In the upper air around fifty miles and higher where the air molecules are far apart, radiation from the
sun strips electrons from oxygen molecules causing the molecules to become ionized forming the
ionosphere. The ionized oxygen molecules and its free electrons float in space forming radio-reflecting
layers. Ionization of the ionosphere varies by the time of day, seasons of the year, and the sunspot cycle.
The strength of ionization also varies from day to day and hour to hour. Since the height of the
ionosphere varies, the higher the ionized layer becomes, the farther the skip will be. We will define skip
in section 5 of part V.
The part of the earths atmosphere called the ionosphere is divided into three layers. The three layers are,
from lowest to highest, the D layer, the E layer, and the F layer. Each layer has a different effect on HF
Being at a lower altitude, the D layer molecules are squeezed closer together by gravity than those in
higher layers, and the free electrons reattach to the molecules easily. The D layer requires constant
radiation from the sun to maintain its ionization. Radio waves at lower frequencies such as the
frequencies of the AM broadcast band cannot penetrate this layer and are absorbed. The higher
frequency signals are able to pass through the D layer. The D layer disappears at night causing AM
broadcast stations to reflect from the higher layers. This is why AM broadcast signals only propagate by
ground wave in the daytime and they can be received from great distances at night. Like the broadcast
band, the D layer absorbs signals on 160 and to a lesser extent 80 meters during the day making those
bands go dead. During solar flares, the D layer becomes ionized so strongly that all high frequency radio
waves are absorbed, causing a radio blackout.
E-layer propagation is not well understood. Being at a lower altitude than F layer, the E layer is
responsible for summertime short skip propagation on the higher high frequency bands. The skip zone is
around 1000 miles, but at times when the E-cloud covers a wide area in the summer, double hops can be
seen. A double hop occurs when the signal reflects from the ionosphere, then returns to the ground,
reflects from the ground back to the ionosphere where it is reflected back to the ground. A double hop
can propagate the signal 2000 miles or more. The E-layer forms mostly during the day, and it has the
highest degree of ionization at noon. The E layer like the D layer disappears at night. Even so, sporadic-
E propagation can and does form at night. There is a minor occurrence of sporadic E propagation
during the wintertime. On rare occasions, sporadic E propagation can surprise you by occurring
anytime regardless of the sunspot cycle or the season of the year.
The F layer is the highest layer and it is divided into two levels: F1 and F2. At night the F1 and F2 merge
into one layer. During the day, the F1 layer doesnt play a part in radio propagation, but F2 does. It is
responsible for most high-frequency long distance propagation on 20 meters and above. However, the F
layer makes it possible for you to work DX on the lower bands at night. Sunspots are responsible for the
ionization layers and in years with high sunspot numbers, worldwide contacts can be made easily on 10-
20 meters by F2 layer propagation. In years of low sunspot numbers, working distant stations is difficult
on those bands. Consequently, ten and fifteen meters will be completely dead most days and twenty
meters will go dead at night. In years of low sunspot numbers DX contacts are easily made at night on
160, 80, and 40 meters. The sunspot numbers increase and decrease in 11-year average cycles.
Since the curvature of the earth averages about 16 feet every 5 miles, an object 5 miles from you on
perfectly flat earth will be 16 feet below the horizon. Because light travels in straight lines, you cannot
see objects beyond the horizon. Radio waves travel in straight lines, but there are ways to get them
beyond the horizon. This is referred to as propagation.