The process of superimposing the audio signal over a high frequency carrier wave is called modulation.
In the process of modulation anyone characteristic of carrier wave is varied in accordance with the instantaneous value of audio signal (modulating signal).
Need of Modulation
(i) Energy carried by low frequency audio waves (20 Hz to 20000 Hz) is very small.
(ii) For efficient radiation and reception of signal. the transmitting and receiving antennas should be very high approximately 5000 m.
(iii) The frequency range of audio signal is so small that overlapping of signals create a confusion.
Types of Modulation
(i) Amplitude Modulation In this type of modulation, the amplitude of high frequency carrier wave is varied in accordance to instantaneous amplitude of modulating signal.
Bandwidth required for amplitude modulation= twice the frequency of the modulating signal.
(ii) Frequency Modulation In this type of modulation, the frequency of high frequency carrier wave is varied in accordance to instantaneous frequency of modulating signal
(iii) Pulse Modulation In this type of modulation, the continuous waveforms are sampled at regular intervals. Information is transmitted only at the sampling times
The process of separating of audio signal from modulated signal is called demodulation.
An antenna converts electrical energy into electromagnetic waves at transmitting end and pick up transmitted signal at receiving end and converts electromagnetic waves into electrical signal.
The term modem is contraction of the term modulator and demodulator. Modem is a device which can modulate as well as demodulate the signal. It connect one computer to another through ordinary telephone lines.
Fax (Facsimile Telegraphy)
The electronic reproduction of a document at a distant place is called FAX
The radio waves are the electromagnetic waves of frequency ranging from 500 kHz to about 1000 MHz. These waves are used In the field of radio communication. With reference to the frequency range and wavelength range, the radio waves have been divided into various categories shown in table.
Frequency Range and Wavelength Range of Radio Waves
3 kHz to 30 kHz
10 km to 100 km
Long distance point to point
Low Frequency (LF)
30 kHz to 300 kHz
1 km to 10 km
300 kHz to 3 MHz
100 m to 1 km
3 MHz to 30 MHz
10 m to 100 m
Communication of all types
30 MHz to 300
1 m to 10 m
T V Radar and air navigation
300 MHz to 3000 MHz
10 cm to 1 m
3 GHz to 30 GHz
1 cm to 10 cm
Radar, Radio relays and navigation
Extremely-High - Frequency (EHF)
30 GHz to 300 GHz
1 mm to 1 cm
Propagation of Radio Waves
The three modes are discussed below.
(i) Ground Wave or Surface Wave Propagation It is suitable for low and medium frequency up to 2 MHz. It is used for local broad casting.
(ii) Sky Wave Propagation It is suitable for radio waves of frequency between 2 MHz to 30 MHz. It is used for long distance radio communication.
Critical Frequency The highest frequency of radio wave that can be reflected back by the ionosphere is called critical frequency.
Critical frequency, vc = 9 (Nmax)1 / 2
Where, Nmax = number density of electrons/metre3.
Skip Distance The minimum distance from the transmitter at which a sky wave of a frequency but not more than critical frequency, is sent back to the earth.
Skip distance (Dskip) = 2h (Vmax / Vc)2 – 1
where h is height of reflecting layer of atmosphere,
Vmax is maximum frequency of electromagnetic waves and Vc is critical frequency.
Fading The variation in the strength of a signal at receiver due to interference of waves, is called fading.
(iii) Space Wave Propagation It is suitable for 30 MHz to 300 MHz. It is used in television communication and radar communication. It is also called line of sight communication.
d = √2RH
It IS carried out between a transmitter and a receiver through a satellite. A geostationary satellite is utilized for this purpose, whose time period is 24 hours.
A communication satellite is a space craft, provided with microwave receiver and transmitter. It is placed in an orbit around the earth. The India remote sensing satellites are
IRS-IA, IRS-IB and IRS-IC
The line-of-sight microwave communication through satellite is possible if the communication satellite is always at a fixed location with respect to the earth, e.g., the satellite which is acting as a repeater must be at rest with respect to the earth. It is so far a satellite known as geo-stationary satellite.
The basic requirements for geostationary satellites are as follows:
The orbit in which the gee-satellite above revolves around the earth is known as geo-synchronous orbit. As its angular speed is synchronized with the angular speed of the earth. therefore, the geo-stationary satellite is also known as geo-synchronous satellite.
Merits of Satellites Communication
Demerits of Satellite Communication
1. If a system on the satellite goes out of order due to environmental stresses, it is almost impossible to repair it.
2. In satellite communication, there is a time delay between transmission and reception, due to extremely large communication path length (greater than 2 x 36000 km). This delay causes a time gap during talking, which proves quite annoying.
It is a technique of observing or measuring the characteristics of the object at a distance. A polar satellite is utilized for this purpose.
Distance upto which a signal can be obtained from an antenna is given by
d = √2hR
where, h is height of antenna and R is radius of earth.
LED and Diode Laser in Communication
Light Emitting. Diode (LED) and diode laser are preferred sources for optical communication links to the following features.
An optical fibre is a long thread consisting of a central core of glass or plastic of uniform refractive index. It is surrounded by a cladding of material of refractive index less than that of the core and a protective Jacket of insulating material.
There are three types of optical fibre configuration
Applications of Optical Fibres