80-meter band

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. 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.

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 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.

In practice, the large physical size of antennas at this frequency (a quarter-wave vertical, for example, is approximately 65 feet/20 meters high) and the difficulty of ensuring significant low angle radiation from antennas to maximize the distance of "hops" are two of the challenges facing amateurs wishing to communicate over longer distances. Most amateurs interested in regional communication use low, wire antennas, such as horizontal dipoles, inverted vee dipole antennas or loop antennas on this band. Horizontally polarized antennas, when close to earth, produce predominantly high-angle radiation. High angles are useful for Near Vertical Incidence Skywave and other short distance propagation modes, but substantial distance can still be covered even 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.

...
Wikipedia