Chapter 18 
ATTENUATION DIAGRAMS FOR SURFACE DUCTS’ 
eg HORIZONTAL ATTENUATION (decibels per unit 
distance) of a signal under assumed propagation 
conditions not only is of intrinsic interest but is one 
of the simplest quantities to verify experimentally. 
In terms of the wave equation formulation this 
attenuation is proportional to the imaginary part of 
the characteristic value associated with the mode 
dominant in the region of space in question. No one 
mode may necessarily be dominant, and in certain 
regions a weakly attenuated mode may be outweighed 
by a mode more strongly attenuated but with a 
@By Lt. William F. Eberlein, USNR, Office of the Chief 
of Naval Operations. 
stronger initial excitation. Well inside the shadow 
zone, however, the “‘first”’ or least attenuated mode 
108 
a: 0,036 FEET "= = 
M-My —> 
\ \ \ 
\ \ \ 
\ M \ 7 
\ 
\ 
SSS \ 
M DEFICIT 
DUCT THICKNESS IN FEET 
Ficure 2. Horizontal attenuation of first mode and trapping index, 10,000 me, standard attenuation (0 M deficit): 
1.83 db per 1,000 yd. 
