Let Your Ears Have Wings Pt II

High frequency is a great distance spanner. Whereas on VHF, aircraft cannot usually communicate with ground stations more than 150-200 nm away, without repeaters, aircraft using HF can communicate directly with ground stations thousands of miles away.

VHF frequencies remain unchanged day and night, but HF frequencies are changed constantly, as they vary in efficiency depending on the time of day, month, year, and sunspot activity (solar storms) on the sun’s surface.

HF frequencies are chosen by reference to prediction charts to fit in with requirements, hour-by-hour. HF radio waves are transmitted into space, where they strike the ionospheric layers, and then bounce back to Earth, sometimes many thousands of miles away, on the opposite side of the globe. This is the phenomenon making long-distance radio communication possible.

International-flying aircraft use primarily 2000, 5000, 8000, 13000 and 17000 Kilohertz (kHz) bands, and en-route frequencies are selected from sets of frequency “families”. They are allocated by Flight service officers, who man the international circuits, or by ATC on VHF control frequencies before the long-distance flight goes beyond the VHF range.

Frequencies are allocated in sets of two, a primary and a secondary, or “backup”. As a general rule, the 2000 Khz and 5000 Khz frequencies are best for usage when the stations are in darkness. The 8000, 13000, and 17000 Khz bands are usually daytime bands when the stations using them have the sun. The 2000 Khz band becomes inefficient after about 500 miles, while 5000 Khz frequencies can span 5000 miles with ease, at night. 8000 Khz channels, depending on time of day, can also span 5000 miles or more. The 13000 Khz band is a good quality daytime band, also able to be used to span 3000-5000 nm. 17000 Khz frequencies are used mainly in autumn or times of high-sunspot activity, when communications are being disrupted, due to storms on the sun’s surface.

Some ground stations use AM on shortwave (HF), but most use SSB, or single-sideband mode of transmission, which has about 9 times the power of AM in relation to the ability to span long distances without losing signal strength. The upper sideband, or USB mode is used at all times for aeronautical HF communications.

Around the world, most ground stations possess USB facilities, but some still have not converted over from the old days of strictly AM radio transmissions. Sydney Radio converted its equipment to USB on February 1, 1979, at its base at the Area Control Centre (ACC), Sydney Kingsford-Smith Airport . . . however, it continued to operate on AM and USB until February 18, 1982. USB only is now used.

For the Technically Minded. . .

Changeover of Am Equipment to SSB During 1982-83 Period

During the period that ground-to-air mobile stations were transmitting in the AM or Double Sideband mode (DSB), aircraft operators in possession of either AM or SSB equipment could communicate with ground stations. Now that AM radio is being phased out, ONLY aircraft equipped with SSB equipment for communications, and possessing the appropriate area frequencies will be given D. O. T. approval to fly international routes.

National airlines such as QANTAS as a general rule utilise SYNTHESISED radio equipment, capable of tuning approximately 30,000 frequencies at will, with easy-to-monitor LED Digital-frequency readout. (Light-emitting-diode). But some computer airlines, who regularly fly over water, along small sections of international routes, merely have 3 or 4 frequencies in the form of radio CRYSTALS, specifically cut by experts to the desired channel frequency. These airlines will in some cases be forced to quickly re-equip themselves to meet the new required standards and to allow them to communicate with ground-to-air stations.

Major Stations Covered in Airband HF Radio Directory

All stations of interest to Australian monitors have been included. However, please note that omissions include a large portion of the Middle East, Europe, Canada and the United States, whose stations (in most cases) use repeater-assisted VHF equipment. Latin American stations have also been purposely omitted as many of their stations use the Spanish language.

Aeronautical mobile transmissions are now made in MODE A3J, Single-sideband, that is Upper-sideband (USS), with carrier wave suppressed.