Electromagnetic Wave Theory II
Lecture 8
Ground Wave Propagation
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Follows contour of the earth
Can Propagate considerable distances
Frequencies up to 2 MHz
Example
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AM radio
Ground Wave Propagation
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Disadvantages
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.Requires relatively high transmission power
.They are limited to very low, low and medium
frequencies which require large antennas
.Losses on the ground vary considerably with
surface material
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Advantages
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Given enough power they can be used to
communicate between any two points in the
world
They are relatively unaffected by changing
atmospheric conditions
Space wave propagation
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This includes radiated energy that travels in the
lower few miles of the earth’s atmosphere. They
include both direct and ground reflected waves.
Direct waves travel in essentially a straight line
between the transmitting and receiving antennas.
The most common name is line of sight
propagation.
The field intensity at the receiving antenna
depends on the distance between the two
antennas and whether the direct and ground
reflected waves are in phase.
Line-of-Sight Propagation
Line-of-Sight Propagation
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Transmitting and receiving antennas must be
within line of sight
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Satellite communication – signal above 30 MHz not
reflected by ionosphere
Ground communication – antennas within effective line
of site due to refraction
Refraction – bending of microwaves by the
atmosphere
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Velocity of electromagnetic wave is a function of the
density of the medium
When wave changes medium, speed changes
Wave bends at the boundary between mediums
Line-of-Sight Equations
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Optical line of sight
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Effective, or radio, line of sight
d  3.57 h
d  3.57 h
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d = distance between antenna and horizon (km)
h = antenna height (m)
K = adjustment factor to account for refraction,
rule of thumb K = 4/3
Line-of-Sight Equations
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Maximum distance between two antennas
for LOS propagation:

3.57 h1  h2
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h1 = height of antenna one
h2 = height of antenna two

LOS Wireless Transmission
Impairments
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Attenuation and attenuation distortion
Free space loss
Noise
Atmospheric absorption
Multipath
Refraction
Thermal noise
Sky Wave Propagation
Sky Wave Propagation
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Signal reflected from ionized layer of atmosphere
back down to earth
Signal can travel a number of hops, back and forth
between ionosphere and earth’s surface
Reflection effect caused by refraction
Examples
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Amateur radio
CB radio
Sky Wave Propagation
 For many years, numerous organisations have been
employing
the
High
Frequency
communicate over long distances.
(HF)
spectrum
to
It was recognised in
the late 30's that these communication systems were
subject to marked variations in performance, and it was
hypothesised that most of these variations were directly
related to changes in the ionosphere.
Sky Wave Propagation
 Considerable effort was made to investigate ionospheric
parameters and determine their effect on radio waves and
the associated reliability of HF circuits. World-wide noise
measurement records were started and steps were taken
to record observed variations in signal amplitudes over
various HF paths.
The results of this research established that ionised regions
ranging from approximately 70 to 1000 km above the earth's
surface provide the medium of transmission for electromagnetic
energy in the HF spectrum (2 to 30 MHz) and that most
variations in HF system performance are directly related to
changes in these ionised regions. The ionisation is produced in
a complex manner by the photoionization of the earth's high
altitude atmosphere by solar radiation.
Sky Wave Propagation
 Within the ionosphere, the recombination of the ions and
electrons proceeds slowly enough (due to low gas densities)
so that some free electrons persist even throughout the
night. In practice, the ionosphere has a lower limit of 50 to
70 km and no distinct upper limit, although 1000 km is
somewhat arbitrarily set as the upper limit for most
application purposes.
Sky Wave Propagation
 The vertical structure of the ionosphere is changing
continuously. It varies from day to night, with the seasons of
the year, and with latitude. Furthermore, it is sensitive to
enhanced periods of short-wavelength solar radiation
accompanying solar activity. In spite of all this, the essential
features of the ionosphere are usually identifiable, except
during periods of unusually intense geomagnetic disturbances.
PREDICTABLE IONOSPHERIC
PARAMETERS
The presence of free electrons in the ionosphere produces the
reflecting regions important to High Frequency (HF) radio-wave
propagation. In the principal regions, between the approximate
heights of 75 km and 500 km, the electrons are produced by the
ionising effect of ultraviolet light and soft x-rays from the sun. for
convenience in studies of radio-wave propagation, the ionosphere is
divided into three regions defined according to height and ion
distribution: the D,E, and F regions.
Sky Wave Propagation
Each region is subdivided into layers called the D,E, Es, F1, and F2
layers, also according to height and ion distribution. These are not
distinctly separated layers, but rather overlapping regions of
ionisation that vary in thickness from a few kilometres to hundreds
of kilometres. The number of layers, their heights, and their
ionisation (electron) density vary both geographically and with time.
At HF, all the regions are important and must be considered in
predicting the operational parameters of radio communication
The D region
The D region lies between the approximate limits of 75 and 90 km
above the earth's surface.
The electron density is relatively small compared with that of the other regions, but, because of
collisions between the molecules of the atmosphere and free electrons excited by the presence of
an electromagnetic wave, pronounced energy loss occurs. This energy loss, dissipated in the
form of thermal energy of the electrons or thermal (electromagnetic) noise, is termed absorption.
Higher in the E and F regions, electron collisions with atmosphere molecules can also affect the
condition for reflection that occurs wherever there is a marked bending of the wave. This is
explained by the fact that as the wave nears its reflecting level, there is a slowing down or
retardation effect, which allows additional time for collisions to occur and thus for absorption to
take place. Absorption of this type is called deviative absorption.
Because of the low electron density, the D region does not reflect useful transmissions in the
frequency range above 1 MHz. However, D-region absorption is important at all frequencies
and, because its ionization is produced by ultraviolet solar radiation, it is primarily a daytime
phenomenon
The degree of absorption is expressed by the absorption factor. After sunset in the D region,
ionization decreases rapidly and non-deviative absorption becomes negligible 2 to 3 hours later.
Non-deviative D-region absorption is the principal cause of the attenuation of HF sky waves,
particularly at the lower frequencies during daylight hours.
THE E REGION
The approximate true height range of the regular E layer is
well established at 90 to 130 km and it is assumed that the
maximum electron density occurs at 110 km and the semithickness is 20 km.
For communication, the most important characteristic feature of the E region is
the temporal and geographic variation of its critical frequency. In almost all other
respects, the features of the E layer are very predictable compared with those of
the F2 layer.
A large volume of vertical-incidence ionosonde data has been collected over
about three solar cycles, and many features of the E region are therefore well
known. The minimum virtual height of the E region and the variation of
maximum electron density within this region as a function of time and
geographic location are readily obtained from the ionograms.
THE F REGION
For HF radio communications, the F region is the most important part
of the ionosphere. It is not regular and because of its variability, short time scale
estimates of the important F-region characteristics are required if predictions of the
operational parameters of HF radio systems are to be meaningful
There are many characteristic features of the F region important to
HF radio communications. This layer is actually divided into two
separate layers, F1 and F2 layers.
The F1 layer is of importance to communication only during daylight
hours or during ionospheric storms; it lies in the height range of about
200 to 250 km and undergoes both seasonal and solar cycle
variations, which are more pronounced during the summer and in
high sunspot periods.
The F2 layer is located between 250 to 350 km above the earth’s
surface. During the night the F1 and F2 layers combine into a single
layer
Effects of the Ionosphere on the Sky wave
If we consider a wave of frequency , f incident on an ionospheric
layer whose maximum density is N then the refractive index of the
layer is given by
81N
n  1 2
f
Critical Frequency
If the frequency of a wave transmitted vertically is
increased, a point will be reached where the wave will not
be refracted sufficiently to curve back to earth and if this
frequency is high enough then the wave will penetrate the
ionosphere and continue on to outer space. The highest
frequency that will be returned to earth when transmitted
vertically under given atmospheric conditions is called the
critical frequency.
fc  9 N
Maximum Usable Frequency
There is a best frequency for communication between any two
points under specific ionospheric conditions. The highest frequency
that is returned to earth at a given distance is called the Maximum
Usable Frequency (MUF).
f muf  9 N sec
Optimum Working Frequency
This is the frequency which provides the most consistent
communication and is therefore the best to use. For transmission
using the F2 layer it is defined as
f owf  0.85  9 N sec
Lowest Usable Frequency
This is set by the attenuation in the ionosphere. A practical
value of this is usually taken as 3 MHz.
Satellite Communication
In these systems a communication satellite is placed into
synchronous orbit about 22 000 mi above the earth’s surface.
The transmitter sends a signal using a highly directional antenna
to the satellite. This signal is reamplified within the satellite and
transmitted back to earth. This allows transoceanic links,
frequencies range from 1 GHz to 40 GHz. The received signals
and the retransmitted signals are usually at different carrier
frequencies.
Satellite-Related Terms
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Earth Stations – antenna systems on or near earth
Uplink – transmission from an earth station to a
satellite
Downlink – transmission from a satellite to an
earth station
Transponder – electronics in the satellite that
convert uplink signals to downlink signals
Ways to Categorize
Communications Satellites
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Coverage area
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Service type
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Global, regional, national
Fixed service satellite (FSS)
Broadcast service satellite (BSS)
General usage
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Commercial, military, amateur, experimental
Classification of Satellite Orbits
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Circular or elliptical orbit
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Orbit around earth in different planes
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Circular with center at earth’s center
Elliptical with one foci at earth’s center
Equatorial orbit above earth’s equator
Polar orbit passes over both poles
Other orbits referred to as inclined orbits
Altitude of satellites
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Geostationary orbit (GEO)
Medium earth orbit (MEO)
Low earth orbit (LEO)
Geometry Terms
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Elevation angle - the angle from the
horizontal to the point on the center of the
main beam of the antenna when the antenna
is pointed directly at the satellite
Minimum elevation angle
Coverage angle - the measure of the portion
of the earth's surface visible to the satellite
Minimum Elevation Angle
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Reasons affecting minimum elevation angle
of earth station’s antenna (>0o)
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Buildings, trees, and other terrestrial objects
block the line of sight
Atmospheric attenuation is greater at low
elevation angles
Electrical noise generated by the earth's heat
near its surface adversely affects reception
GEO Orbit
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Advantages of the the GEO orbit
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No problem with frequency changes
Tracking of the satellite is simplified
High coverage area
Disadvantages of the GEO orbit
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Weak signal after traveling over 35,000 km
Polar regions are poorly served
Signal sending delay is substantial
LEO Satellite Characteristics
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Circular/slightly elliptical orbit under 2000 km
Orbit period ranges from 1.5 to 2 hours
Diameter of coverage is about 8000 km
Round-trip signal propagation delay less than 20
ms
Maximum satellite visible time up to 20 min
LEO Categories
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Little LEOs
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Frequencies below 1 GHz
5MHz of bandwidth
Data rates up to 10 kbps
Aimed at paging, tracking, and low-rate messaging
Big LEOs
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Frequencies above 1 GHz
Support data rates up to a few megabits per sec
Offer same services as little LEOs in addition to voice
and positioning services
MEO Satellite Characteristics
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Circular orbit at an altitude in the range of 5000 to
12,000 km
Orbit period of 6 hours
Diameter of coverage is 10,000 to 15,000 km
Round trip signal propagation delay less than 50
ms
Maximum satellite visible time is a few hours
Frequency Bands Available for
Satellite Communications
Satellite Link Performance
Factors
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Distance between earth station antenna and
satellite antenna
For downlink, terrestrial distance between earth
station antenna and “aim point” of satellite
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Displayed as a satellite footprint (Figure 9.6)
Atmospheric attenuation
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Affected by oxygen, water, angle of elevation, and
higher frequencies
Satellite Footprint
Power Budget for SATCOM
The power relation between a transmitted and received power
of any space wave is given as follows
 Pr 
   Gt dB  Gr dB  32.5  20 log10 d  20 log10 f dB
 Pt  dB
where Pr is the received power
Pt is the transmitted power
Gt is the gain of the transmitting antenna
Gr is the gain of the receiving antenna
d is the distance (km) between the antennas
f is the frequency in MHz
Examples
1. What is the horizon for a transmitting
antenna height 225 feet above ground
level? What is the total horizon if the
receiver is of height 25 feet above ground
level?
2. If the transmitting antenna is 1000ft
above ground level and the receiving
antenna is 20 ft high what is the radio
horizon?
3.
Determine the distance to the radio horizon for an antenna
40 ft above sea level
4.
Calculate the radio horizon for a 500 ft transmitting antenna
and receiving antenna of 20 ft. calculate the required increase in
height for the receiving antenna if a 10% increase in radio horizon
were required.
5. Calculate the power received at a satellite given the following
conditions
Power gain of the transmitting antenna is 30 000
The transmitter drives 2 kW of power into the antenna at a
carrier frequency of 6.21 MHz
The satellite receiving antenna has a power gain of 30
The transmission path is 45 000 km
6.
Determine the maximum distance between identical
antennas equally distant above sea level