PROPAGATION OF ELECTRO MAGNETIC WAVES

Dr. C. SarithaLecturer in

ElectronicsS.S.B.N. CollegeANANTAPUR

DEFINITION OF E.M.WAVES: The wave of the electric field and the wave of the magnetic field

are propagated perpendicularly to the direction of propagation and to each other. At extremely low frequencies, the electric field and the magnetic field are specified separately. At higher frequencies, electric and magnetic fields are not separable, and are named "electromagnetic waves" or "electromagnetic fields".

Nature of Electromagnetic Waves

They are Transverse waves without a medium. (They can travel through empty space)

They travel as vibrations in electrical and magnetic fields.

Have some magnetic and some electrical properties to them.

Speed of electromagnetic waves = 300,000,000 meters/second (Takes light 8 minutes to move from the sun to earth {150 million miles} at this speed.)

Ground-Wave PropagationAt frequencies up to about 2 MHz, the

most important method of propagation is by ground waves which are vertically polarized. They follow the curvature of the earth to propagate far beyond the horizon. Relatively high power is required.

Direction of wave travel

IncreasingTilt

Earth

SPACE WAVE PROPAGATION

The EM wave that propagates from the transmitter to the receiver in the earth’s troposphere is called “Space Wave”.

Troposphere is the region of the atmosphere with in 15km above the surface of the earth.

The Maximun line of sight distance between two antenns depends on the height of each antenna.

SKYWAVE PROPAGATIONSky wave propagation is also called Ionospheric

wave propagation. E.M waves directed upward at some angle from the earth’s Surface are called Skywaves.

Sky wave propagation is usefull in the frequency range of 2 to 30 MHz and for long distance communication

Ionosphere is the upper portion of the atmosphere between 50km and 350km above the earth,which is ionoised by absorbing large quantities of radiation energy from the Sun.

The Major ionisation is from α,β and radiation from the Sun and cosmic rays and meter

ReflectionReflectionRefractionRefractionDiffraction Diffraction PolarizationPolarizationScatteringScattering

Reflection:

Reflection is when waves, whether physical or electromagnetic, bounce from a surface back toward the source. A mirror reflects the image of the observer.

Polarization:

The polarization of an electromagnetic wave indicates the plane in which it is vibrating. As electromagnetic waves consist of an electric and a magnetic field vibrating at right angles to each other it is necessary to adopt a convention to determine the polarization of the signal. For this purpose the plane of the electric field is used.

Refraction is the bending of light as it passes between materials of different optical density

Refraction:

Refraction of a material is the ratio of the speed of light in vacuum to the speed of light in that material:

n =   

Diffraction:

When a travelling water wave hits an obstacle, the wave fronts spreads out round the edge and becomes curved. This phenomenon refers to diffraction

Scattering:The change in direction of a particle or photon because of a collision with another particle or a system.

FADING:Fading effect is nothing but the fluctuation in

received signal strength at the receiver that is, any random variation in the received signal can be termed as fading.

While the microwave signal travel in the medium due to different parameters there is reduction in signal strength, according to that signal fading is divided into seven types: (1) Slow fading (2)Fast fading (3) Interference fading (4)Absorption fading (5)Selective fading (6) Polarization fading (7)Skip fading

Waves or Particles Electromagnetic radiation has properties of

waves but also can be thought of as a stream of particles.

Example: Light Light as a wave: Light behaves as a transverse

wave which we can filter using polarized lenses. Light as particles (photons) When directed at a substance light can knock

electrons off of a substance (Photoelectric effect)

Waves of the Electromagnetic SpectrumElectromagnetic Spectrum—name for the range of

electromagnetic waves when placed in order of increasing frequency

RADIO WAVES

MICROWAVES

INFRARED RAYS

VISIBLE LIGHT

ULTRAVIOLET RAYS

X-RAYS

GAMMA RAYS

RADIO WAVESA. Have the longest wavelengths and lowest

frequencies of all the electromagnetic waves.B. A radio picks up radio waves through an

antenna and converts it to sound waves.C. Each radio station in an area broadcasts

at a different frequency. # on radio dial tells frequency.

D. MRI (MAGNETIC RESONACE IMAGING)Uses Short wave radio waves with a magnet to

create an image

MICROWAVESMicrowaves—have the shortest wavelengths and

the highest frequency of the radio waves.Used in microwave ovens.

Waves transfer energy to the water in the food causing them to vibrate which in turn transfers energy in the form of heat to the food.

Used by cell phones and pagers.RADAR (Radio Detection and Ranging)

Used to find the speed of an object by sending out radio waves and measuring the time it takes them to return.

Infrared= below redShorter wavelength and higher frequency than

microwaves.You can feel the longest ones as warmth on

your skinHeat lamps give off infrared waves.Warm objects give off more heat energy than

cool objects.Thermogram—a picture that shows regions of

different temperatures in the body. Temperatures are calculated by the amount of infrared radiation given off. Therefore people give off infrared rays.

VISIBLE LIGHTShorter wavelength and higher frequency than

infrared rays.Electromagnetic waves we can see.Longest wavelength= red lightShortest wavelength= violet (purple) lightWhen light enters a new medium it bends

(refracts). Each wavelength bends a different amount allowing white light to separate into it’s various colors ROYGBIV.

ULTRAVIOLET RAYSShorter wavelength and higher frequency than

visible lightCarry more energy than visible lightUsed to kill bacteria. (Sterilization of

equipment)Causes your skin to produce vitamin D (good for

teeth and bones)Used to treat jaundice ( in some new born babies.Too much can cause skin cancer. Use sun block to protect against (UV rays)

X- RAYSShorter wavelength and higher frequency than UV-raysCarry a great amount of energyCan penetrate most matter.Bones and teeth absorb x-rays. (The light part of an

x-ray image indicates a place where the x-ray was absorbed)

Too much exposure can cause cancer (lead vest at dentist protects organs from unnecessary

exposure)Used by engineers to check for tiny cracks in

structures.The rays pass through the cracks and the cracks appear dark

on film.

GAMMA RAYSShorter wavelength and higher frequency than X-

raysCarry the greatest amount of energy and

penetrate the most.Used in radiation treatment to kill cancer cells.Can be very harmful if not used correctly.

Brief SUMMARYA. All electromagnetic waves travel at the

same speed. (300,000,000 meters/second in a vacuum.

B. They all have different wavelength and different frequencies.Long wavelength-lowest frequencyShort wavelength highest frequencyThe higher the frequency the higher the

energy.

Power in a waveA wave carries power and transmits

it wherever it goes

The power density per area carried by a wave is given by the Pointing vector.