Figure 7-4. Example RCS-4B Ionogram and Received Signal Power Display

TM 11-5820-917-13

FIGURE 7-4.

E x a m1 l e R C S - 4 B I o n o q r a m a n d R e c e i v e d S i`g n a l

Power Display.

F i g u r e 7-4 shows a classic example i n the band around 6.1 MHz, although the

b a n d from 3.6 to 6 . 4 MHz satisfies t h e a b o v e c r i t e r i a a n d w o u l d p r o v i d e e x c e l l e n t

communication,

7 - 2 1 . T h e most important exception to the above criteria occurs in dealing with

s p o r a d i c E p r o p a g a t i o n . I n g e n e r a l , s p o r a d i c E provides many advantage; for

H F communication, particularly for military applications:

a . I t o f t e n p r o v i d e s s t r o n g--p r o p a g a t i o n a t f r e q u e n c i e s a b o v e t h e F - m o d e M O F

w h e r e a t m o s p h e r i c n o i s e i s l e s s , i n t e r f e r e n c e i s l e s s l i k e l y , a n d antenna systems

are usually more effective.

b . B e c a u s e i t i s n o n - d i s p e r s i v e , s t r o n g Es p r o p a g a t i o n c a n s u p p o r t h i g h e r

data rate communication than normal F-mode propagation.

c . O p e r a t i o n at frequencies above the expected MOF reduces the probability

o f s i g n a l i n t e r c e p t . I n a d d i t i o n , t h e p a t c h y c h a r a c t e r i s t i c o f Es f u r t h e r m i t i -

gates against signal intercept or jamming in directions other than the great

c i r c l e path of the comm circuit. However, the "sporadic" nature of sporadic

E must be fully accounted for in its use for an HF communications circuit.

7 - 2 2 . Figure 7-5 shows an excellent example of the problems one can have with

s p o r a d i c E . T h e upper photo shows strong sporadic E to 28 MHz. Note particu-

l a r l y that the band from 18-22 mHz satisfies all criteria of para. 7-20 above. How-

7-7