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I will try to explain the different ways a radio signal will go from one antenna to another, with this page.

The 3 Main Types of Propagation

Ionospheric  Propagation
     In the HF Band (3-30 Mhz)  Radio Waves  bounce off the Ionosphere, and the earth like giant mirrors.  The Ionosphere is in upper Atmosphere, and is made of Ionized Gass.  The Ionoshere has several layers like an onion.  Some of these layers obsorb RF energy, and some reflect.  The higher the frequency the less obsorbtion.  The portions that obsorb dissapear at night.  As a ball park figure, 1 - 12 Mhz does better at night, and 12 - 30 does better during the day.  As with any rule, there are exceptions.   The strength of Solar radiation, and the stability of the Earths  Magnetisphere also has a play with the condition of Ionosphereic Propagation.  On the Earth there is the "Twilight Zone" or "Terminator."  This is the Gray-Line between night, and Day.  Here there is a lot of reflection, and very little obsorbtion.  Here Propagation is very efficient.
 
Ground-Wave Propagation
     Ground-Wave Propagation is perhaps simpler to explain.  With a vertically polarized antenna, a portion of the RF energy moves along the ground, moving with hills, ect.  The lower the frequency, the farther it will travel, and the higher the frequency the shorter it will travel.  1.8 Mhz will travel about 70 - 100 miles, while 29 Mhz, will only go a few milles.
 
Line-Of-Site Propagation
     Line-Of-Site Propagation is antenna to antenna.  RF will only go from one antenna to another if it can "See" it.  The higher you place a antenna, the farther it will communicate.  As a rule of thumb, each time you double the hight you place a antenna, you will get out 25% farther.  This will still provide for some exotic uses.  A satellite in outerspace, will act as a repeater with a very high-up antenna.  Also some things will act like a giant mirror, like the moon (large amounts of power is needed to make-up for the energy loss).  I hope this helps explain the different types of Propagation.

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Ionoshpereic Propagation by AE4RV Flash Media Presentation