Specifications

HF Propagation
When HF/SSB radio waves are generated by a transceiver there are usually three
components:
Ground-wave
Generally speaking, ground-waves are used to communicate over shorter distances,
usually less than 50km. Because ground-wave follows the contours of the earth, it is
affected by the type of terrain it passes over. Ground wave is rapidly reduced in level
when it passes over heavily forested areas or mountainous terrain.
Direct wave
Line-of-sight is the direct propagation of radio waves between antennas visible to
each other. This is probably the most common form of radio propagation at VHF and
higher frequencies. Because radio signals can travel through various non-metallic
objects, this allows radio transmissions to be received through walls. This is still line-
of-sight propagation. Examples would include propagation between a satellite and a
ground antenna or reception of television signals from a local TV transmitter.
Sky-wave
Travels upward and at an angle from the antenna, until it reaches the ionosphere (an
ionised layer of gas, high above the earth’s surface), and is reflected back down to
earth, to the receiving antenna.
Sky-wave is used to communicate reliably over medium to long distances. Whilst the
nature of sky-wave propagation means the type of terrain as in ground-waves does
not affect it, it is affected by factors involving the ionosphere as described below.
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Radio Operators Training Manual Version 27/11/2007 28