## COMMUNICATION SYSTEM Quiz-15

As per analysis for previous years, it has been observed that students preparing for NEET find Physics out of all the sections to be complex to handle and the majority of them are not able to comprehend the reason behind it. This problem arises especially because these aspirants appearing for the examination are more inclined to have a keen interest in Biology due to their medical background. Furthermore, sections such as Physics are dominantly based on theories, laws, numerical in comparison to a section of Biology which is more of fact-based, life sciences, and includes substantial explanations. By using the table given below, you easily and directly access to the topics and respective links of MCQs. Moreover, to make learning smooth and efficient, all the questions come with their supportive solutions to make utilization of time even more productive. Students will be covered for all their studies as the topics are available from basics to even the most advanced. .

•  Analog data communication
•  Digital data communication
•  Both are equally good
•  Depends on the situation
Solution
Digital data communication is more advantageous.

Q2.To cover a population of 20 lakh, a transmission tower should have a height … (Radius of earth = 6400 km, population per square km =1000)
•  25 m
•  50 m
•  75 m
•  100 m
Solution
Area of region covered = Ï€(2nR) In 1 km2=1000 people 1/1000×20×106=20×106=A ∴20×103=Ï€(2×h×6400) ⇒h= 20×103/Ï€×2×6400 ≈50 m

Q3. A sky wave with a frequency 55 MHz is incident on the D-region of earth's atmosphere at30°. The angle of refraction is (electron density for D-region is 400 electron/cc)
•  60°
•  15°
•  45°
•  30°
Solution
For D-region, v=55×106 Hz;i=30° N=400×106 m-3 Î¼=√1-81.45N/v2 =√(1- 81.45×400×106/( 55×106 )≈1 Also Î¼=sin⁡i/sin⁡r or sin⁡ i=Î¼ sin⁡ r=sin⁡r or i=r=30°

Q4. The fundamental radio antenna is a metal rod which has a length equal to
•  Î» in free space at the frequency of operation
•  Î»/2 in free space at the frequency of operation
•  Î»/4 in free space at the frequency of operation
•  3Î»/4 in free space at the frequency of operation
Solution
Î»/4 in free space at the frequency of operation

Q5.Let A = light obtained by stimulated emission and B = light obtained by spontaneous, then
•  A is incoherent, B is incoherent
•  A is incoherent , B is coherent
•  A is coherent, B is coherent
•  A is coherent, B is incoherent
Solution
A is coherent, B is incoherent

Q6. The antenna current of an AM transmitter is 8 A when only carrier is sent but increases to 8.96 A when the carrier is sinusoidally modulated. The percentage modulation is
•  50%
•  60%
•  65%
•  71%
Solution
We know that (It/Ic )2=1+m2/2 Here, It=8.96A and Ic=8A ∴(8.96/8)2=1+m2/2 or 1.254=1+ m2/2 or m2/2=0.254 or m2=0.508 or m=0.71=71%

Q7.In earth's atmosphere, for F1-layer; the virtual height and critical frequency are
•  150 km and 3 MHz
•  160 km and 3.5 MHz
•  170 km and 4.5 MHz
•  180 km and 5 MHz
Solution
180 km and 5 MHz

Q8. Which one of the following is a full duplex transmission system?
•  TV
•  Telephone
•  Walky-talky (wireless used in the army)
Solution
Telephone is a full duplex transmission system.

Q9.A signal wave of frequency 12 kHz is modulated with a carrier wave of frequency 2.51 MHz. The upper and lower side band frequencies are respectively
•  2512 kHz and 2508 kHz
•  2522 kHz and 2488 kHz
•  2502 kHz and 2498 kHz
•  2522 kHz and 2498 kHz
Solution
Upper side band (USB) frequency =fc+fm =(2.51)MHz+(12)kHz=(2510+12) kHz=2522 kHz Lower side band (LSB) frequency =fc-fm =(2.51)MHz-(12)kHz=(2510-12) kHz=2498 kHz

Q10. In earth's atmosphere, for E-layer, the virtual height and critical frequency are
•  80 km and 3MHz
•  90 km and 3.5 MHz
•  120 km and 4.5 MHz
•  110 km and 4 MHz
Solution
The virtual height and critical frequency of E-layer is 110 km and 4 MHz

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