High Frequency or HF Propagation

Ground Wave Propagation

Ground wave propagation is that which does not depend upon the ionosphere. It can be thought of as "line-of-sight" propagation directly from transmitter to receiver.

The Ionosphere

The ionoshere is a layer of earth's atmosphere extending between 70 and 400 km up. It is so named because gas molecules in this layer of the atmosphere become "ionised" due to radiation from the sun. Ionisation is the process whereby electrons are stripped from gaseous atoms and molecules forming a conducting or partially conducting ionised gas. The ionosphere can be thought of as being composed of a number of "layers" at different heights. These are the D, E, F1 and F2 layers. For hf (the 1 to 30 MHz range for amateur radio) the ionosphere refracts radio waves back to earth at a considerable distance from the transmitter. The relative amounts of ionisation, the thickness of the layers and the height of the layers all tend to vary through the day and lead to differing propagation conditions during the day.

Sunspots

On the surface of the sun are a number of "sunspots". These are visibly darker areas of the sun's surface that are associated with increased ejection of matter and radiation. They increase in number on a 11-year long cycle and have been linked to increases in the ionisation of the upper layers of the atmosphere including the ionosphere. Greater ionisation is generally better for HF radio wave propagation.

Sky Wave Propagation

Propagation that relies on refraction of radio waves by the ionosphere as described above is frequently referred to as "Sky Wave" propagation.

Skip Zone (or Dead Zone) and Skip Distance

Stations located near to a transmitter may be able to hear it by ground wave propagation, and those very far away may be able to hear it by sky wave propagation. In between is the "Skip Zone" or "Dead Zone". The distance covered away from the transmitter by this skip zone is referred to as the "Skip Distance".

Maximum Usable Frequency (MUF)

As greater ionospheric ionisation gives better refraction of higher frequency signals, the amateur radio enthusiast will tend to find that higher frequency radio bands "open" during the day but are "closed" during the night. There is a maximum frequency known as the "Maximum Usable Frequency" that changes throughout the day.

Multiple Hop Propagation

Radio waves are not limited to being refracted by the ionosphere just once. In fact there can be "multiple hops" as the radio waves bounce off the ionosphere and ground in turn. This leads to the ability for world-wide HF communications.

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