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Electromagnetic Radiation

Electromagnetic (EM) radiation is a self-propagatingwave in space with electric and magnetic components.These components oscillate at right angles to each otherand to the direction ofpropagation, and are in phase witheach other. Electromagnetic radiation is classified into

types according to the frequency of the wave: thesetypes include, in order of increasing frequency, radiowaves, microwaves, terahertz radiation, infraredradiation, visible light, ultraviolet radiation, X-rays andgamma rays.

EM radiation carries energy and momentum, which maybe imparted when it interacts with matter. The behaviorof EM radiation depends on its wavelength. Higherfrequencies have shorter wavelengths, and lowerfrequencies have longer wavelengths
http://en.wikipedia.org/wiki/Wave_propagationhttp://en.wikipedia.org/wiki/Wavehttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Oscillatehttp://en.wikipedia.org/wiki/Propagationhttp://en.wikipedia.org/wiki/Phase_(waves)http://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Radio_waveshttp://en.wikipedia.org/wiki/Radio_waveshttp://en.wikipedia.org/wiki/Microwaveshttp://en.wikipedia.org/wiki/Terahertz_radiationhttp://en.wikipedia.org/wiki/Infrared_radiationhttp://en.wikipedia.org/wiki/Infrared_radiationhttp://en.wikipedia.org/wiki/Visible_lighthttp://en.wikipedia.org/wiki/Ultraviolet_radiationhttp://en.wikipedia.org/wiki/X-rayhttp://en.wikipedia.org/wiki/Gamma_rayshttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Momentumhttp://en.wikipedia.org/wiki/Matterhttp://en.wikipedia.org/wiki/Matterhttp://en.wikipedia.org/wiki/Momentumhttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Gamma_rayshttp://en.wikipedia.org/wiki/X-rayhttp://en.wikipedia.org/wiki/X-rayhttp://en.wikipedia.org/wiki/X-rayhttp://en.wikipedia.org/wiki/Ultraviolet_radiationhttp://en.wikipedia.org/wiki/Visible_lighthttp://en.wikipedia.org/wiki/Infrared_radiationhttp://en.wikipedia.org/wiki/Infrared_radiationhttp://en.wikipedia.org/wiki/Terahertz_radiationhttp://en.wikipedia.org/wiki/Microwaveshttp://en.wikipedia.org/wiki/Radio_waveshttp://en.wikipedia.org/wiki/Radio_waveshttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Phase_(waves)http://en.wikipedia.org/wiki/Propagationhttp://en.wikipedia.org/wiki/Oscillatehttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Wavehttp://en.wikipedia.org/wiki/Wave_propagationhttp://en.wikipedia.org/wiki/Wave_propagationhttp://en.wikipedia.org/wiki/Wave_propagation


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EM Radiation

Electromagnetic wave propagation is describedby Maxwells equations, which state that achanging magnetic field produces an electricfield and a changing electric field produces a

magnetic field. Thus electromagnetic waves areable to self-propagate. In electromagnetismsimply any electric charge which accelerates, orany changing magnetic field, produceselectromagnetic radiation.

The concept of electromagnetic field interactionis not entirely new, since electromagnetic fieldsform the basis of all antenna theory


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Theory of EM

Electromagnetic waves were first predicted by

James Clerk Maxwell and subsequently

confirmed by Heinrich Hertz. Maxwell derived a

wave form of the electric and magneticequations, revealing the wave-like nature of

electric and magnetic fields, and their symmetry.

Because the speed of EM waves predicted by

the wave equation coincided with the measuredspeed of light, Maxwell concluded that light itself

is an EM wave.
http://en.wikipedia.org/wiki/James_Clerk_Maxwellhttp://en.wikipedia.org/wiki/Heinrich_Hertzhttp://en.wikipedia.org/wiki/Electromagnetic_wave_equationhttp://en.wikipedia.org/wiki/Electromagnetic_wave_equationhttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Electromagnetic_wave_equationhttp://en.wikipedia.org/wiki/Electromagnetic_wave_equationhttp://en.wikipedia.org/wiki/Heinrich_Hertzhttp://en.wikipedia.org/wiki/James_Clerk_Maxwell


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Theory of EM

According to Maxwell's equations, a time-varying electric field generates a magneticfield and vice versa. Therefore, as an

oscillating electric field generates anoscillating magnetic field, the magneticfield in turn generates an oscillatingelectric field, and so on. These oscillating

fields together form an electromagneticwave. The energy in electromagneticwaves is sometimes called radiant energy.
http://en.wikipedia.org/wiki/Maxwell's_equationshttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Radiant_energyhttp://en.wikipedia.org/wiki/Radiant_energyhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Maxwell's_equations


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EM waves

A wave consists of successive troughs and crests, andthe distance between two adjacent crests or troughs iscalled the wavelength. Waves of the electromagneticspectrum vary in size, from very long radio waves the

size of buildings to very short gamma rays smaller thanatom nuclei. Frequency is inversely proportional towavelength, according to the equation:

v= f

where vis the speed of the wave (cin a vacuum, or less

in other media), fis the frequency and is thewavelength. As waves cross boundaries betweendifferent media, their speeds change but theirfrequencies remain constant.
http://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Wavelength


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EM waves an example

If you are standing on a bridge overlooking a

calm body of water. If you were to drop an object

(which did not float) into the pound, there would

be a path of bubbles generated in the samedirection (vertical) as the object, but there would

also be a circular wave pattern radiating from

the point of impact and spreading horizontally

across the body of water. Electromagnetic andelectrostatic radiation pattern in free space.


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Power Density and Inverse Square

Law

Power Density is defined as radiated power perunit area.

Power density reduced to one-quarter of its

value when distance from the source hasdoubled.

Inverse Square Law states that power density isinversely proportional to the square of the

distance from the source. This law applies to allforms of radiation in free space.


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Power Density and Inverse Square

Law

Where p is the power density at a distancer from an isotropic source.

Pt is the transmitted power.

An isotropic source is one that radiatesuniformly in all directions in space.

Inverse Square law applies also when the

source is not an isotropic one but for goodcalculations and clearing concepts we takeisotropic source.


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Polarization of EM waves

An electromagnetic wave traveling forward, the

electric field might be oscillating up and down,

while the magnetic field oscillates right and left;

but this picture can be rotated with the electricfield oscillating right and left and the magnetic

field oscillating down and up. This is a different

solution that is traveling in the same direction.

This arbitrariness in the orientation with respectto propagation direction is known as

polarization.
http://en.wikipedia.org/wiki/Polarizationhttp://en.wikipedia.org/wiki/Polarization


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Polarization of EM waves

polarization of the received wave and that the polarization of a transmitted

wave is the same as that of the antenna from which it emanated byneglecting any environmental effect.

P = E H The power density on the surface of an imaginary sphere surrounding the

RF source can be expressed as

where dis the diameter of the imaginary sphere, Pis the total power at the

source, and S is the power density on the surface of the sphere in watts/m2


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Types of Polarization

Polarization of wave depends on magnitude and phase relationship between

existing E-field components ( Ex and Ey)

Linear polarization occurs when Ex and/or Ey are in phase regardless of their

relative magnitudes (direction of Linear Polarization wave is the same as E-

field).

Circular polarization occurs when Ex & Ey are out of phase by 90but both

components have equal magnitude.

Elliptical polarization occurs when Ex and Ey are out of phase by 90and

both components have different magnitudes.

Example: use a probe to measure E & H fields of Linear polarized EM wave


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Reception and Radiation

The process of reception is exactly the reverse

of the process of transmission.

Transmitting and receiving antennas are

interchangeable and virtually identicalregardless of use for reception or transmission.

Antennas radiate electromagnetic waves, as a

result electron flow in a suitable conductor as

this is proved mathematically in the Maxwells

Equation.


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Velocity of waves

The velocity of propagation for the

electromagnetic wave can be calculated

as a function of the permittivity and

permeability of the medium.


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Velocity of waves

The velocity of propagation is equal to the

velocity of light in free space divided by

the square root of the product of the

relative permittivity and permeability.

An electromagnetic plane wave traveling

in the positive zdirection can be described

by the following equations:


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Refraction Reflection Diffraction

There are several means of electromagnetic wavepropagation beyond LOS propagation. The mechanismsof non-LOS propagation vary considerably,

Based on the operating frequency. At VHF and UHF

frequencies, indirect propagation is often used.Examples of indirect propagation are cell phones,

pagers, and some military communications. An LOS mayor may not exist for these systems.

In the absence of an LOS path, diffraction, refraction,and/or multipath reflections are the dominantpropagation modes.


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Diffraction is a phenomenon of electromagneticwaves bending at the edge of a blockage,

resulting in the shadow of the blockage being

partially filled-in. Refraction is the bending of electromagneticwaves due to an uneven surface in the medium.

Multipath is the effect of reflections from multiple

objects in the field of view, which can result inmany different copies of the wave arriving at the

receiver.


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Reflection and diffraction around buildings and foliagemay provide enough signal strength for meaningfulcommunication to take place.

The efficiency of indirect propagation depends upon the

amount of margin in the communication link and thestrength of the diffracted or reflected signals.

The operating frequency has a significant impact on the

viability of indirect propagation.

HF frequencies can penetrate buildings and heavyfoliage quite easily. VHF and UHF can penetrate buildingand foliage also, but to a lesser extent.


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At the same time, VHF and UHF will havea greater tendency to diffract around orreflect/scatter off of objects in the path.

Above UHF, indirect propagation becomesvery inefficient and is seldom used. Whenthe features of the obstruction are largecompared to the wavelength, theobstruction will tend to reflect or diffractthe wave rather than scatter it.


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Scattering

Scattering: Scattering occurs when anelectromagnetic wave is incident on a rough orirregular surface.

When a wave is scattered, the resultingreflections occur in many different directions.When looked at on a small scale, the surfacecan often be analyzed as a collection of flat orsharp reflectors.

The determination of when a surface isconsidered rough is usually based on theRayleigh roughness laws


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Absorption

Absorption:Anytime that an

electromagnetic wave is present in a

material other than free space, there will

be some loss of strength with distance

due to ohmic losses and this is termed as

absorption.


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Interference of Electromagnetic

waves

Interference occurs when two waves that left

one source and travel by different paths arrive at

a point.

This can be happen in high frequency sky wavespropagation and microwave space wave

propagation.

The interference produced due to microwave

antenna is located near the ground, and waves

reach after reflection from the ground.


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Frequency Bands

The super-high-frequency (SHF) frequenciesinclude 330GHz and use strictly LOSpropagation. In this band, very small antennascan be employed,or,more typically,moderately

sized directional antennas with high gain. Applications of the SHF band include satellite

communications, direct broadcast satellitetelevision, and point-to-point links.

Precipitation and gaseous absorption can be anissue in these frequency ranges, particularlynear the higher end of the range and at longerdistances.


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Frequency Bands

The extra-high-frequency (EHF) band

covers 30300GHz and is often called

millimeter wave. In this region, much

greater bandwidths are available.

Propagation is strictly LOS, and

precipitation and gaseous absorption are a

significant issue.

Frequency Ban s Ava a e or


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Frequency Ban s Ava a e orMicrowave/Satellite

Communications

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