R&D Report 1951-06
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PRIVATE Bc CONFIDENTIAL . RESEARCH DEI'ARTMENT Report Uo.K.085/3 Serial No.195l/6 SOUTH HAMPSHIRE AJllD ISLE OF VlIGHT TELEVISION SITE TESTSg FINAL REPORT Inves'tigation by Field strength Seotion. Report written by G.I. Ross. April, 1951. REP~, J!TO. ,K. 085h.. Serial No. 1951!~ COllTE1TTS • •• Page 1 • Summary ••• • • 0 .. ... • 0 • 11 I. General .. ... .. ·.. • •• 1. 11. Transmissions and Measuring Techni!lue 11 1. 11 2. Ill. Results • 0 0 • 0 • ·.. tI 2. 111.1 General •• .. • •• ·.. 11 2 & 3. 111.2 Comparison of sites .0 ·.. ·.. IV. Area Ceverage of South Hants and Isle of Wight Sites in Relation to Existing 11 4. and Proposed Transmitters •• 0 • • • 11 V. Conclusions. ... 4'· 11 5 & 6. TABLE I ••• .. o 0 ~o ... ... ... APPENDIX I. Re-Radiation of the Shornhill Signal by Bartley Mast •• , " 7. APPENDIX 11. Position of Sites ••• ... " 8. PRIVATE AlTD CONFIDENTIAL - -~, Figures 1 - 4.' Report No. K~085/3 April, 1951. SOUTH HAMPS]IIRE ill IS}:ftl OF WIGHT TELEVISlbl~ SERVICE SITE TESTS& FINAL REPORT Summary Three television site tests have been carried out in the Isle of Wight ... South Hampshire area. Of these sites, one was looated on the mainland and two were on the island. Interim reports Nos. K.085 and K.085/2 isS'Ued in December 1950 and January 1951 respectively presented the results of two of the sites - one at Shornhill, South Hampshire (near~he B.BoC. station at Bartley) and one at Ruwridge Farm, Cal bourn~, Isle of Wight. ' ' Since these reports were issued a seoond site on the Island (Elm Copse f Calbourne) has been tested and thi's report oollates the results of all three sites and disousses their relative merits. It is reoommended that the stationbe'built on the Rowridge Farm Site. 1. GENERAL, It is proposed that the station on the seleoted site will' ' radiate a horizontally polarised signal on a frequency of 56.75 MC/So It will radiate a peak white power of 5 kW from an aerial, having a gain of 3 db. (relative to a half-wave dipole) on a 750' mas,t. All servioe area oontourmaps and 'field strengths quoted in ,this report refr:-r to this specifioation. A note on nEe-radiation of ,the Shornhill signal by Bartley mast" is given in Appendix I. Details of the ·three sites tested are given in Appendix: II. 11. TRANSMISSION AND MEASURING TECHNIQUE As in earlier site tests, use was ma.de ;ofa balloon to carry the transmitting aerial to a height of 600 feet above ground level. When weather conditions precluded balloon flying, use was made o·f an aerial on the top of a 110 ft. mast. The ballt~ol., aerial radiated 18 watts while the mast aerial radiated 300 watts but all measuremvnts have been corrected to comply With the transmission condi tiors· , specified in the previous paragraph.' Al though both balloon. and' mast' aerials radiated a vertioally polarised signal the field strength fer a hO:ri.zontally polarised transmission would be substantially the s~me f;1.S that measured. The field strength for a 750 ft. mast was -2 found by extrapolation of balloon height versus field strength curvE'S, the field strength being meas1L.'ed at selected points in the service area. Ali e;ignalswere .received on an. end fed dipole elevated 15 ft. above gro'und level buti:ihe field strengtlis quoted have been corrected for a~ aerial at 30 ft. above ground lev.el. A complete account 'of the transmitting and measuring technique will be found in Research Report No. K.076/2. Ill. RESULTS IU.l General The results of the three sites are shown in the fo~m of service 'area bontour maps i1;1 Figs. 1, 2 and 3.' In addit~on, the maximum, m~an and minimum field strength in the more important towns is shown in tabular form under a separate heading for each si te in Table 'I. The field. strengths indic1?_ted on the' service aren map&. of Figs. I and 2 are only average figures and the actual field strength at any giv~n point may vary over a range of ~ 10 db. from the value shown. The figures also show the limits of inter ference for co-channf:ll operation with Kirk 0.' Shotts .... the latter radiating a vertically polarased signal while the proposed station' radiates a, horizontally polarised signnl. ,., The interference limits have been estimated from the latest availnble data on long distance propag8,.tionon frequencies of the order concerned. Tho limits define t:te. perimeter within Which the field strength of the proposed transmitter would be 35 db. above the interfering field from Kirk 0 1 Shotts for at least 95% of the daily trnnsmission time on 90% of the days of the year. III.2 Oomparison of Sites The se,rvice nrea contour map for the Shornhill site is shown in Fig.l nnd the first three columns of Table I show the field st'rength in tl.,1e more importnnt towns. Referring to 'l'alle I, it will be seen that the industrial oentre of ?ortsmouth (1.42 mV/m) is very poorly served, while Southnmpton (31 mV /m), a torm of comparnble size ,viII htwe a field fftrength about six times in excess of thatoonsider~d necessary for an adequate servi6e. Th~s faot in itself indicates that Sho'rnhillis not a sui tp:.ble site for the proposed transmitter. The Bournemouth area, with an n.verage field strength of 1.4 mV/m, is rat::':'er poorly s-orved. Bog-nor (0 .. 47 mV/m) f':.nd Li ttlehampton (0.41 mV /m) get a fnir serv~ce, but viorthing and , Brighton; two highly populated areas, would get a totally inadoqunte signal of the order of 0.1 mV/m. Dorchester (0 .. 24 mV/m) and Weymouth (0.3 mV/m), two towns with no prospect of a service from elsewhere, will also get a poor service. On the otherhan~ the service in Winchester and Salisbury is' much better o:;han that Predicted on Map T86~ From the foregoing it was concluded that Shornhill is not a suitable site for the proposed transmitter, and that" a site on the.Isle of Wight would give ,better results. Rowridge Farm, the first site tested on the island, is higher • and better situated in relation to the densely populated areas under consideration. Referring tu Table I again it will be seen that a transmitter located here will improve the service in these areas. Southampton (7.3' mV/m) and Portsmouth (5.0 mV /m) have a 1st class service, while the field strength in Bournemouth is 3.3 mV/m ... an improvement of 3 db. over that available from Shornhill. The 0 residential areas of Brighton (0~42 mV/m) and Worthing (0.5 mV/m) will get a foair servioe, but in those parts s:ubject to industrial and heavy igni tion interferenoe, the service wioll be poor. The field strength in Sal::'sbury (0,,91 mV/m), while less than that from Shornhill, is, nevertheless, almost adequate for a town Qf its oharacter. It is of interest to note that the field strengthOin WiHchester (2.37mV/m) is slightly greater than that from ShoriiliilI (2.25 mV/m) although the distance from Rowridge is twice as great as t hat from Shornhill. It is probable that this can be aooounted for by the fact that Winchester in relation to Shprnhill" lies immediatQly behind a 350 feet hill while the path from Rowridge lies along the River It'chen valley. From the data and" re suI ts discussed it was considered that a tra'1smittQI' located at Row-ridge would give the best" service to the area under consideration. There were, however, a number of • di.fficul ties in the" way of building a l'!tation here. As a precaution, - therf'fore, a second site on the island, Elm Copse, was selected ::md the fieldstrongth measured in the most populated areas only. The service area oontours are shown in Fig.3, but it mustO be stressed , they are very sketchy on e~ooount of the lirr.i "';ed data available. From the tabulated rest:..lts, however, it will 'be seen that, with the exception of Christchurch (5.2 mV/m) and Bournemouth (4.1 mV/m) the field strength is~ess than that from R('wridg'3 si to. The difference in field strength is greatest in the more' distant towns" e.g.Shaftosbury O.41mV/m compared with 0.81 mV/m, and DorcJ.lester 0 0.45 mV/m comparod with 0.13°mV/m. The field strength in Portsmouth, 3 .. 4 mV /m, is insufficient to provide als~ ol<"ss service, "" 0 "'4- IV~ Coverage of Sou,ji..h Rants and Isle of Wight Sites in " ~lation to Existine; and Pro;eosed Tre.nsmi tters_o The map on Fig.4 ahowa the 0.25 mV/m contour of the, three $i tes under oon'siderati6n in relation to the ourresponding oontoura for Alexandra. Palace, Sutton Ooldfield, Wenvoe (st. Nioholas) an~ South Devon. Oonsidering Shornhill and Rowridge only" the area enolosed by the 0.25 mV/m oontour is almoat identioal for both sites (Rowridge '4570 aquare miles and Shornhi'll 4760 square miles). It muatbe remembered, however, that the tabulated results show, with one or two exoeptions, that the highly populated areas reoeive a better service from Rowridge, and these 0.25mV/m ~ontour maps ,ahould be used only to oonsider the general ooverage of a station in relation to other stations 0 The area enclosed by the 0.25, mV /m oontour ,for Elm Oopse cannot be assessed with any acouracycm ' aC,oount of the sketchiness of the contour, but it is evidently considerably less than that of the other sites.