2.1 Propagation of electromagnetic waves in the atmosphere

These waves are vertically polarized in order to prevent short circuiting of the electric component. These waves induce currents in the ground as they propagate due to which some energy is lost by absorption. Apart from it, as ground wave or surface wave propagates over the surface of earth, the wavefront of the wave gradually tilts over the surface of the earth as shown in the figure and the tilt of the wave front of the wave increases as the wave propagates over the earth.

As a result, the strength of the wave decreases with the propagation of wave along the surface of earth . It is due to this tilt, the propagation of ground wave is limited.

The ground wave propagation is suitable for low and medium frequency, i.e. upto 2 $MHz$ only, hence it is also known as medium wave propagation.

The maximum range of ground or surface wave propagation depends on:

The sky waves are the radio waves of frequency between 2 $MHz$ to 30 $MHz$.

These radio waves can propagate through atmosphere and are reflected back by the ionosphere of earth's atmosphere.

When the sky waves from the transmitting antenna reach the receiving antenna after reflection in the ionosphere, the wave propagation is known as sky wave propagation.

The sky wave propagation is known as ionosphere propagation.

With the help of sky wave propagation, communication over a long distance around the globe is possible.

It is that frequency of radio wave, which when sent straight towards the layer of ionosphere gets reflected from the ionosphere and returns to the earth. If the frequency of the radio wave is more than critical frequency, it will not be reflected by ionosphere.

The critical frequency of a sky wave for reflection from a layer of atmosphere is given by, $${v_c} = 9{\left( {{N_{\max }}} \right)^{\frac{1}{2}}}$$

where $N_{max}$ is the maximum number density of electron per cubic metre $\left( {{m^3}} \right)$.

It is that highest frequency of radio waves which sent at some angle towards the ionosphere, gets reflected from it and returns to the earth.

$$MUF = \frac{{{v_c}}}{{\sin i}} = {v_c}\sec i$$

where $i$ is the angle between normal and the direction of incidence of waves.

It is the smallest distance from the transmitter along the earth's surface at which a sky wave of a frequency not more than critical frequency is sent back to the earth.

The skip distance is given by, $${D_{skip}} = 2h\sqrt {{{\left( {\frac{{{v_0}}}{{{v_c}}}} \right)}^2} - 1} $$

where,

$h$: Height of reflecting layer of atmosphere

$v_0$: Maximum frequency of electromagnetic waves

$v_c$: Critical frequency of that layer

The space waves are the radio waves of very high frequency (i.e. between 30 $MHz$ to 300 $MHz$ or more).

When the space waves from the transmitting antenna reach the receiving antenna either directly or after reflection from the ground in the earth troposphere's region, the wave propagation is known space wave propagation.

Space wave propagation is also known as tropospheric propagation.

Space wave propagation is also known as line of sight propagation.

The space waves travel in straight line from transmitting antenna to receiving antenna.

The space waves are used for the line of sight communication as well as for the satellite communication.

The space wave propagation is used for television broadcast, microwave link and satellite communication.

If $h_t$ is the height of the transmitting antenna, then the distance to the horizon is given by, $$d_t = \sqrt {2h_tR} $$

where $R$ is the radius of the earth.

Area covered $ = \pi {d^2} = \pi 2h_tR$

$${\text{Population covered }} = {\text{ Population density }} \times {\text{ Area covered }}$$

The maximum line of sight distance $d_m$ between two antennas having heights $h_t$ and $h_r$ above the earth is given by,

$${d_m} = \sqrt {2R{h_t}} + \sqrt {2R{h_r}} $$

where,

$h_t$: Height of the transmitting antenna

$h_r$: Height of the receiving antenna

$R$: Radius of the earth