What are ground wave, sky wave and Space waves
Wave which glides over the surface of the earth is called ground wave and the mode of propagation is called as surface wave propagation.
To radiate signals with high frequency, the antennas should have a size comparable to the wavelength λ of the signal on or very near to the ground. In antennas have large physical based vertical towers current in the ground. In standard AM broadcast, ground-based vertical towers are generally used as transmitting antennas.
A wave induces current in the ground over which it passes and it is attenuated as a result of absorption of energy by earth. The attenuation of surface wave increases very rapidly with increase in frequency. The maximum range of coverage depends on the transmitted power and frequency. Ground waves will propagate long distance over sea water due to its high conductivity.
Long distance communication between two points on earth can be achieved by the ionospheric reflection of radio waves are called sky waves.
These waves have a frequency range from a few MHz up to about 30 MHz. The mode of propagation is called sky wave propagation.
The ionospheric layer acts as a reflector for a certain range of frequencies (3 to 30 MHz). Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and escape. The phenomenon of bending of electromagnetic waves is so that they are diverted towards the earth which is similar to a total internal reflection in optics. Above 200 MHz ionosphere becomes completely transparent i.e., the waves of that order cannot be reflected back to earth.
These waves used in shortwave broadcast services.
Space wave communication
The waves which travel in a straight line from transmitting antenna to the receiving antenna are space waves. Communication done by using space waves is known as space wave communication.
At frequencies about 40 MHz communication is limited to line of sight paths. At these frequencies, the antennas are relatively smaller and can be placed at heights of many wavelengths above the ground.
Because of the line-of-sight nature of propagation, direct waves are blocked at some point by the curvature of the earth. If the signal is to be received beyond the horizon, then the receiving antenna must be high enough to intercept the line of sight waves.