This is an area that fascinates me. Most Ham radio operators think of Ionospheric Skip and Skywave propagation as phenomena that happen on the shortwave, or HF, bands. This happens because the Sun’s energy charges up particles (ions) in the atmosphere. The specific region is the F layer (and more specifically it is 2 layers F1 and F2 during the day). The area that is charged up is called the Ionosphere. Depending on the intensity of the charge, the frequency that is able to be reflected back to Earth changes. The more the charge the higher the frequency that can be bent back.
So, why am I talking about Ionospheric Skip while I am trying to talk about VHF propagation? Don’t all VHF signals break through the atmosphere? Even some HF frequencies break through the atmosphere, so why would there be any reason for VHF, even higher frequencies, to be bent back to Earth?
VHF signals can in fact be bent back to Earth! It doesn’t happen nearly as often, or can be predicted nearly as easily, as is done on the HF bands, however.
Some types of propagation that allow VHF signals to come back to Earth are: E-skip, Field Alligned Irregularities (FAI), Aurora, Meteor Scatter, Earth-Moon-Earth (EME), and Transequatorial.
Ill start off with the simple one – E-skip. This is by far the easiest phenomena to take advantage of. This happens most often because of temperature inversions. A temperature inversion is where warm air is above cold air. Its that simple. Typically, as you get higher in altitude the temperature falls. We all know that heat rises, but what creates heat on Earth? The number 1 source is the Sun. The radiation from the sun doesn’t heat up the air at all becaus there is nothing for the energy to run in to that will become energized. The Sun’s energy must reach the Earth before creating heat. The energy hits the ground and energizes it, creating heat. As this heat rises the Sun’s energy, passing right through, still can not heat it up. There is nothing in the air for the energy to hit. Therefore, as heat rises it cools down. The heat looses its energy because there is no energy to keep it going. What creates the temperature inversion, most often, is when a cold front (colder air is more dense and has more power over warm air) is pushing across the land. The higher pressure and higher density of the cold air moving acts like a shovel and scoops up the warmer air near the surface. The warm air gets pushed up above the colder air for it to loose its energy and cool off.
So, what does warm air have to do with radio waves? Warm air carries more water vapor.