From the very beginning of Aeradio in the 1930s until recent times, High Frequency
(HF) radio has played a very important part in the airways system.
The HF band covers the frequency range 2 to 30 MHz (known in earlier times as Mc/s - Megacycles per second). HF offers the possibility of communications over long distances, outside of the line-of-sight limitations of higher frequencies. It achieves this capability by the ability to reflect signals off the ionosphere, a layer of charged particles some 200 Km above the Earth.
Although subject to atmospheric interference and the vagaries of the ionosphere, HF radio communications are still used extensively for aviation purposes in the more remote parts of Australia and in oceanic areas because of its long-range capability.
In the early days all communications with aircraft were by Continuous Wave (CW) using Morse code. Speech was introduced in the late 1930s, but Morse continued to be used into the 1950s.
Another important use for HF radio was for point-to-point communications, the Aeradio/Communications networks relaying aircraft movement messages between ground stations both within Australia and, later, to overseas stations. This network was also used extensively by the airline companies to relay traffic messages between their stations.
At first point-to-point communications were conducted using Morse, but from the post war years these circuits began to be automated using radio-teletype equipment. In some locations HF voice intercom circuits were established between neighbouring air traffic services units, and these existed into the early 1980s.
Today, satellite communications have replaced most of the functions of HF radio, apart from air-ground communications in remote and oceanic areas.
(Photo: CAHS collection)
