Electromagnetic waves are oscillations that transmit through free space with the velocity as that of light. Free space is the space that does not interfere with the normal radiation and propagation of the radio waves. Free space has no magnetic or gravitational field, no solid bodies and no ionized particles. Free space is unlikely exist anywhere. However the concept of free space is used because it simplifies the approach to Wave propagation. Radio waves are electromagnetic in nature, has an electric field and magnetic field. Electromagnetic are transverse wave, that the oscillations are perpendicular to the direction of propagation. Also the direction of electric field, magnetic field and the direction of propagation are mutually perpendicular to each other.

Polarization: Polarization is refers to the physical orientation of the radiated wave in space. A wave is said to be vertically polarized, when all its electric intensity vectors are vertical. A wave is said to be horizontally polarized when all its electric intensity vectors are horizontal. A vertical antenna will radiate vertically polarized wave and a horizontal antenna radiates horizontally polarized waves.

Reflection: Electromagnetic waves will be reflected by a conducting medium. They will be reflected by ground, mountains and buildings. This is much similar to the reflection of light by a mirror.

Refraction: Refraction takes place when electromagnetic waves pass from one propagating medium to another medium having a different density. They will get refracted as they pass through the layers of the atmosphere having different degrees of ionization.

Diffraction: Diffraction of electromagnetic waves occurs due to the presence of small slits in a conducting plane or sharp edges of obstacles. The electromagnetic waves may be diffracted around the tall massive objects.

Attenuation and absorption: The power density of the wave diminishes rapidly with distance from the source of electromagnetic waves. The attenuation is proportional to the square of displacement. In free space, absorption of radio waves does not occur because there is nothing to absorb them. However atmosphere is tend to absorb radio waves because some of the energy from the wave is transferred to the atoms and molecules of the atmosphere. Thus the energy of the waves may be absorbed quite significantly.

Ground Wave Propagation:

Frequency below high frequency range { very low frequency (3 KHz to 30 KHz), low frequency (30 KHz to 300 KHz) and medium frequency (300 KHz to 3 Mhz)} will travel along the curvature of the earth. So these waves are called the ground waves or surface waves. Ground waves are propagated by means of a type of waveguide effect, which uses the earth surface and the lowest ionized layer of the atmosphere as the two waveguide walls. Ground wave propagation is one of the two means of the beyond the horizon propagation. Ground waves must be vertically polarised to prevent the short circuit of the electric field components.

A ground wave is attenuated in two ways.

1. A wave induces currents in the ground over which it passes and they loses some of energy by absorption.

2. Because of the diffraction the wave front gradually tilts over as the wave propagate over the earth, its tilts increases more and more and increasing the tilt causes greater short circuiting of the electric field component of the wave and hence the field strength reduction. Eventually at some distance from the antenna the waves lies down and dies.

Use: Medium wave radio communication

Sky wave propagation

The ionosphere is the upper portion of the atmosphere which absorbs a large quantities of the energy from the sun, it becoming get heated and ionized due to the heat. There were several degrees of ionization at different height. The various layers of ionosphere have specific effects on the propagation of radio waves particularly at high frequency. Under certain conditions, waves in the high frequency range (3 MHz to 30 MHz) are return to the earth by the ionized layers of atmosphere so they are called sky waves. The mechanism involved in this process is refraction. The ionization density increases for a wave approaching the given layer (ionosphere) at an angle, so the refractive index of the layer is reduced. Hence the incident wave gradually bends further and further away from the normal. At a particular layer it will bend downward and finally emerging from the ionized layer at an angle equal to the angle of incidence as shown in figure. This is the second method for the beyond the horizon propagation.

Skip distance

It is the distance upto which sky waves of given frequency cannot be received or minimum distance from transmitting antenna to the point at which sky wave of a given frequency is returned to earth by ionosphere. It depends on frequency of transmission, critical frequency, and height of layer and increases as ionization in the layer reduces. It is the shortest distance measured along the surface of the earth at which a sky wave of fixed frequency will be returned to the earth.

Use: Short wave communication.

Frequency above high frequency range generally travels through the troposphere, the portion of the atmosphere closest to ground. So space waves are sometimes called troposphere waves. Space wave propagation depends on line of sight (LOS) conditions. So the space waves are limited in this propagation by the curvature of earth. When earth curvature can be neglected, space wave propagation take place in the manner illustrated is shown in the figure.

Here energy reaches the receiver in two ways

1. By a ray travelling directly between transmitting and receiving antennas (Direct wave).

2. By a ray that reaches the receiver after reflection from the surface of the ground (Ground reflected wave) or Satellite reflected wave.

The field strength at the receiving antenna is the vector sum of the fields represented by the two rays.