160-meter band

The band was allocated on a worldwide basis by the International Radiotelegraph Conference in Washington, D.C., on 4 October 1927.[2] The allocation at that time was 1.715–2 MHz.

The International Radio Conference of Atlantic City reduced the allocation to 1.8–2 MHz under the provision that amateurs must not interfere with LORAN operation.[3]

As the high frequency (HF) bands were developed in mid-1920s – along with their smaller, more feasible antennas – 160 meters fell into a period of relative disuse. Although there has always been activity on the band, fewer and fewer hams are willing (or able, due to lack of sufficient real estate) to put up the antennas necessary to take advantage of the band's unique properties. For most amateurs, the HF bands are much easier to use, and HF antennas need a lot less real estate.

During World War II all amateur radio licenses were suspended, prohibiting amateur transmission on any radio band.

After World War II, it first seemed that the 160-meter band was not coming back, since a large part of the U.S. 160-meter band was allocated on a primary basis to the LORAN radio-navigation system that began operating in and around the 160-meter band in 1942. Amateurs were relegated to secondary, non-interfering status, with severe regional power limitations and restricted day/night operations on just a few narrow segments of the band.

Many older hams recall, with no great fondness, the ear-shattering buzz-saw racket of high power LORAN stations that began in 1942 until LORAN-A was phased out in North America on 31 December 1980, and most of the world by 1985. LORAN-A was still operating in China and Japan in 1995. Great ingenuity was used to eliminate the pulse noise of the powerful LORAN-A transmitters through such famous circuitry as the "Select-O-Ject" of the late 1950s. The technology was adapted to modern noise blanking circuits used in current amateur receivers and transceivers.

The band experienced a resurgence with the demise of LORAN-A in the United States in December 1980, and the removal of power restrictions below 1.9 MHz soon thereafter. Power restrictions above 1.9 MHz were removed in March 1984, and 160 meters was then no longer regarded as the "abandoned" band, as it had been for more than half a century. Also, with the 21st century allocation of two more bands at lower frequencies, 160 meters is no longer the only mediumwave band, and no longer the lowest-frequency amateur band.

Effective operation on 160 meters can be more challenging than most other amateur bands, because of the overwhelmingly large sizes required for efficient antennas. Full-sized antennas (on the order of a quarter-wavelength or more) are over 130 feet for monopoles, which is also the recommended height for mounting a horizontal dipole antenna, and half-square loops reach nearly 70 feet high. That much real estate may not be feasible for many amateurs, and even with space available, erecting and securing such a large structure is a challenge.

The size of the safety zone around an antenna depends on several factors, including the power fed to the antenna, but is roughly 30–40 m (100–130 ft) from the center of the lowest radiating part of the antenna. If high power is used to compensate for an under-sized antenna, even a small antenna will require a full-sized safety zone around it, free of people and animals. Nevertheless, many radio amateurs successfully communicate over very long distances with relatively small antennas.

During the day propagation is limited to local contacts, but long distance contacts are possible at night, especially around sunrise and sunset and during periods of sunspot minima.

Much about ionospheric and propagation on 160 meters is still not completely understood. Phenomena such as "chordal hop" propagation are frequently observed, as well as other unexplained long-distance propagation mechanisms. Inexplicable radio blackouts occur on 160 meters – sometimes also on the AM broadcast band. Many of these phenomena have been investigated in the scientific community also.