ANTENNA & WAVE

PROPAGATION

sdbanerjee/inst/iriset/sc

ELECTROMAGNETIC

WAVES

ELECTROMAGNETIC WAVES

▪ A Wave is a carrier of energy or information,

which is a function of time and space.

▪ Maxwell predicted the existence of EM

waves and established it through Maxwell’s

Equations.

▪ Examples are : Radio ,Radar Beam ,TV

signals etc.

ELECTROMAGNETIC WAVES

• Oscillations which propagate through free

space with the velocity of speed of light ( i.e

3x10*8 m/s)

• These are transverse waves (Oscillations

perpendicular to the direction of

propagation).

• It has electric field and magnetic field which

are hence perpendicular to the direction of

propagation and also mutually perpendicular .

EMW contd.

• EM waves spread uniformly in all directions

in free space from a point source .

• The plane joining all the points of identical

phase at a particular instance is called a

Wave front .

• In free space it emerges to be spherical .

ELECTROMAGNETIC WAVES

• ELECTROMAGNETIC RADIATIONS

Power escaping into the space is said to be

radiated and is governed by the

characteristics of free space.

• FREE SPACE

Space that does not interfere with the

normal radiation and propagation of radio

waves.

It does not have magnetic or gravitational

fields , solid bodies and ionized particles

• ISOTROPIC SOURCE

Which radiates equally in all directions in

space.

• ISOTROPIC MEDIUM

Medium in which velocity of radiation is

constant all points (as in free space ).

This makes the wave front spherical

WAVES IN FREE

SPACE

WAVES IN FREE SPACE

• P and Q are two wave fronts . The

power ‘Pt’ at point ‘O’ is transmitted

in all directions and is called

isotropic radiation .

• The power density of a wave front

‘P’ is different from the power

density of the wave front ‘Q’

Electrical Field Intensity

• EFI of an EM wave is directly

proportional to the square root of power

density at that point. (Similar to voltage

that is proportional to the square root of

power ) .

• It may be shown that

E F I continued…….

• Comparing with the relation between power and voltage

, it may be shown that

Where Z is the characteristic impedance of Free Space.

APPLICATIONS

POLARIZATION

OF

A WAVE

POLARIZATION

POLARIZATION AND TYPES

• The polarization of a wave is defined as

the direction of of the electric field at a

given point of time .

• Types of polarization

Polarization continued …

• A wave is said to be linearly polarized if the

electric field lies wholly in one plane

containing the direction of propagation .

• If Ey = 0 and Ex is present ,when a wave

travels in Z direction with E field lying in XY

plane , it is said to be horizontally polarized .

• If Ex=0 and Ey is present , then the wave is

vertically polarized .

• If Ex and Ey are present and are in phase,

then the wave is Theta polarized

Polarization continued

• For horizontally polarized wave the electric field

lies in plane parallel to the earth’s surface.

• All the electric intensity vectors are vertical for a

vertically polarized wave .

• The direction of polarization is same as the

direction of antenna .

• Thus ,vertically polarized wave is radiated by

Vertical antenna .

• Horizontally polarized wave is radiated by

Horizontal antenna .

ANTENNA

RADIATION PHENOMENA

INTRO…

• The antenna is the interface between the

transmission line and space

• Antennas are passive devices; the power

radiated cannot be greater than the power

entering from the transmitter

• When speaking of gain in an antenna, gain refers

to the idea that certain directions are radiated

better than others

• Antennas are reciprocal - the same design works

for receiving systems as for transmitting systems

Simple Antennas

• The Isotropic Radiator would radiate all

the power delivered to it and equally in

all directions

• The isotropic radiator would also be a

point source

The Half-Wave Dipole

• A more practical antenna is the half-wave dipole

• Dipole simply means it is in two parts

• A dipole does not have to be one-half wavelength, but that length is handy for impedance matching

• A half-wave dipole is sometimes referred to as a Hertz antenna

Basics of the Half-Wave

Dipole

• Typically, the length of a half-wave dipole is 95% of

one-half the wavelength measured in free space:

=c

f

Radiation Resistance

• The half-wave dipole does not dissipate power, assuming lossless material

• It will radiate power into space

• The effect on the feed point resistance is the same as if a loss had taken place

• The half-wave dipole looks like a resistance of 70 ohms at its feed point

• The portion of an antenna’s input impedance that is due to power radiated into space is known as radiation resistance

Antenna Characteristics

• It should be apparent that antennas

radiate in various directions.

• The terms applied to isotropic and half-

wave dipole antennas are also applied

to other antenna designs.

Radiation Patterns

• Antenna coordinates are

shown in three-

dimensional diagrams

• The angle f is measured

from the x axis in the

direction of the y axis

• The z axis is vertical,

and angle q is usually

measured from the

horizontal plane to the

zenith

Plotting Radiation Patterns

• Typical radiation patters are displayed in a polar

plot

Gain and Directivity

• In antennas, power

gain in one

direction is at the

expense of losses

in others

• Directivity is the

gain calculated

assuming a

lossless antenna

Beamwidth

• A directional antenna can be said to direct a

beam of radiation in one or more directions

• The width of this bean is defined as the angle

between its half-power points

• A half-wave dipole has a beamwidth of about

79º in one plane and 360º in the other

• Many antennas are far more directional than

this

Front-to-Back Ratio

• The direction of maximum radiation is in the horizontal plane is considered to be the front of the antenna, and the back is the direction 180º from the front

• For a dipole, the front and back have the same radiation, but this is not always the case

Major and Minor Lobes

• In the previous diagram, the antenna has one

major lobe and a number of minor ones

• Each of these lobes has a gain and a

beamwidth which can be found using the

diagram.

Effective Isotropic Radiated Power and Effective

Radiated Power• In practical situations, we are more interested in

the power emitted in a particular direction than in

total radiated power

• Effective Radiated Power represents the power

input multiplied by the antenna gain measured

with respect to a half-wave dipole

• An Ideal dipole has a gain of 2.14 dBi; EIRP is

2.14 dB greater than the ERP for the same

antenna combination

Impedance

• The radiation resistance of a half-wave dipole situated

in free space and fed at the center is approximately 70

ohms

• The impedance is completely resistive at resonance,

which occurs when the length of the antenna is about

95% of the calculated free-space, half-wavelength

value

• If the frequency is above resonance, the feedpoint

impedance has an inductive component; if the

frequency is below resonance, the component is

capacitive

Ground Effects• When an antenna is installed

within a few wavelengths of the ground, the earth acts as a reflector and has a considerable influence on the radiation pattern of the antenna

• Ground effects are important up through the HF range. At VHF and above, the antenna is usually far enough above the earth that reflections are not significant

• Ground effects are complex because the characteristics of the ground are variable

Other Simple Antennas

• Other types of simple antennas are :

– The folded dipole

– The monopole antenna

– Loop antennas

– The five-eighths wavelength antenna

– The Discone antenna

– The helical antenna

The Folded Dipole

• The folded dipole is the same length as a standard dipole, but is made with two parallel conductors, joined at both ends and separated by a distance that is short compared with the length of the antenna

• The folded dipole differs in that it has wider bandwidth and has approximately four times the the feedpoint impedance of a standard dipole

The Monopole Antenna

• For low- and medium-frequency transmissions, it is necessary to use vertical polarization to take advantage of ground-wave propagation

• A vertical dipole would be possible, but similar results are available from a quarter-wavelength monopole antenna

• Fed at one end with an unbalanced feedline, with the ground conductor of the feedline taken to earth ground

Loop Antennas

• Sometimes, smaller antennas are required for certain applications, like AM radio receivers

• These antennas are not very efficient but perform adequately

• Two types of loop antennas are:– Air-wound loops

– Ferrite-core loopsticks

The Five-Eighths Wavelength Antenna

• The five-eighths wavelength antenna is used vertically either as a mobile or base antenna in VHF and UHF systems

• It has omnidirectional response in the horizontal plane

• Radiation is concentrated at a lower angle, resulting in gain in the horizontal direction

• It also has a higher impedance than a quarter-wave monopole and does not require as good a ground

The Discone Antenna

• The discone antenna is characterized by very wide bandwidth, covering a 10:1 frequency range

• It also has an omnidirectional pattern in the horizontal plane and a gain comparable to that of a dipole

• The feedpoint resistance is typically 50 ohms

• Typically, the length of the surface of the cone is about one-quarter wavelength at the lowest operating frequency

The Helical Antenna

• Several types of antennas

are classified as helical

• The antenna in the sketch has its maximum radiation along its long axis

• A quarter-wave monopole

can be shortened and

wound into a helix—

common in rubber ducky

antenna used with many

handheld transceivers

Antenna Matching

• Sometimes a resonant antenna is too large to be convenient

• Other times, an antenna may be required to operate at several widely different frequencies and cannot be of resonant length all the time

• The problem of mismatch can be rectified by matching the antenna to the feedline using an LC matching network

Antenna Arrays• Simple antenna elements can be combined to

form arrays resulting in reinforcement in some directions and cancellations in others to give better gain and directional characteristics.

• Arrays can be classified as broadside or end-fire– Examples of arrays are:

– The Yagi Array

– The Log-Periodic Dipole Array

– The Turnstile Array

– The Monopole Phased Array

– Other Phased Arrays

Reflectors

• It is possible to construct a conductive surface that reflects antenna power in the desired direction

• The surface may consist of one or more planes or may be parabolic

• Typical reflectors are:

– Plane and corner Reflectors

– The Parabolic Reflector

Cell-Site Antenna• For cellular radio systems, there is a need for

omnidirectional antennas and for antennas with

beamwidths of 120º, and less for sectorized

cells

• Cellular and PCS base-station receiving

antennas are usually mounted in such a way

as to obtain space diversity

• For an omnidirectional pattern, typically three

antennas are mounted on a tower with a

triangular cross section and the antennas are

mounted at 120º intervals

Mobile and Portable

Antenna• Mobile and portable antennas

used with cellular and PCS systems have to be omnidirectional and small

• The simplest antenna is the quarter-wavelength monopole are these are usually the ones supplied with portable phones

• For mobile phones, and common configuration is the quarter-wave antenna with a half-wave antenna mounted collinearly above it

CONCEPT OF AN ANTENNA

• Antenna is a electrical conductor used

in transmission and reception of EM

energy

Radiation Mechanism

¼ WL OPEN CKTD.XMISSION

LINE

XMISSION LINE do not radiate. Why ?

Opposing Magnetic fields can build

up by the current in the wire and

these cancel each other almost

completely ,so there is no radiation .

¼ WL OPEN CKTD.XMISSION LINE

SET APART

¼ WL OPEN CKTD.XMISSION

LINE SET APART -

TO FORM ANTENNA

Develops

MAX. current and MIN. voltage at

NODE, leads to low IMPEDANCE &

vice versa for ANTINODES

Variation of Reactance

2 WIRE RESONANT FEEDER

SYSTEM

RADIATION PATTERN of ½ , short dipole and

isotropic antenna compared

Example and figure of some

Antennas

a. Dipole Antenna b. Loop Antenna c. Folded dipole d. Folded uni pole

Parabolic & Horn Reflectors

Summary of Antennas

• An antenna is basically a Transducer.

• It converts RF electrical current into an EM wave of the

same frequency.

• It forms a part of a Transmitter as well as a receiver

circuit.

• It is also an impedance matching device . It matches /

couples the transmitter and free space or free space

and receiver.

• A sample antenna is a half wave dipole.

• The shortest length of dipole capable of resonance is an

electrical half wave length.

Summary contd…

• Center Impedance of a simple dipole is approx.

72 ohms ( at resonant frequency) .

• It increases as we go away from center .

• An antenna impedance is required to be

matched with the characteristic impedance of the

feeder so that maximum power transfer takes

place.

• ISOTROPIC antenna radiates equal power in all

directions .

• Practical antenna does not radiate power

equally in all directions .

Length of Antenna

• Practical antennas have length 5% less than the theoretical antennas

• This is due to …

a) Theoretical length is true when antennas are in free space .

b) For practical antennas ,END EFFECT has to be considered which is

caused by

1) Capacitance between pole and antenna

2) Capacitance between antenna and earth

3) Inductive effect in the tightening material of antenna

These effects cause the 5% reduction in length.

The practical length of a half wave dipole is

Lm = (142.5 / F MHz ) meters .

ANTENNA USES

How to improve gain of dipole?

• Add parasitic elements (Directors and

Reflectors)

• Parasitic elements reduce impedance

below 73 ohms . Hence use either

shunt feed or folded dipole

Shunt fed Yagi Antenna

Folded Dipole Yagi Antenna

Antennas for UHF &MW FREQ

Should have High Gain & Directivity

They are

• Yagi Uda

• Grid Pack

• Normal Parabolic

Antennas for UHF & MW FREQ

YAGI GRID

GRID ANTENNAA grid antenna employs welded tube which can

be split into section for ease of transportation

and handling

GAIN & DIRECTIVITY

Gain, Directivity & Efficiency

ANTENNA GAIN

Gain of Parabolic antenna

(7 GHz band)

Front to Back ratio of Antenna : Ratio of the front lobe power to the back lobe power .High Power Beam Width : It is the total angular width of the main beam at the 3dB points . In the figure the angle represents the beam- width . The beam width is represented by φ=22/FD in degree .F= Frequency in GHz and D= diameter of the antenna in meters .

Parabolic Antenna Beam Width

• The 3 dB Bandwidth of main lobe in the

direction XY . In degrees is

Ѳ = BW between half power points

λ =wave length

C=3x10^8 met per second

D =Antenna mouth diameter in meters

F= frequency in Hz.

Ground Plane Antenna

GP antenna is used for HF and VHF communication . The basic design is quarter wave length vertical antenna with four Radials mounted at antenna base . The radials may be made of tube or wire . The coaxial line from transmitter of 50 Ohms characteristics impedance is connected to the GP antenna .

PERFORMANCE

PARAMETERS

• RADIATION / POWER PATTERN

• GAIN

• IMPEDANCE, RADIATION RESISTANCE

• EFFECTIVE APERTURE & LENGTH

• COUPLING

• BANDWIDTH

• SYSTEM CONSIDERATION

PROPAGATION

CHARACTERISTICS

OF EM WAVES

PROPAGATION CHARACTERISTICS

OF EM WAVES

Modes of Propagation

• A part of wave that travels along or near the

surface of the earth .This is Ground Wave.

• Some waves neither follows the earth, nor

moves towards the sky ,but travels directly from

the Tx antenna to the Rx antenna . These are

Space or Tropospheric Waves.

• Some waves travel upwards towards the sky

and get reflected back to the receiver . These

are Sky or Inospheric Waves .

Modes continued ….

FACTORS INFLUENCING

EM WAVE

PROPAGATION

FACTORS…..

• Frequency of operation

• Earth curvature in terms of Conductivity ,

Permitivity and Permeability .

• Polarization of transmitting antenna .

• Height of transmitting antenna

• Transmitter power

• Curvature of the earth

• Obstacles between the transmitter and receiver .

FACTORS contd …..

• Moisture content in the troposphere .

• Electrical characteristics of the atmosphere in the

Tropospheric region .

• Characteristics of the Ionosphere .

• Earths magnetic field .

• Refractive index of the troposphere and

ionosphere.

• Distance between the transmitter and receiver.

• Roughness of the terrain (Hilly, River , Sea ,Forest )

GROUND WAVE

GROUND WAVE

• Best suited for Maritime Communication and

Radio navigation .

• It propagates from transmitter to receiver by

gliding over the surface of the earth .

• It exists when between both the transmitting

and receiving antennas are close to the

surface of the earth and the antennas are

vertically polarized.

• It is limited to only a few kilometers and is of

importance for medium and low frequencies.

GROUND WAVE contd….

• Field strength varies with the characteristics of

earth and is inversely proportional to the

square of the distance and the frequency .

• Requires relatively higher transmitter power

and not effected by the changes in the

atmospheric conditions.

• Horizontally polarized antenna are not

preferred as horizontal components of the

electric field in contact with the earth gets

short circuited and lost.

SKY WAVES

OR

INOSPHERIC WAVES

SKY WAVES OR

IONOSPHERIC WAVES

• It is the upper portion of the atmosphere

between approx. 50Km to 500 Km

above the earth .

• In this region gases get ionized by

absorbing large quantities of radiation

from different layers

• Ionization increases with altitude , and

variation is not linear.

SKY WAVES contd…..

• The amount of ionization depends upon the

rate of formation of ions and the rate of

recombination.

• At lower altitudes since the atmospheric

pressure is large the rate of combination is

large so that ionization is small.

• At higher altitudes since the atmospheric

pressure is low the rate of recombination is

low and so that ionization is high .

SKY WAVES contd…..

• D Layer > 50 t0 90 Kms. above the

surface of the earth and disappears in

night

• E Layer >110 Kms. above the surface of

the earth.

• F Layer > 220 Kms. and 250-350 Kms.

respectively , At night these two layers

combine and become one . The ionization

of this layer is maximum at day time and

minimum at night time

SKY WAVES contd…..

SKY WAVES contd…..

Mechanism of Ionospheric

Propagation • As the waves pass through the ionosphere, the

ionization density increases , and the refractive index

of the layer decreases .hence the incident wave is

gradually bent from the normal

• At a certain point, it finally becomes parallel to the

layer and then bends towards and returns from the

ionized layer . The bending of wave by the

ionosphere follows optical Snell ‘s law

Characteristic Parameters of

Ionospheric Propagation

Virtual Height : It is defined as the height that is

reached by a short pulse of energy which has the

same time delay of original wave .Virtual height of a

layer is always greater than the actual height .

Critical Frequency

Fc for a given layer is defined as the highest

freq that will be reflected to the earth by that

layer at vertical incidence .It is the limiting

frequency below which a wave is reflected

and above which it penetrates through an

ionospheric layer at normal incidence.

Refractive index, by definition ,is equal to the

square root of Dielectric constant

i.e. μ = ( € ) ^1/2

i.e. sin i / sin r = (1- 81 N / F^2)^1/2

N= Electron density and F= Frequency in KHz

Critical Frequency contd…

Above this freq. the wave will not be reflected back to earth .

Maximum Usable Frequency

• It is the highest freq. of wave that is

reflected by the layer at an angle of

incidence other than normal

SKIP DISTANCE

It is the shortest distance from the transmitter that is covered by a fixed

freq.> Fc

Large angle of incidence ray returns to ground at a long distance from

TX .

When angle of incidence is reduced the ray returns to ground at a

shorter distance.

Ultimately , possibility of certain distance not being covered exists ,since

ray escapes .

SKIP FREQUENCY• It is the maximum frequency above which it is not

possible for a signal to reach a point via Ionospheric

reflection .

OPTIMUM WORKING FREQUENCY

•The frequency of wave which is normally used for

ionospheric communication is known as OWF .

•It is generally chosen to be about 15% less than the

MUF .

•It is always desirable to use as high a frequency as

possible ( but not too near the skip frequency or MUF )

since any slight variation in the Ionospheric condition

may cause a loss of signal.

SPACE WAVE

OR

TROPOSPHERIC

WAVE

SPACE WAVE

• Troposphere is the region of atmosphere within

16Kms above the surface of earth .

• The EM waves that propagates from the

transmitter to the receiver in the earth’s

troposphere is called Space wave or

Tropospheric Wave .

• Space wave propagation is useful at frequency

above 30 MHz.

• It is useful for FM ,TV and Radar applications.

EFFECT OF CURVATURE OF EARTH

• The field strength at the receiver becomes

small as the direct wave may not reach the

receiving antenna . The ground reflected rays

diverge after their incidence on earth .

• The curvature of the earth creates shadow

zones also called diffraction zone. These are

the regions where no signal reaches .

• Reduces the possible distance of

communication .

• Field strength available at the receiver

becomes small.

Curvature of Earth and

Shadow Zone

EFFECT OF EARTHS IMPERFECTION

• Earth is basically imperfect and rough .

• For perfect earth, reflection coefficient is unity

,but actual earth makes it different .

• For reflection from perfect earth , phase

change is 180 degrees ,but actual earth

makes it different .

• Amplitude of ground reflected ray is smaller

than that of the direct ray .

• The field strength at receiver is reduced due to

the roughness.

Effect of Hills ,buildings & Obstacles

Height above Earth

These create Shadow zones ,hence possible distance of

transmission is reduced .

Field varies with the height .

Field variation is characterized by the max. ,min. and nulls.

Maxima ,Minima depend upon frequency ,height of

transmitting antenna , ground characteristics and

polarization of the wave.

Field Strength in Practical

Cases

Transition between Ground

and Space wave

• When transmitting antenna is close to earth ,

ground waves exist and field strength is

independent of height of antenna .

• Antenna height has an effect on field strength ,

which depends upon frequency, polarization

and constants of earth .

• At higher heights, space wave dominates .

Atmospheric effects in

Space wave Propagation• Atmosphere consists of gas molecules and

water vapour so density is higher compared

to free space .

• For standard atmosphere , pressure

,temperature , humidity decreases linearly

with altitude.

• The refractive index of air depends on height

and this gives rise to phenomena's like

Reflection , Refraction and Scattering

Reflection• Occurs when waves strike smooth surface,

such as water and smooth earth etc.

• Both reflected wave and direct wave reaching

the receiver ensures reduced signal strength

and they may arrive in phase or out of phase

or partially out of phase .

• For perfectly smooth surfaces ,and under

condition of amplitude being equal and

exactly out of phase at the receiver ,the

received wave may get completely cutoff .

This is FADING

Refraction

• Due to varying refractive indices with height,

the wave does not follow a straight path from

Transmitting to receiving antenna .

• It follows a bent path i.e. follows the curvature

of the earth.

• Hence radius of earth seems to be larger than

actual for the beam .

• Also the path varies during various hours of the

at various places

Scattering / Diffraction

• Obstacles like tall buildings and

hilltops in the path of the wave ,

increase transmission loss .

• Waves arrive at the receiver by the

process of diffraction .

K - FACTOR

Significance of 0.157

• 1/ Earth radius = 1/6378 Kms = 157 x 10^ -6/m

=0.157/ Km i.e

0.157 is the Earth Curvature Factor

• Variation of RI causes curving of beam .

• Curvature of beam can be expressed in terms of

curvature of earth i.e. scaling factor x 0.157 to

have a FEEL of various refractive conditions .

Values of K

Condition Variation of RI

(Expressed in terms of curvature of earth).

• Sub normal (2.4 – 1)x 0.157

• Typically Subnormal (1.5 ) x 0.157

• Normal (1 to 0.64) x 0.157

• Typical normal 0.75 x 0.157

• Super normal (0.64 to 0) x 0.157

K factor Explained

• To correlate between earth’s curvature and the

curvature of MW beam path , it is customary to take

one of the curvatures to be a straight line ( generally

mw beam).

• Due to assumptions , the actual curvature of the

beam will also have to be modified to keep the earlier

correlation same.

• The modification of earth’s curvature is done by

multiplying actual earth’s curvature by K factor .

• K factor depends on atmospheric conditions.

• Hence modified earth’s curvature also changes with

atmospheric conditions.

K factor continued

• The amount and direction of bending subjected by MW

beam is defined

1. Either by refractive index Gradient dN / dH (Where N

is the radio refractivity and h is the height of of the layer

above the surface of earth )

2. Very often by the effective earth radius factor K.

DefinitionK is a factor which when multiplied by actual earth

radius, gives the value of the modified earth radius ,

gives the value of the modified earth’s radius employed

in profile chart to make the MW beam a straight line .

K-Factor =157/(157= dN /dH )

• Here N (Radio refractivity)

= 77.6(P/T) +3.73x10^5(e/ t^2)

Where P= Total atmospheric Pressure in Milli

bar

T=Absolute Temperature in Kelvin

e =Water vapour pressure in Milli bar

Effects with Variations in K

• Changes in value of K from 1 to infinity

have less influence upon the received

signal (excepting multipath fading).

• For K<1 , the path is vulnerable to

extreme multi fading .

• For K= negative , path is susceptible to

black out fading .

Conditions

• When dN/dH= - 40 units /Km ; K=4/3 (This is

standard atmospheric condition)

• When dN/dH= - 157 units /Km ; K= ∞ infinity

(This is super refractive atmospheric condition)

• When dN/dH= 0 units /Km ; K=1 (This is sub

refractive atmospheric condition)

• When dN/dH= +79 units /Km ; K=2/3 (This is

Sub normal refractive atmospheric condition)

SUPER STANDARD REFRACTION

• Arises due to reduction in atmospheric density with

increased height .

• K increases – results in flattening of the effective earths

curvature ,The condition causing it is passage of cool air

over warm body of water .

• Atmospheric density increases near the surface due to

low temperature and high humidity

• High downward bending of the wave is caused .

• In moderate condition when K tends to infinity, the wave

is propagated parallel to earth.

• In extreme conditions when K is negative , it causes a

blackout fade

Summary

SUB STANDARD REFRACTION

• Arises due to increase in atmospheric density

with height .

• Condition causing it --- when fog is formed

with the passage of warm air over cool air or

moist surface .

• Causes upward bending of the beam (Earth’s

buldge)

K with increasing values

• Earth appears to be increasingly flat as the value of k

increases .

• For the value of K= infinity , the earth appears to be

perfectly flat for a microwave beam , since the beam

curves at the same rate as earth .

• The curvature for various values of K can be calculated by

Where h=change in Vertical distance from a horizontal reference line,

d1 = distance from a point to one end of the path in Kms,

d2= distance from a point to the other end of the path in Kms

HOW TO PLOT PATH PROFILES

• Ideally, we have to plot the path taken by the rays for

normal , sub-normal and super-normal conditions .

• If we plot the path profile (using details obtained from

survey maps ) on a plain graph paper, curvature of earth

is not accounted for .

• Hence for convenience of analysis , bending of radio

path to be interpolated in earth curvature for all

conditions ( normal , sub-normal and super-normal) and

using such curved abscissa graph sheets, path profile to

be plotted .

PROFILE CHARTS

• PC’s are for various values of k available .

• Bend of radio path is transferred to earth

radius as per value of K.

• Mark the terrain specific details from

survey maps of these charts.

• Mark the towers / antennas on them.

Profile chart for K=4/3,( Normal condition )

Profile chart for K=2/3,( Sub-Normal condition )

Profile chart for K=infinity,( Super-

Normal condition )

Essential Clearances for

Radio Path

Clearance of Radio path

means..• Clearance of ‘Zone ’ of constructive arrival rays

to the full extent in ‘ normal condition’.

• Clearance of ‘Zone ’ of constructive arrival to

sufficient extent in ‘subnormal condition’

• Clearance of reflection point from reflective

bodies to avoid ground reflected rays

interference and consequent fading

Fresnel Zone

• When MW beam is transmitted from an

antenna, the beam gradually spreads conically

as per Hygen’s principle.

• The total MW energy reaching antenna ‘b’is the

sum of energies passing through various zones

called Fresnel Zone .

• Maximum energy (primary energy)is

concentrated in central zone, called FIRST

FRESNEL ZONE

Concept of ‘Fresnel’Zone

The successive zones have path difference of λ/2

and are180* out of phase when reaching antenna.

Thus the 1st ,3rd ,5th ,7th Fresnel Zone are in phase

and the 2nd ,4th ,6th Fresnel Zones are out of phase.

Observations made …

• Thus we see that the energies are getting diminished

with the higher Fresnel Zones.

• The transmitted wave will have maximum energy if

only first Fresnel zone is cleared .

• More the Fresnel zones less is the strength if signal

reaching the receiver .

• Practically it is not possible to make an antenna

receive only the first Fresnel zone.

So we limit the height of the Tx and Rx antennas so

that 2nd Fresnel zone is obstructed on the lower side

at a certain lower value of K .

Formulas

• Radius of the first Fresnel Zone is

calculated as

F1 = 17.3(d1d2/ F GHz. D Km)^ ½,

d1,d2 is distance in Kms of the towers at

the point where the radius is to be calculated

F GHz = frequency in GHz

• Radius of nth Fresnel Zone is calculated

as

Fn = F1 (n) ^1/2,

(n = Nos. of Fresnel zones to be calculated )

Conditions for clearance..

• If first Fresnel zone is available for K = 4/3, at

least 2/3 of the Fresnel zone should be cleared

for K=1

• Tolerance depends upon the K factor

1.For K = 4/3,Full 1st Fresnel Zone +10 meter

clearance

2.For K =1 ,2/3rd of the 1st Fresnel Zone +10

meter clearance.

3.For K = 2/3,Grazing clearance of 2/3rd of 1st

Fresnel Zone only .

• 1,2,3 are the tolerances for clearing future obstructions

PATH ENGINEERING

SEQUENCE

Step-1 , Plotting path profile

Step- 2, Locating tower and

antennas

Step- 3,Compute first Fresnel Zone

radius at high rise points and check for

clearance

Step- 4 ,Check for Fresnel Zone

clearance for Sub Normal

conditions

Step-5 ,Clearance of points of

Reflections

Step-6,Clearance of Super Normal

Conditions

Step-7 , Check the need for

Diversity

Losses and Gains en-route

Concept of System Gain

Path Loss

Fade Margin

We cannot take the value of fade margin

as 30 dB or 40 dB as granted for Fade

Margin.

What if there is more FM ?

Outage will be less and

Availability will be High.

In other words , FM will be considered

based on the availability requirements.

Choice of Antennaat F = 7GHz and Hop length =40 Km

Fading

Multi path fadingCauses outages .

Duration of fade is independent of

frequency and proportional to the depth of

fade

Frequency Selective Fading

Causes Delay distortion

In digital microwave it manifests ISI .

Strategies to counter fading

• Frequency diversity (5% separation of

carriers)

• Space Diversity (6 meters separation of

antenna)

• Whatever be the technique ensure

hitless switching of digital circuits .

• Space diversity is cost effective and

improves availability to 99.9999 %

Atmospheric Causes

• Attenuation due to rain - when wave length is

close to rain drop size.

1 to 10 dB per Km as per precipitation rate of

rainfall.

Below 15 GHz ,attenuation is less than 1dB .

• Attenuation due to Cloud and Fog- drop sizes

are less than rain drop.

Attenuation is more for 15GHz and above

• Attenuation due to hail & snow –similar to

cause of rain .

Radio path causes

• Insufficient path clearance- Rays getting

obstructed by high rise objects/geographic

features.

• Multi Path Propagation – Destructive

interference of rays reaching on different

paths through atmosphere .

• Fading due to ground reflection- ground

reflected signal of significant strength causes

fading due to interference with normal path

signals .

Classification of fading

• Rapid fluctuations –due to multi path

interference. Occurs for a few seconds

• Short time Fluctuation – due to variation in

characteristics of propagation medium. Occurs

for a few hours .

• Long term Fluctuation – due to seasonal

variation in propagating medium. Occurs over

a few days

• Fade Out(Total Fading)- occurs during sudden

atmospheric disturbances, Sunspot cycles etc.

Freq. Selective Fading

• Alternate points of maximum and minimum i.e

(Reinforcement & cancellation) of signal

strength are encountered during space wave

propagation from Tx to Rx .This phenomenon

is called Selective Fading.

All microwave systems suffer from this type

of fading due to multi path propagation .

• The degree of multi path is highly dependant

on Hop length ,Weather condition and water

logged bodies (paths) .

Flat Fading

As the name implies, it is non frequency

dependant attenuation of the input

signal at the receiver and typically

occurs during periods of heavy rain

particularly at higher Micro Wave

frequencies .

Diversity methods

It is difficult to control short-term and

long- term fluctuations .

Rapid fluctuations can be reduced by

following techniques

• SPACE DIVERSITY

• FREQUENCY DIVERSITY

• POLARITY DIVERSITY

• TIME DIVERSITY

FREQUENCY DIVERSITY

FREQUENCY DIVERSITY

• Advantages

Reliability is more .Equivalent to 100 %

standby hence no need of providing

stand by Tx and Rx .

• Disadvantages

Two freq. are needed. Improvement by

diversity is not much since 5%

separation of frequency is rarely

achieved .

SPACE DIVERSITY

SPACE DIVERSITY

• Advantages - As one freq. is used ,propagation

reliability is improved . By more separation of

antennas improvement factor can be further

improved.

• Disadvantages - As two antennas are mounted

on the same tower and the lower antenna

should be in line of sight with the Tx antenna the

height of the tower is increased thus the cost

increases due to better foundation needed .

Standby is also required for higher reliability

FREQUENCY

ALLOCATIONS

Range Frequency Band Application

VLF 3 KHz – 30 KHz Submarine Application

LF 30KHz to 300 KHz Navigational Application

MF 300KHz to 3MHz Cordless Phones, AM radio

HF 3 MHz to 30 MHz Aeronautical, Amateur radio

VHF 30 MHz to 300 MHz FM Broadcast, TV Applications

UHF 300 MHz to 3GHz TV, Mobile Communication

SHF 3 GHz to 30 GHz Point to Point, Satellite Comm.

EHF 30 GHz to 300 GHz Point to Point microwave

FREQ.BAND & APPLICATIONS

Propagation Mechanism

R F BAND used in RLYs.

GSM & GSM-R

FREQUENCY PLAN

FOR RLYs

There are two distinct patterns of

frequency plans employed in Railways .

They are

• FOUR Frequency plan

• TWO Frequency plan

FOUR FREQ. PLAN

TWO FREQ. PLAN

THANK YOU

FOR

YOUR

INTEREST