Wireless Propagation-Data Communication-Lecture Slides, Slides of Data Communication Systems and Computer Networks

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Lecture- BS(CIS) Semester-IV

Data Communication

Today’s Lecture

• Wireless Propagation  Ground Wave Propagation  Sky Wave Propagation  Line of Sight Propagation  Refraction  Line of Sight Transmission Impairments - Free Space Loss - Atmospheric Absorption - Multi-path - Refraction

Reflection

• Reflection is the abrupt reversal in direction
• Caused by any conductive medium such as  Metal surfaces or  Earth’s surface
• There will normally be a shift in phase
• Coefficient of reflection is less than 1

Complete Reflection

• Complete reflection will occur only in perfect conductors

and when the electric field is perpendicular to the

reflecting element or medium

• Coefficient of Reflection will be 1
• Coefficient of Reflection is the ratio of the reflected wave

intensity to the incident wave intensity

Refraction

• Occurs when the waves pass from one medium to another

whose densities are different

• Coefficient of reflection is less than 1
• The angle of incidence and the angle of refraction is

related by Snell’s Law

Diffraction

• Waves traveling in straight lines bend around

obstacles

• Based on Huygen’s principle (1690)

 Each point on a wavefront can be thought of as an

isotropic point or a source of secondary spherical

energy

• Concepts explains why radio waves can be heard

behind tall mountains or buildings that are

normally considered to block line of sight

transmissions

Ground Wave Propagation

• Properties  Propagates between  Follows curvature of the earth  Reasons - Electromagnetic flux induction with earth, this causes wave front to tilt down - Change in refractive index i.e. refractive index decreases as distance from earth increases  Because of electromagnetic properties these waves do not penetrated in upper atmosphere  Frequency ranges - Wave up-to 2 MHz exhibit these properties  Such waves are scattered by atmosphere that also causes waves to tilt down  Example: AM radio waves

Example-AM-Wave Propagation

• Ionosphere plays an important role in wave propagation
• AM radio station reduces transmitter power with sunset  At night Ionosphere reflects waves  If power is not reduced signal of one station may interfere some other distant AM station

Line of Sight Propagation

• Properties  Frequencies range above 30 MHz  Signals are not reflected form ionosphere at frequencies above 30 MHz  Communication is possible either via satellite or line of sight

Refraction (An Important wave propagation phenomena)

• Velocity of electromagnetic wave is a function of density of material  ~3 x 10^8 m/s in vacuum, less in anything else
• As wave moves from one medium to another, its speed changes  Causes bending of direction of wave at boundary  Towards more dense medium
• Index of refraction (refractive index) is  Sin(angle of incidence)/sin(angle of refraction)  Varies with wavelength
• May cause sudden change of direction at transition between media
• May cause gradual bending if medium density is varying  Density of atmosphere decreases with height  Results in bending towards earth of radio waves

Free Space Loss

• Free loss equation  Pt / Pr = (4* pi * d / λ )^2  20log(f) + 20 log(d) – 147.56 db
• Taking into account the antenna gain  Pt / Pr = (4* pi * d / λ )^2 * 1/GrGt  -20log(f) + 20log(d) – 10log(AtAr) + 169.54 db
• Results  Free space loss increases with the increase in frequency  Antenna gain increases

Atmospheric Absorption

• Atmospheric Absorption  Water vapour and oxygen absorb radio signals  Water greatest at 22GHz, less below 15GHz  Oxygen greater at 60GHz, less below 30GHz  Rain and fog scatter radio waves

Quiz

1. Why antennas are made parabolic, write example to

explain your answer

2. What is meant by the term “antenna gain”

3. What are the differences among different modes of

optical fiber

4. What is the relation between attenuation and frequency