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Research Department

Joint Report by:

Aerial and Field- Strength Sections, Research Department .

REPORT NO . E. 018 27th November, 1943

Serial no . 1943/27 Figures Nos . 1 to 5

Tt-lE PROPOSED 3RECTION OF A ~IINCF..ARGER MAST AT BROOE1'fAl'JS PARK

SUM1"IJARY

Followinf the proposal to erect a 11Jincharger Mast at Broolanans Park, the whole of the aerial system at this station has been reconsidered .

It is concluded that thG use of a Wincharger Mast will not improve the existing

service appreciably . If , however , it is decided to proceed with this proposal , the best position is in the north west corner of the site . The best arr angement v,ill be to use this mast for radiating the 449m service . An improvement of the 342m service is required , and it can best be obtained by increasing the power of the existing transnitter , or by the use of a directional aerial system or both .

1 . Th1TTIODUCTION

Considerat ion has beGn given to the question of the proposed e:>ection of a Wincharger IiIast at Brookmans Park , as described in a memo . fromA .C. (E) to H. R. D. , dated October 26th, 1943 . Special. consideration has been given to the proposal that the Wincl:larger Mast shall radiate either the 449m or the 342m transmission, and tentative conclusions have been drawn as to the likely Gffects that such arrancelTLGnts might have upon the other transmissions .

In ordor to ma~~e any approximation in regard to this question it has been necessary to investigate to some extent the existing conditions and to endeavour to aCCOQ~t for the shape of t he polar diagram resulting from the present arrangement .

The first part of the report deals with a brief summary of the present arrangel1'lent and latterly the likely effect oi' the proposed modification is considered.

A site pl an shoning the position of masts and aerials is attached, (Fie . 5) .

2 . THE PRESENT ARRANGEMENT

2. 1. T!1e N.E . mast (1;1 . 2 .) is at present radiating 449m with an efficiency of 7'Zfl/o . The polar diagram (Fig . 1 ) is not greatly distorted , but the general tendency is an increased field-strr:mgth in the north-easterly and south-westerly directions, which is conSidered clue to the proximity of M. l.

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- 2 -2 . 2 . The N .1N . llJa~t (j\I . 1. ) i s at present radiating 373m with an eff iciency of 86%. There is only slight distortion of the po13.r diagram (Fig . 2). This j.s considered to be mainly due to the 342m r adiator suspended from the t wo southern masts, M. 3. and. M. 4 . I n [;enerrtl , the distortion is such as to provide ml3.ximum r adiation in a _south- G2sterl y direction .

2 . 3 . 'l'he T aerial behTeon t he sout hern masts is at present radiating 342m with an efficiency of 89?~ . The polar diagram is rather badly distorted (Fig . 3 ) . It shOl":.'8 maxi mum radiation in the direction north-east and south-west , and minimum radiation in the direction north-vTest and south- east . The reduced field­streD~th i n the s outh- eastorly direction i s very undesirable, as t he re~ulting service in Kent and some parts of the London area is unsatisfactory.

This condition han been in operation since 1941 and complaints of poor reception in the south-easterl y d.irection are becominG more frequent at night time owing to t he strong indirect r ay i:,_terference from the other stations in the group . Fiel d- strength measurements are available which confirm that the service in parts of Kent i s considerably poorer when compared with the Pre-War "London Regional" Service than -Iloul d be accounted for by the Wartime decrease in power to 50 kW .

At the end of this r eport consideration is given to a proposed modification that might be adopted to improve the service in this direction . It appears that the distortion is mainly due to the S . E. mast , which i s radiating 1500m, and provides a sU9port for t he 342m triatic . The S.W. mast i s free and its effect is t her efore likely to be negligible .

2 . 4 . The (Fig . 4) aerial . VTest and

S . E. Ill3.st i s r adiating 1500m vii th an efficiency of g% . The polar diagr am i s only slightly di storted , this being due to the presence of the 342m T The general tondency is a r educed field- strength in the directions north-

due south , and eome correGPondil~ increases in other directions .

3 . PROPOSED ARRA.N"GEMENT

3 .1. The proposed. arrangement cons i sts of erecting the Wincharger Mast at the northern extremity of the s ite to radiate the 449m transmission , leaving the N. E. mas t , whi ch i s at present r adiating 449m, free for any other requirement . The arr angement is indicated in the attached diagr am of the site plan.

The following broadl y outlines the result of an investigation into the effect of such a modi f icati on on all the transmissions concerned .

3 . 2 • .449m Transmisston . It can be expec t ed that the Wincharger Mast in the proposed posi tton , if providod \"Iith 11 good earthinG syst em, vfill r adiat e 449m with an effic i ency of approximately 85%.

Compar od with the N. E. mast at present tn use on 449m and radiating with an

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efficiency of 7210 , this would represent 0. 6 dB increase in fie l d- s t rength . The polar J.ia.crnr.1 'will be mainly infl uenced by M.l. and M. 2 ., but the distortion is not lih:oly to be serious . In the case of the, existing polar diagram there is a maxiHuJ1l field- strength in Q. north- easterl y direction of approximatel y -t 3 dB relative to the mean field- strength. With the proposed arrangement it is possib l e that ti::) field in this direction will be reduced by 3 dB in spite of the increased effi ciency, but it is unlikely that a serious reduction in field- strength will occur in any other direction . The possible reduction of 3 dB in Q north- easterl y direction will affect places such as Cambridge, Ely und Newmarket . This might be unQesiruble in the case of Cambridge , which has recently lost its "H group" transmittor , but even so the fie l d- strength will be of the or der of 15 mV/m, which is Ctdequate .

The effect of the southern masts and the 342m T aerial 'will certainly 'be negligible by virtue of distance .

3 . 3 . 373m Transmission (M.l.) The effect of the Voftncharger Mast on the 373m

tr<111smissio]1 is likely to be of the same magnitude as that of the 342m T aerial , as doscrHed in §2 . 2 . It is possibl e therefore that <1n additional distortion of +3 dB ':,ll3.jC be introduced , and the overall distortion may therefore be as great as '6 dB in some directions , referred to the mean field- strength . In vi8vJ of this

it may be necessary to include a 373m r ejcctor at the base of the Wincharger 1'Tnst . Thi::: nay also be necessary in order to avoid intermodulatton difficulties .

The effect of fii . 2 . i f left free after the transfer of tilC 449m service to the

Winchurger nlast , will be the same as at present . If M. 2 . is used for any other purpose its effect will be the same as at present , provided that the existing 373m rejector is retained in circuit . The effect of M. 3 . , ];1. 4 ., and the 342m T aerial n ill be ~cl1anged by the proposed arrangement .

3 . 4 . Yk2m Tr:J.nsmissioll-:. (T Aerial - Southern Masts .) As mentioned in §2 . 3 , the 342m trc..nsmission appears to be mainly affected by the S . E. mast (M. 3 .), and the effect of the ,'Jincharger WJB.st will certainly be negligible .

3 . 5 . ,;t.500m Tral13mission (11. 4 .) In the case of the 1500m transmission , the

effectivens3s as a refl ector of the Wincharger Mast will be extremel y poor unless it so happened that a cO~1dition of resonance existed at this wavelength . This of course is very unlikely and it 1ioJQuld be a simple matter to avoid such a condition because of the lmv radiation resistal1ces of all radiators at this l ow frequency .

-1 . Al.,'l'::!.8_' TIVE PROPOSALS

A'" mentioned in §2 . 3 , r eports received indicate t hat reception of the 342m trallsmj.ssion in the south- east is poor, and alternative proposals have been put

fOTIJard with a vim', to improving the service in this area .

-4---!t.l.o To Radiate 3!t2m.. from the \Yincharger Mast crected as pro;p..£S..E..4 As previouslY quoted, t he efficiency of t hc T aerial radiating ~ is 89,% and no appr eciable i ncrease in efficiency could be expected by adopting thc above proposal. Furthermore a t t his wnvelength the increase in the mean field-strength due to t he increased height of the Wincharger Uast would be n~gligible; t he electrical length of the Wincharger a t 342m is onlY O.31Ae The dintortion of the p~lar diagram would be small and the field-strength in the south-caster~ direction w0uld t hus be approximately equal to t he existing mean field-strength.

ReG toratioll of' the polar ~liD.GrQr:l of t ho 34'2m ~ra'Qsm:i.ttor from its present 3hape to somo t~li.!:l" . G.i)p.2ox i l'r.ating to 0. circular diagrarn.,'mll have the following effects .

(a ) kl improved s~rvice in the S .E. direction from which complaints are at pl'e sont rocei ved . There would be an increase of betvveon 2 and 3 .5 dB in field-strength in this [~ener3.1 d.irection , Hhich would provido a noticeable i r 'lp:rovoment in r ocf.pt iO:il in sOr.J.e areas .

(b) .An i I'IprOved service in the N . ~IJ . direction , which appears to be unnecessary,

judgLlb from li steners ' reports . .An increase of field- strength in this direction might in f8.cti prove undesirable on account of its effect on the service area bOIDldaries at night of the remote stations in the group .

(c ) Decrea s e in fj.old-strength in the N. E. and S .W. direct ions with probable com~?laLlts fl'om liGtenors in those directions 'liTho are at prosent satisfied with the sBrvico .

It .!1lUG-G be realised tr..at any change in polar diagram from an established operati:n..g

conditio::" if not accompa:lied by e. corapensating adjustment of power , inevitably results in a poorer service to some areas , particularly when ' the transmitter concernod MS Qn intended service area in all directions . T.Iaking such a change may, hovlever , be justified in cases where it results in a fairer Cistribution of the service to all listeners, in the area , · even at the expense of some districts -,'lhi ch may have hitherto been unduly lucky.

No definite conc l usions have yet been reached rvgarding this matter, Jut it would require considorat i on if this proposal was to be adopted . Consideration vrould o.lso have to be given to the possible effect of the arrangeJ'lent upon the other transmissions; roughly speaking , however, conditions would approximate to those d~scribed in 63 .

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-1 . 2 . To Dnpr.ovG the .!.~hape of the Polar Diagram of the Existing 342 Metre Aerial.

I n section 2 . 3 it is pointed out that the present considerabl e distorti on of the polar d.iagram is duo to the effect of the nast ~II . 3 . i n its present operating condition a:J radiator for thl3 1500m transm:'ssion . If the r"Tincharger ~,~st is used

to radiat3 the 449m transmission it :.jQuld be poss ible to transfer the l500m aerial frO l!1. mast :::. 3 . to mast Ivl . 2., and thus to run mast M. 3 . in the froe condition with

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consequont restoration of the 342m polar diagram to something appr oxi mating to a

circle , the service effects then being as outl ined in section 4 . 1 .

4 . 3 . To Increase the Power of the 342m Transmissions . It 'would be poss i bl e to increase substantially the radiated power of the 342m t r ansmission by interchanging tIle 342J~1 and the 373n tr, nsmi ttors . The shape of the polar diagram would then be illlchanged, but if the field- strength is at present adequate in all but the south­e~sterly direction it l"JOuld seem an inefficient method of 9rovic.ing a remedy .

4 . 4, To Use the S .W. lV.last , which is not at pr~sGnt in usc, as a Radiator for the

1342m Transmission , \\Tit h a Ref lector Suspended from the Existing Triat i c

.l?~tvTeen M. l. and M. 4 . By this moans it woul d be possiblo to provi de a substantial increase of field- strength in the south- easterl y direction i'lithout seriously modifying the fie l d - strength in the north-west . Ther e would undoubtedl y be a tlrop in i'ield- strell-t,;th in the directions north- east and sout h-l/Jest , f or wi th the oxj.sting aerial maximum fie l d- streneth is occurring in these direct ions ; ho·.,rever this arrangement vlOuld permit of conSiderabl e adjustment to the shape Of the polo.r diacram and it woul d largel y be a matter of decidir..g the best compr omise .

A comb ination of the proposed directionn.l systom VIi th an increase in poviGr could

be arranc;ed to give a satisfactory i ncrease in fie l d in a S . E. diroction without too Groat a decrease in the N. E. and S .W. direct i ons . Any change on these lines

"'lOultl of course require Aj r Ministry approval.

Fir..ally it sllould be montioneJ that none of the proposed schemes vlOuld result in the provision of an entj.rely satisfactory Forces service to S . E. Engl and on 342m; the n03.sures proposed would result only in o.n improvement of the existing service .

COl:CLUSIONS

TilO proposed position of the \ilTi:'.1charger Mast at Brookmans Park wi l l be sat i sfactory ~ I

It s use for th.e 449m transmission will not appreciabl y o.ffect the present service , ' thOUGh t~1.0re may be small changes in some directions . The other transmissions will not be seriously affected by the T'Tincharger :Mast , though a re jector for 373 mctr'os may be required .

Hec;ardil1g the 342n service , since the service to the south- east requires improvement , it is conSid.ered that the most satisfactory sol ution v'rill bo to t r ansfer 342m to the S.W. Jltl st and provide 0. reflector suspended from the existing triatic between the N.W. and S . ·W . masts , and .also to increase the povrer of the transmitter .

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FIG. 5 IDCISTI!lG AND PROPOSED LAYour OF L!tl.ST.3 AT BROOL.!:UrJ P.A..m:.

Pos I TI()I'\

Report Ho. 3 .018 serial No. 1943/27

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