A TV trasmission tower has a height of 240 m. Signals broadcast from this tower will be received by LOS communication at a distance of (assume the radius of earth to be $6.4 \times 10^6$ m)

If the whole earth is to be connected by LOS communication using space waves (no restriction of antenna size or tower height), what is the minimum number of antennas required? Calculate the tower height of these antennas in terms of earths radius?

Audio sine waves of 3 kHz frequency are used to amplitude modulate a carrier signal of 1.5 MHz. Which of the following statements are true?

I-V characteristics of four devices are shown in Fig. 15.1 Identify devices that can be used for modulation:

An audio signal is modulated by a carrier wave of 20MHz such that the bandwidth required for modulation is 3kHz. Could this wave be demodulated by a diode detector which has the values of $R$ and $C$ as (iii) $R = 10 \text{ k}\Omega$, $C = 0.1\mu\text{F}$.

On radiating (sending out) an AM modulated signal, the total radiated power is due to energy carried by $\omega_c$, $\omega_c - \omega_m$ & $\omega_c + \omega_m$. Suggest ways to minimise cost of radiation without compromising on information.

A message signal of frequency $\omega_m$ is superposed on a carrier wave of frequency $\omega_c$ to get an amplitude modulated wave (AM). The frequency of the AM wave will be

An amplitude modulated wave is as shown in Fig. 15.4. Calculate (iii) peak value of information voltage.

The maximum frequency for reflection of sky waves from a certain layer of the ionosphere is found to be $f_{\text{max}} = 9\sqrt{N_{\text{max}}}$, where $N_{\text{max}}$ is the maximum electron density at that layer of the ionosphere. On a certain day it is observed that signals of frequencies higher than 5MHz are not received by reflection from the $F_1$ layer of the ionosphere while signals of frequencies higher than 8MHz are not received by reflection from the $F_2$ layer of the inonosphere. Estimate the maximum electron densities of the $F_1$ and $F_2$ layers on that day.

(ii) Attenuation of a signal can be expressed in decibel (dB) according to the relation $\text{dB} = 10 \log_{10} \frac{I_o}{I}$. What is the attenuation in dB/km for an optical fibre in which the intensity falls by 50 per cent over a distance of 50 km?

A male voice after modulation-transmission sounds like that of a female to the receiver. The problem is due to

Which of the following would produce analog signals and which would produce digital signals?
(iii) Light pulse.

A 50 MHz sky wave takes 4.04 ms to reach a receiver via re-transmission from a satellite 600 km above earth’s surface. Assuming re-transmission time by satellite negligible, find the distance between source and receiver. If communication between the two was to be done by Line of Sight (LOS) method, what should size and placement of receiving and transmitting antenna be?

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:

A TV transmission tower antenna is at a height of 20 m. How much service area can it cover if the receiving antenna is (ii) at a height of 25 m? Calculate the percentage increase in area covered in case (ii) relative to case (i).

(ii) Is the plot symmetrical about $\omega_c$? Comment especially about plot in region $\omega < \omega_c$.

A basic communication system consists of
(A) transmitter.
(B) information source.
(C) user of information.
(D) channel.
(E) receiver.
Choose the correct sequence in which these are arranged in a basic communication system:

(iii) Extrapolate and predict the problems one can expect if more waves are to be modulated.

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 5 km, the power of the signal received is [gain in dB = $10 \log \frac{P_o}{P_i}$]

Which of the following would produce analog signals and which would produce digital signals?
(iv) Output of NAND gate.