AWP.pptx

Presented by
Mrs. T Gayatri
Associate Professor, ECE Dept
Antennas & Wave Propagation

UNIT I
ANTENNAS & WAVE PROPAGATION
T Gayatri Assistant Professor

CONTENT
– VARIOUS DEFINITIONS, ANTENNA PARAMETERS
– COMPARISON BETWEEN ANTENNAS &
TRANSMISSION LINES
– TRANSMISSION FORMULA, SOURCES OF
RADIATION
– FREQUENCY SPECTRUM

VARIOUS DEFINITIONS, ANTENNA PARAMETERS
– Antenna
– Radiation Pattern
– Radiation Intensity
– Polarization
– Directivity
– Power gain
– Efficiency
– Effective aperture/area
– Beam width
– Band width
– Radiation resistance
– Effective length
– Reciprocity theorem
– Front to back ratio

An antenna is an electrical device which converts
electric currents into radio waves, and vice versa. It is
usually used with a radio transmitter or radio receiver.
In transmission, a radio transmitter applies an oscillating
radio frequency electric current to the antenna's
terminals, and the antenna radiates the energy from the
current as electromagnetic waves (radio waves).
In reception, an antenna intercepts some of the power of
an electromagnetic wave in order to produce a tiny
voltage at its terminals, that is applied to a receiver to be
amplified. An antenna can be used for both transmitting
and receiving. T Gayatri Assistant Professor

• It is a metallic conductor system capable of radiating and
receiving EM waves.
• Typically an antenna consists of an arrangement of metallic
conductors (“elements"), electrically connected (often through a
transmission line) to the receiver or transmitter.
• An oscillating current of electrons forced through the antenna by
a transmitter will create an oscillating magnetic field around the
antenna elements, while the charge of the electrons also creates
an oscillating electric field along the elements.
• These time-varying fields radiate away from the antenna into
space as a moving electromagnetic field wave.
• Antenna reciprocity : can be used as transmitter and receiver. In
two way communication same antenna can be used as
transmitter and receiver.

• An Isotropic radiator is a theoretical point source of
electromagnetic or sound waves which radiates the same
intensity of radiation in all directions. It has no preferred
direction of radiation. It radiates uniformly in all directions over
a sphere centered on the source. Isotropic radiators are used as
reference radiators with which other sources are compared.
• Omni-directional antennas which receive or radiate more or
less in all directions. These are employed when the relative
position of the other station is unknown or arbitrary.
• Directional antennas which are intended to preferentially radiate
or receive in a particular direction or directional pattern.

VARIOUS DEFINITIONS, ANTENNA
PARAMETERS
• The radiation pattern of an antenna is a plot of the
relative field strength of the radio waves emitted by the
antenna at different angles.

Radiation Intensity
• The radiation intensity is defined as Power per unit solid
angle. It is measure in form of Watts/radian 2
Polarization
• The polarization of an antenna is the orientation of the
electric field (E-plane) of the radio wave with respect to
the Earth's surface and is determined by the physical
structure of the antenna and by its orientation.

Directivity
• The directivity D of an antenna, a function of direction is
defined by the ratio of radiation intensity of antenna in
direction to the mean radiation intensity in all directions.
Gain
• The ratio of maximum radiation intensity in given
direction to the maximum radiation intensity from a
reference antenna produced in the same direction with
same power input.
• Antenna Gain (G) can be related to directivity (D) by

Efficiency
• The efficiency of an antenna is defined as the ratio of power
radiated to the total input power supplied to the antenna.
Effective aperture/area
• It is defined as the ratio of power received at the antenna
load terminal to the power density of the incident wave.

Beam width
• Beam-width of an antenna is defined as angular separation
between the two half power points on power density
radiation pattern
• Angular separation between two 3dB down points on the
field strength of radiation pattern .
• It is expressed in degrees.

Bandwidth
• The bandwidth of an antenna expresses its ability to operate
over a wide frequency range. It is often defined as the range
over which the power gain is maintained to within 3dB of its
maximum value, or the range over which the VSWR is no
greater than 2:1, whichever is smaller.
• The bandwidth is usually given as a percentage of the
nominal operating frequency. The radiation pattern of an
antenna may change dramatically outside its specified
operating bandwidth

Radiation resistance
• The resistive part of the antenna impedance is split into
two parts, a radiation resistance and a loss resistance.
• The power dissipated in the radiation resistance is the
power actually radiated by the antenna, and the loss
resistance is power lost within the antenna itself..
Effective length
• The effective length indicates how far an antenna is
effective in transmitting or receiving the EM wave energy.
Reciprocity theorem
• If a voltage is applied to the terminals of an antenna A and
the current measured at the terminals of another antenna B
then an equal current will be obtained at the terminals of
antenna A if the same voltage is applied to the terminals of
antenna B.

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.

Field Region
• The fields surrounding an antenna are divided into 3
principle regions:
• Reactive Near Field
Radiating Near Field or Fresnel Region
• Far Field or Fraunhofer Region

COMPARISON BETWEEN ANTENNAS & TRANSMISSION LINES
• Ideally all incident energy must be reflected back when
open circuit. escapes from the system that is it gets
radiated.
• This occurs But practically a small portion of
electromagnetic energy because the line of force don’t
undergo complete phase reversal and some of them
escapes..
• Also because two wires are too close to each other,
radiation from one tip will cancel radiation from other
tip.( as they are of opposite polarities and distance
between them is too small as compared to wavelength )
G

COMPARISON BETWEEN ANTENNAS & TRANSMISSION LINES
• To increase amount of radiated power open circuit must
be enlarged , by spreading the two wires.
• Due to this arrangement, coupling between transmission
line and free space is improved.
• Also amount of cancellation has reduced.
• The radiation efficiency will increase further if two
conductors of transmission line are bent so as to bring
them in same line.

DIPOLE ANTENNA

UNIT II
VHF,UHF &MICROWAVE ANTENNAS

• The antennas which are operated b/w the frequency
ranges 30 to 300 MHz & 300 to 3000 MHz are called
vhf and uhf antennas.
• Antennas operating over 3000 MHz are called
microwave antennas.
• Examples: yagi-uda, folded dipoles, ground plane
corner reflectors etc.
• Commonly used for land mobile communication,
public communication and industry.
• Antennas operating in the range 3000-30000 MHz are
called shf antennas.
• Examples: parabolic reflector, horn and lens antennas.

FOLDED DIPOLE ANTENNA

• Basic property of folded dipole is, it is used in wide band of
frequencies.(UHF &VHF)
• Input impedance of folded dipole is high and is used as a
matching transformer.
• It has built in impedance matching transformer.
• A folded dipole, in which 2 half wave dipoles, one is
continuous and the other split at the center have been folded
and joined together in parallel at the ends.
• The radiation pattern of a folded dipole and a conventional
half wave dipole is same, but the I/p impedance of folded
dipole is higher.

FOLDED DIPOLE TRIPOLE

• The input impedance at the terminals of a folded dipole
antenna is equal to the square of the no. of conductors
consisting the antenna times the impedance at the terminals
of a conventional dipole.
• Radiation resistance or terminal impedance or input
impedance of a folded dipole antenna is (2)2 X 73 Ω = 292
Ω.
• Radiation resistance of a Tri pole antenna is
(3)2 X 73 Ω = 657 Ω.

DIFFERENT TYPES OF FOLDED DIPOLES

• If the radii of conductors are made unequal, then the
general expression for the input impedance is given by:
r1 - Radius of the dipole 1
r2 - Radius of the dipole 2
According to Prof. Uda and Mushiake, the impedance is
given by,

• APPLIVATION:
• Most extensively used as a feed
element of TV antennas such as
Yagi-Uda antennas.
• Bandwidth characteristic is far
better than that of single dipole of
same size.
• Used over a wide range of
impedances using different
techniques.

DIPOLES WITH PARASITIC ELEMENTS
• The element in which current is induced due to the field in other
elements is called Parasitic element.
• Such elements does not require transmission line connection for
supplying power.
• One or more elements coupled magnetically with the driven
element forms an array of parasitic elements or parasitic
antenna.
• The effect of parasitic element on the directional pattern of the
antenna depends on the magnitude and the phase of the induced
current in the parasitic element.
• Also the effect of parasitic element on the directional pattern
depends on the spacing between antenna elements and tuning of
the parasitic element. T Gayatri Assistant Professor

DIPOLES WITH PARASITIC ELEMENTS
• λ/2 parasitic element > resonant length ,Inductive nature --
Reflector.
• λ/2 parasitic element < resonant length ,Capacitor nature ---
Director.

YAGI-UDAANTENNA
• First invented by a Japanese prof. s. uda and was described in
English by prof. h. yagi. hence the name yagi- uda antenna.
• It is a high gain and directivity uhf & vhf antenna.
• It is called as super gain antenna, because the gain per unit beam
area is very high.
• It is also known as beam antenna
CONSTRUCTION FEATURES

YAGI-UDAANTENNA CHARACTERISTICS
• It consists of a driven element, one reflector and one or more
directors (Or) an array of one driven element and one or more
parasitic
• The parasitic elements which are continuous are arranged parallel
to the driven element.
• The separation between the successive elements is in the range of
0.1λ to 0.3 λ.
• Properly spaced dipoles longer than λ/2 (Inductive) acts as
reflector and add the fields of the driven element in a direction
away from the reflector towards the driven element.

HORN ANTENNAS
● Simplest form of the microwave antenna.
● It serves as a feed element for large radio astronomy,
communication dishes and satellite tracking.
● It can be considered as a waveguide with hollow pipe of
different cross section which is flared into a large opening.
● It is an aperture antenna which is used to properly match the
waveguide or any guiding system to large radiating aperture
by shaping the transition gradually.
● The large aperture is necessary to improve directivity and to
produce efficient radiation with proper matching with free
space.
● Most useful for broadband signals.

HORN ANTENNAS TYPES
● RECTANGULAR HORN ANTENNAS
SECTORIAL HORN
E- plane sectorial horn
H-plane sectorial horn
PYRAMIDAL HORN
● CIRCULAR HORN ANTENNAS
Conical horn
biconical horn
● TAPERED HORN ANTENNAS
Exponentially tapered pyramidal
Exponentially tapered conical

HORN ANTENNAS TYPES

DESIGN EQUATIONS OF HORN ANTENNAS

DESIGN EQUATIONS OF HORN ANTENNAS
θ= tan-1(h/2L) =cos-1(L/ L +δ) and L=h2/8δ
Here, h= Height of horn antenna, L=Axial length
δ= Permissible phase angle variations expressed as a
fraction of 360° and 1 θ = (1/2) of flare angle.
● For a given aperture distribution, the directivity is
proportional to the aperture area for specified
dimensions.
● If the flare angle is very large, the directivity decreases
with increase in beam width.
● If the flare angle is small, the aperture area is small,
which decrease beam width, resulting increased
directivity.

UNIT III
REFLECTOR ANTENNA
• Important in microwave radiation applications.
• In reflector antenna, another antenna is used to excite it
and hence antennas such as dipoles, horn, slots e t c. are
used to excite and called as primary antenna, while the
reflector is called secondary antenna.
TYPES OF REFLECTORS
• Plane Reflectors
Active corner reflector
Passive corner reflector
• Corner Reflectors
• Parabolic Reflectors
Truncated or cut parabolic reflectors
Parabolic right cylinder
Pill box or cheese antenna

FLAT SHEET OR PLANE REFLECTOR
(TYPES)

CHARACTERISTICS
• It is the simplest form of the reflector antenna
• When the plane reflector is kept in front of the feed, the
energy is radiated in the desired direction.
• To increase the directivity of the antenna, a large flat
sheet can be kept as plane reflector in front of a half
wave dipole.
CORNER REFLECTOR

CORNER REFLECTOR
• A Corner reflector is a reflector consists of two plane
reflectors which are joined to form a corner with some
angle.
• The angle at which two plane reflectors are joined is called
included angle (α).
• In most practical applications, the included angle is 90o .
• Analysis is carried out under the assumption that the two
intersecting planes are perfectly conducting and infinite.
• The corner reflector antenna may also be analyzed by using
the method of images for angles.

DESIGN EQUATIONS
• The important dimensions in the corner reflector antenna are as
follows:
DA = Dimension of aperture
d = Distance between feed and the vertex of the reflector
l = side length of the reflector sheet.
Note: The side length is equal to twice the distance.
d= l/2 or l=2d

PARABOLIC REFLECTOR
• To improve the overall radiation characteristic of the
reflector antenna, the parabolic structure is used.
• A parabola is a locus of a point which moves in such
a way that the distance of the point from fixed point
called focus plus the distance from the straight line
called Directrix constant.

PARABOLOID (PARABOLOIDAL REFLECTOR)
• The three dimensional structure of the parabolic reflector can be
obtained by rotating the parabola around its axis called paraboloid.
• The paraboloid is called Microwave dish which produces sharp
major lobe and smaller minor lobes.

F/D RATIO, SPILLOVER, BACK LOBE
• In paraboloid reflector, the ratio of the focal length ‘f ’ to the
diameter of aperture is also an important design constraint
• It can be designed to obtain pencil shape radiation beam by
keeping the diameter of the aperture fixed and changing the focal
length.
• The three possible cases are:
➢ focal point inside the aperture of paraboloid.
➢ focal point along the plane of open mouth of paraboloid.
➢ focal point beyond the open mouth of paraboloid.

F/D RATIO, SPILLOVER, BACK LOBE
• Effect of variation of focal length keeping diameter of
aperture fixed in paraboloid
NOTE: For practical applications, the value of focal length to
diameter ratio lies between 0.25 and 0.5
• SPILLOVER: in addition to the desired radiation , some of
the desired rays are not captured by the reflector and these
constitutes spillover.
• While receiving , spillover increases noise pick up which is
particularly trouble some in satellite ground stations
• BACKLOBE RADIATION: some radiations from the
primary radiator occur in the forward direction in addition to
the desired parallel beam known as back lobe radiation

TYPES OF PARABOLOID
REFLECTORS
• Truncated or cut paraboloid.
• Parabolic right cylinder
• Pill box or cheese antenna.

FEED SYSTEMS FOR PARABOLIC
REFLECTOR
• Parabolic reflector antenna as a system consists of two basic parts:
• A source placed at the focus is called “Primary Radiator” or “Feed radiator” or simply “Feed”.
• The reflector is called “Secondary Radiator”.

CASSEGRAIN FEED SYSTEM
ADVANTAGES:
• It reduces the spillover and thus minor lobe radiations.
• With this system greater focal length greater than the physical
focal length can be achieved.
• The system has ability to place a feed at convenient place.
• Using this system, beam can be broadened by adjusting one
reflector surfaces
OFFSET FEED SYSTEM

LENS ANTENNA
● An antenna using the collimating properties of an optical lens is
called as Lens antenna.
● It is an optical device like parabolic reflector antenna.
● Lens antenna are used at higher frequencies (around 3 GHz and
above) because at lower frequencies they become bulky and
heavy.
PRINCIPLE OF LENS ANTENNA

TYPES OF LENS ANTENNAS
● Dielectric lens or H plane metal lens or Delay lens:
It is an antenna in which the travelling wave fronts are
retarded or delayed by lens media.
● E plane metal lens antenna or Fast lens:
It is an antenna in which the travelling wave fronts are
spaced up by the lens media.
Delay lens
antenna
Fast lens
antenna

•Lens antennas are suitable for frequencies above 3000
MHz .
•If the frequency is less than 3000 MHz, lens antennas
have more thickness.
•The thickness of lens antennas can be reduced with the
help of zoning. Thickness (t) is given by Where,
t = Thickness, λ= Free space wavelength
μ = Refractive index =(c/v)
ZONING
ADVANTAGES OF
ZONING
• This process reduces the
weight of lens considerably.
• The zoned dielectric lens
antenna ensures that signals
are in phase after
emergence, despite
difference in appearance.
The zoned lens is having
DISADVANTAGE
The zoned lens antennas are
frequency sensitive i.e., they
are dependent on wavelength,
λ

Zoning is classified into two types
Curved surface zoning
• In curved, surface zoning
stepping or zoning is done to
the curved surface of lens
antenna.
• Thickness of curved surface
zoned lens is Curved surface
zoned lens is mechanically
stronger than the plane surface
zoned lens,
• Curved surface zoning lens
antennas have less weight and
less power dissipation
• Example
• In plane surface zoning
stepping or zoning is done to
the plane surface.
Thickness of plane surface
zoned lens is
• Plane surface zoned lens is
less strong
• Here the power dissipation
is more.
• NOTE: Curved surface
zoning is preferable
compared to t he plane
surface zoning
Plane surface
zoning

UNIT IV
ANTENNAARRAYS
The antennas we have studied so far have all been
omnidirectional – no variation in φ. A properly spaced
collection of antennas, can have significant variation in φ
leading to dramatic improvements in directivity.

ANTENNAARRAYS (CONT’D..)
An antenna array can be designed to give a particular shape of
radiating pattern. Control of the phase and current driving each
array element along with spacing of array elements can provide
beam steering capability.
For simplification:
 All antenna elements are identical
 The current amplitude is the same feeding each element.
 The radiation pattern lies only in xy plane, θ=π/2
The radiation pattern then can be controlled by:
 controlling the spacing between elements or
 controlling the phase of current driving for each element

BROADSIDE ARRAY
For simple example, consider a pair of dipole antennas driven
in phase current source and separated by λ/2 on the z axis.
Assume each antenna radiates
independently, at far field point P,
the fields from 2 antennas will be
180 out-of-phase, owing to extra
λ/2 distance travel by the wave
from the farthest antenna  fields
cancel in this direction. At point
Q, the fields in phase and adds.
The E field is then twice from
single dipole, fourfold increase in
power  broadside array 
max radiation is directed
z
z

ENDFIRE ARRAY
Modify with driving the pair of
dipoles with current sources 180 out
of phase. Then along z axis will be in
phase and along y axis will be out of
phase, as shown by the resulting
beam pattern  Endfire array 
max radiation is directed at the ends
of axis containing array elements.
z

ARRAY OF 2-POINT SOURCE
Pair of Hertzian Dipoles
Recall that the far field value of E field from Hertzian dipole at
origin,



 a
sin
4
E 0
0
R
ke
I
j
jkR
s
S



But confining our discussion to the yz plane where phi = π/2,



 a
cos
4
E 0
0
R
ke
I
j
jkR
s
S




ARRAY OF 2-POINT SOURCE (Cont’d..)
Consider a pair of z oriented
Hertzian dipole, with distance
d, where the total field is the
vector sum of the fields for both
dipoles
The magnitude of currents are
the same with a phase shift
between them.
  
 




 a
cos
4
a
cos
4
E
E
E 2
2
2
0
1
1
1
0
2
0
1
0
0
2
1
R
ke
I
j
R
ke
I
j
jkR
s
jkR
s
S
S
tot
S








Where, 
j
s
s e
I
I
I
I 0
2
0
1 


For amplitude variation,
2
1
2
1 R
r
R 


 


For phase variation, 
 cos
2
cos
2
2
1
d
r
R
d
r
R 




Thus, the total E field becomes:
 
   
  






 a
cos
4
E 2
/
cos
2
/
cos
0
0
0





 kd
j
kd
j
jkR
tot
S e
e
R
ke
I
j

With Euler’s identity, the total E field at far field observation
point from two element Hertzian dipole array becomes :
 
 


 


 



 


 


AF
F
jkR
tot
S
kd
R
ke
I
j
unit





 


2
cos
cos
2
a
cos
4
E 0
0
0




 
array factor

  d)
(normalize
2
cos
cos

 

kd
AFn
factor)
(array
element)
single
( AF
E
Etot 

d)
(normalize
n
n
tn AF
E
E 






 

2
cos
cos
cos
E



kd
tn

To find radiated power,
   
 
r
r
tot
S
S
S
kd
R
k
I
E
r
a
2
cos
cos
4
32
a
2
1
H
E
Re
2
1
,
2
,
P
2
2
2
2
2
2
0
0
2
0
0
*





 


























It can be written as:
r
unit AF
F
r a
,
2
,
P 









Unit factor, Funit is the max time averaged power density for an
individual antenna element at θ=π/2









2
2
2
2
2
0
0
32 R
k
I
Funit

 
An array factor, Farray is





 

2
cos
4 2
array
F
Where,

 

 cos
kd
This is the pattern function resulting from an array of two
isotropic radiators. Excitation phase

N - ELEMENT LINEAR ARRAYS
The procedure of two-element array can be extended for an arbitrary
number of array elements, by simplifying assumptions
 The array is linear-antenna elements are evenly spaced, d along a
line.
 The array is uniform-each antenna element driven by same
magnitude current source, constant phase difference, between
adjacent elements.
 

 1
0
2
0
3
0
2
0
1 ,....
,
, 



 N
j
sN
j
s
j
s
s e
I
I
e
I
I
e
I
I
I
I

N - ELEMENT LINEAR ARRAYS (Cont’d..)
The far field electric field intensity :
 
 
  r
N
j
j
j
jkR
tot
S e
e
e
R
ke
I
j a
E 








 1
2
0
0
0 ...
1
4


Where, 
 

 cos
kd
Manipulate this series to get:





 





 

2
sin
2
sin
N
AF
With the max value as :
  N
AF 
max





 





 

2
sin
2
sin
1
N
N
AFn
For smaller value of,
2
2
sin






 

N
N
AFn

(Cont’d..)
Nulls of Array Factor: The nulls of array factor when AFn is set
to 0,
Maxima of Array Factor: The maximum values of the AF is occur
when,

(Cont’d..)
Half Power Beam Width (HPBW) of main lobe
The HPBW can be calculated when A Fn is set to ,
2
1

UNIT V
WAVE PROPAGATION
• The wave propagation characteristics between transmitter and
receiver are controlled by the transmitting antenna, operating
frequencies and media between them.

ELECTROMAGNETIC RADIATION
● Radio: cm to km wavelength
● Microwaves: 0.1 mm to cm
● Infrared: 0.001 to 0.1 mm
● Visible light 0.0004 – 0.0007 mm
● Ultraviolet 10-9 – 4 x 10-7 m
● X-rays 10-13 – 10-9 m
● Gamma Rays 10-15 –10-11 m
Magneti
c Field,
H
Electri
c Field,
E
Direc
of
Propa
n
● Electromagnetic radiation comprises both an Electric and a
Magnetic Field.
● The two fields are at right-angles to each other and the direction of
propagation is at right-angles to both fields.
● The Plane of the Electric Field defines the Polarization of the
wave.

GROUND WAVE
● A wave is said to be ground wave or surface wave when it
propagates from transmitter to receiver by gliding over the
surface of the earth.
● It exists when both the transmitting and the receiving antennas
are close to the earth and antennas are vertically polarized.
● It is useful for communication at VLF, LF and MF.

•Follows contour of the earth
•Can Propagate considerable distances
•Frequencies up to 2 MHz
•Example
•AM radio
GROUND WAVE BETWEEN TRANSMITTING AND
RECEIVING ANTENNAS

GROUND WAVE FIELD STRENGTH
● According to Somerfield analysis, the ground wave field
strength for flat earth is given by: E=AE0/d
E- Field strength at a point, V/m
Eo - Field strength of the wave at a unit distance from the
transmitting antenna V/m
A- factor of the ground losses
d- distance of the point from transmitting antenna
Salient Features
● Ground wave propagates by gliding over the surface of the earth.
● It exists for vertically polarized waves.
● Exists for antennas close to earth.
● Suitable for VLF, LF and MF communications.
● Used at 15 KHz and up to 2 MHz .
● Ground wave field strength varies with characteristics of the
earth.
● Requires relatively high transmitter power.
● Not affected by the changes in atmospheric conditions.
● Used to communicate between two points on the globe if there is

WAVE TILT OF THE GROUND
● It is defined as the change of orientation of the vertically
polarized ground wave at the surface of the earth.
Salient Features Of Wave Tilt:
● Wave tilt occurs at the surface of the earth.
● Depends on conductivity and permittivity of the earth.
● It causes power dissipation.
● Exists both horizontal and vertical components of the electric
field.
● These two components are not in phase.
● Wave tilt changes the originally vertically polarized wave in to
elliptically polarized wave.

SKY WAVE PROPAGATION

CHARACTERISTICS OF IONOSPHERE
● D-Layer:
● Lowest layer of ionosphere, Average height = 70 km.
● Thickness = 10 km, Exists only day time.
● Not useful layer for HF communications, It reflects some VLF
and LF waves, Absorbs MF and HF waves to some extent.
● Electron density, N = 400 electrons/cc, Virtual height is 60 to 80
km
● Critical frequency = 180 KHz.
● E-LAYER
● Exists next to D-Layer and only in day time, Average height = 100
km and Thickness = 25 km,
● The ions are recombined in to molecules due to absence of sun at
night, Its electron density , N = 5 * 10 5 electrons/cc, Virtual height
= 110 km, Critical frequency = 4 MHz
● Maximum single – hop range = 2350 Km.

CHARACTERISTICS OF IONOSPHERE
Es - LAYER
● It is sporadic in nature and if at all it appears, it exists in
both day and night.
● It is a thin layer and ionization density is high.
● If it appears, it provides good reception.
● It is not a dependable layer for communication.
F1 - LAYER
● It exists at a height of about 180 km in day time and
thickness is about 20 km.
● Virtual height = 180 km.
● Critical frequency = 5 MHz .
● Maximum single – hop range = 3000 km.
● HF waves are reflected to some extent and also it absorbs
HF to considerable extent.
● It passes on some HF waves towards F2- layer.
● It combines with F2 – layer during nights.

F2 - LAYER
● It is most important layer for HF communication and topmost
layer of the ionosphere.
● Average height is 325 km in day time and falls to 300 km at
nights as it combines with the F1 – layer.
● Thickness = 200 km.
● It is highly ionized and exists at nights also.
● It offers better HF reflection and hence reception.
● N = 2 * 10 6 electrons/cc.
● h v = 300 km in day time and 350 km in night.
● f c = 8 MHz in day time and 6 MHz at nights.
● Maximum single – hop range = 3800 km in day time and 4100
km at night.

SPACE WAVE
OR
TROPOSPHERIC WAVE PROPAGATION

● The EM wave that propagates from the transmitter to the
receiver is called space wave or tropospheric wave
propagation.
● In space wave propagation, the field strength at the receiver
is contributed by:
Direct ray from transmitter.
Ground reflected wave.
Reflected and refracted rays from the
troposphere.
Diffracted rays around the curvature of the
earth, hills and so on.
SPACE WAVE

CURVATURE OF THE EARTH

REFRACTION IN TROPOSPHERE

DUCT PROPAGATION
● It is a phenomenon of propagation making use of the
atmosphere duct region.
● The duct region exists between two levels where the
variation of modified refractive index with height is
minimum.
● In duct propagation, the ray which is parallel to the earth’s
surface travels round the earth in a series of hops with
successive reflections from the earth.

SALIENT FEATURES OF DUCT PROPAGATION
● It happens when dM/dh is negative.
● It happens when dielectric constant changes with the
height suddenly and rapidly.
● It is similar to waveguide propagation of microwaves.
● It is rare phenomenon and not a standard or dependable
propagation.
● It happens during monsoons, due temperature inversion
and also when low and high moisture regions exist.
● It occurs due to super refraction.

EXAMPLES OF DUCTING

LINE OF SIGHT (LOS)
● It is defined as the distance that is covered by a direct space
wave from the transmitting antenna to the receiving
antenna.
● It depends on:
● Height of the receiving antenna.
● Height of the transmitting antenna.
● Effective earth’s radius factor, K.
● The line of sight distance is given by:

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