54 



TRANSMISSION EXPERIMENTS IN ENGLAND 



Figui-es 6, 7, mid S [,liu\v photographs of sections 200 ft per Iiour and 300 I't |iri- hour rcspcrtively. No 



of the original signal records ilhistrating the main local soundings are availaljle to I'heck tliis hypothesis, 



types of signal which are observed. The type of weather hut the calculated rates of change of heiglit are quite 



involved is shown on the record in each case, also the possible. 



signal calibration. Figure 9 shows a good example of The general meteorological data for the night (illus- 



an effect which is quite often observed, particularly trated in Figure 9) are as follows: cloudless, follow- 



TIME IN HOURS GMT- 



OB/, 



B 



Figure 6. Signal records. (A) June 4, 1944 from midniglit till noon. Weather cloudj^ light W wind, intermittent rain. 

 (B) June 11, 1944, 6 a.m. to 6 p.m. Weather clear in the morning, later cloudy; light W wind, intermittent rain. 



in the latter part of radiation nights. It consists in a 

 regular variation showing the characteristic rounded 

 maxima and sharp minima of interference fading. This 

 is in some cases sujjerimjDosed on a nearly steady high 

 signal ( as in Figure 9 ) . In other cases, as in some fog 

 fades, it is superimposed on variations of a different 

 type. It often starts and stops suddenly, completely 

 changing the character of the record while it lasts, and 

 that time ranges from one or two fading cycles to many 

 cycles. This sort of effect has also been observed on 

 other paths, over sea as well as land. 



The .striking thing about patterns of this sort is that 

 they often correspond to reflection coefficients for the 

 interfering ray which approach unity. In Figure 9 

 the reflection coefficient calculated from the pattern 

 is about 0.8 at the start, al)Out 0.4 in tlic middle, and 

 over 0.9 at the end. It is suggested that reflection at 

 small glancing angles from a sliarp inversion top pro- 

 vides a possiljle explanation and that the first part 

 of the pattern in Figure 9 corresponds to an increase 

 in height (the first deep minimum occurring when 

 the inversion top is just above the transmitter) and 

 the latter part to a decrease in height at a different 

 rate. The rates of climb and fall turn out to be about 



ing a fine day ; temperature inversion of about 6 F in 

 (approximately) the first 500 ft at 0600 GMT ; ground 

 mi.st about dawn. It is clearly desirable to obtain ade- 

 quate soundings at periods when this type of effect is 

 observed, especially on account of the widely held view 

 that the index changes which occur at heights of the 

 order involved (about -500 ft above ground level) are 

 inadequate to account for reflection coefficients of the 

 size implied by the pattern observed here. 



^ '■* Difficulties of Existing Theory 



In tliis section a few general characteristics of the 

 I'adio observations which appear to be at variance with 

 previous theoretical conclusions and which suggest 

 directions in which further work is required will be 

 noted. 



1. The most obvious point as far as the Irish Sea 

 data are concerned is the failure of the .soundings to 

 ]irovide an adequate guide to the signal variations. The 

 fault may lie in the limited nature of the soundings or 

 in the method of interpreting them, but it is clear that 

 the i)roblem is by no means as simple as was supposed 

 when the soundings were started. 



2. The minimum levels obtained (where they are 



