Banerjee, S S

Neogi, G C

English

IACS Institutional Repository

26POLAR DIAGRAMS OF ULTRA-SHORT W AVE HORIZONTAL TRANSMITTING AERIALS*By S. S. BANERJEEANDG. C. NEOGIIR fcn 've c i  lo r  t ' u h ih a l io n ,  F e b r u a r y  ly , loj?)ABSTRACT Polar diagrams for various distanocs nrar horizontal transniitling aerials of iliffnent lengths have been obtained for ultra-short radio waves, A modulated valve oscillator « as used for em itting waves from 3 to 8 metres in length. The field strengths were mca.«ured witli tl.e help of a calibrated ultra-short-wave receiver of the super regenerative type, The ob- -fived values of field strengths were compared with those deduced miitliematically calculated on llie theory of "  induced e m .f.” It has been noticed that aerials of different lengths radiate the energy along tlifferenl channels and the polar diagrams for a particular length of the aerial ilcpcnd on the distance at which the field strengths are measured.i n t r o d u c t i o nThe importance of radio communication with short waves below hundred metres has been realized sufficiently long time back due to the numerous advaii- lagcs which these waves possess. Recently, however, the necessity has been felt of going down to still shorter lengths of the vt'aves specially for television, aircraft communications and upper-air w’eather observations. These .waves which are Ik Iow ten metres in length are knowm as ultra-short waves, and their application^ are still in the early stages of development. Hence the study of these waves has ))cen felt imperative and it has opened a great opportunity for further investi- .gations. The application of the technology of ultra-short waves, as yet largely unexplored, has been proved to lie most essential. The beacons and blin4 landing are well-known uses of these waves and they are also used for sending secret messages-The guiding factor in designing a broadcast transmitter is that it should be able to produce good quality signals within a certain stipulated area afomid the transmitter, A definite knowledge of the field strength, near the trans­mitter, is therefore essential. The bfioadcast engineer tries to design the trans­mitter, so that most of the radiated energy is directed along the ground, in order to secure the largest possible service area.* Coninnintcnted by the Indfim Physical Society,2/2 S. S. Banerjee and G. C. NeogiIt is evident that the field strength will not only depend upon the jmwer of the transmitter Init will also depend upon the radiating system. Therefore it is necessary that the aerial must be carefully designed; and, in the case of ultra- short waves, special tyjres of aerial have to be used due to their short lengths. Oi]account of llic limited range of the transniiltcr, one should be very c^irefu] that the Asaves may be travelling in the right direction. The directivity de])ends only upon the type of the aerial we use, and many investigators from lime to time have tried to develop such directive aerials, and studied their polar diagrams, mostly with vertical aerials.Though various investigations have been made in this direction, it apx^ears that the study of single wire as directive aerial has not received adequate allention. In this cormniinication, polar diagrams for ultra-short wave horizontal tmnsmiiling aerials of different lengths for various distances have been investi­gated. In order to draw the polar diagram the intensities of raBiated field sttengths were determined by an ultra-short wave calibrated receiver of super regenerative type. For the purpose of radiation a iiiodulaled o.scillator generating waves from to S metres in length was employed in conjunction with horizontal aerials of different lengths varying frcjm quarter wave to one wave in length. 'I'he plots for variation of field for different distances have been shown. Hxpeii- mental polar diagrams for difi'erent wave-lengths have been drawn and verified bv those obtained by calculation- Derivations of the necessary mathematical equa­tions involved for calculating the field strength for multiples of quarter wave long aerial have been shown hy the method of induced e.m.f.**T IT K O R  K T  I C A 1/The method of calculating the field strengths for various lengths of aerial has,been showm bcloyv.If we tahe an aerial three-fourths of a wave long, a current anfinode will be formed at the input end- I'he equation giving the current at any point in the aerial is rei)reseiited by ,C O S  m y ( i )where'm —jtt/A and y “ distance along the aerial from the inpul end (hr in Fig. i.Flu.Polar Diagrams oj Vltra-Short Wave Horizontal Aerial '21Now the retarded scalar and vector potentials and A may be denoted bydiikI A - K  f ' j - l  , ,Vdy... (2)-  (.t;ulicrc f*r I nnd J denote the retarded instantaneous chai>;e and current jespec- iively ; K the unit vector in tlie direction parallel to the aerial, and the velocity nf li^llt,1  lie relation hetween and / is ^iven bydi  ^ ily ■iM'oni ciiualion (j) alon^ » with the above relation, we RetandI ]i))l .(r= r' sin n/y.y^ 'Thus from ecjiiations (i), (2), (3) and (4), \ve have A ='I'lit potential parallel to the aerial is given by 1 dA(4), f ' . - j m d sin n/;v . ■ ... (5)/c J dJ  0 , f 1 - 'I J<»lcjmdcos VIV dy. dU(, =  -Oradj,'/' —= I r <J or (it  ^ ““ jmd d, l<oi i ccos my ciy — '^' ,, r  IJ'Substituting 1 =  bi equation (7) wc getF,^jm d 2cos my dy. (7)(S)Now, sei)arating leal and imaginary parts, we havep‘ =; -6 0  (0)V 1' d\ -  2lc/icos &If the aerial is quarter wave long, a current node is formed at the input end. Therefore the current at any point Y  (Fig. i) along the wu're is given by the 1 elationf — sin my. ... (to)2--I423P-- -TV214 S. 5. Banerjee and G, C. NeogiProceeding as above, it can be shown thatE|, =  2j  ^. - jw d i  - i m d 2d -2-cos mlSubslitiiling / =  X/4 in equation (i i) and taking real part only, we have1/ 1 Sin ?nu Ih^==6o I -----.(a)(12)The equations to calculate the field in the vicinity of half wave and one wave- long aerials can lx: obtained from equation ( i i j  by substituting the proper values of the lengths of the aerials. 'I'hiis it may be shown that the potential for half wave aerial will be given by1 ' . = , I Sin ind\ , Sin ■* di“ —2/di costal30 H ------ +  ----I V/'"* + —2/di CO.S )Similarly the potential near full wave aerial will be given by1.- -  ,  1J Sin md I Sin jn v' / ■ + d — 2ld 1 cos 0 } i ' .«  -  3 0  --------- f -I V  l ^  +  d v ^ - a f d i  c o s f i *  )(13)(14)It should be mentioned that the above relations shown in equations (13) and (14) for half wave and full wave aerials respectively have also been deduced by Carter," The polar diagrams for all the above lengths of aerial have been drawn for different distances from the aerial. Four typical diagrams have been shown below with an associated table showing the variation of field strength with angular direction in each case.'I'ablc 1 show's the variation of field strength for an aerial 3A/4 long calculated from equation (9), for different angular directions at a distance of one wave­length from the input end. The corresponding polar diagram is shown in F'ig. 2.<F io . 2Polar diagram o f  fin  fierial 3 v/4 longF i g . 4Polar diagram of  full wave aerialSimilar variations of field strenfitli for half wave and full wave aerials are bhowii in labies II and 111 resi>ectively, while their polar diagrams are shown in I'igs. 3 and 4.TA«t,E IPolar Diagrams o f Ultra-Short Wave Horizontal Aerial ziSAngular direction Field strength in I Angular dirediun ' Field strength inin degrees volts/metre X 1 /A in degrees j volts/mclre x i / \0!—240.0 1 lOU ” 3 ’^ «>10 -198*/] 1 Hu j — 14.820 -  97.1 1 20 i 5*19,7 1 21.3.K' 73 I/JO 1 2g.o76 7 150 , ! 33-160 3‘^  ^ 160 34-470 -  9.0 170 34 38U /|o,8 180 34.390 “* /^ 8.ot a b i .u nAngular direction Field slrenglh in Angular direction Field strength inill degrees volls/inctre x j / a in degreesJvults/iiietre x i /a0 0 luu -2 3 .610 5-1 n o - 2 3 .62u 18 6 120 ’"'20.333-” 130 -^5 440 40 6 j/|() - 1 0  250 .]I.O -  57fif) 25,6 i6ti -  3.570 9 -2 170 “  1.0iSo 0.6 itSo u90 18.2'i'AHUi 111Angular dirceiioii Field sluntdh in Angular direction I hield strength inill degrees. volls/incire x x/A m degrees i1VO Its/met re x j /a0 — 186.0 100 3.910 -153 .1 110 17.620 1 -  7<-94 120 17.230 ! 6.4 130 15^ 340 4o'i 140 Il.O50 2 9 . 3 150 6.4,60 0 i6u 2 .8 ' '70 -- 20.9 170 t>-7 ,80 ■ -  22,7 090 -  10.9216 S, S, Banerjee and C, C. NeogiIt will be observed that Ihu field strength will not sliaw any change lui dideient angttlar directions for a quarter-wave aerial at a fixed distance. Tlie variation of field strength is, therefore, shown in 'fable IV  as the distance from the in|)ut end is altered. The positions of maxinunn field strengths in the presetii case and the nature of tlie curve arc shown in Fig. 5.Pusitions of ninxiaiiim field slrciigtliT ajilk IVDistmioc from input a n dField strength in | volts/mclie X j / A  jDisliincc from input endrAcld strength in voHs/metre x i /aA / 4 11 3A/4 So! A 111) , ,|S1 7« :1 1 IK X  J* K R T M U X T  A Lv\ modulated oscillator of the Hartley type was used for generating the ullni-^  short waves. A constant modulation was effected by a leak and a condenser. A super-regenerative receiver was built for measuring the field strengths. A milliaimneter was put in the anode circuit of the detector stage for observing the change in its deflection due to the incoming signal, 'fhe receiver was calibrated by the same method as described by Banerjee and Parmanand,^ in a previous paper published in this jomnal.The ultra-short wave g e n e r a t o r a s  connected with a horizontal aerial which was placed in a fixed direction and the receiver was moved along the circum­ference of a circle with the input end of llie transmitting aerial as centre. The field strengths were measured at different angular directions with respect to the orientation of transmitting aerial. Special precautions have been taken, as far posfeible^ to. «av:oid the reflection of the waves from the neighbouring walls o f theI'.iboratory. Most of the observations have been taken in clear open space on the tci race of the buikling, the reflection coefficient of which was actually found to be very small. This was evident from the absence of the formation of any inter^ lerencc pattern due to the direct and rellected rays. This pattern could always lie found when such experiments were performed on the round.The wave-lengths used were between to 7 metres and the lengths of the aerials were A, 3A/4, A /’ and A/4.'J'lie observed values of field .strengths in the neighbourhood of a horizontal transmitting aerial, A/e long, at a distance of A from the input end, and for different angular directions, arc shown in 'I'able V below. Tlic [lolai diagiam foi tile same is depicted in Fig. (3.Polar Diagrams of Ullra-Shorf Wave Horizontal Aerial 217F ig . 6Polai diiignim ol liall-wiivf Imi i/onlnl IraiiMnitliiif; .iciiulTAKUi Vin dt*p:ri’esstrength in niilli-volls/niclreAngular distana- in degrcie.sstrenp^th in niini-volth/iuetrc-0 ^3150 287It) 6 i) 6020 161 70 (J230 801^ 0 276 i1S U M M A R Y  A N D  C O N C b U S I O NField strengths have been measured at .short distances in all directions from Imrizontai transniitting aerials of different lengths. Polar diagrams for the aerials have then been shown* In order to verify the results obtained experimentally,2IS S. S. Banerjee and G. C. Neoginew equations have been derived based on the theory of “  induced e .iu .f."  for each length of the aerial. Field  strengths have been calculated from these equa­tions, and theoretical polar diagrams have also been drawn. A  modulated oscib lator was used to energise the transmitting aerial and a calibrated super-regenera- live receiver w-as employed for the purpose of measurements of the field.1 1  may be mentioned here that the equations derived in the jiresent investi­gations could be applied for computing the fields, radiated from quarter-wave transmitting aerials and their odd multiples in addition to half-wave aerials and their multiples as generally u.sed. It has been observed that the polar diagrams for such horizontal aerial will depend mainly on the length of the aerial and on the distances from the input end at which the field strengths are measured.In conclusion, the authors have great pleasure to record their sincere thanks to Prof. Dr. II, Dasannacharya, Head of the Department of Physics, for giving facilities to carry out the above investigation.Wjukuws vSkction,I’ny.sics Dmi'AktmenI',Hknarhs Hjnuu Univeicsitv.R 1$ F n  K K N C li S• A. AlicKMii'i, /')(»( . /. R. I'.., IS, (lya?).V .  Siiutliworlhj /’/111. 1 . R ,  E . ,  18, 150a >' 1 1 .  Ilianiond, I ' l o t . 1. R. E . ,  21, 808 (lys-l)- < V. ('.. Rear, I ’ loc. I. R. h., 22, 8/17 (193/1).<■ 1'. R, Slaiisd, P i o c .  I. R. E., 29, 802* P. S. ChiUt , I'rnf. /. R. I'.., 20, joo/) (1932). ^ S . S . r.niic’i-jc'c and l ’iiiiii.iiiand, I m l i i i i i  J o u r i u  n l  I ’ l i y s . ,  14, 323 (1940)

Polar Diagrams of Ultra-Short Wave Horizontal Transmitting Aerials

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Polar Diagrams of Ultra-Short Wave Horizontal Transmitting Aerials

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