| 词条 | 80-meter band |
| 释义 |
}} The 80 meter or 3.5 MHz band is a band of radio frequencies allocated for amateur radio use, from 3.5 to 4.0 MHz in IARU Region 2, and generally 3.5 to 3.8 or 3.9 MHz in Regions 1 and 3 respectively. The upper portion of the band, which is usually used for phone (voice), is sometimes referred to as 75 meters. In Europe 75m is a shortwave broadcast band, with a number of national radio services operating between 3.9 & 4.0 MHz. Because of high D layer absorption, 80 meters is less reliable for over intercontinental distances, but is the most popular band for regional communications networks through the late afternoon and night time hours{{according to whom|date=February 2016}}. It 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, depending on atmospheric and ionospheric conditions. SummaryThe 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 80 meter band begins at 3.5 MHz and goes up to 4.0 MHz. The upper part of the band, mostly used for voice, is often referred to as 75 meters, since the wavelength is between 80 and 75 meters. Antennas at this frequency are large: A quarter-wave vertical, for example, is approximately 65 feet (20 meters) high. Erecting the 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. None the less, with occasionally favorable propagation conditions substantial distances can still be covered with modest height antennas. 80 meters can be plagued with noise, with the rural noise floor set by propagated noise from distant thunderstorms and man-made noise sources. Urban and suburban noise floor is often established by local noise, and is generally 10–20 dB stronger than the typical rural noise floor. On 80 meters, nearly all areas of the world are subject to local weather induced noises from thunderstorms. The ionosphere's D-layer also affects 80 meters significantly 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 miles (300 km), extending to a few thousand miles or more at night. Global coverage can be routinely achieved during the late fall and winter by a station using modest power and common antennas. The higher background noise on 80 meters, especially when combined with higher ionospheric absorption, causes stations with higher effective radiated power to have a decided advantage in long distance communications. With very high antennas or large vertically polarized arrays and full legal power, reliable worldwide communications occurs over darkness paths. Mobile operation is possible, although the relatively short length of practical mobile antennas – usually less than 10 feet (3 meters) – compared to a quarter-wave antenna results in the need for significant inductive loading to achieve resonance. Since short antennas have very low radiation resistance, their efficiency is typically below 10%. Additionally, the large inductance of the loading coil creates an antenna system with an extremely narrow bandwidth (high Q). History{{Expand section|date=January 2012}}The 80 meter band was made available to amateurs in the United States by the [https://earlyradiohistory.us/1924conf.htm Third National Radio Conference] on October 10, 1924. The band was allocated on a worldwide basis by the International Radiotelegraph Conference in Washington, D.C., on October 4, 1927. PropagationAs the maximum usable frequency for long distance communication seldom dips below 3.5 MHz anywhere on the planet, the main propagation barrier to long distance communication is heavy D-layer absorption during the daytime, ensuring that DX paths must be largely, although not entirely, in darkness. At times, there is pronounced dark-side gray-line propagation, which is most useful on polar routes, away from equatorial thunderstorm activity. At higher latitudes, a noticeable skip zone sometimes appears on the band during darkness hours in midwinter, which can be as much as 300 miles/500 km, rendering communication with some nearby stations impossible. This is not generally a problem at middle or equatorial latitudes, or for large parts of the year anywhere, but it does occasionally limit local wintertime traffic on the band in areas such as Northern Europe, the northern tier of the United States and Canada. During spring and summer (year-round in the tropics), lightning from distant storms creates significantly higher background noise levels, often becoming an insurmountable obstacle to maintaining normal communications. Nearby convective weather activity during the summer months can make the band completely unusable, even for local communications. In the winter months during the peak years of the sunspot cycle, auroral effects can also render the band useless for hours at a time. Frequency allocation{{See also|Amateur radio frequency allocations}}The International Telecommunications Union allocated the whole 500 kHz from 3.5 to 4 MHz to amateurs in the Americas, and 3.5 to 3.8 or 3.9 MHz to amateurs in other parts of the world. However, amateurs outside the Americas must share this useful piece of spectrum with other users, usually on a joint primary basis. As a result, authorities in the affected parts of the world restrict amateur allocations between 3.7 MHz and the top of the band. Some allocations are as follows (in MHz):
Lower band edgeThe lower edge of 80 meters is predominated by CW emissions, with the lower 10 kHz (3.5–3.51 MHz) primarily used for long distance communications. It is common for illegal marine operations, generally using USB voice, to occupy frequencies on the low end of 80 meters. Most operations of this type are from fishing vessels. Most come from SE Asia and South American ports, although some illegal use occurs with vessels from USA and Canadian ports. Upper band edgeFor Canadian and U.S Amateurs with perfect transmitters, the highest usable frequency in the 80 m band for lower side band phone is 3.999 MHz. Depending on quality and condition of radio, audio characteristics, and proper adjustments the bulk of emissions on lower sideband will typically occupy 3.9970–3.9997 MHz. All SSB transceivers have third- and fifth-order products of significant level, typically only 30–35 dB below PEP for third order intermodulation. This means any operation above 3.998 MHz LSB comes with some risk of illegal emissions, even with good equipment. It is a common misconception that using this carrier frequency is not legal as emissions extend beyond the 4 MHz band edge. High quality communications receivers or selective level meters generally have better dynamic range than all but the best spectrum analyzers. Properly used, they do an excellent job of indicating out-of-band emissions. While some people reporting out-of-band operation might use a wide receiver bandwidth, receiver bandwidth adds to transmitter bandwidth, so the perception is bandwidth is wider than it truely is. Any measurement of out-of-band emissions should be made using a receiver bandwidth significantly narrower than the transmitter bandwidth. Inexpensive spectrum analyzers, spectrum scopes, or panadaptors are generally not useful for off-air measurements of bandwidth. Wide detection bandwidth, slow sweep rates, and commonly high local ambient noise levels generally mask weaker emissions. Using other phone modes such as upper side band, amplitude or frequency modulation with 3.999 MHz as the carrier frequency would modulate across the band edge and are not considered legal. Certain data modes and CW are usable as long as the emission bandwidth does not extend across the band edge. Broadcaster interferenceThe European 75 m broadcast band overlaps the North American 80 m ham band allocation. Deutsche Welle legally operates on 3.995 MHz using Digital Radio Mondiale modulation from Sines, Portugal, with an effective radiated power of 90 kW. Their pattern beams mostly on Central Europe. The mode of operation (DRM-17) occupies approximately 9 kHz of bandwidth. See also
References
1 : Amateur radio bands |
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