If you are new to ham radio, you probably glazed over the propagation section of your license manual. I know I did. You learn what you need to. Who would learn more? Well, now is the time to figure it out! You can get on the air and put it to the test. This is a basic introduction to propagation, and hopefully you will learn a little bit more about how an electromagnetic wave moves by the time you're finished. If you have turned the radio on recently, you probably have heard someone cursing about the "sunspot cycle". There is nothing more fundamental to our hobby than sunspots, and nothing quite so annoying. I could throw some large words around, but I won't. Here is why sunspots matter so much...
The atmosphere is the region around the earth. It is comprised of various chemical elements, all designed simply to block harmful radiation from the sun. The layers that concern us the most combine to form the ionosphere. This zone is lazy; it only does the work it wants to do.
The sun occasionally releases energy in the form of sunspots. This releases heat and radiation on such a massive scale that the radiation reaches the earth. This triggers the ionosphere to "ionize", where the ions start moving around and doing what they're supposed to: stop that radiation. This happens all the time. It is happening right now. You can't see it, hear it, or feel it, but it is happening (thank God).
When ionization occurs, nothing gets in, but likewise, nothing gets out - including radio signals. Now that the ionosphere is trapping everything in, your signal will bounce in the form of skywave propagation, travelling further around the earth. When you transmit during periods of low ionization, your signal cannot bounce and flies out into space, similar to VHF propagation.
This means that during times when there are very few sunspots, high frequency terrestrial activity is limited. So, if you weren't cheering for radiation to come hurtling towards earth before, you should be now. Well, at least in limited quantities.
The good news is that the sunspot cycle is very much predictable. Not only can we measure them, we can actually take pictures of them. In August 2006, there was actually a phenomenon known as a "backwards sunspot", where the natural magnetic polarity was reversed. The technology is advanced, and you can see it for yourself. Knowing what is happening in the ionosphere can make the DX that much easier to get.
How does the ionosphere reflect the signals, though?
That gets a little more complicated. The layer we most deal with is known as the F-Layer. This layer is the topmost layer in the ionosphere, where your radio wave is being bombarded with ions, to the point where they are refracted back towards earth.
The refractive index of the ionosphere is directly related to the density of free-moving electrons, which means that sometimes the radio waves don't even get to the F-Layer. In the F-Layer, there is one trillion electrons per cubin meter (10^12cm^3). Should the ionization be great in a lesser layer, the signal will be reflected there (i.e. in the E- or D-Layer). More information is available on refraction and multi-hop paths here.
Another factor determining just how a signal will be refracted is the critical frequency. This is somewhat of a theoretical frequency, since it involves perfect vertical incidence. It does help to predict behavior, however. The critical frequency is the highest frequency that returns echoes at vertical incidence. A signal above the critical frequency may still be propagated, however, given that it enters the ionosphere at an oblong angle. The point beyond which no signal may be propagated is knows as the maximum usable frequency.
Realize, though, that the ionosphere is far from perfect. Radio energy does dissipate. In fact, radio energy will dissipate at the square of the distance from its own source. Seems very abbrasive? It is. The dissipation is known as "free space attenuation".
Geomagnetic conditions and disturbances can effect free space attenuation greatly. To keep aprised of the current conditions, we refer to the "K Index". Very quiet conditions are represented as 0 or 1. Geomagnetic storms can begin at 4. K indices that indicate average conditions are indicated as "Kp". We also use the "A index". Basically, tha A index shows what happened yesterday.
Used in conjunction with current conditions (such as the A- and K-index), an understanding of basic propagation can amplify operating experience, success, and enjoyment.