SKYWAVE PROPAGATION - WHICH FREQUENCY TO
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 surface. This reflect-
ing of signals makes communications possible 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 ionosphere, HF com-
munications require more analysis of conditions and operational deci-
sions (such as frequency selection) than VHF communications.
The ionosphere is a multi-layered band of electrically charged particles
surrounding the earth. It varies in height above the surface of the earth
from approximately 30 to over 400 miles. The height and intensity 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 ultraviolet radiation, it's thick-
ness changes in relation to the amount of sunlight passing through it.
Sunlight-induced ionization increases the particle 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 signals can
also suffer interference from such atmospheric disturbances as precipita-
tion 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 1050
may be accompanied by a considerable amount of static from atmos-
pheric disturbances, or it may fade in and out at times because each
radio wave which hits the changing ionosphere may be reflected differ-
ently. 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 ionosphere. One of the best things
the pilot can do to assure the best possible HF communications, based
on existing HF propagation conditions, is to select the proper frequency.
A good rule of thumb for the time of day is that the higher frequencies
are best during daylight (10 to 29.9999 MHz) and lower frequencies work
best at night (2 to 10 Mhz).
Revision 0 Mar/2003
KHF 1050/PS440 Pilot's Guide