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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