Section 7. FREQUENCY MANAGEMENT OF HF NETS USING AN/TRQ-35(V) EQUIPMENT (cont)

TM 11-5820-917-13

ing electron density and RF signal wavelength (or decreasing signal frequency),

the electron content in the D-region establishes the lowest propagating frequency

for a given transmitter output. D-region absorption peaks around noontime. At

night, the natural recombination process depletes the D-region electrons, and low-

er-frequency signals can propagate virtually unattenuated.

7-8. The E-region is of little interest to HF communicators except for a phenomen

which generally occurs near its boundary with the D-region. It is at this altitude

(approximately 90 km) that the "sporadic E" (Es) layer is observed.

7-9. Sporadic E appears not to be caused directly by the effects of solar radiation,

but rather by secondary effects such as wind shears. The result is a layer which

is generally extremely thin (perhaps tens or hundreds of meters thick), highly

ionized (causing partial reflection of signals at frequencies far higher than the

normal ionosphere will support), and spatially dependent (visible on one path, but

not present on another). On occasion, Es can be so strong as to "blanket" the

upper ionospheric layers (i.e., prevent radio waves from reaching higher alti-

tudes); while at other times, RF energy easily penetrates the Es and also propa-

gates via normal F-region modes.

7-10. Because of its variability, Es has been examined largely by statistical

means. At mid-latitudes, Es is observed on at least half of the days for most

hours during the summer months becoming less frequent as autumn and winter

pass, then increasing again in late spring. It is observed more often in daytime

than at night. At high latitudes, seasonal variations are less, and Es is observed

more often at night. There is no firm indication of any correlation of Es with the

11-year solar cycle, varying correlation statistics related to solar storm activity,

and data showing significant year-to-year variation of occurrence over a given

location.

7-11. Radio propagation in the F-region is a direct function of the electron con-

tent in the region, which continually varies. In general, F-region ionization is

a superposition of three cycles: a diurnal cycle, a seasonal cycle, and the 11-year

solar activity cycle. Ionization usually builds quickly after dawn in the ionosphere

and peaks around midday. It falls off more slowly as the afternoon and evening

passes reaching a minimum in the hours after local midnight. Nighttime ioniza-

tion is higher in summer than winter. However, daytime ionization is often greater

in winter than summer - a phenomenon called the "winter anomaly."

7-12. The effect of increasing electron content in the ionosphere is to increase

the maximum radio frequency that will propagate to any range. Thus, the maxi-

mum propagating frequency, less affected by D-region than lower frequencies,

is generally established by the ionosphere and not by transmitter power or

antenna characteristics (although these latter factors affect the strength of the

signal relative to received noise).

7-13. Figure 7-1 illustrates several important features of HF propagation that

should be kept in mind:

a. At lower frequencies, ionization levels in the ionosphere usually permit

signal propagation by one-hop or multiple-hop modes. The lowest observed fre-

7-2