180 TECHNICAL SURVEY 
model one should first approximate the actual M 
curve by a bilinear curve and then enter the diagram 
with the values of M deficit-and duct thickness 
corresponding to the zdealized curve. How to make the 
best bilinear approximation to a given M curve is 
100 
M DEFICIT 
| 
=—=——DB_ ATTENUATION / THOUSAND YARDS i 
—-— TRAPPING INDEX 
(Nine ONII SIS 
TASS 
FEISS 
A AAW 
an important but still open question. 
Except for the 0.01- and 0.001-db contours, which 
were computed by an asymptotic method, the attenu- 
ation contours were cross-faired from preliminary 
Radiation Laboratory calculations. 
= Yd mss 
— 
NNN 
LWA SS 
1000 
OUCT THICKNESS IN FEET 
Figure 4. Horizontal attenuation of first mode and trapping index, 200 me, standard attenuation (0 M deficit): 0.498 db 
per 1,000 yd. 
