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STANLEY L. AQUINOInstructor, ECE

ELECTRONIC W VE PROP G TIO

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In Earths atmosphere, ray wavefron

propagation may be altered from freespace behavior by optical effects such

Refraction (Bending)

Reflection (Bouncing)Diffraction (Scattering)

Interference (Colliding)

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Optical properties can be analyzed

completely by application of Maxwellequation, this is necessarily complex.

For most applications, geometric raytracing can be substituted for analysis

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Refraction is sometimes referred to a

bending of the radio-wave path. How

the ray does not actually bend.

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Electromagnetic refraction is actuallychanging of direction of an electroma

ray as it passes obliquely from one me

into another with different velocities

propagation.

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Refraction of electromagnetic waves c

expressed in terms of the refractive in

of the atmosphere. Refractive index is

square root of the dielectric constant

=

=

= refractive index

= equivalent dielectric constant relative

= number of electrons per cubic centim

= frequency (kHz)

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Whenever a ray passesfrom a less de

a more dense medium, it is effectively

toward the normal.

The normal is simply an imaginary line d

perpendicular to the interface at the poincidence.

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Conversely, whenever a ray passesfro

more dense to a less dense medium, i

effectively bent away from the norma

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The angle of incidence is the angle fo

between the incident wave and the n

The angle of refraction is the angle fo

between the refracted wave and thenormal.

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The refractive index (index of refracti

the two materials is simply the ratio ovelocity of propagation of a light ray i

space to the velocity of propagation o

light ray in a given material. =

= refractive ind = speed of ligh

= speed of ligh

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How an electromagnetic wave reacts w

it meets the interface of two transmis

materials that have different indexes o

refraction can be explained with Snell

=

=

1 = refractiv

2 = refractiv

1 = angle of

2 = angle of

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and because the refractive index of a

material is equal to the square root o

dielectric constant,

=

1 = dielectric co

2 = dielectric co

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Refraction also occurs when a wavefrpropagates in a medium that has a de

gradient that is perpendicular to the

direction of propagation (i.e., parallel

wavefront).

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Reflect means to cast or turn back, an

reflection is the act of reflecting.

Electromagnetic reflection occurs wh

incident wave strikes a boundary of tw

media and some or all of the incidentpower does not enter the second mat

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The angle of reflection is equal to the

of incidence = .

However, the reflected voltage field

intensity is less than the incident voltintensity.

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The ratio of the reflected to the incidevoltage intensities is called the reflect

coefficient, (sometimes called the

coefficient of reflection).

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For a perfect conductor, = . is u

indicate both the relative amplitude oincident and reflected fields and the p

shift that occurs at the point of reflec

=

=

() = reflect = incide

= reflec

= incide

= reflec

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The portion of the total incident power t

not reflected is called the power transm

coefficient (or simply the transmissicoefficient)

For a perfect conductor, = . The law

conservation of energy states that for aperfect reflective surface, the total reflec

power must equal the total incident pow

+ =

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If medium 2 is not a perfect conducto

some of the incident waves penetrateand are absorbed.

The fraction of power that penetratesmedium 2 is called the absorption

coefficient.

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Reflection also occurs when the reflecsurface is irregular or rough; howeve

a surface may destroy the shape of th

wavefront.

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When an incident wavefront strikes a

irregular surface, it is randomly scattemany directions. Such condition is cal

diffuse reflection, whereas reflection

a perfectly smooth surface is calledspecular (mirrorlike) reflection.

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Surfaces that fall between smooth anirregular are called semirough surface Semirough surfaces cause a combination of diffus

specular reflection. A semirough surface will not t

destroy the shape of the reflected wavefront. Howthere is a reduction in the total power.

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The Rayleigh criterion states that a

semirough surface will reflect as if it w

smooth surface whenever the cosine angle of incidence is greater than /8

where d is the depth of the surface

irregularity and is the wavelength ofincident wave.

>

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Diffraction is defined as the modulatiredistribution of energy within a wave

when it passes near the edge of an op

object.

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When a wavefront passes near an obs

or discontinuity with dimensionscomparable in size to a wavelength, s

geometric analysis cannot be used to

explain the results, and Huygens prin

(which is deduced from Maxwells

equation) is necessary.

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Huygens principle states that every p

on a given spherical wavefront can beconsidered as a secondary point sour

electromagnetic waves from which ot

secondary waves (wavelets) are radiaoutward.

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Diffraction occurs around the edge ofobstacle, which allows secondary wav

sneak around the corner of the obs

into what is called the shadow zone.

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Refraction, reflection and diffraction a

categorized as a geometric optics.

Interference, on the other hand, is su

to the principle of linear superpositioelectromagnetic waves.

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If the difference in distance travelled

odd integral multiple of one-half

wavelength, reinforcement takes plac

If the difference is an even integral m

of one-half wavelength, total cancella

occurs.

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