14092_ch1(wave propagation)

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CHAPTER 1

WAVE PROPAGATION

The eyes and ear s of a sh ip or sh ore stat ion dependon sophist icated, highly computerized electronicsystems. The one thing all of these systems have incomm on is that th ey lead to an d from antennas. Shipsoperators who must communicate, navigate, and beready to fight the ship 24 hours a day depend on youto keep these emitters and sensors operational.

In this volume, we will review wave propagation,antenna characteristics, shore-based and shipboardcommunications antennas, matching networks, antennatuning, radar antennas, antenna safety, transmissionlines, connector instal lat ion and weatherproofing,

waveguides, and waveguide couplings. When youhave completed this chapter, you should be able todiscuss the basic principles of wave propagation andth e at mospheres effects on wave propagation.

THE EARTHS ATMOSPHERE

While radio waves traveling in free space have

little outside influence to affect them, radio wavest r a v e li n g in t h e e a r t h s a t m os p h e r e h a v e m a n yinfluences that affect them. We have all experiencedp rob lems wi th r ad io waves , c aused by ce r t a in

atmospheric condit ions complicating what at f irstseemed to be a relatively simple electronic problem.These problem-causing conditions result from a lack of un iformity in th e eart hs at mosphere.

Many factors can affect atmospheric conditions,either positively or negatively. Three of these areva r i a t i ons i n geog raph ic he igh t , d i f f e r ences i ngeographic location, and changes in time (day, night,season, year).

To understand wave propagation, you must haveat least a basic understa nding of the ear ths atm osphere.The eart hs atm osphere is divided into th ree separa teregions, or layers. They are the troposphere, t h estratosphere, and the ionosphere. These layers areillustrated in figure 1-1.

T R O P O S P H E R E

Almost all weather phenomena take place in thetroposphere. The temperature in this region decreasesrapidly with altitude. Clouds form, and there may bea lo t of turbulence because of var ia t ions in thetemperature, pressure, and density. These conditionshave a profound effect on the propagation of radiowaves, as we will explain later in this chapter.

S T R ATO S P H E R E

The stratosphere is located between the troposphereand the ionosphere. The temperature throughout this

region is almost constant and there is little water vaporpresent. Because it is a relatively calm region withli t t le or no temperature change, the stratosphere hasalmost no effect on radio waves.

I O N O S P H E R E

This is the m ost importa nt region of the ea rt hsat mosphere for long dist an ce, point -to-point comm un i-cations. Because the existence of the ionosphere isdirectly related to radiation emitted from the sun, the

movement of the earth about the sun or changes inthe suns ac t iv ity wi l l resul t in var ia t ions in theionosphere. These var iations ar e of two general types:(1) those that more or less occur in cycles and,therefore, can be predicted with reasonable accuracy;and (2) those that are irregular a s a result of abnormalbehavior of the sun and, therefore, cannot be predicted.Both regular an d irregular variat ions have importan teffects on radio-wave propagation. Since irregularvariations cannot be predicted, we will concentrateon regular variations.

Regu la r Var i a t ions

The regular variations can be divided into fourmain classes: daily, 27-day, seasonal, and 11-year.We will concentr at e our discussion on da ily var iations,

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Figure 1.1Atmospheric layers.

since they have the greatest effect on your job. Daily of the ultraviolet energy that initially set them freevariations in the ionosphere produce four cloud-likelayers of electrically-charged gas atoms called ions,which enable radio waves to be propagated greatdistances around the earth. Ions are formed by aprocess called ionization.

Ion iza t ion

In ionization, high-energy ultraviolet light wavesfrom t he su n per iodically enter the ionosphere, strikeneutral gas atoms, and knock one or more electronsfree from each a tom. When t he electr ons ar e knockedfree, the atoms become positively charged ( po s i t i v ei ons ) and remain in space, along with the negatively-charged free electrons. The free electrons absorb some

and form an ionized layer.

Since th e at mosphere is bombarded by ultr avioletwaves of differing frequencies, several ionized layersare formed at different altitudes. Ultraviolet wavesof higher frequencies penetrate the most, so theyproduce ionized layers in the lower portion of theionosphere. Conversely, ultraviolet waves of lowerfrequencies penetrate the least, so they form layersin the upper regions of the ionosphere.

An important factor in determining the densityof these ionized layers is the elevation angle of thesun. Since this angle changes frequently, the heightand thickness of the ionized layers vary, depending

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on the time of day and the season of the year.Another important factor in determining layerdensity is known as recombin ation.

Recombina t ion

Recombination is the reverse process of ionization. It occurs when free electrons and positiveions collide, combine, and return the positive ions to

their original neutra l state.

Like ionization, the recombination processdepends on the time of day. Between early morningan d late a fter noon, the ra te of ionization exceeds th era te of recombinat ion. Dur ing th is period the ionizedlayers reach their greatest density and exertma ximum influence on r adio waves. However, duringthe late afternoon and early evening, the rate of recombination exceeds the rate of ionization, causingthe densities of the ionized layers to decrease.Throughout the night, density continues to decrease,reaching its lowest point just before sunrise. It isimportant to understand that this ionization andrecombination process varies, depending on theionospheric layer and the time of day. The followingparagraphs provide an explanation of the fourionospheric layers.

Ionospher i c Laye r s

The ionosphere is composed of three distinctlayers, designated from lowest level to highest level(D, E, and F) as s hown in figur e 1-2. In a ddition, th e

F layer is divided into two layers, designated F1 (thelower level) an d F2 (the higher level).

The presence or absence of these layers in theionosphere and their height above the earth varywith the position of the sun. At high noon, radiationin the ionosphere above a given point is greatest,while at night it is minimum. When the radiation isremoved, many of the particles that were ionizedrecombine. During the time between these twoconditions, the position and number of ionized layerswithin th e ionosphere change.

Since the position of the sun varies daily,monthly, and yearly with respect to a specific pointon earth, the exact number of layers present isextremely difficult to determine. However, thefollowing general statements about these layers canbe made.

D LAYER. Th e D layer ranges from about 30to 55 miles above th e ear th . Ionization in th e D layeris low becau se less ultra violet light penetr at es t o thislevel. At very low frequencies, the D layer and theground act as a huge waveguide, ma king comm unica-tion possible only with large antennas and high-power transmitters. At low and medium frequencies,the D layer becomes highly absorptive, which limitsthe effective daytime communication range to about200 miles. At frequencies above about 3 MHz, the Dlayer begins to lose its absorptive qualities.

Figure 1-2 .Layers of the ion osph ere .

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Long-distance communication is possible atfrequencies as h igh as 30 MHz. Waves at frequenciesabove this range pass through the D layer but areattenuated. After sunset. the D layer disappearsbecause of the rapid recombination of ions. Low-frequency and medium-frequency long-distancecommunication becomes possible. This is why AMbehaves so differently at night. Signals passingthrough the D layer normally are not absorbed but

ar e propagat ed by th e E a nd F layers.

E LAYER. Th e E layer ranges from approxi-mately 55 to 90 miles above the earth. The rate of ionospheric recombination in this layer is ratherrapid after sunset, causing it to nearly disappear bymidnight. The E layer permits medium-rangecommunications on the low-frequency through very-high-frequen cy ban ds. At frequ encies above about 150MHz, radio waves pass th rough the E layer.

Sometimes a solar flare will cause this layer toionize at night over specific areas. Propagation in thislayer during this time is called SPORADIC-E. Therange of communication in sporadic-E often exceeds1000 miles, but the range is not as great as with Flayer propagat ion.

F LAYER. Th e F layer exists from a bout 90 to240 miles above the eart h. During daylight hours, th eF layer separates into two layers, F1 a nd F2. Duringthe night, the F1 layer usually disappears, The Flayer produces maximum ionization during theafternoon hours, but the effects of the daily cycle arenot as pronounced as in the D and E layers. Atoms in

the F layer sta y ionized for a longer tim e after su nset,and during maximum su nspot a ctivity, they can stayionized a ll night long.

Since the F layer is the highest of theionospheric layers, it also has the longest propagationcapability. For horizontal waves, the single-hop F2distance can reach 3000 miles. For signals topropagate over greater distances, multiple hops arerequired.

Th e F layer is responsible for most high-frequency, long-distance communications. Themaximum frequency that the F layer will returndepends on the degree of sunspot activity. Duringmaximum sunspot activity, the F layer can return

signals at frequencies as high a s 100 MHz. Duringminimum sunspot activity, the maximum usablefrequen cy can dr op to as low as 10 MHz.

ATMOSPHERIC PROPAGATION

Within the atmosphere, radio waves can berefracted, reflected, and diffracted. In the followingparagraphs, we will discuss these propagation

characteristics.

REFRACTION

A radio wave transmitted into ionized layers isalways refracted, or bent. This bending of radiowaves is called refraction. Notice the radio waveshown in figur e 1-3, tra veling thr ough t he ea rt hsatmosphere at a constant speed. As the wave entersthe denser layer of charged ions, its upper portionmoves faster t ha n its lower portion. The abr upt speedincrease of the upper part of the wave causes it tobend back toward the earth. This bending is alwaystoward the propagation medium where the radiowaves velocity is t he lea st .

Figure 1-3 .Radio-wav e refract ion .

The am ount of refraction a ra dio wave un dergoesdepends on th ree ma in factors.

1. The ionizat ion den sity of th e layer

2. The frequen cy of th e ra dio wave

3. The angle at which the radio wave enters thelayer

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Figure 1 -4 .Effec t s o f i on ospher i c d ens i ty on r ad io wa ves .

Laye r Dens i ty

Figure 1-4 shows the relationship betweenradio waves and ionization density. Each ionizedlayer has a middle region of relatively denseionization with less intensity above and below. Asa radio wave enters a region of i nc reas ingionization, a velocity increase causes it to bendback t oward the earth. In the highly densemiddle region, refraction occurs more slowlybecause the ionization density is uniform. As thewave enters the upper less dense region, thevelocity of the upper part of the wave decreasesand t he wave is bent away from th e earth .

F r e q u e n c y

The lower the frequency of a radio wave, the

more rapidly the wave is refracted by a givendegree of ionization. Figure 1-5 shows threeseparate waves of differing frequencies enteringthe ionosphere a t t he sam e angle. You can see t ha tthe 5-MHz wave is refracted quite sharply, whilethe 20-MHz wave is refracted less sharply andreturns to earth at a greater distance than the 5-MHz wave. Notice that the 100-MHz wave is lost

into space. For an y given ionized layer, t here is afrequency, called the escape point, at which ener gytransmitted directly upward will escape intospace. The maximum frequency just below theescape point is called the c r i t ica l f r equency . Inth is example, t he 100-MHz waves frequen cy isgreat er th an t he critical frequency for t ha t ionizedlayer.

Figure 1-5.Frequ enc y versu s ref ract iona n d d i s t a n c e .

The critical frequency of a layer depends uponth e layer s density. If a wave pass es th rough a

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particular layer, it may still be refracted by ahigher layer if its frequency is lower than thehigher layer s critical frequen cy.

Angle of Incid en ce an d Cri t ica l Angle

When a radio wave encounters a layer of theionosphere, that wave is returned to earth at thesame angle (roughly) as its angle of incidence.Figure 1-6 shows three radio waves of the samefrequency entering a layer at different incidenceangles. The angle at which wave A strikes thelayer is too nearly vertical for the wave to berefracted to earth, However, wave B is refractedback to earth. The angle between wave B and theearth is called the c r i t i ca l ang le . Any wave, at agiven frequency, that leaves the antenna at anincidence angle grea ter th an th e critical a ngle willbe lost int o spa ce. This is why wave A was notrefracted. Wave C leaves the antenna at thesma llest a ngle that will allow it to be refra cted andstill return to earth. The critical angle for radiowaves depends on the layer density and thewavelength of the signal.

Figure 1 -6 .Inc idence ang les o f r ad io w aves .

As the frequency of a radio wave is increased,th e critical an gle must be r educed for refraction t ooccur. Notice in figure 1-7 that the 2-MHz wavestr ikes the ionosphere at th e critical angle for t hatfrequency and is refracted. Although the 5-MHz

line (broken line) strikes the ionosphere at a lesscritical angle, it still penetrates the layer and islost As the angle is lowered, a critical angle isfinally reached for the 5-MHz wave and it isrefracted back to ear th.

Figure 1-7 .Effect of f reque ncy o n the cr i t ica l ang le .

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density is the greatest. As a radio wave passes intothe ionosphere, it loses some of its energy to the freeelectr ons an d ions pr esent t here. Since the am ount of absorption of the radio-wave energy varies with thedensity of the ionospheric layers, there is no fixedrelationship between distance and signal strength forionospher ic propagat ion. Absorption fadin g occurs fora longer period than other types of fading, sinceabsorption takes place slowly. Under certain

conditions, the absorption of energy is so great thatcommunication over any distance beyond the line of sight becomes difficult .

Although fading because of absorption is themost serious type of fading, fading on the ionosphericcircuits is ma inly a r esult of multipa th propagation.

Mul tipa th Fad in g

MULTIPATH is simply a term used to describethe multiple paths a radio wave may follow betweentransmitter and receiver. Such propagation pathsinclude the ground wave, ionospheric refraction,reradiation by the ionospheric layers, reflection fromthe ea rt hs sur face or from m ore t ha n one ionosphericlayer, and so on. Figur e 1-11 shows a few of the pa th stha t a signal can tra vel between two sites in a t ypicalcircuit. One path, XYZ, is the basic ground wave.Another path, XFZ, refracts the wave at the F layeran d passes it on to the r eceiver a t point Z. At point Z,the received signal is a combination of the groundwave and the sky wave. These two signals, havingtr aveled different pat hs, ar rive at point Z at differenttimes. Thus, the a rr iving waves may or ma y not be inphas e with ea ch other. A similar situa tion may resu ltat point A. Another path, XFZFA, results from agreater angle of incidence and two refractions fromthe F layer. A wave traveling that path and onetraveling the XEA path may or may not arrive atpoint A in phase. Radio waves that are received inpha se reinforce each other a nd produce a str ongersignal at the receiving site, while those that arereceived out of phase produce a weak or fadingsignal. Small alterations in the transmission pathma y cha nge the ph ase r elationship of the two signa ls,caus ing periodic fading.

Figu re 1-11.Mult ipath t ran smiss ion .

Multipath fading ma y be minimized by pra cticescalled SPACE DIVERSITY and FREQUENCYDIVERSITY In space diversity, two or more receivingantennas are spaced some distance apart . Fadingdoes not occur simulta neously at both ant enna s.Therefore, enough output is almost always availablefrom one of th e an tenn as t o provide a useful signa l.

In frequency diversity, two transmitters and tworeceivers are used, each pair tuned to a different

frequency, with the same information beingtransmitted simultaneously over both frequencies.One of the two receivers will almost always produce auseful signal.

Se lec t ive Fad ing

Fading resulting from multipath propagationvaries with frequency since each frequency arr ives atthereceiving point via a different radio path. When awide band of frequencies is transmittedsimultaneously,

each frequency will vary in the amount of fading.This variation is called SELECTIVE FADING. Whenselective fading occurs, all frequencies of thetransmitted signal do not retain their original phasesand relative amplitudes. This fading causes severedistortion of the signal and limits the total signaltransmitted.

Frequency shifts and distance changes becauseof daily variations of the different ionospheric layersar e summ ar ized in table 1-1.

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Table 1-1.Daily Ionospheric Communications

D LAYER: reflects vlf waves for long-rangecommunications; refracts lf and mf forshort-range communications; has littleeffect on vhf and above; gone at night.

E LAYER: depends on the angle of the sun:refracts hf waves during the day up to 20MHz to distances of 1200 miles: greatlyreduced at night.

F LAYER: structure and density depend onthe time of day and the angle of the sun:consists of one layer at night and splitsinto two layers during daylight hours.

F1 LAYER: density depends on the angle ofthe sun; its main effect is to absorb hfwaves passing through to the F2 layer.

F2 LAYER: provides long-range hf communica-tions; very variable; height and densitychange with time of day, season, and sun-spot ac t iv i ty.

Figure 1-12.Ionosphericlayers.

OTHER PHENOMENA THAT AFFECTof these layers is greatest during the summer. The

C O M M U N I C AT I O N S F2 layer is just the opposite. Its ionization is greatestduring the winter, Therefore, operating frequencies

Although daily changes in the ionosphere have for F 2 layer propagation are h igher in the winter tha n

the greatest effect on communications, other phenom-ena also affect communications, both positively andnegatively. Those phenomena are discussed brieflyin the following paragraphs.

SEASONAL VARIATIONS IN THEI O N O S P H E R E

Seasonal variat ions a re th e result of the ea rth srevolving around t he su n, becau se th e relative positionof the sun moves from one hemisphere to the otherwith the changes in seasons. Seasonal variations of the D, E, and F1 layers are directly related to thehighest angle of the sun, meaning the ionization density

in the summer.

S U NSP OT S

One of the most notable occurrences on the surfaceof the sun is the appearance and disappearance of dark,i r regular ly shaped areas known as SUNSPOTS.

Sunspots are believed to be caused by violent eruptionson the sun and are characterized by strong magneticfield s. T h es e su n s pot s ca u s e va r ia t ion s in t h eionization level of the ionosphere.

Sunspots tend to appear in two cycles, every 27days and every 11 years.

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Twenty-Seven Day Cycle

The number of sunspots present at any one timeis constantly changing as some disappear and new onesemerge. As the sun rotates on its own axis, thesesunspots are visible at 27-day intervals, which is theapproximate period for the sun to make one completerevolution. During this time period, the fluctuationsin ionization are greatest in the F2 layer. For this

reason, calculating critical frequencies for long-distancecommunications for the F2 layer is not possible andallowances for fluctuations must be made.

Eleven-Year Cycle

Suns pots can occur u nexpectedly, and t he life spano f i n d i v i d u a l sunspots is va r iable. Th e

ELEVEN-YEAR SUN SPOT CYCLE is a regularcycle of sunspot activity that has a minimum andmaximum level of activity that occurs every 11 years.During periods of maximum activity, the ionizationdensity of all the layers increases. Because of this,the a bsorption in th e D layer increases a nd th e criticalfrequencies for the E, F1, and F2 layers are higher.During th ese times, higher operat ing frequencies mustbe used for long-range communications.

IRREGULAR VARIATIONS

Irregular variations are just that, unpredictablechanges in the ionosphere that can drastically affectour abil i ty to communicate. The more common

variations are sporadic E, ionospheric disturbances,and ionospheric storms.

Sporadic E

Irregular cloud-l ike patches of unusually highionization, called the sporadic E, often format heightsnear the normal E layer. Their exact cause is notknown and their occurrence cannot be predicted.However, sporadic E is known to vary significantlywith lat i tude. In the northern lat i tudes, it a ppears tobe closely related to the aurora borealis or northernlights.

The sporadic E layer can be so thin that radiowaves penetrate i t easi ly and are retu rned to earth bythe upper layers, or it can be heavily ionized and

extend up to several hundred miles into the ionosphere.This condition may be either harmful or helpful toradio-wave propagation.

On the harmful side, sporadic E may blank outthe use of higher more favorable layers or causeadditional absorption of radio waves a t some frequen-cies. It can also cause additional multipath problemsand delay the arrival times of the rays of RF energy.

On the helpful side, the critical frequency of thesporadic E can be greater than double the criticalfrequency of the normal ionospheric layers. This maypermit long-distance communications with unusuallyhigh frequencies. I t may also permit short-distancecommunications to locations that would normally bein the skip zone.

Sporadic E can appear and disappear in a shorttime during the day or night and usually does not occurat same t ime for all trans mitting or receiving sta tions.

S u d d e n I o n o s p h e ri c D i s t u rb a n c e s

Commonly known as SID, these disturbances mayoccur without warn ing and m ay last for a few minutesto several hours. When SID occurs, long-range hf communications are almost totally blanked out. Theradio operator listening during this time will believehis or her receiver has gone dead.

The occurrence of SID is caused by a bright solar

erupt ion producing an unusual ly in tense burs t of ultraviolet light that is not absorbed by the F1, F2,or E layers. Instead, it causes the D-layer ionizationdensity to greatly increase. As a result, frequenciesabove 1 or 2 megahertz are unable to penetrate theD layer and are completely absorbed.

Ionospher ic S torms

Ionospheric storms are caused by disturbances inth e eart hs ma gnet ic field. They ar e associated withboth solar eruptions and the 27-day cycle, meaningth ey are relat ed to the r otation of the sun . The effectsof ionospheric storms are a turbulent ionosphere andvery erratic sky-wave propagation. The storms affectmostly the F2 layer, reducing its ion density andcausing the cri t ical frequencies to be lower than

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normal. What this means for communication purposesis that the range of frequencies on a given circuit issmaller than normal and that communicat ions arepossible only at lower working frequencies.

Weather

Wind, air temperature, and water content of thea tmosphere can combine e i ther to ex tend rad io

communications or to greatly attenuate wave propaga-t i o n . m a k i n g n o r m a l c o m m u n i c a t i o n s e x t r e m e l ydi ff icu l t . Prec ip i ta t ion in the a tmosphere has i t sgrea tes t e ffec t on the h igher f requency ranges .Frequencies in the hf range and below show little effectfrom this condition.

RAIN. Atten uat ion because of raindrops is greaterthan attenuation for any other form of precipitation.R a i n d r o p a t t e n u a t i o n m a y b e c a u s e d e i t h e r b yabsorption, where the raindrop acts as a poor dielectric,absorbs power from the radio wave and dissipates thep o w e r b y h e a t l o s s ; or by scattering (fig. 1-13).Raindrops cause greater attenuation by scattering thanby absorption at frequencies above 100 megahertz.At f requenc ies above 6 g igaher tz , a t tenua t ion byraindrop scat ter is even greater.

Figure 1-13.Rf energy losses fromscattering.

FOG. Since fog remains suspended in theatmosphere, the at tenuation is determined by thequantity of water per unit volume (density of the fog)and by the size of the droplets. Attenuation becauseof fog has little effect on frequencies lower than 2gigaher tz , bu t can cause se r ious a t tenua t ion byabsorption at frequencies above 2 gigahertz.

SNOW. Since snow has about 1/8 the densityof rain, and because of the irregular shape of the

snowflake, the scattering and absorption losses aredifficult to compu te, but will be less t ha n t hose causedby raindrops.

HAIL. Attenuation by hail is determined by thesize of the stones and their density. Attenuation of radio waves by scattering because of hailstones isconsiderably less than by rain.

TEMPERATURE INVERSION

When layers of warm air form above layers of cold air, th e condition known as temper at ur e inversiondevelops. This phenomenon causes ducts or channelsto be formed, by sandwiching cool air either betweenthe surface of the earth and a layer of warm air, orbetween two layers of warm air. If a transmittingantenna extends into such a duct, or if the radio waveenters the duct at a very low angle of incidence, vhf and uhf transmissions may be propagated far beyondnormal line-of-sight distances. These long distancesare possible because of the different densities andrefractive qualities of warm and cool air. The suddenchange in densities when a radio wave enters the warmair a bove the duct causes t he wave to be refracted back toward earth. When the wave str ikes the earth or awarm layer below the duct, it is again reflected orrefracted upward and proceeds on through the ductwith a multiple-hop type of action. An example of radio-wave propagation by ducting is shown in figure1-14.

Figure 1-14.Duct effect cau sed by te mperatu reinversion.

TRANSMISSION LOSSES

All radio waves propagated over the ionosphereunder go energy losses before a rr iving a t t he r eceivingsite. As we discussed earlier, absorption and lower

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atmospheric levels in the ionosphere account for alarge par t of th ese energy losses. There a re t wo oth ertypes of losses that also significantly affectpropagation. These losses are known as ground reflection losses and freespace loss. The combinedeffect of absorption ground reflection loss, andfreespace loss account for most of the losses of radiotr ans missions propagated in th e ionosphere.

GROUND R EFLECTION LOSS

When propagation is accomplished via multihoprefraction, rf energy is lost each time the radio waveis reflected from th e ear th s su rface. The a mount of energy lost depen ds on th e frequency of the wa ve, theangle of incidence, ground irregularities, and theelectrical condu ctivity of th e point of reflection.

FREESP ACE LOSS

Normally, the major loss of energy is because of the spreading out of the wavefront as it travels fromthe transmitter. As distance increases, the area of thewavefront spreads out, much like the beam of aflashlight. This means the amount of energycontained within any unit of area on the wavefrontdecreases as distance increases. By the time theenergy arrives at the receiving antenna, thewavefront is so sprea d out t ha t th e receiving anten naextends into only a small portion of the wavefront.This is illustrated in figure 1-15.

FREQUENCY SELECTION

You must have a thorough knowledge of radio-wave propagation to exercise good judgment whenselecting transmitting and receiving antennas andoperating frequencies. Selecting a usable operatingfrequency within your given allocations andavailability is of prime importance to maintainingreliable communications.

For successful communication between any twospecified locations at an y given t ime of the da y, thereis a maximum frequency, a lowest frequency and anoptimum frequency that can be used.

Figure 1-15.Frees pace los s pr inciple .

MAXIMUM USABLE FRE QUEN CY

The higher the frequency of a radio wave, thelower the rate of refraction by the ionosphere.Therefore, for a given angle of incidence and time of

day, there is a ma ximum frequency tha t can be usedfor comm un ications between two given locat ions. Th isfrequency is known as the MAXIMUM USABLEFREQUENCY (muf).

Waves at frequencies above the muf arenorma lly refracted so slowly th at they ret urn to eart hbeyond the desired location or pass on through theionosphere an d ar e lost. Variat ions in t he ionospherethat can raise or lower a predetermined muf mayoccur at an ytime. his is especially tru e for the highlyvariable F2 layer.

LOWEST U SABLE FR EQUENCY

Just as there is a muf that can be used forcommunications between two points, there is also aminimum operating frequency that can be usedknown a s t he LOWEST USABLE F REQUEN CY (luf).As t he frequency of a ra dio wave is lowered, the ra teof refraction increases. So a wave whose frequency isbelow the esta blished luf is refra cted back to ear th ata short er distan ce th an desired, as sh own in figure 1-16.

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Figure 1-16.Refract ion o f f requ en cies be lowthe low es t u sab le f requen cy ( lu f) .

As a frequen cy is lowered, a bsorption of th e ra diowave increases. A wave whose frequency is too low isabsorbed to such an extent that it is too weak forreception. Atmospheric noise is also greater at lowerfrequencies. A combination of higher absorption andatmospheric noise could result in an unacceptablesignal-to-noise ra tio.

For a given angle ionospheric conditions, of incidence and set of the luf depends on t he refraction

properties of the ionosphere, absorptionconsiderat ions, an d th e am ount of noise present .

OPTIMUM WORKING FREQUEN CY

The most practical operating frequency is onethat you can rely onto have the least number of problems. It should be high enough to avoid theproblems of multipath fading, absorption, and noise

encount ered at th e lower frequencies; but not so highas to be affected by the adverse effects of rapidchanges in th e ionosphere.

A frequency that meets the above criteria isknown as the OPTIMUM WORKING FREQUENCYIt is abbreviated fot from the initial letters of theFrench words for optimum working frequency,frequence optimum de travail. The fot is roughlyabout 85% of the muf, but the actual percentagevaries and may be considerably more or less than 85percent.

In t his chapter , we discussed t he ba sics of radio-wave propagation and how atmospheric conditionsdetermine the operating para meters needed to ensuresuccessful communications. In chapter 2, we willdiscuss basic antenna operation and design tocomplete your understanding of radio-wavepropagation.

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14092_ch1(wave propagation)

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