## Analog Communications Questions and Answers Part-14

1. Radiation resistance of an antenna is ________
a) a dc resistance
b) an ac resistance
c) a constant value
d) neither ac nor dc resistance

Explanation: Radiation resistance is that part of an antenna that is caused by the radiation of electromagnetic waves from the antenna. It is considered to be an equivalent resistance dissipating the same amount of power as an actual antenna would have radiated. Radiation resistance of an antenna is usually an ac resistance.

2. What do you understand by isotropic antenna?
a) it radiates its power uniformly in all directions
b) it radiates its power non-uniformly in all directions
c) it radiates its power specifically in one direction only
d) it does not radiate any power

Explanation: An isotropic antenna is considered to be an ideal antenna that radiates power uniformly in all directions. Generally, this type of antenna does not exist. However, it is often used to determine certain antenna characteristics like antenna gain.

3. The spectrum of white noise and impulse noise is similar in terms of ________
a) magnitude spectrum
b) phase spectrum
c) both magnitude spectrum and phase spectrum
d) amplitude spectrum

Explanation: White noise is a random signal having equal intensities of different frequencies. Impulse noise is a type of noise consisting of sudden sounds, which includes unwanted and instantaneous signals. Thus, they have only their magnitude in common.

4. The amount of power and bandwidth necessary to be transmitted for a given amount of information are reduced in _________
a) Single Sideband Modulation
b) Double Sideband Modulation
c) Vestigial Sideband Modulation
d) Amplitude Modulation

Explanation: In SSBSC, the carrier is suppressed and only either of the two sidebands is transmitted. This reduces the power consumption and also lessens the bandwidth. While in DSBSC, the carrier is suppressed but both the sidebands are transmitted, whereas in AM, the carrier as well as both the sidebands are transmitted.

5. A long wave AM broadcast transmitter needs _________
a) very small carrier power
b) very large carrier power
c) small carrier power
d) large carrier power

Explanation: In Communication Systems, a transmitter produces radio waves, which are the modulated message signals, and are transmitted via the antennae. These message signals carry information which are modulated using a carrier wave, having frequency higher than the message signal frequency. Thus, for transmitting information to a large distance, it is necessary that a very large carrier wave is used for modulation.

6. In FM, if we decreases modulating frequency then the modulation index ________
a) will increase, if the modulating voltage amplitude increases
b) will decrease, if the modulating voltage amplitude increases
c) will increase, if the modulating voltage amplitude remains constant
d) will decrease, if the modulating voltage amplitude remains constant

Explanation: In frequency modulation, the modulation index is the ratio of frequency deviation to modulating frequency. Therefore, modulation index can be made inversely proportional to the modulating frequency by keeping amplitude constant. Thus, if we decrease modulating frequency then the modulation index will increase, with amplitude remaining constant.

7. Which of the following modulation system is used for video-modulation?
a) DSB-SC
b) SSB-SC
c) VSB
d) FM

Explanation: Vestigial Sideband Modulation (VSB) is a type of amplitude modulation in which the carrier and only one sideband is completely transmitted and the other sideband is partly transmitted. Thus, video signals are transmitted using VSB modulation.

8. In SSB modulation, only a single sideband is transmitted leaving the other sideband and the carrier.
a) True
b) False

Explanation: In SSBSC, the carrier is suppressed and only either of the two sidebands is transmitted. This reduces power consumption and also lessens the bandwidth.

9. For demodulation of PCM, it is first converted into __________
a) PDM
b) PWM
c) PPM
d) PAM