《航海学》课程参考文献（地文资料）CHAPTER 10 RADIO WAVES

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 cur￾rent 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 be￾tween a conductor and a magnetic field, such as is the case in a rotating machine called a generator, then the resulting cur￾rent changes direction in the conductor as the polarity of the electromotive force changes with the rotation of the genera￾tor’s rotor. This is known as alternating current. The energy of the current flowing through the conduc￾tor 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 orienta￾tion 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 great￾ly 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, propor￾tional 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 “detach￾es” 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 conduc￾tor is directly proportional to the magnitude of the current flowing through the conductor. Recall the discussion of al￾ternating 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 revolu￾tion, and falls to zero when the rotor completes one half of its revolution. The current then approaches a negative max￾imum; 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 am￾plitude. The forward side of any wave is called the wave front. For a nondirectional antenna, each wave proceeds out￾ward 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 sec￾ond. 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*4.8x1015 MHz* Values approximate.Table 1002.Electromagnetic spectrum

166 RADIO WAVES is stated in circular measure, a complete cycle being consid￾ered 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 frequen￾cies is called the electromagnetic spectrum. The frequency range suitable for radio transmission, the radio spectrum, extends from 10 kilohertz to 300,000 mega￾hertz. 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, X￾rays, 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 “verti￾cally polarized,” and if horizontal, “horizontally polarized.” A wave traveling through space may be polarized in any di￾rection. 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 sur￾face, and divergence of the reflected ray. Lower frequency results in greater penetration. At very low frequencies, us￾able 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 re￾flection) 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 in￾teraction of waves is called wave interference. A phase difference may occur because of the change of phase of a re￾flected 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 dif￾ference 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 fre￾quencies, but becomes more prevalent as frequency increases. In radio communication, it can be reduced by us￾ing 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, par￾ticularly 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, refrac￾tion in the lower layer of the atmosphere extends the radio horizon to a distance about 15 percent greater than the vis￾ible 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 be￾comes 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 hor￾izontally 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-re￾fraction. 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 cool￾er 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 prop￾erties 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 nega￾tively 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 atmo￾sphere, 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 numer￾ous positive and negative ions and unattached electrons is called the ionosphere. The extent of ionization depend￾supon 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 elec￾tronsrecombine 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 ion￾ization 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 de￾creases 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 en￾counters ozone for the first time. The D layer is the principal source of absorption of HF waves, and of reflec￾tion 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 vibrat￾ing particle absorbs electromagnetic energy from the radio wave and radiates it. The net effect is a change of polariza￾tion 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 to￾ward 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 hor￾izontal 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 fre￾quency increases, the angle becomes smaller. At frequencies greater than about 30 MHz virtually all of the energy pene￾trates 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 re￾ceived 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 max￾imum usable frequency because this frequency fluctuates with changes of intensity within the ionosphere. During mag￾netic 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 oc￾curring at each reflection, the various signals arriving at a receiver have different phase relationships. Since the densi￾ty 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 to￾tal 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. Sig￾nals 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 one￾hop 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 polariza￾tion 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 obsta￾cle. However, some energy does enter the shadow area because of diffraction. This is explained by Huygens’ prin￾ciple, which states that every point on the surface of a wave front is a source of radiation, transmitting energy in all direc￾tions ahead of the wave. No noticeable effect of this principle is observed until the wave front encounters an ob￾stacle, 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 out￾side 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 dis￾tance 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 diffract￾ed 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 ground￾waves 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 fre￾quency. 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, wa￾ter 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-

170RADIOWAVESOBSTACLETRANSMITTERFigure1008.Diffractionence is called noisedischarges into the atmosphere.Under suitable conditionsthisbecomesvisibleandisknownasSt.Elmosfire,whichNoise may originate within the receiver.Hum is usual-is sometimes seen at mastheads,theends ofyardarms,etclytheresult ofinductionfrom neighboring circuits carryingalternating current.Irregular crackling or sizzling soundsAtmospheric noiseoccursto someextentatall fre-maybecausedbypoorcontacts orfaultycomponents with-quencies but decreases with higher frequencies. Aboveinthe receiver.Stray currents innormal components causesabout 30 MHz it is not generally a problem.some noise.This source sets the ultimate limit ofsensitivitythat can be achieved in a receiver. It is the same at any1011.AntennaCharacteristicsfrequency.Noise originating outside the receiver may be eitherAntenna design and orientation have a marked effectman-made or natural.Man-made noises originate in electri-upon radio wave propagation.Fora single-wireantenna,cal appliances,motorand generator brushes,ignitionstrongest signals aretransmitted along theperpendiculartosystems,andother sources of sparks whichtransmitelectro-the wire,and virtually no signal in thedirection of the wiremagnetic signals thatare picked upbythe receiving antenna.For a vertical antenna, the signal strength is the same in allhorizontal directions.Unless thepolarization undergoes aNatural noise is caused principally by discharge of stat-change during transit, the strongest signal received from aic electricity in the atmosphere.This is called atmosphericvertical transmitting antenna occurs when thereceiving an-noise, atmospherics, or static.An extreme example is atennaisalsovertical.thunderstorm. An exposed surface may acquire a consider-For lower frequencies the radiation of a radio signalable charge of static electricity.This may be caused byfriction ofwater orsolidparticlesblown againstoralongtakes placeby interaction between the antenna and thesucha surface.Itmayalsobe causedbysplitting ofa waterground.For a vertical antenna, efficiency increases withdroplet which strikes the surface, one part ofthe droplet re-greater length of the antenna.For a horizontal antenna, ef-quiring a positive charge and the other a negative chargeficiency increases with greater distance between antennaThese charges maybe transferred to the surface.The chargeand ground. Near-maximum efficiency is attained whentends to gather at points and ridges of the conducting sur-this distance is one-half wavelength. This is the reason forface, and when it accumulates to a sufficient extent toelevating low frequency antennas to great heights. Howev-overcomethe insulatingpropertiesof theatmosphere,iter, at the lowest frequencies, the required height becomes

170 RADIO WAVES ence is called noise. Noise may originate within the receiver. Hum is usual￾ly the result of induction from neighboring circuits carrying alternating current. Irregular crackling or sizzling sounds may be caused by poor contacts or faulty components with￾in the receiver. Stray currents in normal components causes some noise. This source sets the ultimate limit of sensitivity that can be achieved in a receiver. It is the same at any frequency. Noise originating outside the receiver may be either man-made or natural. Man-made noises originate in electri￾cal appliances, motor and generator brushes, ignition systems, and other sources of sparks which transmit electro￾magnetic signals that are picked up by the receiving antenna. Natural noise is caused principally by discharge of stat￾ic electricity in the atmosphere. This is called atmospheric noise, atmospherics, or static. An extreme example is a thunderstorm. An exposed surface may acquire a consider￾able charge of static electricity. This may be caused by friction of water or solid particles blown against or along such a surface. It may also be caused by splitting of a water droplet which strikes the surface, one part of the droplet re￾quiring a positive charge and the other a negative charge. These charges may be transferred to the surface. The charge tends to gather at points and ridges of the conducting sur￾face, and when it accumulates to a sufficient extent to overcome the insulating properties of the atmosphere, it discharges into the atmosphere. Under suitable conditions this becomes visible and is known as St. Elmo’s fire, which is sometimes seen at mastheads, the ends of yardarms, etc. Atmospheric noise occurs to some extent at all fre￾quencies but decreases with higher frequencies. Above about 30 MHz it is not generally a problem. 1011. Antenna Characteristics Antenna design and orientation have a marked effect upon radio wave propagation. For a single-wire antenna, strongest signals are transmitted along the perpendicular to the wire, and virtually no signal in the direction of the wire. For a vertical antenna, the signal strength is the same in all horizontal directions. Unless the polarization undergoes a change during transit, the strongest signal received from a vertical transmitting antenna occurs when the receiving an￾tenna is also vertical. For lower frequencies the radiation of a radio signal takes place by interaction between the antenna and the ground. For a vertical antenna, efficiency increases with greater length of the antenna. For a horizontal antenna, ef￾ficiency increases with greater distance between antenna and ground. Near-maximum efficiency is attained when this distance is one-half wavelength. This is the reason for elevating low frequency antennas to great heights. Howev￾er, at the lowest frequencies, the required height becomes Figure 1008. Diffraction

171RADIOWAVESprohibitively great. At 10 kHz it would be about 8 nauticalseparation of the stable groundwave pulse from the variablemiles for a half-wavelength antenna.Therefore, lowerfre-skywavepulseupto1,500km,and upto2,000kmforover-water paths.Thefrequencyfor Loran C is in the LF band.Thisquency antennas are inherently inefficient.This is partlyband is also useful for radio direction finding and timeoffsetbythegreaterrangeofalowfrequencysignalofthesametransmittedpoweras oneof higherfrequencydisseminationAthigherfrequencies,thegroundisnotused.bothcon-Medium Frequency (MF, 300 to 3,000 kHz): Ground-ducting portions being included in a dipole antenna. Notwavesprovidedependableservice,buttherangeforagivenonly can such an antenna bemade efficient, but it can al-power is reduced greatly.This range varies from about 400sobe made sharplydirective,thusgreatlyincreasingthemiles atthelowerportion of theband toabout15miles atstrength of the signal transmitted in a desired directionthe upper end for a transmitted signal of 1kilowatt.TheseThe power received is inversely proportional to thevaluesare influenced, however,bythepower of thetranssquareofthedistancefromthetransmitter,assumingtheremitter, the directivity and efficiency of the antenna,and theis noattenuationduetoabsorption or scatteringnature of the terrain over which signalstravel.Elevating theantennatoobtaindirectwavesmayimprovethetransmis-1012.Rangesion. At the lower frequencies of the band, skywaves areavailable both day and night. As the frequency is increased.ionospheric absorption increasesto a maximum ataboutThe range at which a usable signal is received depends1,400kHz.Athigherfrequencies the absorption decreasesupon thepower transmitted,the sensitivityofthereceiverpermitting increased use of skywaves.Since the ionospherefrequency,route of travel,noise level, and perhaps otherchanges with the hour, season, and sunspot cycle,the reli-factors.For the sametransmitted power, both theground-ability of skywave signals is variable.By careful selectionwave and skywave ranges aregreatest at thelowestoffrequency,rangesofasmuchas8,000mileswith1kilo-frequencies,butthis is somewhat offsetbythe lesser effiwatt of transmitted power are possible,using multihopciency of antennas for these frequencies.At highersignals. However, the frequency selection is critical. If it isfrequencies, onlydirect waves are useful,and the effectivetoo high, the signals penetrate the ionosphere and arelost inrangeisgreatlyreduced.Attenuation,skipdistance,groundspace.Ifit is too low, signals are too weak. In general, sky-reflection,waveinterference,condition oftheionospherewavereception is equallygood byday ornight, but loweratmospheric noise level, and antenna design all affect thefrequencies are needed at night.The standard broadcastdistance at which useful signals can be received.bandforcommercialstations(535to1,605kHz)isintheMFband.1013.RadioWavePropagationHigh Frequency (HF, 3 to 30 MHz): As with higher me-Frequency is an importantconsideration in radio wavedium frequencies, the groundwave range of HF signals ispropagation.Thefollowing summary indicates the principal ef-limitedtoafewmiles,buttheelevationoftheantennamay in-crease the direct-wave distance of transmission.Also, thefects associatedwith thevariousfrequencybands,startingwithheight ofthe antenna does have an important effect upon sky-the lowest and progressing tothe highest usableradiofrequencywavetransmission becausethe antenna hasan“image"withinVeryLowFrequency(VLF.10to30kHz):TheVLFtheconductingearth.Thedistancebetween antenna and imagesignals propagate between the bounds ofthe ionosphere andisrelatedtotheheightoftheantenna.andthisdistanceisasthe earth and are thus guided around the curvature of thecritical asthedistancebetweenelements ofanantennasystemearthtogreatdistanceswithlowattenuationandexcellentMaximum usable frequencies fall generally within the HFstability.Diffraction is maximum.Becauseof the longband.Byday this maybe10to30MHz,but during thenightwavelength, large antennas are needed, and even these areit maydrop to8to 10 MHz.The HF band is widelyused forinefficientpermittingradiationofrelativelysmallamountsship-to-ship and ship-to-shore communication.of power.Magnetic stormshavelittleeffectupontransmis-sion because of the efficiency of the“earth-ionosphereVeryHighFrequency(VHF.30to300MHz):Com-waveguide."During such storms,VLF signals may consti-munication is limited primarily to the direct wave, or thetute the only source of radio communication over greatdirect wave plus a ground-reflected wave.Elevating the an-distances.However,interferencefrom atmospheric noisetenna to increase thedistanceat whichdirect waves can bemaybetroublesome.Signals maybereceived from belowused results in increased distance ofreception,even thoughthe surface of the sea.some wave interference between direct and ground-reflect-LowFrequency (LF,30to300kHz):As frequencyis ined waves ispresent.Diffraction ismuchlessthan withcreased to theLF band and diffraction decreases, there islowerfrequencies,but itis mostevidentwhen signals crossgreater attenuation with distance,andrangefor agiven powersharpmountain peaksorridges.Under suitableconditionsoutputfalls off rapidly.However,this ispartly offsetbymorereflectionsfromtheionospherearesufficientlystrongtobeefficienttransmitting antennas.LF signals aremost stableuseful, but generally they are unavailable.There is relativewithingroundwave distanceofthetransmitter.Awiderband-ly little interferencefrom atmosphericnoise inthis bandwidth permits pulsed signals at 100 kHz. This allowsReasonablyefficientdirectional antennas arepossiblewith

RADIO WAVES 171 prohibitively great. At 10 kHz it would be about 8 nautical miles for a half-wavelength antenna. Therefore, lower fre￾quency antennas are inherently inefficient. This is partly offset by the greater range of a low frequency signal of the same transmitted power as one of higher frequency. At higher frequencies, the ground is not used, both con￾ducting portions being included in a dipole antenna. Not only can such an antenna be made efficient, but it can al￾sobe made sharply directive, thus greatly increasing the strength of the signal transmitted in a desired direction. The power received is inversely proportional to the square of the distance from the transmitter, assuming there is no attenuation due to absorption or scattering. 1012. Range The range at which a usable signal is received depends upon the power transmitted, the sensitivity of the receiver, frequency, route of travel, noise level, and perhaps other factors. For the same transmitted power, both the ground￾wave and skywave ranges are greatest at the lowest frequencies, but this is somewhat offset by the lesser effi￾ciency of antennas for these frequencies. At higher frequencies, only direct waves are useful, and the effective range is greatly reduced. Attenuation, skip distance, ground reflection, wave interference, condition of the ionosphere, atmospheric noise level, and antenna design all affect the distance at which useful signals can be received. 1013. Radio Wave Propagation Frequency is an important consideration in radio wave propagation. The following summary indicates the principal ef￾fects associated with the various frequency bands, starting with the lowest and progressing to the highest usable radio frequency. Very Low Frequency (VLF, 10 to 30 kHz): The VLF signals propagate between the bounds of the ionosphere and the earth and are thus guided around the curvature of the earth to great distances with low attenuation and excellent stability. Diffraction is maximum. Because of the long wavelength, large antennas are needed, and even these are inefficient, permitting radiation of relatively small amounts of power. Magnetic storms have little effect upon transmis￾sion because of the efficiency of the “earth-ionosphere waveguide.” During such storms, VLF signals may consti￾tute the only source of radio communication over great distances. However, interference from atmospheric noise may be troublesome. Signals may be received from below the surface of the sea. Low Frequency (LF, 30 to 300 kHz): As frequency is in￾creased to the LF band and diffraction decreases, there is greater attenuation with distance, and range for a given power output falls off rapidly. However, this is partly offset by more efficient transmitting antennas. LF signals are most stable within groundwave distance of the transmitter. A wider band￾width permits pulsed signals at 100 kHz. This allows separation of the stable groundwave pulse from the variable skywave pulse up to 1,500 km, and up to 2,000 km for over￾water paths. The frequency for Loran C is in the LF band. This band is also useful for radio direction finding and time dissemination. Medium Frequency (MF, 300 to 3,000 kHz): Ground￾waves provide dependable service, but the range for a given power is reduced greatly. This range varies from about 400 miles at the lower portion of the band to about 15 miles at the upper end for a transmitted signal of 1 kilowatt. These values are influenced, however, by the power of the trans￾mitter, the directivity and efficiency of the antenna, and the nature of the terrain over which signals travel. Elevating the antenna to obtain direct waves may improve the transmis￾sion. At the lower frequencies of the band, skywaves are available both day and night. As the frequency is increased, ionospheric absorption increases to a maximum at about 1,400 kHz. At higher frequencies the absorption decreases, permitting increased use of skywaves. Since the ionosphere changes with the hour, season, and sunspot cycle, the reli￾ability of skywave signals is variable. By careful selection of frequency, ranges of as much as 8,000 miles with 1 kilo￾watt of transmitted power are possible, using multihop signals. However, the frequency selection is critical. If it is too high, the signals penetrate the ionosphere and are lost in space. If it is too low, signals are too weak. In general, sky￾wave reception is equally good by day or night, but lower frequencies are needed at night. The standard broadcast band for commercial stations (535 to 1,605 kHz) is in the MF band. High Frequency (HF, 3 to 30 MHz): As with higher me￾dium frequencies, the groundwave range of HF signals is limited to a few miles, but the elevation of the antenna may in￾crease the direct-wave distance of transmission. Also, the height of the antenna does have an important effect upon sky￾wave transmission because the antenna has an “image” within the conducting earth. The distance between antenna and image is related to the height of the antenna, and this distance is as critical as the distance between elements of an antenna system. Maximum usable frequencies fall generally within the HF band. By day this may be 10 to 30 MHz, but during the night it may drop to 8 to 10 MHz. The HF band is widely used for ship-to-ship and ship-to-shore communication. Very High Frequency (VHF, 30 to 300 MHz): Com￾munication is limited primarily to the direct wave, or the direct wave plus a ground-reflected wave. Elevating the an￾tenna to increase the distance at which direct waves can be used results in increased distance of reception, even though some wave interference between direct and ground-reflect￾ed waves is present. Diffraction is much less than with lower frequencies, but it is most evident when signals cross sharp mountain peaks or ridges. Under suitable conditions, reflections from the ionosphere are sufficiently strong to be useful, but generally they are unavailable. There is relative￾ly little interference from atmospheric noise in this band. Reasonably efficient directional antennas are possible with

172RADIOWAVESVHF.TheVHFband ismuchused for communication.1015.TypesOf RadioTransmissionUltraHighFrequency(UHF,300to3.000MHz):Aseriesofwavestransmittedat constantfrequencyandSkywavesarenotused intheUHFbandbecausetheiono-sphere is not sufficiently dense to reflect the waves, whichamplitudeiscalledacontinuouswave(CW).Thiscannotbeheard except at thevery lowest radio frequencies,when itpass through it into space.Groundwaves and ground-re-flected waves are used, although there is some wavemay produce, in a receiver, an audible hum of high pitchinterference.Diffraction is negligible, but the radio horizonAlthough a continuous wavemay be used directly,asextends about15percent beyond the visiblehorizon, dueinradiodirectionfinding or Decca,it is more commonlyprincipallyto refraction.Reception ofUHF signals is virtu-modified in some manner.This is called modulationWhen this occurs,the continuous wave serves as a carrierallvfreefromfadingandinterferencebyatmosphericnoise.Sharply directiveantennas can beproduced for transmis-wavefor information.Anyof several typesof modulationsion in this band, which is widely used for ship-to-ship andmay be usedship-to-shorecommunicationIn amplitude modulation (AM) the amplitude of theSuperHighFrequency (SHF,3.000 to30.000MHz)carrier wave is altered in accordance with theamplitudeofIntheSHFband,alsoknown asthemicrowaveorastheamodulating wave,usuallyofaudiofrequency,as shown inFigure 1015a.In the receiver the signal is demodulated bycentimeterwaveband.therearenoskywaves.transmissionbeing entirely by direct and ground-reflected waves.Dif-removingthemodulatingwaveand converting itbackto itsoriginal form.This form of modulation is widely used infraction and interferenceby atmospheric noise arevirtuallynonexistent.Highly efficient, sharply directive antennasvoiceradio,as in the standard broadcastband of commer-cial broadcasting.can be produced.Thus,transmission in this band is similarto that ofUHF,but with the effects ofshorter waves beingIf the frequency instead of the amplitude is altered ingreater.Reflectionbyclouds,waterdroplets,dustparticles,accordancewiththeamplitudeoftheimpressedsignal.asetc.,increases, causing greater scattering,increased waveshown in Figure 1015a, frequency modulation (FM)oc-interference,and fading.TheSHF band is used for marinecurs.This is used for commercial FM radio broadcasts andnavigationalradar.the sound portion oftelevision broadcasts.Extremely HighFrequency (EHF,30,000 to300,000Pulse modulation (PM) is somewhat different, thereMHz):Theeffects of shorterwavesaremorepronounced inbeing no impressed modulating wave.Inthisform oftransthe EHF band, transmission being freefrom wave interfer-mission, very short bursts of carrier wave are transmitted,separated by relatively long periods of “silence," duringence,diffraction,fading,and interferencebyatmosphericnoise.Only direct and ground-reflected waves are avail-which there is notransmission.This type of transmission.illustrated inFigure 1015b, is used in some common radioable.Scattering and absorption in theatmosphere arenavigational aids,including radar and Loran-C.pronounced and mayproducean upper limittothefrequen-cyuseful in radio communication.1016.Transmitters1014.RegulationOf FrequencyUseAradio transmitter consists essentially of(1)a powersupply to furnish direct current, (2) an oscillator to convertWhile thecharacteristics ofvarious frequencies are im-direct current into radio-frequency oscillations (the carrierportant to the selection of the most suitable one for anywave), (3)a device to control the generated signal, and (4)given purpose,these arenot the only considerations.Con-an amplifier to increase the output of the oscillator.Forfusion and extensive interference would result if everysometransmittersamicrophoneisneededwithamodulatouserhad complete freedom of selection.Some form ofreg-and final amplifier to modulate the carrier wave.In addi-ulation is needed.The allocation of various frequencytion, an antenna and ground (for lowerfrequencies)arebands to particular uses is a matter of international agree-needed to produce electromagnetic radiation.These com-ment.WithintheUnitedStates,theFederalponents are illustrated diagrammatically in Figure 1016.CommunicationsCommissionhasresponsibilityforautho-rizinguseof particularfrequencies.Insomecasesagiven1017.Receiversfrequencyisallocatedtoseveral widelyseparatedtransmit-tersbutonlyunderconditionswhichminimizeWhen a radio wave passes a conductor, a current is in-interference, such as during daylight hours.Interference be-tween stations is further reduced by the use of channels,duced in that conductor.Aradio receiver is a device whicheachofanarrowband offrequencies.Assignedfrequenciessenses the power thus generated in an antenna, and transareseparatedbyanarbitrarybandof frequenciesthatareforms it into usableform.It is ableto select signals ofanot authorized for use. In the case of radio aids to naviga-singlefrequency(actually a narrow band of frequencies)tionand shipcommunications bands ofseveral channels arefromamongthemanywhichmayreachthereceivingantenna.The receiver is able to demodulate the signal andallocated,permitting selectionof band and channelbytheuser.provideadequate amplification.The output ofa receiver

172 RADIO WAVES VHF. The VHF band is much used for communication. Ultra High Frequency (UHF, 300 to 3,000 MHz): Skywaves are not used in the UHF band because the iono￾sphere is not sufficiently dense to reflect the waves, which pass through it into space. Groundwaves and ground-re￾flected waves are used, although there is some wave interference. Diffraction is negligible, but the radio horizon extends about 15 percent beyond the visible horizon, due principally to refraction. Reception of UHF signals is virtu￾ally free from fading and interference by atmospheric noise. Sharply directive antennas can be produced for transmis￾sion in this band, which is widely used for ship-to-ship and ship-to-shore communication. Super High Frequency (SHF, 3,000 to 30,000 MHz): In the SHF band, also known as the microwave or as the centimeter wave band, there are no skywaves, transmission being entirely by direct and ground-reflected waves. Dif￾fraction and interference by atmospheric noise are virtually nonexistent. Highly efficient, sharply directive antennas can be produced. Thus, transmission in this band is similar to that of UHF, but with the effects of shorter waves being greater. Reflection by clouds, water droplets, dust particles, etc., increases, causing greater scattering, increased wave interference, and fading. The SHF band is used for marine navigational radar. Extremely High Frequency (EHF, 30,000 to 300,000 MHz): The effects of shorter waves are more pronounced in the EHF band, transmission being free from wave interfer￾ence, diffraction, fading, and interference by atmospheric noise. Only direct and ground-reflected waves are avail￾able. Scattering and absorption in the atmosphere are pronounced and may produce an upper limit to the frequen￾cy useful in radio communication. 1014. Regulation Of Frequency Use While the characteristics of various frequencies are im￾portant to the selection of the most suitable one for any given purpose, these are not the only considerations. Con￾fusion and extensive interference would result if every userhad complete freedom of selection. Some form of reg￾ulation is needed. The allocation of various frequency bands to particular uses is a matter of international agree￾ment. Within the United States, the Federal Communications Commission has responsibility for autho￾rizing use of particular frequencies. In some cases a given frequency is allocated to several widely separated transmit￾ters, but only under conditions which minimize interference, such as during daylight hours. Interference be￾tween stations is further reduced by the use of channels, each of a narrow band of frequencies. Assigned frequencies are separated by an arbitrary band of frequencies that are not authorized for use. In the case of radio aids to naviga￾tion and ship communications bands of several channels are allocated, permitting selection of band and channel by the user. 1015. Types Of Radio Transmission A series of waves transmitted at constant frequency and amplitude is called a continuous wave (CW). This cannot be heard except at the very lowest radio frequencies, when it may produce, in a receiver, an audible hum of high pitch. Although a continuous wave may be used directly, as in radiodirection finding or Decca, it is more commonly modified in some manner. This is called modulation. When this occurs, the continuous wave serves as a carrier wave for information. Any of several types of modulation may be used. In amplitude modulation (AM) the amplitude of the carrier wave is altered in accordance with the amplitude of a modulating wave, usually of audio frequency, as shown in Figure 1015a. In the receiver the signal is demodulated by removing the modulating wave and converting it back to its original form. This form of modulation is widely used in voice radio, as in the standard broadcast band of commer￾cial broadcasting. If the frequency instead of the amplitude is altered in accordance with the amplitude of the impressed signal, as shown in Figure 1015a, frequency modulation (FM) oc￾curs. This is used for commercial FM radio broadcasts and the sound portion of television broadcasts. Pulse modulation (PM) is somewhat different, there being no impressed modulating wave. In this form of trans￾mission, very short bursts of carrier wave are transmitted, separated by relatively long periods of “silence,” during which there is no transmission. This type of transmission, illustrated in Figure 1015b, is used in some common radio navigational aids, including radar and Loran-C. 1016. Transmitters A radio transmitter consists essentially of (1) a power supply to furnish direct current, (2) an oscillator to convert direct current into radio-frequency oscillations (the carrier wave), (3) a device to control the generated signal, and (4) an amplifier to increase the output of the oscillator. For some transmitters a microphone is needed with a modulator and final amplifier to modulate the carrier wave. In addi￾tion, an antenna and ground (for lower frequencies) are needed to produce electromagnetic radiation. These com￾ponents are illustrated diagrammatically in Figure 1016. 1017. Receivers When a radio wave passes a conductor, a current is in￾duced in that conductor. A radio receiver is a device which senses the power thus generated in an antenna, and trans￾forms it into usable form. It is able to select signals of a single frequency (actually a narrow band of frequencies) from among the many which may reach the receiving an￾tenna. The receiver is able to demodulate the signal and provide adequate amplification. The output of a receiver

173RADIOWAVESCARRIERAMPLITUDE MODULATEDWAVEWAVECARRIERFREQUENCY MODULATEDWAVEWAVEFigure1o15a.Amplitudemodulation (upperfigure)andfrequencymodulation (lowerfigure)bythesamemodulatingwave.NOTRANSMISSIONNOTRANSMISSIONFigure1015b.Pulsemodulation.MREUNWOscilttaRFAntieAimpifihodulutsSupplFigure1016.Components of a radiotransmitter.maybepresentedaudiblybyearphonesorloudspeaker,orquency,(3)sensitivity,the ability to amplify a weak signalvisually on a dial, cathode-ray tube, counter, or other dis-tousable strengthagainst a background of noise;(4)stabil-play.Thus, the useful reception of radio signals requiresity,theabilityto resistdriftfrom conditionsorvaluestothree components: (1)an antenna, (2)a receiver,and(3)awhichset,and(5)fidelity,thecompletenesswithwhichthedisplay unit.essential characteristics of the original signal are repro-duced.Receivers may have additional features such as anRadio receivers differ mainly in(1)frequency range,automaticfrequency control,automatic noiselimiter,etc.the range of frequencies to which they can be tuned; (2) se-lectivity,theabilityto confine reception tosignals of theSome of these characteristics are interrelated.For in-desired frequency and avoid others of nearly the same fre-stance,ifareceiverlacksselectivity,signalsofafrequency

RADIO WAVES 173 may be presented audibly by earphones or loudspeaker; or visually on a dial, cathode-ray tube, counter, or other dis￾play. Thus, the useful reception of radio signals requires three components: (1) an antenna, (2) a receiver, and (3) a display unit. Radio receivers differ mainly in (1) frequency range, the range of frequencies to which they can be tuned; (2) se￾lectivity, the ability to confine reception to signals of the desired frequency and avoid others of nearly the same fre￾quency; (3) sensitivity, the ability to amplify a weak signal to usable strength against a background of noise; (4) stabil￾ity, the ability to resist drift from conditions or values to which set; and (5) fidelity, the completeness with which the essential characteristics of the original signal are repro￾duced. Receivers may have additional features such as an automatic frequency control, automatic noise limiter, etc. Some of these characteristics are interrelated. For in￾stance, if a receiver lacks selectivity, signals of a frequency Figure 1015a. Amplitude modulation (upper figure) and frequency modulation (lower figure) by the same modulating wave. Figure 1015b. Pulse modulation. Figure 1016. Components of a radio transmitter

174RADIOWAVESdiffering slightly from those to which the receiver is tunedfull range of those of the desired signal. Thus, the fidelitymay be received.This condition is called spillover,and themaybe reduced.resulting interference is called crosstalk.If the selectivity isAtransponderisatransmitter-receivercapableofac-increasedsufficientlytopreventspillover,itmaynotpermitcepting the challengeof an interrogatorand automaticallyreceipt of agreat enough band of frequencies to obtain thetransmittinganappropriatereplyU.S.RADIONAVIGATIONPOLICY1018.TheFederalRadionavigationPlannavigation systems.Eachsystem utilizedthelatesttechnologyavailableatthetimeof implementation and hasbeenThe Federal Radionavigation Plan (FRP) is producedupgraded astechnologyand resourcespermitted.TheFRPbytheU.S.DepartmentsofDefenseandTransportation.Itaddressesthelengthoftime each system should bepartofthesystemmix.The1992FRP setsforththefollowing sySestablishesgovernmentpolicyonelectronicnavigationsys-tems, ensuring consideration of national interests andtem policyguidelines:efficientuseofresources.ItpresentsanintegratedFederalRADIOBEACONS:Bothmaritimeand aeronauticalplanforall common-use civilian and militaryRadionaviga-tion systems,outlines approaches for consolidation ofradiobeacons providethe civilian community with a low-systems,provides information and schedules,defines andcost, medium accuracy navigation system.They will re-clarifies newor unresolved issues,andprovidesafocalpointmain part of the radionavigation mix at least until the yearforuser input.TheFRPis areviewofexisting and planned200o.Those radiobeacons suitable for supporting Differen-radionavigation systems used in air, space, land, and marinetial GPS (DGPS) will remain well into the next century.navigation.It is availablefrom theNational Technical Infor-Manyof theremainingmaritime radiobeaconsmaybedis-mation Service,Springfield,Virginia,22161continuedaftertheyear2000.Thefirstedition of theFRP was released in1980aspart of a Presidential reporttoCongress.It marked thefirstLORANC:LoranCprovidesnavigation,location,andtimethat a jointDepartmentofTransportation/Departmenttiming services for both civil and military air, land, and seaof Defenseplan hadbeendevelopedfor systems usedbyusers. It is the federally provided navigation system for thebothdepartments.TheFRPhashad international impactonmaritime Coastal Confluence Zone; it is alsoa supplementalnavigation systems, it has been distributed to the Interna-airnavigationsystem.TheLoranCsystem servingthe con-tional MaritimeOrganization (IMO),the International CiviltinentalU.S.,Alaska,and coastalareas with the exception ofAviation Organization (ICAO), the International Associa-Hawai, is expected to remain in place through the yeartionof LighthouseAuthorities(IALA),andother2015.Military requirements for Loran C ended in 1994, andinternational organizations.U.S.-maintained stations overseas and in Hawaii will beDuring a national emergency.any orall of the systemsphased out. Discussions between the U.S. and foreign gov-may be discontinued due to a decision by theNationalernments may resultin continuation of certain overseasCommandAuthority(NCA).TheNCA'spolicyis tocon-stationsafterterminationofthemilitaryreguirementstinuetooperateradionavigationsvstemsaslongastheU.sOMEGA: Omega serves civilian and military mari-and its allies derive greater benefit than adversaries.Oper-ating agencies may shut down systems or change signaltime and air navigation.The military requirement forformats andcharacteristics during such an emergency.Omegaendedin1994:thesvstemmaybemaintainedforTheplan is reviewed continuallyand updated biennial-civil users at least until the year 2005.Replacement ofly.Industry, advisory groups, and other interested partiesequipment at some stations may result in disruption or re-provide input.Theplan considers governmental responsi-duction of service in some areas. Also, the Omega systembilities for national security,public safety,andrelies on support from several foreign nations whose coop-transportation system economy.It is the official sourceoferationmaynotbeforthcomingradionavigation systemspolicyand planningfor the UnitedTRANSIT: The Transit satellite system will end oper-States.Systems coveredbytheFRPinclude,RadiobeaconsOmega,TACAN,MLS,GPS,LoranC,VOR/NOR-DMEations inDecember1996.VORTAC,ILS,andTransit.GPS:TheGlobal Positioning System, or GPS,will be1019. Individual System Plansthe military's primary radionavigation system well into thenext century.It is operated by the U.S. Air Force, and it willIn order to meet both civilian and military needs,theprovidetwobasiclevels ofpositioning servicefederal government has established a number of differentStandardPositioning Service(SPS)isapositioningand

174 RADIO WAVES differing slightly from those to which the receiver is tuned may be received. This condition is called spillover, and the resulting interference is called crosstalk. If the selectivity is increased sufficiently to prevent spillover, it may not permit receipt of a great enough band of frequencies to obtain the full range of those of the desired signal. Thus, the fidelity may be reduced. A transponder is a transmitter-receiver capable of ac￾cepting the challenge of an interrogator and automatically transmitting an appropriate reply. U.S. RADIONAVIGATION POLICY 1018. The Federal Radionavigation Plan The Federal Radionavigation Plan (FRP) is produced by the U.S. Departments of Defense and Transportation. It establishes government policy on electronic navigation sys￾tems, ensuring consideration of national interests and efficient use of resources. It presents an integrated Federal plan for all common-use civilian and military Radionaviga￾tion systems, outlines approaches for consolidation of systems, provides information and schedules, defines and clarifies new or unresolved issues, and provides a focal point for user input. The FRP is a review of existing and planned radionavigation systems used in air, space, land, and marine navigation. It is available from the National Technical Infor￾mation Service, Springfield, Virginia, 22161. The first edition of the FRP was released in 1980 as part of a Presidential report to Congress. It marked the first time that a joint Department of Transportation/Department of Defense plan had been developed for systems used by both departments. The FRP has had international impact on navigation systems; it has been distributed to the Interna￾tional Maritime Organization (IMO), the International Civil Aviation Organization (ICAO), the International Associa￾tion of Lighthouse Authorities (IALA), and other international organizations. During a national emergency, any or all of the systems may be discontinued due to a decision by the National Command Authority (NCA). The NCA’s policy is to con￾tinue to operate radionavigation systems as long as the U.S. and its allies derive greater benefit than adversaries. Oper￾ating agencies may shut down systems or change signal formats and characteristics during such an emergency. The plan is reviewed continually and updated biennial￾ly. Industry, advisory groups, and other interested parties provide input. The plan considers governmental responsi￾bilities for national security, public safety, and transportation system economy. It is the official source of radionavigation systems policy and planning for the United States. Systems covered by the FRP include, Radiobeacons, Omega, TACAN, MLS, GPS, Loran C, VOR/VOR-DME/ VORTAC, ILS, and Transit. 1019. Individual System Plans In order to meet both civilian and military needs, the federal government has established a number of different navigation systems. Each system utilized the latest technol￾ogy available at the time of implementation and has been upgraded as technology and resources permitted. The FRP addresses the length of time each system should be part of the system mix. The 1992 FRP sets forth the following sys￾tem policy guidelines: RADIOBEACONS: Both maritime and aeronautical radiobeacons provide the civilian community with a low￾cost, medium accuracy navigation system. They will re￾main part of the radionavigation mix at least until the year 2000. Those radiobeacons suitable for supporting Differen￾tial GPS (DGPS) will remain well into the next century. Many of the remaining maritime radiobeacons may be dis￾continued after the year 2000. LORAN C: Loran C provides navigation, location, and timing services for both civil and military air, land, and sea users. It is the federally provided navigation system for the maritime Coastal Confluence Zone; it is also a supplemental air navigation system. The Loran C system serving the con￾tinental U.S., Alaska, and coastal areas with the exception of Hawaii, is expected to remain in place through the year 2015. Military requirements for Loran C ended in 1994, and U.S.-maintained stations overseas and in Hawaii will be phased out. Discussions between the U.S. and foreign gov￾ernments may result in continuation of certain overseas stations after termination of the military requirements. OMEGA: Omega serves civilian and military mari￾time and air navigation. The military requirement for Omega ended in 1994; the system may be maintained for civil users at least until the year 2005. Replacement of equipment at some stations may result in disruption or re￾duction of service in some areas. Also, the Omega system relies on support from several foreign nations whose coop￾eration may not be forthcoming TRANSIT: The Transit satellite system will end oper￾ations in December 1996. GPS: The Global Positioning System, or GPS, will be the military’s primary radionavigation system well into the next century. It is operated by the U.S. Air Force, and it will provide two basic levels of positioning service. Standard Positioning Service (SPS) is a positioning and