TRUE/FALSE
1. James Clerk Maxwell predicted radio waves in 1865 based on a set of equations he derived.
ANS: T
2. Marconi demonstrated the existence of radio-waves in 1887.
ANS: F
3. Electromagnetic radiation is in the form of TEM waves.
ANS: T
4. Electromagnetic radiation is in the form of particles called photons.
ANS: T
5. X rays are not electromagnetic radiation.
ANS: F
6. Gamma rays are not electromagnetic radiation.
ANS: F
7. Radio waves are electromagnetic radiation.
ANS: T
8. Like light rays, Radio waves can be reflected.
ANS: T
9. Free, empty space has a characteristic impedance of 377 ohms.
ANS: T
10. Radio waves, like all other waves, must travel through some sort of medium.
ANS: F
11. An isotropic radiator radiates in one direction only.
ANS: F
12. A "point source" would be an isotropic radiator.
ANS: T
13. The wavefront of a point source radiator would be a sphere.
ANS: T
14. Beyond some distance from the source, we can approximate a wavefront by assuming it is flat.
ANS: T
15. The polarization of a plane wave is the direction of its magnetic field.
ANS: F
16. Polarization is either vertical or horizontal.
ANS: F
17. For best results, the transmitting and receiving antennas should have opposite polarizations.
ANS: F
18. As radio waves travel outward, they are effectively attenuated by the "spreading out" of the wavefront.
ANS: T
19. A power density of 100 ´ 10-9 W/m2 would be a strong radio signal.
ANS: T
20. An antenna is a passive device.
ANS: T
21. An antenna can have a gain specification greater than one.
ANS: T
22. The power absorbed by a receiving antenna depends on its shape, not its size.
ANS: F
23. Reflection of plane waves off a smooth surface is called "specular" reflection.
ANS: T
24. Radio waves can be reflected, but not diffracted.
ANS: F
25. Space waves work on a "line-of-sight" basis.
ANS: T
26. Ground waves can follow the curvature of the earth to travel over the horizon.
ANS: T
27. Ground waves are not reliable because they are affected by bad weather.
ANS: F
28. Because of refraction in ionized layers of air, sky waves appear to "bounce" off the sky and come back down to earth far from the source.
ANS: T
29. Because of diffraction, radio waves can sometimes "bend" around a corner.
ANS: T
30. At gigahertz frequencies, radio waves are absorbed by the ionosphere.
ANS: F
31. Transmission of high-frequency (HF) radio waves is affected by "spots" on the surface of the sun.
ANS: T
32. It's possible for a radio signal to be detected 1000 miles from the transmitter but not detected 500 miles from the transmitter.
ANS: T
33. There is no effective way to compensate for fading.
ANS: F
34. Communication by "bouncing" radio signals off the ionization trails of meteors is routinely done by some organizations.
ANS: T
MULTIPLE CHOICE
1. Radio waves were first predicted mathematically by:
| a. | Armstrong | c. | Maxwell |
| b. | Hertz | d. | Marconi |
ANS: C
2. Radio waves were first demonstrated experimentally by:
| a. | Armstrong | c. | Maxwell |
| b. | Hertz | d. | Marconi |
ANS: B
3. The technology that made cell phones practical was:
| a. | the microprocessor chip | c. | high-power microwave transmitters |
| b. | the miniature cell-site | d. | all of the above |
ANS: A
4. Cell phones reduce much of the problems of mobile communications with:
| a. | high power levels | c. | reuse of frequencies |
| b. | high antennas | d. | all of the above |
ANS: C
5. Which of the following are electromagnetic:
| a. | radio waves | c. | gamma waves |
| b. | light | d. | all of the above |
ANS: D
6. The electric and magnetic fields of a radio wave are:
| a. | perpendicular to each other | c. | both a and b |
| b. | perpendicular to the direction of travel | d. | none of the above |
ANS: C
7. TEM stands for:
| a. | Transverse Electromagnetic | c. | True Electromagnetic |
| b. | Transmitted Electromagnetic | d. | none of the above |
ANS: A
8. In free space, radio waves travel at a speed of:
| a. | 3 ´ 106 meters per second | c. | 3 ´ 106 miles per second |
| b. | 300 ´ 106 meters per second | d. | 300 ´ 106 miles per second |
ANS: B
9. Which is a possible polarization for an electromagnetic wave:
| a. | vertical | c. | circular |
| b. | horizontal | d. | all of the above |
ANS: D
10. Which polarization can be reasonably well received by a circularly polarized antenna:
| a. | vertical | c. | circular |
| b. | horizontal | d. | all of the above |
ANS: D
11. The number of circular polarization modes (directions) is:
| a. | 1 | c. | 3 |
| b. | 2 | d. | many |
ANS: B
12. An antenna has "gain" as compared to:
| a. | an isotropic radiator | c. | a ground-wave antenna |
| b. | a vertically polarized radiator | d. | none of the above |
ANS: A
13. EIRP stands for:
| a. | the E and I fields of the Radiated Power |
| b. | the Effective Isotropic Radiated Power |
| c. | the Effective Internal Reflected Power |
| d. | the Electric-field Intensity of the Radiated Power |
ANS: B
14. The "attenuation of free space" is due to:
| a. | losses in the characteristic impedance of free space |
| b. | losses due to absorption in the upper atmosphere |
| c. | the decrease in energy per square meter due to expansion of the wavefront |
| d. | the decrease in energy per square meter due to absorption of the wavefront |
ANS: C
15. Ground waves are most effective:
| a. | below about 2 MHz | c. | at microwave frequencies |
| b. | above about 20 MHz | d. | when using horizontally polarized waves |
ANS: A
16. Radio waves would most strongly reflect off:
| a. | a flat insulating surface of the right size | c. | a flat metallic surface of the right size |
| b. | a flat dielectric surface of the right size | d. | a flat body of water |
ANS: C
17. Radio waves sometimes "bend" around a corner because of:
| a. | reflection | c. | refraction |
| b. | diffusion | d. | diffraction |
ANS: D
18. Space waves are:
| a. | line-of-sight |
| b. | reflected off the ionosphere |
| c. | same as sky waves |
| d. | radio waves used for satellite communications |
ANS: A
19. Sky waves:
| a. | are line-of-sight |
| b. | "bounce" off the ionosphere |
| c. | are same as space waves |
| d. | are radio waves used for satellite communications |
ANS: B
20. Sky waves cannot be "heard":
| a. | close to the transmitter | c. | in the "silent" zone |
| b. | far from the transmitter | d. | in the "skip" zone |
ANS: D
21. A 20-dB reduction in the strength of a radio wave due to reflection is called:
| a. | fading | c. | frequency diversity |
| b. | diffraction | d. | spatial diversity |
ANS: A
22. "Ghosts" on a TV screen are an example of:
| a. | fading | c. | multipath distortion |
| b. | diffraction | d. | cancellation due to reflection |
ANS: C
23. A "repeater" is used to:
| a. | send a message multiple times over a channel |
| b. | send a message over multiple channels at the same time |
| c. | extend the range of a radio communications system |
| d. | cancel the effects of fading |
ANS: C
24. Cellular phone systems rely on:
| a. | high power | c. | the radio horizon |
| b. | repeaters | d. | the reuse of frequencies |
ANS: D
25. If the number of cell-phone users within a cell increases above some limit:
| a. | the cell area is increased | c. | the power levels are increased |
| b. | the cell area is split | d. | the number of channels is reduced |
ANS: B
26. As a cell-phone user passes from one cell to another:
| a. | a "handoff" process occurs | c. | both cells will handle the call |
| b. | a "sectoring" process occurs | d. | nothing occurs |
ANS: A
27. To receive several data streams at once, a CDMA spread-spectrum system uses:
| a. | a "funnel" receiver | c. | multiple receivers |
| b. | a "rake" receiver | d. | none of the above |
ANS: B
28. The troposphere is the:
| a. | highest layer of the atmosphere | c. | lowest layer of the atmosphere |
| b. | middle layer of the atmosphere | d. | the most ionized layer of the atmosphere |
ANS: C
29. Meteor-trail propagation is:
| a. | used for radio telephony | c. | also called "ducting" |
| b. | used to send data by radio | d. | not possible |
ANS: B
COMPLETION
1. Radio waves were mathematically predicted by ____________________.
ANS: Maxwell
2. Radio waves were first demonstrated by ____________________.
ANS: Hertz
3. Radio waves are ____________________ electromagnetic waves.
ANS: transverse
4. The propagation speed of radio waves in free space is ____________________ m/sec.
ANS: 300 ´ 106
5. Electromagnetic radiation can be thought of as a stream of particles called ____________________.
ANS: photons
6. Unlike sound or water waves, radio waves do not need a ____________________ to travel through.
ANS: medium
7. The dielectric strength of clean dry air is about ____________________ volts per meter.
ANS: 3 ´ 106
8. Waves from an ____________________ source radiate equally in all directions.
ANS: isotropic
9. The wavefront of a point source would have the shape of a ____________________.
ANS: sphere
10. At a far distance from the source, a radio wavefront looks like a flat ____________________-wave.
ANS: plane
11. The polarization of a radio wave is the direction of its ____________________ field.
ANS: electric
12. The electric field of a radio wave is ____________________ to its magnetic field.
ANS: perpendicular
13. Both the electric and magnetic fields of a radio wave are ____________________ to its propagation direction.
ANS: perpendicular
14. With ____________________ polarization, the direction of a radio wave's electric field rotates as it travels through space.
ANS: circular
15. An antenna is said to have ____________________ in a certain direction if it radiates more power in that direction than in other directions.
ANS: gain
16. The watts per square meter of a radio wave ____________________ as the wave-front moves through space.
ANS: decrease
17. Reflection of plane-waves from a smooth surface is called ____________________ reflection.
ANS: specular
18. ____________________ is the "bending" of radio waves as they travel across the boundary between two different dielectrics.
ANS: Refraction
19. The process of ____________________ makes radio waves appear to "bend around a corner".
ANS: diffraction
20. ____________________ waves travel from transmitter to receiver in a "line-of-sight" fashion.
ANS: Space
21. ____________________ waves are vertically polarized radio waves that travel along the earth's surface.
ANS: Ground
22. ____________________ waves are radio waves that "bounce off" the ionosphere due to refraction.
ANS: Sky
23. The ____________________ zone is a region where sky waves cannot be received.
ANS: skip
24. "Ghosts" on a TV screen are an example of ____________________ distortion.
ANS: multipath
25. The "fast fading" seen in mobile communications is caused by ____________________ waves interfering with direct waves.
ANS: reflected
26. Cell phones typically operate at a ____________________ power level.
ANS: low
27. The ____________________ of frequencies allows many cell-phone users to share a geographical area.
ANS: reuse
28. ____________________ is when a cell-site uses three directional antennas, each covering a third of the cell area, to reduce interference.
ANS: Sectoring
29. The use of ____________________ chips makes cell phones a practical technology.
ANS: microprocessor
SHORT ANSWER
1. A certain dielectric has permittivity of 6.3 ´ 10–10 F/m and the same permeability as free space. What is the characteristic impedance of that dielectric?
ANS:
45 ohms
2. If a point source of radio waves transmits 1 watt, what is the power density 10,000 meters from the source?
ANS:
796 pW/m2
3. What power must a point-source of radio waves transmit so that the power density at 3000 meters from the source is 1 mW/m2?
ANS:
113 watts
4. If a radio receiver needs 1 nW/m2 of power density to function, how far away from a 1-watt point source will it continue to work?
ANS:
8.9 km
5. A line-of-sight radio link over flat terrain needs to use antenna towers 50 km apart. What, approximately, is the minimum height for the towers assuming all the towers are the same?
ANS:
37 meters
6. A mobile radio is being used at 1 GHz in an urban environment with lots of reflecting structures. If the car is traveling 36 km/hour, what is the expected time between fades?
ANS:
15 msec
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