Physics > Communication Systems > 1.0 Elements of a communication system
Communication Systems
1.0 Elements of a communication system
1.1 Terminology used in electronic communication system
1.2 Bandwidth of signals
1.3 Bandwidth of transmission medium
1.4 Earth's atmosphere
2.0 Radio waves
3.0 Modulation
1.4 Earth's atmosphere
1.2 Bandwidth of signals
1.3 Bandwidth of transmission medium
1.4 Earth's atmosphere
The gaseous envelope which surrounds the earth is known as earth's atmosphere.
The earth atmosphere mainly consists of nitrogen 78%, oxygen 21% along with a little portion of argon, carbon dioxide, water vapour,
hydrocarbons, sulphur compounds and dust particles.
The density of the atmospheric air goes on decreasing as we go up.
The electrical conductivity of the atmospheric air increases as we go up.
The various regions of earth's atmosphere are as follows,
- Troposphere: It extends upto a height of 10 $km$ over earth's surface.
- Stratosphere: It extends from 10 $km$ to 50 $km$. There is an ozone layer in stratosphere which mostly absorbs high energy radiations like ultraviolet radiations etc. coming from outer space.
- Mesosphere: It extends from 50 $km$ to 65 $km$.
- Ionosphere: It extends from 65 $km$ to 400 $km$.
In Ionosphere, the temperature rises to some extent with height, hence it is also known as thermosphere.
The ionosphere which is composed of ionised matter (i.e. electron and positive ions) plays an important role in space communication.
The ionosphere is subdivided into four main layers as $D$, $E$, $F_1$ and $F_2$ as shown in the table.
| S.No. | Name of the layer | Approximate height over earth's surface | Exists during | Frequencies most affected |
| 1 | $D$ | 65 - 75 $km$ | Day only | Reflects LF, absorbs MF and HF to some degree |
| 2 | $E$ | 100 $km$ | Day only | Helps surface waves, reflects HF |
| 3 | $F_1$ | 170 - 190 $km$ | Day time merges with $F_2$ at night | Partially absorbs HF waves yet allowing them to reach $F_2$ |
| 4 | $F_2$ | 300 $km$ at night, 250 - 400 $km$ during day time | Day and night | Efficiently reflects HF waves, particularly at night |
