Unguided Media

The wireless communication between a transmitting and a receiving station utilizing the space around the earth, i.e. atmosphere is called space communication.

The earth’s atmosphere plays a very interesting role the propagation of e.m. waves from one place to another due to change in air temperature, air density, electrical conductivity and absorption characteristics with height. For example, most of the radiations in infrared region are absorbed by the atmosphere. The ultraviolet radiations are absorbed by the ozone layer.

Five layers are considered to play main role in communication:

1. C layer at about 60 km above the surface of earth reflects e.m. waves in the frequency range 3 kHz – 300 kHz. It is therefore used for direct long range communication.

2. D layer at a height of about 80 km reflects e.m. waves in the low frequency range (3 kHz – 300 kHz) but absorbs waves in the medium frequency range (300 kHz – 3 MHz) and high frequency range (3 – 30 MHz).

3. E layer at a height of about 110 km helps in propagation of waves in the medium frequency range but reflects waves in the high frequency range in the day time.

4. F1 layer at a height of about 180 km lets most of the high frequency waves to pass through.

5. F2 layer (at a height of 300 km in daytime and 350 km at night) reflects e.m. waves up to 30 MHz and allows waves of higher frequencies to pass through.

Atmosphere

Based on the variation of temperature, air density and electrical conductivity with altitude, the atmosphere is thought to be made up of several layers. The atmospheric layer close to the earth called the troposphere extends up to about 12 km above the sea level. The temperature in troposphere vary between 290 K (at the equator) to 220 K (at tropopause). The air density is maximum but electrical conductivity is the least compared to other layers.

The next layer up to about 50 km is called the stratosphere. An ozone layer is in the lower stratosphere extends from about 15 km to about 30 km.

The layer above the stratosphere and up to about 90 km is called the mesosphere. The minimum temperature in mesosphere in about 180 K.

Beyond mesosphere up to 350 km, there is a zone of ionized molecules and electrons called the ionosphere. In ionosphere, temperature increases with height to about 1000 K. The ionosphere affects the propagation of radio waves. It is divided into D, E, F and F2 regions based on the number density of electrons, which increases with height from about 109 m–3 in D region to 1012 m–3 in F2 layer.

These variations in temperature, density and conductivity arise due to different absorption of solar radiations at different heights and changes in composition, etc.

The essential feature of space communication is that a signal emitted from an antenna of the transmitter has to reach the antenna of the receives. Depending on the frequency of radio wave, it can occur as ground wave, space wave, sky wave and via satellite communication.