question_answer

What does the term LOS communication mean? Name the type of waves that are used for this communication. Give typical example with the help of suitable figure of communication systems that use space wave mode propagation.

Or

(i) Describe briefly the three factors which justify the need for translating a low frequency signal into high frequencies before transmission.

(ii) Figure shows a block diagram of a detector for AM signal.

Detector for AM signal

Draw the waveforms for the

(a) input AM wave at A.

(b) output B at the rectifier.

(c) output signal at C.

Answer:

Line of Sight Communication (LOS) LOS is a type of communication where data can be transmitted and received only if the transmission and receiving stations are in view of each other without any sort of obstacle between them. Space waves are used for this communication FM radio, microwave and satellite transmission are examples of LOS communication. Line of sight communication by space waves. \[{{d}_{T}}\] is called the radio horizon of the transmitting antenna, \[{{d}_{M}}\] is called the maximum line of sight distance between the two antennas. The space wave propagation is used for television broadcast, microwave link and satellite communication. It is the mode of wave propagation in which the radio waves emitted from the transmitter antenna reach the receiving antenna through space. These radio waves are called space waves. The space waves are the radio waves of frequency range from 54 MHz to 4.2 GHz. Space waves travel in straight lines. The communication through space wave between transmitter and receiver is limited to line of sight path. The line of sight communication is limited by (a) the line of sight distance and (b) the curvature of the earth. The space waves following the line of sight propagation get blocked at some point by the curvature of the earth as shown in the figure. Or (i) Need for Modulation A message signal is also called a base band signal, which usually spreads over a range of frequencies, called the signal bandwidth. Suppose we wish to transmit an electronic signal in the audio frequency AF range (20 Hz to 20 kHz) over a long distance directly. This cannot be done due to the following factors (a) Size of the Antenna or Aerial An antenna is needed both for transmission and reception. Each antenna should have a size comparable to the wave length of the signal (at least \[\lambda \,/\,4\,in\]size). For an electromagnetic wave of frequency 20 kHz, wavelength is 15 km. \[\therefore \] Length of antenna \[=\frac{15}{4}km=3.75\,km\] Obviously, such a long antenna is not possible. Hence, antenna length can be made reasonable, if the frequency is high. Hence, there is need to convert low frequency signal into high frequency signal before transmission. (b) Effective Power Radiated by an Antenna Power \[P\] radiated from a linear antenna of length \[l\] is proportional to \[{{\left( \frac{l}{\lambda } \right)}^{2}},\] i.e. \[p\propto {{\left( \frac{l}{\lambda } \right)}^{2}}.\] As, high powers are needed for good transmission, therefore for given antenna length, wavelength l should be small or frequency n should be high. This also points towards the need of using high frequency transmission. (c) Mixing up of Signals from Different Transmitters Suppose many people are talking at the same time or many transmitters are transmitting baseband information signals simultaneously. All these signals will get mixed up and there is no simple way to distinguish between them. The possible solution is to use communication at high frequencies and allotting a band of frequencies to each message signal for its transmission. (ii)