# Topic: Communications (Test 4)

Topic: Communications
Q.1
In the following scheme, if the spectrum M(f) of m(t) is as shown, then the spectrum Y(f) of y(t) will be A. B. C. D. Explaination / Solution:   Workspace
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Q.2
An air-filled rectangular waveguide has inner dimensions of 3 cm × 2 cm. The wave impedance of the TEmode of propagation in the waveguide at a frequency of 30 GHz is (free space impedance η0 = 377Ω)
A. 308 Ω
B. 355 Ω
C. 400 Ω
D. 461 Ω
Explaination / Solution: Workspace
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Q.3
A λ/2 dipole is kept horizontally at a height of λ0/2 above a perfectly conducting infinite ground plane. The radiation pattern in the lane of the dipole ( plane) looks approximately as
A. B. C. D. Explaination / Solution: Workspace
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Q.4
For a Hertz dipole antenna, the half power beam width (HPBW) in the E-plane is
A. 3600
B. 1800
C. 900
D. 450
Explaination / Solution:

The beam-width of Hertizian dipole is 1800 and its half power beam-width is 900

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Q.5
A memory less source emits n symbols each with a probability p. The entropy of the source as a function of n
A. increases as log n
B. decreases as log(1/n )
C. increases as n
D. increases as nlogn
Explaination / Solution: Workspace
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Q.6
The amplitude of a random signal is uniformly distributed between -5 V and 5 V.If the signal to quantization noise ratio required in uniformly quantizing the signal is 43.5 dB, the step of the quantization is approximately
A. 0.033 V
B. 0.05 V
C. 0.0667 V
D. 0.10 V
Explaination / Solution:  Workspace
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Q.7
At 20 GHz, the gain of a parabolic dish antenna of 1 meter and 70% efficiency is
A. 15 dB
B. 25 dB
C. 35 dB
D. 45 dB
Explaination / Solution: Workspace
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Q.8
A band-limited signal with a maximum frequency of 5 kHz is to be sampled. According to the sampling theorem, the sampling frequency which is not valid
A.

5kHZ

B.

12kHZ

C.

15kHZ

D.

20kHZ

E.

Explaination / Solution:

Given, the maximum frequency of the band-limited signal
fm = 5 kHz
According to the Nyquist sampling theorem, the sampling frequency must be greater than the Nyquist frequency which is given as So, the sampling frequency fs must satisfy only the option (A) does’nt satisfy the condition therefore, 5K HZ is not a valid sampling frequency.

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Q.9
A transmission line of characteristic impedance 50Ω is terminated by a 50 W load. When excited by a sinusoidal voltage source at 10 GHz, the phase difference between two points spaced 2 mm apart on the line is found to be π/4 radians. The phase velocity of the wave along the line is
A.

0.8 × 108 m/s

B.

1.2 × 108 m/s

C.

1.6 × 108 m/s

D.

3 × 108 m/s

Explaination / Solution:

We have d = 2 mm and f = 10 GHz Workspace
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Q.10
The Column-I lists the attributes and the Column-II lists the modulation systems. Match the attribute to the modulation system that best meets it Column-I P. Power efficient transmission of signals Q. Most bandwidth efficient transmission of voice signals R. Simplest receiver structure S. Bandwidth efficient transmission of signals with Significant dc component Column-II 1. Conventional AM 2. FM 3. VSB 4. SSB-SC
A. P-4;Q-2;R-1;S-3
B. P-2;Q-4;R-1;S-3
C. P-3;Q-2;R-1;S-4
D. P-2;Q-4;R-3;S-1