Which of the following frequencies will be suitable for beyond the horizon communication using sky waves.

1

(a)

10 kHz

(b)

10 MHz

(c)

1 GHz

(d)

1000 GHz

Frequencies in the UHF range normally propagate by means of:

1

(a)

Ground waves

(b)

sky waves

(c)

surface waves

(d)

space waves

Three waves A, B and C of frequencies 1600 kHz, 5 MHz and 60 MHz, respectively are to be transmitted from one place to another. Which of the following is the most appropriate mode of communication:

1

(a)

A is transmitted via space wave while B and C are transmitted via sky wave.

(b)

A is transmitted via ground wave, B via sky wave and C via space wave.

(c)

B and C via ground wave while A is transmitted via sky wave.

(d)

B is transmitted via ground wave while A and C are transmitted via space wave.

A 100m long antenna is mounted on a 500 m tall building. The complex can become a transmission tower for waves with \(\lambda \).

1

(a)

\(\sim \) 400 m

(b)

\(\sim \) 25 m

(c)

\(\sim \) 150 m

(d)

\(\sim \) 2400 m

A 1 kW signal is transmitted using a communication channel which provides attenuation at the rate of -2dB per km. If the communication channel has a total length of 5km, the power of the signal received is \([gain \ in \ dB=10\log { \left( \frac { { P }_{ 0 } }{ { P }_{ i } } \right) } ]\)

1

(a)

900 W

(b)

100 W

(c)

990 W

(d)

1010 W

What is audio frequency range?

1

(a)

The audio frequency signals range from 20 Hz to 20 kHZ.

What is modulating signal?

1

(a)

The audio frequency signals are the modulating signals.

What is the range of values of modulation index of an AM wave?

1

(a)

Modulation index of an AM wave lies between 0 and 1

What is the total modulation index when a carrier wave is modulated by a number of sine waves?

1

(a)

\(\mu _{ A }=\sqrt { { \mu }_{ 1 }^{ 2 }+{ \mu }_{ 2 }^{ 2 }+{ \mu }_{ 3 }^{ 2 }+..... } \)

What is varied in an FM wave?

1

(a)

In frequency modulation,the frequency of carrier wave is varied in accordance with the instantaneous value of the modulating voltage. 

Why are micro wave used in radars?

1

(a)

In a radar, a beam signal is needed in particular direction which is possible if wavelength of signal waves is very small. Since the wavelength of microwave is a few millimeter, hence they are used in radar.

Why sky waves are not used in the transmission of television signals?

1

(a)

The television signals have frequencies in 100-200 MHz range. As ionosphere cannot reflect radio waves of frequency greater than 40 M back to earth, the sky waves cannot be used in the transmission of TV signals.

What should be the desirable characteristic of a diode detector?

1

(a)

A diode detector should have the following characteristic for proper detection a) high rectification efficiency. b) negligible loading effect on previous stage c) low distortion

Give a velocity factor of a line

1

(a)

Velocity factor (VF) of a cable is the ratio of reduction speed of light in the dielectric of the cable. Velocity of light in vacuum is 3 x10⁸ m/sec. It reduces when light passes through a medium. Velocity of light in a medium is given
by v=c / \(\surd \) k & where, c-velocity of light in vacuum and
k- is the dielectric constant of the medium Vf. =v/c=l/\(\surd \)k
For a line velocity factor is generally of the order of 0.6 to 0.9

Why is delta modulation a convenient method of digital modulation

1

(a)

Delta modulation involves simple pulse coding and decoding methods. A simple delta modulation uses just one bit per sample i.e. a ‘non-zero’ sample or one per sample. Thus, this method is convenient to use.