Saturday, June 25, 2005

An introduction to Longwave frequencies

"Longwave" refers to all frequencies below the lower end of the AM broadcasting band at 540 kHz. The lower limit of what frequencies constitute "radio" is not precisely defined, but 5 kHz is a widely accepted starting point for the radio spectrum.

For many years, radio hobbyists ignored longwave because most commonly available communications receivers only tuned down to 540 kHz. However, most new receivers today tune down to at least 150 kHz and longwave DXing is enjoy new popularity.

One big problem when tuning longwave is electrical noise from power lines, electrical devices, motors, etc. Longwave is far more susceptible to such noise than higher frequencies, and you might hear only a loud "buzz" when you tune across longwave from your location. Also, static crashes from thunderstorms can be severe, especially in summer. To combat noise, many longwave DXers use an indoor "loop" antenna that allows rejection of nearby electrical noise sources. Other longwave DXers use special phasing units to reduce noise levels.

Reception distance on longwave is similar to that on the AM broadcast band, as are reception patterns. Greater range is possible when the signal is reaching you over a water path, as is often the case in coastal regions. At night, reception of stations from hundreds or even thousands of miles away is possible. Night reception on longwave is better in winter than in summer, and the equinoxes often give the best propagation.

Unlike the shortwave frequencies above 1700 kHz, the longwave spectrum is allocated on a more "ad hoc" basis, with different users and services frequently sharing the same frequency range. Here is a general description of the world below 540 kHz:

Below 155 kHz: Signals below 155 kHz don't propagate very well via the ionosphere; the absorption is too great even at night during winter. These signals can travel for thousands of miles via ground wave, but high transmitter powers are required. Signals at very low frequencies, about 50 kHz and lower, can penetrate sea water very well. As a result, these frequencies are used by military forces of the major powers, especially for communication with submarines. The U.S. Navy's "Omega" navigation system is found on 10.2, 12, and 13.6 kHz. The Russian navy operates a similar system on 15.62 kHz. The U.S. Air Force has a FSK-based communications system on 29.5 and 37.2 kHz. This system was established to provide a backup in case nuclear explosions rendered the ionosphere useless for propagation. Miscellaneous FSK-based stations are found here for direct communications with submarines and naval forces.

150 to 175 kHz: In the United States, this range is used by the U.S. Air Force's ground wave emergency network (GWEN), a packet-based network to provide communications during a nuclear war. Transmitters are kept continuously operational here on a "standby" basis, and it's easy to hear their loud, "raspy" signal bursts.

155 to 281 kHz: This is another AM broadcasting band in Europe and parts of Asia. In Europe, there are numerous high powered (1,000,000 watts or more) stations here. These stations are capable of covering an entire European nation like France or Germany with reliable signals around the clock. Although ionospheric propagation is not good at these frequencies, the high powers used means that many of these broadcasters can be heard along the Atlantic seaboard during the fall and winter. Best reception is usually from local sunset to about 0600 UTC. A few longwave stations in Asiatic Russia can be heard on the Pacific Coast beginning an hour or so before local sunrise.

160 to 190 kHz: In the United States, this range is open to unlicensed experimental transmissions. Transmitter power is restricted to one watt, and the maximum antenna length (including feedline) can be no more than 50 feet. Any mode can be used. Some of these "lowfer" (as they are known) unlicensed stations have been heard several hundreds of miles away under favorable conditions.

200 to 430 kHz: This range is used mainly by navigation beacons, which continuously repeat their call signs in Morse code. Call signs do not follow the international allocations given elsewhere on this site. Instead, the call signs usually give an idea of the location of the beacon. For example, beacon "FT" on 365 kHz is located at Fort Worth, Texas.

430 to 500 kHz: This range is used for two-way Morse code communications between ships at sea and shore stations. Shore stations use three-letter callsigns, while ship station callsigns consist of four letters. All callsigns are from international allocations. The number of stations you can hear in this range is rapidly declining due to a shift in maritime communications to satellites and shortwave frequencies. After February, 1999, radio operators skilled in Morse code will no longer be required on ships sailing in international waters, and the number of stations you can hear in this range should drop precipitously after that date.

500 kHz: This is an international ship calling and distress frequency for maritime communications in Morse code. It is very lightly used, and after February, 1999, ship stations and shore stations will no longer be required to monitor this frequency for calls.

500 to 540 kHz: This segment is populated by miscellaneous beacons and stations. Perhaps the most interesting frequency here is 518 kHz, used for transmission of maritime safety and navigation information via FSK. This system is known as NAVTEX, and includes weather bulletins as well as notices of missing and overdue vessels. 530 kHz is used in the United States and Canada for low powered road and traffic information broadcasts.

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