The Internet Encyclopedia (Volume 3)

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Ugweje WL040/Bidgolio-Vol I WL040-Sample.cls June 19, 2003 17:10 Char Count= 0

184 RADIOFREQUENCY ANDWIRELESSCOMMUNICATIONS

Scattering Object

Diffraction

Base

Station

Diffraction

& Reflection

Reflection

from building

Reflection

from House

Direct path

Reflection

from House

Reflection of

moving truck

Mobile Receiver

Factory

Figure 5: Illustration of reflection, refraction, diffraction, scattering, and absorption.

All radio waves will undergo reflection if the propagation

medium undergoes abrupt changes in its physical prop-

erties. This is illustrated in Figure 5. The more abrupt

the discontinuity, the more pronounced the reflection. De-

pending on the type of object, the RF energy can be par-

tially reflected, fully reflected, or absorbed. It is possible to

compute the amount of reflection from the properties of

the two media. This is known as the reflection coefficient,

=(η 2 −η 1 )/(η 2 +η 1 ), whereη 1 andη 2 are the intrinsic

impedance of the two media. Note that depending on the

values ofη 1 andη 2 , there could be partial reflection, full

reflection, or no reflection at all. If the incident object is a

good conductor, the wave is totally reflected and the angle

of incidence is the same as the angle of reflection.

Refraction

Refraction (see Figure 5) occurs at the boundary between

two dielectrics, when the incident wave propagates into

another medium at an angle. When radio waves propagate

from a medium of one density to a medium of another

density, the wave speed changes. This change in speed

will cause the wave to bend at the boundary between the

two media. The wave will always bend toward the denser

medium.

Diffraction

Diffraction of radio waves occurs when the waves en-

counter some obstruction along their path and tend to

propagate around the edges and corners and behind the

obstruction. This is illustrated in Figure 5. The height or

dimension of the obstruction has to be comparable to

thewavelengthof the transmission. The same obstruction

height may produce lower diffraction loss at higherwave-

lengththan at lowerwavelength. The result of this effect is

that the object shadows the radio wave. The field strength

of the wave decreases as the receiver moves deeper into a

shadowed region.

Scattering

Scattering is also illustrated in Figure 5. It is due to

small objects and irregularities in the channel, rough in-

cident surfaces, or particles in the atmosphere. When

the radio wave encounters objects or particles with di-

mension smaller than thewavelengthof the wave, scat-

tering occurs, which causes the signal to spread in all

directions.

Interference

Interference can occur when the transmitted radio wave

arrives at the same location via two or more paths (multi-

path). One of the ways this can happen is illustrated in

Figure 6. This figure shows three waves arriving at a

mobile receiver (the car) after traveling slightly different

paths. Due to their phase differences, the radio waves can

add either constructively or destructively at the receiver.

If the phase shift experienced by the propagating waves

Base

Station

time t: time t:+α

1

1

2

3

3

2

Figure 6: Interference of radio wave.