SKY WAVE PROPAGATION
The mode of propagation in which the electromagnetic waves radiated from an antenna, directed upwards at large angles, gets reflected by the ionosphere back to earth is called sky wave propagation or ionospheric propagation. The corresponding waves are called sky waves (Figure 10.5(b)).
The frequency range of EM waves in this mode of propagation is 3 to 30 MHz. EM waves of frequency more than 30 MHz can easily penetrate through the ionosphere and does not undergo reflection. It is used for short wave broadcast services. Medium and high frequencies are for long-distance radio communication. Extremely long- distance communication is also possible as the radio waves can undergo multiple reflections between the earth and the ionosphere. A single reflection helps the radio waves to travel a distance of approximately 4000 km.
Ionosphere acts as a reflecting surface. It is at a distance of approximately 50 km and spreads up to 400 km above the Earth surface. Due to the absorption of ultraviolet rays, cosmic ray, and other high energy radiations like α, β rays from sun, the air molecules in the ionosphere get ionized. This produces charged ions and these ions provide a reflecting medium for the reflection of radio waves or communication waves back to Earth within the permitted frequency range. The phenomenon of bending the radio waves back to earth is nothing but the total internal reflection.
This is the reason why the EM waves are transmitted at a critical angle to ensure that the waves undergo total reflection and reaches the ground without escaping into space.
The shortest distance between the transmitter and the point of reception of the sky wave along the surface is called as the skip distance shown in Figure 10.5(b).
The electromagnetic waves are transmitted from the ground at particular angles. When the angle of emission increases, the reception of ground waves decreases. At one point, there will be no reception due to ground waves and marked as A in the Figure 10.5(b).
If the angle of emission is increased further, the reception of sky waves starts at point B in the Figure 10.5(b). There is a zone (in between A and B) where there is no reception of electromagnetic waves neither ground nor sky, called as skip zone or skip area.
The higher the frequency, higher is the skip distance and for a frequency less than the critical frequency, skip distance is zero.