Electromagnetic Wave Theory II

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Electromagnetic Wave Theory II

• Lecture 8

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Ground Wave Propagation

• Follows contour of the earth
• Can Propagate considerable distances
• Frequencies up to 2 MHz
• Example
• AM radio

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Ground Wave Propagation

• Disadvantages
• .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
• Given enough power they can be used to
  communicate between any two points in the world
• They are relatively unaffected by changing
  atmospheric conditions

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Space wave propagation

• This includes radiated energy that travels in the
  lower few miles of the earths 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.

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Line-of-Sight Propagation
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Line-of-Sight Propagation

• Transmitting and receiving antennas must be
  within line of sight
• 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
• 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

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Line-of-Sight Equations

• Optical line of sight
• Effective, or radio, line of sight
• d distance between antenna and horizon (km)
• h antenna height (m)
• K adjustment factor to account for refraction,
  rule of thumb K 4/3

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Line-of-Sight Equations

• Maximum distance between two antennas for LOS
  propagation
• h1 height of antenna one
• h2 height of antenna two

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LOS Wireless Transmission Impairments

• Attenuation and attenuation distortion
• Free space loss
• Noise
• Atmospheric absorption
• Multipath
• Refraction
• Thermal noise

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Sky Wave Propagation
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Sky Wave Propagation

• Signal reflected from ionized layer of atmosphere
  back down to earth
• Signal can travel a number of hops, back and
  forth between ionosphere and earths surface
• Reflection effect caused by refraction
• Examples
• Amateur radio
• CB radio

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Sky Wave Propagation

• For many years, numerous organisations have been
  employing the High Frequency (HF) spectrum to
  communicate over long distances. 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.

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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.

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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.
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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.

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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.

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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.
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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 circuits.
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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.
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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 semi-thickness 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.
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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 earths surface. During
the night the F1 and F2 layers combine into a
single layer
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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
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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.
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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).
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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
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Lowest Usable Frequency
This is set by the attenuation in the
ionosphere. A practical value of this is usually
taken as 3 MHz.
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Satellite Communication
In these systems a communication satellite is
placed into synchronous orbit about 22 000 mi
above the earths 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.
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Satellite-Related Terms

• 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

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Ways to CategorizeCommunications Satellites

• Coverage area
• Global, regional, national
• Service type
• Fixed service satellite (FSS)
• Broadcast service satellite (BSS)
• General usage
• Commercial, military, amateur, experimental

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Classification of Satellite Orbits

• Circular or elliptical orbit
• Circular with center at earths center
• Elliptical with one foci at earths center
• Orbit around earth in different planes
• Equatorial orbit above earths equator
• Polar orbit passes over both poles
• Other orbits referred to as inclined orbits
• Altitude of satellites
• Geostationary orbit (GEO)
• Medium earth orbit (MEO)
• Low earth orbit (LEO)

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Geometry Terms

• 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

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Minimum Elevation Angle

• Reasons affecting minimum elevation angle of
  earth stations antenna (gt0o)
• 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

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GEO Orbit

• Advantages of the the GEO orbit
• No problem with frequency changes
• Tracking of the satellite is simplified
• High coverage area
• Disadvantages of the GEO orbit
• Weak signal after traveling over 35,000 km
• Polar regions are poorly served
• Signal sending delay is substantial

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LEO Satellite Characteristics

• 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

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LEO Categories

• Little LEOs
• Frequencies below 1 GHz
• 5MHz of bandwidth
• Data rates up to 10 kbps
• Aimed at paging, tracking, and low-rate messaging
• Big LEOs
• 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

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MEO Satellite Characteristics

• 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

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Frequency Bands Available for Satellite
Communications
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Satellite Link Performance Factors

• Distance between earth station antenna and
  satellite antenna
• For downlink, terrestrial distance between earth
  station antenna and aim point of satellite
• Displayed as a satellite footprint (Figure 9.6)
• Atmospheric attenuation
• Affected by oxygen, water, angle of elevation,
  and higher frequencies

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Satellite Footprint
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Satellite Network Configurations
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Power Budget for SATCOM
The power relation between a transmitted and
received power of any space wave is given as
follows
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
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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?
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3. Determine the distance to the radio horizon
for an antenna 40 ft above sea level 4. Calculat
e 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.
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• 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