58 



TRANSMISSION EXPERIMENTS IN ENGLAND 



Tl ME IN HOURS CMT- 



M U A 01 



Figure 9. Signal record. August 26 to 27, 1944, showing examples of interference fading in radiation nights. 



iEarch 1943, and during the intervening 18 mouths 

 there lias been a seasonal variation, with the average 

 daily vahie in November and December at least 10 db 

 below the value in July and August. Two reports^"'" 

 have already been issued on the results obtained from 

 an analysis of the records, and a tliird will be pre- 

 pared shortly. 



Among the main cniichisions so far reached are the 

 following. Cloudy weather, either Ity day or night, 

 tends to produce a signal steady to within about 2 db, 

 while on days of clear and variable skie.s, the signal 

 exhibits slow (period .5 to 10 minutes) fluctuations of 

 the order of 3 or 4 db, with .sharper and more rapid 

 fluctuations superposed. These rapid fluctuations are 

 accentuated by the presence of strong wiiul. On nights 

 of clear or variable skies with temperature inversions 

 near the ground, the field intensity is from 5 to 10 

 db above the daytime level and is usually accompanied 

 by variations in the alisence of wind. On clear radia- 

 tion nights, when the wind is too .strong to permit the 

 development of a temperature inversion, tlie peak of 

 signal intensity does not occur. Fog affects the field 

 strength differently according to its depth. A .shallow 

 autumn fog causes a .sharp decrease in signal strength, 

 while the widespread and more established type of fog 

 experienced in winter may sometimes cause marked 

 interference type of fading with unusually high peak 

 values and at other times may liave no apparent effect 

 on the field strength. 



The sending and receiving stations on this link arc 

 now lieing eqiripped with field strength monitoring 

 arrangements to improve the overall accuracy of the 

 radio recording, and a daily statement of the meteoro- 

 logical conditions over the path is being supplied to 

 supplement the ground station records already avail- 

 able. It is contemplated that this link should remain 

 in operation for a further period of 6 to 13 months. 



S3 FADING IN A LINE-OF-SIGHT 



EXPERIMENT IN ENGLAND<^ 



This experiment was carried out between Aberporth, 

 South Wales, and the summit of ilt. Snowdon (3,600 

 ft). The transmitters were mounted at a height of 

 120 ft above sea level at Aberporth, the receivers being 

 on Mt. Snowdon. The length of path was aljout 60 

 miles as compared with 8-5 miles optical range. 



Two .separate radio circuits were used, one on a 

 wavelength of 9 cm and the other on 10 cm. For a 

 standard atmosphere the phase difference between the 

 ray reflected from the sea and the direct ray was about 

 4.5 radians on 10 cm and about 5 radians on 9 cm, 

 while under ''flat earth" conditions the corresponding 

 phase differences were about 12 radians and 13 radians 

 respectively. As the atmospheric conditions varied, an 

 interference pattern was obtained arising from varia- 

 tions in the jjhase difference. The chief characteristic 

 of the interference pattern was that in a plot of radio 

 signal strength against time the peaks are broad and 

 flat and the minima are sharp and deep. The provision 

 of the two circuits insured that the variations due to 

 alterations in the phase difference could easily be 

 distinguished from variations due to other causes 

 (described below). The reason for this is that the 

 interference minima on one circuit tend to occur at 

 times when the signal strength is high on the other 

 circuit. 



The length of time rocpiircd for the radio signal 

 strength to complete a cycle of the interference pat- 

 tern was usually about 2 hours. The receivers and 

 transmitters were very carefully calibrated in order 

 to show that the received field strength at interference 

 maximum was ecpial to twice the free space field. This 



'By F. Ho3'le, Ultra Short Wave Panel, Ministrj- of Supply, 

 England. 



