Радиочастотный диапазон 80 метров: характеристики и особенности использования.

Radio frequency band
The 80 meter or 3.5 MHz band is a span of radio frequencies allocated for amateur use, from 3.5–4.0 MHz in North and South America (IARU and ITU Region 2); generally 3.5–3.8 MHz in Europe, Africa, and northern Asia (Region 1); and 3.5–3.9 MHz in south and east Asia and the eastern Pacific (Region 3). The upper portion of the band, which is usually used for phone (voice), is sometimes referred to as 75 meters; however, in Europe, "75 m" is used to name an overlapping shortwave broadcast band between 3.9–4.0 MHz used by a number of national radio services. Because high absorption in the ionosphere's Sun activated D layer persists until nightfall, 80 meters is usually only good for local communications during the day, and hardly ever good for communications over intercontinental distances during daylight hours. But it is the most popular band for regional communications networks from the late afternoon through the night time hours. At night, 80 m is usually reliable for short to medium distance contacts, with average distances ranging from local contacts within 200 miles / 300 km out to a distance of 1,000 miles / 1,600 km or more at night – even worldwide – depending on atmospheric and ionospheric conditions.

The nominal "80 meter" band begins at 3.5 MHz (85.7 m wavelength) and goes up to 4.0 MHz (74.9 m wavelength). The upper part of the band, mostly used for voice, is often referred to as 75 meters, since in Region 2, the wavelengths in that section are between 80–75 meters (adjacent to or overlapping a shortwave broadcast band called by the same name: "75 m").

80 meters can be plagued with noise: The same ionospheric refraction that makes long distance shortwave propagation possible also traps terrestrial noise under the ionosphere, preventing it from dissipating into space, which quiets down radio bands at higher frequencies, above ~20 MHz. The 80 m rural noise floor is mostly determined by noise produced by distant tropical thunderstorms and cumulative regional sources of human made static. The urban and suburban 80 m noise floor is typically set by the amount of noise generated locally, from electrical machinery and household electronics, and is generally 10–20 dB stronger than typical rural noise. On 80 meters, nearly all areas of the world are subject to weather induced noise from regionally local thunderstorms, and combined distant lightning strikes from tropical thunderstorms that perpetually supply world wide a continuous source of radio static.

The 80 meter band is favoured for ragchews between amateurs within a range of 500 miles / 800 km. During contests the band is filled with activity beginning before sunset and continuing all through the night. The ionosphere's D layer significantly affects the 80 meter band by absorbing signals. During the daylight hours, a station in middle or high latitudes using 100 watts and a simple dipole antenna can expect a maximum communication range of 200 mile (322 km), extending to a few thousand miles or more at night. Global coverage can be routinely achieved at high latitudes during the late fall and winter, by stations using modest power and common antennas. The higher background noise on 80 meters, especially when combined with higher ionospheric absorption, causes transmitting stations with higher effective radiated power to have a decided advantage in being heard by long distance receiving stations. With very high transmitting antennas or large vertically polarized arrays and full legal power, reliable worldwide communications occurs over nighttime paths. Good receiving antennas have far more modest requirements to reliably get signals from worldwide sources.

Antennas for 3.5 MHz are large: For example, a quarter wave vertical sized to resonate at 3.6 MHz is approximately 65 feet (20 m) high; for reasonable antenna efficiency, even a reduced size antenna needs to be a large fraction of that height, which is still a formidable construction project for an amateur. Erecting such large antennas and ensuring the antennas radiate significant power at low angles are two of the challenges facing amateurs wishing to communicate over long distances. Amateurs interested in regional communication can use low wire antennas, such as horizontal dipoles, inverted vee dipole antennas or loop antennas on this band. Horizontally polarized antennas closer than a quarter wave to earth produce predominantly high angle radiation, which is useful for short distance propagation modes, such as near vertical incidence skywave. Nonetheless, occasional favorable propagation conditions make substantial distances still possible with modest height antennas. Mobile radio operation with portable antennas is still possible, but the relatively short length of practical mobile antennas compared to a quarter wave antenna – usually under 10 feet (3 m) vs. around 65 feet (20 m) tall – results in the need to compensate with a large inductive loading coil to bring the antenna to resonance. However a large coil looses power through resistive heating of its wire, and that wire resistance is always high enough to compete for RF power against the antenna's meager effective radiation resistance: Since short antennas have very low radiation resistance, the lion's share of their fed power is lost to heat, and their efficiency is typically below 10%, with roughly 90% of the power input lost to wire and ground resistance. Additionally, the large inductance of the loading coil creates an antenna system with an extremely narrow bandwidth (very high Q), which can be good for reducing received noise, but makes changing frequency a challenge, since one must retune the loading coil's inductance.

The 80 meter band was made available to amateurs in the United States by the Third National Radio Convention in 1924.