SPACE WAVE PROPAGATION
The process of sending and receiving information signal through space is called space wave communication (Figure 10.5(c)). The electromagnetic waves of very high frequencies above 30 MHz are called as space waves. These waves travel in a straight line from the transmitter to the receiver. Hence, it is used for a line of sight communication (LOS).
For high frequencies, the transmission towers must be high enough so that the transmitted and received signals (direct waves) will not encounter the curvature of the Earth and hence travel with less attenuation and loss of signal strength. Certain waves reach the receiver after getting reflected from the ground.
The communication systems like television telecast, satellite communication and RADAR are based on space wave propagation. Microwaves having high frequencies (super high frequency band) are used against radio waves due to certain advantages: larger bandwidth, high data rates, better directivity, small antenna size, low power consumption, etc.
The range or distance (d) of coverage of the propagation depends on the height (h) of the antenna given by the equation,
where R is the radius of the Earth and it is 6400 km.
The distance of coverage is shown pictorially in Figure 10.6.
A transmitting antenna has a height of 40 m and the height of the receiving antenna is 30 m. What is the maximum distance between them for line-of-sight communication? The radius of the earth is 6.4×106 m.
The total distance d between the transmitting and receiving antennas will be the sum of the individual distances of coverage.
d = d1 +d2
= √(2Rh) + √(2Rh2)
= √2√R (√h1 + √h2)
= √[2 × 6.4 ×106] × ( √40 + √30 )
=16 ×102 √5 ×(6.32 + 5.48)
=42217m= 42.217 km