radio wave propagations

7/29/2019 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.

http://scienceworld.wolfram.com/physics/SnellsLaw.htmlhttp://scienceworld.wolfram.com/physics/SnellsLaw.html


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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.

radio wave propagations

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