JEE Main & Advanced Physics Communication System / संचार तंत्र Space Wave Propagation

The space waves are the radio waves of very high frequency (30 MHz to 300 MHz) ultra high frequency (300 MHz to 3000 MHz) and microwave (more than 3000 MHz). At such high frequencies, the sky wave as well as ground wave propagation both fails.

These waves can be transmitted from transmitting to receiving antenna either directly or after reflection from the ground or in troposphere, the wave propagation is called space wave propagation.

The space wave propagation is also called as line of sight propagation. The line of sight distance is the distance between transmitting antenna and receiving antenna at which they can see each other.

Space wave propagation can be utilised for transmitting high frequency TV and FM signals.

(1) Television signal propagation : Frequency range for propagation is 80 MHz to 200 MHz

Height of transmitting antenna : \[h=\frac{{{d}^{2}}}{2R}\] (d = distance covered by the signal, R = Radius of the Earth)

Area covered : \[A=\pi {{d}^{2}}=2\,\pi Rh\]

Population cover : Population density ´ Area covered

(2) Microwave communication : Microwave communication systems are used for long distance communication. Since at microwave frequencies, electromagnetic waves cannot bend across the obstacles, such as the top of the buildings, mountains etc., it is therefore necessary that microwave transmission is in line-of-sight.

Due to curvature in the surface of earth, the range of microwave transmission is very small \[(\approx 50\,\,km)\]. The range of microwave transmission is also limited by the fact that signals gets weaker and weaker as it propagates. However, these problems are overcome by using repeaters (A repeater is basically an amplifier, which amplifies the attenuated signal and then retransmits it.) at intervals between the transmitter and the receiver. Due to this, the cost of transmission of signal between the two stations increases.

The problems faced in a microwave communication system are solved to a large extent by using a geostationary satellite as a communication satellite.