RADIO AND RADAR TRANSMISSION MEASUREMENTS 



11 



mission) and both one-way and radar transmission on 

 X band. The signal occurring with these M inver- 

 sions less than 20 ft high is usually the high, steady 

 type. It is generally not quite so high in average level 

 as that characterized by roller fades found with larger 

 M inversions. 



3. The high, steady signal occurring witli very 

 low M inversions reveals the only clear-cut cases of 

 frequency diversity between S and X bands. In this 

 case a variety of combinations has been found : nearly 

 standard signal on S band with X-l)and signal from 

 10 to 30 db above standard; S band 10 to 1-5 db above 

 standard with X band 30 or so db above standard ; 

 and finally S band about 20 or more dl) above stand- 

 ard and steady while X band changes to the first sig- 

 nal type : high with fades. 



4. One of the most interesting features of the trans- 

 mission is the fact that at any given location, for a 

 fixed frequency, the increase in field strength is lim- 

 ited ; that is, no matter how much the M inversion 

 increases in height or in strength beyond a certain 

 value (which is as yet unsisecified) the average value 

 of the signal strength does not continue to increase 

 but rather remains the same within about 10 db. This 

 "saturation" level is of the order of the free space 

 value. (Maximum level goes up to 13 to 15 db above 

 free space but only infrequently.) Consequently, the 

 level reached on a given path appears to be indepien- 

 dent of the receiver height (within the height range 

 covered in these measurements), the height-gain effect 

 which exists under standard conditions being essen- 

 tially eliminated when shore trapping takes place. 

 Under some conditions, especially when the signal is 

 high with fading but has not yet reached the satura- 

 tion level, the lower of the two receivers has been ob- 

 served to receive higher signal than the higher one. 

 With stronger signal the values on the two become 

 nearly identical, as has been stated. 



These results agree with unpublished calculations 

 made for several values of duct height and M deficit 

 for S band, of the first transmission mode alone, 

 wliich indicate that the height-gain effect should dis- 

 appear and the signal approach a certain saturation 

 level. Thereafter, calculations show, the contrilmtion 

 of the first mode decreases, but the observations sug- 

 gest that perhaps the other modes continue to cause 

 the average level to reach approximately the same 

 value, as duct height and M deficit continue to in- 

 crease. 



It has been found that, with an M inversion over 

 only a portion of the path and a standard curve on at 



least a small part of it, the signal type may be high 

 with roller fades and the average level high, so that 

 the record is indistinguishable from that which oc- 

 curs with more uniform conditions. 



^'^'^ Radar Transmission 



From Eace Point, targets were available over water 

 at ranges of 20 to several hundred miles along the 

 coast of Massachusetts and Maine, phis some addi- 

 tional targets inland and whatever shipping was in 

 the vicinity. Of the coastal targets four were chosen 

 for regular observation. These were fairly isolated 

 fixed targets, the echoes from which appeared to be 

 relatively steady in several days' observations, at 

 ranges of 22, 41, 65, and 73 statute miles. Absolute 

 power measurements of the returns of each of these 

 targets (whenever visible) were made hourly by com- 

 parison with a signal generator. Each measurement 

 represents the maaimtim value of the signal during 

 a period of 1 to 3 minutes. This differs rather essen- 

 tially from the hourly averages of the one-way data. 



In addition to signal strength measurements, hour- 

 ly observations were also made of the maximum ranges 

 obtained on surface targets, and plan position indi- 

 cator [PPI] iDliotographs were made which reveal 

 at a glance many interesting features of the radai' 

 coverage which are hard to describe briefly in words. 

 The maximum sweep length available on the PPI 

 was 140 miles for the S-band set and 115 miles at X 

 band. Additional range was available on the delayed 

 A-scope sweeps, so that the maximum was 180 miles 

 for most of the period of observations. This was ex- 

 tended to 280 miles for the last week of the test. 



In addition a portable K-band radar set was set up 

 near Eace Point Light only 17 ft above mean sea 

 level and regular observations of range were made and 

 shipping tracked. 



Takget Signal Strengths 



The strength of the echoes from the four targets, 

 including the nearest one which is ordinarily visible 

 both optically and by radar, varied from below mini- 

 mum detectable to at least 60 db above for the two 

 nearer targets and about 35 db al)ove for the two 

 more distant targets, at both frequencies. In general 

 the values of the signal strength were higher for the 

 nearer targets, but there were some interesting cases 

 when the more distant targets were visible while the 

 nearer ones were either not seen or were very weak. 

 This may occur at times when the M curve varies 



