20 



TRANSMISSION EXPERIMP;NTS NEAR SAN DIEGO 



quciK-y is used to cliofk the plane's bearing during 

 flight and keep the transmitting antennas pointed at 

 the plane. Bearing checks have been consistently ob- 

 tained at ranges np to 130 miles. The d-f bearings 

 agree qnite well with those obtained by use of a type 

 FC fire control radar. 



''■^■^ One-Way Fixed Link Data 



Eay Theory — Geometkic Optics 



On the basis of i-ay theory, when it is assumed im- 

 plicitly that the energy follows the rays, the modilied 

 index criterion for trapping should be expected to 

 agree with experience. Ray tracing theory states that 

 when the modified index at some elevation above a 

 transmitter attains a value ecpuil to or less than its 

 value at the transmitter height trapping can occur. 



As a preliminary check on this criterion the maxi- 

 mum field strength observed during the hour in which 



Ih'Iow detection for all llie f iTqiicncies used on the 80- 

 niile o\er-water link. This lias been conftnned experi- 

 mentally. On Xovember 5, 1944 a front passed accom- 

 panied by heavy rain which dissipated all low-level 

 inversions, a)ul a standard condition resulted. During 

 this period all the signals decreased below detection. 

 Figure "2 shows the above field strength data plotted 

 against the height of the base of the temperature in- 

 version. (The curves appearing in this figure will be 

 ex]»lained later.) Although both the thickness of the 

 inversion layer and the strength of the inversion vary 

 considerably, the correlation of signal strength with 

 the height of the base of the inversion is quite remark- 

 able. The S4T-mc signal decreases below detection as 

 the layer heights increase above 3,000 ft; whereas the 

 100- and oS-mc signals are still relatively strong when 

 the inversion base is above this altitude. These lower 

 fi'e(iiu'ncies do show a decreasing trend as the layer 

 continues to rise, going completely out, as stated above, 

 when the low-level inversion is washed out. 



I- 



UJ 



ui 



z 



111 



< 

 m 



UI 



o 



3 



547 MC 



• SlNOlEOO-RATSONa 



WIRED SONDE 



1 AIRPLANE SOUNDING 

 -THEORl' 

 iN'SOXIC 

 0= SJOFEET 



100 MC 





« • I* 



-«-i- 



-L^ 



•SO •«0 -50 -20 10 



DB ABOVE FREE SPACE FIELD 



Figure 2. Maximum received signal vensus altitude of base of temperature inversion. 



the meteorological sounding was taken is plotted 

 against Ail/ in Figure 1. When the minimum value 

 of M in the refracting stratum is less than its value 

 at the transmitter elevation, AM is negative, and the 

 trapping condition is fulfilled. The 52-, 100-, and 547- 

 mc links all show strong fields for large positive AM. 

 These data are not compatible with the assumption 

 that the enersiv follows the rays. The diffracted field is 



Figure 3 shows a condensed log of the field strength 

 data taken on the one-way link. Maximum and mini- 

 mum field strengths during successive 3-hour intervals 

 are plotted, thus showing the general level and fading 

 range for each frequency during a 6-week period. The 

 corresponding elevation of the base of the tempera- 

 ture inversion is shown by the discrete points in the 

 upper part of Figure 3. It is at once apparent that the 



