BendixKing KHF 990 - Manual (Page 14)

SKYWAVE PROPAGATION - WHICH FREQUENCY TO
USE?
As mentioned earlier, HF’s primary method of travel or propagation is
via skywaves which are radio waves that start out radiating into
space and are reflected off the ionosphere back to the earth’s sur-
face. This reflecting of signals makes communications over very long
distances-under ideal conditions more than 4,000 miles and typically
in excess of 2,000 miles-possible. Because of variations in the ionos-
phere, HF communications require more analysis of conditions and
operational decisions (such as frequency selection) than VHF com-
munications.
The ionosphere is a multi-layered band of electrically charged parti-
cles surrounding the earth. It varies in height above the surface of the
earth from approximately 30 to over 400 miles. The height and inten-
sity varies from one location to the next and according to the season
of the year and the time of day.
Because HF radio waves depend upon the ionosphere for reflection,
their propagation is affected by changes in the ionosphere. It is
changes in the density of the electrically charged particles in the
ionosphere which cause propagation to improve or deteriorate. Since
the ionosphere is formed primarily by the action of the sun’s ultravio-
let radiation, its thickness changes in relation to the amount of sun-
light passing through it. Sunlight-induced ionization increases the par-
ticle density during the day and the absence of it reduces the particle
density at night. At midday, when the sun’s radiation is at its highest,
the ionosphere’s thickness may expand into four layers of ionized
gas. During the nighttime hours, the ionosphere diminishes, normally
merging into just one layer.
Solar disturbances including solar flares and magnetic storms can
cause propagation of HF radio waves to deteriorate rapidly. HF sig-
nals can also suffer interference from such atmospheric disturbances
as precipitation and thunderstorms.
The net result of all these factors is that because the ionospheric and
atmospheric conditions are constantly changing, HF communications
can vary in quality and strength. The signal received on the KHF
950/990 may be accompanied by a considerable amount of static
from atmospheric disturbances, or it may fade in and out at times
because each radio wave which hits the changing ionosphere may
be reflected differently. Your reception and transmission success may
vary from loud and clear to nonexistent depending on your selection
of frequency and the conditions in the atmosphere and the ionos-
Description
1-3 KHF 950/990 Pilots Guide
Rev. 0
Dec/96