physics : class12 Radio-Wave Fading : Radio Waves : Electromagnetic Waves MCQ Questions and Answers
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Radio-Wave Fading
MCQ questions and answers.
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MCQ Questions
1. A receiver gets two radio-wave signals of equal amplitude by two different paths. If their path difference is 150 m and the frequency is 1 MHz, what type of interference occurs at the receiver? Take the wave speed as 3 × 10^8 m/s.
A.
Complete constructive interference
B.
Complete destructive interference
C.
No interference because the paths are different
D.
Partially constructive interference with maximum amplitude
Correct Answer:
Complete destructive interference
Explanation:
At 1 MHz, the wavelength is 300 m. The path difference is 150 m, equal to half a wavelength, so the two equal-amplitude waves arrive 180° out of phase and cancel, producing deep radio-wave fading.
2. A radio receiver experiences periodic fading when direct and ionospherically reflected waves reach it with slowly varying path difference. If the path difference changes by one wavelength in 0.20 s, what is the fading frequency?
A.
0.20 Hz
B.
2.0 Hz
C.
5.0 Hz
D.
10 Hz
Correct Answer:
5.0 Hz
Explanation:
A change of one wavelength changes the phase difference by 2π, producing one complete cycle of constructive and destructive interference. Thus the fading frequency is 1/0.20 s = 5.0 Hz.
3. Two equal-amplitude radio signals arrive at a receiver through different paths. Which path difference produces the deepest possible fade?
A.
An integral multiple of the wavelength
B.
An odd multiple of half the wavelength
C.
One-fourth of the wavelength
D.
A path difference much greater than the wavelength only
Correct Answer:
An odd multiple of half the wavelength
Explanation:
For equal-amplitude signals, destructive interference occurs when the path difference is (2n + 1)λ/2. The resulting cancellation can produce a deep fade or, ideally, zero received amplitude.
4. A radio link receives two components with amplitudes 4 mV/m and 3 mV/m. During a change in ionospheric conditions, their phase difference changes from 0 to π. What is the change in the resultant field amplitude?
A.
It changes from 7 mV/m to 1 mV/m
B.
It changes from 1 mV/m to 7 mV/m
C.
It remains 5 mV/m throughout
D.
It changes from 4 mV/m to 3 mV/m
Correct Answer:
It changes from 7 mV/m to 1 mV/m
Explanation:
For phase difference zero, the fields add to give 4 + 3 = 7 mV/m. For phase difference π, they subtract to give |4 − 3| = 1 mV/m. The changing phase caused by varying propagation paths produces fading.
5. Why can a small change in ionospheric electron density produce severe fading in a shortwave radio signal?
A.
It changes the radio-wave speed in vacuum directly
B.
It changes the refractive path and therefore the phase difference between multipath components
C.
It converts the radio wave completely into a sound wave
D.
It prevents all radio waves from undergoing reflection
Correct Answer:
It changes the refractive path and therefore the phase difference between multipath components
Explanation:
Ionospheric electron-density changes alter refraction and reflection conditions, changing the lengths and phases of different propagation paths. Their interference at the receiver can therefore shift from constructive to destructive, causing fading.
6. A receiver obtains signals at 6.00 MHz and 6.01 MHz that follow paths differing by nearly one wavelength at 6.00 MHz. Which effect is most likely when the received signal is narrowband?
A.
Frequency-selective fading because the phase relation varies with frequency
B.
No fading because both frequencies are radio waves
C.
Complete absorption of both frequencies by free space
D.
A permanent increase in received power at both frequencies
Correct Answer:
Frequency-selective fading because the phase relation varies with frequency
Explanation:
Different frequency components acquire different phase differences over unequal paths. Consequently, some frequencies may interfere destructively while nearby frequencies interfere constructively, producing frequency-selective radio-wave fading.
7. A radio link has a direct wave and a ground-reflected wave of comparable strength. Which change most directly converts a strong signal into a fade without significantly changing transmitter power?
A.
A phase change of approximately π between the two received waves
B.
A reduction of the speed of light in vacuum
C.
An increase in the charge of the electron at the transmitter
D.
A change in the unit used to measure field strength
Correct Answer:
A phase change of approximately π between the two received waves
Explanation:
When the direct and reflected waves have comparable amplitudes, a phase change of about π changes constructive interference into destructive interference. The resulting cancellation causes a sharp decrease in received signal strength, called fading.
8. A mobile radio receiver moves through a region where multipath components have nearly equal amplitudes. If the receiver moves by approximately half a wavelength along the direction of the path difference change, what is the likely result?
A.
The phase relation can change by about π, changing a maximum toward a minimum
B.
The phase relation cannot change because wavelength is fixed
C.
The received power must remain constant because transmitter power is unchanged
D.
The radio wave must be absorbed completely by the atmosphere
Correct Answer:
The phase relation can change by about π, changing a maximum toward a minimum
Explanation:
A path-length change of λ/2 corresponds to a phase change of π. Thus, as the receiver moves, constructive interference can become destructive interference, producing rapid spatial radio-wave fading.
9. Which receiver arrangement is most effective for reducing the effect of multipath radio-wave fading when two receiving antennas are separated by several wavelengths?
A.
Select or combine the antenna branch having the stronger instantaneous signal
B.
Connect both antennas with exactly opposite phase at all times
C.
Use only the antenna that is closest to the transmitter permanently
D.
Reduce the carrier frequency to zero
Correct Answer:
Select or combine the antenna branch having the stronger instantaneous signal
Explanation:
Fading is often different at spatially separated antennas because their multipath phases differ. Diversity reception reduces fading by selecting or combining branches so that a deep fade in one branch is less likely to occur in all branches simultaneously.
10. A radio signal has a received power of 1 mW before fading and 1 μW at the bottom of a fade. What is the fade depth in decibels?
A.
10 dB
B.
20 dB
C.
30 dB
D.
60 dB
Correct Answer:
30 dB
Explanation:
Fade depth is 10 log10(P_before/P_during). Here, the power ratio is 1 mW divided by 1 μW, equal to 1000. Therefore, fade depth = 10 log10(1000) = 30 dB.
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