Radio wave propagation

Prepared by:
Abrham Sorecha
Menbere Tsige
Surafel Alemu
Yalemzewud Tadesse
 Point to be discussed
 Radio wave propagation
• RF propagation in general
• Modes of propagation
• Fresnel zone
• Propagation Effects as a Function of Frequency
 Antenna and link budget
• Antenna
• Link budget and fade margin
• Decibel mathematics
• Radio mobile
 RF propagation in general

 Propagation is the study how radio waves travel from a Tx to Rx

through the atmosphere.
 The events occurring in the transmission path between two
stations affect the communications between the stations.
 EM waves travel in straight lines, unless acted upon by some
outside force.
 They travel faster through a vacuum than through any other
medium.
 Propagation modes

 Ground-wave propagation
 Sky-wave propagation
 Line-of-sight propagation
 Ground wave propagation

 A method of RF propagation that uses the area between the

surface of the earth and the ionosphere for transmission.
 Follows the contour of the earth.
 Can Propagate a considerable distances over the earth's surface
particularly in the low and medium frequency band.
 In this mode the radio wave propagates by interacting with the
semi-conductive surface of the earth.
 Used for AM radios and submarine communication.
 Con…

 A ground wave is composed of two separate component waves-the

surface wave and the space wave.
Surface wave:

 Follows the curvature of the Earth due to the process of diffraction.

 As a surface wave passes over the ground, it induces voltage into Earth and
the induced voltage takes energy away from the surface wave.
 Thereby weakening (attenuating) the wave as it moves away from Tx
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.
 A horizontally polarized wave means, the wave's electric field is parallel
with the surface of the Earth and constantly in contact with it.
 Con…

 This attenuation also depends up on the conductivity of the earth terrain.

 The better the conductivity, the less attenuation and the better the
propagation
 Different types of earth terrain has different degree of conductivity and
propagation.
Example: Sea water …….….Good
Wet soil ………..…fair
Flat, loamy soil .….fair
Rocky terrain……..poor
Desert…………….very poor
Jungle…………….extremely poor
Space wave:

 The space wave follows two distinct paths from Tx antenna to Rx

antenna:
1. Through the air directly to the receiving antenna(direct
wave).
2. By reflection from the ground to the receiving antenna(ground-
reflected wave ).
 Although space waves suffer little ground attenuation, they are
susceptible to fading.
 Since space waves actually follow two paths of different length to
the receiving site, they may arrive in or out of phase.
 Con…

 If these two component waves are received in phase, the result is a

stronger signal.
 Conversely, if they are received out of phase, they tend to cancel one
another, resulting in a weak or fading signal
 Factors that affect ground wave propagation

1. Frequency: Small frequencies results in less ground loss and

increases range.
2. Antenna characteristics: Using vertical polarization, when
possible, reduces the effect of the Earth "shorting out" the
electric field of the wave.
3. Power: Increasing the power output result in greater distance.
4. Time of day: Sources of noise affect radio wave propagation
at different times of the day.
5. Terrain: The best propagation is achieved over conductive
terrain.
 Sky wave propagation

 Sky wave propagation is used to communicate over a long distances.

 Signal reflected from ionized layer of atmosphere back down to earth, or
even thousands of miles away.
 Signal can travel a number of hops, back and forth between ionosphere and
earth’s surface.
 Usually, the HF band is used for sky wave propagation.
 Reflection effect caused by refraction
 Examples :- Amateur radio.
:- CB radio.
:- International broadcasts.
 Con…

Sky wave propagation

 Ionosphere region

 Is the region of the atmosphere that extends from 49 km to about

402 km above the Earth's surface.
 It gets its name because it consists of several layers of
electrically charged atoms called ions
 Two common process take place in this region
1. Ionization
2. Recombination
 They are basic parameter for sky wave propagation.
Ionization

 Ions are formed by a process called ionization

 When high energy UV light waves from the sun enter the
ionosphere region, they strike gas atoms and makes negative electrons
free.
 Normally, atoms are electrically neutral but when they lose an
electron, atoms become positively charged and are called positive ions.
 This process of upsetting electrical neutrality is known as ionization.
 The rate at which ionization occurs depends on the density of atoms in
the atmosphere and the intensity of the UV light wave.
Recombination

 Reverse process in which free electrons and positive ions collide each other.
 Both ionization and recombination depends on time of the day.
 Between early morning and late afternoon, the rate of ionization exceeds
the rate of recombination.
 Hence It is during this period that the ionized layers reach their greatest
density and exert maximum influence on radio waves.
 Conversely, between the late afternoon and early evening, the rate of
recombination exceeds the rate of ionization.
 Here the density of the ionized layers begins to decrease.
 We can better appreciate this phenomena by listening to a far away
commercial AM radio station at night and at sunrise.
 Con…
 The D region ranges up to 88 km above the Earth's surface.
Has low ionization density and refracts MF signals but HF pass through
it.
Disappears after sunset because of recombination.
 The E region ranges from about 88 to 144 km in altitude.
Used during the day for HF radio transmissions
Almost gone by midnight.
 The F region ranges from about 144 to 400 km high.
Very useful for HF long-distance radio communications.
During day time the F region separates into two layers-the F1 and F2
layers.
 Con…

 The lower the frequency of a radio wave, the more rapidly the wave
is refracted by a given degree of ionization.
 The rate at which a wave of a given frequency is refracted by an
ionized layer depends on the angle at which the wave enters the
layer.
Critical frequency
  It is the highest frequency that is returned by a layer at vertical


incidence.
 Frequencies higher than the critical frequency will pass through the
layer and will not be refracted.
n=sin where n =refractive index
= angle of incidence
 The electron density N at the that point satisfies the following relation.
= sin N=
 If N at some level in a layer is sufficient great to satisfy the above
condition then the wave will be returned to the earth
 Con…
  When =900 , then n=1 and N=0


The wave is traveling horizontally

 When =00 , then n=0 and N =
There is the largest electron density in the ionosphere.
The wave is traveling vertically
 N= → Nmax= →
where fcr= Critical frequency for the layer
Nmax= Maximum ionization density (electrons per cubic meter)
 Con…
  
MUF = fcr sec
The largest angle of incidence that can be obtained in F-layer reflection is of the
order of 74°.
MUF =
MUF = 3.6 fcr

 fcr in F1 region ranges from 5 MHz to 7 MHz at noon time.

 fcr inF2 region ranges from 5 MHz to 12 MHz (basically 10 MHz)
 Line of sight propagation

 Above 30 MHz neither ground nor sky wave propagation operates.

 Transmitting and receiving antennas must be within line of sight.
Satellite communication –signal above 30 MHz not reflected by
ionosphere
Ground communication – antennas within effective LOS
 For the most part, VHF and UHF waves travel by LOS.
 More susceptible for free space loss.
 Con..

 Maximum distance between two antennas for LOS propagation :

D=

where h1= height of antenna Tx(m)

h2= height of antenna Rx(m)
D= distance between Tx and Rx
K= called k factor= 1.33
 Fresnel zone

 A Fresnel zone is a three dimensional cylindrical ellipse drawn

between transmitter and receiver.
 Used by propagation theory to calculate reflections and
diffraction loss between Tx and Rx.
 The size of the ellipse is determined by the frequency of operation
and the distance between the two sites.
 Con…

 When a radio signal travels between transmitter and receiver, it can

travel in several ways.
Directly between transmitter and receiver (main signal).
Signal can reflect off the ground.
It can go left or right and be reflected back by a hill (another
reflected signal).
 Con…

Why is this important?

 The receiver antenna cannot differentiate between a main and
reflected signal.
 It receives both main and reflected signals.
 If the two signals are 360º shifted (in phase), there is no issue.
 However, if the signals are 180º apart (opposite phase), they will
cancel and the receiver will receive nothing.
Fresnel Zone Clearance

 The diameter of the Zone of the elliptical cylinder can be calculable,

based on frequency of the signal and distance between the 2 endpoint
radio sites.
 The distance between the obstacle and the actual main signal path is
known as Fresnel Zone clearance.
 Con...

 The Fresnel zone radius R can be determined from the equation:

Where λ is wavelength of the signal (m).

d1 is the distance from the first end to the obstacle (km)
d2 is the distance from the second end to the obstacle (km)
 If the ratio of

Is greater than 60% the radio path is considered “clear, line of sight” and incurs
no diffraction loss.
 Con…

 There are an infinite number of Fresnel zones, however, only the first 3 have
any real effect on radio propagation.
 Con…

 Each Fresnel Zone number sequentially increases the phase reversal and
adds a 180° phase shift.
Fresnel zone Phase shift Path length Total phase shift
caused by phase shift
reflection
Fresnel zone1(F1) 180º 1*180° =180º 360º
in phase

Fresnel zone1(F2) 180º 2*180° = 360º 540° (same as 180°)

out of phase

Fresnel zone1(F3) 180º 3*180° = 540º 740° (same as 360°)

in phase
 Con…

LOS propagation Transmission Impairments

Attenuation
Distortion
Fading
Noise
Other effects(rain attenuation & atmospheric attenuation)
Attenuation
 Loss of Strength of signal due to distance over transmission medium
 It is a function of distance, transmission medium, as well as the
frequency of the underlying transmission.
Distortion
 Is the alteration of the original shape (or other characteristic) of the
signal such as phase or amplitude of the signal.
 Signals at higher frequencies distort more than that of lower
frequencies.
Fading
 Fading refers to the fluctuations in signal strength when received at the
receiver.
 Variation over time or distance of received signal power caused by changes in
the transmission medium or path.
 In a fixed environment:
-Changes in atmospheric conditions
 In a mobile environment:
-Multipath propagation
 Fading can be classified into
Fast fading(small scale fading)
Slow fading(large scale fading)
 Con…

Fast fading
 Refers to the rapid fluctuations in the amplitude, phase or
multipath delays of the received signal.
 Can be due to the interference between multiple versions of
the same transmitted signal arriving at the receiver at slightly
different times.
 The multipath propagation which causes fast fading are
Reflection Diffraction and Scattering.
 Con…

Slow fading
 As the name indicates slow fading implies that the signal fades
away slowly.
 The features of slow fading are as given below.
The occurrence of absorbing material between Tx and Rx
Slow fading may occur when the receiver is inside a building
and the radio wave must pass through the walls of a
building.
Slow fading may cause the received signal power to vary,
though the distance remains the same.
 Indirect or Obstructed Propagation

 Indirect propagation describes terrestrial propagation where the LOS is

obstructed.
 The operating frequency has a significant impact on the viability of indirect
propagation, with lower frequencies working the best.
 HF frequencies can penetrate buildings and heavy foliage quite easily.
 VHF and UHF can penetrate building and foliage also, but to a lesser extent.
 At the same time, VHF and UHF will have a greater tendency to diffract/
reflect/scatter off of objects in the path.
 In the absence of a LOS path, multipath propagation is the dominant
propagation modes.
 Above UHF, indirect propagation becomes very inefficient and is seldom used.
 Multipath propagation

Reflection
 Occurs when the signal encounters a large solid surface, whose size is much
larger than the wavelength of the signal.
Diffraction
 Occurs when the signal encounters an edge or a corner, whose size is smaller than
the wavelength of the signal.
 The lower the frequency (The longer the wavelength), the greater the bending of
the wave.
 Therefore, radio waves are more readily diffracted than light waves.
Scattering
 Occurs when the signal encounters small objects of size smaller than the
wavelength of the signal.
Con…
 Con…
 Examples of indirect propagation are cell phones, pager and some
military communications.
Effects of multipath propagation
 Multiple copies of a signal may arrive at different phases
-If phases add destructively, the signal level relative to noise declines,
making detection more difficult.
 Inter-symbol interference (ISI)
- One or more delayed copies of a pulse may arrive at the same time as
the primary pulse for a subsequent bit.
Propagation Effects as a Function of Frequency

Very low frequency band(VLF)

 Covers a very small frequency 3–30 kHz (large wave length)
 Requires large antennas to achieve a reasonable efficiency
 Because antenna must be on the order of one-tenth of a wavelength or more
in size to provide efficient performance.
 Only permits narrow bandwidth to be used (the entire band is only 27 kHz
wide).
 The primarily mode of propagation in the VLF range is ground-wave
propagation.
 VLF has been successfully used with underground antennas for submarine
communication
Low(LF) and medium-frequency(MF) bands

 Cover the range from 30 kHz to 3 MHz .

 Have smaller wavelength than the VLF band.
 These bands still require very large antennas.
 Both bands use ground-wave propagation and some sky wave.
 These frequency permit slightly greater bandwidth than the VLF
band.
 Uses include broadcast AM radio
The high-frequency (HF) band

 Cover the range from 3 MHz to 30 MHz .

 Support some ground-wave propagation, but most HF
communication is via sky wave.
 HF sky waves are the primary means of long-distance
communication.
 The HF band includes citizens’ band (CB) radio at 27 MHz and
for amateur radio.
 The advantages of the HF band include inexpensive and widely
available equipment and reasonably sized antennas.
Very high frequency (VHF) and ultra-high frequency (UHF) band

 Cover frequencies from 30 MHz to 3 GHz.

 VHF and UHF systems can employ very small and moderately
sized antennas.
 For the most part, VHF and UHF waves travel by LOS and
ground-bounce propagation.
 Applicable for FM radio broadcast, aircraft radio, cellular
telephones, public service radio such as police and fire
departments, and the Global Positioning System (GPS).
Super-high-frequency (SHF) band

 Covers frequencies from 3GHz to 30 GHz and use strictly LOS

propagation.
 Very small antennas can be employed, or more typically, moderately
sized directional antennas with high gain.
 Applicable for satellite communications, direct broadcast satellite
television, and point-to-point links.
Extra-high-frequency (EHF) band
 Covers 30–300 GHz and is often called millimeter wave.
 Much greater bandwidths are available.
 Propagation is strictly LOS, and gaseous absorption is a significant
issue.
Frequency bands
 Noise in the RF link

 It is correct to say that noise is any signal that is not the signal of
interest.
 Thus, noise can be narrowband, wideband, structured, or unstructured
 RF link cannot be created over infinite distances and will be limited
in range by background noise even if there are no interferers.
 Such noise is always present and it is more prevalent at lower
frequencies than at higher ones.
 Noise arises from various sources, both external to a system as well as
internal to that system.
 Internal noise sources

Thermal noise
 Caused by the random motion of electrons that are excited by being raised above
absolute zero temperature.
 Each circuit in the receiver generates such thermal noise, some of which is added
to the signal.
 It can be radiated or carried along wires just as other EM energy can.
 It characterized by its wide bandwidth and it is actually the product of three
values
PN=k *T* B Where K= Boltzmann's constant 1.38 X 10-23 Joule/ok)
T = Operating temperature in degrees Kelvin;
B= Noise bandwidth
 External noise sources

Man-made noise:
 Are generated due to machinery or other man-made devices producing
frequency.
 Examples automobile ignitions, welding machines, and microwave ovens.
 Dominant source of noise cities than in the country.
Atmospheric noise:
 Due to certain amount of heat in the atmosphere surrounding the Earth.
 This heat energy warms up the electrons in air, which in turn radiate a certain
amount of thermal noise.
 This noise is picked up by the antennas and is manifest as thermal noise at the
system inputs.
 Con…

Lightning strikes from thunderstorms:

 This source of noise is prevalent in the HF frequency range.
Celestial noise sources:
 Due to energy source such as Sun and stars.
 Directional antennas pointed directly at the sun will be subject to this
noise.
 Interference in RF link

 Defined as unwanted contributions from other intended radio systems.

 This is distinct from noise, which is regarded as contributions from
unintended radio frequency sources.
 Intentional jamming of an EW system can be considered as
interference
 We can split interference sources into different categories:
Co-channel interference
Adjacent channel interference
Antenna and link budget
 Antennas
 An Antenna is a transducer, which converts electrical power into
electromagnetic waves and vice versa.
 An Antenna can be used either as a transmitting antenna or a
receiving antenna.
 A transmitting antenna is one, which converts electrical signals
into electromagnetic waves and radiates them
 A receiving antenna is one, which converts electromagnetic waves
from the received beam into electrical signals.
 In two-way communication,. the same antenna can be used for
both transmission and reception
 Basic parameters
Frequency:-
 The rate of repetition of a wave over a particular period of time, is called as
frequency.
Wavelength:-
 The distance between two consecutive maximum points (crests) or between
two consecutive minimum points (troughs) is known as the wavelength.
 Con…

Impedance Matching:-
 The approximate value of impedance of a transmitter, when equals the
approximate value of the impedance of a receiver, or vice versa, it is
termed as Impedance matching
VSWR & Reflected Power:-
 The ratio of the maximum voltage to the minimum voltage in a
standing wave.
 If the impedance of the antenna, the transmission line and the
circuitry do not match with each other, then the power will not be
radiated effectively. Instead, some of the power is reflected back.
 Con..

Bandwidth:-
 A band of frequencies in a wavelength, specified for the particular
communication, is known as bandwidth.
Bandwidth is the band of frequencies between the higher and lower
frequencies over which a signal is transmitted.
The bandwidth once allotted, cannot be used by others.
The whole spectrum is divided into bandwidths to allot to different
transmitters.
Radiation Intensity:-
 Radiation emitted from an antenna which is more intense in a particular
direction, indicates the maximum intensity of that antenna.
 Con…
Directivity:-
 The ratio of maximum radiation intensity of the subject antenna to the
radiation intensity of an isotropic or reference antenna, radiating the same
total power
Aperture Efficiency:-
 Is the ratio of the effective radiating area (or effective area) to the physical
area of the aperture.
 Con…

Antenna Efficiency:-
 Is the ratio of the radiated power of the antenna to the input
power accepted by the antenna.
 Con..

Gain:-
 The ratio of the radiation intensity in a given direction to the radiation intensity
that would be obtained if the power accepted by the antenna were radiated
isotropically.
 The term antenna gain describes how much power is transmitted in the direction of peak
radiation to that of an isotropic source.
 Gain is usually measured in dB.
 Unlike directivity, antenna gain takes the losses that occur also into account and hence focuses
on the efficiency.
Where
 G is gain of the antenna.
 ηe is the antenna’s efficiency.
 D is the directivity of the antenna.
 Con…
Near and Far Fields:-
 Near Field:-The field, which is nearer to the antenna.
 It has an inductive effect and hence it is also known as inductive
field, though it has some radiation components.
 Far field:-The field, which is far from the antenna.
 It is also called as radiation field, as the radiation effect is high in
this area.
 Radiation Pattern:- Radiation Patterns are diagrammatical
representations of the distribution of radiated energy into space, as
a function of direction.
 Con…

 In the representation of radiation pattern, we often come across different

shapes, which indicate the major and minor radiation areas, by which the
radiation efficiency of the antenna is known.
 Con…

 A clear peak in the radiation intensity surrounded by regions of weaker

radiation intensity.
Main Lobe (major lobe, main beam) :-
 radiation lobe in the direction of maximum radiation.
Minor Lobe:-
 Any radiation lobe other than the main lobe.
Side Lobe:–
 A radiation lobe in any direction other than the direction(s) of intended
radiation.
Back Lobe :–
 The radiation lobe opposite to the main lobe.
 Con…
 Half-Power Beam width (HPBW) - the angular width of the main beam at
the half-power points.
 First Null Beam width (FNBW) - angular width between the first nulls on
either side of the main beam.
 Isotropic Radiation

 Is the radiation from a point source, radiating uniformly in all directions,

with same intensity regardless of the direction of measurement.
 The isotropic radiator has unity gain, which means having a gain factor of 1
in all directions. In terms of dB, it can be called as 0dB gain (zero loss).
 Equivalent Isotropic Radiated Power:- The amount of power that an
isotropical antenna radiates to produce the peak power density observed in
the direction of maximum antenna gain
 Effective Radiated Power:- If the radiated power is calculated by taking
half-wave dipole as the reference, rather than an isotropic antenna, then it
can be termed as ERP
Gain and directivity
 Antenna Polarization
 The direction of the E field of a radio wave, relative to the ground,
determines the polarization of the wave
 Polarization can either be horizontal, vertical, or circular.
Vertical polarization:-
 Con…

Vertical polarization
 The low frequency vertically polarized waves are advantageous for
ground wave transmission
 These are not affected by the surface reflections
Horizontal polarization
 Horizontal polarization makes the wave weak, as the reflections
from the earth surface affect it.
 They are usually weak at low frequencies below 1GHz.
 Con…

Horizontal polarization:-
 Link budget and fade margin
 When planning an RF (radio frequency) link:-
One begins with the output power capacity of the transmitter
and sums the system gains and losses to determine the level of
power actually delivered to the receiver.
To ensure a reliable link, the level of power available to the
receiver should be in excess of that required for a minimum
level of performance.
An account of all the various gains and losses between the
transmitter and the receiver is referred to as the link budget.
Con…
 Con…

Where:
PTX = the transmit power in dBm .
LTX = the total system loss in dB at the transmitter.
GTX = the antenna gain in dBi at the transmitter.
LPATH = the total propagation losses in dB between the Tx and Rx
antennas.
GRX = the antenna gain in dBi at the receiver.
LRX = the total system loss in dB at the receiver.
PRX = the receive power in dBm.
 Why decibel?

 RF propagation involves a complicated multiplication and division

along a radio path.
 using these operation with linear scale make calculation more
complex.
 But dB expression is much easier to work with large number using
logarithmic scale instead of linear scale.
 So taking logarithms of all gain and loss values allows them to be
simply added and subtracted.
 Making convenient to compare values that may differ by many
order of magnitude
 Con…

 The level of received power in excess of that required for a

specified minimum level of system performance is referred to as
the fade margin.
 It was so called, because it provides a margin of safety in the
event of a temporary attenuation or fading of the received signal
power.
 Sensitivity specifies the minimum RF input power required to
produce a useable output signal.
 Link budget parameters

Transmit power
System loss
Antenna gain
Path loss
System loss:- System loss is the sum of the total insertion loss in the
transmission line plus any loss due to an impedance mismatch with
the antenna.
Antenna Gain:- By convention, antenna gain figures used in a link
budget are expressed in units of dBi; gain relative to a theoretical
isotopic radiator.
 Con…

 The following equation is used to convert gain in units of dBd to

units of dBi:
dBi = dbd + 2.15
Path Loss:- path loss is the principal contributor to loss in the link
budget.
 It is the sum of free space loss plus additional losses induced by the
interaction of the EM (electromagnetic) wave front with the terrain
and/or obstructions along the path of propagation.
 Line-of-Sight Path of Propagation

 For the majority of RF links, the primary mode of EM wave

propagation is line of sight, a direct, unobstructed path between
the transmitting and receiving antennas.
 Therefore, for a single antenna, the maximum path of
propagation is limited by the distance to the RF horizon.
 Con…
 An RF LOS follows a curved path that is initially parallel to the earth’s
surface but is progressively bent toward the surface due to the refractive
properties of the atmosphere
 For a standard atmosphere (standard refraction = k = 1.33) over a smooth
earth, the distance to the RF horizon is related to the height of the antenna
as follows:
Dhor=4.124√ℎ
 Where:
Dhor= distance in kilometers to the RF horizon
h = the antenna height in meters above a smooth earth
 Con…
 For an RF link, the maximum line-of-sight path distance is equal
to the sum of the RF horizon distance for both the transmitting
and receiving antennas:
 Free Space Propagation Model

 As an EM wave propagates in free space, the power density per unit area
decreases in proportion to the frequency and the square of the distance
traveled.
 This gives to:-
 Con…

 But this does not represent a realistic environment for earth-based

RF links and, for many path scenarios, the use of free space loss
alone will not result in a realistic link budget.
2-Ray Multipath Propagation Model
 The resultant signal delivered to the connected receiver will be the
vector sum of the relative amplitude and phase of the direct and
reflected wave fronts.
 For an unobstructed LOS path over relatively flat terrain, the
primary source of reflections is the earth’s surface.
 Con…
 The effect of the ground reflected wave front on the received signal is largely
dependent on :-
The distance between the transmitting and receiving antennas,
The relative height of the antennas,
And the reflective properties of the earth’s surface.
Con…
 Propagation model
Near earth propagation model
Foliage propagation Propagation model Terrain propagation
models in building area models

Weissberger’s models  Young model  Egli model

Early ITU vegetation  Okumura model  ITU terrain model
model  Hata model  Longley - rice
Updated ITU  Cost 231 model models

vegetation model  Lee model

 Radio mobile
 Radio Mobile Online is a radio wave propagation prediction tool.
 It uses digital terrain information and a mathematical model to
simulate radio transmissions between two fixed sites (radio link) or
between a fixed site and a mobile (Radio coverage).
 The digital terrain information comprises two databases: Elevation
and land cover.
 Radio Mobile is a popular tool for RF studies.
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