GROUND WAVE PROPAGATION
If the electromagnetic waves transmitted by the transmitter
glide over the surface of the earth to reach the receiver, then the propagation
is called ground wave propagation. The corresponding waves are called ground
waves or surface waves. The pictorial representation is
shown in Figure 10.5(a).
Both transmitting and receiving antennas
must be close to the earth. The size of the antenna plays a major role in
deciding the efficiency of the radiation of signals.
During transmission, the electrical
signals are attenuated over a distance. Some reasons for attenuation are as
follows:
• Increasing distance: The attenuation
of the signal with distance depends on (i) power of the transmitter (ii)
frequency of the transmitter and (iii) condition of the Earth surface.
• Absorption of energy by the Earth: When the transmitted signal in the form of EM wave is in
contact with the Earth, it induces charges in the Earth and constitutes a
current. Due to this, the Earth behaves like a leaky capacitor which leads to
the attenuation of the wave.
• Tilting
of the wave: As the wave progresses, the
wavefront starts gradually tilting according to the curvature of the Earth.
This increase in the tilt decreases the electric field strength of the wave.
Finally at some distance, the surface wave dies out due to energy loss.
The frequency of the ground waves is
mostly less than 2 MHz as high frequency waves undergo more absorption of
energy at the earth’s atmosphere. The medium wave signals received during the
day time use surface wave propagation.
It is mainly used in local
broadcasting, radio navigation, for ship-to-ship, ship- to-shore communication
and mobile communication.
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