Figure 7-1. Simplified Characteristics of HF Propagation.

TM 11-5820-917-13

EL9TE032

F I G U R E 7-1.

S i m p l i f i e d Characteristics of HF Propagation.

quency (LOF) is set by signal attenuation caused primarily by D-region absorp-

t i o n . The maximum observed frequency (MOF) is generally set by the electron

d e n s i t y i n t h e Es o r F l a y e r s .

b . A s r a d i o f r e q u e n c y i n c r e a s e s , t h e signal energy propagating via F modes

penetrates deeper into the ionosphere. If sufficient refraction occurs to return

t h e signal to earth, the result is generally an increasing propagation time delay

with frequency.

S i g n a l e n e r g y p r o p a g a t i n g v i a Es will always arrive earlier than energy

v i a t h e F r e g i o n . I n addition, because the Es layer is so thin, propagation time

delay is essentially independent of frequency.

d . S p o r a d i c E can vary greatly in spatial extent. Thus, great care must be

t a k e n in assuming that, because Es is observed o n o n e p a t h , i t m u s t b e p r e s e n t

on another path.

7 - 1 4 . In addition to the above items, there are s e v e r a l other propagation factors

to remember:

a. The ionosphere is an atmospheric region in near-constant turbulence.

I o n i z a t i o n concentration and depletion regions constantly occur creating focusing

a n d d e f o c u s i n g o f r a d i o e n e r g y . T h e result is a near-continuous variation of the

MOF (usually over small limits) and signal fading on any frequency.

7-3