When the electromagnetic waves are propagated from the transmitter antenna, it travels in a free space, up to receiver antenna. The movement of theses electromagnetic waves took place by different ways. Each way of propagation is explained as below.
- Ground waves
- Sky Waves
- Space Waves
When the electromagnetic waves are propagated and if it travels along the surface of the earth towards the receiver antenna, this type of propagation is called ground waves propagation, and this type of electromagnetic waves are known as ground waves. The frequency of these waves is up to 3 MHz this type of waves is also known as surface waves.
The distance covered by ground waves from transmitter antenna to receiver antenna is depends on following factors.
Power of Transmitter
If the output power of transmitter is high the distance covered by the ground waves will be more.
Frequency of Transmitter
If the ground waves are propagated at high frequency, its wave length will be small and the distance covered will be less, if the frequency of the ground waves is less and wave length is long the distance covered will be more.
Condition of Earth Surface
If the earth surface is plane or smooth while the propagation of electromagnetic waves take place the distance covered will be more. It there is buildings, trees, hills etc. in a way of the ground waves the attenuation will take place and the distance covered by those waves will be less.
The electromagnetic waves which travels towards the sky after propagation and reflects back to the earth from the ionosphere is known as the sky waves. The distance traveled by the sky waves from the transmitter antenna depends upon the following factor.
Angle of Radiation
If the angle of radiation from the transmitter antenna towards the sky is less, the sky waves will be received for the shorter distance.
If the angle of radiation from the transmitter antenna towards the sky is more, the sky waves will be received from the longer distance from transmitter
Strength of Ionosphere
If the strength of the ionosphere is more, the distance covered by the sky waves from transmitter to receiver will be more, because attenuation of sky waves in the ionosphere will be less. If the strength of the ionosphere is less, the distance covered by the sky waves from transmitter to receiver will be less because the attenuation will be more in the ionosphere.
Power of Transmitter
If the output power of the transmitter is high, the distance covered by the sky waves form transmitter to receiver will be more and vice versa.
When the electromagnetic waves are propagated from the transmitter antenna and it travels in a straight line without any reflection or refraction. It is known as space wave.
This type of transmission is also called line of sight transmission. The frequency of the space waves is above 30 MHz and the distance covered by these waves is about 100 miles or 121 Km.