WAVE PROPAGATION

The way in which a Radio wave travels, is termed as wave propagation.

GROUND WAVE PROPAGATION

Principle: Diffraction

The ground wave follows the surface of the earth partly because of diffraction, a phenomenon associated with all wave motion which causes the wave to bend around any obstacle it passes.

In addition, the wave of the magnetic field cuts the earth’s surface, so causing currents to flow. The required power for these currents must come from the wave, thus a flow of energy from wave to earth takes place. This is called Surface Attenuation.

Due to the bending property, the range of Ground waves is low.

Ground waves are used for V.L.F, L.F. and lower end of M.F.

SKY WAVE PROPAGATION

Principle: Reflection but prone to refraction

When there is a change in the density of the medium through which the EM wave is propagating, the wave changes direction. This phenomenon is similar to the refraction of light. When EM waves of H.F. and upper end of M.F. bands travel towards the sky, they enter ionosphere.

Radio waves striking the ionosphere (lying between 50 and 500 km above the earth’s surface) are refracted by an amount depending on the frequency of the incident wave.

Under favorable circumstances the wave will return to the earth, which can said to be reflected by the ionosphere. Such reflected back waves are called sky waves

SKIP DISTANCE

The shortest distance between transmitter and the point of return (one hop) along the surface of the earth is known as the skip distance.

CRITICAL FREQUENCY

The maximum frequency that will be reflected back to the earth by the ionosphere.

A higher frequency will get refracted at higher layer of ionosphere whereas a lower frequency will get refracted at lower layers.

At V.L.F. the ionosphere causes reflection rather than refraction, with little loss in energy. Thus V.L.F. navigation aids of extremely long range may be used.

CRITICAL ANGLE

The maximum angle which will get reflected back to the earth by the ionosphere.

SINGLE HOP AND MULTIPLE HOP

The stronger sky wave reflected by the ionosphere can again be reflected back by the earths surface. This is known as Single hop. Multiple hops may occur giving a very long range.

Sky wave propagation is useful for H.F. communication but can cause problems with L.F. and M.F. navigation aids since the sky wave and ground wave may combine at the receiver in such a way as to cause fading, false direction of arrival or false propagation time measurements.

FADING

When two waves of the same signal leave the transmitter at the same time but reach the receiver through different paths, will have difference in phase. If both are

• in phase – strength of signal is more

• Out of phase- strength of signal is less

If both the waves are exactly out of phase and of same amplitude they may completely cancel out each other.

This fluctuation of signal strength when it arrives at the receiver is called fading

The ionosphere consists of 4 layers:

1. D layer: Height- 70kms & Thickness- 10kms

2. E layer: Height- 100 kms & Thickness- 25 kms

3. F1 layer: Height- 180 kms & Thickness- 20 kms

4. F2 layer: Height- 250 to 400 kms & Thickness- 200 kms

At night, the layers D and F2 vanishes, which results in the reflection of the Sky waves from layers E and F1.

Therefore, At night for similar point of reflection we use a higher frequency in comparsion to day time frequency.

This is what is done in H.F. communication

SPACE WAVE PROPAGATION

Above 30 MHz, space waves, sometimes called line of sight waves/communication, are utilized.

In this type of communication, the transmitter must be in direct LOS for the waves to reach. These waves do not bend and will pass through space. Therefore any object infront (including thunderstorms and other aircrafts) will deteriorate the wave from receiving the receiver.

If an obstruction is infront, the wave will not be able to propagate, and therefore the area after the obstacle is termed as Shadow zone

From about 100 MHz to 3 GHz, the transmission path is highly predictable and reliable, and little atmospheric attenuation occurs.

Above 3 GHz attenuation and scattering occur, which become limiting factors above about 10 GHz.

SUPERREFRACTION (DUCTING)

During temperature inversion, at about 30 metres from the ground, complete bending down of space wave take place from the layer of atmosphere. Hence, the waves are refracted back to the earth and reflected back by the surface continuously, which allows them to propagate around the curvature of the earth for long distances which sometime exceeds 1000 kms.

This phenomenon is called superrefraction or ducting ( the earth’s surface and the refracting layer of the atmosphere acts like a duct for propagation of waves)