These links are also found down in the text of this primer...they'll make more sense if you follow them when you come across them.
The ionoshpere is a layer of the atmosphere between 30 and 370 miles above the surface of the earth. The ionopshere is made up mostly of O2 and N2. Solar energy, in the form of ultraviolet light (UV) and x rays, ionize these gases, allowing electrons to float freely. These "ionic" gases (so called because they are ionized) subsist in three major layers:
These layers have pronounced effects on the transmission and reception of signals in the bands below 30 MHz.
Ionization is affected by the sun. When there is more activity on the sun, there is usually more atmospheric ionization. If the "noise floor" is low enough, this increased activity improves communications on many bands. Unfortunately, if the sun is active (i.e. solar flares and storms) then the noise floor is increased, diminishing the effectiveness of the increased atmospheric ionization. In this case, even though the ionosphere is in better condition to propagate waves far, the noise floor does not allow the ham to hear those signals. Sort of a catch twenty two. More information on this matter can be found on A and K index help.
Quite simply, the ionosphere "bends" radio signals below 30 MHz to varying degrees. However, it is much easier to think of these bends as reflections. A signal goes up and is reflected back down to the surface of the earth. Basically, the reflections occur because the free electrons mentioned in the section above act on the signal in such a way that they are reflected.
Because the ionosphere is made up of gases, it is free to move about. As a result, conditions are always changing. This constant movement causes a correlated change in the bending characteristics of the ionosphere. For example, a station in New York may be talking to Florida one minute and then 5 minutes later find a station from Ohio more readable. It is not because the station in Ohio is more readable, it is because the propagation "gods" see it fit to change the different concentrations of ionized gases. This is known, less technically, as "propagation shifting." The higher in frequency (below 30 MHz) the more one finds this "shift" occurs.
The following are the different regions of the ionosphere. As you can see, the F region is made up of two regions that combine during certain conditions and separate during others. Each region is discussed below, although the F region is given only one section because it is commonly refered to as a single entity that splits.
The F region is the very thickest region of the ionosphere, which makes the F region special. It is the only layer of the ionosphere that is subdivided into two parts, the F1 layer and the F2 layer. During the day the F region ionizes at different rates, due to the thickness. As a result two characteristic changes occur: 1) during the day, the region splits up and 2) at night the two layers slowly recombine. The F1 layer is relatively unimportant to hams. On the other hand, the F2 layer is very important to hams.
During the daylight hours, the F2 layer forms. The F2 layer is on top of the F1 layer (making it situated closer to the sun). As a result of being closer to the sun, it comes into contact with more of the UV and x ray energy. Because of this, the F2 layer becomes more ionized than the F1 layer. When night settles in, the F1 layer quickly loses its energy while the F2 layer loses its energy much more slowly--usually at the lowest point right before sun-up. It is the F2 layer of the ionosphere that provides the capacity of the ionosphere to reflect radio energy (this is why the F2 layer is more important to hams).
Some of the properties of the F2 layer include reflecting radio signals up to distances of 2500 miles (in a single bound) and extend to globe-circling "long-path" communications (all the way around the world). F region ionization is always greatest when the sun is directly overhead (as are all ionospheric layers). F2 ionization is also directly related to UV radiation: the greater the UV, the greater the ionization (note that this means the greater the UV above the cloud cover, and is irrelevant when considered at a "ground level"). Thus, in the sumer F region propagation is greatest and in the winter it is least.
There are many more technical aspects of F region propagation to go into, but as of this edition, I will only go as far as I have, in an attempt to not scare off any more people. For more in depth information, see another of my pages on the F Region.
The E layer is not very important, for most practical purposes. It can be very effective when it occurs though. On the VHF bands E layer propagation, better known as E-skip, provides exceptional communication range and clarity. For instance, a station in St. Louis, Missouri may be speaking on a local simplex frequency when all of a sudden a station from Chicago, Illinois comes in over the local station. Sometimes during the summer months here in St. Louis, a local repeater experiences interference from a Kansas City repeater--some 250 to 300 miles away! E-skip is very exciting and occurs on frequencies ranging from around 10 meters (29 MHz) and up.
E layer ionization is not as long-lasting, nor as energetic as F region ionization (except in the case of E-skip). Somehow, the E layer is "super ionized" which provides this fantastic propagation, when mixed with the correct weather. QST printed articles concerning the E layer ionization. I would highly recommend reading them if your interest has not been quelched.
The D layer is not very interesting. During the daylight hours it serves to absorb most energy below about 7 MHz. During the night hours, it totally disappears, making 80 meter (3.5 MHz) communications once again usable. It quickly reaches full ionization soon after sun-up, and almost immediately loses its energy after the sun goes down. It does nothing in the way of reflecting signals--as far as science knows.
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Last Modified: 4/10/98