80-meter 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 Region 2), generally 3.5–3.8 MHz in Europe, Africa, and western Asia (Region 1) and 3.5–3.9 MHz in south and east Asia and the eastern Pacific (Region 3), with some different national allocations in each region. 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 the day. 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.

Overview

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").

Natural and human-made noise

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.

Daytime and nighttime use

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 miles (320 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.

Cumbersome large antennas

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. Nonetheless, with occasionally favorable propagation conditions substantial distances can still be covered with modest height antennas.

Mobile operation with portable antennas is possible, although the relatively short length of practical mobile antennas compared to a quarter-wave antenna – usually less than 10 feet (3.0 meters) vs. around 65 feet (20 meters) 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 the coil's wire, and that wire resistance is high enough to out-compete the antenna's effective radiation resistance for RF power. 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 put in, 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).

History

The 80 meter band was made available to amateurs in the United States by the Third National Radio Convention in 1924. The band was allocated on a worldwide basis by the International Radiotelegraph Convention in 1927.

Propagation

As 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 nighttime hours in midwinter, which can be as much as 300 miles / 500 km, rendering communication with closer 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 Telecommunication Union allocated the whole 500 kHz from 3.5–4.0 MHz to amateurs in the Americas, and 3.5–3.8 MHz or 3.5–3.9 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:

Country or Region	Allocation(s) (in MHz)	Refs
Argentina	3.500–3.750, 3.790–3.800
Australia	3.500–3.700, 3.776–3.800
Canada	3.500–4.000
Europe	3.500–3.800
India	3.500–3.700, 3.890–3.900
Japan	3.500–3.580, 3.599–3.612, 3.662–3.687, 3.702–3.716, 3.745–3.770, 3.791–3.805
Korea	3.500–3.550, 3.790–3.800
New Zealand	3.500–3.900
Russia	3.500–3.800
United States	3.500–4.000

Lower band edge

The 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 edge

For 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 truly 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.

Broadcast interference

The European 75 m broadcast band overlaps the North American 80 m ham band allocation. When it is night on both ends of the transmission path some broadcasters in Asia and Europe can be heard in North America between 3.9–4.0 MHz. On an SSB receiver this produces a tone in the received audio when the station is broadcasting with conventional amplitude modulation or white noise if the station is using Digital Radio Mondiale modulation. Setting the receiver to the exact frequency of the AM carrier can eliminate the tone but an audio program may still be heard. If the DRM signal is strong enough the noise may mask weak amateur signals. Most DRM signals occupy 9 or 10 kHz of bandwidth.

See also

• Shortwave bands

References

"Ham Radio QRP". ac6v.com. Archived from the original on 24 September 2005. Retrieved 3 August 2005.