14 



TRANSMISSION EXPERIMENTS OVER MASSACHUSETTS BAY 



and 117-mc radiation over only the liigii patli. Numer- 

 ous meteorological surface measurements and low-level 

 soundings were made, and essentially through compar- 

 isons with these measurements the following correla- 

 tions for microwave transmission and surface M curves 

 were obtained. 



AVith positive M deiicits, or M inversions, two cases 

 were found. 



1. Low ducts, less than 50 ft thick, resulted in a very 

 steady signal at levels well above standard. The in- 

 crease in signal level took place although the terminals 

 were as much as 100 ft alwve the top of the M inver- 

 sion. Such low ducts caused greater increases in the 

 signal level on X band than on S band. 



3. High ducts, 100 ft thick or more, resulted in very 

 high signal levels on the average, but with deep fad- 

 ing. The signal level did not continue to increase with 

 increasing duet height but instead "saturated" near 

 the free space level. No frequency diversity between 

 S and X bands was found in this case. 



With negative M deficits, or substandard M curves, 

 the signal was always below standard. 



In November 1944 no correlations with M curves 

 had lieen obtained for the 117-mc signal, and a clear 

 lack of correlation with the microwaves had been 

 noted. 



A detailed analysis has since been undertaken 

 which is as yet far from complete. This paper descrilies 

 the method in use and presents some additional results. 



In studying the fundamental phenomena of propa- 

 gation the method employed was to tie the complete 

 representative M curve to the observed transmission 

 results liy means of the wave theory. A threefold at- 

 tack was used : 



1. The meteorologists studied each situation in de- 

 tail to determine a representative il/ curve and its 

 clianges with position and time. 



3. Theoretical field strengths were found by put- 

 ting the representative M cur\-e, or a close approxi- 

 mation to it, back into the wave equation. These theo- 

 retical values were then compared with the observa- 

 tions. 



3. Emj)irical correlations were then made between 

 the M curves and the transmission i-esults. This was 

 done l)ecause the theory is applicable only to the sim- 

 plest il/ curves and to uniform conditions. 



This approach was employed in an effort to find 

 parameters in terms of which predictions of range or 

 field strength can be uuide for operational use. It is 

 not considered a suitable method in itself for use in 

 the field. 



The meteorological part of the program has not in 

 general received sufficient attention. Spot measure- 

 ments at a given time and place do not necessarily 

 give an adequate description of prevailing conditions. 

 A thorough meteorological analysis of the entire period 

 of transmission is therefore under way. For each case 

 the synoptic situation is studied to find the trajectory 

 of the air over the path at the time in question. Kadio- 

 .sondes, surface measurements, winds aloft, measured 

 water temperatures, and all available low-level sound- 

 ings are studied and the characteristics of the air over 

 the water determined. Then representative low-level 

 soundings are constructed. Such so-called synthetic 

 soundings for the path midpoint are l)eing drawn for 

 6-hour intervals for each day of operation. In addition, 

 estimates are made of the departures from uniformity 

 over the path and of the times of occurrence of marked 

 changes. 



All the radio analysis has l)een Ijased upon these 

 synthetic soundings and the accompanying discussion. 

 The meteorological analysis is at first made completely 

 independent of the radio data, with minor revisions 

 when necessary after consideration of the transmis- 

 sion data. It is l)elieved that full use of transmission 

 data can be made only through such close cooperation 

 of the persons engaged in both the meteorological and 

 the radio work, not only in the measurements but also 

 in the analysis. 



Perhaps the most striking information which has 

 so far resulted from the detailed analysis is the em- 

 pirical correlation of the 117-mc performance with 

 M curves. Increases in signal level abo.ve the standard 

 are found to result from either large surface ducts 

 (300 ft or more thick) or elevated superstandard 

 layers which do not necessarily show overhanging M 

 curves. Such layers occur frequently over Massachu- 

 setts Bay, mainly as a result of nocturnal cooling over 

 land. Those which affect the 117-mc transmission 

 occur below about 1,500 ft. Their strength is usually 

 doubtful in view of the lack of accurate information 

 on conditions over land in radiation inversions. 



Figure 10 shows the correlation diagrams obtained 

 when, first, all points are included, and second, all 

 cases of elevated superstandard i)/ layers are omitted. 

 (Standard values are —130 db for 117 mc and —80 

 (11) for S band.) The first diagram obviously shows no 

 correlation and is tlie sort of diagram ol)tained last 

 fall. The second, however, is just what sliould be ex- 

 pected for the correlation with surface phenomena. 

 The S-band signal rises to the free space value as the 

 duct height goes up to about 100 ft and then "satu- 



