AIM OF THE SUBJECT WILL BE COVERED IN THE FOLLOWING PHASES :- PHASE-I :-INTRODUCTION PHASE-II :-IMPORTANT TERMINOLOGY USED PHASE-III :-PROPAGATION OF ELECTROMAGNETIC.

AIM OF THE SUBJECT WILL BE COVERED IN THE FOLLOWING PHASES :-

PHASE-I :-INTRODUCTIONPHASE-II :-IMPORTANT TERMINOLOGY USEDPHASE-III :-PROPAGATION OF

ELECTROMAGNETIC WAVEPHASE-IV :-SKY WAVE PROPAGATIONPHASE-V :-SPACE WAVE PROPAGATION PHASE-VI :-GROUND WAVE PROPAGATIONPHASE VII:- SUMP UP

PHASE -I Electromagnetic radiation is energy, which is sent through free space or through a material medium in the form of electromagnetic waves. An electromagnetic wave is a transverse wave in that the electric field and the magnetic field are perpendicular to each other as well as to the direction of the wave movement, at any point or time in the wave. However, electromagnetic waves are not affected by normal electric or magnetic fields.  Electromagnetic waves have properties similar to other forms of wave such as reflection, refraction, diffraction and interference. 

Electromagnetic waves were first postulated by James Clark Maxwell and subsequently confirmed by Heinrich Hertz.

Electromagnetic waves - can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This diagram shows a plane linearly polarized wave propagating from right to left. The electric field is in a vertical plane, the magnetic field in a horizontal plane.

PHASE II

IMPORTANT TERMINOLOGY USED –FREQUENCY:-

The frequency of a wave is its rate of oscillation and is measured in HERTZ, the unit of frequency, where one hertz is equal to one oscillation per second. RELATION:- Velocity = Frequency X Wavelength WAVELENGTH :-

A wave consists of successive troughs and crests, and the distance between two adjacent crests or troughs is called the wave length

REFLECTION - When light is incident upon some surfaces light can be reflected from it The direction that the light reflects is so that it comes off the surface at the same angle as shown in the diagram. This is known as the law of reflection.i.e. Angle of Incidence = Angle of ReflectionNote that the angles are measured from a line that is at right angles to the surface.  This is known as the Normal.

REFRACTION When light is able to pass through some materials such as glass or Perspex a process known as refraction occurs.In this process the light is changing medium.  This results in the wave slowing down when moving to a medium of higher refractive index (kind of like a material with higher density) Devices that have glass lenses have been developed to make use of the principle of refraction. Glass can be used to bend the light through them to magnify images in the case of telescopes or microscopes and can also be used in eye glasses to correct problems focusing at different distances

Refraction 

Diffraction is when a wave goes through a small hole and has a flared out geometric shadow of the slit. Diffraction is a characteristic of waves of all types. We can hear around a corner because of the diffraction of sound waves. For instance, if a wall is next to you when you yell, the sound will parallel the wall. The wall may stop, but the voice doesn't; sound will almost turn the corner of the wall. This is diffraction.

Electromagnetic interference  (or EMI, also called  radio frequency interference or RFI . when in high frequency or radio frequency) is disturbance that affects an electrical circuit due to either electromagnetic induction or  electromagnetic radiation emitted from an external source.EMI can be intentionally used for radio jamming as in some forms of electronic warfare , or can occur unintentionally.  

POLORIZATION:-

The physical orientation of electrical field with respect to the direction of propagation in free space is known as polarization.

 The act or process of propagating, especially the process by which a disturbance, such as the motion of electromagnetic or sound waves, is transmitted through a medium such as air or water.

CRITICAL FREQUENCY - Critical frequency of sky wave is that maximum frequency in which the sky wave after reflecting from the ionosphere return back to earth. If the frequency of the sky wave is increased then the critical frequency, it will not come back to earth after reflection.

SKIP DISTANCE:- Skip distance is the shortest distance from a transmitter measured along with the surface of the earth at which a sky wave of fixed frequency will be returned to earth.SKIP ZONE The skip zone is the area in which no usable radio signal can be received because it is shorter than the skip distance but longer than the ground wave range

MAXIMUM USABLE FREQUENCY:-

It is defined as the highest frequency that can be used for sky wave communication between the two points on the earth. It follows that there are different value of M U F for different pair of stations on the globe.

Radio frequencies and their primary mode of propagation

Band Frequency Wavelength Propagation via

VLF Very Low Frequency 3–30 kHz 100–10 kmGuided between the earth and the ionosphere.

LF Low Frequency 30–300kHz 10–1 km

Guided between the earth and the D layer of the ionosphere.Surface waves.

MF Medium Frequency 300–3000kHz 1000–100 m

Surface waves.E, F layer ionospheric refraction at night, when D layer absorption weakens.

HFHigh Frequency (Short Wave)

3–30 MHz 100–10 m

E layer ionospheric refraction.F1, F2 layer ionospheric refraction.

VHF Very High Frequency 30–300MHz 10–1 m

Infrequent E ionospheric refraction. Extremely rare F1,F2 layer ionospheric refraction during high sunspot activity up to 80 MHz. Generally direct wave. Sometimestropospheric ducting.

UHF Ultra High Frequency 300–3000MHz 100–10 cmDirect wave. Sometimes tropospheric ducting.

SHF Super High Frequency 3–30 GHz 10–1 cm Direct wave.

EHF Extremely High Frequency 30–300GHz 10–1 mmDirect wave limited by absorption.

Propagation of an Electromagnetic WaveElectromagnetic waves are waves which can travel through the vacuum of outer space. Mechanical waves, unlike electromagnetic waves, require the presence of a material medium in order to transport their energy from one location to another. Sound waves are examples of mechanical waves while light waves are examples of electromagnetic waves.Electromagnetic waves are created by the vibration of an electric charge. This vibration creates a wave which has both an electric and a magnetic component. The mechanism of energy transport through a medium involves the absorption and reemission of the wave energy by the atoms of the material

How electromagnetic waves propagate in Atmosphere?

The magnetic field and electric field oscillations perpendicular to each other or in the direction same as that of the wave propagation as basically known as Electromagnetic waves. The nature of electromagnetic waves is traverse. The speed of propagation of the electromagnetic waves is same as that of velocity of light..Different types of propagation depending upon the properties are:1. Sky wave propagation2. Space wave propagation3. Ground wave propagation

SKY WAVE PROPAGATION:-

The sky waves have frequency range between 2MHz to 30MHz. These radio waves have the ability to pass through earth’s atmosphere. The ionosphere of our earth reflects these rays very efficiently. When these rays move along the atmosphere then their movement is from transmitter towards the receiver antenna. This is called as sky wave propagation of the waves.

LAYERS OF IONOSPHERE :-The ionosphere is constituted of the following layers D ,E F1,F2 . Layer F1 and F2 combines together during night and makes a single layer .

D LAYER :-The D layer is the lowest existing at an average height of 70 KM with an average thickness of 10 KM. The degree of its ionization depends on the altitude of the sun above the horizon and thus disappears during night. Least important from the view of HF propagation.

E LAYER :- The E layer is next in height existing at about 100 km with the thickness of 25 kms. Like D layer it all but disappears at night. The reason of disappearance is the recombination of the ions into molecules . This is due to the absence of the sun at night . The main effect of E layer are reflect some HF waves in day time.

F1 LAYER:-

F1 Layer exists at a height of 180 KM in day time and combines with F2 Layer at night . Its Day time thickness is about 25 Kms. Although some HF waves are reflected from it , most passes to be reflected from F2 Layer.

F2 LAYER :-

The F2 layer is the most reflecting medium for high frequency waves. Its approx thickness can be up to 200 Kms and its height ranges from 250 kms to 400 kms in day time . During night it fails to a height of 300 kms where it combines with F1 Layer. It is most noticeable that F layer as whole remains present in night also and most useful layer for HF communication.

Space wave propagation -The radio waves having high frequencies are basically called as space waves. These waves have the ability to propagate through atmosphere, from transmitter antenna to receiver antenna. These waves can travel directly or can travel after reflecting from earth’s surface to the troposphere surface of earth. So, it is also called as Tropospherical Propagation.

There are some limitations of space wave propagation.1. These waves are limited to the

curvature of the earth.2. These waves have line of sight propagation, means their propagation is along the line of sight distance.

Ground wave propagation:

Ground wave propagation is particularly important on the LF and MF portion of the radio spectrum. Ground wave radio propagation is used toprovide relatively local radio communicationscoverage, especially by radio broadcaststations that require to cover a particular locality. Ground wave radio signal propagation isideal for relatively short distance propagation on these frequencies during the daytime.

Ground wave or surface wave propagation in which part of wave travels along or near the surface of the earth. In ground wave propagation the transmitting and receiving Antenna should be closed to the surface of the earth and the antenna should be vertically polarized

Sky-wave ionosphere propagation is notpossible during the day because of the attenuation of the signals on these frequencies caused by the D region in the ionosphere.In view of this, radio communicationsstations need to rely on the ground-wavepropagation to achieve their coverage

SUMUP

THANKS