Question: Q. 2. What is space wave propagation? Which systems of communication use space waves ? What is ‘radio horizon’ of a transmitting antenna of height $h$ ? Why is space wave propagation suitable for frequencies above $40 \mathrm{MHz}$ ?
U] [2017 Foreign Set-III]
Q. 3. Draw a block diagram of a generalized communication system. Write the functions of each of the following :
(i) Transmitter
(ii) Channel
(ii) Receiver
U [2017, OD Set-III]
[Also accept the following diagram
Information
source $\rightarrow$\begin{tabular}{c}
Communication
channel
$\rightarrow \rightarrow$
Receiver of
Information
\end{tabular}
(i) Transmitter : A transmitter processes the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception.
(ii) Channel : It carries the message signal from a transmitter to a receiver.
(iii) Receiver : A receiver extracts the desired message signals from the received signals at the channel output. $1 / 2$
[CBSE Marking Scheme, 2017]
Commonly Made Error
- Few students couldn’t write the function of channel.
Q. 4. (i) Which mode of propagation is used by shortwave broadcast services having frequency range from a few $\mathrm{MHz}$ up to $30 \mathrm{MHz}$ ? Explain diagrammatically how long distance communication can be achieved by this mode.
(ii) Why is there an upper limit to frequency of waves used in this mode?
U] [O.D. I, II, III, 2016$]$
Show Answer
Solution:
Ans. (i) Sky wave propagation.
Long distance communication can be achieved by reflection of radio waves by the ionosphere, back towards the Earth. This ionosphere layer acts as a reflector only for a certain range of frequencies. (few $\mathrm{MHz}$ to $30 \mathrm{MHz}$ )
(ii) Electromagnetic waves of frequencies higher than $30 \mathrm{MHz}$, penetrate the ionosphere and escape whereas the waves less than $3 \mathrm{MHz}$ are reflected back to the earth by the ionosphere.
