Ionosphere Layers
The study of Sky Wave Propagation is an understanding of how RF is reflected by the various layers in the ionosphere. The Earth's ionosphere is part of the atmosphere around the planet. The ionosphere includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is a shell of electrons and electrically charged atoms and molecules around our planet that is ionized by solar radiation and influences radio propagation to distant places.
Ionosphere Layers
Scientists have divided the ionosphere into layers, with each layer having specific behavioral characteristics dependent on solar radiation.
There are three main regions within the ionosphere that impact RF communication
- D Region - Causes attenuation of RF signals, low frequencies are attenuated more than higher ones.
- E Region - HF radio signals are reflected here back towards the Earth and sometimes higher frequencies
- F Region - Most forms of skywave propagation use the normal and cyclic ionization properties of this layer
Technically, the signals are not "reflected" as in a bounce, but they are bent around so that they return to the earth. So it is a "bend" rather than a "bounce." However, with that being said, scientists believe that the E layer is thin and dense and therefore may actually reflect rather than bend. The other other layers bend the wave.
Medium wave signals use ground propagation only during the day. The D region will absorb any signals it meets. At night the D region becomes invisible and the waves pass though to the E region. The E region reflects the signal back to the ground. Higher frequencies are not reflected by the E region. As frequency goes up, reflection goes down. E region reflection tends to make for shorter hops, meaning shorter distance DX.
When frequency is high enough it penetrates the E region it then may be reflected back by the F1 region, the first part of the F region of the ionosphere. Then with higher frequency F1 is penetrated and F2 is reflective. With each increase the next layer provides greater distance DX. The maximum skip distance for the E region is around 1550 miles and 3100 miles for the F2 region.
Signals can hop more than once. They can be bounced back to the ground, then reflected back up to the ionosphere once again for another hop, sometimes several more times. This is how DX is achieved from the other side of the globe. Water (the ocean) reflects the signal much better than land. The surface of the Earth and the D layer will weaken signal strength during each hop.
Several factors can contribute to signal fading, however, the lower the frequency the less fading will occur.
Sporadic E
Sporadic E or Es can allow VHF signals (30-300 MHz) to propagate over long distances via a reflection from the E layer. Sporadic E is generally classified into three types by, Mid-latitude, Equatorial and Auroral. Sporadic E propagation bounces signals off smaller "clouds" of unusually ionized atmospheric gas in the lower E region. Sporadic E activity peaks predictably in the summertime in both hemispheres. In North America, the peak is most noticeable in mid-to-late June, trailing off through July and into August.
For a signal to propagate via Es a cloud of sufficient ionization must exist at the mid-path point. Just like the F layer, a sporadic E cloud will support a maximum frequency, above this frequency signals will not be reflected to earth. Because of the limited size of the cloud the areas where propagation occurs are also limited.
