TRANSMISSION E 
path'’ °° 80 miles long with both terminals at an 
elevation of 100 ft, which were thus well below the 
optical horizon. Three fairly low frequencies, 52, 
100, and 647 me, were used. Figure 13 shows a field 
strength diagram of bihourly means for a periad of 
about six weeks in the early fall of 1944. At the top 
of these diagrams is shown the height of the base 
of the temperature inversion, which is a quantita- 
tive measure of the height of the elevated duct. In 
order to compare these data with the results of duct 
theory, Figure 14 shows the number of lowest modes, 
trapped in the elevated duct, plotted against the 
signal strength. For each point indicated, the number 
of trapped modes is calculated by simple waveguide 
theory from the measured M curves while the field 
strength is that simultaneously measured on the 
transmission path. For the lowest frequency, 52 me, 
the duct is always beyond cutoff and no trapping 
should occur; nevertheless, the field strength record 
shows considerable fluctuation. 
As seen from Figure 14 there is no correlation be- 
tween the field strength and the number of modes 
Bec 
ueaes 
AY 
\ 
2 2600) ea 
ere ESS SX 
: Ee KIN 
RANGE IN THOUSANDS OF YARDS 
NEPEE 
SY 
Se 
vy 
Y 
MN 
Ni 
AY 
VAM 
fy 
ae 
a 
XPERIMENTS 35 
TRAPPED 
+ ww an @ 
NUMBER OF MODES 
ol 
-30 -20 +10 O 
DB ABOVE FREE SPACE 
10 20 -30 -20 -I0 O 
Figure 14. Computed number of modes trapped versus 
observed field strength, San Diego Bay. 
that, theoretically, are transmitted by the duct. On 
the other hand, there is a very pronounced inverse 
correlation between the height of the inversion layer 
and the strength of the received signal. This is just 
what should be expected on the basis of reflection, as 
WA 
Y 
eo 
wa 
= 
a 
od 
iN 
N 
NV 
NK 
[\ 
ns 
Ne 
ON 
KS 
oC 
A 
ai 
IN 
\\ 
Y 
\ 
VX/ 
V 
A 
U 
A/\ 
LL 
V 
VN 
Figure 15. Ray tracing diagram including rays reflected from elevated inversion layer, San Diego Bay. M changes by 
50 units through the inversion. 
