I .

.ADJUSTh[E~_..QF THE lMAST. LOADING COII@

AT START POINT b .. ___ _

Report No. E.035 Serial No. 1948/2;;

Research Department

Investication by:

H. Eyet I • F. N CW'l11:'1ll

H. Page

REPORT NO. E.032

Serial No. 1948.(25

August, 1948

FiS. Nos..l-5.

.AD;JUSIJ}:lE.NT 01i1 T"BE 11AST LOADJNG cons AT START POmT

This report de~0ribestho results obtained when series loadinG coils were installed in the mast racUe.tor at start Point in November, 1947.

Ir:~edrulce-frequency charaqteristics of the l~ast were measured with different loading inductances, and the optimum setting deduced. The horizontal polar diaer['Jl1 was then adjusted by making local field strength measurements.

During the course of the measurements there was evidence of radiation from other masts and short wave arrays on the srune site. In order to obtain tho best performance from the aerial system it is consi dcred essential to remove all other re-radiators. This proposal ~~s now been agreed, but until it is put into effect, it is not considered worth vvhile car~Jing out an investigation of the improvement in the service area clue to the uSe of mast loading coils.

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1. JNTRODUCTION

Under the post-war distribution soheme' it was decido<l to use the start Point transmitter on a wavelength of 307m., to serve the populated areas of the South Ooast region from Penzance to Worthing.

The Start Point masts arc 450 ft. high vl'ith an insulated gap at a height of 310 ft. This gap h'ld been short-circuited dur:lng the war, and the IilD.St Vlent into service on 307m. in this condition when the post-war distribution scheme croae i.."'1to operation on July 29th, 1945. It was knavvn that the mast was too short to give optimwn performance on 307m., and that the vertical polar diagram could be improved by the use of series loading coils, but those were not available at the time.' . ArrLmgements Y{ere later made to provide a suitable coil for each mast, and these vrere 4

installed by P.I.D. by Noveml)er, 1947.

This report describes the measurements which were made in order to determine the optimum loading inductance, and the consequent re-adjustment of the horizontal polar,diagram. ,

2. ADJ,YST1\[ENT OF M'J¥3..l SERIES LOADING 001L§.

An impedance-frequency characteristic of one of the masts, vvith the gap short-circuited, is shown in F{g. 1. \Vhen this is corrected to take account of the effect of the leads connecting the mast tc the measuring bridge, the follovvin.c: information can bc deduced:- .

~ resonmlce frequency 495kC/s.

Oharacteristic impedance •• , ••••••••• 250 ohms.

Veloci ty of propagation •••••••••.••• 0'90 (c ::: velocity of light)

Physical height at .307ra. . o •••••••••• 0·45'A.

Effective height deduced from bri<lge n;asurements, at .307m. . ..... VPD height at 307m. . ............... .

The VPD height is defined as the height of a hypothetical mast, having a velocity of propagation equal to that of light, and the same vertical polar diaermn as the actual mast. The VP.D height of an unlonded. mast is the mean of the phYsical height and that dElduood from bridge measurements.

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A considern tion of the predicted service area cf the transm;i tter indicated that the optimumVPD height should be 0·55\,representing on increase in height over thcmiloaded mast in the ratic 1.17 :1.

Measurcnents were next made with 4 turns in the mas t loading coil, the re suI ts being shown in Fig. 2; the corresponding results using 6 turns arc ShovVll jn Fig. 3. It is no longer quite so simple to determine the VPD height in these cases, since the effect of .the loading coil changes with frequency. Moreover, the vertical polar cliagram of a loaded mast is not the some alFl that of an unloaded mast having tho. same effective height as deduced from bridge meastiremen ts ..

To determine the increase in effective height of the mast, the hlpeCkmce .... frequency characteristic. was examined at the \/2 end 3\A resonen t frequencies, and compared ,vi th the calculated increase based on the measured coil inductance. Reasonable agreement was obtained from these different methods of approach.

The derived info'rmation is smrmarised in the fqllowing table:-

No. of turns in mast ..

;Loa§inccoi 1

Height deduc~;:l. from bri(l.j~e measurements .. ,307m. •

0·49\

0'5.45\.

0.59\

~eirht for 1:£fto.e d. mas t , velOCity of propagation 0'90

0.47\

-0·51\ ,

0-54\

I1l1 edance from ATH other mast earthed

e+2p~j564 = 2BOwj320

740p ... j,3oB ;; 109-j262

B50p-j20B a 48~j196

. . 1 I

From the above table it appears that approximately ~ turns of loading coil exe required to give the optinuru VPD height of 0·55\. It should, however, be noted that,as the coil turns are increased the parallel resistance Cane!. hence the base voltage) is rising, '\iv-hile the series resistance is falling, with a consequent increase in aerial . current.

For 6 turns the base voltage for a power of 120 kW is 10 KV RMS, and the base current 50 am?s., both of which were considered reasonable. At this setting flashover difficulties on high modulation were experienced and overcome, but in addition the high base current ignited the aerial inductance coil former. At the time it was decided to compromise by reducing the number of coil.turns to 5, the estimated

3.

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VPD height being 0 • .53'" in this oondition; it is considered, however," that 6 turns would give an improved vertical polar diagt:'am and this decision will be reconsidered when a oomplete survey of t~ service area has been made.

.ADJUSTMENT OF HORIZON'IAL POL1~ DIAGRAM - - ..... ..- --A theoretical investigation had indicated that a parasitic

reflector would produce a satisfactory horizontal polar diagram, using mast A as the fu~iver and B as the reflector (Fig. 4-); the predicted curve for this arrangement is shovvnby the full line in Fig • .5. The deoisionto use such a diagram VIas based on a study of the service area, vrhich indicated the desirability of obtaining the mnximum fonvard gain, without allowing the field strength at bearings t 90 0

from the line of masts to be less than that for 0. single aerial radiating the name power.

The Start Point site is on tho ooast and the fanvard radiation can be measured only over an angular arc of about 1000

; the backward radiation can be measured only at sea, which is inconvenient. It was therefore decided to obtain the desired polar diagram by making A the parasitic reflector, adjustinG for rninimunl field in the required diJ:'Qction, and then to interchance the circui tsat the bases of the masts. This method was adopted, o.s the position of the polar diagram rninimum gives a much better indication of the relative phase of the aerial currents than does the mnximu;m.

Mast A wo.s accordingly usod as 0. parasitic reflector 1 and terminated in 0. coil, the number of turns being changed (maintaining the input power to the system approxlinatcly constant) until a field s trene;th minim~ was obtained in the des ired direction; this was achieved with B2' turns in use, A horizontal polar diagram was then measured and compared with '!:;hat for mast A earthed, i.e. nominally inoperative.

The results are shovn1by the lm~ part of,the dotted curve in 1:'i8. 5, Dnd compnring this with the theoretica.l diagram, it is seen that the measurec. minimum is sharper than the theoretical. It was concluded that other masts and short wa.ve arrays on the site were acting as re-radiators, and controlling the horizontal polar diagram to some extent.

This was confirmed when the functions of Jn9.sts A and. B were interchanged. With ~ turns in the coil at the base of mast B, ,the field . strength at 960 to the line of masts, compared with a single mast radiating the same povver, was 0.81; ; this low ratio and the shape of' thp horizontal polar diagr:oam, when compared wi th the fUll line curve of .Fig, 5, indicates that the phase of' the refleotor current is

incorrect. Increasing the number of turns in the·reflector terminating coil to 9 cave the diagram shovvn' as the upper dotted curve in Fie. 5, which is. considered satisfactory.

The theoretical value of ED/Iki" (field strength in mY/m x mstance in l<m for 1 kW ramated) assuming no earth losses, ;is 377 without the mast loadinc coil and 396 for a mast loading coil of 5 turtlS, an increase in ED//kW" of 5%. It should be noted that the VPD mast height was left at O.53'A. in order to achieve suitable driving point conditions. 'If the ideal VPD height of O.55'A. could have been achievecl, the :increase in ED//kW would have been 8%.

The :increase of.5% is too small to be measured accurately, but the measurenents so far made :inclicate that a small improvement in ED//kW' has been obtained.

4.. FIELD STRENGTH SURVEY OF SERVICE .AREA -- • .*

For the reasons stated e~lier, it has not yet been considered worth while to carI"'J out a complete survey of the Start Point service area.

The 11 spot" measurements which have so far been made in Oornwall and Devon confirm that the gr'ound ray field strength predictions shown in Research Department :Map A.36 have been obtained. It seems unlikely that the arr\:;ifading limits sl:J.own on this map have yet been achieved, in view of the probable distortion of thc vertical polar diagr'Dm discussed in Section 3. After the site has been cleared of existing obstructions a full survey will be made to determine the antifamng limits, when other work permits.

5. OONOLUSIONS

The limited measur{."Znents so far made at start Point indicate tha t the series loading coils D.re giving satisfactory perfonllance. The adjustment for optimum mast height has not been attempted, in vieW of the unsuitable driving conditions; but a reasonably close approXimation to this has been made. This point will be reconsidered when a survey of the service area has been made.

The horizontal polar diagram is satisfactory and the l:imited measurements possible incHcate reasonable agreement with theory. There was, however, evidence that the horizontal polar diagram is being affected to same extent by re-radiators' on the site, and clearance of all unwanted raas ts and arrays was recommended, and has ru:JW' been agreed.

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A oomplete survey of the servioe area from this transmitter MS not yet been made, but the "spot" measurements in Cornwall nnd Devon ina.ioate reasonable agreement withthc theoretioal ground ray prodiotions.

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ISSUE 4

25 -0 - 46

AD'-"USTMEN, OF THE. MAST LO~D\NG COlLS AT STAR, POlNT.

DEPT REPORT

E,035'5 5 SHE.ET5