Chapter5

WAVE PROPAGATION

Input

Transdu

cer

Transmit

terChannel Receiver

Output Transduce

r

Basic block diagram of Communication

System

2

Electromagnetic wave

• Both electric fields and magnetic fields are simultaneously present • Both fields are perpendicular to each other and direction of

propagation.• Electromagnetic wave travels through space at the speed of light.

Electromagnetic polarization

• The polarization of EM wave is the orientation of the electric field vector with respect to the earth surface

Vertically polarized Horizontally polarized

Electromagnetic polarization

Elliptical polarization

Properties of EM Wave1. Refraction is the change of direction of an electromagnetic ray or wave

travelling in one medium when it encounters another medium and the twomedia support different velocities of propagation The diagram shows how aray in one medium is refracted as it enters a different medium.

2.Reflection is the change in direction of a wavefront at an interface between

two different media so that the wavefront returns into the medium from which it originated.

3.Diffraction refers phenomena that occur when a wave encountersan obstacle or a slit. It is defined as the bending of light around thecorners of an obstacle or aperture into the region of geometricalshadow of the obstacle

Diffraction of EM wave

4.EM Wave Interference will take place when two or morewaves combine together in such way that system performancedegrades.

Radiation and Propagation

The process of signal travel from the transmitter to receiver is divided in to two parts1.Radiation of signal2.Propagation

1.Radiation-Whenever high frequency current flows to the conductor, the power measured on both the sides of the conductor is not same. a part of the power is dissipated in the resistance of the conductor and a part of it escapes in to the free space. This escape of power is known as radiation.

2.Propagation-the radiated power is then propagates in space in the form of electromagnetic waves. The radiation and propagation of the radio waves cannot be seen

Modes of propagation of EM wave

Frequency

30MHz.

3MHz.

30kHz.

Space Wave or line of sight

Sky wave Propagation

Ground wave propagation

Ground Wave Propagation

1.Ground Wave Propagation• The EM waves leaves antenna and remains closed to the earth surface.it

follows the curvature of the earth surface.

• The ground wave propagation is strongest at low and medium frequency ranges i.e. between 30kHz and 3MHz.

• The Ground wave should be vertically polarized to prevent short circuiting of the electric field component.

• While passing over the earth surface ,the ground waves induce some current in to it. Thus they loose some energy due to absorption.

• Due to diffraction EM wave tilt over the surface of the earth and this angle increases with distance and frequency so, ground wave propagation should be used up to MW frequency band.

Advantages of ground wave propagation1.The atmospheric condition do not affect ground wave propagation.2.If the transmitted power is large enough then it can be used for communication between any two point.

Disadvantages of ground wave propagation1.Frequency range is limited.2.For low operating frequency height of antenna is increased.3.High transmission power is necessary.

Applications-1.For AM radio broad cast in MW band.2.in ship communication such as radio navigation and marine mobile communication

2.Sky wave propagation

• In sky wave propagation the transmitted signal travels in to the upper atmosphere where it is reflected back to the earth.

• Reflection takes place due to presence of allayer in the upper atmosphere called ionosphere.

Ionosphere layers

Ionosphere layersLayer Height above earth

surfaceThickness of layer

D Layer 70km 10km

E Layer 100km 25km

F1 Layer 180km 20km

F2 Layer 250km-400km 200km

Virtual heightThe incident EM wave returns back to earth due to refraction. In the process it bends down gradually and not sharply. but incident and reflected wave follow exactly the same paths as those if the signal would have been reflected from surface located at greater height. This height is called virtual height.

If the virtual height of layer is known then it is possible find the angle of incidence required to return the wave to the ground at selected point.

Critical frequency of layer

It is defined as the maximum frequency that is returned back to the earth by that layer, when the wave is incident at an angle 90degree to it.

The maximum usable frequency(MUF)

The MUF is defined for certain value of angle of incidence rather than at 90 degree.According to this if angle is increased then it is possible to operate at higher freuencythan critical frequency fc.

MUF= critical frequency/cosαMUF=FC*SECα

MUF is the the highest operating frequency between tow points on the earth.

Skip distanceThe skip distance is the shortest distance from a transmitter, measured along the surface of the earth at which a sky wave of fixed frequency returned back to the earth.

Skip zone

A skip zone, also called a silent zone or zone of silence, is a region where a radio transmission can not be received. The zone is located between regions both closer and farther from the transmitter where reception is possible.

Effect of variation in the operating frequency

Single hop sky wave propagation -4000km distance is covered.Multiple hop sky propagation-200000km diatance

Fading

Fading means variation or fluctuation in signal strength at the receiver.it takes place due to the interference between two waves which follow differen tpaths to travel from transmitter toreceiver