Test: Communication Systems - 2 - CUET Humanities MCQ

The perceived frequency of sky wave for reflection from an ionospheric layer is
vc = 9n^1/2
where, n is the number density of electrons/m³.
Given, n = 10¹¹/m³
∴ v_c = 9 x (10¹¹) ^1/2
= 2.8 MHz

In sky-wave propagation, the radio waves which have frequencies between 2 MHz to 30 MHz, are reflected back to the ground by the ionosphere. But radio waves, having frequency greater than 30 MHz cannot be reflected by the ionosphere because at this frequency, they penetrate the ionosphere. It makes the sky-wave propagation less reliable for propagation of TV signals having frequency greater than 30 MHz.
Critical frequency is defined as the highest frequency that is returned to the earth by the ionosphere. Thus, above this frequency, a wave, whether or not it is electromagnetic, will penetrate the ionosphere.

Radio waves can be sent from one place to another by using all of the given modes.

• Ground wave propagation: Ground wave propagation is a type of radio propagation which is also known as surface wave. These waves propagate over the earth's surface in low and medium frequencies. These are mainly used for transmission between the surface of the earth and the ionosphere. The waves follow the curvature of the earth enabling them to cover beyond the horizon.
• Sky wave propagation: Sky wave propagation, also known as skip, is a type of radio wave propagation. It is either the reflected or refracted back waves to the earth from the ionosphere which is an electrically charged layer of the upper atmosphere. Medium and shortwave frequencies can be refracted back to the earth which is beyond the horizon, which makes them useful in the transcontinental transmission of the waves.
• Space wave propagation: Space wave propagation is defined for the radio waves that occur within the 20 km of the atmosphere, i.e. troposphere, comprising direct and reflected waves. These waves are also known as tropospheric propagation as they can travel directly from the earth's surface to the troposphere surface of the earth. It is also known as a line of sight propagation as the signals are sent in a straight line from the transmitter to the receiver.

In any communication system, information (a physical quantity) is first converted into an electrical signal by a device called transducer. So, a transducer is a device which converts a physical signal into electrical signal and vice versa. Most of the speech or information signal cannot be directly transmitted to long distances. For this, an intermediate step of modulation is necessary in which the information signal is loaded or superimposed on a high frequency wave which acts as a carrier wave.

Given: β = 62, R₀ = R_L = 5000Ω, R_i = 500Ω

AM receivers are simple as compared to FM receivers, but they are noise prone.

λ =  = 3 x 10⁵ m = 300 km

Ground wave propagation is effective for frequencies in the range of 500 kHz to 1500 kHz. The correct choice is (2).

Here, h = 100 m, R = 6.4 x 10⁶ m
Using d = , we get:
d = 
d = 3.58 x 10⁴ m = 35.8 km
 Communication is possible up to 35.8 km.
Area covered,
A = πd²
A = 3.14 x (35.8)²
A = 4024.35 km²

For TV transmission, the frequency employed is normally 30 MHz – 300 MHz.