Antennas and Propagation
Review/Recap
Lecture 17
Overview
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Antenna Functions
Isotropic Antenna
Radiation Pattern
Parabolic Reflective Antenna
Antenna Gain
Signal Loss in Satellite Communication
Noise Types
Refraction
Fading
Diffraction and Scattering
Fast and Slow Fading
Flat and Selective Fading
Diversity Techniques ……
2
Review Question: Antenna Functionality
Q:- What two functions are performed by an antenna?
3
Antenna Definition
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An antenna is defined as
usually a metallic device (as a
rod or wire) for radiating or
receiving radio waves.
The IEEE Standard Definitions
of Antenna defines the antenna
or aerial as “a means for
radiating or receiving radio
waves.” In other words the
antenna is the transitional
structure between free-space
and a guiding device, as shown
in Figure
4
Why Antennas of Different Shapes
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In addition to receiving or transmitting energy, an antenna in an
advanced wireless system is usually required to optimize
or
accentuate the radiation energy in some directions and suppress it in
others.
Thus the antenna must also serve as a directional device in addition
to a probing device.
It must then take various forms to meet the particular need at hand,
and it may be a piece of conducting wire, an aperture, a patch, an
assembly of elements (array), a reflector, a lens, and so forth.
For wireless communication systems, the antenna is one of the most
critical components. A good design of the antenna can relax system
requirements and improve overall system performance.
The antenna serves to a communication system the same purpose that eyes
5
and eyeglasses serve to a human
Basic Antenna Functions
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As Antenna resides between cable/waveguide and the medium
air, the main function of antenna is to match impedance of the
medium with the cable/waveguide impedance. Hence antenna
is impedance transforming device.
The second and most important function of antenna is to
radiate the energy in the desired direction and suppress in the
unwanted direction. This basically is the radiation pattern of the
antenna. This radiation pattern is different for different types of
antennas.
6
7
The Role of Antennas
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Antennas serve four primary functions
Spatial filter
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Polarization filter
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polarization-dependent sensitivity
Impedance transformer
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directionally-dependent sensitivity
transition between free space and transmission line
Propagation mode adapter
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from free-space fields to guided waves
(e.g., transmission line, waveguide)
8
Spatial filter

Antennas have the property of being more sensitive in one direction than in
another which provides the ability to spatially filter signals from its
environment.
Radiation pattern of directive antenna.
Directive antenna.
9
Polarization filter
Antennas have the property of being more sensitive to
one polarization than another which provides the ability
to filter signals based on its polarization.
Incident
E-field
vector

E  ẑ E 0
Dipole antenna
 
V  hE

h  ẑ h
+
_
V = h E0
Incident
E-field
vector

E  ŷ E 0
Dipole antenna
 
V  hE

h  ẑ h
+
_
z
y
V=0
z
x
y
x
In this example, h is the antenna’s effective height whose units
are expressed in meters.
10
Impedance transformer

Intrinsic impedance of
free-space, E/H
0   0  0
 120 
 376.7 
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Characteristic impedance
of transmission line, V/I
A typical value for Z0 is
50 .
Clearly there is an
impedance mismatch that
must be addressed by the
antenna.
11
Propagation Mode Adapter
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In free space the waves spherically expand following Huygens
principle: each point of an advancing wave front is in fact the
center of a fresh disturbance and the source of a new train of
waves.
Within the sensor, the waves are guided within a transmission
line or waveguide that restricts propagation to one axis.
12
Propagation Mode Adapter
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During both transmission and receive operations the antenna
must provide the transition between these two propagation
modes.
13
Antenna purpose
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Space wave
Transformation of a guided EM
wave in transmission line
(waveguide) into a freely
propagating EM wave in space (or
vice versa) with specified
directional characteristics
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Guided wave
Transformation from time-function in
one-dimensional space into timefunction in three dimensional space
The specific form of the radiated wave
is defined by the antenna structure
and the environment
14
Antenna functions
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Transmission line
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Radiator
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Power transport medium - must avoid power
reflections, otherwise use matching devices
Must radiate efficiently – must be of a size
comparable with the half-wavelength
Resonator
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Unavoidable - for broadband applications
resonances must be attenuated
15
Review Question: Antenna Functionality
Q:- What two functions are performed by an antenna?
Ans:- Two functions of an antenna are:
(1) For transmission of a signal, radio frequency electrical energy
from the transmitter is converted into electromagnetic energy
by the antenna and radiated into the surrounding environment
(atmosphere, space, water);
(2) For reception of a signal, electromagnetic energy impinging on
the antenna is converted into radio-frequency electrical energy
and fed into the receiver.
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17
Isotropic Antenna
Q:- What is an isotropic antenna?
18
Isotropic Antenna
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Isotropic antenna or isotropic radiator is
a hypothetical (not physically realizable)
concept, used as a useful reference to
describe real antennas.
Isotropic antenna radiates equally in all
directions.
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Its radiation pattern is represented by a
sphere whose center coincides with the
location of the isotropic radiator.
19
Reference Antenna for Gain
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Gain is Measured Specific to a Reference Antenna
isotropic antenna often used - gain over isotropic
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Isotropic antenna – radiates power ideally in all directions
Gain measured in dBi
Test antenna’s field strength relative to reference isotropic antenna at
same power, distance, and angle
-Isotropic antenna cannot be practically realized
e.g.
A lamp is similar to an isotropic antenna
20
Isotropic
21
An Isotropic Source: Gain
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Isotropic sphere
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Every real antenna radiates more
energy in some directions than in
others (i.e. has directional properties)
Idealized example of directional
antenna: the radiated energy is
concentrated in the yellow region
(cone).
Directive antenna gain: the power flux
density is increased by (roughly) the
inverse ratio of the yellow area and the
total surface of the isotropic sphere

Gain in the field intensity may also be
considered - it is equal to the square
root of the power gain.
22
Antenna Gain Measurement
Reference
antenna
Po = Power
delivered to
the reference
antenna
Step 1: reference
Measuring
equipment
S0 = Power
received
(the same
in both
steps)
Actual
antenna
P = Power
delivered to
the actual
antenna
Measuring
equipment
S = Power
received
(the same in
both steps)
Step 2: substitution
Antenna Gain = (P/Po) S=S0
23
Isotropic Antenna
Q:- What is an isotropic antenna?
Isotropic sphere
Ans:- An isotropic antenna is a point in space that radiates power
in all directions equally.
24
25
Review: Radiation Pattern
Q:- What information is available from a radiation pattern?
26
Radiation Pattern
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In the field of antenna design the term radiation pattern (or antenna pattern or farfield pattern) refers to the directional (angular) dependence of the strength of the
radio waves from the antenna or other source.
Particularly in the fields of fiber optics, lasers, and integrated optics, the term
radiation pattern may also be used as a synonym for the near-field pattern or
Fresnel pattern. This refers to the positional dependence of the electromagnetic field
in the near-field, or Fresnel region of the source. The near-field pattern is most
commonly defined over a plane placed in front of the source, or over a cylindrical or
spherical surface enclosing it.
The far-field pattern of an antenna may be determined experimentally at an antenna
range, or alternatively, the near-field pattern may be found using a near-field
scanner, and the radiation pattern deduced from it by computation. The far-field
radiation pattern can also be calculated from the antenna shape by computer
programs such as NEC. Other software, like HFSS can also compute the near field.
27
Antenna Radiation Pattern
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Radiation pattern
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Graphical representation of radiation properties of an antenna
Depicted as two-dimensional cross section
The radiation pattern of an antenna is a plot of the far-field
radiation from the antenna. More specifically, it is a plot of the
power radiated from an antenna per unit solid angle, or its
radiation intensity U [watts per unit solid angle]. This is arrived
at by simply multiplying the power density at a given distance
by the square of the distance r, where the power density S
[watts per square metre] is given by the magnitude of the
time-averaged Poynting vector:
U=r²S
28
Radiation pattern
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The radiation pattern of antenna is a representation (pictorial
or mathematical) of the distribution of the power out-flowing
(radiated) from the antenna (in the case of transmitting
antenna), or inflowing (received) to the antenna (in the case of
receiving antenna) as a function of direction angles from the
antenna
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Antenna radiation pattern (antenna pattern):
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is defined for large distances from the antenna, where the spatial (angular)
distribution of the radiated power does not depend on the distance from the
radiation source
is independent on the power flow direction: it is the same when the antenna is
used to transmit and when it is used to receive radio waves
is usually different for different frequencies and different polarizations of radio
wave radiated/ received
29
Power Pattern Vs. Field pattern
Auxiliary
antenna
Antenna
under test
Large distance
Power or
field-strength meter
Generator
Turntable
•
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The power pattern and the field
patterns are inter-related:
P(θ, ϕ) = (1/)*|E(θ, ϕ)|2 = *|H(θ,
ϕ)|2
P = power
E = electrical field component vector
H = magnetic field component vector
 = 377 ohm (free-space, plane wave
impedance)

The power pattern is the
measured (calculated) and
plotted received power:
|P(θ, ϕ)| at a constant
(large) distance from the
antenna
The amplitude field
pattern is the measured
(calculated) and plotted
electric (magnetic) field
intensity, |E(θ, ϕ)| or
|H(θ, ϕ)| at a constant
(large) distance from the
antenna
30
Normalized pattern
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Usually, the pattern describes the normalized field (power)
values with respect to the maximum value.
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Note: The power pattern and the amplitude field pattern are the same
when computed and when plotted in dB.
31
3-D pattern
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Antenna radiation
pattern is
3-dimensional
The 3-D plot of antenna
pattern assumes both
angles θ and ϕ varying,
which is difficult to
produce and to interpret
3-D pattern
32
2-D pattern
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Usually the antenna
pattern is presented as a
2-D plot, with only one of
the direction angles, θ or ϕ
varies
It is an intersection of the
3-D one with a given
plane

usually it is a θ = const
plane or a ϕ= const plane
that contains the pattern’s
maximum
Two 2-D patterns
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Example: a short dipole on z-axis
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Principal Patterns
Principal patterns are the 2-D patterns of linearly
polarized antennas, measured in 2 planes
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1.
2.
the E-plane: a plane parallel to the E vector and
containing the direction of maximum radiation, and
the H-plane: a plane parallel to the H vector, orthogonal
to the E-plane, and containing the direction of maximum
radiation
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Example
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Antenna Mask (Example 1)
Typical relative
directivitymask of
receiving
antenna (Yagi
ant., TV dcm
waves)
-5
-10
-15
180
120
60
0
-60
-120
-20
-180
Relative gain, dB
0
Azimith angle, degrees
37
Antenna Mask (Example 2)
0
0dB
RR/1998 APS30 Fig.9
Relative gain (dB)
-10
COPOLAR
-3dB
-20
Phi
-30
-40
CROSSPOLAR
-50
0.1
1
10
100
Phi/Phi0
Reference pattern for co-polar and cross-polar components for satellite transmitting
antennas in Regions 1 and 3 (Broadcasting ~12 GHz)
38
Review: Radiation Pattern
Q:- What information is available from a radiation pattern?
Radiation Patterns in Polar and Cartesian Coordinates Showing Various Types of Lobes
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Ans:- A radiation pattern is a graphical representation of the
radiation properties of an antenna as a function of space
coordinates.
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40
Parabolic Reflective Antenna
Q:- What is the advantage of a parabolic reflective antenna?
41
Two Main Purposes of Antenna
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Impedance matching: matches impedance of transmission line
to the intrinsic impedance of free space to prevent wanted
reflection back to source.
Antenna must be designed to direct the radiation in the desired
direction.
So a parabolic antenna
 is a high gain reflector antenna. It is used for television, radio
and data communications. It may also be used for radar on the
UHF and SHF sections of the electromagnetic spectrum.
42
Reflector Antenna
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Reflector antenna such as parabolic antenna are composed of
primary radiator and a reflective mirror.
43
Parabolic Reflector Antenna
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Any electromagnetic wave incident upon the paraboloid surface
will be directed to the focal point.
Primary antenna is used at the focal point of the parabolic
reflector antenna instead of isotropic antenna. The isotropic
antenna would radiate and receive radiation from all directions
resulting in spillover.
Primary antenna should be designed to “illuminate” just the
reflector uniformly.
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Loss
45
Characteristics
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A  r ^2
Aperture:
r= radius of the diameter
Larger dish has more gain than smaller
Clear line of sight is important
46
Calculations
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Physical area: Ap 
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Effective area:
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D= Diameter
D ^ 2
4
Ae  i * Ap
 = illumination efficiency
c
Wavelength:  
f
4Ae
Gi

Gain:
 ^2
3db beamwidth:  3dB  70

D
Degrees
47
Half Power Beamwidth
The half power graph showing the angle between
the half power point on either side of maximum
48
Radiation Pattern for Parabolic Antenna
49
Advantage of a Parabolic Antenna
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The advantage of a parabolic antenna is that it can be used as primary mirror for all the
frequencies in the project, provided the surface is within the tolerance limit; only the feed
antenna and the receiver need to be changed when the observing frequency is changed.
An advantage of such a design is the small irradiation loss, which allows for an optimum
antenna gain.
It is an advantage of such an arrangement that the exciter system and/or the exciter 3
are/is protectively located within the parabola or the parabolic reflector.
Parabolic antenna is the most efficient type of a directional antenna - large front/back ratio,
sharp radiation angle and small side lobes. It fits well for noisy locations where other antennas
factually do not work.
The antenna's Gain is adequate to the area of the reflector. The reflector can be centralfocused(the focus is in the center of the dish) or offset (the focus is off the axis of the dish).
In general, they serve for connection of end users (so-called last mile) to a wireless network.
However, in areas with lower intensity of Wi-Fi networks, they can be successfully used also for
back-bone links. In fact, this frequency is used for connections up to maximum 10 km
50
Parabolic Reflective Antenna
Q:- What is the advantage of a parabolic reflective antenna?

A parabolic antenna creates, in theory, a parallel beam without
dispersion. In practice, there will be some beam spread.
Nevertheless, it produces a highly focused, directional beam. 51
52
Antenna Gain
Q:- What factors determine antenna gain?
53
Antenna Gain
Change in coverage by focusing the area of RF propagation
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Antenna Gain (Directivity)
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Power output, in a particular direction, compared to that produced in any
direction by a perfect omnidirectional antenna [usual reference is an isotropic
antenna (dBi) but sometimes a simple ½  antenna is a more practical
reference; good sales trick to use an isotropic reference when a dipole is inferred
resulting in a 1.64 power gain]
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Antenna gain doesn’t increase power; only concentrates effective radiation
pattern
Effective area (related to antenna aperture) Expressed in terms of effective area
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Related to physical size and shape of antenna related to the operational
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wavelength of the antenna
Passive Gain
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Focusing isotropic energy in a specific pattern
Created by the design of the antenna
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Uses the magnify glass concept
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Passive Gain…
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Antennas use passive gain
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Total amount of energy emitted by antenna doesn’t increase – only the
distribution of energy around the antenna
Antenna is designed to focus more energy in a specific direction
Passive gain is always a function of the antenna (i.e.
independent of the components leading up to the antenna
56
Passive Gain…
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Advantage…
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Does not require external power
Disadvantage…
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As the gain increases, its coverage becomes more focused
Highest-gain antennas can’t be used for mobile users because of their
tight beam
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Active Gain
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Providing an external power source
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Amplifier
High gain transmitters
Active gain involves an amplifier
58
Antenna Gain
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Relationship between antenna gain and effective area
G=





4pAe
l
2
4pf 2 Ae
=
2
c
G = antenna gain
Ae = effective area
f = carrier frequency
c = speed of light (≈ 3 x 108 m/s)
 = carrier wavelength
59
Antenna gain

Antenna gain is increased by focusing the antenna

The antenna does not create energy, so a higher gain in one
direction must mean a lower gain in another.

Note: antenna gain is based on the maximum gain, not the
average over a region. This maximum may only be achieved
only if the antenna is carefully aimed.
This antenna is narrower and results in 3dB higher gain than the
dipole, hence, 3dBD or 5.14dBi
This antenna is narrower and results in 9dB higher gain than the
dipole, hence, 9dBD or 11.14dBi
60
Antenna gain
Instead of the energy going in all horizontal
directions, a reflector can be placed so it only
goes in one direction => another 3dB of gain,
3dBD or 5.14dBi
Further focusing on a sector results in more gain.
A uniform 3 sector antenna system would give
4.77 dB more.
A 10 degree “range” 15dB more.
The actual gain is a bit higher since the peak is
higher than the average over the “range.”
Mobile phone base stations claim a gain of 18dBi
with three sector antenna system.
• 4.77dB from 3 sectors – 13.33 dBi
• An 11dBi antenna has a very narrow range.
61
Antenna Gain
The power gain G, or simply the gain, of an antenna is the ratio of
its radiation intensity to that of an isotropic antenna radiating the
same total power as accepted by the real antenna. When antenna
manufacturers specify simply the gain of an antenna they are
usually referring to the maximum value of G.
62
Antenna gain and effective areas
Type of antenna
Effective area
Power gain
Isotropic
‫ג‬2/4
1
Infinitesimal dipole or loop
1.52/4
1.5
Half-wave dipole
1.642/4
1.64
Horn, mouth area A
0.81A
10A/ 2
Parabolic, face area A
0.56A
7A/ 2
turnstile
1.152/4
1.15
63
Antenna Gain
Q:- What factors determine antenna gain?
Ans:- Effective area and wavelength
64
65
Satellite Communication
Q:- What is the primary cause of signal loss in satellite
communications?
66
Basics: How do Satellites Work
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


Two Stations on Earth want to communicate through radio
broadcast but are too far away to use conventional means.
The two stations can use a satellite as a relay station for their
communication
One Earth Station sends a transmission to the satellite. This is
called a Uplink.
The satellite Transponder converts the signal and sends it down
to the second earth station. This is called a Downlink.
67
Basics: Advantages of Satellites

The advantages of satellite communication over terrestrial
communication are:




The coverage area of a satellite greatly exceeds that of a terrestrial
system.
Transmission cost of a satellite is independent of the distance from the
center of the coverage area.
Satellite to Satellite communication is very precise.
Higher Bandwidths are available for use.
68
Basics: Disadvantages of Satellites

The disadvantages of satellite communication:



Launching satellites into orbit is costly.
Satellite bandwidth is gradually becoming used up.
There is a larger propagation delay in satellite communication than in
terrestrial communication.
69
Basics: Factors in Satellite Communication

Elevation Angle: The angle of the horizontal of the earth
surface to the center line of the satellite transmission beam.


This effects the satellites coverage area. Ideally, you want a elevation
angle of 0 degrees, so the transmission beam reaches the horizon visible
to the satellite in all directions.
However, because of environmental factors like objects blocking the
transmission, atmospheric attenuation, and the earth electrical
background noise, there is a minimum elevation angle of earth stations.
70
Basics: Factors in satellite communication ….







Coverage Angle: A measure of the portion of the earth surface
visible to a satellite taking the minimum elevation angle into
account.
R/(R+h) = sin(π/2 - β - θ)/sin(θ + π/2)
= cos(β + θ)/cos(θ)
R = 6370 km (earth’s radius)
h = satellite orbit height
β = coverage angle
θ = minimum elevation angle
71
Basics: Factors in satellite communication….

Other impairments to satellite communication:



The distance between an earth station and a satellite (free space loss).
Satellite Footprint: The satellite transmission’s strength is strongest in
the center of the transmission, and decreases farther from the center as
free space loss increases.
Atmospheric Attenuation caused by air and water can impair the
transmission. It is particularly bad during rain and fog.
72
Atmospheric Losses

Different types of atmospheric losses can disturb radio wave
transmission in satellite systems:



Atmospheric absorption
Atmospheric attenuation
Traveling ionospheric disturbances
73
Atmospheric Absorption


Energy absorption by atmospheric
gases, which varies with the frequency
of the radio waves.
Two absorption peaks are observed
(for 90º elevation angle):



22.3 GHz from resonance absorption in
water vapour (H2O)
60 GHz from resonance absorption in
oxygen (O2)
For other elevation angles:

[AA] = [AA]90 cosec 
Source: Satellite Communications, Dennis Roddy, McGraw-Hill
74
Atmospheric Attenuation


Rain is the main cause of atmospheric attenuation (hail, ice and
snow have little effect on attenuation because of their low
water content).
Total attenuation from rain can be determined by:



A = L [dB]
where  [dB/km] is called the specific attenuation, and can be calculated
from specific attenuation coefficients in tabular form that can be found in
a number of publications
where L [km] is the effective path length of the signal through the rain;
note that this differs from the geometric path length due to fluctuations
in the rain density.
75
Traveling Ionospheric Disturbances


Traveling ionospheric disturbances are clouds of electrons in
the ionosphere that provoke radio signal fluctuations which can
only be determined on a statistical basis.
The disturbances of major concern are:




Scintillation;
Polarisation rotation.
Scintillations are variations in the amplitude, phase,
polarisation, or angle of arrival of radio waves, caused by
irregularities in the ionosphere which change over time.
The main effect of scintillations is fading of the signal.
76
Satellite Communication
Q:- What is the primary cause of signal loss in satellite
communications?
Ans:- Free space loss.
77
78
Impairments
Q:- Name and briefly define four types of noise.
79
Transmission Impairments

Signal received may differ from signal transmitted causing:



Analog - degradation of signal quality
Digital - bit errors
Most significant impairments are



Attenuation and attenuation distortion
Delay distortion
Noise
80
Noise


Signal strength falls off with distance over any transmission medium
Varies with frequency
Unwanted signals inserted between transmitter
and receiver
is the major limiting factor in communications
system performance
81
Categories of Noise
Intermodulation noise
• produced by nonlinearities in the
transmitter, receiver, and/or
intervening transmission medium
• effect is to produce signals at a
frequency that is the sum or
difference of the two original
frequencies
82
Categories of Noise

Impulse Noise:


Crosstalk:


a signal from one line is picked
up by another
can occur by electrical coupling
between nearby twisted pairs or
when microwave antennas pick
up unwanted signals



caused by external
electromagnetic interferences
noncontinuous, consisting of
irregular pulses or spikes
short duration and high
amplitude
minor annoyance for analog
signals but a major source of
error in digital data
83
Noise






Thermal noise due to thermal agitation of electrons.
Present in all electronic devices and transmission media.
As a function of temperature.
Uniformly distributed across the frequency spectrum, hence
often referred as white noise.
Cannot be eliminated – places an upper bound on the
communication system performance.
Can cause erroneous to the transmitted digital data bits.
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Noise: Noise on Digital Data
Error in bits
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Thermal Noise

The noise power density (amount of thermal noise to be found
in a bandwidth of 1Hz in any device or conductor) is:
N 0  kT W/Hz 
N0 = noise power density in watts per 1 Hz of
bandwidth
k = Boltzmann's constant = 1.3803  10-23 J/K
T = temperature, in kelvins (absolute
temperature)
0oC = 273 Kelvin
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Thermal Noise
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Noise is assumed to be independent of frequency
Thermal noise present in a bandwidth of B Hertz (in watts):

N  kTB
or, in decibel-watts (dBW),
N  10 log k  10 log T  10 log B
 228.6 dBW  10 log T  10 log B
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Noise Terminology

Intermodulation noise – occurs if signals with different
frequencies share the same medium
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Interference caused by a signal produced at a frequency that is the sum
or difference of original frequencies
Crosstalk – unwanted coupling between signal paths
Impulse noise – irregular pulses or noise spikes


Short duration and of relatively high amplitude
Caused by external electromagnetic disturbances, or faults and flaws in
the communications system
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Impairments
Q:- Name and briefly define four types of noise.
Ans:- Thermal noise is due to thermal agitation of electrons.
Intermodulation noise produces signals at a frequency that is the
sum or difference of the two original frequencies or multiples of
those frequencies. Crosstalk is the unwanted coupling between
signal paths. Impulse noise is noncontinuous, consisting of
irregular pulses or noise spikes of short duration and of relatively
high amplitude.

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Refraction?
Q:- What is refraction?
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Law of refraction
A refracted ray lies in the plane of
incidence and has an angle θ2 of
refraction that is related to the
angle of incidence θ1 by:
the symbols n1 and n2 are dimensionless
constant, called the index of refraction
c
ni 
vi
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Refraction
Refraction occurs when an RF signal changes speed and is
bent while moving between media of different densities.
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Refraction
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Refraction?
Q:- What is refraction?
Ans:- Refraction is the bending of a radio beam caused by
changes in the speed of propagation at a point of change in the
medium
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Fading
Q:- What is fading?
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Fading in a Mobile Environment



The term fading refers to the time variation of received signal
power caused by changes in the transmission medium or paths.
Atmospheric condition, such as rainfall
The relative location of various obstacles changes over time
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Types of Fading

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Fast fading
Slow fading
Flat fading
Selective fading
Rayleigh fading
Rician fading
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Fading in the Mobile Environment








Fading: time variation of received signal power due to changes
in the transmission medium or path(s)
Kinds of fading:
Fast fading
Slow fading
Flat fading  independent from frequency
Selective fading  frequency-dependent
Rayleigh fading  no dominant path
Rician fading  Line Of Sight (LOS) is dominating + presence
of indirect multipath signals
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Fading
Q:- What is fading?

Ans:- The term fading refers to the time variation of received
signal power caused by changes in the transmission medium or
path(s).
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102
Q:- What is the difference between diffraction and scattering?
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Diffraction
Diffraction is a change in the direction and/or intensity of a
wave as it passes by the edge of an obstacle.
Diffraction occurs because the RF signal slows down as it
encounters the obstacle and causes the wave front to change
directions
Diffraction is often caused by buildings, small hills, and other
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larger objects in the path of the propagating RF signal.
Diffraction

Diffraction - occurs at the edge of an impenetrable body that is
large compared to wavelength of radio wave
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Scattering
Scattering happens when an RF signal strikes an uneven surface
causing the signal to be scattered. The resulting signals are less
significant than the original signal.
Scattering = Multiple Reflections
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Scattering


Scattering – occurs when incoming signal hits an object whose
size in the order of the wavelength of the signal or less.
Irregular objects such as walls with rough surfaces,furniture
and vehicles and foliage and the like scatter rays in all the
direction in the form of spherical waves.
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Multipath Propagation
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Diffraction and Scattering
Q:- What is the difference between diffraction and scattering?
Ans:- Diffraction occurs at the edge of an impenetrable body that
is large compared to the wavelength of the radio wave. The edge
in effect become a source and waves radiate in different
directions from the edge, allowing a beam to bend around an
obstacle. If the size of an obstacle is on the order of the
wavelength of the signal or less, scattering occurs. An incoming
signal is scattered into several weaker outgoing signals in
unpredictable directions
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
Summary










Antenna Functions
Isotropic Antenna
Radiation Pattern
Parabolic Reflective Antenna
Antenna Gain
Signal Loss in Satellite Communication
Noise Types
Refraction
Fading
Diffraction and Scattering
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111
Complimentary Session for Antennas and Propagation
(Lecture 17)
112
Antenna Gain (Q)
Where
Sol
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114
Q
Q:- For each of the antenna types listed in Table above , what is
the effective area and gain at a wavelength of 30 mm? Repeat for
a wavelength of 3 mm. Assume that the actual area for the horn
and parabolic antennas is m2 .
115
Antenna Gain
Where
116
Ans
Q:- For each of the antenna types listed in Table above , what is
the effective area and gain at a wavelength of 30 mm? Repeat for
a wavelength of 3 mm. Assume that the actual area for the horn
and parabolic antennas is m2 .
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118
Q
Question:Solution
119
120
Q
Question
121
122
Thermal Noise
Where
Question:-
123
124
The Expression Eb /N0
in decibel notation,
Question:-
125
126
Q:- It is often more convenient to express distance in km rather
than m and frequency in MHz rather than Hz. Rewrite Equation
using these dimensions.
Solution:- The equations from Text Book are
Solution:-
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128
Q
Q:- Suppose a transmitter produces 50 W of power.
a.
Express the transmit power in units of dBm and dBW.
b.
If the transmitter's power is applied to a unity gain antenna
with a 900-MHz carrier frequency, what is the received power
in dBm at a free space distance of 100 m?
c.
Repeat (b) for a distance of 10 km.
d.
Repeat (c) but assume a receiver antenna gain of 2.
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Q/A
Q:- Suppose a transmitter produces 50 W of power.
a. Express the transmit power in units of dBm and dBW.
b. If the transmitter's power is applied to a unity gain antenna with a 900MHz carrier frequency, what is the received power in dBm at a free space
distance of 100 m?
c. Repeat (b) for a distance of 10 km.
d. Repeat (c) but assume a receiver antenna gain of 2.
a)
.
b)
Therefore, received power in dBm = 47 – 71.52 = –24.52 dBm
130
Q/A
Q:- Suppose a transmitter produces 50 W of power.
a. Express the transmit power in units of dBm and dBW.
b. If the transmitter's power is applied to a unity gain antenna with a 900MHz carrier frequency, what is the received power in dBm at a free space
distance of 100 m?
c. Repeat (b) for a distance of 10 km.
d. Repeat (c) but assume a receiver antenna gain of 2.
c)
d)
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132
Free Space Loss
133
Free Space Loss
134
135
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