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Fo .. po~t No. E .03 2 SeriRl No. 1952 1. "--- -

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Research Department JonU2XY, 1952. 11'igs ~ Nos. 1-17

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.An earlier report, No. E.0.39,' Serial No. 1951/8, described the developrlent of a Ions' slot aerial which ladiates horizontally polarised vlaves, a:'l.d has a' substantially uniforL1 horizontal radiation pattern anc3. yfi th [',]1 ac1n.i ttance-frequoncy characteristic sui table for vvide-b2J'l("J transYc1issions.

The possi1)ili ties of this aerial for applications where a ,directional radiation pattern is specifiod have been explored, Dnd experiments have sh6vm thn-t vii th the ac1di tion of a group of par8.si tic rod reflectors a yfidc variety of horizontal patterns is obtai..'1.able~ Several a.rrongenonts sui table for television and multi-chcnnol' f .m. transr:1i,ssions are c3.escribed.

Special consideration has been given to cp.ses where a service is required in Ghe direction of relatively shOrp minima in the horizontal rp.diation pattelns. 'In these cases a severe limitation is inposed, since the rac"Liatec1 field must remain constant in ~':lpli tude and phase within narrow I imi ts over the requisi to bend of frequencies.

Experiments carried out on a small scale modeloporating on 3.

frequency of aPl)roxiY'la tely 400 l\lC/S have shovm that this a~rial system C211 be a:lapted to satisfy these requirements for several type$ of directional patterns.

It is assuned that the reader is fm-:1iliDr with the contents of Research Dopartncnt ReP9r~t No. E~039, 8erial No., 1951/8, . (Fi:r;'st Intc:-m Report ori the Lon3; Slot Aerial), which describecL the devoloJ?1:J.ent of tho basic aerial structure sui tD.ble for widc­band transmission syste:,s in which [l substantially uniforn horizontD.l racliation pattorn is specific(l~. The aerial is siniliJ..I' to tbat descr:;'bod by Alforcl;ll:. bU)G tbe dimensions were chosen to give [JJ1 impec-:'anco charncterietic sui table for "\vicle-bend trans .... missions such as teleVision. Experiments carried out on sT'lall scalo nodcJls t+sin~: a sinGle s'lot approxinrttely two wavelengths

li . A.Alford - Long Slot Antennas, P:t-oc. Nat. Electronics, Oonf'erence, Vol~2, p.l4-.3 1946.

-2-

long, hwe shO'wn that a men.n power gain of l.,. db relative to a half-wave dipole can be achieved; the reflection coefficient doe s not exceed 4>~& for a .:!:. 476 varia tien in frequency.

For some broe.dcasting applicat~ons, airectional horizontal radiation patterns rire required; in the oase of theconr,10n channel opera tion of tVfO spc.cedtransmi tters, for exanple, it r:w.y be necessary to reduoe mutue1 interferenoe by limiting the racliD. ted field in the r.:.ppropriate direction; alternatively, if the trans'­'nutter is located 011 the ed.:;e of' the service Draa, a directional pattern might be prefm.~red.

The potentiali tios of a J.on[; slot aorial to provicle directional.· patterns ha.ve been recognisccl elsewhere ['.rid sevcrnl nethods havo been described for particular applicntions. Alford in the United stntes attached metallic flanGes or refloctinGscroensto the slot edg&s, which reduoed the radiation in a direction opposite to that of the slot, giving patterns charc.oteristic of cylinders' Vii th large clirunoters.

Unfortunately, tha radiation pattern exhibi'ts tvvo sn'lrp ninima which vary considerably in both phMc and Drlpli tude fot' snaIl ohanges of frequency. For the application cor.sic1a!'od 'by Alforl1. thore appeared to be no i:::1tention of provi(ling 0.. tolevision service in, the reverse directions, and conseqnently thearranGcment Vias satisfactory. In general terr:J.s, continuous reflecting or diffro..cting burfaces adjacent to the slot :cesul t in patte;rns of a ch:lracteri3tic np.ture which effectively limits the sector of uml.istortod tranmnission. for wide -banG. signals.

• In cOr1lYlon wi th other parts of a wide -band transmission system the characteristic8 of the aerif'.lmust remain substontially unchanged over a frequency band detenained by the mQXimum modulation frequency to be transmitted. Toavnid distortion, the radiation in every direction frorn. the aerial must remain constant in amplitude ,md pr-ase over the I'f'lquisite frequency band, ['. condition v{hich may prove difficult· to satisfy :for directional patterns in which there are large variations of field streng'~h 'with direction.

·The system described in this report uses D. [';l'oup of parasi tio reo reflectors oi "::hor adjaoent to or atJGached to the slotted oylinc1er, A "viele vo.rioty of radin.tion patterns can be obtained with different e,rranGements and tlll"'ee mD.in systems have been investigated

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expe:r:imentally; in. eE~ch cn.se the electrical cooracteristics ore sui to.blafor 'Vvide-bE~n(l operation. For the application in vievf, namely undistorted transLussion in 0.11 horizonto.l directions from the o.e1'ial, . those nrr['n[~er:lents Ell'e consiclurecl superior to those .proviously described.

Alti10ush thero were nO specific praoticQl requirernents when theihvestiC;Qtion vvas undor~tjaken, it '\vas necessG.!'Y to ho.ve in viCf\V' suitable horizontal racli8.tion pattern shQpes. If the front/ back ratio is tbo ST:1.'lll the pr,)tection o.[3ainst interferonce a.f't'::Jrded in the service area of fl. common charmel tr[:l.llsni tter is correspondingly STIle,:.ll; if the rdio is made too [;roat the difficulty of rQdiating Qll

. undistortecl . signal in the clirection of the minimum is also grer'.t. ConsiderQtion of the conflictine; requirements led to the c1ecisio'n toain at Q front/back ratio in the nei~;hbourhoocl of 10 db.

A description of the nethod of oper~\tion of the basic long slot qerial 'WQS included in th,o previous report. The horizontQl .. dii\tion patter.£1 of' vertical cylincJorsnot oxceedinG O.13'A. ir:.. diameter is aP2roxil:lately circular. A p['I'E'.sitic rod eleraent ple.ced behind the slot vvi th its n::::is pr'.rallel to the electric vector Cfu"1 be adjusted in length so tp.o.. t thoro-radiatecL field will partially canccl the i·ncident field behind. the slct but reinforce the fielcL in front of the slot. A gr'Oup of parn:).lel pDxasi tic eler'lonts is usee. spaced ono-half wavelength apDrtGO give zero rac1L'.tion in the vertical cQrection; a minimum of four elum0nts is requirod to maintain adequate cUrecti vi ty.

The experimental results arc presented 1)y consiclerin~s three arrangeaents using PQrasitic reflectors. In the first, dGnignatcd lI:Jirectional liJystom Aft, the [>;roup of four eleilem;s is mountod . behind the slot and spaced one-quartor wavelength from the nearest point of the cylinder. Fig.l shows an irometric si."8tch of this arran:::;eraent, which gives. the hori30nto.l radiation pattern shovm in Fig.20

For the second system (Directional Systen B) the reflectors 2~e attaohed directly to the cylinder as sho;,.-'.£1 in Fig.8, the radial extensions of enchref'lector bein,'!, O.2'A.; Tho hClrizontal rneliation patterns, ohovm in Fig.9, !:Fe similar to those characteristic of largo diametercylfuclers. A i'eature of this

.J+_

arrn.n2,'ement is the compensation exerted by the parasitic elements on the input a&nittanco, which remains substantially constant over a.±. 8% frequency band.

The third systo!U described is a oombination of the two forer~oins systems and is designated !tDire8tional System e". The two groups of refleotors are used as shown in Fig.12 and the resultant horizontal pattorn is apprmdmately elliptioal in form (FiS.13). This arrange-ment gives a horizontal radiation patten which is substantially constant in shape ovur the frequency band; in addition, a flat L'1lpedance-f'requency characteristic results. I

Finally, Fig.17 ShOV1S the horizontal radiation patterns of some' miscellaneous arrangements of parasitic elements in conjunction with u. long slot.

The experimental results included in this report have boen obtained using a

l small soale model operating at a mid-ba:'1.d fro:g,uency

of approximately 375 Mc/s. The methods of meD.s:xdng radiation . patterns and impedance characteristics have beon described in earlier reports.i in addition new equipment, by meEl1lS of which the cC'mpletc . radiation patterns wo~e displayed on a cathode ray tubo, has been used and has proved extre;nely vaiuable in the rapid cheoking of radiation patterns.

The refleotor system comprises a group of' parasitic rod elements mounted horizontally in a vertical stack and spcwed ITom tho cylinder as shovm in Fig.l. Four eiements are required to give the necessary front/back ra.tio and to maintain the optimum vertico.l radiation pattern. It Can be shovm that the field behinCl. the cylinder risos from zero at the surface to a maximum value approximately ono quarter-wavelength o.way and this influonces the choice of position ef t~1o reflectors. As tho spacing of the reflectors increases to ono quarter4'ravelength, tho front/back ro.tio of the 'cardioid! type po.ttorn increases and at .the smne time the variations with fl'Cquency aro reduoed, the incre8.sing field intensity permitting the reflectors to be tuned well away from roson~ce.

i

T):-...3 experimental resul t5 of' the qut=trter-wave spacing condition are :included as representative of this type 01' system; this gives a suitable horizontal r':ld.iaJ~ion pattern and leaves the input admittance of the slot substantially unaltered by the introduction of the reflecting ~ods.

The paxa·si tic reflectocs are shown a ttaehed to the cylinder by horizontal metallic rods. As an alternative a separate' vertic8l mast is permissible with or Vii thout horizontal rods used as bracing mombers.

Fig.2 shows the horizontal :radiation patterns taken o.t the 'ViVO freque.i.1.cies which delimit a .:t. 4% freqLl_encv b811d.,

The P'l ttcrns arc of f cDrdioid I sh"_pe -vii th [e Edngle nl1.nDnum instead of a sirigle zero, the mean front/back ratio beinG 13 db. The maxim'UJl1 variD.tion in nny r.;.orizontal direction over the requisite + 4% frequency band is + 1.4 db which doss not' cause ,any signif=_cant d.isto:.~tion of the -tele-v"isiGh signal. The gain of the system relative to a hfllf-wo.ve:Length dipole in vo.rious directions is . '.

Be8ring (00 in front ef slot)

, 0 90

Ro.dio.tcid field. relative to a haIl:i' -wo.velength dipole (mean over working frequQl~cy

banrl )

. +8.1 d::,

+2.7 db

-1.0 db

o , The aVOrllse field. radiated in e, + 15 Sl!ctor in the bD,ck

direction is -12 db relative to the m';;dmum.

Fig.3 shows the patterns measured i"'l a plane containing the axis of. the cylinder. and the slot, and Fig.Lt. the pa-ctorns in the normal pl311e. The acldit:wn of the parasitic group results in increased side lobe DJllpli tudes; this ho.s been counturc,cteo. by a small reduction in slot length which enduros that the aorial gain is a maximum at the mid-band frequency.

Figs. 5 and 6 show the input acJmi tt311ce in co.rtesian and po19.r form rospecti velY, in both Cases normalised to 70 ohrn.s. For comparison purposes, J?ig.5 shows' 8.:'so the corresponding charactor­:~.stic of the oonic:iroctional long slot acriD.l, v,thich illustrates the relatively small effect of the po.rasitic group on the input adnuttance.

The lengths of the pox8.sitic oloBents v{Ore 2.djustecl exporimen:j;ally, the .tvvo outer elements being mc'.de slightly shorte!' than the inner pair. This implies that the relative phD.so of the inciclent field varies from one pair to the ether which is consistent vvHh ti1.0 fp-ct that the phase of the field across the slot is retarded as the distnnce from the feed point is incroasoQ.

Fig.7 shows the tr.eo~etical .. input admi tto.nce of the aerial sys tem together with the nk"tchirl£,; network shovm in the Ci5ngI'am, basod on the measured acJmittance of Fig.6. The ne~vork comprines a mis­matched line of 70 ohms impedance, shu.'1ted n.t the i':lput with a negative susceptance. This system is sitlilc>.r. to the experimentally matched omnidirectional acr~al described in Report No.E039. The rosu~ts indicate that for 8. frequency v2.riation of + 4% the ren.cction co­efficient of the matched aerial sys tern rel2. ti ve to a 70 obr:13 co:uci8.1 feeder 'would not exceed. 5~,g.

Various methods of matching are possiblo, dependinG on the characteristic impodiUlCe of the m;:dn ·'cr2.n3YDission line. . -

The horizontL11 rD.cli2.tion patterDs of vertic[',l slotted cylinders depart considerabl;y frf)r,] the circular fonn as the c1iru~'1eter is increased 3lld this is an obvious method of redUCing the field in desired directions. Large str'.lctw'os mi8ht be nochMic,uly inconven­ient, howoveJ:', anC:. 'the phase velocity l.uside the cy:!.incler vvould be

•

/

·-7 ...

r3duoed considerably; two cylinders, one wi'thin the other would overcome the latter difficulty but complicate -the structUre.

The eecond system to be described enables patterns characteristic of large cylinders to be obtained using an arrang~ne4t similex to Directional System A except that the parasitic reflectors are mounted directly on the cylinder as sho,m in Fig.8 which is a relati'lelyconvenient ferm mechanically.

Fig.9 shows the measured horizontal radiation patterns vdth the lengths of the radial extensions of the parasitio reflectors adjusted to be 0.21 A. The patterns shov~ apply to a total frequenoy variation of .±. 7%; the variation of shape w~th frequency is about the SDnle as for Directional Systen A. The patterns are equivalent to those obtained with a plain slotted cylinacr of diameter 0.3A.

Figs. 10 and 11 show the input a&ni ttance of the arranr;oment in cartesian ancl polar form respectively. The outstanding feature of this arrangement is the slM.ll variation of impedonGe with'frequenG~; for a frequency variation of at least.±. 7% the input ac3mi ttariC€ ~emains substanti.ally unchanged. This ·feature can be exploited in combination vvith other arrangements, as for instance in Dirdc"-;ional System O~

3.3. Di~ectional System C

Directional System 0 shown in Fig.12, is a c0mbination of the twoprev:l.ous arrangements, designed to give a hori.zontal radiation pa~~te~n with consideJ;'able dii ... ecti 7i ty with a substantially constant input admittance,

The first group of reflectors is spaced o.li\. behind the cylinder and the lengths of the individual elements adjusted to be approximately 0.6i\.. The second group of parasHic reflectol's is attached directly to the cylinder, the radial extensions being adjusted too .19i\..

3.3.1. tIP..z:i,Eontal RadiatJ-on Patterns

Fig.13 shovJS the horizontal patterns obtained "¥i th . this arrangement for a frequency rarJ.ge of + 3.5%. The sbape is approximately elliptical vdth an average 'front/back ratio of 11 db.

The relative field streU[~th of a half-4i\fo.velength dipole radiatins the same pOvYer is also shovm in the cliaL:7'1:'JH and the ["orial [',Edns in vorious horizontal directions are as folloW"s:-

oBearins (0 . in front of slot)

300

60°

90°

120°

150°

1800

Rarlio.ted field rel3.ti ve to .half-wavelength dipole (mean value over worldng frequency band)

+7.7 ab

+3~O db

-0.4 ab

-3.9 clb

-3.6 db

Fi.c;s. 14 Mcl 15 show the mOJ.sured: input ncm ttance in c['.rtesian and polo.r form res pec ti vely; . the reln:'i:;i vcly small voXiations Vii th frequency illust't"ate the practicabili.ty of usin;s Q combiw.tion of ~oups of reflcctors as described.

Fig.16 shoVJS El. measured inp~t aCmi tta:llce \'vi th the aeria: matched to a 70' ohr1scoo.xial foeder in r.. r;J.anner not previo:J.Sly described in this report. The slot is :.. .... olded onc1 tho ,coo.xiB.l feedm attached betvlCon the c1ntre conductor .and the ec1[;0 of the slot. The lencth of this conductor provides a control for the phase rmcle of the aclmi ttnnce whilst the position relntiye to the slot ed2;C varies the transformC'. tion ratio. This nethocl hr..s the o.clvaniage of extrerle mechnnical siT'1plid:ty coupled with ease of ac1justnent, a factor of consid.er3.ble importnnce in the final, o.c1justmonJ.;'3. In this p['xticul['.,r instD.nce the refh)ction coefficient does not exceed 51& for a frequency variation of .:±:. 47'6... This methocl of matchins mo.y be convenient for. other ::;.rroncenents.

-9-

Fig,,17 sr()ws the horizonb.l IDclia-t;ion patterns obJvained vdth three further combinations 01 slotted cylinder end parasitic :cflectors. The first resembles Directiork~l System A ~ith the spacing of the reflectors from the cylinder reduced from 0.25X to 0.125X. The pattern,is of the Icardioi~1 +ype bu+' the front/back ratio is now reduced to approximately S' db. The dotted cirules represent field strl-rgths of -6db and -10 db relative to the maximum valuo.

The second arrongement utilises r8clb.1 refleotors as in the case of Directiono..l Syst'31n B Gut incli1J.oc1 forvlO.rct at n.n angle of 450

• This reduce<=, the radiation o..t right., D.Y1Cles to the plane conto..inine; the <1.~:is 01' tne cylinclor o..nd the slot, giving a moctifiod !figure of eight! patte~n. ~ the final c[':se the p3.ra.sitic olemen'cs ['.re loc3.ted 3.S in t:le previous arr3.nf.~ement but the individual mOTJ'bers of the radiD..l pairs are miequal in length, and th:"s l:osults in an f1.:...yn;metrico..l p[tttorn. These exo..mpler e.j.'e inclua.ed 'tJ illustrllte '~~lA wicle vllr:i.ety of pJttern types obtainable which remain subs.!-;antic.lly unchanged fd:>:> a consider.lble vario.. tioL in freq',lCJncy c

4 J Conclu2i.on

The development of 0.. lonG sl:)t aerio..J. system has been continued '\id th len investig'ltl.on of the use of parhE.i tic rod reflectors to provicle directilma: horizontd raclio:don patterns. EX'periments UE.VG shmm that, wi ~h simple arr'o..l1[;ements, a wide vEI.I'iety o~ patterns is obb.ill3.ble; 'noreovcr, tlW aeri..d ~yste:r:11.1 are sui to..i:lle lor +'he transmissir;n of signals whj.ch oc<:!'Llpy a vlide band of frequencies such as tolevisi.)r and mul ti-channel sound services in e~hA existing v.h.f chD:'1l.els.

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24.9.51

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AERIAL DIMENSIONS (MID-BAND FREQUENCY 370 Mc/s ".SI cm)

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FIG. I GENERAL ARRANGEMENT

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SECOND INTERIM REPORT ON THE LONG SLOT AERIAL

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ISSUE 1

24 - 9 -51

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FIG.6

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ADMITTANCE - FREQUENCY CHARACTERISTIC

SECOND INTERIM REPORT ON THE LONG SLOT AERIAL

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24-9- 51

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FIG.7

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SECOND INTERIM REPORT ON THE LONG SLOT AERIAL

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ISSUE 1

24- 9 ·51

BBC

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CYLINDER INSIDE DIA. WALL THICKNESS

SLOT lENGTH SLOT WIDTH

SIDE REFLECTORS LENGTH (0) DIAMETER

REAR REFL ECTORS LENGTH (f ) DIAMETER SPACING FROM CYLINDER (5) VERTICAL SPACING (h)

FIG.12 GENERAL ARRANGEMENT

DIRECTIONAL SYSTEM "C '

10-5 cm = 0-13)" 16 SWG =0-0025)..

170em = 2 ·09 X. I -35 cm = a-a 17>"

16 cm = 0·19)" 0·9cm - 0-011>"

SOem = 0-6 )" 0-5 = 0:006 x. 8 cm = 0-096 )"

41'Sem = 0-51X.

SECOND INTERIM REPORT ON THE LONG SLOT · AERIAL

FIG.l3 HORIZONTAL RADIATION PATTERNS

ISSUE , 24-9-51

• I · i

BBC OS/44/ P

FIG. 14 ADMITTANCE - FREOUENCY CHARACTERISTIC

SECOND INTERIM REPORT ON THE LONG SLOT AERIAL

RESEARCH DEPT REPORT

~i~9Jf

BBC .SECOND INTERIM REPORT ON · THE LONG SLOT AERIAL

ADMITTANCE MEASURED AT THIS POINT

ISSUE 1

24-9- 51

BBC

"

,_ .. - . - .- .4-1. T--+--

. .LL

FIG. 16

O'3S(:m I·Ocm 4t~

MATCHING SECTION 15cm

I ISS.W.G.

ADMITTANCE MEASURED A THIS POIN

lo:iQ OHMS

DIRECTIONAL SYSTEM C

ADMITTANCE - FREQUENCY CHARACTERISTIC (MATCHING SECTION INCLUDED)

SECOND INTERIM REPORT ON THE LONG SLOT AERIAL

ISSUE 1

24-9 ~ 51

1;1

... Jt!1 -11 :'1 .. jJ ~I

'.111 BBC

0°

2700 90° h ,J.

1800

2700 _900

I h

Y

DOTIED CIRCLES INDICATE FIELD STRENGTHS OF-6db AND-IOdb RELATIVE TO FULL LINE CURVE

FIG.l7 HORIZONTAL RADIATION PATTERNS, MISCELLANEOUS ARRANGEMENTS

SECOND INTERIM REPORT ON THE LONG SLOT AERIAL

00

t

lj - £

PL AN

s = 0 · 1>'" .f :: 0·5 >" £, = 0-55 A. tz " 0-65 A.

0°

1t~ "'vi

e = 45

l :: 0-36 A. h = 0-5>'"

8 :: 45 tl = 0·19 A..

~2 = 0-36 A h=0·5>"