TRANSMISSION CHARACTERISTICS OF AN OVER-WATER PATH 



13 



rises above the standard level no appreciable increase 

 in radar range occurs at first. Only when the one-way 

 signal strength has become fairly high do the radar 

 ranges begin to increase. Then the entire gamut of 



100 



60 60 



DB BELOW 1 WATT 

 SIGNAL STRENGTH 



40 



Figure 8. Relation between maximum radar ranges 

 and one-way transmission. 



long radar ranges, from about 40 to 380 miles, takes 

 place while the one-way signal strength changes only 

 slightly. This is another manifestation of the satura- 

 tion of the signal at a high value. 



Summary 



Two major conclusions may be drawn from this 

 preliminary survey : 



1. Standard signal is the exception rather than the 

 rule for microwave radiation on this over-water path 

 during the summer and fall. AVitli the high M inver- 

 sions which occur with warm, dry air over water, 

 signal strengths 30 to 45 db above the standard occur 

 about equally often on both, the upper limit being 

 approximately the free space value, and radar ranges 



on surface targets are extended to five to ten times 

 their normal values. On the other hand, with the low 

 M inversions (less than 20 ft, say) whicli occur with 

 air colder than the water, X band is aiiected more tlian 

 S. Both may experience increases in signal level of 10 

 to 30 db above the standard, but the X-band signal 

 is liigh more often than the S and at any given time 

 usually reaches a higher level. Radar ranges on sur- 

 face targets are extended by as much as 30 to 35 per 

 cent above normal, and again X band experiences 

 more effect. These increases in signal strength can 

 be of great importance for communications, beacons, 

 or any other application involving one-way transmis- 

 sion of microwaves, such as countermeasure. It should 

 also be remembered whenever secrecy is required. 



3. Substandard conditions may be present for sev- 

 eral days at a time if the air is warm and moist. The 

 reduction in signal strengths and radar ranges on 

 surface targets which accompanies substandard con- 

 ditions does not seem to be markedly frequency sensi- 

 tive. It should be stressed that variations in one-way 

 signal strength of at least 90 db have been observed. 

 The radar ranges have also varied from roughly 10 or 

 15 miles up to at least 380 miles. These changes are 

 not rare occurrences; deviations from the standard 

 account for the major percentage of the time, especial- 

 ly during warm weather, and at the higher frequencies 

 even during the fall. 



1 ' TRANSMISSION CHARACTERISTICS 

 OF AN OVER-WATER PATH'' 



Results were previously reported of some prelim- 

 inary analyses of one-way radio transmission on a 41- 

 mile over- water path from Provincetown to Gloucester, 



300 



200- 



100- 



Profii* of Transmission Path 

 Raca Point To Eastern Point 



Standard Refraction 

 (Msosuro Hoigllt Vtrticolly) 



— 1« 



-300 

 -ZOO 



100 



Figure 9. Transmission path profiles. (Heights in feet.) 



with terminals well below the horizon. S- and X-band 

 radiations were transmitted over the double patlis in- 

 dicated in Figure 9 to both "high" and "low" receivers. 



■'By P. J. Rubenstein and W. T. Fishback, Radiation Lab- 

 oratory, MIT. 



