26 



TRANSMISSION EXPERIMENTS NEAR SAN DIEGO 



Treating the elevated i-efraetiiig stratum as a plane 

 reflecting layer seems to agree in general with experi- 

 ence, for the following reasons. (1) The observed fre- 

 quency sensitivity of the reflecting layer is predicted. 

 (2) The observed fading characteristics of the differ- 

 ent frequencies is again in the right direction, the 

 higher the frequency the greater the fading. (3) 

 Strong fields well lielow the horizon under conditions 

 of high layers cannot be explained on the basis of 

 refraction alone. 



22 THE CORRELATION OF CALCULATED 

 AND MEASURED FIELD STRENGTHS" 



Since the time of issue of reference 3, the impor- 

 tance of further experimental check against the cal- 

 culated patterns has been fully realized. 



The field strength cross sections recently obtained 

 by airplane-ljorne receivers have made possible such 

 a check. 



For anything mui'e than a rough qualitative corre- 

 lation it was soon apparent that quantitative field 

 strength analyses were needed for the actual observed 

 meteorological conditions. 



Because of the clearly apparent influence of high 

 level inversion layers ou the observed radiation fields, 

 this type of condition was selected. Consider, for ex- 

 ample, the il/ curve at 50-mile range obtained on 

 September 39 reduced to three linear segments as 

 shown in Figure 9."= It is clear that the M curves at 

 10 and 100 miles are not seriously different. 



We thus have a condition in which M — il/o de- 

 creases l)y 50 units in a 300-ft interval of altitude 

 attaining the minimum value of -|-50 at 3,000-ft 

 elevation. 



Figure 10 shows the ray diagram constructed for 

 the analysis. The diagoiuil lines below 4,000 ft rep- 

 resent the positions at which field strengths were 

 measured and calculated. 



The actual size of the ray diagrams is 27x40 in. 

 Eays in the region of standard refraction have a 58-in. 

 radius. Through the transition layer the radius is 4 

 in. The above radii are determined by the vertical and 

 horizontal scaling factors and are approximately one 

 ten millionth of the curvature as given by dM/dli. 

 Note that the downward curvature of the earth and 



''By V. R. Abbott, U. S. Navy Radio and Sound Labo- 

 atory, San Diego, California. 



''See discussion of Figure 9 in Section 2.1.2. 



upward curvature of rays in the standard propaga- 

 tion regions are made equal, thus reducing the slopes 

 of the rays and resulting errors inherent with de- 

 formed scale graphical methods. 



Since the tangent ray (shown with short dashes) 

 intercepts only a small part of the fourth and none 

 of the fifth section of measurements, the analysis 

 methods employed in radar coverage diagrams had 

 to be extended. Specifically, the coverage diagram 

 analysis at NIJSL has applied to fields l)etween 85 and 

 100 db below that at a distance of one meter from the 

 transmitter. This largely excliules consideration of 

 any but interference and trapping zones. 



The measurements with which correlation was de- 

 sired extended to about 30-db weaker fields so that 

 partial reflection and diffraction fields were involved. 



Proceeding with the ray tracing analysis, the in- 

 terference field was calculated at points of intersec- 

 tion of the direct and sea-reflected rays. Path diflfer- 

 ences were determined using a map measure and a 

 jjlanimeter as exjjlained in reference 4. Pay densities 

 were measured for the direct and reflected compo- 

 nents, and the associated fields were added with re- 

 spect to the phase. The diffraction field below the 

 tangent ray was calculated by Norton's method. 



Keflected rays from the layer were introduced as 

 originating at the center of the layer. The reflection 

 coefficients for the angles of incidence were calculated 

 as described in i-eference 5 for the case of a mono- 

 tonic transition layer in which the refractive index 

 decreases by 50 X 10~". In the terms of field intensity 

 the reflection coeflncient values ranged from 0.2 to 

 0.1 at 63 mc and from 0.01 to 0.003 at 524 mc. 



In Figure 9 the calculated normal interference 

 and dift'raction fields are shown dotted beside the 

 measured values except at 3,250 mc on which the 

 30- and 45-mile sections have been displaced for 

 clarity. At 63 mc there is an apparent displacement 

 of al)out 3 db which is probably associated with the re- 

 duction in measurements to the decibels Ix'low the field 

 at a distance of 1 m from the transmitter. Note that 

 at 60 miles the interference pattern of the diffraction 

 and partial reflection fields as caleidated appears with 

 a jiliase displacement of about 180 degrees from the 

 observed field. The phase relation.ship depends, of 

 course, on an assumed \alue of 90 degree change of 

 phase on reflection. 



At 170 mc there is a displacement of about 10 db 

 due to difficulty of reduction in measurements. In- 

 troducing a 10-db correction, all values at 170 mc 



