CHAPTER10RADIOWAVESELECTROMAGNETICWAVEPROPAGATION1000.SourceOfRadioWavesinducethe current.The current starts at zero, increases toamaximum asthe rotor completes onequarterof its revolu-Considerelectric current as aflowof electrons alongation,andfallstozerowhentherotor completesonehalfofconductor between points of differing potential. A direct cur-its revolution.The current then approaches a negativemax-rent flows continuously in the same direction.This wouldimum,then it once again returns to zero.This cycle can beoccurif thepolarityof theelectromotiveforce causingtherepresented by a sine function.electronflowwereconstant,suchas isthecasewithabatteryThe relationship between the current and the magneticIfhowever.thecurrentisinducedbytherelativemotionbe-field strength induced in the conductor through which thetween a conductor and a magnetic field, such as is the case incurent is flowing is shown in Figure 1001.Recall from thea rotating machine called a generator, then the resulting cur-discussion above thatthis field strength is proportional to therent changesdirection in theconductor as thepolarityof themagnitude of the current, that is, if the current is representedelectromotiveforce changes withthe rotationof thegenera-by a sine wave function, then so too will be the magnetic fieldtor's rotor.This is known as alternating current.strength resultingfrom that current.This characteristic shapeTheenergy of thecurrent flowing through the conduc-ofthefieldstrengthcurvehasledtotheuseofthetermtor is either dissipated as heat (an energy loss proportional"wave"when referring to electromagneticpropagation.Theto both the current flowing through the conductor and themaximumdisplacementofapeakfromzero iscalledtheam-conductor'sresistance)orstoredinanelectromagneticfieldplitude.The forward side of any wave is called the waveoriented symmetrically about the conductor.The orienta-front.For a nondirectional antenna,each waveproceeds out-tion of this field is a function of the polarity of the sourceward as an expanding sphere (or hemisphere).producing the current. When the current is removed fromOnecycleisacompletesequenceofvalues,asfromcrestthe wire, this electromagnetic field will, after a finite timeto crest.The distance traveled by the energy during one cyclecollapsebackintothewireis the wavelength, usually expressed in metric units (meters,What would occur should the polarity of the currentcentimeters,etc.).Thenumberofcvclesrepeatedduringunitsource supplying the wire be reversed at a rate whichgreat-time (usually1 second)is the frequency.This is given in hertzly exceeds the finite amount of time required for the(cycles per second).A kilohertz (kHz) is 1,000 cycles per sec-electromagnetic field to collapse back upon the wire? In theond.Amegahertz(MHz)is1,000,000cyclesper secondcaseof rapidpolereversal, anothermagneticfield,propor-Wavelengthand frequencyareinverselyproportionaltional in strength but exactly opposite in magneticThe phase of a wave is the amount by which the cycleorientationtotheinitialfield.willbeformeduponthewireThe initial magnetic field, its current source gone, cannothasprogressedfroma specifiedorigin.Formostpurposes itcollapseback uponthewirebecauseoftheexistenceofthissecond, oriented electromagnetic field.Instead, it“detach-1CYCLEes"from the wire and propagates out into space.This is thebasicprincipleof a radio antenna,which transmits a waveWAVELENGTHNat a frequencyproportional to the rateofpole reversal andCrestEAMPLITUDEat a speed equal tothe speed of light.Peal1001.Radio Wave Terminology0The magnetic field strength in the vicinity ofa conduc-tor is directly proportional to themagnitude of the currentTroughflowing throughthe conductor.Recall thediscussion of al--TIMEORternating current above.A rotating generator producesDISTANCEcurrent intheform ofa sinewave.That is,the magnitude ofthecurrentvariesasafunctionoftherelativepositionoftheFigure1001.Radiowaveterminology.rotatingconductorandthestationarymagneticfieldusedto165
165 CHAPTER 10 RADIO WAVES ELECTROMAGNETIC WAVE PROPAGATION 1000. Source Of Radio Waves Consider electric current as a flow of electrons along a conductor between points of differing potential. A direct current flows continuously in the same direction. This would occur if the polarity of the electromotive force causing the electron flow were constant, such as is the case with a battery. If, however, the current is induced by the relative motion between a conductor and a magnetic field, such as is the case in a rotating machine called a generator, then the resulting current changes direction in the conductor as the polarity of the electromotive force changes with the rotation of the generator’s rotor. This is known as alternating current. The energy of the current flowing through the conductor is either dissipated as heat (an energy loss proportional to both the current flowing through the conductor and the conductor’s resistance) or stored in an electromagnetic field oriented symmetrically about the conductor. The orientation of this field is a function of the polarity of the source producing the current. When the current is removed from the wire, this electromagnetic field will, after a finite time, collapse back into the wire. What would occur should the polarity of the current source supplying the wire be reversed at a rate which greatly exceeds the finite amount of time required for the electromagnetic field to collapse back upon the wire? In the case of rapid pole reversal, another magnetic field, proportional in strength but exactly opposite in magnetic orientation to the initial field, will be formed upon the wire. The initial magnetic field, its current source gone, cannot collapse back upon the wire because of the existence of this second, oriented electromagnetic field. Instead, it “detaches” from the wire and propagates out into space. This is the basic principle of a radio antenna, which transmits a wave at a frequency proportional to the rate of pole reversal and at a speed equal to the speed of light. 1001. Radio Wave Terminology The magnetic field strength in the vicinity of a conductor is directly proportional to the magnitude of the current flowing through the conductor. Recall the discussion of alternating current above. A rotating generator produces current in the form of a sine wave. That is, the magnitude of the current varies as a function of the relative position of the rotating conductor and the stationary magnetic field used to induce the current. The current starts at zero, increases to a maximum as the rotor completes one quarter of its revolution, and falls to zero when the rotor completes one half of its revolution. The current then approaches a negative maximum; then it once again returns to zero. This cycle can be represented by a sine function. The relationship between the current and the magnetic field strength induced in the conductor through which the current is flowing is shown in Figure 1001. Recall from the discussion above that this field strength is proportional to the magnitude of the current; that is, if the current is represented by a sine wave function, then so too will be the magnetic field strength resulting from that current. This characteristic shape of the field strength curve has led to the use of the term “wave” when referring to electromagnetic propagation. The maximum displacement of a peak from zero is called the amplitude. The forward side of any wave is called the wave front. For a nondirectional antenna, each wave proceeds outward as an expanding sphere (or hemisphere). One cycle is a complete sequence of values, as from crest to crest. The distance traveled by the energy during one cycle is the wavelength, usually expressed in metric units (meters, centimeters, etc.). The number of cycles repeated during unit time (usually 1 second) is the frequency. This is given in hertz (cycles per second). A kilohertz (kHz) is 1,000 cycles per second. A megahertz (MHz) is 1,000,000 cycles per second. Wavelength and frequency are inversely proportional. The phase of a wave is the amount by which the cycle has progressed from a specified origin. For most purposes it Figure 1001. Radio wave terminology
166RADIOWAVESis stated in circularmeasure,a completecyclebeingconsid1004.Reflectionered 360°.Generally, the origin is not important, principalinterest being the phase relative to that of some other wave.When radio waves strike a surface, the surface reflectsThus, two waves having crests 1/4cycle apart are said tobethem in the samemanner as light waves.Radiowaves of all90o"out of phase." If the crest of one wave occurs at thefrequencies are reflected by the surface of the earth.Thetrough of another, the two are180°out of phase.strength of the reflected wave depends upon grazing angle(the angle between the incident ray and the horizontal), type1002.ElectromagneticSpectrumofpolarization,frequency,reflectingpropertiesofthesur-face, and divergence of the reflected ray.Lower frequencyTheentire range of electromagnetic radiationfrequen-results in greater penetration.Atvery low frequencies,us-cies is called the electromagnetic spectrum. Theable radio signals can be received some distancebelowthefrequency range suitablefor radio transmission, the radiosurface of the seaspectrum, extends from 10 kilohertz to 300,000 mega-Aphasechangeoccurswhenawaveisreflectedfromhertz.It is divided into a number of bands, as shown inthe surface of theearth.TheamountofthechangevarieswithTable1002.Belowtheradiospectrum,butoverlapping it,the conductivity oftheearth and the polarization of thewaveistheaudiofrequencyband,extendingfrom20to20,000reaching a maximum of 180° for a horizontally polarizedhertz.Above theradio spectrum areheat and infrared, thewave reflected from sea water (considered to have infinitevisible spectrum (light in its various colors), ultraviolet, X-conductivity).When direct waves (those traveling fromrays,gamma rays,and cosmic rays.These areincluded intransmitter to receiver in a relatively straight line,without re-Table 1002.Waves shorter than 30centimeters are usuallyflection)and reflected waves arrive at a receiver, the totalcalled microwavessignal is the vector sum ofthetwo.Ifthe signals are in phase,they reinforce each other, producing a stronger signal. If1003.Polarizationthere is a phase difference, the signals tend to cancel eachother,thecancellationbeingcompleteifthephasedifferenceRadio waves producebothelectric andmagnetic fieldsis180°and thetwosignals havethesameamplitude.This in-The direction oftheelectric component ofthefield is calledteraction of waves is called wave interference. A phasethe polarization of the electromagnetic field. Thus, if thedifference may occurbecause of the change of phaseofa re-electric component is vertical, the wave is said to beverti-flected wave,orbecauseofthelongerpathfollowedbyitcally polarized,"and if horizontal,"horizontally polarized."The second effect decreases with greater distance betweenAwavetravelingthroughspacemaybepolarizedinanydi-transmitter and receiver,for under these conditions the dif-rection.Onetravelingalongthesurfaceoftheearthisference inpath lengths is smaller.At lowerfrequencies therealways vertically polarized becausethe earth,a conductor,is no practical solution to interference caused in this way.Forshort-circuits any horizontal component. The magnetic fieldand the electric field are always mutually perpendicular.VHFand higherfrequencies,the condition can be improvedBandAbbreviationRange of frequencyRange of wavelengthAF20 to 20,000 HzAudio frequency15,000,000 to 15,000mRF10kHzto300,000MHz30,000mto0.1cmRadio frequencyVLFVerylowfrequency10to30kHz30,000to10,000mLFLowfrequency30to300kHz10,000to1,000mMFMedium frequency300to3,000kHz1,000to100mHFHigh frequency3to30MHz100to10mVHFVery high frequency30to300MHz10to1mUHFUltra high frequency300 to 3,000 MHz100 to 10cmSuper high frequencySHF3,000 to30,000MHz10 to1 cmExtremely highEHF30,000 to 300,000 MHzI to0.1cmfrequencyHeat and infrared*106to3.9x108MHz0.03 to 7.6x10-5 cmVisible spectrum*3.9x108to7.9x108MHz7.6x10-5to3.8x10-5cmUltraviolet*7.9x108to2.3×1010MHz3.8x10-5 to 1.3x10-6cmX-rays*2.0x109to3.0x1013MHz1.5x10-5 to 1.0x10-9cmGamma rays*2.3x1012 to 3.0×1014MHz1.3x10-8 to1.0x10-10cmCosmic rays*<6.2x10-12 cm>4.8x1015 MHz* Values approximate.Table 1002.Electromagnetic spectrum
166 RADIO WAVES is stated in circular measure, a complete cycle being considered 360°. Generally, the origin is not important, principal interest being the phase relative to that of some other wave. Thus, two waves having crests 1/4 cycle apart are said to be 90° “out of phase.” If the crest of one wave occurs at the trough of another, the two are 180° out of phase. 1002. Electromagnetic Spectrum The entire range of electromagnetic radiation frequencies is called the electromagnetic spectrum. The frequency range suitable for radio transmission, the radio spectrum, extends from 10 kilohertz to 300,000 megahertz. It is divided into a number of bands, as shown in Table 1002. Below the radio spectrum, but overlapping it, is the audio frequency band, extending from 20 to 20,000 hertz. Above the radio spectrum are heat and infrared, the visible spectrum (light in its various colors), ultraviolet, Xrays, gamma rays, and cosmic rays. These are included in Table 1002. Waves shorter than 30 centimeters are usually called microwaves. 1003. Polarization Radio waves produce both electric and magnetic fields. The direction of the electric component of the field is called the polarization of the electromagnetic field. Thus, if the electric component is vertical, the wave is said to be “vertically polarized,” and if horizontal, “horizontally polarized.” A wave traveling through space may be polarized in any direction. One traveling along the surface of the earth is always vertically polarized because the earth, a conductor, short-circuits any horizontal component. The magnetic field and the electric field are always mutually perpendicular. 1004. Reflection When radio waves strike a surface, the surface reflects them in the same manner as light waves. Radio waves of all frequencies are reflected by the surface of the earth. The strength of the reflected wave depends upon grazing angle (the angle between the incident ray and the horizontal), type of polarization, frequency, reflecting properties of the surface, and divergence of the reflected ray. Lower frequency results in greater penetration. At very low frequencies, usable radio signals can be received some distance below the surface of the sea. A phase change occurs when a wave is reflected from the surface of the earth. The amount of the change varies with the conductivity of the earth and the polarization of the wave, reaching a maximum of 180° for a horizontally polarized wave reflected from sea water (considered to have infinite conductivity). When direct waves (those traveling from transmitter to receiver in a relatively straight line, without reflection) and reflected waves arrive at a receiver, the total signal is the vector sum of the two. If the signals are in phase, they reinforce each other, producing a stronger signal. If there is a phase difference, the signals tend to cancel each other, the cancellation being complete if the phase difference is 180° and the two signals have the same amplitude. This interaction of waves is called wave interference. A phase difference may occur because of the change of phase of a reflected wave, or because of the longer path followed by it. The second effect decreases with greater distance between transmitter and receiver, for under these conditions the difference in path lengths is smaller. At lower frequencies there is no practical solution to interference caused in this way. For VHF and higher frequencies, the condition can be improved Band Abbreviation Range of frequency Range of wavelength Audio frequency AF 20 to 20,000 Hz 15,000,000 to 15,000 m Radio frequency RF 10 kHz to 300,000 MHz 30,000 m to 0.1 cm Very low frequency VLF 10 to 30 kHz 30,000 to 10,000 m Low frequency LF 30 to 300 kHz 10,000 to 1,000 m Medium frequency MF 300 to 3,000 kHz 1,000 to 100 m High frequency HF 3 to 30 MHz 100 to 10 m Very high frequency VHF 30 to 300 MHz 10 to 1 m Ultra high frequency UHF 300 to 3,000 MHz 100 to 10 cm Super high frequency SHF 3,000 to 30,000 MHz 10 to 1 cm Extremely high frequency EHF 30,000 to 300,000 MHz 1 to 0.1 cm Heat and infrared* 106 to 3.9×108 MHz 0.03 to 7.6×10-5 cm Visible spectrum* 3.9×108 to 7.9×108 MHz 7.6×10-5 to 3.8×10-5 cm Ultraviolet* 7.9×108 to 2.3×1010 MHz 3.8×10-5 to 1.3×10-6 cm X-rays* 2.0×109 to 3.0×1013 MHz 1.5×10-5 to 1.0×10-9 cm Gamma rays* 2.3×1012 to 3.0×1014 MHz 1.3×10-8 to 1.0×10-10 cm Cosmic rays* >4.8×1015 MHz <6.2×10-12 cm * Values approximate. Table 1002. Electromagnetic spectrum
RADIOWAVES167byelevatingthe antenna,if the wave isverticallypolarized1.000 to5,000 feet,due to the settling ofa large airmassAdditionally,interferenceat higherfrequenciescanbemoreThis is a frequent occurrence in Southern California andcertain areas of the Pacific Ocean.nearlyeliminatedbecauseofthegreater easeof beamingthesignal toavoidreflection.A bending in the horizontal plane occurs when aReflectionsmayalsooccurfrommountains,trees,andgroundwave crosses a coast atan obliqueangle.This is dueotherobstacles.Suchreflection is negligiblefor lowerfre-toamarkeddifference in theconducting andreflecting prop-quencies,but becomes more prevalent as frequencyerties of theland and water over which the wave travels.Theincreases.In radio communication,itcanbe reduced by us-effect isknown as coastal refraction or land effect.ing directional antennas, but this solution is not alwaysavailablefornavigational systems.1006.The IonosphereVarious reflecting surfaces occur in the atmosphere. Athigh frequencies,reflections take place from rain.At stillSinceanatomnormallyhas an equal number of negahigherfrequencies,reflectionsarepossiblefromclouds,partively charged electrons and positively charged protons, itticularly rain clouds.Reflections may even occur at a sharplyiselectricallyneutral.Anion is anatomorgroupofatomsdefined boundary surface between air masses, as whenwhich has become electricallycharged,either positively orwarm.moistairflowsovercold,drvair.Whensuchasurfacenegatively,bythe loss or gain ofone or more electrons.is roughly parallel to the surface of the earth, radio wavesLoss ofelectrons may occur in a varietyof ways.In themaytravel forgreater distances than normal Theprincipalatmosphere,ionsareusuallyformedbycollisionofatomssource of reflection in the atmosphere is the ionosphere.withrapidlymovingparticles, or by the action of cosmicrays or ultraviolet light. In the lower portion of the atmo-1005.Refractionsphere,recombination soon occurs,leavinga smallpercentage ofions.In thin atmospherefar above the surfaceRefraction of radio waves is similartothatof lightof theearth, however,atoms arewidely separated and awaves.Thus, as a signal passes from air of one density tolargenumberof ionsmaybepresent.Theregion of numerthat ofadifferentdensity,the directionof travel is alteredouspositiveandnegativeionsandunattachedelectronsisThe principal cause of refraction in the atmosphere is thecalled the ionosphere.The extent of ionization depend-difference intemperatureandpressureoccurring at varioussupon thekinds of atoms present in the atmosphere, theheights and in differentairmasses.density of theatmosphere,and theposition relativeto theRefractionoccurs at allfrequencies,butbelow30MHzsun (time of day and season). After sunset, ions and elec-tronsrecombine faster than they are separated, decreasingthe effect is small as compared with ionospheric effectstheionizationoftheatmospherediffraction, and absorption.Athigherfrequencies,refrac-tion in the lower layer of the atmosphere extends the radioAn electron can be separated from its atom only by thehorizon to a distance about 15percentgreaterthanthe vis-application of greater energy than that holding theelectronible horizon.The effect is the same as if the radius of theSince the energy oftheelectron depends primarily upon theearthwereaboutone-thirdgreaterthanitisandtherewerekindofan atom of which it is apart, and its positionrelativenorefractionto the nucleus of thatatom,differentkindsofradiationmaySometimesthe lowerportion of theatmospherebe-causeionizationofdifferentsubstancescomes stratified.This stratification results in nonstandardIntheoutermostregionsoftheatmosphere,thedensitytemperature and moisture changeswith height.If there is aissolowthatoxygenexistslargelyas separateatoms,rathermarkedtemperatureinversionora sharpdecreaseinwaterthan combining as molecules as itdoesnearerthe surface ofvapor content withincreased height,ahorizontal radioductthe earth.At great heights the energy level is low and ion-maybeformed.Highfrequency radio wavestraveling hor-ization from solar radiation is intense.Thisisknownastheizontally within theduct are refracted to suchan extentthatFlayer.Above this level the ionization decreases becausethey remain within the duct, following the curvature of theofthelackofatomstobeionized.Belowthislevelitdeearth for phenomenal distances.This is called super-re-creases because the ionizing agent of appropriate energyfraction. Maximum results are obtained when bothhas alreadybeen absorbed.During daylight, two levels oftransmittingandreceivingantennasarewithintheductmaximumFionizationcanbedetected.theFlaveratabout125 statute miles above the surface ofthe earth,and theFThere is a lower limitto thefrequency affected by ducts.Itvariesfromabout200MHztomorethan1,000MHzlayer at about 90 statute miles.At night, these combine toform a singleFlayerAt night,surface ducts may occur overland due toAt a height of about 60 statute miles, the solar radiationcoolingofthesurface.Atsea,surfaceductsabout50feetthick may occurat anytime in thetrade wind belt.Surfacenot absorbed by the Flayer encounters, for the first time, largeductsio0feetormoreinthicknessmayextendfromlandnumbers of oxygenmolecules.Anewmaximum ionizationouttoseawhenwarmairfrom theland flows overthecool-occurs, known as the E layer. The height of this layer is quiteer ocean surface.Elevated ducts from a fewfeet tomoreconstant, in contrast with thefluctuating Flayer.At night thethan 1.000feetin thicknessmay occur at elevations ofE layer becomes weaker by two orders ofmagnitude
RADIO WAVES 167 by elevating the antenna, if the wave is vertically polarized. Additionally, interference at higher frequencies can be more nearly eliminated because of the greater ease of beaming the signal to avoid reflection. Reflections may also occur from mountains, trees, and other obstacles. Such reflection is negligible for lower frequencies, but becomes more prevalent as frequency increases. In radio communication, it can be reduced by using directional antennas, but this solution is not always available for navigational systems. Various reflecting surfaces occur in the atmosphere. At high frequencies, reflections take place from rain. At still higher frequencies, reflections are possible from clouds, particularly rain clouds. Reflections may even occur at a sharply defined boundary surface between air masses, as when warm, moist air flows over cold, dry air. When such a surface is roughly parallel to the surface of the earth, radio waves may travel for greater distances than normal The principal source of reflection in the atmosphere is the ionosphere. 1005. Refraction Refraction of radio waves is similar to that of light waves. Thus, as a signal passes from air of one density to that of a different density, the direction of travel is altered. The principal cause of refraction in the atmosphere is the difference in temperature and pressure occurring at various heights and in different air masses. Refraction occurs at all frequencies, but below 30 MHz the effect is small as compared with ionospheric effects, diffraction, and absorption. At higher frequencies, refraction in the lower layer of the atmosphere extends the radio horizon to a distance about 15 percent greater than the visible horizon. The effect is the same as if the radius of the earth were about one-third greater than it is and there were no refraction. Sometimes the lower portion of the atmosphere becomes stratified. This stratification results in nonstandard temperature and moisture changes with height. If there is a marked temperature inversion or a sharp decrease in water vapor content with increased height, a horizontal radio duct may be formed. High frequency radio waves traveling horizontally within the duct are refracted to such an extent that they remain within the duct, following the curvature of the earth for phenomenal distances. This is called super-refraction. Maximum results are obtained when both transmitting and receiving antennas are within the duct. There is a lower limit to the frequency affected by ducts. It varies from about 200 MHz to more than 1,000 MHz. At night, surface ducts may occur over land due to cooling of the surface. At sea, surface ducts about 50 feet thick may occur at any time in the trade wind belt. Surface ducts 100 feet or more in thickness may extend from land out to sea when warm air from the land flows over the cooler ocean surface. Elevated ducts from a few feet to more than 1,000 feet in thickness may occur at elevations of 1,000 to 5,000 feet, due to the settling of a large air mass. This is a frequent occurrence in Southern California and certain areas of the Pacific Ocean. A bending in the horizontal plane occurs when a groundwave crosses a coast at an oblique angle. This is due to a marked difference in the conducting and reflecting properties of the land and water over which the wave travels. The effect is known as coastal refraction or land effect. 1006. The Ionosphere Since an atom normally has an equal number of negatively charged electrons and positively charged protons, it is electrically neutral. An ion is an atom or group of atoms which has become electrically charged, either positively or negatively, by the loss or gain of one or more electrons. Loss of electrons may occur in a variety of ways. In the atmosphere, ions are usually formed by collision of atoms with rapidly moving particles, or by the action of cosmic rays or ultraviolet light. In the lower portion of the atmosphere, recombination soon occurs, leaving a small percentage of ions. In thin atmosphere far above the surface of the earth, however, atoms are widely separated and a large number of ions may be present. The region of numerous positive and negative ions and unattached electrons is called the ionosphere. The extent of ionization dependsupon the kinds of atoms present in the atmosphere, the density of the atmosphere, and the position relative to the sun (time of day and season). After sunset, ions and electronsrecombine faster than they are separated, decreasing the ionization of the atmosphere. An electron can be separated from its atom only by the application of greater energy than that holding the electron. Since the energy of the electron depends primarily upon the kind of an atom of which it is a part, and its position relative to the nucleus of that atom, different kinds of radiation may cause ionization of different substances. In the outermost regions of the atmosphere, the density is so low that oxygen exists largely as separate atoms, rather than combining as molecules as it does nearer the surface of the earth. At great heights the energy level is low and ionization from solar radiation is intense. This is known as the F layer. Above this level the ionization decreases because of the lack of atoms to be ionized. Below this level it decreases because the ionizing agent of appropriate energy has already been absorbed. During daylight, two levels of maximum F ionization can be detected, the F2 layer at about 125 statute miles above the surface of the earth, and the F1 layer at about 90 statute miles. At night, these combine to form a single F layer. At a height of about 60 statute miles, the solar radiation not absorbed by the F layer encounters, for the first time, large numbers of oxygen molecules. A new maximum ionization occurs, known as the E layer. The height of this layer is quite constant, in contrast with the fluctuating F layer. At night the E layer becomes weaker by two orders of magnitude
168RADIOWAVESBelowtheElayer,aweak D layer forms at a heightofRefertoFigure1007a,in whicha single layerof theabout 45 statute miles, where the incoming radiation en-ionosphere is considered.RayI enters the ionosphere atcounters ozone for the first time.The D layer is thesuch an angle that its path is altered, but it passes throughprincipal source of absorption of HF waves, and of reflec-and proceeds outward into space. As the angle with the hor-tionof LFandVLFwaves duringdaylight.izontal decreases,a critical value is reached where ray 2 isbent or reflected back toward the earth.As the angle is still1007.Thelonosphere And RadioWavesfurther decreased, such as at 3, the return to earth occurs atagreaterdistancefromthetransmitter.When a radio wave encounters a particle having anA wavereachinga receiver by way of the ionosphereelectric charge,it causes thatparticleto vibrate.The vibrat-is called a skywave.This expression is also appropriatelying particle absorbs electromagnetic energy from the radioapplied to a wave reflected from an air mass boundary.Incommon usage,however,it isgenerallyassociated withthewave and radiates it. The net effect is a change of polariza-tion and an alterationofthepath of thewave.That portionionosphere.Thewave which travels along thesurface oftheof the wave in a more highly ionized region travels faster,earth is called a groundwave.At angles greater than thecritical angle, no skywave signal is received. Therefore,causing thewavefrontto tilt and the wave tobe directedto-wardaregion of less intenseionizationthere is a minimum distancefrom the transmitter at whichFigure1007a.Theeffect of theionosphereon radio waves.LAYERELAYERGnoutREFLECTONFigure1007b.Variouspaths by which a skywave signal might bereceived
168 RADIO WAVES Below the E layer, a weak D layer forms at a height of about 45 statute miles, where the incoming radiation encounters ozone for the first time. The D layer is the principal source of absorption of HF waves, and of reflection of LF and VLF waves during daylight. 1007. The Ionosphere And Radio Waves When a radio wave encounters a particle having an electric charge, it causes that particle to vibrate. The vibrating particle absorbs electromagnetic energy from the radio wave and radiates it. The net effect is a change of polarization and an alteration of the path of the wave. That portion of the wave in a more highly ionized region travels faster, causing the wave front to tilt and the wave to be directed toward a region of less intense ionization. Refer to Figure 1007a, in which a single layer of the ionosphere is considered. Ray 1 enters the ionosphere at such an angle that its path is altered, but it passes through and proceeds outward into space. As the angle with the horizontal decreases, a critical value is reached where ray 2 is bent or reflected back toward the earth. As the angle is still further decreased, such as at 3, the return to earth occurs at a greater distance from the transmitter. A wave reaching a receiver by way of the ionosphere is called a skywave. This expression is also appropriately applied to a wave reflected from an air mass boundary. In common usage, however, it is generally associated with the ionosphere. The wave which travels along the surface of the earth is called a groundwave. At angles greater than the critical angle, no skywave signal is received. Therefore, there is a minimum distance from the transmitter at which Figure 1007a. The effect of the ionosphere on radio waves. Figure 1007b. Various paths by which a skywave signal might be received
RADIOWAVES169skywaves can be received.This is called the skip distance,polarization error a maximum.This is called night effectshown inFigure 1007a.If thegroundwave extends outforlessdistance than theskip distance,a skipzone occurs, in1008.Diffractionwhich no signal is received.The critical radiation angle depends upon the intensityWhenaradio waveencounters an obstacle,its energyisofionization,and thefrequencyoftheradiowave.Asthefrereflected or absorbed,causing a shadowbeyond theobsta-cle.However, some energy does enter the shadow areaquency increases, the angle becomes smaller.At frequenciesgreater than about30 MHzvirtually all of theenergy pene-because of diffraction.This is explained byHuygens"prin-trates through or is absorbed by the ionosphere.Therefore, atciple,which states thateverypoint on the surfaceofawaveany given receiverthere is a maximum usablefrequency iffrontisasourceofradiation,transmittingenergyinalldirecskywaves are to be utilized. The strongest signals are retions ahead of the wave.No noticeable effect of thisceived at or slightly below this frequency.There is also aprinciple is observed until the wavefront encounters an ob-lowerpracticalfrequencybevondwhichsignalsaretooweakstacle,whichintercepts aportion ofthewave.From theedgetobeofvalue.Withinthisbandtheoptimumfrequency canof the obstacle,energy isradiated into the shadowarea,andalso outsideofthe area.The latter interacts with energyfrombeselectedtogivebestresults.Itcannotbetoonearthemaximum usable frequencybecausethis frequencyfluctuatesother parts of thewavefront,producingalternatebands inwith changes of intensity within the ionosphere.During mag-whichthesecondaryradiationreinforcesortendstocanceneticstormstheionospheredensitydecreases.Themaximumtheenergy oftheprimaryradiation.Thus,thepractical effectusablefrequency decreases,andthe lower usablefrequencyof an obstacle is a greatly reduced signal strength in theshadowarea,andadisturbedpatternfora shortdistanceout-increases.ThebandofusablefrequenciesisthusnarrowedUnderextremeconditionsitmaybecompletelyeliminatedsidetheshadowarea.ThisisillustratedinFigure1008isolating the receiver and causing a radio blackoutTheamountof diffractionisinverselyproportionaltothefrequency,being greatest at very lowfrequencies.Skywavesignals reachinga given receivermayarrivebyanyof several paths,asshown inFigure1007b.Asignal1009.AbsorptionAnd Scatteringwhich undergoes a single reflection is called a“one-hop”signal, one which undergoes two reflections with a groundreflection between is called a“two-hop”signal, etc. AThe amplitude of a radio wave expanding outward"multihop"signal undergoes several reflections.The layerthrough space varies inversely with distance, weakeningatwhichthereflection occurs is usually indicated,also,aswith increased distance.The decrease of strength with dis-"one-hopE,"“two-hopF,"etc.tance is called attenuation. Under certain conditions theBecause of thedifferentpaths andphasechangesoc-attenuation is greater than in free space.A wave traveling along the surface of the earth loses acurring at each reflection, the various signals arriving at areceiver havedifferentphase relationships.Sincethedensi-certain amount of energyto the earth.The wave is diffract-ed downward and absorbed by the earth.As a result ofthistyof the ionosphere is continuallyfluctuating,the strengthand phaserelationships of thevarious signals may undergoabsorption,theremainderofthewavefronttiltsdownwardanalmostcontinuous change.Thus,thevarious signalsmayresultinginfurtherabsorptionbytheearth.Attenuationisreinforceeachotheratonemomentandcanceleachothergreaterover a surface which is a poor conductor.Relativelyatthe next,resulting influctuationsofthestrengthoftheto-littleabsorption occurs over seawater,which isanexcellenttal signal received.This is called fading.This phenomenonconductor at lowfrequencies,and low frequencyground-mayalsobecausedbyinteractionofcomponentswithinawaves travel great distances over water.single reflected wave, or changes in its strength due toAskywave suffers anattenuation loss inits encounterchangesinthereflectingsurface.Ionosphericchangesarewith the ionosphere.The amount depends upon the heightassociatedwithfluctuations intheradiationreceivedfromand compositionof the ionosphereas well as thefrequencythe sun, since this is the principal cause of ionization, Sigoftheradiowave.Maximumionosphericabsorptionoccursnals fromtheFlayerareparticularly erraticbecauseof theatabout1.400kHz.rapidlyfluctuating conditions within the layer itselfIngeneral,atmosphericabsorption increases withfre-Themaximum distanceatwhicha one-hopE signalcan bequency. It is a problem only in the SHF and EHF frequencyreceived is about 1,400 miles.At thisdistance the signal leavesrange.At thesefrequencies,attenuation is further increasedthetransmitterin approximatelya horizontal direction.Aone-by scattering duetoreflectionby oxygen,watervapor,wa-hopFsignal canbereceivedouttoabout2,500miles.Atlowterdroplets,andrainintheatmospherefrequencies groundwaves extend outforgreat distances1010.NoiseA skywave may undergo a change of polarization duringreflectionfromthe ionosphere,accompaniedbyan alterationin the direction of travel of the wave.This is called polariza-Unwanted signals ina receiver are called interferenceThe intentional productionof such interference toobstructtion error.Near sunrise and sunset, when rapid changes areoccurringintheionosphere,receptionmaybecomeerraticandcommunication is called jamming.Unintentional interfer-
RADIO WAVES 169 skywaves can be received. This is called the skip distance, shown in Figure 1007a. If the groundwave extends out for less distance than the skip distance, a skip zone occurs, in which no signal is received. The critical radiation angle depends upon the intensity of ionization, and the frequency of the radio wave. As the frequency increases, the angle becomes smaller. At frequencies greater than about 30 MHz virtually all of the energy penetrates through or is absorbed by the ionosphere. Therefore, at any given receiver there is a maximum usable frequency if skywaves are to be utilized. The strongest signals are received at or slightly below this frequency. There is also a lower practical frequency beyond which signals are too weak to be of value. Within this band the optimum frequency can be selected to give best results. It cannot be too near the maximum usable frequency because this frequency fluctuates with changes of intensity within the ionosphere. During magnetic storms the ionosphere density decreases. The maximum usable frequency decreases, and the lower usable frequency increases. The band of usable frequencies is thus narrowed. Under extreme conditions it may be completely eliminated, isolating the receiver and causing a radio blackout. Skywave signals reaching a given receiver may arrive by any of several paths, as shown in Figure 1007b. A signal which undergoes a single reflection is called a “one-hop” signal, one which undergoes two reflections with a ground reflection between is called a “two-hop” signal, etc. A “multihop” signal undergoes several reflections. The layer at which the reflection occurs is usually indicated, also, as “one-hop E,” “two-hop F,” etc. Because of the different paths and phase changes occurring at each reflection, the various signals arriving at a receiver have different phase relationships. Since the density of the ionosphere is continually fluctuating, the strength and phase relationships of the various signals may undergo an almost continuous change. Thus, the various signals may reinforce each other at one moment and cancel each other at the next, resulting in fluctuations of the strength of the total signal received. This is called fading. This phenomenon may also be caused by interaction of components within a single reflected wave, or changes in its strength due to changes in the reflecting surface. Ionospheric changes are associated with fluctuations in the radiation received from the sun, since this is the principal cause of ionization. Signals from the F layer are particularly erratic because of the rapidly fluctuating conditions within the layer itself. The maximum distance at which a one-hop E signal can be received is about 1,400 miles. At this distance the signal leaves the transmitter in approximately a horizontal direction. A onehop F signal can be received out to about 2,500 miles. At low frequencies groundwaves extend out for great distances. A skywave may undergo a change of polarization during reflection from the ionosphere, accompanied by an alteration in the direction of travel of the wave. This is called polarization error. Near sunrise and sunset, when rapid changes are occurring in the ionosphere, reception may become erratic and polarization error a maximum. This is called night effect. 1008. Diffraction When a radio wave encounters an obstacle, its energy is reflected or absorbed, causing a shadow beyond the obstacle. However, some energy does enter the shadow area because of diffraction. This is explained by Huygens’ principle, which states that every point on the surface of a wave front is a source of radiation, transmitting energy in all directions ahead of the wave. No noticeable effect of this principle is observed until the wave front encounters an obstacle, which intercepts a portion of the wave. From the edge of the obstacle, energy is radiated into the shadow area, and also outside of the area. The latter interacts with energy from other parts of the wave front, producing alternate bands in which the secondary radiation reinforces or tends to cancel the energy of the primary radiation. Thus, the practical effect of an obstacle is a greatly reduced signal strength in the shadow area, and a disturbed pattern for a short distance outside the shadow area. This is illustrated in Figure 1008. The amount of diffraction is inversely proportional to the frequency, being greatest at very low frequencies. 1009. Absorption And Scattering The amplitude of a radio wave expanding outward through space varies inversely with distance, weakening with increased distance. The decrease of strength with distance is called attenuation. Under certain conditions the attenuation is greater than in free space. A wave traveling along the surface of the earth loses a certain amount of energy to the earth. The wave is diffracted downward and absorbed by the earth. As a result of this absorption, the remainder of the wave front tilts downward, resulting in further absorption by the earth. Attenuation is greater over a surface which is a poor conductor. Relatively little absorption occurs over sea water, which is an excellent conductor at low frequencies, and low frequency groundwaves travel great distances over water. A skywave suffers an attenuation loss in its encounter with the ionosphere. The amount depends upon the height and composition of the ionosphere as well as the frequency of the radio wave. Maximum ionospheric absorption occurs at about 1,400 kHz. In general, atmospheric absorption increases with frequency. It is a problem only in the SHF and EHF frequency range. At these frequencies, attenuation is further increased by scattering due to reflection by oxygen, water vapor, water droplets, and rain in the atmosphere. 1010. Noise Unwanted signals in a receiver are called interference. The intentional production of such interference to obstruct communication is called jamming. Unintentional interfer-