List Of American Shortwave Broadcasters – Shortwave radio is radio broadcasting that uses shortwave (SW) radio frequencies. There is no official definition of the band, but the band always includes the high frequency (HF) band, which extends from 3 to 30 MHz (100 to 10 meters); Above the medium frequency (MF) band, below the VHF band.
Radio waves in the shortwave band can be reflected or refracted from a layer of electrically charged atoms in the atmosphere called the ionosphere. Therefore, small waves directed at an angle in the sky can be reflected back to the earth at great distances beyond the horizon. This is called airwave or “skip” propagation. Shortwave radio can therefore be used for communication over very long distances, unlike high-frequency radio waves, which travel in straight lines (line-of-sight propagation) and are limited by the visual horizon, about 64 km (40 mi).
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Shortwave broadcasting of radio programs played an important role in the early history of radio. In World War II it was used as a propaganda tool to an international audience. The heyday of international shortwave broadcasting was during the Cold War between 1960 and 1980.
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Shortwave broadcasting has lost importance due to the widespread application of other technologies for the distribution of radio programs, such as satellite radio and cable broadcasting, as well as IP-based transmission. Initiatives for digitization of broadcasting also failed to bear fruit, and so on until 2022
However, shortwave remains important in war zones such as the Russo-Ukraine war, and shortwave transmissions can be broadcast thousands of kilometers from a single transmitter, making it difficult for government authorities to intercept.
The name “shortwave” comes from the beginning of radio in the early 20th century, where the radio spectrum was divided into longwave (LW), mediumwave (MW) and shortwave (SW) bands based on wavelength. . Shortwave radio gets its name because the wavelength of this band is shorter than 200 m (1,500 kHz), marking the original upper limit of the medium frequency band first used for radio communication. The broadcast medium wave band now spans the 200 m/1, 500 kHz range.
Early long-distance radio telegraphy used long waves, below 300 kilohertz (kHz). Disadvantages of this system include a very limited spectrum available for long distance communication, and highly explosive transmitters, receivers and massive antennas. Long waves are also difficult to beam directionally, resulting in a large loss of energy over long distances. Before the 1920s, shortwave frequencies above 1.5 MHz were considered useless for long-distance communications and were designated for amateur use in many countries.
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Guglielmo Marconi, the inventor of radio, commissioned his assistant Charles Samuel Franklin to conduct a large-scale study of the transmission properties of short-wavelength waves and determine their suitability for long-distance transmission. Franklin built a large 25 kW antenna at Pauldhu Wireless Station, Cornwall. In June and July 1923, night wireless transmissions on 97 meters (about 3 MHz) were completed from Poldhu in the Cape Verde Islands to Marconi’s yacht Elettra.
In September 1924, Marconi arranged for a day-and-night transmission on his yacht on 32 meters (9.4&MHz) from Poldhu to the port of Beirut, where he was sailing, and was “amazed” that he could receive a signal . “All day long”.
In July 1924 Marconi entered into a contract with the British General Post Office (GPO) to install high-speed short-wave telegraph circuits from London to Australia, India, South Africa and Canada as a key component of the Imperial Wireless chain. The UK to Canada shortwave “Beam Wireless Service” began commercial operations on 25 October 1926. In 1927, Beam wireless service was introduced from the UK to Australia, South Africa and India.
By 1928, more than half of long-distance communications had shifted from transoceanic cable and longwave wireless services to shortwave, and the overall volume of transoceanic shortwave communications had greatly increased. Shortwave stations had cost and efficiency advantages over massive longwave wireless installations.
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However, some commercial long-wave communication stations were in use until the 1960s. Long-distance radio circuits have also reduced the need for new cables, although cables have the advantage of higher security and maintain a much more reliable and better quality signal than shortwave.
In 1927, the cable companies began to lose a lot of money. A severe financial crisis threatened the viability of cable companies vital to strategic British interests. The British government held the Imperial Wireless and Cable Conference
In 1928 to “investigate the situation created by the competition of beam wireless with cable services”. In 1929 it recommended and obtained official approval for all foreign cable and wireless sources in the Empire in a system controlled by a newly formed company, Imperial and International Communications Limited. The company’s name was changed to Cable and Wireless Limited. In 1934.
The revival of long-distance cables began in 1956 with the laying of TAT-1 across the Atlantic Ocean, the first voice frequency cable on this route. It provided 36 high-quality telephone channels and was soon followed by high-capacity cables around the world. Competition from these cables soon reduced the economic viability of shortwave radio for commercial communications.
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Amateur radio operators have also discovered that long distance communication is possible in the shortwave bands. Early long distance services used surface wave propagation at very low frequencies,
Which are connected along the way at wavelengths shorter than 1,000 m. Longer distances and higher frequencies using this method mean more signal loss. This, and the difficulties in determining and detecting higher frequencies, made the discovery of shortwave propagation difficult for commercial services.
200 meters operates in the medium wave band (about 1500 kHz within the modern AM broadcast band), which was the lowest wavelength / highest frequency available in amateur radio at the time. Hundreds of North American amateurs were heard in Europe on 200 meters in 1922, and at least 20 North American amateurs heard amateur signals from Europe. The first two-way contact between North American and Hawaiian amateurs began in 1922 at 200 meters. Although operation on wavelengths below 200 meters was technically illegal (but tolerated at the time because authorities mistakenly believed that such frequencies were useless for commercial or military use), shortly after World War II, amateurs began experimenting with those wavelengths using newly available vacuum tubes. a.m
Extreme interference at the long end of the 150–200 meter band – the official wavelength assigned to amateurs by the Second National Radio Conference
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In 1923 – amateurs were forced to move to shorter and shorter wavelengths; However, amateurs are restricted by regulation to wavelengths greater than 150 meters (2 MHz). A few lucky amateurs who received special permission for experimental communications on wavelengths below 150 meters completed hundreds of long-distance two-way communications on 100 meters (3 MHz) in 1923, including the first transatlantic two-way communication.
By 1924, many additional specially trained amateurs were making regular transatlantic communications over distances of 6,000 miles (9,600 km) and more. On September 21, 1924, several amateurs in California completed two-way communication with an amateur in New Zealand. On October 19, amateurs in New Zealand and Gland completed a 90-minute two-way call almost halfway around the world. On October 10, the Third National Radio Conference made three shortwave bands available to American amateurs
80 meters (3.75 MHz), 40 meters (7 MHz) and 20 meters (14 MHz). It was allocated worldwide, while the 10 meter band (28 MHz) was created by the Washington International Radiotelegraph Conference.
On November 25, 1927. The 15 meter band (21 MHz) was opened to amateurs in the United States on May 1, 1952.
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Shortwave radio frequency energy is able to reach anywhere on Earth because it is affected by ionospheric reflection back to Earth through the ionosphere (a phenomenon known as “sky wave propagation”). A common occurrence of shortwave propagation is the appearance of a skip zone where reception fails. With a fixed operating frequency, large changes in ionospheric conditions can create skip zones at night.
Due to the multi-layered structure of the ionosphere, spread through ‘E’ or ‘F’ layers and with different number of hops, a phonon that can be disturbed for certain techniques propagates simultaneously in different paths. Absorption of radio frequency energy in the lowest ionospheric layer, the ‘D’ layer, can impose a severe limit, especially for lower frequencies in the shortwave band. This is due to collisions of electrons with neutral molecules, which absorb some of the energy of a radio frequency and convert it into heat.
Amplitude modulation is the simplest type and is most commonly used for shortwave broadcasting. The instantaneous amplitude of the carrier is controlled by the amplitude of the signal (for example speech or music). At the receiver, a simple detector recovers the desired modulation signal from the carrier.
Single sideband transmission is a form of amplitude modulation but effectively filters the modulation result. An amplitude modulated signal has frequency components above and below the carrier frequency. If one set of these elements is omitted along with the remaining carrier, only the remaining set is transmitted. This reduces transmission power, since about 2⁄3 of the energy through an AM signal is in the carrier, which does not need to recover the information contained in the signal. It also reduces
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