Radio Wave propagation

RADIO WAVE PROPAGATION

Submitted by, Ritika De 11900313040Under the guidance of, Associate Prof Dr. Sudipta Chattopadhyay

CONTENTSIntroductionRadio wave propagationClassification of radio wavesDifferent modes of propagationGround Wave propagationSky wave propagationSpace wave propagationConclusionBibilography

INTRODUCTION

The term Radio waves arbitrarily applied to electromagnetic waves in the frequency range of 0.001 hertz to hertz

and the wavelength ranging from 3× m to 3× m.

Radio waves comprises of both Electric and Magnetic fields.

The two fields are at right-angles to each other and the direction of propagation is at right-angles to both fields.

X

Y

Electric field , E

Direction of propagation

RADIO WAVE PROPAGATIONRadio Waves propagate outward from an antenna , at the speed of

light. The exact nature of these waves is determined by the transmission medium. i.e., In free space they travel in straight lines , whereas in the atmosphere they generally travel in curved path.

In a unguided medium, radio waves propagate in TEM mode while in confined or guided medium radio waves do not propagate in the TEM mode but in TM or TE mode.

Radio waves can be reflected and refracted in a manner similar to light . They are effected by ground terrain, atmosphere and other objects.

CLASSIFICATION OF RADIO WAVESBased on Frequency:

DIFFERENT MODES OF PROPAGATIONRadio waves

Ground waves Space waves or Line of Sight waves

Direct waves

Ground reflected waves

Sky waves

MODES OF PROPAGATION

FREQUENCY

APPLICATION

1. Ground waves VLFLFMF

1.Submarine Communication2.AM ,FM and television broadcasting

2. Space or line of sight waves

HFVHFUHF

1.Satellite communication2.Mobile communication

3. Sky waves 2Mhz-30Mhz

1.Microwave link2.Radar communication

GROUND WAVE PROPAGATION It propagates from transmitter to receiver by gliding over the surface of the

earth in which both antennas are close to the surface of the earth. It follows the curvature of the earth.

Earth is assumed to be an ideal conductor ,EM waves cannot penetrate ideal conductor only exist in dielectric medium above conducting surface that’s why ground wave propagation is possible.

The Ground waves are traverse in nature. Horizontal polarised antennas are not preferred as the horizontal component of the electric field in contact with the earth is short circuited by the earth.

WAVE TILTThe earth is not an ideal conductor , so there will remain a tangential

component of electric field resulting wave tilt.

The earth is characterised by σ and εr then Surface impedanc for earth:

Intrinsic impedance for dielectric medium:

Since there is surface impedance which is non-zero there will be tangential component of electric field i.e., EH and Ev be vertical component of electric field and if H is the magnetic field

then, and

Then,

From the equation we see that the two component are not in phase and as in general

The resultant electric field is elliptically polarised with plane of polarisation in longitudinal direction .as conductivity is infinite for ideal conductor the wave is vertically polarised as the conductivity decreases the ellipse become more tilted. This phenomena is known as WAVE TILT.

wave tilt for decreasing value of conductivity

SKY WAVE PROPAGATION• The sky waves are reflected from ionized layer of atmosphere back down

to the earth surface and is useful for very long range distance communication

• They can travel a number of layers, back and forth between ionosphere and earth’s surface.

• The sky wave, often called the ionospheric wave.

11

D layer – 50 to 90 km.

Disappears at night.

N=400e/cc

fc=180KHz

Mainly reflects ULF and LF

E layer – 90 to 140 km

Disappears at night.

N=2* 10^5 e/cc

fc=4mhz

IONOSPHERIC LAYERS

Sporadic E layer:

Temporary layer

Summer time

thin layer of intense ionisation clouds.

reflect frequency up to 224 MHz

90-130 km

F1 layer- 140 to 250km

fc=5Mhz

N=2*10^5 to 4*10^5 e/cc

F2 layer- 250 km and beyond

fc=8Mhz at day and

6Mhz at night

N=2*10^6 e/cc

CRITICAL FREQUENCYCritical frequency (fc) for given layer is defined as the highest frequency of wave that will be returned to the earth by that layer having been beamed normal ly at it.

Nmax = maximum ionisation density i.e., no. of electrons per unit volume .

MUF is defined as the highest frequency at which it is reflected by the ionospheric layer at the angle of incidence other than normal incidence.

MUF depends on time ,day ,distance direction and solar activity. MUF is the highest frequency that can be used by sky waves.

MAXIMUM USABLE FREQUENCY(MUF)

SKIP DISTANCE The minimum distance from the transmitter to the point on

ground at which of a given frequency will return to the earth by the ionosphere is called skip distance.

For flat earth: 1.Dskip=skip distance

2.h=height at which the reflection occurs.

3.fMUF=maximum usable frequency.4.fc= Critical

frequency

Dskip

h

i

T R

ionosphere

VIRTUAL HEIGHTThe virtual height of an ionospheric layer is the

equivalent altitude of a reflection that would produce the same effect as the actual refraction.c=speed of light=3×10⁸T=round trip time

APPLICATION OF SKY WAVES:1.Satellite Communication2. Mobile communication

FADING:Fading is defined as the fluctuation in the received signal strength at

the receiver or a random variation in the received signal.Fading may be classified in terms of duration of variation in signal

strength as : 1) Rapid Fluctuations2)Short Term Fluctuations3)Long term Fluctuations

The various types of fading are as follows: Selective Fading: At high frequency. Interference Fading Absorption Fading Polarization Fading Skip Fading

SPACE WAVE PROPAGATION(line of sight)These waves occur within the lower 20 km of the

atmosphere i.e. troposphere . In this mode of wave propagation electromagnetic waves

after transmitted from transmitting antenna reaches the receiving antenna either directly or after reflection from earth’s surface and tropospheric region. i.e., direct wave and ground reflected wave or indirect wave.

SHADOW ZONEIdeally field strength of receiving antenna should be

But sometimes when receiving antenna and transmitting antenna is not in l.o.s a zone is created where the field radiations are not utilised. This is called Shadow zone.

RADIO HORIZONThe radio horizon of an antenna is defined as the distant points locus at which

antenna’s direct ray become tangential to planetary surface.

a

Let h1=transmitting antenna’s heighth2= receiving antenna’s heighta= radius of the earth

Duct propagationDuct is a leaky waveguide through which E.M waves move in the air by

successive reflection and refraction. When the signal move through different layers, signal may suffer from some loss.

APPLIATION OF SPACE WAVES:1.Radar communication2.Microwave linking3.Mobile systems and satellite systems.

CONCLUSIONRadio wave signals are mainly used for communication purpose.

Startingfrom transmission of television broadcast signals to the

communicationwith aircrafts, every mode of communication and transportation isdependent on signals transmitted in radio wave frequency .The

mainobjective of radio wave propagation is to transmit signals

securelywithout any error. Mobile phones use radio signals for

communicationpurpose. It can be concluded that Radio wave Propagation is anImportant branch of Communication studies.

BIBILOGRAPHY

https://en.wikibooks.org/wiki/Communication_Systems/Wave_Propagation

http://wikipedia.org/radio wave propagation.Electronic communications by DENNIS ROODY ,JOHN

COOLENwww.radioelectronics.com.

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