radio wave propagations
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Radio Wave Propagation Lecture notes. By M.D.Kabadi 2012
DAR ES SALAAM INSTITUTE OF TECHNOLOGY
DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATIONS
MODULE: WAVE PROPAGATIONS AND ANTENNAE.
Lecture: Radio wave propagations
BY, M.D. KABADI
2012
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LECTURE 1: RADIO WAVE PROPAGATION
Electromagnetic waves and radio propagation
Wireless communication is facilitated by electromagnetic waves. An electromagnetic wave
consists of a time varying electric field traveling through space with a time varying magnetic
field. The two fields are perpendicular to each other and the direction of propagation. Radio
signals exist as a form of electromagnetic wave. These radio signals are the same form of
radiation as light, ultra-violet, infra-red, etc., differing only in the wavelength or frequency of
the radiation.
Electromagnetic waves have two elements. They are made from electric and magneticcomponents that are inseparable. The planes of the fields are at right angles to each other and
to the direction in which the wave is travelling.
An electromagnetic wave
It is useful to see where the different elements of the wave emanate from to gain a more
complete understanding of electromagnetic waves. The electric component of the wave
results from the voltage changes that occur as the antenna element is excited by the
alternating waveform. The lines of force in the electric field run along the same axis as the
antenna, but spreading out as they move away from it. This electric field is measured in terms
of the change of potential over a given distance, e.g. volts per metre, and this is known as the
field strength. This measure is often used in measuring the intensity of an electromagnetic
wave at a particular point. The other component, namely the magnetic field is at right angles
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to the electric field and hence it is at right angles to the plane of the antenna. It is generated
as a result of the current flow in the antenna.
Like other forms of electromagnetic wave, radio signals can be reflected, refracted and
undergo diffraction. In fact some of the first experiments with radio waves proved these
facts, and they were used to establish a link between radio waves and light rays.
Wavelength, frequency and velocity
There are a number of basic properties of electromagnetic waves, or any repetitive waves for
that matter that are particularly important.
One of the first that is quoted is their speed. Radio waves travel at the same speed as light.
For most practical purposes the speed is taken to be 300 000 000 metres per second although
a more exact value is 299 792 500 metres per second. Although exceedingly fast, they still
take a finite time to travel over a given distance. With modern radio techniques, the time for
a signal to propagate over a certain distance needs to be taken into account. Radar for
example uses the fact that signals take a certain time to travel to determine the distance of a
target. Other applications such as mobile phones also need to take account of the time taken
for signals to travel to ensure that the critical timings in the system are not disrupted and that
signals do not overlap.
Another major element of a radio wave is its wavelength. This is the distance between a
given point on one cycle and the same point on the next cycle as shown. The easiest points to
choose are the peaks as these are the easiest to locate. The wavelength was used in the early
days of radio or wireless to determine the position of a signal on the dial of a set. Although it
is not used for this purpose today, it is nevertheless an important feature of any radio signal
or for that matter any electromagnetic wave. The position of a signal on the dial of a radio set
or its position within the radio spectrum is now determined by its frequency as this provides a
more accurate and convenient method for determining the properties of the signal.
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Wavelength of an electromagnetic wave
Finally the frequency of a radio signal or electromagnetic wave is of great importance. This
is the number of times a particular point on the wave moves up and down in a given time
(normally a second). The unit of frequency is the Hertz and it is equal to one cycle per
second. The frequencies used in radio are usually very high. Accordingly the prefixes kilo,
Mega, and Giga are often seen. 1 kHz is 1000 Hz, 1 MHz is a million Hertz, and 1 GHz is a
thousand million Hertz i.e. 1000 MHz. Originally the unit of frequency was not given a name
and cycles per second (c/s) were used. Some older books may show these units together with
their prefixes: kc/s; Mc/s etc. for higher frequencies.
Electromagnetic and the radio spectrum
Electromagnetic waves have an enormous range, and as a result it is very convenient to see
where each of the different forms of radiations fits within the spectrum as a whole. It can be
seen that radio signals have the lowest frequency, and hence the longest wavelengths. Above
the radio spectrum, other forms of radiation can be found. These include infra red radiation,
light, ultraviolet and a number of other forms of radiation.
The spectrum of electromagnetic waves
Even within the radio spectrum there is an enormous range of frequencies. It extends over
many decades. In order to be able to categorise the different areas and to split the spectrum
down into more manageable sizes, the spectrum is split into different segments.
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The spectrum of electromagnetic waves
Table: Radio waves at different frequencies propagate in different ways and its
applications
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Electromagnetic waves interact with objects
As electromagnetic waves, and in this case, radio signals travel, they interact with objects
and the media in which they travel. As they do this the radio signals can be reflected,
refracted or diffracted. These interactions causes the radio signals to change direction, and to
reach areas which would not be possible if the radio signals travelled in a direct line.
Reflection
Reflection of light is an everyday occurrence. Mirrors are commonplace and can be seen in
houses and many other places. Shop windows also provide another illustration for this
phenomenon, as do many other areas. Radio waves are similarly reflected by many surfaces.
When reflection occurs, it can be seen that the angle of incidence is equal to the angle of
reflection for a conducting surface as would be expected for light. When a signal is reflected
there is normally some loss of the signal, either through absorption, or as a result of some of
the signal passing into the medium.
A variety of surfaces can reflect radio signals. For long distance communications, the sea
provides one of the best reflecting surfaces. Other wet areas provide good reflection of radio
signals. Desert areas are poor reflectors and other types of land fall in between these two
extremes. In general, though, wet areas provide better reflectors.
For relatively short range communications, many buildings, especially those with metallic
surfaces provide excellent reflectors of radio energy. There are also many other metallic
structures such as warehouses that give excellent reflecting surfaces. As a result of this
signals travelling to and from cellular phones often travel via a variety of paths. Similar
effects are noticed for Wi-Fi and other short range wireless communications. An office
environment contains many surfaces that reflect radio signals very effectively.
Refraction
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When radio waves pass from one material to another, they change direction at the interface
between the two materials. This is called refraction. The angles of incidence and refraction
are related to the refractive indices of the two media by Snells law:
n1sin1 = n2sin2
Figure 3. Refraction of Radio Waves
Variables n1 and 1 are the refractive index and direction of travel in the incident medium and
n2 and 2 are the refractive index and direction of travel in the refracting medium. Refraction
is an important aspect of radio wave propagation. At frequencies between 30 and 30 MHz,
the ionosphere refracts RF and redirects the waves back towards the earth's surface. Above
100 MHz; The refractive index of air is dependent on the temperature and relative humidity
of the air. A temperature inversion can cause RF waves to be bent just enough to follow the
curvature of the earth and travel for hundreds of miles with little loss. For radio signals there
are comparatively few instances where the signals move abruptly from a region with one
refractive index, to a region with another. It is far more common for there to be
comparatively gradual change. This causes the direction of the signal to bend rather than
undergo an immediate change in direction.
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Diffraction
Radio signals may also undergo diffraction. It is found that when signals encounter an
obstacle they tend to travel around them. This can mean that a signal may be received from a
transmitter even though it may be "shaded" by a large object between them. As a result the
long wave transmissions can be heard in many more places than transmissions on VHF FM.
The amount of scattering depends on the size of the electromagnetic wave relative to the size
of the object. For example, an interstate underpass is dark underneath, because its size
(~10m) is millions of times larger than light waves (~0.5 m). The bridge casts a sharp
shadow and there is little illumination. However, FM radio waves, whose wavelength is
about 3m are diffracted significantly by the bridge and it is possible to receive FM signals on
a car radio while driving under the bridge.
The degree of diffraction also depends on the sharpness of the edges of the object. A
gradually sloping hill does not diffract radio waves much and the shadow zone behind it is
quite small. On the other hand, a sharply defined cliff or mountain causes significant
diffraction and a sizeable shadow zone.
For a radio signal a mountain ridge may provide a sufficiently sharp edge. A more rounded
hill will not produce such a marked effect. It is also found that low frequency signals diffract
more markedly than higher frequency ones. It is for this reason that signals on the long wave
band are able to provide coverage even in hilly or mountainous terrain where signals at VHFand higher would not.
Figure 4: Sometimes, effects of diffraction help to receive radio waves in areas located in the
"shadow" of obstacles like behind a hill. Signals will be weak but readable
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Polarization of electromagnetic waves and their importance in radio wave propagation
The polarization of electromagnetic waves often has a significant effect on the way in which
radio wave propagate. While it is important to match the polarization of the transmitting and
receiving antennas, the choice of polarization is also important for the signal propagation.
What is polarization?
The polarization of an electromagnetic wave indicates the plane in which it is vibrating. As
electromagnetic waves consist of an electric and a magnetic field vibrating at right angles to
each other it is necessary to adopt a convention to determine the polarization of the signal.
For this purpose the plane of the electric field is used.
Vertical and horizontal polarizations are the most straightforward forms and they fall into a
category known as linear polarization. Here the wave can be thought of as vibrating in one
plane, i.e. up and down, or side to side. This form of polarization is the most commonly used,
and the most straightforward.
However this is not the only form as it is possible to generate waveforms that have circular
polarization. Circular polarization can be visualized by imagining a signal propagating from
an antenna that is rotating. The tip of the electric field vector can be seen to trace out a helix
or corkscrew as it travels away from the antenna. Circular polarization can be either right or
left handed dependent upon the direction of rotation as seen from the transmitting antenna.
It is also possible to obtain elliptical polarization. This occurs when there is a combination of
both linear and circular polarization. Again this can be visualized by imagining the tip of the
electric field tracing out an elliptically shaped corkscrew.
Importance for propagation
For many terrestrial applications it is found that once a signal has been transmitted then its
polarization will remain broadly the same. However reflections from objects in the path can
change the polarization. As the received signal is the sum of the direct signal plus a number
of reflected signals the overall polarization of the signal can change slightly although it
usually remains broadly the same. When reflections take place from the ionosphere, then
greater changes may occur.
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In some applications there are performance differences between horizontal and vertical
polarization. For example medium wave broadcast stations generally use vertical polarization
because ground wave propagation over the earth is considerably better using vertical
polarization, whereas horizontal polarization shows a marginal improvement for long
distance communications using the ionosphere. Circular polarization is sometimes used for
satellite communications as there are some advantages in terms of propagation and in
overcoming the fading caused if the satellite is changing its orientation.
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LECTURE 2: MODE OF RADIO WAVE PROPAGATION
Electromagnetic (radio) energy travels from a transmitting antenna to a receiving antenna in
one of three ways:
1. Ground wave propagation
2. Space wave (direct wave) propagation
3. Sky wave propagation
Figure 1: Ground waves, Space waves and sky waves.
1. GROUND WAVE PROPAGATION
Ground Waves are radio waves that follow the curvature of the earth. Ground waves are
always vertically polarized, because a horizontally polarized ground wave would be shorted
out by the conductivity of the ground. Because ground waves are actually in contact with the
ground, they are greatly affected by the grounds properties. Because ground is not a perfect
electrical conductor, ground waves are attenuated as they follow the earths surface. This
effect is more pronounced at higher frequencies, limiting the usefulness of ground wave
propagation to frequencies below 2 MHz. Therefore, Ground wave propagation is
particularly important on the LF and MF portion of the radio spectrum. Ground wave radio
propagation is used to provide relatively local radio communications coverage, especially by
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radio broadcast stations that require covering a particular locality. Ground waves will
propagate long distances over sea water, due to its high conductivity.
Ground wave radio signal propagation is ideal for relatively short distance propagation on
these frequencies during the daytime.
A ground wave radio signal is made up from a number of constituents. If the antennas
are in the line of sight then there will be a direct wave as well as a reflected signal. As the
names suggest the direct signal is one that travels directly between the two antenna and is not
affected by the locality. There will also be a reflected signal as the transmission will be
reflected by a number of objects including the earth's surface and any hills, or large
buildings. That may be present.
In addition to this there is surface wave. This tends to follow the curvature of the
Earth and enables coverage to be achieved beyond the horizon. It is the sum of all these
components that is known as the ground wave.
Beyond the horizon the direct and reflected waves are blocked by the curvature of the Earth,
and the signal is purely made up from the diffracted surface wave. It is for this reason that
surface wave is commonly called ground wave propagation.
As shown in Figure 2, the surface wave travels along the surface of the ground. A
surface wave flow the curvature of the Earth due to the process of diffraction.
Figure 2: Surface wave propagation.
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The Earth's atmosphere:
Earth's atmosphere is the medium through which radio signals are transmitted, it follows that
these signals are affected by varying conditions (weather, electrical activity in the upper
regions, and solar eruptions). The atmospheric conditions vary with changes in altitude,
geographical location, and changes in time (day, night, season, year).
The three separate regions (layers) of the Earth's atmosphere are the troposphere, the
stratosphere, and the ionosphere. Figure 3 illustrates the layers of the atmosphere.
Figure 3 Layers of the Earth's atmosphere
a. The troposphere extends from the face of the Earth to an altitude of about 7
miles at the north or south poles and 11 miles at the equator. The Earth's
weather activity occurs in this region. It is very unstable due to the
temperature variations, density, and pressure, and these atmospheric
conditions greatly affect radio wave propagation.
b. The stratosphere is located above the troposphere. It extends from a height of
7 miles at the poles (11 miles at the equator) to a height of about 31 miles.
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There is little water vapor present and the temperature is almost constant;
consequently, this region has relatively little affect on radio waves.
c. The ionosphere extends from about 31 miles to a height of about 250 miles. Four
layers of electrically charged ions enable radio waves to be propagated to great
distances around the Earth through reflection and refraction. This region of the
atmosphere is the most important because of its use for long-distance, point-to-
point communications.
As shown in Figure 2, the surface wave travels along the surface of the ground. A
surface wave flow the curvature of the Earth due to the process of diffraction.
As a surface wave passes over the ground, it induces voltage into Earth. The induced voltage
takes energy away from the surface wave, thereby weakening (attenuating) the wave as it
moves away from the transmitting antenna. To reduce attenuation, the amount of induced
voltage must be reduced. This is done by using vertically polarized waves, which minimize
the extent to which the electric field of the wave is in contact with the Earth. When the wave
is horizontally polarized, the wave's electric field is parallel with the surface of the Earth and
constantly in contact with it. As a transmission is made, the signal (horizontally polarized
wave) is completely attenuated within a short distance from the transmitting site.
Conversely, a vertically polarized surface wave has its electric field perpendicular to the
Earth and merely dips onto and off of the Earth's surface. Because of the lower signal loss,
vertical polarization is vastly superior to horizontal polarization for surface wave
propagation.
The amount of attenuation that a surface wave undergoes due to the induced voltage
in the Earth also depends, to a considerable extent, on the electrical properties of the terrain
over which the wave travels. The best type of surface is one which has good electrical
conductivity. The better the conductivity, the less attenuation and the better the propagation.
Table 1 shows the relative conductivity of various surfaces of the Earth.
Each type of terrain shown has a different degree of conductivity-the ease at which radio
waves propagate. Salt water has the best degree of conductivity. Because salt enhances
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conductivity, it can be used in the field when grounding a communications assemblage or
generator. Moist land surfaces provide fair conductivity, while dry terrain provides poor
conductivity and thus, impedes wave propagation. Jungle terrain is the worst environment,
as the jungle vegetation absorbs the radio waves, reducing transmission range.
A surface wave component, generally transmitted as a vertically polarized wave, remains
vertically polarized at appreciable distances from the antenna. As mentioned before,
vertically polarized waves do not lose power (attenuate) like horizontally polarized waves.
The better the conducting surface, the less energy lost. Since no surface is a perfect
conductor, any loss retards the grounded edge of a wave front, causing it to bend forward in
the direction of travel so that successive wave fronts have a forward tilt. The Earth's surface
guides the wave, and the tilt has the effect of propagating the energy in the direction of wave
travel. Poor conducting surfaces cause a high loss of energy and greater tilt. The result is
total absorption of wave energy. As frequency increases, the angle of tilt increases. A 20
MHz signal propagating over sea water has a very small tilt (one degree). Over dry ground,
the same signal is tilted about 35 degrees.
Table 1. Surface conductivity
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Frequency is a factor in surface wave attenuation.
(a) The higher a radio wave's frequency, the shorter its wavelength will be. These
high frequencies, with their shorter wavelengths, are not normally diffracted, but are
absorbed by the Earth at points relatively close to the transmitting site. As a surface wave's
frequency is increased, the more rapidly the surface wave will be absorbed, and attenuated,
by the Earth. Because of this loss by absorption, the surface wave is impractical for long-
distance transmissions with frequencies above 2 MHz.
(b) When a surface waves frequency is low enough to have a very long
wavelength, the Earth appears to be very small, and diffraction is sufficient for propagation
well beyond the horizon. In fact, by lowering the transmitting frequency into the VLF range
and using very high-powered transmitters, the surface wave can be propagated over great
distances.
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Therefore, Ground waves are used primarily for local AM broadcasting and communications
with submarines. Submarine communications takes place at frequencies well below 10 KHz,
which can penetrate sea water and which are propagated globally by ground waves.
.
There are a number of factors that affect ground wave propagation. Some of these are:
a. Frequency. Using lower frequencies will result in less ground loss.
b. Antenna characteristics. Using vertical polarization, when possible, reduces
the effect of the Earth "shorting out" the electric field of the wave.
c. Power. Increasing the power output result in greater distance.
d. Time of day. Sources of noise (natural and manmade) affect radio wave
propagation at different times of the day.
e. Terrain. The best propagation is achieved over conductive terrain.
Conductive terrain absorbs less wave energy.
ADVANTAGES AND DISADVANTAGES OF GROUND WAVES PROPAGATION
The advantages of ground waves propagation are as follows:
1. Given enough transmit power; ground waves can be used to communicate between any
two locations in the world.
2. Ground waves are relatively unaffected by changing atmospheric conditions.
The disadvantages of ground-wave propagation are as follows:
1. Ground wave requires a relatively high transmission power.
2. Ground waves are limited to low and very low frequencies (LF AND VLF), facilitating
large antennas
3. Ground losses vary considerably with surface material
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Polarisation & ground wave propagation
The type of antenna and its polarisation has a major effect on ground wave propagation.
Vertical polarisation is subject to considerably less attenuation than horizontally polarised
signals. In some cases the difference can amount to several tens of decibels. It is for this
reason that medium wave broadcast stations use vertical antennas, even if they have to be
made physically short by adding inductive loading. Ships making use of the MF marine
bands often use inverted L antennas as these are able to radiate a significant proportion of the
signal that is vertically polarised.
At distances that are typically towards the edge of the ground wave coverage area, some sky-
wave signal may also be present, especially at night when the D layer attenuation is reduced.
This may serve to reinforce or cancel the overall signal resulting in figures that will differ
from those that may be expected.
2. SPACE (DIRECT) WAVE PROPAGATION
Space Waves, also known as direct waves, are radio waves that travel directly from the
transmitting antenna to the receiving antenna. In order for this to occur, the two antennas
must be able to see each other; that is there must be a line of sight path between them. The
figure 4 shows a typical line of sight. The maximum line of sight distance between two
antennas depends on the height of each antenna. If the heights are measured in feet, the
maximum line of sight, in miles, is given by:
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Figure 4: Space wave propagation.
Because a typical transmission path is filled with buildings, hills and other obstacles, it is
possible for radio waves to be reflected by these obstacles, resulting in radio waves that
arrive at the receive antenna from several different directions. Because the length of each
path is different, the waves will not arrive in phase.
They may reinforce each other or cancel each other, depending on the phase differences. This
situation is known as multipath propagation. It can cause major distortion to certain types of
signals. Ghost images seen on broadcast TV signals are the result of multipath one picture
arrives slightly later than the other and is shifted in position on the screen.
Multipath is very troublesome for mobile communications. When the transmitter and/or
receiver are in motion, the path lengths are continuously changing and the signal fluctuates
wildly in amplitude. For this reason, Narrow Band Frequency Modulation ( NBFM) is used
almost exclusively for mobile communications. Amplitude variations caused by multipath
that make AM unreadable are eliminated by the limiter stage in an NBFM receiver.
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3. SKY WAVE PROPAGATION
INTRODUCTION
The sky wave, often called the ionospheric wave, is radiated in an upward direction and
returned to Earth at some distant location because of refraction from the ionosphere.
Therefore,Sky wave propagation allows transmitted signals to be reflected (bounced) off a
portion of the Earth's ionosphere and picked up at a receiver hundreds, or even thousands of
miles away and it is relatively unaffected by the earths surface. A common example of this
phenomenon is heard on the AM broadcast band, when many distant stations can be heard
after sunset or in the evening hours . Hence, Sky wave propagation is used to communicate
over long distances. Usually the high frequency (HF) band is used for sky wave propagation.
The sky waves have frequency range between 2MHz to 30MHz. These radio waves have theability to pass through earths atmosphere.
STRUCTURE OF THE IONOSPHERE
As we stated earlier, the ionosphere is the region of the atmosphere that extends from about
30 miles above the surface of the Earth to about 250 miles. At this altitude, high energy solar
radiation can ionize the atmosphere. Therefore, this region is known as the ionosphere
because it consists of several layers of electrically charged gas atoms called ions. The ions
are formed by a process called ionization. This region can bend and attenuate radio waves
that travel through it, causing some to be returned to earth and others simply to disappear. At
frequencies above 200 MHz, the ionosphere becomes completely transparent to radio waves
and has little effect on them. Below 30 MHz the ionosphere exerts a profound effect on radio
waves, creating many of the propagation phenomena observed at HF, MF, LF and VLF
frequencies.
Layers of Ionosphere
The ionosphere is composed of three distinct layers, designated from lowest level to highest
level (D, E, and F) as shown in figure 5 and figure 6.
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Figure 5: A simplified view of the layers in the ionosphere over the period of a day
In addition, the F layer is divided into two layers, designated F1 (the lower level) and F2 (the
higher level). The presence or absence of these layers in the ionosphere and their height
above the earth vary with the position of the sun. At high noon, radiation in the ionosphere
above a given point is greatest, while at night it is minimum. When the radiation is removed,
many of the particles that were ionized recombine. During the time between these two
conditions, the position and number of ionized layers within the ionosphere change. Since the
position of the sun varies daily, monthly, and yearly with respect to a specific point on earth,
the exact number of layers present is extremely difficult to determine. However, the
following general statements about these layers can be made.
D LAYER: The D layer ranges from about 30 to 55 miles above the earth. Ionization in the
D layer is low because less ultraviolet light penetrates to this level. At very low frequencies,
the D layer and the ground act as a huge waveguide, making communication possible only
with large antennas and high power transmitters. At low and medium frequencies, the D layer
becomes highly absorptive, which limits the effective daytime communication range to about
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200 miles. At frequencies above about 3 MHz, the D layer begins to lose its absorptive
qualities.
Long-distance communication is possible at frequencies as high as 30 MHz. Waves at
frequencies above this range pass through the D layer but are attenuated. After sunset. The D
layer disappears because of the rapid recombination of ions. Low frequency and medium-
frequency long-distance communication becomes possible. This is why AM behaves so
differently at night. Signals passing through the D layer normally are not absorbed but are
propagated by the E and F layers.
E LAYER: The E layer ranges from approximately 55 to 90 miles above the earth. The rate
of ionospheric recombination in this layer is rather rapid after sunset, causing it to nearly
disappear by midnight. The E layer permits medium-range
communications on the low-frequency through very high-frequency bands. At frequencies
above about 150 MHz, radio waves pass through the E layer.
Sometimes a solar flare will cause this layer to ionize at night over specific areas.
Propagation in this layer during this time is called SPORADIC-E. The range of
communication in sporadic-E often exceeds 1000 miles, but the range is not as great as with
F layer propagation.
F LAYER: The F layer exists from about 90 to 240 miles above the earth. During the
daylight hours, the F layer separates into two layers, the F1 and F2 layers. The ionization
level in these layers is quite high and varies widely during the day. At noon, this portion of
the atmosphere is closest to the sun and the degree of ionization is maximum. Since the
atmosphere is complex at these heights, recombination occurs slowly after sunset. Therefore,
a fairly constant ionized layer is always present. The F layer produces maximum ionization
during the
afternoon hours, but the effects of the daily cycle are not as pronounced as in the D and E
layers. Atoms in the F layer stay ionized for a longer time after sunset, and during maximum
sunspot activity, they can stay ionized all night long. Since the F layer is the highest of the
ionospheric layers, it also has the longest propagation capability.
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For horizontal waves, the single-hop F2 distance can reach 3000 miles. For signals to
propagate over greater distances, multiple hops are required.
The F layer is responsible for most high frequency, long-distance communications. The
maximum frequency that the F layer will return depends on the degree of sunspot activity.
During maximum sunspot activity, the F layer can return signals at frequencies as high as
100 MHz. During minimum sunspot activity, the maximum usable frequency can drop to as
low as 10 MHz.
Figure6: Regions of the Atmosphere
ATMOSPHERIC PROPAGATION
Within the atmosphere, radio waves can be refracted, reflected, and diffracted.
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REFRACTION
A radio wave transmitted into ionized layers is always refracted, or bent. This bending of
radio waves is called refraction.
Notice the radio wave shown in figure7 traveling through the earths
atmosphere at a constant speed.
Figure7: Radio wave refraction
As the wave enters the denser layer of charged ions, its upper portion moves faster than its
lower portion. The abrupt speed increase of the upper part of the wave causes it to bend back
toward the earth. This bending is always toward the propagation medium where the radio
waves velocity is the least.
The amount of refraction a radio wave undergoes depends on three main factors.
1. The ionization density of the layer
2. The frequency of the radio wave
3. The angle at which the radio wave enters the layer
LayerDensity
Figure 8 below shows the relationship between radio waves and ionization density. Each
ionized layer has a middle region of relatively dense ionization with less intensity above and
below. As a radio wave enters a region of increasing ionization, a velocity increase causes it
to bend back toward the earth. In the highly dense middle region, refraction occurs more
slowly because the ionization density is uniform. As the wave enters the upper less dense
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region, the velocity of the upper part of the wave decreases and the wave is bent away from
the earth.
Figure 8
Frequency
The lower the frequency of a radio wave, the more rapidly the wave is refracted by a given
degree of ionization. The Figure 9 below shows three separate waves of differing
frequencies entering the ionosphere at the same angle. You can see that the 5-MHz wave is
refracted quite sharply, while the 20-MHz wave is refracted less sharply and returns to earth
at a greater distance than the 5- MHz wave. Notice that the 100-MHz wave is lost into space.
For any given ionized layer, there is a frequency, called the escape point, at which energy
transmitted directly upward will escape into space. The maximum frequency just below the
escape point is called the critical frequency. In this example, the 100-MHz waves frequency
is greater than the critical frequency for that ionized layer.
The critical frequency of a layer depends upon the layers density. If a wave passes through a
particular layer, it may still be refracted by a higher layer if its frequency is lower than the
higher layers critical frequency.
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Figure 9
Angle of Incidence and Critical Angle
When a radio wave encounters a layer of the ionosphere, that wave is returned to earth at thesame angle (roughly) as its angle of incidence. Figure 10 below shows three radio waves of
the same frequency entering a layer at different incidence angles.
Figure 10
The angle at which wave A strikes the layer is too nearly vertical for the wave to be
refracted to earth, However, wave B is refracted back to earth. The angle between wave B
and the earth is called the critical angle. Any wave, at a given frequency, that leaves the
antenna at an incidence angle greater than the critical angle will be lost into space. This is
why wave A was not refracted. Wave C leaves the antenna at the smallest angle that will
allow it to be refracted and still return to earth. The critical angle for radio waves depends on
the layer density and the wavelength of the signal.As the frequency of a radio wave is
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increased, the critical angle must be reduced for refraction to
occur. Notice in figure 11 belowthat the 2-MHz wave strikes the ionosphere at the critical
angle for that frequency and is refracted. Although the 5-MHz line (broken line) strikes the
ionosphere at a less critical angle, it still penetrates the layer and is lost As the angle is
lowered, a critical angle is finally reached for the 5-MHz wave and it is refracted back to
earth
Figure 11
DIFFRACTION
Diffraction is the ability of radio waves to turn sharp corners and bend around obstacles.
Diffraction results in a change of direction of part of the radio-wave energy around the edges
of an obstacle. Radio waves with long wavelengths compared to the diameter of an
obstruction are easily propagated around the obstruction. However, as the wavelength
decreases, the obstruction causes more and more attenuation, until at very-high frequencies a
definite shadow zone develops. The shadow zone is basically a blank area on the opposite
side of an obstruction in line-of-sight from the transmitter to the receiver. Diffraction can
extend the radio range beyond the horizon. By using high power and low-frequencies, radio
waves can be made to encircle the earth by diffraction.
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PROPAGATION TERMS AND DEFINITION
The CRITICAL FREQUENCY is the maximum frequency that a radio wave can be
transmitted vertically and still be refracted back to Earth.
The CRITICAL ANGLE is the maximum and/or minimum angle that a radio wave can be
transmitted and still be refracted back to Earth.
SKIP DISTANCE AND ZONE
The skip zone is a zone of silence between the point where the ground wave is too weak for
reception and the point where the sky wave is first returned to earth. The outer limit of the
skip zone varies considerably, depending on the operating frequency, the time of day, the
season of the year, sunspot activity, and the direction of transmission.
Skip Distance
Look at the relationship between the sky wave skip distance, skip zone, and ground wave
coverage shown in figure below.
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Figure: Relationship between skip zone, skip distance and ground wave
The skip distance is the distance from the transmitter to the point where the sky wave first
returns to the earth. The skip distance depends on the waves frequency and angle of
incidence, and the degree of ionization. At very-low, low, and medium frequencies, a skipzone is never present. However, in the high frequency spectrum, a skip zone is often present.
As the operating frequency is increased, the skip zone widens to a point where the outer limit
of the skip zone might be several thousand miles away.
At frequencies above a certain maximum, the outer limit of the skip zone disappears
completely, and no F-layer propagation is possible. Occasionally, the first sky wave will
return to earth within the range of the ground wave. In this case, severe fading can result
from the phase difference between the two waves (the sky wave has a longer path to follow).
The MAXIMUM USABLE FREQUENCY is the highest frequency that can be used for
communications between two locations at a given angle of incidence and time of day.
The higher the frequency of a radio wave, the lower the rate of refraction by the ionosphere.
Therefore, for a given angle of incidence and time of day, there is a maximum frequency that
can be used for communications between two given locations. This frequency is known as
the MAXIMUM USABLE FREQUENCY (muf). Waves at frequencies above the muf are
normally refracted so slowly that they return to earth beyond the desired location or pass on
through the ionosphere and are lost. Variations in the ionosphere that can raise or lower a
predetermined muf may occur at anytime. This is especially true for the highly variable F2
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layer.
OPTIMUM WORKING FREQUENCY
The most practical operating frequency is one that you can rely onto have the least number of
problems. It should be high enough to avoid the problems of multipath fading, absorption,
and noise encountered at the lower frequencies; but not so high as to be affected by the
adverse effects of rapid changes in the ionosphere. A frequency that meets the above criteria
is known as the OPTIMUM WORKING FREQUENCY. It is abbreviated fot from the
initial letters of the French words for optimum working frequency, frequence optimum de
travail. The fot is roughly about 85% of the muf, but the actual percentage varies and may
be considerably more or less than 85 percent.
LOWEST USABLE FREQUENCY
Just as there is a muf that can be used for communications between two points, there is also a
minimum operating frequency that can be used known as the LOWEST USABLE
FREQUENCY (luf). As the frequency of a radio wave is lowered, the rate of refraction
increases. So a wave whose frequency is below the established luf is refracted back to earth
at a shorter distance than desired. As a frequency is lowered, absorption of the radio wave
increases. A wave whose frequency is too low is absorbed to such an extent that it is too
weak for reception. Atmospheric noise is also greater at lower frequencies. A combination of
higher absorption and atmospheric noise could result in an unacceptable signal-to-noise ratio.
For a given angle ionospheric conditions, of incidence and set of the luf depends on the
refraction properties of the ionosphere, absorption considerations, and the amount of noise
present.
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ATMOSPHERIC EFFECTS ON PROPAGATION
1. DAILY PROPAGATION EFFECTS
D LAYER: reflects VLF waves for long-range communications; refracts lf and mf for short-
range communications; has little effect on VHF and above; gone at night.
E LAYER: depends on the angle of the sun: refracts hf waves during the day up to 20 MHz
to distances of 1200 miles: greatly reduced at night.
F LAYER: structure and density depend on the time of day and the angle of the sun: consists
of one layer at night and splits into two layers during daylight hours.
F1 LAYER: density depends on the angle of the sun; its main effect is to absorb HF waves
passing through to the F2 layer.
F2 LAYER: provides long-range HF communications; very variable; height and density
change with time of day, season, and sunspot activity
2. SEASONAL PROPAGATION EFFECTS
Seasonal variations are the result of the earths revolving around the sun, because the relative
position of the sun moves from one hemisphere to the other with the changes in seasons.
Seasonal variations of the D, E, and F1 layers are directly related to the highest angle of the
sun, meaning the ionization density of these layers is greatest during the summer. The F2
layer is just the opposite. Its ionization is greatest during the winter; therefore, operating
frequencies for F2 layer propagation are higher in the winter than in the summer.
3. GEOGRAPHICAL VARIATION
The suns ionizing radiation is most intense in the equatorial regions and least intense in the
polar regions. As a result, the daytime MUF of the E and F1 layers is highest in the tropics.
4. SUNSPOTS
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One of the most notable occurrences on the surface of the sun is the appearance and
disappearance of dark, irregularly shaped areas known as SUNSPOTS. Sunspots are believed
to be caused by violent eruptions on the sun and are characterized by strong magnetic fields.
These sunspots cause variations in the ionization level of the ionosphere. Sunspots tend to
appear in two cycles, every 27 days and every 11 years
5. IRREGULAR VARIATIONS
Irregular variations are just that, unpredictable changes in the ionosphere that can drastically
affect our ability to communicate. The more common variations are sporadic E, ionospheric
disturbances, and ionospheric storms.
6. WEATHER
Wind, air temperature, and water content of the atmosphere can combine either to extend
radio
communications or to greatly attenuate wave propagation making normal communications
extremely difficult. Precipitation in the atmosphere has its greatest effect on the higher
frequency ranges. Frequencies in the HF range and below show little effect from this
condition.
TROPOSPHERIC SCATTER
Regional over the horizon communications are possible through a sky wave technique called
tropospheric scatter (troposcatter or just tropo). As shown in the diagram below, the
troposphere, which is the layer of the atmosphere closest to the ground, has pockets or cells
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of air within it that have a different water vapor content and therefore a different refractive
index for radio waves. As a result, radio waves are scattered by the cells over the horizon.
This scatter occurs at frequencies of 0.3 10 GHz. Operation above 10 GHz is not possible
because water vapor in the air strongly absorbs the signals This scattering process is not
efficient and very little of the transmitted signal is scattered in the direction of the receiver.
High power transmitters and sensitive receivers are required.
The troposphere contains almost all of the earths weather patterns, which makes the
tropospheres properties quite variable. This makes troposcatter communications subject to
weather induced fading and communications blackouts. To improve the reliability of
troposcatter links, a technique called diversity operation is used. There are three types of
diversity:
Frequency Diversity two frequencies simultaneously transmit the same signal
Polarization Diversity radio waves of both polarizations are transmitted simultaneously
Space Diversity pairs of widely separated antennas are used for transmitting and receiving
Diversity operation greatly increases the reliability of troposcatter links, but it comes at a
significant cost, because at least double the amount of equipment is needed at each
installation.
Factors of Degradation of the Signal or Signal path loss basics
The transmitted signal doesn't arrive to the receiver with the same power as when it was
transmitted. The signal path loss is essentially the reduction in power density of an
electromagnetic wave or signal as it propagates through the environment in which it is
travelling. There are many reasons for the radio path loss that may occur:
Fading
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The fading refers to any fluctuation or variation in the signal intensity that occurs in the
receiver during its trajectory since the transmitter and this is known as fading effect. The
fading can occur in any time where both the ground wave and the sky wave are received. In
this case, the two waves can arrive having phase difference, causing the cancellation of the
signal. In regions where only arrives the sky wave, the fading can be caused by two sky
waves following different paths, arriving with a phase difference between them. If the
frequencies of the sky waves will be high, then the fading effect will increase. Errors in data
transmissions and data retrievals are also caused by fading. Fading basically varies with time
Fading of signals is the effect at a receiver do to a disturbed propagation path. A local
station will come in clearly, a distant station may rise and fall in strength or appear
garbled.
Fading may be caused by a variety of factors:
A reduction of the ionospheric ionization level near sunset.
Multi-path propagation: some of the signal is being reflected by one layer of the
ionosphere and some by another layer. The signal gets to the receiver by two
different routes. The received signal may be enhanced or reduced by the waveinteractions. In essence, radio signals' reaching the receiving antenna by two or
more paths. MULTIPATH is simply a term used to describe the multiple paths a
radio wave may follow between transmitter and receiver. Such propagation paths
include the ground wave, ionospheric refraction, reradiation by the ionospheric
layers, reflection from the earths surface or from more than one ionospheric layer,
and so on. This causes include atmospheric ducting, ionospheric reflection and
refraction, and reflection from terrestrial objects, such as mountains and buildings.
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Increased absorption as the D layer builds up during the morning hours.
Multipath: In a real terrestrial environment, signals will be reflected and they will
reach the receiver via a number of different paths. These signals may add or subtract
from each other depending upon the relative phases of the signals. If the receiver is
moved the scenario will change and the overall received signal will be found vary
with position. Difference in path lengths caused by changing levels of ionization in
the reflecting layer.
E layer starts to disappear radio waves will pass through and be reflected by the F
layer, thus causing the skip zone to fall beyond the receiving station.
Selective fading: similar to Multi-path propagation, creates a hollow tone common
on international shortwave AM reception. The signal arrives at the receiver by two
different paths, and at least one of the paths is changing (lengthening or shortening).
This typically happens in the early evening or early morning as the various layers in
the ionosphere move, separate, and combine. The two paths can both be sky wave or
one can be ground wave.
Absorption: occurs when radio waves are transmitted from one medium to another, with a
resultant loss of energy. For example, if a radio signal is propagated through trees during the
summer months, the foliage will absorb some of the energy of the signal. The same signal
transmitted during the winter months may pass because the trees have shed their leaves and
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do not absorb the signal. A receiving antenna should be erected so that it is in the best
position possible to absorb incoming electromagnetic energy.
Attenuation: Like the light, the intensity of the field decreases in following the inverse-
distance law : when the distance double, the signal becomes half less strong. This is true in
free space but on earth this attenuation is much stronger due to obstacles placed between
emitter and receiver and to the fact that travelling around the earth radio waves lost their
energy as they forced to bend to follow the earth curvature.
Free space loss: The free space loss occurs as the signal travels through space
without any other effects attenuating the signal it will still diminish as it spreads out.
This can be thought of as the radio communications signal spreading out as an ever
increasing sphere. As the signal has to cover a wider area, conservation of energy tells
us that the energy in any given area will reduce as the area covered becomes larger.
Diffraction: Diffraction losses occur when an object appears in the path. The signal
can diffract around the object, but losses occur. The loss is higher the more rounded
the object. Radio signals tend to diffract better around sharp edges.
Terrain: The terrain over which signals travel will have a significant effect on the
signal. Obviously hills which obstruct the path will considerably attenuate the signal,
often making reception impossible. Additionally at low frequencies the composition
of the earth will have a marked effect. For example on the Long Wave band, it is
found that signals travel best over more conductive terrain, e.g. sea paths or over
areas that are marshy or damp. Dry sandy terrain gives higher levels of attenuation.
Buildings and vegetation: For mobile applications, buildings and other obstructions
including vegetation have a marked effect. Not only will buildings reflect radio
signals, they will also absorb them. Cellular communications are often significantly
impaired within buildings. Trees and foliage can attenuate radio signals, particularly
when wet.
Atmosphere: The atmosphere can affect radio signal paths. At lower frequencies,
especially below 30 - 50MHz, the ionosphere has a significant effect, reflecting (or
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more correctly refracting) them back to Earth. At frequencies above 50 MHz and
more the troposphere has a major effect, refracting the signals back to earth as a result
of changing refractive index. For UHF broadcast this can extend coverage to
approximately a third beyond the horizon.
SUMMARY
RADIO WAVES are electromagnetic waves that can be reflected, refracted, and diffracted
in the atmosphere like light and heat waves.
REFLECTED RADIO WAVES are waves that have been reflected from a surface and are
180 degrees out of phase with the initial wave.
The Earth's atmosphere is divided into three separate layers: The TROPOSPHERE,
STRATOSPHERE, and IONOSPHERE.
The TROPOSPHERE is the region of the atmosphere where virtually all weather
phenomena take place. In this region, rf energy is greatly affected.
The STRATOSPHERE has a constant temperature and has little effect on radio waves
.
The IONOSPHERE contains four cloud-like layers of electrically charged ions which aid in
long distance communications.
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GROUND WAVES and SKY WAVES are the two basic types of radio waves that
transmit energy from the transmitting antenna to the receiving antenna.
GROUND WAVES are composed of two separate component waves: the SURFACE
WAVE and the SPACE WAVE.
SURFACE WAVES travel along the contour of the Earth by diffraction.
SPACE WAVES can travel through the air directly to the receiving antenna or can be
reflected from the surface of the Earth.
SKY WAVES, often called ionospheric waves, are radiated in an upward direction and
returned to Earth at some distant location because of refraction.
NATURAL HORIZON is the line-of-sight horizon.
RADIO HORIZON is ONE-THIRD farther than the natural horizon.
The IONOSPHERE consists of several layers of ions, formed by the process called
ionization.
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.
The D LAYERis the lowest region of the ionosphere and refracts signals of low frequencies
back to Earth.
The E LAYERis present during the daylight hours; refracts signals as high as 20 megahertz
back to Earth; and is used for communications up to 1500 miles.
The F LAYERis divided into the F1 and F2 layers during the day but combine at night to
form one layer. This layer is responsible for high-frequency, long-range transmission.
The CRITICAL FREQUENCY is the maximum frequency that a radio wave can be
transmitted vertically and still be refracted back to Earth.
The CRITICAL ANGLE is the maximum and/or minimum angle that a radio wave can be
transmitted and still be refracted back to Earth.
SKIP DISTANCE is the distance between the transmitter and the point where the sky wave
first returns to Earth.
SKIP ZONE is the zone of silence between the point where the ground wave becomes too
weak for reception and the point where the sky wave is first returned to Earth.
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FADING is caused by variations in signal strength, such as absorption of the rf energy by the
ionosphere.
MULTIPATH FADING occurs when a transmitted signal divides and takes more than one
path to a receiver and some of the signals arrive out of phase, resulting in a weak or fading
signal.
Some TRANSMISSION LOSSES that affect radio-wave propagation are ionospheric
absorption, ground reflection, and free-space losses.
ELECTROMAGNETIC INTERFERENCE (emi), both natural and man-made, interfere
with radio communications.
The MAXIMUM USABLE FREQUENCY is the highest frequency that can be used for
communications between two locations at a given angle of incidence and time of day.
The LOWEST USABLE FREQUENCY (luf) is the lowest frequency that can be used for
communications between two locations.
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OPTIMUM WORKING FREQUENCY (fot) is the most practical operating frequency and
the one that can be relied on to have the fewest problems.
PRECIPITATION ATTENUATION can be caused by rain, fog, snow, and hail; and can
affect overall communications considerably.
TEMPERATURE INVERSION causes channels, or ducts, of cool air to form between
layers of warm air, which can cause radio waves to travel far beyond the normal line-of-sight
distances.
TROPOSPHERIC PROPAGATION uses the scattering principle to achieve beyond the
line-of-sight radio communications within the troposphere.
Virtual height: The point of the ionosphere from which a radio wave appears to have been
refracted is called the virtualheightof the ionosphere. Thus, virtual height is the altitude that
refraction occurs.
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