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 , 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 nighttime paths. Mobile operation is possible, although the relatively short length of practical mobile antennas – usually less than – 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 The 80-meter band was made available to amateurs in the United States by the 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. 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