Shortwave Radio - Propagation Characteristics

Propagation Characteristics

Shortwave radio frequency energy is capable of reaching any location on the Earth as it can be refracted back to the earth by the ionosphere, (a phenomenon known as "skywave propagation"). A typical phenomenon of shortwave propagation is the occurrence of a skip zone (see first figure on that page) where reception fails. With a fixed working frequency, large changes in ionospheric conditions may create skip zones at night.

As a result of the multi-layer structure of the ionosphere, propagation often simultaneously occurs on different paths, scattered by the E or F region and with different numbers of hops, a phenomenon that may be disturbed for certain techniques. Particularly for lower frequencies of the shortwave band, absorption of radio frequency energy in the lowest ionospheric layer, the D layer, may impose a serious limit. This is due to collisions of electrons with neutral molecules, absorbing some of a radio frequency's energy and converting it to heat. Predictions of skywave propagation depend on:

• The distance from the transmitter to the target receiver.
• Time of day. During the day, frequencies higher than approximately 12 MHz can travel longer distances than lower ones. At night, this property is reversed.
• With lower frequencies the dependence on the time of the day is mainly due to the lowest ionospheric layer, the D Layer, forming only during the day when photons from the sun break up atoms into ions and free electrons.
• Season. During the winter months of the Northern or Southern hemispheres, the AM broadcast band tends to be more favorable because of longer hours of darkness.
• Solar flares produce a large increase in D region ionization so high, sometimes for periods of several minutes, all skywave propagation is nonexistent.