52 



TRANSMISSION EXPERIMENTS IN ENGLAND 



oi' pailiculai ly ^(hmI wciiilici' and (2) during I'oggy 

 coiidition.s which iiic likely to \iv associated with a 

 sub.staiidani iiidrx gradient. StJ'iking examples of the 

 latter occurred on June 4, o (Figure 2) and August 

 10, 11 (Figure 3). On the last occasion the BC level 

 went about '20 ilh helow standard. 



The X-band results for the 5T-mile paths are gen- 

 erally similar to those for S liand as regards the major 

 variations, but the I'ange of variation is noticeably 

 larger, particularly on paths BC and AD, which are 

 not much longer than optical range: the increase in 

 range of variation for these jiaths is of the same order 

 as the difference in standard level for the two wave- 

 lengths. In general, short-period variations are larger 

 and more rajiid for the shorter wavelength. 



Figure 4 shows the results for both S and X bands 

 over the 200-mile paths AE (high stations) and BF 

 (low stations) for part of the same period as shown 

 in Figure 3. After allowing for the estimated atmos- 

 pheric absorption (6 dl) for S, 16 db for X) the free 

 space levels are similar for the two wavelengths, ex- 

 perimental uncertainty being apprecial)ly greater at 

 X band as regards absolute values. The standaril 

 le\els are. of course, far below the receiver threshold, 

 actually about 2?.-> db for path AE on 8 and 4i)0 db 

 for ]iath F>F on X. 





H 



/"V 



^^w^W 



^TRCE SPACC 



PATH A— E 



S BAND 



08 



30 



^o 



10 

 



^FRtE SPACE 



£1 



fr 



RATH A— E 



X BAND 



SIGNAL STRENGTH IN DB ABOVE [/^V RECEIVER INPUT 





-FREE SPACE 



PATH 8 — r 



S BAND 



t 



— FREE SPACE 



20 



10 







40 

 30 



PATH 



X BAND 



AUC. 8 19 4.1 



Fi(;uRE 4. Hourly mean values of signal strength on 

 S and X band. 



The most striking cliaracterislic of the i'esulls is 

 perliaiis the similarity in magnitude of the signals 

 botli for tlie two paths and for the two wavelengths. 

 In general, signals are measurable for a greater frac- 

 tion of the time on the longer wavelength and for 

 the higher sites. (The dift'erence of alxiut 5 db in re- 

 ceiver threshold sensitivity between S and X has only 

 a slight eifect on this.) At the peak of good periods 

 tlie lower sites and shorter wavelengths sometimes 

 reach rather higher levels, as in the case of the 57- 

 mile paths the maximum signal level is frequently 

 com])arable with free space; rather rarely it exceeds 

 free space level by something of the order of 10 dh. 

 For the 200-mile path the possil)le error in the esti- 

 nuite of atmospheric absorption is rather more serious, 

 particularly for X Ijand, but it seems improbaljle that 

 this could alter the general character of the results. 



Comparison with the results for the .j?-mile BD 

 path in the bottom record of Figure 3 is interesting 

 and is reasonably typical of the extent to which the 

 performance of the long path can be predicted from 

 the perfornumce of the shorter one. It should be em- 

 phasized that this was a period of good summer 

 weather apart from the break on August 10-11. 



5.1.3 Overland Measurements: Wliitwell 

 Hatch to Wembley 



A single S-liand link has been in continuous opera- 

 lion ii\er this :iiS-mile jiath since Mai'ch l!ib'i. Its 

 terminals, with the transmitter in one of the Ad- 

 miralty Signal Establishment buildings and the re- 

 cener at General Electric Company lie.search Labora- 

 toi'ies. were chosen for operating convenience rather 

 than to meet any special requirements for the j)atli, 

 as an important subsidiary jjurpose was to provide for 

 controlled long-period tests on equipment developed 

 For use in the less readily accessible stations of the 

 Irish Sea program. Apart from routine checks the 

 equi])ment normally operates unattended; automatic 

 frequency control at the receiver has been in operation 

 since June 1943. But the receiver is provided with a 

 relay-operated alarm which can be set to operate on an 

 abnormal cliange of received level in either direction 

 (normally d(.)wiiwards ) and this has proved valuable 

 in calling attention to both faults and unusual |iropa- 

 gation conditions. 



The transmitter is on a hill T2.") Ft aliove sea level 

 and the path runs northwards across the Thames 

 valley and the western outskirts of London to the 

 receiver, which is onlv 170 ft alrove sea level, in low, 



