COMMUNICATION

INTRODUCTION TO COMMUNICATION ENGG:

Communication is the conveyance of a message from one entity, called the source or transmitter, to another, called the destination or receiver, via a channel of some sort. A simple example of such a communication system is conversation; people commonly exchange verbal messages, with the channel consisting of waves of compressed air molecules at frequencies which are audible to the human ear. The term communication in electronics refers to sending, receiving and processing of information by electronic means.
Communication can be from a source to a single destination, known as point-to-point or unicast, or to multiple destinations, known as point-to-multipoint or multicast. A special case of multicast is the conveyance of a message from a source to every possible destination, which is referred to as broadcast.
The primary difference between multicast and broadcast is that multicast communication is targeted at specific destinations, regardless of location, while broadcast communication is targeted at all possible destinations within the range of the source.
A communication channel can be,
i. Simplex, in which only one party can transmit
ii. Full-duplex, in which both correspondents can transmit and receive simultaneously, or
iii. Half-duplex, in which the correspondents alternate between transmitting and receiving states.
Even though the channel might be capable of supporting full-duplex communication, if the corresponding entities are not capable of transmitting and receiving simultaneously, the communications system will be half-duplex.
Communication started with wire telegraphy in the eighteen fourties , developing with telephony some decades later and radio at the beginning of this century. Radio communication is made possible by the invention of vacuum triodes and radio system become even more popular and developed through the invention of Transistors, ICs and other semiconductor devices. The use of satellite, wireless technologies and fiber optics has made communication even more wide spread, with an increasing emphasis on computers and other data communications. In this module we are discussing the different terms associated with communication like Modulation, Propagation of waves, Frequency bands, Band width and different communication systems like Radio, TV, Mobile, Microwave etc



PROPAGATION OF WAVES:

Propagation of waves can take place in different ways.
i. Ground wave propagation:
A ground or surface wave is a form of propagation when the transmitting and receiving antennas are close to the surface of the earth and are vertically polarized. This wave, supported at its lower edge by the presence of the ground, is of practical importance at broadcast at lower frequencies
The ground wave is actually composed of two separate component waves. These are known as the SURFACE WAVE and the SPACE WAVE. The determining factor in whether a ground wave component is classified as a space wave or a surface wave is simple. A surface wave travels along the surface of the Earth. A space wave travels over the surface.

a. Surface wave:- The surface wave reaches the receiving site by traveling along the surface of the ground as shown in figure (1). A surface wave can follow the contours of the Earth because of the process of diffraction. When a surface wave meets an object and the dimensions of the object do not exceed its wavelength, the wave tends to curve or bend around the object. The smaller the object, the more pronounced the diffractive action will be.

b. Space propagation:- Here the radio signals travel in free space, or away from other objects which influence the way in which they travel. It is a form of propagation where the energy reaching the receiving antenna arrives through the earth's troposphere, i.e. the portion of the earth's atmosphere in the first 15km adjacent to the earth's surface. At frequencies above about 20 - 30 MHz the ionosphere is not able to refract energy to earth.
The space wave follows two distinct paths from the transmitting antenna to the receiving antenna - one through the air directly to the receiving antenna, the other reflected from the ground to the receiving antenna.


ii. Sky wave:
It is a form of propagation where the energy reaching the receiving antenna arrives as a result of a bending of the wave path introduced by the ionization in the upper atmosphere. This ionized region, termed the ionosphere, extends from about 50km to about 400km above the earth's surface, and accounts for practically all very long-distance HF (3 to 30 MHz) radio communications.