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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.
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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.
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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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Refraction Reflection Diffraction
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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Refraction Reflection Diffraction
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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Refraction Reflection Diffraction
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