When a wave from a medium strikes a surface & bent back into the same medium then the wave is said to be reflected wave. Wave reflects from a surface where the angle of incidence, θi = the angle of reflection, θr i.e. in reflection θi= θr

What is Wave Propagation? Definition, Equation, & Types

Wave Propagation is any of the ways in which wave travel . Single wave Propagation can be calculated by 2nd order wave equation ( standing wave field ) or 1st order one-wave equation.

With respect to the direction of the oscillation relative to Propagation direction, we can distinguish between longitudinal wave and transverse waves.

For electromagnetic wave Propagation may occur in a vacuum as well as in the material medium . Other wave types cannot propagate through a vacuum and need a transmission medium to exist.

Reflection

When a wave from a medium strikes a surface & bent back into the same medium then the wave is said to be reflected wave.
Wave reflects from a surface where the angle of incidence, θi = the angle of reflection, θr i.e. in reflection θi= θr

Radio waves will bend or refract when they go from one medium (with refractive index, n1)to another (with refractive index, n2).

Snell’s Law states that:

That is in this θi = θr

Diffraction

Diffraction is the phenomenon which results in radio waves that normally travel in a straight line to bend around an obstacle.

Types of Waves

Troposphere

Extends from the surface of earth to a height of about 12km.
This is the lowest atmospheric layer and is about seven miles (11 km) thick.
Most clouds and weather are found in the troposphere.
The troposphere is thinner at the poles (averaging about 8km thick) and thicker at the equator (averaging about 16km thick).
The temperature decreases with altitude

Stratosphere

The stratosphere is found from about 12-50 km above the earth’s surface.
Located b/w troposphere & ionosphere.
Temp. Is considered to be almost constant & there is little water vapour present. So it has little effect on radio waves.
In this region of the atmosphere, there is the ozone layer, which absorbs most of the harmful ultraviolet radiation from the Sun.
The temperature increases slightly with altitude in the stratosphere.

Ionosphere

Extends upwards from about 50 km to a height of about 400 km.
The ionosphere overlaps the other atmospheric layers, from above the Earth.
The air is ionized by the Sun’s ultraviolet light.
The ionized layer enables the radio waves to be propagated to great distances around the earth.
Different ioniosphere layers are the D, E and F.

Ground Wave propagation or Surface Wave propagation

Propagate in the frequency range of 30khz to 3Mhz.
Ground waves are radio waves that travels near the earth’s surface.
In this propagation waves leave antenna & remain close to the earth.
A surface wave follows the curvature of earth due to the process of diffraction.
Due to less frequency range it is also called medium wave propagation.

in this, intensity of the signal falls with distance due to its absorption by ground.
GWs propagates along the surface of earth & must be vertically polarized to prevent short circuiting the electric component.
A wave induces currents in the ground over which it passes & thus loses some energy by absorption.
While propagating, wave tilts over more & more. This causes greater short circuiting of the EF components of the wave.
Eventually at some distance from antenna, the wave lies down & dies.

Transmission paths & angle of tilt

Advantages

Using very high powered transmitters the surface wave can be propagated over long distances.
Vertical polarization is superior to horizontal polarization for GWP.

Disadvantages

GW are limited to very low & medium freqs. Requiring very large antenna.
High powered transmitters are required to cover an adequate range.

Applications

AM broadcasting over medium wave band.
In military communication.

Sky Wave Propagation (Ionospheric propagation)

The sky waves are in the frequency range of 3 Mhz to 30 Mhz.
Due to their high freq., the sky waves are called short waves.
Propagation of radio waves from one point to another via reflection from ionosphere is known as SWP.
With multiple reflections sky wave reaches the receiver.
As a wave travels up in the ionosphere, it finds itself travelling from denser to rarer medium. That means in the last it suffers TIR to reach back to the earth.
Extends from a height of about 50 km to over 500 km, most of the molecules of the atmosphere are ionised by radiation from the Sun. This region is called the ionosphere.
Ionisation is the process in which electrons, which are negatively charged, are removed from neutral atoms or molecules to leave positively charged ions and free electrons. It is the ions that give their name to the ionosphere, but it is the much lighter and more freely moving electrons.
The free electrons in the ionosphere cause HF radio waves to be refracted (bent) and eventually reflected back to earth. The greater the density of electrons, the higher the frequencies that can be reflected.
The ionosphere absorbs large quantites of energy from the sun, becoming heated & ionized.
The ionizing agents are ultraviolet & α, β & γ radiation from the sun.
During the day there may be four regions present called the D, E, F1 and F2 regions. Their approximate height ranges are:
D region 50 to 90 km
E region 90 to 140 km
F1 region 140 to 210 km
F2 region over 210 km.

Layers of Ionosphere

The D region is the lowest in altitude 60 km (37 mi) to 90 km (56 mi) above the surface of the Earth. It is present during the day when radiation is beaming in from the sun. After sunset when there is no radiation from the sun the D layer effectively disappears. This layer acts as an attenuator, especially at low frequencies. This is why low-frequency signals are prevented from reaching the higher layers, except at night when the D layer disappears.
E layer is the next in height, existing 90 km (56 mi) to 150 km (93 mi) above the surface of the Earth. Like D layer it disappears at night. The main effect of E layer is to aid MF surface wave propagation & to reflect some HF waves in daytime.
The F1 layer exists at a height of 140km in daytime & combines with the F2 layer at night. Main effect of F1 layer is to provide more absorption for HF waves. Although some HF waves are reflected from it, most pass through to be reflected from F2 layer.
F2 layer ranges from 250km to 400km in daytime. This is the highly ionized layer. The lifetime of free electrons is greatest in the F2 region which is one reason why it is present at night. The F2 region is the most important region for HF radio propagation bcoz: it is present 24 hours of the day; • its high altitude allows the longest communication paths; • it reflects the highest frequencies in the HF range.

Bending of radio waves in the ionosphere

As the ionization density increases, the refractive index of the layer gets reduced. Hence incident wave is gradually bent as shown in figure. When the electron density is large, the angle of refraction becomes 90˚ & wave then travel towards the earth.
Figure shows that angle of refraction gradually increases & the wave goes on bending. When the angle becomes 90˚ wave travels parallel to the earth’s surface i.e. R=90˚
Beyond this point the wave tend to move towards the earth.

Critical frequency

The frequency where the angle of refraction becomes 90˚ is called critical frequency. At this frequency the wave becomes parallel with the earth surface

Virtual height

The virtual height is the height from which the radio wave appears to be reflecting.

Maximum Usable Frequency

Maximum usable frequency (MUF) is the highest frequency at which propagation exists between two points. Frequencies higher than the MUF pass through the ionosphere into space & are lost.
MUF = fc sec qi

Skip Distance

The SKIP DISTANCE is the distance from the transmitter to the point where the sky wave is first returned to Earth.
The size of the skip distance depends on the frequency of the wave, the angle of incidence, and the degree of ionization present.
The SKIP ZONE is a zone of silence between the point where the ground wave becomes too weak for reception and the point where the sky wave is first returned to Earth.
The size of the skip zone depends on the extent of the ground wave coverage and the skip distance. When the ground wave coverage is great enough or the skip distance is short enough that no zone of silence occurs, there is no skip zone.

Multiple hop propagation

MULTIPATH is simply a term used to describe the multiple paths a radio wave may follow between transmitter and receiver.

Space-Wave Propagation

SWP propagates in the freq. Range above 30Mhz to 300Mhz.
It is also called Line of Sight (LOS) propagation.
In this both transmitting & receiving antennas are in sight of each other.

This propagation may follow the direct air path to the receiving antenna or other reflected from the ground to the receiving antenna.

space-Wave Propagation

The approximate maximum distance of communication is given by:

Reflections from a relatively smooth surface, such as a body of water, could result in partial cancellation of the direct signal - a phenomenon known as fading. Also, large objects, such as buildings and hills, could cause multipath distortion from many reflections.

Limitations

These waves are limited to the curvature of the earth.
Needs LOS propagation.

Applications

TV & FM radio broadcasting
For microwave communication.
For satellite communication.

Duct Propagation

Under normal atmospheric conditions, the warmest air is found near the surface of earth.
This air gradually becomes cooler as altitude increases.
Thus an unusual situation developed in which layers of warm air are formed above layers of cool air, often over the surface of water.
The result is that the refractive index will decreases with height.
This happens near the ground, often within 30m of it.
This condition is known as temperature inversion.
These temp. Inversions causes channels or ducts of cool air to be sandwiched b/w the surface of earth & a layer of warm air or b/w two layers of warm air.
This layer returns the signals to the earth.
The signals can then propagate over long distances by alternately reflecting from the earth and refracting from the superrefractive layer.
These long distances are possible bcoz of the different densities & refractive qualities of warm & cool air.
The sudden change in density when a radio wave enters the warm air above a duct causes the wave to be refracted back toward earth.