Figure 7-6. Example of Strong Es propagation extending well past the F-mode MOF

TM 11-5820-917-13

ever, the lower photo shows that 20 minutes later the MOF has dropped to 15 MHz,

and propagation in the 18-22 MHz band is nil. Interestingly, in the next 10 minutes,

conditions returned to those of 0125 local time.

7-23. Figure 7-5 and 7-6 show very clearly how easy it is to recognize sporadic

E propagation on the RCS-4 CRT display. Sporadic E will nearly always (exceptions

are very rare) show two important characteristics:

a. Es will be the earliest arriving skywave mode and will be the lowest trace

on the display.

b. The Es trace appears as a straight line with near-constant time delay vs

frequency as contrasted with F-mode propagation which will generally show variable

time delay particularly near the MOF of each mode.

7-24. For short path lengths, care must be taken to avoid confusion between Es

and surface wave propagation as their appearance on the CRT is similar. Figure

7-7 shows ionograms taken on two circuits near local midnight. The upper photo

shows 1-hop F and Es propagation; the lower photo shows 1-hop F and surface

wave propagation. There is little to separate the surface wave from the Es

except for the slightly greater time delay spread between the surface wave and the

F-mode propagation and that surface wave propagation shows significantly less

variation with time than Es. For an 80-km circuit, the following typical signal

propagation time delays can be expected.

1,2,3, AND 4 HOP F MODES

FREQUENCY (MHz)

EL9TE038

Example of Strong Es Propagation extending well

past the F-mode MOF.

7-9