chapter 01(wavepropagation)

7/29/2019 Chapter 01(WavePropagation)

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Wave PropagationPage 1

MW Transmission Network Planning & Pathloss Training

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WAVE PROPAGATIONOVERVIEW



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TOPICS IN THIS SECTIONELECTROMAGNETIC RADIATION

WAVE MODEL

PARTICLE MODEL

THE SPECTRUM

RADIO WAVE PROPAGATION

ABSORPTION

RAINREFRACTION

REFRACTION MODESSUPER REFRACTIONDUCTINGSUB-REFRACTION

NOISEDIFFRACTIONREFLECTIONMULTIPATH PROPAGATIONDOPPLER EFFECT

TYPES OF PROPAGATION

SCATTER LINKS

PROPAGATION VIA IONOSPHEREGROUND (SURFACE) WAVE


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ELECTROMAGNETIC RADIATIONElectromagnetic waves were first postulated by James Clerk Maxwell andsubsequently confirmed by Heinrich Hertz. Maxwell derived a wave form ofthe electric and magnetic equations, revealing the wave-like nature ofelectric and magnetic fields, and their symmetry. Because the speed of EM

waves predicted by the wave equation coincided with the measured speedof light, Maxwell concluded that light itself is an EM wave.

According to Maxwell's equations, a spatially-varying electric fieldgenerates a time-varying magnetic field and vice versa. Therefore, as anoscillating electric field generates an oscillating magnetic field, themagnetic field in turn generates an oscillating electric field, and so on.These oscillating fields together form an electromagnetic wave.

A quantum theory of the interaction between electromagnetic radiationand matter such as electrons is described by the theory of quantumelectrodynamics.

Electromagnetic waves can be imagined as a self-propagating

transverse oscillating wave of electric and magnetic fields. This

diagram shows a plane linearly polarized wave propagating from

right to left. The electric field is in a vertical plane, the magnetic

field in a horizontal plane.

Electromagnetic radiation is classified into several types according to the frequency of its wave; these types include (in order ofincreasing frequency and decreasing wavelength): radio waves, microwaves, terahertz radiation, infrared radiation, visible light,ultraviolet radiation, X-rays and gamma rays. A small and somewhat variable window of frequencies is sensed by the eyes of variousorganisms; this is what we call the visible spectrum, or light.

EM radiation carries energy and momentum that may be imparted to matter with which it interacts.


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WAVE MODELMaxwell's equations are a set of four partial differential equations that describe the properties of the electric and magnetic fieldsand relate them to their sources, charge density and current density. These equations are used to show that light is anelectromagnetic wave. Individually, the equations are known as Gauss's law, Gauss's law for magnetism, Faraday's law of induction,and Ampre's law with Maxwell's correction. The set of equations is named after James Clerk Maxwell.

These four equations, together with the Lorentz force law are the complete set of laws of classical electromagnetism. The Lorentzforce law itself was actually derived by Maxwell under the name of "Equation for Electromotive Force" and was one of an earlier setof eight equations by Maxwell.

fD

0B

t

BE

t

DJH f

Gauss's law relates electric charge contained within a closed surface (Gaussian surface) to the surroundingelectric field. It describes with mathematical clarity how the divergence of an electrical field is affected bycharges (electric field lines diverge from positive charges and are drawn towards negative charges). It also statesthat the total electric flux through a Gaussian surface is unrelated to the shape and size of that surface.

Gauss's law for magnetism states that the total magnetic flux through a Gaussian surface is zero. This is due toreal world magnetic charges coming in pairs (referred to as dipoles), with the two charges giving rise to opposite

magnetic field divergences which cancel each other out. The theoretical single magnetic charge is referred to asa magnetic monopole. Magnetic monopoles have never been observed, but if they do exist, this law would need tobe modified.

Faraday's law of induction describes how a changing magnetic field can create an electric field. This is, forexample, the operating principle behind many electric generators: Mechanical force, such as water falling onturbines in a hydroelectric dam, spins a huge magnet, and the changing magnetic field creates an electric fieldwhich drives electricity through the power grid.

Ampre's law with Maxwell's correction states that magnetic fields can be generated in two ways: By electrical

current (this was the original "Ampre's law") and by changing electric fields. The idea that a magnetic field canbe induced by a changing electric field follows from the modern concept of displacement current which wasintroduced to maintain the solenoidal nature of Ampre's law in a vacuum capacitor circuit. This moderndisplacement current concept has the same mathematical form as Maxwell's original displacement current.Maxwell's current applies to the polarization current in a dielectric medium, and it sits adjacent to the moderndisplacement current in Ampre's law.


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PARTICLE MODELElectromagnetic radiation has particle-like properties as discrete packets ofenergy, or quanta, called photons. The frequency of the wave is proportional tothe particle's energy. Because photons are emitted and absorbed by chargedparticles, they act as transporters of energy. The energy per photon can becalculated from the PlanckEinstein equation:

where E is the energy, h is Planck's constant, and f is frequency. This photon-energy expression is a particular case of the energy levels of the more generalelectromagnetic oscillator whose average energy, which is used to obtain Planck'sradiation law, can be shown to differ sharply from that predicted by theequipartition principle at low temperature, thereby establishes a failure ofequipartition due to quantum effects at low temperature.

As a photon is absorbed by an atom, it excites an electron, elevating it to ahigher energy level. If the energy is great enough, so that the electron jumps to ahigh enough energy level, it may escape the positive pull of the nucleus and beliberated from the atom in a process called photoionisation. Conversely, anelectron that descends to a lower energy level in an atom emits a photon of lightequal to the energy difference. Since the energy levels of electrons in atoms arediscrete, each element emits and absorbs its own characteristic frequencies.

Together, these effects explain the emission and absorption spectra of light. Thedark bands in the absoption spectrum are due to the atoms in the interveningmedium absorbing different frequencies of the light. The composition of themedium through which the light travels determines the nature of the absorptionspectrum. For instance, dark bands in the light emitted by a distant star are due

to the atoms in the star's atmosphere. These bands correspond to the allowedenergy levels in the atoms. A similar phenomenon occurs for emission. As theelectrons descend to lower energy levels, a spectrum is emitted that representsthe jumps between the energy levels of the electrons. This is manifested in theemission spectrum of nebulae. Today, scientists use this phenomenon to observewhat elements a certain star is composed of. It is also used in the determinationof the distance of a star, using the red shift.

f.hE


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Frequency

band Designation, use and propagation

330 kHz

Very low frequency (VLF). Worldwide and long distance

communications. Navigation. Submarine communications.Surface

wave.

30300 kHz

Low frequency (LF). Long distance communications, time and

frequency standard stations, long-wave broadcasting. Ground

wave.

3003000 kHz

Medium frequency (MF) or medium wave (MW). Medium-wave

local and regional broadcasting. Marine communications. Ground

wave.

330MHzHigh frequency (HF). Short-wave bands. Long distancecommunications and short-wave broadcasting. Ionospheric sky

wave.

30300MHzVery high frequency (VHF). Short range and mobilecommunications, television and FM broadcasting. Sound

broadcasting. Space wave.

3003000MHz

Ultra high frequency (UHF). Short range and mobilecommunications. Television broadcasting. Point-to-point links.

Space wave. Note: The usual practice in the USA is to designate

3001000MHz as UHF and above 1000MHz as microwaves.

330 GHzMicrowave or super high frequency (SHF). Point-to-point links,

radar, satellite communications. Space wave.

Above 30GHzExtra high frequency (EHF). Inter-satellite and micro-cellular radio-telephone. Space wave.

THE SPECTRUMEM waves are typically described by any of the followingthree physical properties: the frequency f, wavelength, or photon energy E. Frequencies range from 2.4x1023Hz (1 GeV gamma rays) down to tiny fractions of Hertz

(nanohertz for astronomical scale waves). Wavelength isinversely proportional to the wave frequency, so gammarays have very short wavelengths that are fractions ofthe size of atoms, whereas wavelengths can be as longas the universe. Photon energy is directly proportionalto the wave frequency, so gamma rays have the highestenergy (around a billion electron volts) and radio waveshave very low energy (around femto electron volts).These relations are illustrated by the followingequations:

Whenever electromagnetic waves exist in a medium with matter, theirwavelength is decreased. Wavelengths of electromagnetic radiation, nomatter what medium they are traveling through, are usually quoted in termsof the vacuum wavelength, although this is not always explicitly stated.

Generally, EM radiation is classified by wavelength into radio wave,microwave, infrared, the visible region we perceive as light, ultraviolet, X-

rays and gamma rays. The behavior of EM radiation depends on itswavelength. When EM radiation interacts with single atoms and molecules,its behavior also depends on the amount of energy per quantum (photon) itcarries.

c

fh

Ef

c.hE

c = 299,792,458 m/s (speed of light in vacuum)

h = 6.62606896(33)10-34 Js (Planck's constant)

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RADIO WAVE PROPAGATIONThe troposphere is the lowest part of the atmosphere, where all weather phenomena occur. It extends on the poles to about 9 km andon the equator to about 17 km. The troposphere is inhomogeneous and constantly changing. Temperature, pressure, humidity, andprecipitation affect the propagation of radio waves. In the troposphere the radio waves attenuate, scatter, refract, and reflect; theamplitude and phase of the received signal may fluctuate randomly due to multipath propagation; the polarization of the wave maychange; noise originating from the atmosphere is added to the signal.

ABSORPTIONAt frequencies above a few gigahertz, theattenuation due to atmospheric absorption andscattering must be taken into account. Thisattenuation can be divided into two parts:attenuation due to clear air and attenuation dueto precipitation (raindrops, hail, and snow flakes)and fog. Attenuation of the clear air is mainly dueto resonance states of oxygen (O2) and watervapor (H2O) molecules. An energy quantum

corresponding to the resonance frequency maychange the rotational energy state of a gasmolecule. When the molecule absorbs an energyquantum, the molecule is excited to a higherenergy state. When it returns back to equilibriumthat is, drops back to the ground stateit radiatesthe energy difference, but not necessarily at thesame frequency because returning to equilibriummay happen in smaller energy steps. Underpressure the molecular emission lines have a widespectrum. Therefore the energy quantum is lostfrom the propagating wave, and for the samereason the atmosphere is always noisy at all

frequencies.

Atmospheric Absorption

The lowest resonance frequencies of oxygen are 60 GHz and 119 GHz, and those of water vapor are 22, 183, and 325 GHz. The amountof oxygen is always nearly constant, but that of water vapor is highly variable versus time and location. The attenuation constant dueto water vapor is directly proportional to the absolute amount of water vapor, which is a function of temperature and humidity. Abovefigure presents the clear air attenuation versus frequency. Between the resonance frequencies there are so-called spectral windowscentered at frequencies 35, 95, 140, and 220 GHz. At resonance frequencies the attenuation may be tens of decibels per kilometer.


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RADIO WAVE PROPAGATIONRAINFigure on right presents attenuation due torain and fog. Attenuation of rain is mainlydue to scattering: The electric field of the

radio wave polarizes the water molecules ofthe raindrop, and then the raindrop acts likea small electric dipole radiating over a largesolid angle. A heavy rain makes long radiohops impossible at frequencies above 10GHz.In a moderate rain (5 mm/hr) attenuation is0.08 dB/km at 10 GHz and 3 dB/km at 100GHz. In a pouring rain (150 mm/hr) thesevalues are about tenfold, but on the otherhand the time percentage of such strongrains is small. In a heavy rain the drops arelarge and their shape is ellipsoidal. Then a

horizontally polarizedwave attenuates more than a verticallypolarized wave. This phenomenon,depending on the wind speed, also causesdepolarization of the wave, if the electricfield is not along either axis of the raindrop.Depolarization results in unwanted couplingbetween orthogonally polarized channels andan extra loss in reception because thereceiving antenna can accept only thatpolarization for which it is designed.

Rain Attenuation

The attenuation constant due to fog and clouds is nearly directly proportional to the amount of water. Both real and imaginary parts of

the dielectric constant of ice are clearly smaller than those of water. Therefore, attenuation due to dry snow is low. Wet snow causesmore attenuation, and its attenuation is directly proportional to the amount of water.Turbulence in the troposphere may cause also scintillation, that is, random changes in amplitude and phase of the wave as it propagatesvia different routes due to turbulence (refractive index may vary strongly over short distances).


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Effects of Refraction

REFRACTIONAs radio waves travel more slowly in dense media andthe densest part of the atmosphere is normally thelowest, the upper parts of a wave usually travel fasterthan the lower. This refraction has the effect of bendingthe wave to follow the curvature of the earth andprogressively tilting the wavefront until eventually thewave becomes horizontally polarized and short-circuitedby the earths conductivity.

Waves travelling above the earths surface (spacewaves) are usually refracted downwards, effectivelyincreasing the radio horizon to greater than the visual.

The refractive index of the atmosphere is referred to asthe K factor; a K factor of 1 indicates zero refraction.

Most of the time K is positive at 1.33 and the wave isbent to follow the earths curvature.

The radio horizon is then 4/3 times the visual. However, the density of the atmosphere varies from time to time and in different partsof the world. Density inversions where higher density air is above a region of low density may also occur. Under these conditions the Kfactor is negative and radio waves are bent away from the earths surface and are either lost or ducting occurs. A Kfactor of 0.7 is theworst expected case.Ducting occurs when a wave becomes trapped between layers of differing density only to be returned at a great distance from itssource, possibly creating interference.


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REFRACTION MODESRefraction is the mechanism for most tropospheric propagation phenomena.The dielectric properties of the air, which are set mostly by the moisture content, are a primary factor in tropospheric refraction.Refraction occurs in both light or radio wave systems when the wave passes between mediums of differing density. Under that

situation, the wave path will bend an amount proportional to the difference in density of the two regions.The general situation is typically found at UHF and microwave frequencies. Because air density normally decreases with altitude, thetop of a beam of radio waves typically travels slightly faster than the lower portion of the beam. As a result, those signals refract asmall amount. Such propagation provides slightly longer surface distances than are normally expected from calculating the distance tothe radio horizon. This phenomenon is called simple refraction, and is described by the K factor.


Figure 1: An example of super refraction

Hot LandMass Cold Sea

SUPER REFRACTIONA special case of refraction called super refraction occurs in areas of theworld where warmed land air flows out over a cooler sea (Figure 1).Examples of such areas are deserts that are adjacent to a large body ofwater: the Gulf of Aden, the southern Mediterranean, and the Pacific Oceanoff the coast of Baja, California. Frequent VHF/UHF/microwavecommunications up to 200 miles are reported in such areas, and up to 600miles have reportedly been observed.

DUCTINGAnother form of refraction phenomenon is weather related. Called ducting, this form of propagation is actually a special case of superrefraction. Evaporation of sea water causes temperature inversion regions to form in the atmosphere. That is, layered air masses inwhich the air temperature is greater than in the layers below it (note: air temperature normally decreases with altitude, but at theboundary with an inversion region, it begins to increase). The inversion layer forms a duct that acts similarly to a waveguide. Ductingallows long distance communications from lower VHF through microwave frequencies; with 50MHz being a lower practical limit, and 10GHz being an ill-defined upper limit. Airborne operators of radio, radar, and other electronic equipment can sometimes note ducting

at even higher microwave frequencies.Antenna placement is critical for ducting propagation. Both the receiving and transmitting antennas must be either:(a) Physically inside the duct (as in airborne cases), or (b) able to propagate at an angle such that the signal gets trapped inside theduct.The latter is a function of antenna radiation angle. Distances up to 2500 miles or so are possible through ducting.Certain paths, where frequent ducting occurs, have been identified: in the United States, the Great Lakes region to the southeasternAtlantic seaboard; Newfoundland to the Canary Islands; across the Gulf of Mexico from Florida to Texas; Newfoundland to theCarolinas; California to Hawaii; and Ascension Island to Brazil.


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RADIO WAVE PROPAGATIONSUBREFRACTIONAnother refractive condition is noted in the polar regions, where colder air fromthe land mass flows out over warmer seas (Figure 2).Called subrefraction, this phenomena bends EM waves away from the Earths

surface thereby reducing the radio horizon by about 30 to 40%.All tropospheric propagation that depends upon air-mass temperatures andhumidity shows diurnal (i.e. over the course of the day) variation caused by thelocal rising and setting of the sun. Distant signals may vary 20dB in strengthover a 24-hour period. These tropospheric phenomena explain how TV, FMbroadcast, and other VHF signals can propagate great distances, especiallyalong seacoast paths, sometimes weak and sometimes nonexistent.

Figure 2 An example of sub refraction

Cold Land

Mass Warm Sea

NOISE

The quality of radio signals is not only degraded by the propagation losses: natural or manmade electrical noise is added to them,reducing their intelligibility.Atmospheric noise includes static from thunderstorms which, unless very close, affects frequencies below about 30MHz and noise fromspace is apparent at frequencies between about 8MHz to 1.5 GHz.A type of noise with which radio engineers are continually concerned is thermal. Every resistor produces noise spread across the wholefrequency spectrum. Its magnitude depends upon the ohmic value of the resistor, its temperature and the bandwidth of the followingcircuits. The noise voltage produced by a resistor is given by:

where

En = noise voltage, V(RMS)

k= Boltzmanns constant (1.38 10-23 joules/kelvin)

T= temperature in degrees K

B = bandwidth of measurement (Hz)

R = resistance ()

An antenna possesses resistance and its thermal noise, plus that of a receiver input circuit, is a limiting factor to receiver performance.Noise is produced in every electronic component. Shot noise it sounds like falling lead shot caused by the random arrival of electronsat, say, the collector of a transistor, and the random division of electrons at junctions in devices, add to this noise.

kTBR4En


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RADIO WAVE PROPAGATIONDIFFRACTIONWhen a wave passes over on the edge of an obstacle some ofits energy is bent in the direction of the obstacle to provide asignal in what would otherwise be a shadow. The bending is

most severe when the wave passes over a sharp edge.

As with light waves, the subsequent wavefront consists ofwavelets produced from an infinite number of points on thewavefront, rays a and b (Huygens principle). This produces apattern of interfering waves of alternate addition andsubtraction.

According to the ray theory, it is enough that just a ray canpropagate unhindered from the transmitting antenna to thereceiving antenna. In reality a radio wave requires much morespace in order to propagate without extra loss. The free spacemust be the size of the so-called first Fresnel ellipsoid.

Figure 3 shows the extra attenuation due to a knife-edgeobstacle in the first Fresnel ellipsoid. According to the raytheory, an obstacle hindering the ray causes an infiniteattenuation, and an obstacle just below the ray does not haveany effect on the attenuation. However, the result shown inFigure 3 is reality and is better explained by Huygensprinciple: Every point of the wavefront above the obstacle is asource point of a new spherical wave. This explains diffraction(or bending) of the wave due to an obstacle.Diffraction also helps the wave to propagate behind theobstacle to a space, which is not seen from the original point

of transmission. When the knifeedge obstacle is just on theLOS path (h=0), it causes an extra attenuation of 6 dB. Whenthe obstacle reaches just to the lower boundary of the firstFresnel ellipsoid, the wave arriving at the receiving point maybe even stronger than that in a fully obstacle-free case. Ifthere is a hill in the propagation path that cannot beconsidered a knife-edge, extra attenuation is even higher thanin case of a knife-edge obstacle.

Diffraction

A B

Figure-3

Extra attenuation due to the knife-edge diffraction
http://upload.wikimedia.org/wikipedia/commons/6/60/Refraction_on_an_aperture_-_Huygens-Fresnel_principle.svghttp://upload.wikimedia.org/wikipedia/commons/6/60/Refraction_on_an_aperture_-_Huygens-Fresnel_principle.svghttp://upload.wikimedia.org/wikipedia/commons/6/60/Refraction_on_an_aperture_-_Huygens-Fresnel_principle.svg


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RADIO WAVE PROPAGATIONREFLECTIONIn an LOS path, the receiving antenna often receives, besides a direct wave, waves that are reflected from the ground or fromobstacles such as buildings.Let us consider a simple situation, where there is a flat, smooth ground surface between the transmitting and receiving antenna

masts, which are located so that their distance is d, and the distance between the transmitting and receiving points is r0 , aspresented in Figure 5.Let us further assume that the transmitting antenna radiates isotropically. The electric field strength due to the direct wave at thedistance r0 from the transmitter is E0 . Taking also the reflected wave into account, the total electric field strength at the receivingpoint is:

FIGURE 4

A direct and reflected wave

021 rrrj0 e1EE

where is the reflection coefficient of the ground surface, and we have assumed that r 1 + r2 r0 .

The reflection coefficient depends onthe electric properties of the surfaceand on the polarization of the wave.At frequencies above 30 MHz thereflection coefficient can be assumedto be -1 if the polarization ishorizontal (or perpendicular) and theantenna heights are much less thanthe distance d ; that is, the grazingangle is small. For the vertical orparallel polarization the amplitudeand phase angle of the reflectioncoefficient vary rapidly at smallangles .

In the following we assume a horizontal polarization. In practice the antenna heights h1 and h2 are often small compared to thedistance d, and therefore the angle is small and;

Now the field strength is:

d

hh2rrr 21021

d

hh2j

0

21

e1EE


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The actual radiation pattern corresponds to that of the arrayformed by the transmitting antenna and its mirror image. Theelectric field strength Evaries as a function of distance betweenvalues 0 and 2E0 , as shown in Figure 5A. Note that E0 decreasesas 1/d. The nulls of the field strength (maxima for verticalpolarization in case of an ideal conducting surface) are atheights;

where n = 0, 1, 2, . . . ,

As shown in Figure 5B. The receiving antenna height must beselected correctly. In order to reduce the effects of reflections,it is possible to use a transmitting antenna that has radiationnulls at directions of the reflection points. Also, a forest at thetheoretical reflection points helps because it effectivelyeliminates reflections.

FIGURE 5A

FIGURE 5B

1

2h2

dnh

Field strength (a) as a function

of the distance d when the

receiving antenna height is h2 =

10m and (b) as a function of the

receiving antenna height h2

when d = 10 km. The situation

resembles Figure 4: = -1, f=

500 MHz, h1 = 300m.


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RADIO WAVE PROPAGATIONMULTIPATH PROPAGATIONReflection, refraction and diffraction mayprovide signals in what would otherwise beareas of no signal, but they also produce

interference.Reflected or diffracted signals may arrive atthe receiver in any phase relationship with thedirect ray and with each other. The relativephasing of the signals depends on the differinglengths of their paths and the nature of thereflection.When the direct and reflected rays havefollowed paths differing by an odd number ofhalf-wavelengths they could be expected toarrive at the receiver in anti-phase with acancelling effect. However, in the reflectionprocess a further phase change normally takes

place. If the reflecting surface had infiniteconductivity, no losses would occur in thereflection, and the reflected wave would haveexactly the same or opposite phase as theincident wave depending on the polarization inrelation to the reflecting surface. In practice,the reflected wave is of smaller amplitude thanthe incident, and the phase relationships arealso changed. The factors affecting the phasingare complex but most frequently, in practicalsituations, approximately 180 phase changeoccurs on reflection, so that reflected waves

travelling an odd number of half-wavelengthsarrive in phase with the direct wave whilethose travelling an even number arrive anti-phase.

Calculation uses observed high-resolution (10 m) radiosonde data from Gardermoen/Norway for

atmospheric modeling and a typical terrain of the region.(*)


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RADIO WAVE PROPAGATIONAs conditions in the path between transmitter and receiverchange so does the strength and path length of reflectedsignals. This means that a receiver may be subjected tosignal variations of almost twice the mean level andpractically zero, giving rise to severe fading. This type of

fading is frequency selective and occurs on troposcattersystems and in the mobile environment where it is moresevere at higher frequencies. A mobile receiver travellingthrough an urban area can receive rapid signal fluctuationscaused by additions and cancellations of the direct andreflected signals at half-wavelength intervals. Fading due tothe multi-path environment is often referred to as Rayleighfading and its effect is shown in next figure. Rayleigh fading,which can cause short signal dropouts, imposes severerestraints on mobile data transmission.

Amplitude(dB)

Distance ()

Average Signal Level

Receiver Threshold

Receiver Noise

Multipath Fading

DOPPLER EFFECTDoppler effect is an apparent shift of the transmitted frequency which occurs when either the receiver or transmitter is moving. Itbecomes significant in mobile radio applications towards the higher end of the UHF band and on digitally modulated systems.When a mobile receiver travels directly towards the transmitter each successive cycle of the wave has less distance to travel beforereaching the receiving antenna and, effectively, the received frequency is raised. If the mobile travels away from the transmitter,each successive cycle has a greater distance to travel and the frequency is lowered. The variation in frequency depends on thefrequency of the wave, its propagation velocity and the velocity of the vehicle containing the receiver. In the situation where thevelocity of the vehicle is small compared with the velocity of light, the frequency shift when moving directly towards, or away from,the transmitter is given to sufficient accuracy for most purposes by:

td

f

C

Vf

where

fd = frequency shift, Hz

ft = transmitted frequency, Hz

V= velocity of vehicle, m/s

C= velocity of light, m/s

Examples are: 100 km/hr at 450 MHz, frequency shift = 41.6Hz 100 km/hr at 1.8 GHz GSM frequency shift = 166.5Hz Train at 250 km/hr at 900MHz GSM frequency shift = 208HzWhen the vehicle is travelling at an angle to the transmitter the frequency shift is reduced. It iscalculated as above and the result multiplied by the cosine of the angle of travel from the directapproach.


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TYPES OF PROPAGATIONSCATTER LINKSInhomogeneities of the atmosphere cause scattering. Scatteringmeans that part of the coherent plane wave is transformed into

incoherent form and will radiate into a large solid angle.Normally, scattering will weaken the radio link by attenuatingthe signal. However, the scattered field may also be useful: Ascatter link takes advantage of scattering.In radio links, three kinds of scattering are utilized: tropospheric,ionospheric, and meteor scattering. The tropospheric scatteringis due to turbulence in the troposphere and is the most widelyutilized scattering mechanism in communication. This mayprovide a 500-km hop in the microwave region. The ionosphericscattering is caused by the cloudlike structures of the lowestlayers of the ionosphere, and the meteor scattering is caused bythe ionized trails of meteors. (The ionized trail is due to thehigh temperature produced by friction as the meteor enters the

atmosphere at high velocity.) These scattering mechanisms mayprovide radio hops of 2,000 km at frequencies from 30 to 80 MHz.Furthermore, raindrops and snowflakes cause scattering, whichmay introduce interference between different radio links.

A radio path utilizing tropospheric scattering: a scatter link.

Fading or a random variation of the signal power level is typical for radio systems based on scattering. This phenomenon may bedivided into a slow fading, which is due to large changes in the propagation conditions, and into a fast fading, which is due tomultipath propagation. Fading can be partially compensated by a feedback, which is used to stabilize the signalpower level, or byusing diversity.

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TYPES OF PROPAGATIONPROPAGATION VIA IONOSPHEREThe highest layers of the Earths atmosphere are called the ionosphere,because they contain plasma, which is ionized gas (free electrons andions).

The ionosphere extends from 60 to 1,000 km. Below 60 km the ionizationis insignificant because the solar ionizing radiation is getting weaker dueto absorption in the higher layers, and because recombination of plasmais fast due to high density of molecules. Above 1,000 km the density ofmolecules is too low for a significant phenomenon. It is possible todistinguish different layers in the ionosphere, as shown in Figure; theyare called D, E, F1 , and F2 layers. The electron density and the heightof these layers depend on the solar activity, on the time of day andseason, and on the geographical location. During night the D layer nearlydisappears, and the F1 and F2 layers merge together. The highestelectron density is about 1012 electrons/m3, and it can be found atdaytime at the altitude of about 250 to 400 km in the F2 layer.Via the ionosphere it is possible to obtain a radio hop to a distance of

4,000 km just with one reflection. Also, longer hops are possible if thewave reflects from the ground back to the ionosphere. In a given radiolink the frequency must be higher than the lowest usable frequency(LUF) but lower than the maximum usable frequency(MUF). The LUF andMUF depend on the temporary characteristics of the ionosphere.

Electron density in the ionosphere versus altitude.

GROUND (SURFACE) WAVEIf the transmitting and receiving antennas are close (in comparison to a wavelength) to the ground, the wave propagates bound to the

ground, as a surface wave. The electric field strength of the wave decreases rapidly as the distance from the surface increases. At lowfrequencies the attenuation of such a ground wave is small, and the wave can propagate beyond the horizon thousands of kilometers,especially over seawater. However, the attenuation of a ground wave increases rapidly with frequency. Therefore this propagationmechanism is useful only below 10 MHz.

chapter 01(wavepropagation)

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