Radio Wave Propagation - Antenna Fundamentals

RADIOWAVE PROPAGATION
• Modes of propagation, Ground Wave Propagation,
• Structure of troposphere and ionosphere,
• Characteristic of Ionospheric layers,
• Sky wave propagation, Definitions for Virtual height,
• Maximum Usable Frequency (MUF) and Skip distance,
• Optimum Working Frequency (OWF), Fading,
• Ionospheric absorptions, Multi-hop propagation,
• Space wave propagation and Super refraction.
Prepared By:
Prof. Sandeep J Rajput
Assistant Professor
E&C Engg. Dept.
GEC, Gandhinagar.

SIGNAL PROPAGATION RANGES
• Transmission range
• communication possible
• low error rate
• Detection range
• detection of the signal
possible
• communication possible
• Interference range
• signals may not be
detected
• signals add to the
background noise
distance
sender
transmission
detection
interference
Note: These are not perfect spheres in real life!

SIGNAL PROPAGATION
Receiving power additionally influenced by
• Fading (frequency dependent)
• Shadowing (blocking)
• Reflection at large obstacles
• Refraction depending on the density of a medium
• Scattering at small obstacles
• Diffraction at edges
reflection scattering diffractionshadowing refraction

PROPAGATION MODES
• Ground-wave (< 2MHz) propagation
• Sky-wave (2 – 30 MHz) propagation
• Line-of-sight (> 30 MHz) propagation

PROPAGATION MODES
• When wave leaves a vertical antenna, the radio wave resembles
a huge doughnut lying on the ground, with the antenna in the
hole at the center.
• Part of the wave moves outward in contact with the ground to
form the GROUND WAVE.
• The rest of the wave moves upward and outward to form the
SKY WAVE.

• The GROUND and SKY portions of the radio wave responsible for
two different METHODS of carrying the messages from transmitter to
receiver.
• The GROUND WAVE is used for SHORT-RANGE
COMMUNICATION at high frequencies with low power, and for
LONG-RANGE COMMUNICATION at low frequencies and very
high power.
• Day-time reception from most commercial stations is carried by the
ground wave.
• The SKY WAVE is used for long-range, high-frequency daylight
communication.
• At night, the sky wave provides a means for long-range contacts at
LOWER FREQUENCIES.
PROPAGATION MODES

GROUNDWAVE PROPAGATION
• Follows the contour of the earth
• Can propagate considerable distances
• Frequencies up to 2 MHz
• Example
• AM radio
• submarine communication (long waves)

• The ground wave is made up of four parts-DIRECT, GROUND-
REFLECTED, TROPOSPHERIC, and SURFACE waves.
• The relative importance and use made of each part is dependent on
several factors.
• The main factors are-frequency, distance between the transmitting and
receiving antennas, height of the antenna, the nature of the ground
over which the wave travels, and the condition of the atmosphere at
the lower levels.
• The DIRECT WAVE travels directly from the transmitting antenna to
the receiving antenna.
• This direct wave is not influenced by the ground, but may be affected
by the atmospheric conditions through which the wave travels.
GROUND WAVE PROPAGATION

• The GROUND-REFLECTED WAVE arrives at the receiving antenna
after being reflected from the earth's surface.
• BOTH direct waves and ground-reflected waves, the signals may be
either reinforced or weakened, depending upon the relative phases of
the two waves.
• The TROPOSPHERIC WAVE is the part of the wave that is subject to
the influences of the atmosphere at the low altitudes. The effects of the
atmosphere on this type of wave propagation are most pronounced at
frequencies above the high end of the H-F band.
• The SURFACE WAVE brings most of the low and medium frequency
broadcasts to your receiver.
• These frequencies are low enough to permit this wave to follow the
surface of the earth.

• The intensity of the surface wave decreases as it moves outward from
the antenna. This ATTENUATION-rate of decrease-is influenced
mainly by the conductivity of the ground or water and the frequency of
the wave.
• As it passes over the ground, the surface wave induces a voltage in the
earth, setting up eddy currents.
• The ENERGY to create these currents is PIRATED or taken away
from the surface wave.
• In this way, the surface wave is weakened as it moves away from the
antenna.
• Increasing the frequency rapidly increases the rate of attenuation.
Hence surface wave communication is limited to the lower frequency.

• Shore establishments are able to furnish long-range ground-wave
communication by using frequencies between about 18 and 300 kHz.
with EXTREMELY HIGH POWER.
• Since the electrical properties of the earth over which the surface
waves travel are relatively constant, the signal strength from a given
station at a given point is nearly constant.
• This holds true in practically all localities, except those that have
distinct rainy and dry seasons.
• The difference in the amount of moisture will cause the soil's
conductivity to change.

• It is interesting to note that the conductivity of salt water is 5,000
times as great as that of dry soil.
• The superiority of surface wave conductivity by salts water explains
why high-power, low-frequency transmitters are located as close to
the edge of the ocean as practicable.
• Do not think that the surface wave is confined to the earth's surface
only.
• It also extends a considerable distance up into the air, but it drops in
intensity as it rises.

SKY WAVE PROPAGATION
• Signal reflected from ionized layer of atmosphere back down
to earth
• Signal can travel a number of hops, back and forth between
ionosphere and the earth surface
• Reflection effect caused by refraction
• Examples
• amateur radio
• International broadcasts

• The part of the expanding lobe that moves toward the sky
"bumps" into an IONIZED layer of atmosphere, called the
IONOSPHERE, and is bounced or bent back toward the earth.
• If your receiver is located in the area where the returning wave
strikes, you will receive the program clearly even though you
are several hundred miles beyond the range of the ground
wave.
SKY WAVE PROPAGATION

• The ionosphere is found in the rarified atmosphere, approximately 30-
350 miles above the earth.
• It differs from the other atmosphere in that it contains a higher
percentage of positive and negative ions.
• The ions are produced by the ultra violet and particle radiations from
the sun.
• The rotation of the earth on its axis, the annual course of the earth
around the sun, and the development of SUN-SPOTS all affect the
number of ions present in the ionosphere, and these in turn affect the
quality and distance of radio transmission.
THE IONOSPHERE

• You must understand that the ionosphere is constantly changing. Some
of the ions are re-combining to form atoms, while other atoms are
being split to form ions.
• The rate of formation of ions and recombination depends upon the
amount of air present, and the strength of the sun's radiations.
• At altitudes above 350 miles, the particles of air are too sparse to
permit large-scale ion formation.
• At about 30 miles altitude, few ions are present because the rate of
recombination is too high.
• Also few ions are formed, because the sun's radiations have been
materially weakened by their passage through the upper layers of the
ionosphere with the result that below 30 miles, too few ions exist to
affect materially sky wave communication.
THE IONOSPHERE

• Different densities of ionization make the ionosphere appear to have
layers.
• Actually there is no sharp dividing line between layers. But for the
purpose of discussion a sharp markation is indicated.
• The ionized atmosphere at an altitude of between 30 and 55 miles is
designated as the D-LAYER.
• Its ionization is low and has little effect on the propagation of radio
waves except for the ABSORPTION of energy from the radio waves
as they pass through it.
• The D-layer is present only during the day. This greatly reduces the
field intensities of transmissions that must pass through daylight
zones.
LAYERS OF THE IONOSPHERE

• The band of atmosphere at altitudes between 55 and 90 miles contains
the E-LAYER.
• It is a well-defined band with greatest density at an altitude of about
70 miles. This layer is present during the daylight hours, and is also
present in PATCHES, called "SPORADIC E," both day and night. The
maximum density of the regular E-layer appears at about noon, local
time.

D Layer
• Region below E layer
• Responsible for attenuation of high frequency in day time.
• Lower most region of Ionosphere.
• Height range of 50 km to 90 km.
• Present at day
• Disappears at night
• Critical frequency is 100 KHz.
• Electron density is ranging from 1014 to 1016 per cm3
• Reflecting – VLF Signals.
• Absorbing – HF signals.
• Ionization increases with solar activity.

E Layer
• Layer occurs during day light hours.
• Electron density
• Day – 105 to 4.5x105 per cm3
• Night – 5x103 to 104 per cm3
• Max electron density at noon & summer(at 110 km)
• Useful in long distance communication in day hours
• Height range from 90 to 140 km.
• Critical frequency – 3 MHz to 5 MHz.
• Also known as Kennelly Heaviside layer.

F Layer
• Height- 140 km to 400 km from earth surface.
• Top most layer & highly ionized layer.
• Remains ionized irrespective of hours.
• Critical frequency is 5MHz to 7MHz.
• During day, F layer is found to split up into two layers called,
• F1 Layer
• F2 Layer

F1 Layer
• Upper most region
• Height – 140 km to 250 km.
• Critical frequency at noon 5 MHz to 7 MHz
• Electron Density 2x105 to 4.5x 105 per cm3
• Formed by ionization of Oxygen atoms.
• More absorption of HF waves.
F2 layer
• Upper most region(above F1)
• Height – 250km to 400 km.
• Critical frequency at noon 10 MHz to 12 MHz
• Electron Density 3x105 to 2x 106 per cm3
• Formed by ionization of UV & X rays.
• More important reflecting medium for HF Radio Waves.

• The ionization of the E-layer at the middle of the day is sufficiently
intense to refract frequencies up to 20 MHz back to the earth. This is
of great importance to daylight transmissions for distances up to 1,500
miles.
• The F-layer extends from the 90-mile level to the upper limits of the
ionosphere. At night only one F-layer is present. But during the day,
especially when the sun is high, this layer separates into two
parts, F1 and F2.
• As a rule, the F2-layer is at its greatest density during early afternoon
hours. But there are many exceptions of maximum F2 density existing
several hours later. Shortly after sunset, the F1 and F2 layers
recombine into a single F-layer.

• In addition to the layers of ionized atmosphere that appear regularly,
erratic patches occur at E-layer heights much as clouds appear in the
sky.
• These clouds are referred to as SPORADIC-E IONIZATIONS. These
patches often are present in sufficient number and intensity to enable
good radio transmission over distances where it is not normally
possible.
• Sometimes sporadic ionizations appear in considerable strength at
varying altitudes, and actually prove harmful to radio transmissions.
SPORADIC E LAYER

• The ionosphere has three effects on the sky wave. It acts as a
CONDUCTOR, it ABSORBS energy from the wave, and it
REFRACTS or BENDS the sky wave back to the earth.
EFFECT OF IONOSPHERE ON THE SKY WAVE
• When the wave from an antenna strikes the ionosphere, the wave
begins to bend. If the frequency is correct, and the ionosphere
sufficiently dense, the wave will eventually emerge from the
ionosphere and return to the earth.

• If your receiver is located at either of the points B, you will receive
the transmission from point A.
• The ability of the ionosphere to return a radio wave to the earth
depends upon the ANGLE at which the sky wave strikes the
ionosphere and upon the FREQUENCY of the transmission.
EFFECT OF IONOSPHERE ON THE SKY WAVE

• For discussion, the sky wave is assumed to be composed of four rays.
The angle at which ray 1. strikes the ionosphere is too nearly vertical
for the ray to be returned to the earth. This ray is bent out of line, but
it passes through the ionosphere and is lost.
• The angle made by ray 2 is called the CRITICAL ANGLE for that
frequency. Any ray that leaves the antenna at in angle GREATER than
theta (θ) will penetrate the ionosphere.
• Ray 3 strikes the ionosphere at the SMALLEST ANGLE that will be
refracted and still return to the earth. Any smaller angle, like ray 4,
will be refracted toward the earth, but will miss it completely.

• As the FREQUENCY INCREASES, the size of the CRITICAL
ANGLE DECREASES.
• Low frequency fields can be projected straight upward and will be
returned to the earth.
• The HIGHEST FREQUENCY that can be sent directly upward and
still be returned to the earth is called the CRITICAL FREQUENCY.
• At sufficiently high frequencies, the wave will not be returned to the
earth, regardless of the angle at which the ray strikes the ionosphere.
• The critical frequency is not constant. It varies from one locality to
another, with differences in time of day, with the season of the year,
and according to sunspot cycle.

• This variation in the critical frequency is the reason why you should
use issued predictions FREQUENCY TABLES or NOMOGRAMS to
determine the MAXIMUM USABLE FREQUENCY (MUF) for any
hour of the day.
• Nomograms and frequency tables are prepared from data obtained
experimentally from stations scattered all over the world.
• All this information is pooled and you get the results in the form of a
long-range prediction that removes most of the guess work from radio
communication.

• The area between points B and C will receive the transmission via the
REFRACTED SKY WAVE.
• The area between points A and E will receive its signals by GROUND
WAVE.
• All receivers located in the SKIP ZONE between points E and B will
receive NO transmissions from point A, since neither the sky wave
nor the ground wave reaches this area.

VIRTUAL HEIGHT
• Virtual height is the height above Earth’s surface from which a
refracted wave appears to have been reflected.
• The radiated wave is refracted back to Earth and follows path B.
• The actual maximum height that the wave reached is height ha.
However, path A shows the projected path that a reflected wave
could have taken and still be returned to Earth at the same
location.
• The maximum height that this hypothetical reflected wave would
have reached is the virtual height (hv).

• The INCREASED IONIZATION during the day is responsible for
several important changes in sky-wave transmission.
• First-It causes the sky-wave to be returned to the earth NEARER to
the point of transmission.
• Next-The EXTRA ionization increases the ABSORPTION of energy
from the sky-wave. If the wave travels a sufficient distance into the
ionosphere, it will lose all its energy.
• And-The presence of the F1- and E-layers with the F2-layer make
long-range, high-frequency communication possible by all three
layers, provided the correct frequencies are used.
EFFECT OF DAYLIGHT ON WAVE PROPAGATION

• In figure, you see the results of daylight in increasing refraction and
absorption.
• These two factors usually combine to reduce the effective daylight
communication range of low-frequency and medium frequency
transmitters to surface wave ranges.
EFFECT OF DAYLIGHT ON WAVE PROPAGATION

• The high frequency electromagnetic wave is not reflected back by the
ionosphere, so to use high frequency electromagnetic wave in
communication we used space wave propagation.
• Space waves are used in two types of communication :
• Line-of-sight (LOS) propagation.
• Satellite communication
SPACE WAVE PROPAGATION

LINE-OF-SIGHT PROPAGATION
• In line of sight propagation a space wave travels in a straight line
from transmitting antenna to the receiving antenna.
• For this type of propagation there should be no obstacle between the
transmitting antenna and the receiving antenna.
• In line-of-sight propagation, space waves are very powerful, the
signals are very clear, the bandwidth is very large and a huge amount
of information can be transmitted.
• Mobile phone systems, satellite systems, cordless phones, etc.

• In line-of-sight propagation, direct waves get blocked at some point
by the curvature of the earth.
• If the signal is to be received beyond the horizon then the receiving
antenna must be high enough to intercept the line-of-sight waves.
• Range of transmission is dependent upon the height of the antenna,
relation between range and height of antenna is given by

• From figure,
(R + h)2 = R2 + r2
∴ R2 + 2hR + h2 = R2 + r2
• As we know, radius of earth is approximately 6400km while the
height of the antenna is few meters. So we can neglect h2
∴ 2hR = r2
∴ r = √2hR
Where,
r = range
h = height of antenna
R = radius of earth

• For a greater range of electromagnetic wave, an antenna of large
height is required.
• Distance between two antennas for line-of-sight propagation is given
by , r1 = √2h1R
r2 = √2h2R
∴ (r1 + r2) = √2R(h1 + h2)
Where,
r1 = range of antenna 1, r2 = range of antenna 2
h1 = height of antenna 1, h2 = height of antenna 2
R = radius of earth
• Area covered by a transmitting antenna is given by −
Area = π r2
We have, r2 = 2hR
∴ Area = π 2hR

FRIIS FREE SPACE EQUATION
• As only a small fraction of radiated power is received at the receiver
from an isotropic radiator in free space, but the received signals, must
be 10-20 dB above the receiver noise to complete the link between
transmitter (TX) and receiver (RX) antenna.
• The amount of received power depends on transmitted power, gains of
transmitter and receiver antennas and separation between them,
operating frequency and path attenuation.
• Thus in order to describe the characteristics of wave propagation, it is
necessary to derive equation relating to these parameters.
• The expression relating these parameters is known as Friis free space
wave equation.

• Let us take an isotropic radiator transmitting power in free space, so
the medium surrounding is homogeneous and non absorbing with
dielectric constant unity. The power density at distance d from the
center of the radiator will be,
PD = Pt /4πd2 …(1) (a)
where PD = Power density (W/m2)
Pt= Transmitted power (W)
d = Distance between transmitter and receiver (km)
4πd2 = Spherical surface area (m2)

• If a directional antenna is used at Transmitter, the power density will
increase by the multiple of gain of the transmitting antenna. i.e.,
PD = Pt Gt /4πd2
where Gt = maximum directive gain of the TX antenna and
= 6(R/λ)2 in the case of microwave dish antenna in which R
is larger aperture of antenna and λ is operating wavelength.
• If a is attenuation of the medium the power density is modified to
PD = PtGt /4πd2a

• As the transmitted power spreads over a spherical area of many
kilometers, the receiving antenna picks up only a small fraction of the
radiated power.
• The amount of power at the RX antenna will be area of the receiving
antenna (A) times the power density of the TX antenna.
Pr= PtGtA/4πd2a …(1) (b)
As the gain of receiving antenna is
Gr = 4πA/λ2
then, A = Gr λ2/4π …(2)

• The power received at the receiver will be Pr = PDA
Put equation (1.b) and (2)
Pr = PtGtGra( λ2/4πd2) …(3)
or Pr = Pt Gt Gr a 1/Lp
Where LP = 4πd2/ λ2 = Free space path loss.
• In equation (3) all the parameters can be determined easily except
attenuation a.
• The a depends upon atmospheric conditions that vary with time and
local weather.

FREE SPACE PATH LOSS
• Free space path loss, ideal isotropic antenna
• Pt = signal power at transmitting antenna
• Pr = signal power at receiving antenna
•  = carrier wavelength
• d = propagation distance between antennas
• c = speed of light (3x108 m/s)
where d and  are in the same units (e.g., meters)
   
2
2
2
2
44
c
fdd
P
P
r
t 




FREE SPACE PATH LOSS IN DB
• Free space path loss equation can be recast (decibel version):








d
P
P
L
r
t
dB
4
log20log10
    dB98.21log20log20  d
    dB56.147log20log20
4
log20








df
c
fd

LOS WIRELESSTRANSMISSION IMPAIRMENTS
• Attenuation and attenuation distortion
• Free space loss
• Atmospheric absorption
• Multipath (diffraction, reflection, refraction)
• Noise
• Thermal noise

ATTENUATION
• Strength of signal falls off with distance over transmission medium.
• As a wavefront moves away from the source, the continuous
electromagnetic field that is radiated from that source spreads out.
• That is, the waves move farther away form each other and
consequently, the number of waves per unit area decreases.
• The wave simply spreads out or disperses over a larger area, decreasing
the power density.
• The reduction in power density with distance is equivalent to a power
loss and is commonly called wave attenuation.
• The attenuation is due to the spherical spreading of the wave, it is
sometimes called space attenuation of the wave.

ATMOSPHERIC ABSORPTION
• Absorption of radio frequencies in a normal atmosphere depends on
frequency and is relatively significant above approximately 10 Ghz.
• Water vapor and oxygen contribute most
• Water vapor: peak attenuation near 22GHz, low below 15Ghz
• Oxygen: absorption peak near 60GHz, lower below 30 GHz.
• Rain and fog may scatter (thus attenuate) radio waves.
• Low frequency band usage helps.

MULTI-PATH PROPAGATION
• Signal can take many different paths between sender and receiver due
to reflection, scattering, diffraction
• Time dispersion: signal is dispersed over time
• Interference with “neighbor” symbols, Inter Symbol Interference
(ISI)
• The signal reaches a receiver directly and phase shifted
• distorted signal depending on the phases of the different parts
signal at sender
signal at receiver
LOS Pulses
Multipath
Pulses

FADING
• FADING is the result of variations in signal strength at the receiver.
There are several causes. Some are easily understood, others are more
complicated.
• One cause is probably the direct result of interference between single-
hop and double-hop transmissions.
• If the two waves arrive IN PHASE, the signal strength will be
increased, but if the phases are opposed, they will cancel each other
and weaken the signal.
• Interference fading is also severe in regions where the ground and sky
wave are in contact with each other. This is especially true if the two
are approximately of equal strength.

FADING
• Fluctuations of the sky wave with a steady ground wave can cause
worse fading than sky-wave transmission alone.
• The way the waves strike the antenna and the variations in absorption
in the ionosphere are also responsible for fading.
• Occasionally, sudden ionospheric disturbances will cause complete
absorption of all sky-wave radiations.
• Receivers that are located near the outer edge of the skip zone are
subjected to fading as the sky wave alternately strikes and skips over
the area.
• This type of fading is sometimes so complete that the signal strength
may fall to near zero level.

TYPES OF FADING
• Short term (fast) fading - Quick changes in the power received
• Long term (slow) fading - Slow changes in the average
Power received
• Flat fading – Across all frequencies
• Selective fading – only in some frequencies
• Rayleigh fading – no LOS path, many other paths
• Rician fading – LOS path plus many other paths

DEALINGWITH FADING CHANNELS
• Error correction
• Adaptive equalization
• Attempts to increase signal power as needed
• Can be done with analog circuits or DSP (digital signal processor)

SUPER REFRACTION (DUCTS)
• Normally, you will find the warmest air near the surface of the water.
The air gradually becomes cooler as you gain altitude. However,
unnatural situations often develop where WARM bands of air are
above the COOLER layers. This unusual situation is called a
TEMPERATURE INVERSION.
• Whenever TEMPERATURE INVERSIONS are present, the
AMOUNT OF REFRACTION-called INDEX OF REFRACTION-is
different for the air trapped WITHIN the inversion than it is for the air
outside the inversion.

• The differences in the index of refraction form CHANNELS or
DUCTS that will pipe V.H.F. and U.H.F. signals many miles beyond
the assumed normal range.
• Sometime these ducts will be in contact with the water and may
extend a few hundred feet into the air.
• At other times the duct will start at an elevation of about 500 to 1,000
feet, and extend an additional 500 to 1,000 feet in the air.
• If an antenna extends into the duct or if. wave motion lets the wave
enter a duct after leaving an antenna, the transmission may be
conducted long distances to another ship whose antenna extends into
the duct.

• These ducts occur over relatively long distances and at varying
heights from almost ground level to several hundred meters above the
earth's surface.
• This propagation takes place when hot days are followed by rapid
cooling at night and affects propagation in the 50 MHz - 450 MHz.
Signals can propagate hundreds of kilometers up to about 2,000
kilometers (1,300 mi).

Radio Wave Propagation - Antenna Fundamentals

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