48 



TRANSMISSION EXPERIMENTS IN ENGLAND 



Some ditticulties were encountered early in 1U42 

 in iiiiding sites for the stations wliich were accept- 

 able from all [loints of view, and the field work done 

 in tliat year consisted of several short-period trials 

 over a rather wide variety of land and sea paths. In 

 spite of many limitations, in particular as regards 

 detailed meteorological data, the general conclusions 

 reached in these trials" have been largely substan- 

 tiated by later measurements. Table 1 gives details of 

 the sites finally ado]ited. 



Table 1 



Station 



Height, ft 



S. Wales (transmitters) 



A. Garn-Fawr 



B. Strunible Head 

 N. Wales (receivers) 



C. Rhiw 



D. Aberdaron 

 Scotland (receivers) 



E. Knockharnahan 



F. Portpatrick 



540 

 90 



825 

 95 



375 

 95 



The path length I'lom South Wales (A and B) to 

 North Wales ((' and D) is 5? statute miles and tliat 

 to Scotland ( K and F) is WO statute miles. The jiatli 

 lengths in terms of geometrical optical range for the 

 eight ])ossible paths are shown in Table 2. 



111 the original scheme all the paths were to be 

 collinear, but this could ]iot he realized with the 

 sites finally adopted; the South Wales to Scotland 

 paths differ by about ^7° in bearing from the South 

 Wales to North Wales paths, the bearing of the 

 former being within a fraction of a degree of true 

 north. A scheme for recording data over all paths 

 (though necessarily not continuously) was evolved; 

 each transmitter beam was aimed for half the time 

 along each of tlie two l>earings 17° apart (a 7Vo- 

 minute period was found the most satisfactory, and 

 a small change in frequem-y (o to 10 mc) was made 

 automatically when the beams switched over. 



At each frequency the transmitted signal consisted 

 of square pulses, at equal on/off ratio, with a repeti- 

 tion frequency of 1000 c. The "standard" power out- 

 put in the "on" period was 0.6, 0.3, and 0.15 w for 9, 

 6, and 3 cm, respectively; the signal records were 



corrected for any significant departure from these 

 ]>owers. Paraboloid mirrors 48 in. in diameter were 

 used for all transmitters and receivers; these were 

 mounted inside the stations behind large cauvas- 

 co\ered "windows." The increase in mirror gain with 

 frequency more than made up for the reduction in 

 transmitter power, in spite of the less effective utiliza- 

 tion of the mirror area. In the receivers the 1,000-c 

 component of the modulation was rectified to operate 

 the recording milliammeters. Provision had been made 

 for monitoring the field radiated from the trans- 

 mitters and the sensitivity of the receivers, in terras 

 of a standard radiated field. This scheme was brought 

 into operation as the National Physical Laboratory 

 ('(piipment became available; other less complete 

 methods of monitoring the Iransmitters and check- 

 ing the receivers had been in ojjeration from the start. 

 ( Data for the 5-cm equipment are included here 

 although, as will Ije noted, it was not used.) 



Padiotelephone communication between the North 

 Wales and South Wales stations has been maintained 

 satisfactorily for two periods of several months each 

 using first S- and later X-band equipment, essen- 

 tially the same as that used for the signal measure- 

 ments, arranged for duplex operation. A meter-wave 

 system (which gives more continuous service over 

 long nonoptical paths) is now being installed by Ad- 

 miralty Signal Establishment to link all the stations; 

 it is already operating satisfactorily over the 57-mile 

 path, and a relay link from North Wales to Scotland 

 is being pro\ided. 



On S band, operation on all four links across the 

 57-mile path commenced in November 1943, although 

 the two from Station A (high site) had been running 

 since July. During the preliminary period, up to the 

 beginning of 1944, in which a number of practical 

 difficulties had to be overcome, the radio results were 

 subject to rather more uncertainty than was the case 

 in the earlier measurements where a concentration of 

 experienced personnel was possible for the short peri- 

 ods involved, and detailed analysis of these results has 

 not yet been attempted. One S-band receiver was in 

 operation in Scotland (Station F, low site, 200 miles) 

 from the end of August 1943, but apart from one 

 brief period during Septendier, no signals were re- 

 ceived until March 1944, just before the second S- 

 l)and receiver (Station E, high site) was installed. 



On X band all the stations were in operation by 

 duly 1944, operation on the 200-miie links having 

 stai'ted a month earlier. 



