The Ionosphere

The Ionosphere

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

radio propagation.

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.