Honeywell KHF 1050 Pilot's Manual page 22

You should also be aware of the possibility that you or the ground station
you are calling may be in a quiet zone. The linear distance from the
point of transmission to the point where the skywave returns to earth is
called the skip distance. There may be a quiet zone between the end of
the ground wave and the return of the skywave. No communication can
take place in this area. At any time, day or night, there is a "window" of
useable frequencies created by the reflecting properties of the ionos-
phere. At night this "window" will normally be in the lower range of HF
frequencies, and during the day it will be in the higher range of frequen-
cies.
Normally you will not know what the MUF is at any particular time and
location unless you have a table of propagation forecasts. Just remem-
ber that the higher frequencies in the "window" of useable frequencies
are likely to be the most effective. The closer a frequency is to the MUF,
the better it is likely to be.
The effect of solar disturbances including solar flares and magnetic
storms is to change the particle density in the ionosphere. Therefore, the
"window" of useable frequencies may begin to close, with radio waves of
frequencies in the lower range dropping out first as they are absorbed by
the ionosphere.
Next, the radio waves of upper frequencies in the useable "window" may
start to penetrate the ionosphere and go into outer space. It is even pos-
sible for the entire "window" to close, particularly if you are flying in a
polar region in latitudes above 60 degrees north or 60 degrees south.
Solar disturbances have the most negative effects on HF communica-
tions in these regions.
If you are flying in polar regions and are having difficulty raising any
ground station located in the same region, remember this: even though
the "window" of useable frequencies may have closed in the polar
regions, another "window" may be open in regions closer to the equator
which are less affected by solar disturbances. Try calling a station closer
to the equator in latitudes lower than 60 degrees north or 60 degrees
south, and use a higher frequency. If you can raise a station in these
areas, that station may be able to relay your message.
There are even times when solar disturbances improve the usability of
higher frequencies in the HF band, particularly in equatorial regions.
Another phenomenon which occurs during solar disturbances may allow
you to communicate with a station even though the "window" is closed.
This is known as scatter propagation, in which a radio wave is broken up
in the ionosphere and scatters in various directions.
Because frequency propagation cannot be predicted with total accuracy,
ground stations responsible for aircraft HF communications will typically
operate on several different frequencies within the HF band. The pilot is
then able to choose the optimum communication frequency for the exist-
ing ionospheric conditions.
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Revision 0 Mar/2003
KHF 1050/PS440 Pilot's Guide