 km apart and are to communicate by means of HF. Given that this is to be a single-hop transmission, the critical frequency at that time is 7 MHz and conditions are idealized, calculate the MUF for those t hose two points if the height of the ionospheric layer is 300 km.
7) A microwave link consists of repeater s at 40-km intervals. What must be the minimum height of transmitting and receiving antennas above ground level (given that they are the same) to ensure line-ofline -ofsight conditions?
REVIEW QUESTIONS:
1) Electromagnetic waves are said to be transverse; what does this mean? In what way ar e transverse waves different from longitudinal waves?
2) Define the terms power density, and explain why it is inversely proportional to the square of the distance from the source.
3) Explain what is meant by the terms isotropic source and isotropic medium.
4) Define and explain field intensity. Relate it to power density with the concept of characteristic impedance of free space.
5) Explain fully the concept of linear polarization. Can longitudinal waves be polarized? Explain.
6) Why does the atmosphere absorb some power from waves propagating through it? At what frequencies does this absorption become apparent? Illustrate with a sketch combining the absorption by
oxygen and water vapour versus frequency, labelling the “windows”.
7) Prove that when electromagnetic waves are reflected from a perfectly conducting medium, the angle of reflection is equal to the angle of incidence. Hint: Bear in mind that all parts of the wave front travel tr avel with the same velocity, and consider what would happen if the two angles were not equal.
8) What is refraction? Explain under what circumstances it occurs and what causes it.
9) Prove, with a diagram, that an electromagnetic waves passing from a denser to a rarer medium are bent away from the normal.
10) What is interference of radio waves? What are the conditions necessary for it to happen?
11) What is meant by the t he diffraction of radio waves? Under what conditions does it arise? Under what condition does it not arise?
12) Draw up a table shown radio-frequency ranges, the means whereby they propagate and the maximum terrestrial distances achievable under normal conditions.
13) Describe ground-wave propagation. What is the angle of “tilt”? How does it affect field strength at a distance from the transmitter?
14) Describe briefly the strata of the ionosphere and their effects on sky-wave propagation. Why is this propagation generally better at night than during the day?
15) Discuss the reflection mechanism whereby electromagnetic waves are bent back by a layer of the ionosphere. Include in your discussion a description of the virtual height of a layer. The fact that the virtual height is greater than the actual height proves something about the reflection mechanism. What is this?
16) Show with the aid of a suitable sketch, what happens as the angle of incidence of a radio wave, using sky-wave propagation is brought closer and closer to the vertical. Define skip distance, and show how it is related to the maximum usable fre quency.
17) What is fading? List its major c auses.
18) Describe: Virtual height, critical frequency, maximum usable frequency, skip distance and and fading. fading.
19) Describe the main ionospheric variations, including a brief mention of t he interference that may be caused by the sporadic E layer.
20) In connection with space-wave propagation. What is the radio horizon? How does it differ from the optical horizon?
