This rule of thumb can be explained by a mirror analogy. It is the electri-
cally charged particles in the ionosphere which reflect or bend radio
waves back toward earth like a mirror reflects light. Sunlight induces ion-
ization and increases the density of these particles in the ionosphere dur-
ing the day. The mirror becomes thicker and it reflects higher frequen-
cies 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 reflected
back to earth (Figure 1A). 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).
Figure 1A - Effects Of Different Skywave Paths
Changing the frequency under the same conditions will change the criti-
cal 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 communications 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 ionos-
Revision 0 Mar/2003
KHF 1050/PS440 Pilot's Guide