Propagation

Wikipedia has a good article concerning Radio waves propagation, see Wikipedia:Radio_propagation.

Propagation Theory
Radio frequencies form a fragment of the entire electromagnetic spectrum. It is generally considered that radio frequencies are found between 10 kHz and 300 GHz. 10 kHz is considered to be the lower end of the electromagnetic spectrum. Above 300 GHz infrared begins.

Electromagnetic energy travels as waves that move at the speed of light (300 000km/s), normally in a straight line. Atmospheric conditions can affect radio transmissions, and are responsible for transmissions between ham radio stations that are not a "line of sight" distance apart.

Electromagnetic waves have two components, electric and magnetic. These are at right angles to each other. Unlike sound waves, electromagnetic waves do not need a medium through which to pass. Hence radio transmissions to and from deep space are possible.

DX (long distance) communications are possible when radio waves are reflected off the E and F layers in the the ionosphere. The E and F layers contain high electron densities which coincide with high temperatures.



The D layer can be found between 50km to 90km above earth. Propagation by reflection from this layer is quite rare as it is highly absorbent of RF energy. The thickness of the D layer decreases during the night, allowing more RF to travel through to the E and F layers, hence propagation often improves during the night.

D-Region Absorption Prediction Center

The E layer can be found between 90km and 120km above earth. Reflection of RF waves by this layer is responsible for most of the long distance propagation valued by hams. The E layer both thins out at night, and rises, again resulting in increased propagation distances. E layer propagation is most useful for frequencies below 10MHz.

Sporadic E-Skip is caused by "clouds" of especially intense ionisation. These are highly reflective and allow DX communication from 25MHz to 225MHZ. The cluods are usually relatively short lived, and occur mostly during summer months.

Th F-layer (between 120km and 400km above earth) is contains the highest densities of ionised particles, is the most reflective of RF energy, and hence is responsible for most DX propagation of HF communications. During the day, energy from the sun splits the F layer into two distinct parts known as F1 and F2.

Propagation Prediction
Australian Space Weather Agency (IPS)

Radiowave Propagation Center