Almost all HF propagation that takes place over 2000 miles is the result of multiple bounces off of the ionosphere. As described earlier, the signals are for all practical purposes reflected off of the ionosphere. Multihop occurs when the signal is bounced between the ionosphere and the ground (or a body of water). Of course, the radio signal is weaker. In fact, every time the signal hits the ground, it loses energy. Also, the signal can be bounced inside of the F-layer. When this occurs, the signal never has to "touch" the ground. Of course, not nearly as much radio energy is lost when this occurs. Both methods can reflect signals around the world. There really is not too much more to the multihop method.
Long path propagation is, as the name implies, over a long path--i.e. around the world. Now, specifically, long path refers to the path of communications that is the longest that one could choose...normally one would choose the shortest path to get the job done because that also implies using the least amount of power. For example, instead of a ham in England pointing his beam west to a station in New York, the English ham would point his antenna to the east--180 degees opposite the direction he wishes to have a contact in. The station in New York would have to point his antenna to the west in order to hear the English station--also 18 degrees opposite the direction the station he wishes to contact. By doing this, if the conditions are working with the two hams, they should be able to have a contact, although the signals will be distinctly weak due to the large losses that accompany such long distances. Due to these long distances, long path communications are usually accomplished using two beam antennae, one on end of the contact (although it is not unheard of for one of the stations to have a simple wire antenna of some sort rather than a beam--this is meant to give the large majority of hams who read this some inspiration).
Grey line propagation is a little different. It is also ususally a little more common...simply because the opportunity to communicate using this method exists every day. How can this be? I have only one question for you to answer to find out if you can participate using this method: does the sun set where you live? If you answered yes to this quetion then you can communicate with this method (I thought you could).
Why does it matter that the sun set in your area? Grey-line propagation is the propagation that accompanies every sunset of every day. It is due to certain qualities being brought out by the absence of Mr. Sun. Things like less absorption. The grey line being refered to is the period of dusk during the late afternoon. During this period, increased communications to areas north and south exist due to the rapidly decreasing D layer. While the D layer is decreasing so quickly, the F layer is still stratiated--dual layered--which means that for a few precious moments, relatively unimpeded transmission can take place between stations north and south of each other (i.e. stations that are experiencing dusk at the same time).
An interesting phenomenon occurs: stations located rougly around the Tropics of Cancer and Capricorn will have much better luck with grey line, due to atmospheric conditions--the sun is closer to these two locations year 'round, therefore it should stand that they should get the most ionization, and thus the best results. That does not mean that northern areas won't realize this method...it just means that areas between the tropics will get better results.
Backscatter is an interesting phenomenon. Consider that a radio wave has a particular "skip" zone depending upon conditions. That is, if I transmitted a signal right now and if I knew the conditions affecting that signal I could very easily predict where the signal would "touch down" on the earth--where the ground signal received by another station would be greatest. If it has a 300 mile skip on a particular day, then from my station in St. Louis Missouri I could have a nice strong signal in Kansas City, Missouri. In between St. Louis and Kansas City though, the signal would not be very strong--note that I am effectively removing groundplane radiation for the sake of arguement. Now, if I were to increase the transmitter power to such a level that the ionospheric region that my signal was "bouncing" from became saturated, some of the signals woud "bounce" back into areas between Kansas City and St. Louis. That is, some of the signals would "scatter" themselves all around in a somewhat random pattern--I know, that is an oxymoron, but so what. Instead of the signals going following a predictable path, they spread out and go different ways. That is the principle of backscatter--apply alot of power and hope the signal gets to an area other than just the predicted "touchdown" point.
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Last Modified: 4/10/98