Radio Wave Propagation 11.1, INTRODUCTION O far, we have been discussing the antennas and the process of radiation from them in which a high frequency currents in the antennas produce detachment of electromagnetic energy in the form of waves of thesame natureas light. This chapter is mainly concerned with the phenomena that occur in the media between a transmitter and a receiver. When a radio wave is radiated from the transmitting antenna, it spreads in all directions decreasing in amplitude with increasing distance because of spreading of the speaking electromagnetic energy through larger and larger surface areas. Electomagnetic wave in the frequency spectrum of 0.001 to 10" Hertz are arbitrarily termed as radio waves, although broadly speaking by radio waves, in this chapter, mean that band of electromagnetic energy which covers the frequency range from few kHz (very low frequencies or very long waves) to a few MHz (super high frequencies or microwaves). In this chapter, for discussion purpose, the electromagnetic wave and radio wave have the same meaning and hence this chapter could have also been named as "Propagation of Electromagnetics waves” or simply wave pro- pagation. depend on their radio waves In an earth environment, the electromagnetic waves propagate in ways that not only x, the ofl own properties but are also dictated by the environment itself. The actual environment in which the are propagated may have obstacles, discontinuities and propagation medium variations. Howevet environment of importance for practical radio-wave propagation is that of the earth and its immé¢ ‘x surroundings. Of course, in the recent years, radio has invaded "out space" too. In many respects, the Con of free space propagation is realized in the space far from the earth, Free space is the space which does interfere with the normal radiation and propagation of radio waves. In other words, in free space. 10 ™! or gravitational fields or solid bodies or ionized particles are assumed to exist. The concept of fee propagation simplifies the approach to wave propagation.. ‘ti Bj 01 il de useful oF Not. The portion of the s # signal We mode of : received energy at es eros Dropagations. The prominent modes ot tant esving Pere are nf e Ground wave oF surface wave propagation, (uy Sky wave oF ionospheric propagation, (@ Space wave Propagation, say be ned that these modes of propagation largel cay veloped means of Propagation beyond the horizon ane pidees ‘upon the frequency in use, Besides ns All these modes of propagation willbe discussed in epee eee tion of radio waves are not only use sath emt ot and long iste but alsin rae, dheston indy ouch reg fea het: Propagation phenomenon of electromagnetic waves ar also happening in edmme pane sores et. Dut we shall restrict only to unguided propagation and all lier wil be omited Nidion w these, special propagation i.e. super refraction or duct propagation, ad's aa retin propagation like. MUF, critical frequency, skip distance fading and rmeasuretnens wil also be ssused. 12, ELECTROMAGNETIC OR RADIO WAVES Elecuromagnetic waves are nothing but oscillations which propagate with the velocity of light aayroximately,¢ = 3 X 10° m/s) in free space. Electromagnetic waves consist of moving fields of electric sragnetc forces. The lines of force inthe electric and magnetic fields are atright angle and they bath => trait ange tothe direction of propagation, As illustrated in Fig. 11.1, the direction of the “electric field’, te magnetic field” and ‘propagation’ are mutually perpendicularin electromagnetic waves, ‘The electomag- vem are averse in nature i.e. oscillations are perpendicular to the direction of propagation of ‘aves, A simple electromagnetic wave is shown in Figure 11.1 in which electric lines are perpendicular to the tah and magnetic lines horizontal. However, they can assume any orientation wt. earth. Arrows indicate ‘i oni Det ins a5 53k 5 553 Seems) EER ese pa ae 7 TAS ¥ € a “grec lines (Horizomal) w rRec TION OF : PROPAGATION Fig. 11.1. Simple electromagnetic waves Fig, 112. EM. Waves in free space isunancous direction of fields for a wave travelling wowards reader aig TM ABNet fields surround the wise and perpendicular toi tea ite. This configuration is tue © is electric field is parallel to the eter the radiation from antenna and PAs the pion (et 0.625) The orientation of Electric field wt earth gives the polarization. In Figs. 11 at 112 ‘scaly polarized, The intial polarization is determined by the ean of amen a ae ‘ual , then it will produce respectively vertically polarized oF ‘horizontally eee wa ern? ill antan its polarization during the aves Wash err Sie Space is assumed to be present so e.m. waves spread ‘uniformly in all directA,B,C, D = Wave fronts Fig. 115 ean epee ae ° Fig. 11.4. Spherical wave fr point source, . Pron represent The wave front is, thus, spherical as illustrated in Fig. U3 and actual es 180 shown in ‘Sometimes for convenience even the "rays" ~~ lsa:imagined which radiates Point source in a de, and they are every where perpendicular to ane of wave front. The optical phenomena of reflection, refraction, diffraction and polarization ae alg . followed by e.m. waves, . -AMENTAL EQUATION FOR FREE-SPACE PROPAGATION a "Remipefmandjoneoreavedpwerceaelberaooseaea loss The bac ‘estimating radio transmission losis the loss expected in free space. in a GON Of al obese wat absorb or reflect radio energy. This concept is essentially the inverse square law in optics aplied o transmission, sure ean fees nsmison oc propagation, Est cons power Wr Pac ne pe salina dretons Gsotopical). is further asuned tat an radiator is placed in free space, a homogeneous and non absorbing medium Of dielectric constant unig, Ty radiations willbe spread out spherically ast travels away from the source so thatdistance r, the powerdasy inthe wave, which isthe power per unit area of wavefront, will be Wr 2 Pre= > W/m (ll 4nd ! where 4 x d* is the surface area of we sphere of radia Since all the practical antennas fbave direction, antenna at the same distance fed with the ‘Same input power, Let Gr = Maximum directivity 'y Bain of the transmitting antenna. ‘Then, power density along the maximum radiation is given by Gp = —Po(test antenna ) = Pie Py (isotropic antenna) or Po = Gr- P, 0 Wr a Po=G, Wr "and oe! Now a receiving antenna may be Positioned so that it maximum power from UH ¥ Wr be the power delivered by the an iar ° 30 post itenna to the load i condition. When ‘he antenna can be considered to have an effec 4 oeeive") under matched ‘etive Aperture Area (A, ) so that we We= Pp. a,PR a j _ SrWra, | —_ we Sid ws 7 a, the maxi J put for any antenn: ‘imum directivity gain and effective (1a) 7 area are related as Ga FA oF “ ® G4 4, ~ Wavelength being radiated ais) _ Maximum directivity gain of the receiving antenna, Thei ur Tey) n from eqn. 11.5 we have Ac = Gr agin eqn, HA we get /———_*_] ptt cn We = SLOT xv and (% Wr _ Gr: Gr( 22-\ ] Wr and az} Watts eae) . leguatinn fir fren oo qn 11.6is the fundamental equation for free space transmission or pr is equation in SI unit fr antereas Propagation. This is also called ans FREE SPACI antennas in a loss-free medi it f poeta fre ium. Writing eqn. 11.6 in an Wr ad We _ GG, (2 Wr ~ Ot Gr (aa) 17@ And lL ( # J ® vise Wr = Radiated power in Watts ; Wr = Received power in Watts ; Gr = Maximum directivity anol transmitting antenna; Gr = Maximum directivity gain of receiving antenna; 2. = wavelength in neers=f/eandd = distance in meters. This apparent from the eqn. 11.7 (b) that larger the denominator, smaller the value of received power iy and hence we may describe L= (4) .. (118) ‘the spatial attenuation which is distinct from dissipative or absorption losses. a Eqn, 11.7 is the propagation equation and is usually expressed in logarithmic. form in order 10 ofthe decibel notations. The reference units for the decibel notation are either the appropriate SI unit OF ‘ne peered arbitrary value, Accordingly, in case of latter the reference must be specified Cane Sample if we wish to express, electric field strength in deeibels with respect to 1 pV/m aga in), Now (iL mW, we must write E (dB v/m) or W (dB mW) oF simply E( dB 1) or W(aB men, 11.7 @) A 10 logio (#) = 10 logio Gr + 10 logio Gr + wnse( 4a) ftATRIA 8 WA Ry, Cites in (db) — LCs a " i yr (dW) + Gr( db) + Ga , Ry or [Wacanw) = Wea ; ; i ay. with | acs) = 10 tome za} ne (ara) | Les yab ~ 24S + 20 logies + 20 oR . N 's expressed in MHz, and dist This obi as follows, Nomally, frequency Ws expressed i MHZ diane | further known that a4 mi _ 3x08 fli) of x 10° (MHz) or . 9 x 10 ;} 1 = Or Gn | TE 98596 x 10° (a) (app [We _ GrGx x 0570s x07 GG / | Wr (ary ~ (un Expressing above power ratio in decibels, we get ( 10 ee it} 10 logie Gr+ 10 log Ge + 10 logi0 0.57 +10 logi0 10"? 10 togs0d*—10 og = Gr( db) +Gr(db)+ 10x 1.7559 - 10 x 3 logio 10-102 logiod-2x 10 logs = Gr( db )+ Ga (db )+ 3.5590 — 30.0000 - 20 logio d—20 lopiof F ~ 3010000 = Gr (do) + Gr (db ) - 32.45 - 20 logio d- 20 logiof + 35590 = Gr (db) + Gx( db) ~{32.45 +20 logiod +20 login} Se (we) = Gr(db) + Ge(db) - L(s) db where | L (sa = 3245 + 20logied + 20 logis in km and f in MHz, This is the loss in decibels, resulting from the spreadi i and fran , ls, Spreading of the wave as it propagates outw: the source. L (sis known as spatial attenuation or transmission path loss, At microwave frequencies, he wile Eater because of atmospheric absorption, It may be noled that decibels ae defined terms otros ratios as such, it is not correct to express logarithmic ratio of fi distance as decibels a" *Pressions should always be writen 3820 logiof of 20 loge d en ne Ss d etc. Often, itis required to know the electric field 1 ving antenna. THs’ bined as follows. By Poynting Theorem, Strength of the wave at the receivingand 20% | by eqn, 112 “a vm | we.) fundamental equation which gi zsis the funda a ives the field strength at a _ropio® conditions. A receiving antenna, therefore, has an eine antenna for free stve et and hence open circuit emf of the receiving antenna is given by ight 1, analogous o“ Voc = E+ le i i i i we (1 gan. 11-11 is also written sometimes in terms ofthe field strength Eo at unit distance i ne ae km we have gee ne (11.13) puting eqn. 11.13 into eqn. 11.11, we have 2-0 G By, da d we 11.14 (@) = i(m) e E=Eyx a(m) v/m ww 11.14 (0) runher, Eqn. 11.11 is also written in an alternative form when choice of reference having distance sgesed in |km and transmitted power is 1 kW. Then V30WrGr _ 30 x 1000 x 1 x Wr_ 100 x V3. x Wr vm d ~ 1000 ~ 1000 = L132Wn ~ 9.173 WW v/m 10 Eo = 173 VWr mv/m| if Wr = 1kW « Ee = 173 mv/m Hence, it can be written, in general, as | Be(mV/m) = Bo (11.15) 14, MODES OF PROPAGATION The radio waves from the transmitting antenna be following modes of propagations depending upon ‘ween transmitting and receiving antennas €1C. LAA, Ground Wave or Surface Wave Propagation (Upto 2 MHD (AMIETE, May 1976,77, 78, 79,93,Nov. 1977 8, pe a spout a0und wave or surface wave (sometimes aso called as Norton’s surface wave) i of Pe tures broadcast and lower frequencies i. for medium WOKS tong waves and vr 8 yasgtive isa wave that is guided along the surface of the éarth just as an electromagn " mete transmission line. Surface wave permits the propagation around = Of propagations exist when the transmitting and receiving antennas are its usually, Be ‘suport ats lower edie bythe presence ofthe ground, The ground Wav (AMIETE, Dec. 1992) may reach to the receiving antenna following any of several factors like frequency of operation, distancea = —— va vector of ©. WAVES ARE Vertical yp, eR = Bileciri field ontact with the earth is ¢h "dry ; tically polarized i circ field in CO ith the earths sho Bg, y peal aoa re horizontal component on as of the wave, induce charges in the ii as . 8 I id el 1 W by the earth, The ground wave Propagation ‘a oe 1. While carrying is es — earth wt Fer ety aera here, he car OST tes in cata Ths Fehaviurof earth as a conductor may be dese Y add Capacitor. This behaviour ‘ constant k energy isabstracted from th su rface ofthe ear ac. When the surface wave ies ring over the face ofthe e27th, he urface way von, to supply the losses in the earth. Thus while pass replenished to a certain extent, by the energy ¢i ‘me is, however, | jl fi i fone erties pae fi a front present some what above the immediate Tite, jown 7 n ant cart, The atenuaton while propagating lng fore, suffers varying amount o mv ecu car depending pon ogre, sure meulris,permiiity and condi. ang cients increases and hence the mode of propagation is suitable for low = frequency ce upo2 Miz only. At higher equency, wave attenuation by ground is much more , jim wave propagation andi used invari frequency over to» same ground. Its also called as m wave in broadcasting Ail die broadcast signals received during day time is due to ground wave propagation, ee 11.5 path A, represents the ground or surface wave propagation. ‘DNOSPHERE PROPAGATION, DIRECTION —= —\TILT ANGLES. Ve VM eA al" & DIRECT WAVE INDIRECT i ‘GROUND REFLECTED TRANSMITTING wave ANTENNA eae | © Ground wave or SURFACE WAVE Tx = Transmitting antenna. Rx = Receiving antenna. Path A = Ground wave propagation, Path B = Sky or ionospheric propagation. W = Successive wave fons inl Penseonrapagaton = Tiangles ince nde naming anager ean path frm ig. 11.6. Tilting wave wave fronts in ransmiting antenna to receiving antenna round wave progagation. Besides ground atenuaion, there is still another way in which surface wave is attenuated i. due 0 crea oe eta radualy king more and more. Thisineone ene cof wave causes more shit sree dient tel component and hence the fed strength goes on reducing. Ulimately, a sm the transmitting antenna in wavel t f the losses mentioned above. : length, the surface wave dies because o but Saar pee He or ra 3P8¢ of surface wave propagation depends not only on the frequen? the VIF band ae a eae fans Can be increased by increasing the power ofthe tsi Not be effect “ + diffraction is more effective, O10» effective atthe MF band (higher side) where te iting 6 ‘The field strength ata dis fr i c: sed from the Maxwell eqns. as “om the ansmitting antenna due to ground wave has been callwe = HORM hy, ; 7105) | hao ‘meter | (11.16) 377, Intrinsic impedance of free space. Effective heights of transmitting and receiving antennas. Antenna currents. 2 = wavelength. a distance between transmitting and receiving points. the distance dis fairly large, the reduction in the field strength duc to ground attenuation ‘orption increases and thus the actual voltage received at receiving point decreases. Thi Feyg strength than that shown by eqn. 11.16, ee yj,howevel> soospherc abs jn less ° " ccording to Sommerfeld, the field strength for ground wave propagation for a flat earth is given by ws Ee d quay fp = Ground wave field strength at the surface of earth, at unit distance from the nee transmitting antenna, Earth losses not accounted. £, = Ground wave field strength, 'A = Factor accounting for earth losses called attenuation factor. d = Distance from transmitting antenna expressed in the same unit as Eo. unit distance field strength E depends upon {i Power radiation of transmitting antenna, (i) Directivity in vertical and horizontal planes. Ifthe antenna is non-directional in the horizontal plane, producing a radiated field which is propor- 1p the cosine of the angle of elevation (as in case of short vertical antenna), then the field at unit distance « km) fora radiated power of 1 kW is given by the general formula _ 300 VP _ 30011, _ Eo= d V/m = “1000 V/m = 300 mV/m 11.17 (@) = radiatea power in kiloWatts and = distance in kilometers. This is because, for a short vertical unipole antenna (grounded antenna), the field strength Eo at a dsuunce of don a hypothetical flat perfectly conducting earth is | Eo= oF yolts/metres ws 11,18 @) ¥ P = radiated power in Watts ad = d = distance in metres. “=P isexpressed in kilowatts (kW) and d in kilometres (km), then eqn. 11.18 (4) reduced to | gy = Ox Px 1000 aon | Sea L kW = 1000 Watts d and d = 1 km = 1000m 300 VP da Eo = v/m If, is expressed in ausles, then Ey = ae mV/miles = 186.45 mV/miles a. 11.18 (b) LS 1609or —— TERM 8 WARE i, radiated power of | kW, Fy = 300 mV/m ava dance of I km and Ey. ) 5 Ponieg we smile For other values of radiated power, Eo will be proportional inthe sua 45, power Pond wll accordingly be modified in accordance with ihe directly in horizontal plans’ ne (ded dct when the field ine proportional tthe cosine of the angle Of elevation “ing PP being effective power radiated in kiloWatt and d, the distance in kilometers The reas, A ineqn. 11.17, accounting for earth losses too, depends on om (i) frequency (ii) dielectric constant (tii) conductivity of the earth A Gna t,o above factor, expend in LMS Of aay yn, ‘mumerical distance p and phase constant b. ; te ‘These two constants p and bare dvermine bythe frequency, distance and dielectric a, of ground considered as a conductor of radio frequency currents and are given as follows, re, (© For Vertically polarised wave. The reduction factor A is expressed in terms OF 0 ag Parameters p and b. ‘The parameter p and b are related as a Noy 7 (MHz) (@) For horcontaly polarized wave The parameters p and b ae given by = ed a Plea reas ~(U12 b= 180" - Uy snd by = tan? (3) 19 wie b= wt a = Power factor angle of the impedance offered by the earth othe flow of cure. where J = frequency, in Hz, © = the conductivity of the earth, in mhos/em, & = dielectric constant of the earth relative to air. A. = wavelength in same unit as d. 11.4.2. The Ground wave Attenuation facior A. ‘The relation between nunerical disux®? hase constant is shown in Fg 11.7. The numerical distance p depends upon te fequency 28 ie freuen and ae nyt 1 the ansmitr 1s proportional tothe distance and ei frequency and varies almost inversely with the ower- factor angle of the earth, The i Forb< 5 Az —2+03p uy 2+p+ 067 a’ = . (11.26) 8 a resistive impedance to the i i gah offers a resis flow of ado agian b = 180" for horizontal polarizations ofers 9 eon CUTER when = 0 for vera yt olaization. The study of Fig.11.8 shows that Pacitive impedance when b = 9° for "0 1001000 PINUMERICAL oistancey ——» 7 Shows graph of the approximate values of ground ~ensaton facior A against numerical distance p based Fig. 11.8, Variation of ground attenuation factor A with onegn. 11.24. numerical distance p for different values ofb | Forp <1. The ground attenuation factor A differs i) Forp > 1. As the numerical distance p becomes greater than unity, the atenuation factor A ses rapidly. (ti) Forp > 10. Theattenuation factor Ais almost exactly inversely proportional to actual physical sat Hence for p > 10, the field strength of the ground wave is inversely proportional to the square of tance ‘The value of numerical distance «effect of diffraction and ink P of the plane earth (eqn 11.25) ground wave attenuation factor ground permittivity. The variation of p and ground attenuation factor A is 'g 11.1. This gives realistic answers for the distance less than dinax Where (11.27) fu is in km and f is in MHz. Typical value of the maximum distance is 125 km to 90 km “Tesponding to a frequency range of f = 0.5 MHz to 1.5 MHz. . - ‘emit of di the A slightly more accurate solution is obtained within the same limit of distance by oes te lative Permitivity €, of the ground path. This is achieved through yet another au ned byEPROP, [ ny = = DL ~ tan no withthe auxiliary parameter for numerical distance reduced 10 __ 08824 - f cos DY an s “May, where d is in km, f is in MHz and ois in mS/m. is ‘These assumptions assume a plane earth, A The expression for Ais then changed toeqn. 11.26. Th a a Veni Polarization and distances restricted 10 dmux . The difference in above equations and equations shown Cathieg is only of units. (ay ‘Sometimes, ground wave propagation is sub-divided into surface wave and space waye tion following sommerfeld. According to him, space wave predominates at a larger distance abovetnens where as, the surface wave is the larger near the surface of the earth. 114.3. Sky wave or Ionospheric Wave Propagation. (Between 2 to 30 MHz). The sky Waves ae Of practical importance at medium and high frequencies (i.e. at medium waves and short waves} for 7 distance radio communications, In this mode of propagation electromagnetic waves reach the Teceiving poiny after reflection from the ionized region in the upper atmosphere called ionosphere-situated between 59, 400 km above earth surface-under favourable conditions. This is illustrated by the path (B) in Fig. 11.5 ry ionosphere acts like a reflecting surface and is able to reflect back the electromagnete waves of frequencies te ost DOME Electromagnetic waves of fequency more than 30 MHz are not eflecea ee Further, since sky wave propagation takes place after reflection ons from the ionosphere, 50 it is also called as ionospheric propagation. Since long distance point wo point communication is possible with sky conve Propagation, $0 itis also called as point to point propagation or fommunication by engineers and scientists. Extremely long distance #4, round the globe communication is also possible with the multiple rections of sky waves as shown in Fig 11.9. In asiugle reflection frem he lonosphere the radio waves cover a distance not more than 4000 Ion ‘The signals received due to sky wave Propagation are, however, M Propagation of practical de ULF, and microwaves and communications © televisions, radar, frequency modulations etc.. utilize this mode of propagation. In, this mode of mer es ectomagnetc waves from the transmit antenna ey the receiving antenna either directly from 's troposphere region, Troposphere- is that portion of she : 11.5, path C), Space wave consists, i a rect ie. ground reflected components, It means in the meiavereaches directly rom the ransmiting antennato receiving antenna and in latter, the wave reaches flection a the mnt Tefetion from the ground, where the phase change of 180" is also introduced due vn a the ground, in the Sround reflected wave, Although both the waves (direct and indirect) leave rane eral antenna atthe same time with the Same phase but may reach the receiving antenna either it © OF out of phase, because the two waves travel different path lengths. The Strength of the eee ror i ing upon wh Yo waves are adding OF opposing in phase. Atrocinng ao ie Path alone depending upon whsai ivingantenna can usually "see" each other, a the line of sight distance i.e, range of com- i Scan also be increase by increasing the a gansmiting and receiving antennas as webs “sin Fig. 11.10. The curvature ofthe earth igysoaed ih espe eight of the transmitting and receiving + gots determines maximum range of com- a through direct waves. atl Fig. 11.10, Direct ray propagation.. inthe Fig, 11.10, with the heightof transmitting antenna, h ivi ; samuiaton range is TR. As the range is increased. eel Pe a thedliest wave anfedwhen eine sight distance from Two $ willjust graze overthe surnceof oath hen TS feos a ” 0 |. Then TS represents she maximum range of line of sight distance upto which communication is possible with the transmitting and -eeiving antennas of height ky and ,,. This line of sight distance can still be increased if heights ‘of euher tnenna is increased further (say hs) point U which means the range has increased from TS to TU. Lastly if tr neeiving antenna is moved to a distance which is not inthe line of sight distance just like point V of the came height h,, , then no direct wave signal reception is possible. In fact, the line of sight distance has now been extended by what is known as Space Communication «rsecaly Satellite Communication which has facilitated trans-oceanic propagation of microwaves with the preniaty of large bandwidth, By space communication we mean te radio traffic belween 8 ground station tnatelteor space probe, between satellites or space probes and also between ground stations itself via man rade communication satellites or natural space body (eg. the sun, the moon, the venus etc.) 1.48. Tropospheric Scatter Propagation or Forward Scatter Propagation (UHF and Microwaves ie above 300 MHz) (AMIETE, June 1974, 80,91, 92, 93, Dec. 87, 89,82) propagation is of practical importance at VHF. UHF et ped much beyond the line of sight ties, It uses certain properties of Forward scatter propagation or simply td microwaves. UHF and microwaves signals were found to be propagst Feeton though the forward seattering in the tropospheric iregulai posphere and is also known as Troposcatter as ik \ 4 . ‘wate in Fig, 11.11. This has also read to the dis- wa NO SCATTERING ‘eof ionosphere scatter propagation for signal “swaciss inthe lower end of VHF band. Therefore habe years, it has been established that it is comme tieve a very reliable ‘communication over snus range of 160 km to 1600 km by Using CQRNe wansmiuer and high gain antennas 1 0 ELecys ELECTRON ELECTRON ~ DENSI SiN) DENSITYIN) (b) night, Fig. 11.14. Electrons density variation durino /2\ 4...SOPACATION aE PRISTICS OF DiftEmae an ; Tz ar ACTERISTICS OF DIFFERENT 1ONIZED REGIONS pifferent regions will be discussed one by one, (AMIETE, May 1973, 69, Nov, 1964) D-region. depends on the alti est Thi 4 titude of th re pbination increases —— Vanishing of D-region all together. The ionization density, however is rim at noon and its electron density is ranging from 10! ip 10° per cubic centimeter, This D-region nator have the wel defined maximum asin other layer, " : does ‘ 7 iani. p-region is believed to be ionized by photo-ioization of NO which Prevails in that height. This xizationis produced by La (Lyman alpha) radiation from the Sun. A comprehensive view is that D-region is jew proto ionization of oxygen ‘molecule (02 ) at its first ionization Potential. Itisnot important from the HF communication is maximum in summer. The absorption i.e. ionization also increases with solar activity. Sometimes a peak in clecton density has been observed in | 11.6.2 Normal E-region. (AMIETE, Dec. 1989) E-egion (normal) lies as narrow layer of ionization just above the D-region in the height range of 90 ‘9140 km, having maximum at 110 km from the earth surface (Fig 11.13). Two scientists A.E. Kennelly in (USA) and Oliver Heaviside (in England) independently predicted (in 1902) tha there must be densely ion tr which acts as mirror for turning electromagnetic waves back to earth by reflection or :efraction, This is the E-tegion isalso called as ‘Kennelly Heaviside region or ayer’. The word layer is however leading ‘thas no sharp boundaries. This layer occurs during day light hours and has its maximum density at the sverageheight 110km which has a ippreciable effect on the direction of propagation of radio waves, The lower {imitis not sharply defined. During night hours E-region remains weekly ionized and during day light hours ‘s eights remains practically constant. The day to day variations are not much, Since obliquity ofthe sun ‘uesionization and hence electron density islow in winter and high in summer comparatively. The eleciron densityof E-region ranges from I 0° t0.4.5 x 10° during day and from5 x 10° to 10! at night. tis maximum ‘itoonin summer and increases with increased solar activity (ie. sunspot cycle). Critical frequency of E-layer Si the range of 3 MHz to. 5 MHz at noon in low latitudes and it varies ver) little from day 10 day variations. “tion is formed By ionization of all gases by soft X-ray radiations. roma 2¥2t is the most useful layer for long distance radio propagation during day light hours, although “lion trough F-region also takes place during day hours. The main function of E-layer i to reflect Some HE ‘waves in day hours,” 1163, Sporadic E-region (E,). (AMIETE, June 1979, Dec. 1989) “med ge it®S More stable regions like D, E and F in the ionosphere there exists an anomalous ionization Much ge POrAdlc E-region or layer and is denoted by E,. Since the presence of sporadic E-region is very ho Bula, hence it ig termed as sporadic. E-layer. Itusually occurs in the form of clouds, varying in a insisy wert M10 several hundred km across. its presence also is purely regional »nd its occurrence and edggy Mzation has no connection with sun radiaiion. The occurrence of sporadic layer is quite “and it may be observed both in day and night hours, and in any season of the E, isavery‘ANTENNA & WAVE PRop, Ge —— —_— = — Aga be even 10 times to that of normaly i on density (electron density may 1 rma ‘ mnt On th EE height and its height may differ from E-layer by not an vant caows to ons the sporadic E-layer occurs ‘ Wed, vaions On he ter and in equation appa See Posi gt occurrence increases with geomagnetic latitude. It appears in the a wet several fom? ten during which E, exists varies within broad limit but does not oa : cues pa hors ! sporadic — E-layer formation is still un 7 § TeVeal thar of E, is dhe ovitbeen casa dierent snes Sonedines they are produced by meteoric nian, at other times due to vertical transport of ion clouds i.e. formation of E, is by the infilteration Of charg Particles from the overlying F2 layer due to turbulent motion of air masses. At times, thunder Som op Bcomagnetic disturbances are also the causes for E, formation, 7 114s not important in long distance propagation but sometimes it allows unexpectedly goog, Mostly ionosphei efections fem 7, and from higher F-regions are observed simultaneously, iflyeya ‘smaller compared with wavelength. E, helps long distance : ‘Scatter propagation of V.HLF. signals. E, region also some- F limes produces M_ type of relection i. the signal path is like letter M. Two reflections are at F-layer and one at the {op of the E, region as shown in Fig, 11.15. 41.6.4. Fy, Fz and F Regions or Appleton ¢ Regions, ~_ (AMIETE, Dec. 1989)" : * ou) Ry __ Theregion ofthe ionosphere lying between 140km F'8- 1115. M type reflection from Ey and layers, to 400 km from earth surface is called as F-region or layer. Its average height is around 270 km. It i the day or sane 2nd region and isthe only egion which always remains ionized inrespective of hous ot day or seasons of the years. Appleton in 1925 showed that there was a further densely ionized layer ata sill Breater height than the Kennelly - Heaviside layer. He concluded this by hearing echo effecs by sending fs During day, sometimes after sunrise, the F-region is found to split up into two layers called 1 and F2 in low latitude stations throughout the year and in high latitude stations only in summet Fig. 11.14), ‘Frayer isthe uppermost region situated height Tange of 140 km to 250 km with average height ts behaviour is like E-region (normal), approximately follows the Chapman’ 7 According to an accepted View, F; layer is formed waves ae reflected from the F layer but moslly pes = the Fr layer. Hence main effect fF; layers to provide more absorption or HP veItfalls to ed : ae density of Fr-layerisranging from3 x 105 yg 3 x 108. why. F, ‘ phen ie eis wegon thatthe ionization disappears very so Being highest in height, the air density ~ layer is formed by the ionization of UV, 2 65. Outer Atmosphere or G-Region. ‘The away from the 400 km is known as "G-region" ‘wie bls girdling the earth and consisting ofthe ai having the shape of magnetic lines of force, 117. SKY WAVE PROPAGATION “"* Asmentioned already, the propagation of space and ground waves are limited by the curvature of the ach and hence these modes of propagations fail for communication over large distances. Therefore, pagnton overlong distance of thousand kilometer or more are almost exclusively performed by the sky vars orionospheric waves. The sky waves are reflected from some ofthe ionized layers of ionosphere and sum bck to earth either in single hop or in multiple-hops of reflections Fig 11.16. Thus fora sky wave of sishe frequency it is possible to cover any distance round the earth. Radio wave of frequency 2 MHz 10 30 Stie(ie. HF signals or short waves) is reflected from the ionosphere butin the day time the lower frequencies of2-30 MHZ are highly attenuated and hence efficient log distance communication or broadcasting is per- fomed inthe frequency range of 10 MHz to 30 MHz. Sincein night higher frequencies around 30 MHzis not ‘tallrefleted back to earth, so during night some what ‘ovr fequency is utilized for long distance or broad- Casing. Further sky waves follows different paths in a and at receiving point, the received ine vector sum of all, so fading occurs which ip. 17,16 single and mulpleshos reflections fork ‘minimized by A.V.C. or Diversity reception. Met cote —s ihe po¥ He Sky wave propagation, by neglecting carih's magnetic field frst and then by considering it : be discussed, Besides, the various terminology grown up around ionosphere and sky wave propagation int ital height, critical frequency, MUF skip distance, fading and also various ionospheric variation Fete. would be studied, Fat: Propagation of Radio Waves through the Tonosphere (Neglecting Earth's Magnetic or of Becles and Larmor) or Expression forthe Refractive Index of the Ionosphere, (AMIETE, Dec. 1979, 78, 1988, June 87, Agra Univ, M.Sc. Phy. 1986, Ghy. Univ. 1971) nized medium having free electrons, and ions when the radio waves passes through, it sot these ages ‘in motion, Since the mass of the ions are much heavier than the electrons so their motions loca all and neglected forall practical purposes, The radio wave passing through the ionsophere se cea theclectrons only andthe electric field of radio waves setelectrons of the ionosphere in motion, then vibrate simultaneously along paths parallel to the electric field of the radio waves and Sapna ‘Wons represent’ an a.c, current proportional in the velocity of vibration, Here the effect of field on the vibrations of ionospheric electrons lags behind thé electric field of the wave, thus "upper limit ofthe ionosphere is not known but region The outer-region of the ionosphere is occupied by the charged particles trapped by the terrestrial magnetic Tan toyANTENA & WAVE Papp 1120 ductive. The actual current flowing through a volume of yy, i recomponents¢-g. He us ca es | \Olage yg ei and hence Sul citys “ voltage by 90° and hence, activ caren, 8 cea A en cae cma eles in pein absence of HN TM rn i nee weet “Sei aa mt y to lower electron density Letan clectric field of value C = Exsint volts/motre / a sntheionospherewwhere istheangular velocity and, yy 1% field on each electron is given by ain, |e = charge of an electron in coulomb - isacting acrossa cubic metre of space amplitude. Force exerted by electric F = -eE Newton Lat us agume that there is no collision smeters/se in the direction opposite to the field. Force = Mass x Acceleration a | m= mass of electron in kg; 47 = Acceleration 1149 , wl then the electron will haye an instantaneous os a iy -£e= ! Ee « # oat Integrating both sides — E e ‘ far--[tavy=-Z fee smord, | by em ny En cos OT - -kt “no or | Eqeoswt| (N35) | mo . . constants of integration is set 2670. TEN be the numberof electron per cubic metre, then instantaneous electric current constituted by Bes N electrons moving with instantaneous velocity v is Ne. |—— 0) le (zis Jose (11368) or a me) which shows current je lags behind the electric field E = (Eq sin) by 90°. Besides this inductive curent (or conduction current component obtained by ionization of exis presence of electron and its motion), there is usual capaciti ic : . pacitive current ( ic) (or displacement cure an un-ionized air). The capacitance of unit volume is ky = 8.854 x 10? F/m Hence the capacitive or displacement current through this capacitance is yegn =D d i = ay (HOE) = bo 7 (Emsin wt)B= hE; bo = constan dé oro } ‘thus total current i that flows Uhrough a cubic mete of ionized medium ig = (11.38) Ee ie + ie = bE: cosa, Ne mo Emcosar 5 Ne = Femara ME] a ) “ bet Me ly we no mo ky Hence the relative dielectric constant w.r.2. vacuum or air Ne nae ‘Thus relative refractive index (j.) of the ionosphere wart. vacuum or =VWealk Ne ee ae | If we put the values ie, m = 9.107 x 10-*'kg; € = 1.602 x 10" coulombs air (j.e. un-ionized air) B= Ve. 1140) 1 ; = 8.854 x 10°? = —~____ ym, and w = 2ef (vide Ex, 11.1) i 36n x 10° we ‘eetthe desired expression for the refractive index of ionosphere as. u=¥ aN 11.4000) Pade Number of electrons per cubic metre or ionic density and f = frequency in Hz. If N is in ‘®icem, thea frequency is in kHz, then this relation still holds good. . Eqn. Tabada en hs ain i is less than one where that of un-ionized, medium ot Thus presence of electrons in ionosphere reduces the refractive index of the air and reduction is higher ‘tons are more. While deriving this eqn., it is assumed that the electrons do not undergo any in-elastic "sonduring thei motion and there is no dissipative loss of energy. 117.2 Mechanism of Radio Wave bending by the ionosphere _ (AMIETE, Dec. 1978) . indy bending of radio waves at the ionosphere can readily be understood with the help of refractive'y' mula of the ionized medium 11.40 i. J wn (1 wh wave =i - 2 (1140) ere, ‘ = ionic : = , in Hz : tutelage nae 1.40 shows that real values of refractive index Pinna 8 PEF Cubic cc. then f will be in kHz. Eqn 11 So deny Pte is always less than unity and the deviation ofp. from becomes greater, ei od |S higher nd frequency is lower. If f? < 81 N, then the refrac aa ae imaginary wich "nip fa condition the radio waves are attenuated at this fea 'nd the radio waves.- nm a = aa f radio waves by the ionosphere is gorenet by te = mae on ) and refraction ( r ) at any point is aw. the angle of incidence ( i Ea * sinr i angle of refraction ionosphere, so sin i < sin ie. an Since ji. < 1. for the ionospt x Fi Will B9 on rarer medium as illustrated in Fig. 11,17 Tf succggn Vig, ormal acer ceive iors ae of highs ances density ie. Ne > Ns > Ne >Na >No > Ny iy mean i : , My, Sees 4 n Fig. 11.17. Refraction of radio waves in the ionosphere i ing i. > Hs > Hs. Thus rl Bon decreasing and decreasing i. i > ua > Us > te willbe deviating more and more and a point will each where it travels parale] angle of refraction is 90° and the point Pm is the highest Kun be refractive index become A Wave enters a aes pDeath (ary) i ‘point in the lonosphere reached by: andNm be the maximum electron density at the Point P,, then ea deg * sin = sin ggr _ (thay to oe ugh itis actually a point of ref this point tol intemal reflection takes place and the wave Sets bent earthward and uhimately retuns) at point P, leave the ionosphere at point Q fier slighy in (0 the ionosphere and thus radio waves are elected back 10 earth after successive refraction it ie ionosphere 11.73. Crticai Prequeney, 18) (AMIETE, Dec. 1990, 84, 93, 1591, 1980, 72, 71, May 1978, 76,70, 69,91, PSC 8 The critical frequency of an ionized layer of the ionosphere is defined as the highest frequen jon Het ehecled bya particular ayer oa Vertical incidence. This highes frequency is called ce ihe for that particular layer ang i Is different for different layers. It is usually denoted by fo 0 f are for the particular regular layer rn ; {yet proportional to the square roo ofthe weve inthe layer as shown below, From 40. 11.40 and 11.41 we can rite as BY definition, at vertical incidence Angle of incidence 2j = 9 iNs can AS the angle of incidence 0 On decreas:on increasing and reaches to maximum elec. ens) 80 ack by the ionosphere i electron densi, oe rected is one for which nsity (Nm ). Then the m refractive index highest frequency tha H becomes zero, -y that can sing -——— sing = V1 - ote « f= 9VNy 11.44 (@) sadio waves of frequency greater than critical frequency will also be req ; ; n siidenc () is sficiently glancing so that eqn, 11 42s stisied atthe frequency moliea ake a oe will penetrate the layer concerned. However, it may be reflected back by a stl higher layer, Thus for awaveof frequency greater than critical frequency to be reflected, the conditicn is yer. sind > pm from eqn. 11.42 81 Nm >VI- f? (11.45) sini > 1 - & 81Nm Nme oe = w+ (11.46) ff mar phy 18. EFFECT OF THE EARTH'S MAGNETIC FIELD ON IONOSPHERIC RADIO WAVE PROPAGATION (AMIETE, May 1970, 76, 0, Dec. 1971, 76, 73, 1993) Aradio wave propagating in an atmosphere which is not ionized is not affected by the earths magnetic ii However, inthe ionized medium i. ionosphere the electrons are set in motion by the electri field of te adio wave and the earth’s magnetic field, then, exerts a force on the vibrating electrons producing twisting “con heir paths. This reacts on the incident radio waves. seo Thus the earth ic field splits up the incident radio waves into wo components e.g. the ordinary and the cara ordinary weves, The reperis of the ordinary wave are same as the waves Cae ups Mgnt field. The extra-ordinary wave is distinguishable from the ordinary wave only ne SPP ia Of layer or F layer. The two rays bend different amounts by the ono a hence: aan gene slightly different paths, The rates of energy absorption and velocities also ai pie oa "ctns. The two waves (ordinary and extra-ordinary) have eliptical polarization ere te a The phenomenon of spliting of wave into two different Ce vtinary wave depend das : 1 he peed aa splitting pe Sa ane aif ot extra-ordinay wave is always higher ( ‘of magnetic effects. (Sanam pi ie OEE aay as Maenete we tg of a eauonottie lina aio wave. Tecteerons sein simple harmorc fecting:motion (when m ieee ic field as iste’ in Fig. 11.18. v in substantially effects the radio wave PRON. t directions. The earth’s magnenc field causes the ef constant in different the frequency of which (ce. moving i.e. offering different dielectric ere rajectOrYs ton, e the complicated traject to mass ratio) of the particle, Foe ae TO ions in ionosphere to tract o ores) depends 00 the magnetic field and the ratio ‘e/m (charge earth's magnetic flux density B= 05 x 10% wobers/metre as gyro-frequency (fe )-Itis defined asthe i is about 1e4 MHz ands called: vency ( whose peried iseaual 0 the period of revolution of an electron in its circular orbit under the ing nae of the earth’s magnetic field of the flux B. e i e Thus o=a(E}sr20n (5) 1 ,(e)_ Be . otal} Putting the values of m, ¢, B we get _ 05 x 10° x 1.602 x 10°? t= a B14 x 9.107 x 10" _ 05 x 1.602» gt = _ fe = CEASE x 10H = 1417 MHz = 14 MH (14 log fy = (1.6990 + 0.2095 - 0.7980 - 0.9593) x 10° = (1.9085 — 1.7573) x 10 = (2.1512) x 10° fe = A’ log 2.1512 x 10° = 0.01417 x 10° Hz = 1.417 MHz ew ott the ae (f) of the indent radio wave is equal or nearly equal to f, (gyro-frequency) then ee Eresmanee ner cite oscillating electrons receive more and more energy from the inci and hence they have more chances of having ine oe ne eo ee em de ede eis eee re oi Collisions, thereby dissipate a large amount of energy Seta ase eee teuation is maximum near gyro-frequency (when f = f,) and a nd propagation work, During Yo a inDegion where collision frequency is high and thus n0kY used for communication, . Bey e frequency about 2 MHz, the attenuation is smal and hen Athigh frequene ies, when > f, the electron motions follows an elliptical path Fig. 11.18 (>)="4 0 6 4 (a) High frequency f> fy Critical, 2) | Critical frequency f = low frequency ANTENA & WAVE FRc cr wan, ea (AEBS Hike wave normal sani: paral othe ca gy (©) Longitudinal propagation (¥ < 1) Se eld, Vy = 0. 8 — 4 egn. 11.48 becomes . sme TATA 4 This has been discussed by Lorentz We This formula gives two straight lines cor 4 responding to X= 14 ¥ and X= 1 ¥ While for the line throughX = 1 — ¥, the wave 's circularly polarized in right handed sense and for the line through X = 1 — ¥ the wave is ° circularly polarized in a left-handed sense. This 1s illustrated in Fig. 11.19 (a) (©) Transverse propagation (Y < 1) When the wave normal is perpendicular to the cae agi Hild, Ye =O and ence thee 1719 fy Variation of? with inlongdina case hey ©qn. 11.48 reduces 10 or ~ 1170) and ~170(5 In this case the wave is nearly polarized with its eleciic field parallel to the X-axis. Thus the ony if the qu imparted to the electrons is parallel to the earth's magnetic field and the wave behaves as it would, if the fields were not present. Equation 11.70 (has zeros where X = 1 + Y and infinity where *=1~Y. Thisis between X = 1 — Y and X = las¥ <1. ‘The ordinary and extra-ordinary components to which the incident wave is split up, also suffer diferent Mounts of absorption in the ionosphere, In order to evaluate the absorption suffered by a wave while passing through the ionized region, it is better to consider Separately the absorption suffered in the region i. which reflects the wave in the lower region and which it passes through : (@) The absomption which suffered in the reflecting (or deviating) region is called as deviating absorption where << 1, (b) The absorption suffered in the lower region (where the wave is not reflected and He = 1)iscalledas the ‘non-deviating absorption, _, The non-deviating absorption takes place in the lower most D-region where collision frequency is ry high and in the case of absorption extra-ordinary wave Suffers more absorption than ordinary wave. Since extra ordinary wave normally suffers more and more absorption (except near the penetration quency of the ordinary wave), sky wave Propagation almost always occur through the ordinary wave. przaton of thereflecied waves arealsocffcted by thecnrn magnetic field. Foraplane poli! dent wave the reflected wave is elipicaly polarized, in penn the dretion ofthe propagation as cule angle with the direction ofthe magnetic field, the Ordinary component is polarized in left-hand © and extra-ordinary component is polarized in right-handed sense. The reverse is tre ifthe angle mad?WAVE *AGATION , Hence in northern hemisphere, i i. oA ing waves while that in Southem ner ey n —— aon te ae ine FECTIVE DIELECTRIC ¢ MEMEEC, it conforma eens OMIM to the et case, for the i z P CONSTAN, id case, 11! TONOSPHERE a AOLLISION FRE auneyin? Conductiyrry (8) OFTHE NETE, Nov. 1979, 77 C¥ (Y) . . . 7 71,77 . ies vy ions Te con ways occur d be ne JORT, Meecut Univ. M.Sc, Phy. 1985) es 7 i € 7 soos oe eorption of energy takes place hye Aireet momentum (atleast in p ofthe ps etal pitence bon rene taken Place om the radio wavey when the later parser, vibrating eectons A 0 inelastic cojisi, ‘illecs lonosphere. ee i Collision of oscillating electrons with particles of D = Average dissipative cot ‘Then eqn. 11.34 (b), modified as ative collision frequency di cE = mo + my GLI or , > tere = = Velocity of electron = yy J! ate = collision frequency of the electrons E=Eyd®' _ harmonically varying electric field Pn ary dv _, orn, From eqn. 11.72 “dt = IM we = jay Hence eqn. 11.71 assume the form as eE =m (jov) + mv =m (jor ») ___e€ or ve m(jo +o) rationalizing, we get eE x (v ~ jo) m(v + jo)(v - jo) eE(v - jo) OF (e+ oF) | i i jing with E due to This shows v has two components i.e. one is in phase with E and the other lagging : (11.74) collisions. ing i hich has : ie having ions, atoms, electrons) w' i it jlasma (i.e. ionosphere ‘The conduction current density in a pl n mn number of ionized particles per cubic metre with charge can be expressed Ne-eE o J=Ne-v= m(v? + a) But from Maxwell’s eqns. + (11.75) oD = bE | VxH=d+5 ope pe oE =J+ koa a : © = jokod*'=joE “Or NEV -~J®) 4 jak bo m(v +m)(3736 jo@-Ne m(v +o) or we = joke [1 - kom(W? + @) { 2 Neve who {1 - —Ne_ 5 “aera! ohb(? + a) m VxH=o-E+jokk EF This suggests that conductivity and relative permittivity are Nev _ conductivity m(v +o) Net = Permittivity (Relative ) and el ato) LO Hence in the presence of collision the refractive index Ne -Weyi-—_ 2 an mko(v? + a) ~ah Ne BET be (0? +o) (173) where v = collision frequency Fora given frequency cw, the maximum value of conductivity 6 will occur when © = v. Hence de to inelastic collisions of electrons, the ionosphere not only has reduced refractive index but also gives a fine Conductivity which gives a loss of energy, The collision frequency is a function of height as it depends on he thermal agitation and gas pressure. small and » << @, the conductivity becomes negligibly small and At great heights where v is “Hfective dielectric constant ie. refractive index will be given by eqn. 11.40 e.g. E “M6 “ly, ~Ung, 11.12, DEFINITIONS yssed The terminology around ionosphere and sk ion whi oped will be disc BE By aro y Wave developed wi now. The important being virtual height, MUR, skip distance, LP ON ” ae Virtual Height, (AMIETE, June 1983 92, De.c 1987, 91, UPSC ae be understood with help of diagram drawn in 2, i. st Eee ur wn in Fig, 117 that as the wavt 7 from the layer, itis bent down gradually rather sharply. a aa Fig. showst vat nthe ionized layers Curve and is due to the refiaction of the Wave, as happens in cace af path of th ¢ shanriem Since it is ™