BendixKing KHF 990 - Manual (Page 15)

phere. One of the best things the pilot can do to assure the best pos-
sible HF communications, based on existing HF propagation condi-
tions, 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).
This rule of thumb can be explained by a mirror analogy. It is the
electrically charged particles in the ionosphere which reflect or bend
radio waves back toward earth like a mirror reflects light. Sunlight
induces ionization and increases the density of these particles in the
ionosphere during the day. The mirror becomes thicker and it reflects
higher frequencies better. When the sun goes down the density of
charged particles decreases and the ionosphere becomes a mirror
that can only reflect lower frequencies in the HF band.
For any one particular frequency, as the angle at which an HF radio
wave hits a layer of the ionosphere is increased, a critical angle will
be reached from which the wave will just barely manage to be reflect-
ed back to earth
(Figure 1-1). Waves entering at sharper angles than
this will pass through this layer of the ionosphere and be lost in space
(or may reflect off another layer of the ionosphere).
Changing the frequency under the same conditions will change the
critical angle at which the HF radio waves will be reflected back to
earth. The highest frequency which is reflected back to the earth is
called the maximum useable frequency (MUF). The best HF commu-
nications are usually obtained using a frequency as close to the MUF
as possible since radio waves higher than this frequency are not
reflected and radio waves lower than this frequency will be partially
absorbed by the ionosphere.
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 ionosphere. At night this “window” will nor-
mally be in the lower range of HF frequencies, and during the day it
will be in the higher range of frequencies.
Normally you will not know what the MUF is at any particular time and
location unless you have a table of propagation forecasts. Just
remember that the higher frequencies in the “window” of useable fre-
Description
1-4 KHF 950/990 Pilots Guide
Rev. 0
Dec/96