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Mediumwave

Mediumwave

Mediumwave radio transmissions (sometimes called Medium frequency or MF) are those between the frequencies of 300 kHz and 3000 kHz. In most of the world, mediumwave serves as the most common band for broadcasting. The standard AM broadcast band is 525 kHz to 1715 kHz in North America, but remains only up to 1615 kHz elsewhere. Mediumwave signals have the property of following the curvature of the earth (the groundwave) at all times, and also reflecting off the ionosphere at night (skywave). This makes this frequency band ideal for both local and continent-wide service, depending on the time of day. For example, during the day a radio receiver in the state of Maryland is able to receive reliable but weak signals from high-power stations WFAN, 660 kHz, and WOR, 710 kHz, 400 km away in New York City, due to groundwave propagation. The effectiveness of groundwave signals largely depends on ground conductivity—higher conductivity results in better propagation. At night, the same receiver picks up signals as far away as Mexico City and Chicago reliably. Many stations are required to shut down or reduce power at night in order to make way for clear channel stations that can then be received over a wider range. In the Americas, mediumwave stations are separated by 10 kHz and have two sidebands of ±5 kHz. In the rest of the world, the separation is 9 kHz, with sidebands of ±4.5 kHz. Both provide adequate audio quality for voice, but are insufficient for high-fidelity broadcasting, which is common on the VHF FM bands. In the US the maximum transmitter power is restricted to 50 kilowatts, while in Europe there are medium wave stations with transmitter power up to 2.5 megawatts. Stereo transmission is possible and offered by some stations in the U.S., Australia, South Africa, and France. However, there are multiple standards for AM stereo with C-QUAM being the legal one in the United States, and receivers that actually implement the technologies are relatively rare but not uncommon. Failed systems include Kahn Powerside and others. In September 2002, the United States Federal Communications Commission approved the iBiquity in-band on-channel (IBOC) system of digital audio broadcasting, which is meant to improve the audio quality of signals. The Digital Radio Mondiale (DRM) IBOC system has been approved by the ITU for use outside the Americas.

Antennas

As aerials mostly mast radiators are used. Stations broadcasting with low power commonly use masts with heights of a quarter wavelength, while high power stations mostly use half wavelength. The usage of masts longer than 5/8 of radiated wavelength gives a bad radiation pattern. Usually mast antennas are insulated against ground and show a high voltage against ground during transmission, which complicates maintenance, installation of air safety warning lights or using the mast as a tower for UHF/VHF-radio, but there are several ways to use grounded masts or towers. wavelength If grounded masts or towers are required, than cage aerials or longwire aerials are used. Another possibility consists of feeding the mast or the tower by cables running from the tuning unit to the guys or crossbars in a certain height. Directional aerials consist of multiple masts, which need not to be from the same height. It is also possible to realize directional aerials for mediumwave with cage aerials where some parts of the cage are fed with a certain phase difference. Other type of aerials sometimes used for mediumwave are T- and L-aerials. The kind used depends on the need for grounded or insulated towers. In some cases dipole aerials are used, which are spun between two masts or towers. Such aerials radiate toward the sky. The mediumwave transmitter at Berlin-Britz for transmitting RIAS used a cross dipole mounted on five 30.5 metre high guyed masts to transmit the skywave up to the ionosphere at nighttime.

Non-broadcast use

For most of the 20th century, the radio frequency 500 kHz was reserved world wide as the Morse code international calling and distress frequency for ships on the high seas. The frequency 2182 kHz is still used for this purpose, but employing voice transmission. Other services that operate in medium wave include Navtex and the Amateur Radio 160-meter band. The obsolete LORAN-A system used medium wave.

Radio

Radio is the wireless transmission of signals, by modulation of electromagnetic waves with frequencies below those of light.

Radio waves

Radio waves are a form of electromagnetic radiation, created whenever a charged object (e.g. an electron) accelerates with a frequency that lies in the radio frequency (RF) portion of the electromagnetic spectrum. In radio, this acceleration is caused by an alternating current in an antenna. Radio frequencies occupy the range from a few tens of hertz to a few hundred gigahertz. Other types of electromagnetic radiation, with frequencies above the RF range are infrared, visible light, ultraviolet, X-rays and gamma rays. Since the energy of an individual photon of radio frequency is too low to remove an electron from an atom, radio waves are classified as non-ionizing radiation.

non-ionizing radiation
Radio transmission diagram and electromagnetic waves.

Electromagnetic radiation travels (propagates) by means of oscillating electromagnetic fields that pass through the air and the vacuum of space equally well, and does not require a medium of transport (such as the aether). When radio waves pass an electrical conductor, the oscillating electric or magnetic field (depending on the shape of the conductor) induces an alternating current and voltage in the conductor. This can be transformed into audio or other signals that carry information. Although the word 'radio' is used to describe this phenomenon, the transmissions which we know as television, radio, radar, and cell phone are all classed as radio frequency emissions.

History and invention

The identity of the original inventor of radio, at the time called wireless telegraphy, is contentious. The controversy over who invented the radio, with the benefit of hindsight, can be broken down as follows: :Q1: Who invented 'wireless transmission of data' (spark-gap radio)? :A1: Alexander Popov, Guglielmo Marconi, Nikola Tesla (possibly in that order). :Q2: Who invented amplitude-modulated (AM) radio, so that more than one station can send signals (as opposed to spark-gap radio, where one transmitter covers the entire bandwidth of the spectrum)? :A2: Reginald Fessenden _{[http://www.invent.org/hall_of_fame/59.html]} and Lee de Forest. :Q3: Who invented frequency-modulated (FM) radio, so that an audio signal can avoid "static," that is, interference from electrical equipment and atmospherics? :A3: Edwin H. Armstrong and Lee de Forest. Early radios ran the entire power of the transmitter through a carbon microphone. While some early radios used some type of amplification through electric current or battery, through the mid 1920s the most common type of receiver was the crystal set. In the 1920s, amplifying vacuum tubes revolutionized both radio receivers and transmitters.

Discovery and development

The theoretical basis of the propagation of electromagnetic waves was first described in 1873 by James Clerk Maxwell in his paper to the Royal Society A dynamical theory of the electromagnetic field, which followed his work between 1861 and 1865. In 1878 David E. Hughes was the first to transmit and receive radio waves when he noticed that his induction balance caused noise in the receiver of his homemade telephone. He demonstrated his discovery to the Royal Society in 1880 but was told it was merely induction. It was Heinrich Rudolf Hertz who, between 1886 and 1888, first validated Maxwell's theory through experiment, demonstrating that radio radiation had all the properties of waves (now called Hertzian waves), and discovering that the electromagnetic equations could be reformulated into a partial differential equation called the wave equation. William Henry Ward was issued on April 30, 1872. Mahlon Loomis was issued on July 30, 1872. Landell de Moura, a Brazilian priest and scientist, conducted experiments after 1893 (but at least by 1894). He did not publicize his achievement until 1900. Claims have been made that Nathan Stubblefield invented radio before either Tesla or Marconi, but his device seems to have worked by induction transmission rather than radio transmission.

Wireless age

In 1893 in St. Louis, Missouri, Tesla made devices for his experiments with the electricity. Addressing the Franklin Institute in Philadelphia and the National Electric Light Association, he described and demonstrated in detail the principles of their work. [http://www.ieee-virtual-museum.org/collection/people.php?taid=&id=1234597&lid=1] They contained all the elements that were later incorporated into radio systems before the development of the vacuum tube. He initially experimented with magnetic receivers, unlike the coherers used by Marconi and other early experimenters. [http://www.teslasociety.com/teslarec.pdf]. Tesla is usually considered the first to apply the mechanism of electrical conduction to wireless practices. On 19 August 1894, British physicist Sir Oliver Lodge demonstrated the reception of Morse code signalling using radio waves using a detecting device called a coherer, a tube filled with iron filings which had been invented by Temistocle Calzecchi-Onesti at Fermo in Italy in 1884. Edouard Branly of France and Popov of Russia later produced improved versions of the coherer. Alexander Popov, who was the first to develop a practical communication system based on the coherer, is usually considered to have been the inventor of radio. In 1894 he built a coherer and presented it to the Russian Physical and Chemical Society on May 7 1895 [http://www.ieee.org/organizations/history_center/milestones_photos/popov.html]. In March 1896, he effected transmission of radio waves between different campus buildings in Saint Petersburg, but didn't care to apply for a patent. The Indian physicist, Jagdish Chandra Bose, during the years 1894-1900, performed pioneering research on radio waves and created waves as short as 5 mm. [http://www.ieee-virtual-museum.org/collection/people.php?taid=&id=1234735&lid=1] In November 1894 J.C. Bose ignited gunpowder and rang a bell at a distance using electromagnetic waves, confirming that communication signals can be sent without using wires. But he was not interested in patenting his work too. In 1896 Marconi was awarded what is sometimes recognised as the world's first patent for radio with British Patent 12039, Improvements in transmitting electrical impulses and signals and in apparatus there-for. In 1897 he established the world's first radio station on the Isle of Wight, England. The same year in the U.S., some key developments in radio's early history were created and patented by Tesla. The U.S. Patent Office reversed its decision in 1904, awarding Marconi a patent for the invention of radio, possibly influenced by Marconi's financial backers in the States, who included Thomas Edison and Andrew Carnegie. Some believe this was made for financial reasons, allowing the U.S. government to avoid having to pay the royalties that were being claimed by Tesla for use of his patents. In 1909, Marconi, with Karl Ferdinand Braun, was also awarded the Nobel Prize in Physics for "contributions to the development of wireless telegraphy". However, Tesla's patent (number 645576) was reinstated in 1943 by the U.S. Supreme Court, shortly after his death. This decision was based on the fact that prior art existed before the establishment of Marconi's patent. Some believe the decision was also made for financial reasons, to allow the U.S. government to avoid having to pay damages that were being claimed by the Marconi Company for use of its patents during World War I.

"Wireless" factories and vacuum tubes

Marconi opened the world's first "wireless" factory in Hall Street, Chelmsford, England in 1898, employing around 50 people. Around 1900, Tesla opened the Wardenclyffe Tower facility and advertised services. By 1903, the tower structure neared completion. Various theories exist on how Tesla intended to achieve the goals of this wireless system (reportedly, a 200 kW system). Tesla claimed that Wardenclyffe, as part of a World System of transmitters, would have allowed secure multichannel transceiving of information, universal navigation, time synchronization, and a global location system. The next great invention was the vacuum tube detector, invented by a team of Westinghouse engineers. On Christmas Eve, 1906, Reginald Fessenden (using his heterodyne principle) transmitted the first radio audio broadcast in history from Brant Rock, Massachusetts. Ships at sea heard a broadcast that included Fessenden playing O Holy Night on the violin and reading a passage from the Bible. The world's first radio news program was broadcast August 31, 1920 by station 8MK in Detroit, Michigan. The world's first regular wireless broadcasts for entertainment commenced in 1922 from the Marconi Research Centre at Writtle near Chelmsford, England.

20th century

Developments in the early 20th century (1900-1959):
- Aircraft used commercial AM radio stations for navigation. This continued through the early 1960s when VOR systems finally became widespread (though AM stations are still marked on U.S. aviation charts).
- In the early 1930s, single sideband and frequency modulation were invented by amateur radio operators. By the end of the decade, they were established commercial modes.
- Radio was used to transmit pictures visible as television as early as the 1920s. Standard analog transmissions started in North America and Europe in the 1940s.
- In 1954, Regency introduced a pocket transistor radio, the TR-1, powered by a "standard 22.5V Battery". Developments in the latter half of the 20th century (1960-1999):
- In 1960, Sony introduced their first transistorized radio, small enough to fit in a vest pocket, and able to be powered by a small battery. It was durable, because there were no tubes to burn out. Over the next twenty years, transistors displaced tubes almost completely except for very high power, or very high frequency, uses.
- In 1963 color television was commercially transmitted, and the first (radio) communication satellite, TELSTAR, was launched.
- In the late 1960s, the U.S. long-distance telephone network began to convert to a digital network, employing digital radios for many of its links.
- In the 1970s, LORAN became the premier radio navigation system. Soon, the U.S. Navy experimented with satellite navigation, culminating in the invention and launch of the GPS constellation in 1987.
- In the early 1990s, amateur radio experimenters began to use personal computers with audio cards to process radio signals. In 1994, the U.S. Army and DARPA launched an aggressive, successful project to construct a software radio that could become a different radio on the fly by changing software.
- Digital transmissions began to be applied to broadcasting in the late 1990s.

Uses of radio

software radio software radio Many of radio's early uses were maritime, for sending telegraphic messages using Morse code between ships and land. One of the earliest users included the Japanese Navy scouting the Russian fleet during the Battle of Tsushima in 1905. One of the most memorable uses of marine telegraphy was during the sinking of the RMS Titanic in 1912, including communications between operators on the sinking ship and nearby vessels, and communications to shore stations listing the survivors. Radio was used to pass on orders and communications between armies and navies on both sides in World War I; Germany used radio communications for diplomatic messages once its submarine cables were cut by the British. The United States passed on President Woodrow Wilson's Fourteen Points to Germany via radio during the war. Broadcasting began to become feasible in the 1920s, with the widespread introduction of radio receivers, particularly in Europe and the United States. Besides broadcasting, point-to-point broadcasting, including telephone messages and relays of radio programs, became widespread in the 1920s and 1930s. Another use of radio in the pre-war years was the development of detecting and locating aircraft and ships by the use of radar (RAdio Detecting And Ranging). Today, radio takes many forms, including wireless networks, mobile communications of all types, as well as radio broadcasting. Read more about radio's history. Before the advent of television, commercial radio broadcasts included not only news and music, but dramas, comedies, variety shows, and many other forms of entertainment. Radio was unique among dramatic presentation that it used only sound. For more, see radio programming. There are a number of uses of radio:

Audio

- AM broadcast radio sends music and voice in the Medium Frequency (MF—0.300 MHz to 3 MHz) radio spectrum. AM radio uses amplitude modulation, in which louder sounds at the microphone causes wider fluctuations in the transmitter power while the transmitter frequency remains unchanged. Transmissions are affected by static because lightning and other sources of radio add their radio waves to the ones from the transmitter.
- FM broadcast radio sends music and voice, with higher fidelity than AM radio. In frequency modulation, louder sounds at the microphone cause the transmitter frequency to fluctuate farther, the transmitter power stays constant. FM is transmitted in the Very High Frequency (VHF—30 MHz to 300 MHz) radio spectrum. FM requires more radio frequency space than AM and there are more frequencies available at higher frequencies, so there can be more stations, each sending more information. Another effect is that shorter VHF radio waves act more like light, travelling in straight lines, hence the reception range is generally limited to about 50-100 miles. During unusual upper atmospheric conditions, FM signals are occasionally reflected back towards the Earth by the ionosphere, resulting in Long distance FM reception. FM receivers are subject to the capture effect, which causes the radio to only receive the strongest signal when multiple signals appear on the same frequency. FM receivers are relatively immune to lightning and spark interference.
- FM Subcarrier services are secondary signals transmitted "piggyback" along with the main program. Special receivers are required to utilize these services. Analog channels may contain alternative programming, such as reading services for the blind, background music or stereo sound signals. In some extremely crowded metropolitan areas, the subchannel program might be an alternate foreign language radio program for various ethnic groups. Subcarriers can also transmit digital data, such as station identification, the current song's name, web addresses, or stock quotes. In some countries, FM radios automatically retune themselves to the same channel in a different district by using sub-bands.
- Aviation voice radios use VHF AM. AM is used so that multiple stations on the same channel can be received. (Use of FM would result in stronger stations blocking out reception of weaker stations due to FM's capture effect). Aircraft fly high enough that their transmitters can be received hundreds of miles (kilometres) away, even though they are using VHF.
- Marine voice radios can use AM in the shortwave High Frequency (HF—3 MHz to 30 MHz) radio spectrum for very long ranges or narrowband FM in the VHF spectrum for much shorter ranges.
- Government, police, fire and commercial voice services use narrowband FM on special frequencies. Fidelity is sacrificed to use a smaller range of radio frequencies, usually five kilohertz of deviation (5 thousand cycles per second), rather than the 75 used by FM broadcasts and 25 used by TV sound.
- Civil and military HF (high frequency) voice services use shortwave radio to contact ships at sea, aircraft and isolated settlements. Most use single sideband voice (SSB), which uses less bandwidth than AM. SSB sounds like ducks quacking on an AM radio. Viewed as a graph of frequency versus power, an AM signal shows power where the frequencies of the voice add and subtract with the main radio frequency. SSB cuts the bandwidth in half by suppressing the carrier and (usually) lower sideband. This also makes the transmitter about three times more powerful, because it doesn't need to transmit the unused carrier and sideband.
- TETRA, Terrestrial Trunked Radio is a digital cell phone system for military, police and ambulances.
- Commercial services such as XM and Sirius offer digital Satellite radio.

Telephony

- Cell phones transmit to a local cell transmitter/receiver site, which connects to the public service telephone network through an optic fiber or microwave radio. When the phone leaves the cell radio's area, the central computer switches the phone to a new cell. Cell phones originally used FM, but now most use various digital encodings.
- Satellite phones come in two types: INMARSAT and Iridium. Both types provide world-wide coverage. INMARSAT uses geosynchronous satellites, with aimed high-gain antennas on the vehicles. Iridium provides cell phones, except the cells are satellites in orbit.

Video

- Television sends the picture as AM, and the sound as FM, on the same radio signal.
- Digital television encodes three bits as eight strengths of AM signal. The bits are sent out-of-order to reduce the effect of bursts of radio noise. A Reed-Solomon error correction code lets the receiver detect and correct errors in the data. Although any data could be sent, the standard is to use MPEG-2 for video, and five CD-quality (44.1 kHz) audio channels (center, left, right, left-back and right back). With all this, it takes only half the bandwidth of an analog TV signal because the video data is compressed.

Navigation

- All satellite navigation systems use satellites with precision clocks. The satellite transmits its position, and the time of the transmission. The receiver listens to four satellites, and can figure its position as being on a line that is tangent to a spherical shell around each satellite, determined by the time-of-flight of the radio signals from the satellite. A computer in the receiver does the math.
- Loran systems also used time-of-flight radio signals, but from radio stations on the ground.
- VOR systems (used by aircraft), have a antenna array that transmits two signals simultaneously. A directional signal rotates like a lighthouse at a fixed rate. When the directional signal is facing north, an omnidirectional signal pulses. By measuring the difference in phase of these two signals, an aircraft can determine its bearing from the station. An aircraft can get readings from two VORs, and locate its position at the intersection of the two beams.
- Radio direction-finding is the oldest form of radio navigation. Before 1960 navigators used movable loop antennas to locate commercial AM stations near cities. In some cases they used marine radiolocation beacons, which share a range of frequencies just above AM radio with amateur radio operators.

Radar

- Radar detects things at a distance by bouncing radio waves off them. The delay caused by the echo measures the distance. The direction of the beam determines the direction of the reflection. The polarization and frequency of the return can sense the type of surface.
- Navigational radars scan a wide area two to four times per minute. They use very short waves that reflect from earth and stone. They are common on commercial ships and long-distance commercial aircraft
- General purpose radars generally use navigational radar frequencies, but modulate and polarize the pulse so the receiver can determine the type of surface of the reflector. The best general-purpose radars distinguish the rain of heavy storms, as well as land and vehicles. Some can superimpose sonar data and map data from GPS position.
- Search radars scan a wide area with pulses of short radio waves. They usually scan the area two to four times a minute. Sometimes search radars use the doppler effect to separate moving vehicles from clutter.
- Targeting radars use the same principle as search radar but scan a much smaller area far more often, usually several times a second or more.
- Weather radars resemble search radars, but use radio waves with circular polarization and a wavelength to reflect from water droplets. Some weather radar use the doppler to measure wind speeds.

Emergency services

- emergency position-indicating rescue beacons (EPIRBs), emergency locating transmitters or personal locator beacons are small radio transmitters that satellites can use to locate a person or vehicle needing rescue. Their purpose is to help rescue people in the first day, when survival is most likely. There are several types, with widely-varying performance.

Data (digital radio)

- The oldest form of digital broadcast was spark gap telegraphy, used by pioneers such as Marconi. By pressing the key, the operator could send messages in Morse code by energizing a rotating commutating spark gap. The rotating commutator produced a tone in the receiver, where a simple spark gap would produce a hiss, indistinguishable from static. Spark gap transmitters are now illegal, because their transmissions span several hundred megahertz. This is very wasteful of both radio frequencies and power.
- The next advance was continuous wave telegraphy, or CW, in which a pure radio frequency, produced by a vacuum tube electronic oscillator was switched on and off by a key. A receiver with a local oscillator would "heterodyne" with the pure radio frequency, creating a whistle-like audio tone. CW uses less than 100Hz of bandwidth. CW is still used, these days primarily by amateur radio operators (hams). Strictly, on-off keying of a carrier should be known as "Interrupted Continuous Wave" or ICW.
- Radio teletypes usually operate on short-wave (HF) and are much loved by the military because they create written information without a skilled operator. They send a bit as one of two tones. Groups of five or seven bits become a character printed by a teletype. From about 1925 to 1975, radio teletype was how most commercial messages were sent to less developed countries. These are still used by the military and weather services.
- Aircraft use a 1200 Baud radioteletype service over VHF to send their ID, altitude and position, and get gate and connecting-flight data.
- Microwave dishes on satellites, telephone exchanges and TV stations usually use quadrature amplitude modulation (QAM). QAM sends data by changing both the phase and the amplitude of the radio signal. Engineers like QAM because it packs the most bits into a radio signal. Usually the bits are sent in "frames" that repeat. A special bit pattern is used to locate the beginning of a frame.
- Systems that need reliability, or that share their frequency with other services may use "corrected orthogonal frequency-division multiplexing" or COFDM. COFDM breaks a digital signal into as many as several hundred slower subchannels. The digital signal is often sent as QAM on the subchannels. Modern COFDM systems use a small computer to make and decode the signal with digital signal processing, which is more flexible and far less expensive than older systems that implemented separate electronic channels. COFDM resists fading and ghosting because the narrow-channel QAM signals can be sent slowly. An adaptive system, or one that sends error-correction codes can also resist interference, because most interference can affect only a few of the QAM channels. COFDM is used for WiFi, some cell phones, Digital Radio Mondiale, Eureka 147, and many other local area network, digital TV and radio standards.
- Most new radio systems are digital, see also:Digital TV, Satellite Radio, Digital Audio Broadcasting.

Heating

Radio-frequency energy generated for heating of objects is generally not intended to radiate outside of the generating equipment, to prevent interferance with other radio signals.
- Microwave ovens use intense radio waves to heat food. (Note: It is a common misconception that the radio waves are tuned to the resonant frequency of water molecules. The microwave frequencies used are actually about a factor of 10 below the resonant frequency.)
- Diathermy equipment is used in surgery for sealing of blood vessels.
- Induction furnaces are used for melting metal for casting.

Mechanical Force

- Tractor beams: Radio waves exert small electrostatic and magnetic forces. These are enough to perform station-keeping in microgravity environments.
- Conceptually, Spacecraft propulsion: Radiation pressure from intense radio waves has been proposed as a propulsion method for an interstellar probe called Starwisp. Since the waves are long, the probe could be a very light-weight metal mesh, and thus achieve higher accelerations than if it were a solar sail.

Other

solar sail
- Amateur radio is a hobby where enthusiasts who purchase or build their own equipment and use radio for their own enjoyment. They may also provide an emergency and public-service radio service. This has been of great use, saving lives in many instances. Radio amateurs are able to use frequencies in a large number of narrow bands throughout the radio spectrum. Radio amateurs use all forms of encoding, including obsolete and experimental ones. Several forms of radio were pioneered by radio amateurs and later became commercially important, including FM, single-sideband AM, digital packet radio and satellite repeaters.
- Personal radio services such as Citizens' Band Radio, Family Radio Service, Multi-Use Radio Service and others exist in North America to provide simple, (usually) short range communication for individuals and small groups, without the overhead of licensing. Similar services exist in other parts of the world.
- Wireless energy transfer: A number of schemes have been proposed that transmit power using microwaves, and the technique has been demonstrated. (See Microwave power transmission). These schemes include, for example, solar power stations in orbit beaming energy down to terrestrial users.
- Radio remote control: Use of radio waves to transmit control data to a remote object as in some early forms of guided missile, some early TV remotes and a range of model boats, cars and aeroplanes. Large industrial remote-controlled equipment such as cranes and switching locomotives now usually use digital radio techniques to ensure safety and reliability.

External links

- [http://www.satelliteradionews.net/ Satellite Radio News.Net] Everything you need to know about Satellite Radio.
- Horzepa, Stan, "[http://www.arrl.org/news/features/2003/10/10/1/ Surfin': Who Invented Radio]?". Arrl.org. 10 October 2003.
- IAteacher: [http://www.iateacher.com/Lesson%206/L6P1-Title.htm Interactive Explanation of Radio Receiver Construction]
- U.S. Supreme Court, "[http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=us&vol=320&invol=1 Marconi Wireless Telegraph co. of America v. United States]". 320 U.S. 1. Nos. 369, 373. Argued 9 April-12, 1943. Decided 21 June 1943.
- Radio Locator: [http://www.radio-locator.com/ Find a radio station in your area]
- On The Radio.Net: [http://www.ontheradio.net/ Find phone numbers and websites for commercials you heard on the radio!]
- [http://www.ovrc.org/ Ottawa Vintage Radio Club of Canada]
- [http://www.xmradio.com XM Satellite Radio]
- [http://www.oldradio.com The Broadcast Archive - Radio History on the Web!]
- [http://ndaeuro.online.fr/gargot/index.htm Radiozone]
- Directories
- [http://www.looksmart.com/eus1/eus317828/eus317855/eus52445/ LookSmart - Radio]
- [http://dmoz.org/Arts/Radio/ Open Directory Project - Radio]
- [http://dir.yahoo.com/News_and_Media/Radio/ Yahoo! - Radio] Category:Radio Category:Sound ja:放送 simple:Radio th:วิทยุ

Hertz

:See also the car rental company, The Hertz Corporation, and Hertz (disambiguation). ---- The hertz (symbol: Hz) is the SI unit of frequency. It is named in honor of the German physicist Heinrich Rudolf Hertz who made important scientific contributions to electromagnetism.

Definition

One hertz is defined as one cycle per second. :1 Hz = 1 s⁻¹

SI multiples

Explanation

One hertz simply means "one per second" (1 / s); 100 Hz means "one hundred per second", and so on. The unit may be applied to any periodic event – for example, a clock might be said to tick at 1 Hz, or a human heart might be said to beat at 1.2 Hz. Frequency of random events, such as radioactive decays, is expressed in becquerels. The name hertz was adopted by the CGPM (Conférence générale des poids et mesures) in 1960, replacing the previous name for the unit, cycles per second (cps), along with its related multiples, primarily kilocycles (kc) and megacycles (Mc). Hertz largely replaced cycles in common use by 1970.

AM broadcasting

AM radio is radio broadcasting using Amplitude Modulation.

History

AM was the dominant method of broadcasting during the first two thirds of the 20th century and remains widely used into the 21st. The Central Intelligence Agency World Factbook list approximately 16,265 AM [http://www.odci.gov/cia/publications/factbook/fields/2013.html stations worldwide]. AM radio began with the first, experimental broadcast in 1906 by Reginald Fessenden, and was used for small-scale voice and music broadcasts up until World War I. The great increase in the use of AM radio came the following decade. The first commercial radio services began on AM in the 1920s (the first American radio station was started by Frank Conrad: KDKA in Pittsburgh, Pennsylvania). Radio programming boomed during the "Golden Age of Radio" (1920s–1950s). Dramas, comedy and all other forms of entertainment were produced, as well as broadcasts of news and music. see History of radio for main article

Operation

AM radio technology is simpler than either FM radio or DAB. An AM receiver detects amplitude variations in the radio wave. It then amplifies changes in the signal voltage to drive a loudspeaker or earphones. The earliest crystal radio receivers used a crystal diode detector with no amplification.

Frequencies

AM radio is broadcast in on several frequency bands:
- Long wave is 153–279 kHz; it is not available in the Western Hemisphere, and European 9kHz channel spacing is generally used.
- Medium wave is 530–1,710 kHz in the Americas and 530-1620 in other parts of the world. In the Americas 10kHz spacing is used; elsewhere it is 9kHz.
- Short wave is 2,300–26,100 kHz, divided into 15 broadcast bands. Shortwave broadcasts generally use a narrow 5kHz channel spacing. The allocation of these bands is governed by the ITU's Radio Regulations and, on the local level, by each country's national telecommunications administration — for instance, in the U.S., the FCC.
- Long wave is used for commercial radio broadcasting in Europe, Africa, Asia, and Australasia (ITU regions 1 and 3). In the Americas this band is reserved for aeronautical navigation. Due to the propagation characteristics of long wave signals, the frequencies are used most effectively in latitudes north of 50°.
- Medium wave is by far the most heavily used band for commercial broadcasting. This is the "AM radio" that most people are familiar with.
- Short wave is used by radio services intended to be heard at great distances from the transmitting station. The long range of short wave broadcasts comes at the expense of lower audio fidelity. The mode of propagation for short wave is different (see high frequency). AM is used mostly by broadcast services — other shortwave users may use a modified version of AM such as SSB or an AM-compatible version of SSB such as SSB with carrier reinserted. In many parts of the world short wave radio also carries audible, encoded messages of unknown purpose from numbers stations. Frequencies between the broadcast bands are used for other forms of radio communication, such as baby monitors, walkie talkies, cordless telephones, radio control, "ham" radio, etc.

Limitations of AM radio

Because of its susceptibility to atmospheric interference and generally lower-fidelity sound, AM broadcasting is better suited to talk radio and news programming, while music radio and public radio mostly shifted to FM broadcasting in the late 1960s and 1970s. Medium wave and short wave radio signals act differently during daytime and nighttime. During the day, AM signals travel by groundwave, refracting around the curve of the earth over a distance up to a few hundred kilometres (or miles) from the signal transmitter. However, after sunset, changes in the ionosphere cause AM signals to travel by skywave, enabling AM radio stations to be heard much farther from their point of origin than is normal during the day. This phenomenon can be easily observed by scanning an AM radio dial at night. As a result, many broadcast stations are required as a condition of license to reduce their broadcasting power significantly after sunset, or even to suspend broadcasting entirely during nighttime hours. (Such stations are commonly referred to as daytimers.) Some other radio stations are granted clear channel rights, meaning that they broadcast on frequencies whose use is restricted and thus relatively unaffected by interference from other stations. The hobby of listening to long distance signals is known as DX or DX'ing, from an old telegraph abbreviation for "distant". Several non-profit hobbyist clubs are devoted exclusively to DXing the AM broadcast band, including the National Radio Club and International Radio Club of America. Similarly, people listening to short wave transmissions are SWLing. AM radio signals can be disrupted in large urban centres by skyscrapers and other sources of radio frequency interference (RFI). FM signals, however, are not affected as much by these types of interference. As a result, AM radio has lost its dominance as a music broadcasting service, and in many cities is now relegated to news, sports and talk radio stations.

Other distribution methods

Stereo transmissions are possible (see AM stereo), and there is work underway to add digital radio services to currently existing AM transmissions. In the United States, the iBiquity company is developing a proprietary standard for medium wave transmissions, while Digital Radio Mondiale is a more open effort often used on the shortwave bands, and can be used alongside many AM broadcasts. While FM radio can also be received by cable, AM radio cannot be, although an AM station can be converted into an FM cable signal. In Canada, cable operators that offer FM cable services are required by the CRTC to distribute all locally available AM stations in this manner.

External link

- [http://www.salestores1.com/woreltab.html Table of Voltage, Frequency, TV Broadcasting system, Radio Broadcasting, by Country].
- [http://www.dxtuners.com Listen to live AM radio transmissions]. Category:Radio

North America

North America is a continent in the northern hemisphere bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the south by the Caribbean Sea, and on the west by the North Pacific Ocean. It covers an area of 24,497,994 km² (9,458,728 sq mi), or about 4.8% of the Earth's surface. As of July 2002, its population was estimated at more than 514,600,000. It is the third largest continent in area, after Asia and Africa, and is fourth in population after Asia, Africa, and Europe. Both North and South America are named after Amerigo Vespucci, who was the first European to suggest that the Americas were not the East Indies, but a previously undiscovered (by Europeans) New World. North America occupies the northern portion of the landmass generally referred to as the New World, the Western Hemisphere, the Americas, or simply America. North America's only land connection is to South America at the narrow Isthmus of Panama. (For geopolitical reasons, all of Panama – including the segment east of the Panama Canal in the isthmus – is often considered a part of North America alone.) According to some authorities, North America begins not at the Isthmus of Panama but at the Isthmus of Tehuantepec, with the intervening region called Central America and resting on the Caribbean Plate. Most, however, tend to see Central America as a region of North America, considering it too small to be a continent on its own. Greenland, although a part of North America geographically, is not considered to be part of the continent politically.

Physical features

Greenland, plutonic, metamorphic rock types of North America. ]] Plate tectonics recognizes the vast majority of North America as being the surface of the North American Plate. Parts of California and western Mexico are known for being the edge of the Pacific Plate, with the two plates meeting along the San Andreas fault. The continent can be divided into four great regions (each of which contains many sub-regions): the Great Plains stretching from the Gulf of Mexico to the Canadian Arctic; the geologically young, mountainous west, including the Rocky Mountains, the Great Basin, California and Alaska; the raised but relatively flat plateau of the Canadian Shield in the northeast; and the varied eastern region, which includes the Appalachian Mountains, the coastal plain along the Atlantic seaboard, and the Florida peninsula. Mexico, with its long plateaus and cordilleras, falls largely in the western region, although the eastern coastal plain does extend south along the Gulf. The western mountains are split in the middle, into the main range of the Rockies and the coast ranges in California, Oregon, Washington, and British Columbia with the Great Basin – a lower area containing smaller ranges and low-lying deserts – in between. The highest peak is Denali in Alaska. Since 1931, Rugby, North Dakota, has officially been recognized as being at the geographic center of North America. The location is marked by a 4.5 metre (15 foot) field stone obelisk. Image:North america terrain 2003 map.jpg|North America bedrock and terrain. Image:North america basement rocks.png|North American cratons and basement rocks. Image:North America Tectonic Elements.jpg|Tectonic elements of North America Image:North america craton nps.gif|North American craton.

Territories and regions

craton On the main continent landmass, there are three large and relatively populous countries:
- Canada - many large islands off the shore of North America belong to Canada, including Vancouver Island and the Queen Charlotte Islands on the west, Prince Edward Island, Newfoundland and Cape Breton Island on the east, and the Canadian Arctic islands (including Ellesmere Island, Baffin Island, and Victoria Island) in the north
- Mexico - the Revillagigedo archipelago and numerous smaller islands off its coast belong to Mexico
- The United States - the 48 contiguous states and Alaska are part of North America, while the state of Hawaii in the Pacific Ocean is not; the Aleutian Islands south of Alaska also belong to the U.S. At the southern end of the continent, in a relatively small area known as Central America, are the countries of:
- Belize
- Costa Rica
- El Salvador
- Guatemala
- Honduras
- Nicaragua
- Panama ¹ At the southeastern end of the continent lies a chain of islands territories called the Antilles, the Caribbean or the West Indies, which include the countries:
- Antigua and Barbuda
- Bahamas
- Barbados
- Cuba
- Dominica
- Dominican Republic
- Grenada
- Haiti
- Jamaica
- Saint Kitts and Nevis
- Saint Lucia
- Saint Vincent and the Grenadines
- Trinidad and Tobago ¹ And the dependencies:
- Anguilla (British overseas territory)
- Aruba ² (part of the Kingdom of the Netherlands)
- Cayman Islands (British overseas territory)
- Guadeloupe (French région d'outre-mer)
- Martinique (French région d'outre-mer)
- Montserrat (British overseas territory)
- Navassa Island (U.S. territory)
- Netherlands Antilles ¹ (part of the Kingdom of the Netherlands)
- Puerto Rico (U.S. commonwealth)
- Turks and Caicos Islands (British overseas territory)
- British Virgin Islands (British overseas territory)
- U.S. Virgin Islands (territory of the USA) Lying in the Atlantic Ocean but considered part of the continent are the dependencies:
- Bermuda, a British overseas territory found about 1,072 km (670 mi.) southeast of New York City
- Greenland, the largest island in the world and a self-governing dependency of Denmark, which is located in the far north of the continent to the east of Nunavut.
- Saint Pierre and Miquelon, a French collectivité d'outre-mer off the south coast of Newfoundland, is the last of France's once vast possessions in America north of the Caribbean. ¹ These states and dependencies have territory both in North and South America.
² These dependencies lie in South America, but are considered North American because of cultural and historical reasons.
See here for details.

Usage

The United States, Canada, and the other English-speaking nations of the Americas (Belize, Guyana, and the Anglophone Caribbean) are sometimes grouped under the term Anglo-America, while the remaining nations of North and South America are grouped under the term Latin America. Alternatively, Northern America is used to refer to Canada and the U.S. together (plus Greenland and Bermuda), while Central America is mainland North America south of the United States. The West Indies generally include all islands in the Caribbean Sea. In this respect, Latin America generally includes Central America and South America and, sometimes, the West Indies. The term Middle America is sometimes used to refer to Mexico, Central America, and the Caribbean collectively. The term "North America" may mean different things to different people. The term in common usage is often taken to mean "the United States and Canada, only" by some people of the United States and Canada, excluding Mexico and the countries of Central America, unless the context makes it clear that they are to be included (such as with specific reference to Mexico, when talking about NAFTA). For example, guides to wild flora and fauna published by the National Audubon Society for "North America" frequently include only species found in Canada and the U.S. This may be attributed to the fact that culturally and economically, the U.S. and Canada are more alike to each other than they are to the rest of North America. Mexicans, however, are acutely aware that Mexico is a part of North America and object to this usage. Central Americans, however, are generally content to be called Central Americans – largely because of their shared history, which includes several attempts at supranational integration in the region and in which Mexico, their much larger northern neighbor, was never involved.

Political divisions and regions

Notes:
¹ Continental regions as per UN categorisations/map.
² Depending on definitions, Aruba, Netherlands Antilles, Panama, and Trinidad and Tobago have territory in one or both of North and South America.
³ Due to ongoing activity of the Soufriere Hills volcano beginning 1995, much of Plymouth, Montserrat's de jure capital, was destroyed and government offices relocated to Brades.

External links

- http://www.america-norte.com/america-norte-mapa.htm Category:Continents Category:North America zh-min-nan:Pak Bí-chiu ko:북아메리카 ja:北アメリカ simple:North America th:ทวีปอเมริกาเหนือ

Ionosphere

The Ionosphere is the part of the atmosphere that is ionized by solar radiation. It forms the inner edge of the magnetosphere and has practical importance because it influences high-frequency (HF) (3–30 MHz) radio propagation to distant places on the Earth.

Geophysics

The lowest part of the Earth's atmosphere is called the troposphere and it extends from the surface up to about 10 km (6 miles). The atmosphere above 10 km is called the stratosphere, followed by the mesosphere. It is in the stratosphere that incoming solar radiation creates the ozone layer. At heights of above 80 km (50 miles), in the thermosphere, the atmosphere is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. The number of these free electrons is sufficient to affect radio propagation. This portion of the atmosphere is ionized and contains a plasma which is referred to as the ionosphere. In a plasma, the negative free electrons and the positive ions are attracted to each other by the electromagnetic force, but they are too energetic to stay fixed together in an electrically neutral molecule.
plasma
Solar radiation at ultraviolet (UV) and shorter X-Ray wavelengths is considered to be ionizing since photons of energy at these frequencies are capable of dislodging an electron from a neutral gas atom or molecule during a collision. At the same time, however, an opposing process called recombination begins to take place in which a free electron is "captured" by a positive ion if it moves close enough to it. As the gas density increases at lower altitudes, the recombination process accelerates since the gas molecules and ions are closer together. The point of balance between these two processes determines the degree of ionization present at any given time. The ionization depends primarily on the Sun and its activity. The amount of ionization in the ionosphere varies greatly with the amount of radiation received from the sun. Thus there is a diurnal (time of day) effect time and a seasonal effect. The local winter hemisphere is tipped away from the Sun, thus there is less received solar radiation. The activity of the sun is associated with the sunspot cycle, with more radiation occurring with more sunspots. Radiation received also varies with geographical location (polar, auroral zones, mid-latitudes, and equatorial regions). There are also mechanisms that disturb the ionosphere and decrease the ionization. There are disturbances such as solar flares and the associated release of charged particles into the solar wind which reaches the Earth and interacts with its geomagnetic field.

The Ionospheric Layers

geomagnetic
Solar radiation, acting on the different compositions of the atmosphere with height, generates layers of ionization:

D Layer

The D layer is the innermost layer, 50 km to 90 km above the surface of the Earth. Ionization here is due to Lyman series-alpha hydrogen radiation at a wavelength of 121.5 nanometre (nm) ionizing nitric oxide (NO). In addition, when the sun is active with 50 or more sunspots, Hard X-rays (wavelength < 1 nm) ionize the air (N₂, O₂). During the night cosmic rays produce a residual amount of ionization. Recombination is high in this layer, thus the net ionization effect is very low and as a result the high-frequency (HF) radio waves aren't reflected by the D layer. The frequency of collision between electrons and other particles in this region during the day is about 10 million collisions per second. The D layer is mainly responsible for absorption of HF radio waves, particularly at 10 MHz and below, with progressively smaller absorption as the frequency gets higher. The absorption is small at night and greatest about midday. The layer reduces greatly after sunset, but remains due to galactic cosmic rays. A common example of the D layer in action is the disappearance of distant AM broadcast band stations in the daytime.

E Layer

The E layer is the middle layer, 90km to 120km above the surface of the Earth. Ionization is due to Soft X-Ray (1-10 nm) and far ultraviolet (UV) solar radiation ionization of molecular oxygen (O₂). This layer can only reflect radio waves having frequencies less than 10 MHz. It has a negative effect on frequencies above 10 MHz due to its partial absorption of these waves. During the daytime the solar wind presses this layer closer to the Earth, thereby limiting how far it can reflect radio waves. On the night side of the Earth, the solar wind drags the ionosphere further away, thereby greatly increasing the range which radio waves can travel by reflection.

E_S

The E_s layer or sporadic E-layer. Sporadic E propagation is characterized by small clouds of intense ionization, which can support radio wave reflections from 25 – 225 MHz. Sporadic-E events may last for just a few minutes to several hours. There are multiple causes of sporadic-E that are still being pursued by researchers. This propagation occurs most frequently during the summer months with major occurrences during the summer, and minor occurrences during the winter. During the summer, this mode is popular due to its high signal levels. The skip distances are generally around 1000km (620 miles).

F Layer

The F layer or region, also known as the Appleton layer, is 120km to 400km above the surface of the Earth. Here extreme ultraviolet (UV) (10-100 nm) solar radiation ionizes molecular oxygen (O₂). The F region is the most important part of the ionosphere in terms of HF communications. The F layer combines into one layer at night, and in the presence of sunlight (during daytime), it divides into two layers, the F₁ and F₂. The F layers are responsible for most skywave propagation of radio waves, and are thickest and most reflective of radio on the side of the Earth facing the sun.

Anomalies to the Ideal Model

The statements above assumed that each layer was smooth and uniform. In reality the ionosphere is a lumpy, cloudy layer with irregular patches of ionization.

Winter Anomaly

At mid-latitudes, the F₂ layer daytime ion production is higher in the summer, as expected, since the sun shines more directly on the earth. However, there are seasonal changes in the molecular-to-atomic ratio of the neutral atmosphere that cause the summer ion loss rate to be even higher. The result is that the increase in the summertime loss overwhelms the increase in summertime production, and total F₂ ionization is actually lower, not higher, in the local summer months. This effect is known as the winter anomaly. The anomaly is always present in the northern hemisphere, but is usually absent in the southern hemisphere during periods of low solar activity.

Equatorial Anomaly

radio
Within approximately ± 20 degrees of the magnetic equator, is the Equatorial Anomaly. It is the occurrence of a trough of concentrated ionization in the F₂ layer. The Earth's magnetic field lines are horizontal at the equator. Solar heating and tidal oscillations in the lower ionosphere move plasma up and across the magnetic field lines. This sets up a sheet of electric current in the E region which, with the horizontal magnetic field, forces ionization up into the F layer, concentrating at ± 20 degrees from the magnetic equator. This phenomenon is known as the equatorial fountain.

Ionospheric Perturbations

X-rays: Sudden Ionospheric Disturbances (SID)

When the sun is active, strong solar flares can occur that will hit the Earth with hard X-rays on the sunlit side of the Earth. They will penetrate to the D-region, release electrons which will rapidly increase absorption causing a High Frequency (3-30 MHz) radio blackout. During this time Very Low Frequency (3 - 30 kHz) signals will become reflected by the D layer instead of the E layer, avoiding the signal loss through the D layer. As soon as the X-rays end, the sudden ionospheric disturbance (SID) or radio black-out ends as the electrons in the D-region recombine rapidly and signal strengths return to normal.

Protons: Polar Cap Absorption (PCA)

Associated with solar flares is a release of high-energy protons. These particles can hit the earth within 15 minutes to 2 hours of the solar flare. The protons spiral around and down the magnetic field lines of the Earth and penetrate into the atmosphere near the magnetic poles increasing the ionization of the D and E layers. PCA's typically last anywhere from about an hour to several days, with an average of around 24 to 36 hours.

Geomagnetic Storms

A geomagnetic storm is a temporary intense disturbance of the Earth's magnetosphere.
- During a geomagnetic storm the F₂ layer will become unstable, fragment, and may even disappear completely.
- In the Northern and Southern pole regions of the Earth aurora will be observable in the sky.

Radio Application

DX communication, popular among amateur radio enthusiasts, is a term given to communication over great distances. When using High-Frequency bands, the ionosphere is utilized to reflect the transmitted radio beam. The beam returns to the Earth's surface, and may then be reflected back into the ionosphere for a second bounce. Radio waves "hop" from the Earth to the ionosphere and back to the Earth. When a radio wave reaches the ionosphere, the electric field in the wave forces the electrons in the ionosphere into oscillation at the same frequency as the radio wave. Some of the radio wave energy is given up to this mechanical oscillation. The oscillating electron will then either be lost to recombination or will re-radiate the original wave energy back downward again. Total reflection can occur when the collision frequency of the ionosphere is less than the radio frequency, and if the electron density in the ionosphere is great enough. The critical frequency is the limiting frequency at or below which a radio wave is reflected by an ionospheric layer at vertical incidence. If the transmitted frequency is higher than the plasma frequency of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal. It is calculated as shown below: :

f = 9 \times 10^ \sqrt

where N = electron density per cm³ and f_critical is in MHz. The Maximum Usable Frequency (MUF) is defined as the upper frequency limit that can be used for transmission between two points at a specified time. :

f = \frac

where I = angle of attack, the angle of the wave relative to the horizon, and sin is the sine function. The cutoff frequency is the frequency below which a radio wave fails to penetrate a layer of the ionosphere at the incidence angle required for transmission between two specified points by reflection from the layer.

Other Applications

The open system space tether, which uses the ionosphere, is being researched. The space tether uses plasma contactors and the ionosphere as parts of a circuit to extract energy from the Earth's magnetic field by electromagnetic induction.

Measurements

Ionograms

Ionograms show the virtual heights and critical frequencies of the ionospheric layers and which are measured by an ionosonde. An ionosonde sweeps a range of frequencies, usually from 0.1 to 30 MHz, transmitting at vertical incidence to the ionosphere. As the frequency increases, each wave is refracted less by the ionization in the layer, and so each penetrates further before it is reflected. Eventually, a frequency is reached that enables the wave to penetrate the layer without being reflected. For ordinary mode waves, this occurs when the transmitted frequency just exceeds the peak plasma, or critical, frequency of the layer. Tracings of the reflected high frequency radio pulses are known as ionograms.

Solar Flux

Solar Flux is a measurement of the intensity of solar radio emissions at a frequency of 2800 MHz made using a radio telescope located in Ottawa, Canada. Known also as the 10.7 cm flux (the wavelength of the radio signals at 2800 MHz), this solar radio emission has been shown to be proportional to sunspot activity. However, the level of the sun's ultraviolet and X-ray emissions is primarily responsible for causing ionization in the earth's upper atmosphere. We now have data from the GOES spacecraft that measures the background X-Ray flux from the sun, a parameter more closely related to the ionization levels in the ionosphere.
- The A and K indices are a measurement of the behavior of the horizontal component of the geomagnetic field. The K index uses a scale from 0 to 9 to measure the change in the horizontal component of the geomagnetic field. A new K index is determined at the Table Mountain Observatory, north of Boulder, Colorado.
- The geomagnetic activity levels of the earth are measured by the fluctuation of the Earth's magnetic field in a unit called Gauss. The earth's magnetic field is measured around the planet by many observatories. The data retrieved is processed and turned into measurement indices. Daily measurements for the entire planet are made available through an estimate of the ap index, called the planetary A-index (PAI).

Scientific research on Ionspheric propagation

Scientists also are exploring the structure of the ionosphere by bouncing radio waves of different frequencies from it, and using special receivers to detect how the reflected waves have changed from the transmitted waves. Project HAARP (High Frequency Active Auroral Research Program) investigations focus on studying the properties and behavior of ionospheric plasma, with particular emphasis on being able to understand and use it to enhance communications and surveillance systems for both civilian and defense purposes. It started in 1993 for a proposed twenty year experiment. CUTLASS (Co-operative UK Twin Located Auroral Sounding System) researches the high latitude ionosphere using radar. Scientists are also examining the ionosphere by the changes to radio waves from satellites and stars passing through it. The Arecibo radio telescope located in Puerto Rico, was originally intended to study Earth's ionosphere.

History

In 1899, Nikola Tesla researched ways to utilize the ionosphere to transmit energy wirelessly over long distances. In his experiments, he transmitted extremely low frequencies between the earth and ionosphere, up to what is called the Kennelly-Heaviside Layer (Grotz, 1997). Tesla made mathematical calculations and computations based on his experiments. He predicted the resonant frequency of this area within 15% of modern accepted experimental value. (Corum, 1986) In the 1950s, researchers confirmed the resonant frequency was at the low range 6.8 Hz. Guglielmo Marconi received the first trans-Atlantic radio signal on December 12, 1901, in St. John's, Newfoundland (now in Canada) using a 400-foot kite-supported antenna for reception. The transmitting station in Poldhu, Cornwall used a spark-gap transmitter to produce a signal with a frequency of approximately 500 kHz and a power of 100 times more than any radio signal previously produced. The message received was three dots, the Morse code for the letter S. To reach Newfoundland the signal would have to bounce off the ionosphere twice. Dr. Jack Belrose has recently contested this, however, based on theoretical work as well as an actual experiments. However, Marconi did achieve transatlantic wireless communications beyond a shadow of doubt in Glace Bay one year later. In 1902, Oliver Heaviside proposed the existence of the Kennelly-Heaviside Layer of the ionosphere which bears his name. Heaviside's proposal included means by which radio signals are transmitted around the Earth's curvature. Heaviside's proposal, coupled with Planck's law of black body radiation, may have hampered the growth of radio astronomy for the detection of electromagnetic waves from celestial bodies until 1932 (and the development of high frequency radio transceivers). Also in 1902, Arthur Edwin Kennelly discovered some of the ionosphere's radio-electrical properties. In 1912, the U.S. Congress imposed the Radio Act of 1912 on amateur radio operators, limiting their operations to frequencies above 1.5 MHz (wavelength 200 meters or smaller). The government thought those frequencies were useless. This led to the discovery of HF radio propagation via the ionosphere in 1923. Edward V. Appleton was awarded in 1947 a Nobel Prize for his confirmation of the existence of the ionosphere in 1927. Lloyd Berkner first measured the height and density of the ionosphere. This permitted the first complete theory of short wave radio propagation. Maurice V. Wilkes and J. A. Ratcliffe researched the topic of radio propagation of very long radio waves in the ionosphere. Vitaly Ginzburg has developed a theory of electromagnetic wave propagation in plasmas such as the ionosphere. In 1962 the Canadian satellite Alouette 1 was launched to study the ionosphere. Following its success were Alouette 2 in 1965 and the two ISIS satellites in 1969 and 1971, all for measuring the ionosphere.

References

- Corum, J. F., and Corum, K. L., "A Physical Interpertation of the Colorado Springs Data". Proceedings of the Second International Tesla Symposium. Colorado Springs, Colorado, 1986.
- Grotz, Toby, "The True Meaing of Wireless Transmission of power". Tesla : A Journal of Modern Science, 1997.
- Leo F. McNamara. (1994) ISBN 0-89464-807-7 Radio Amateurs Guide to the Ionosphere.
- Davies, K., 1990. Peter Peregrinus Ltd, London. ISBN 0-86341-186-X Ionospheric Radio.

External links

- Gehred, Paul, and Norm Cohen, [http://www.sec.noaa.gov/radio/radio.html SEC's Radio User's Page].
- [http://geomag.usgs.gov USGS Geomagnetism Program]
- [http://www.sec.noaa.gov/SWN/ Current Space Weather Conditions]
- [http://www.sec.noaa.gov/rt_plots/xray_1m.html Current Solar X-Ray Flux] Category:Radio frequency propagation Category:Nikola Tesla Category:Atmosphere Category:Space plasmas Category:Plasma physics ko:전리층 ja:電離層

WOR (AM)

WOR is a class A (nighttime clear channel), AM radio station located in New York, New York, USA, operating on 710kHz. The station has a talk format and has been owned by Buckley Broadcasting since 1987, after the station was sold by the now-defunct RKO. WOR began broadcasting on February 22, 1922 using a 500 watt transmitter on 833kHz and located in the Bamberger's Department Store in Newark, New Jersey. Bamberger's sale of radio sets to consumers explained their affiliation with the station. The WOR call sign was reissued from the U.S. maritime radio service. The station initially operated limited hours, sharing time with two other stations, WDT-AM and WJY-AM, which also operated on 833kHz. After changing frequency to 740kHz in 1924 and sharing time with WJY, WOR occupied its current channel in 1927 on a full-time basis. In 1934, WOR formed the Mutual Broadcasting System and became its keystone station. In 1941 the station changed its city of license from Newark to New York City. In 1959 WOR left Mutual and became an independent station. In 1949 WOR started a sister TV station, WOR-TV, on channel 9. This station became WWOR-TV after it and WOR were sold to separate companies in 1987. From the 1930s to the early 1980s, WOR was a free-flowing full service station. There was an emphasis on news reports and talk programs, but music was played also, usually pop standards. From 1983 to around 1985, WOR stopped playing music altogether, as they evolved to an almost complete talk format. WOR's most renowned program was its morning show, Rambling with Gambling, which aired continuously from March 1925 to September 2000 across three generations of hosts: John B. Gambling, John A. Gambling, and John R. Gambling. After John R. Gambling's edition of the show was unceremoneously dropped, he moved to WABC where he hosts a late morning show. Other notable WOR program hosts have included Arlene Francis, Michael Strange, Carlton Fredericks, Jinx Falkenberg, "Uncle Don" Carney, Jean Shepherd, Bob and Ray, Long John Nebel, Joy Browne, Michael Savage, Bill O'Reilly, and Bob Grant. Today, WOR is a news and talk radio station. It broadcasts 24 hours per day with 50,000 watts using a three-tower directional antenna with a single radiation pattern, both day and night. Its transmitter is located in Lyndhurst, New Jersey. It is the only New York City AM station to have retained its original three-letter call sign, which are the oldest continually-used ones in the New York City area. On April 30, 2005, WOR moved its offices and studios from 1440 Broadway in Midtown Manhattan where it had been based for 80 years to a new facility at 111 Broadway in Downtown Manhattan. In a press release, Richard Buckley, president of WOR's owner, Buckley Broadcasting, cited that the main reason WOR moved was because the new New York Times building currently being built on 8th Avenue would block WOR's STL (Studio to Transmitter Link) from the main studio in Manhattan to their transmitter.

External links

- [http://www.wor710.com/ WOR AM]
- [http://www.wor710.com/History/history.htm WOR AM History]
- [http://www.nyradionews.com/wor/ WOR News Historical Profile & Interviews - 1978]
- Category:Class A radio stations in North America

Chicago, Illinois

Chicago, colloquially known as the "Second City" and the "Windy City", is the third-largest city in population in the United States, following New York City and Los Angeles, and the largest inland city in the country. Chicago is located in the Midwestern state of Illinois along the southwestern shore of Lake Michigan. It is the largest city and the county seat of Cook County. When combined with its suburbs and eight surrounding counties, the greater metropolitan area known as Chicagoland encompasses a population greater than 9 million people. Growing from a frontier town in 1833 to one of the world's premier cities, Chicago is ranked as one of 10 "Alpha" (most influential) world cities by the Globalization and World Cities Study Group & Network. Today, Chicago is the financial, transportation, and cultural capital of the American Midwest. The city has long been known around the world as a financial, industrial, and transportation center and for its ethnic diversity. Chicago's skyscrapers, local cuisine, political traditions, and sports teams are some of the most recognized symbols of the city. A variety of colloquial nicknames reflect Chicago's unique character. A resident of Chicago is referred to as a Chicagoan. About one-third of Chicagoans are White, another third African-American, and the rest Hispanic or from other ethnic groups. Chicago also has many dozen distinct neighborhoods to match the ethnic diversity; the city is divided into 77 official community areas.

History

Early days

During the mid 1700s, the Chicago area was inhabited primarily by Potawatomis, who took the place of the Miami and Sauk and Fox who had controlled the area previously. The name Chicago originates from "Checagou" (Chick-Ah-Goo-Ah) or "Checaguar," which in the Potawatomi language means "garlic" (not "onions" or "skunk"). The area was so named because of the smell of rotting marshland wild leeks (ramps) that once covered it. The first non-native settler in Chicago was Jean-Baptiste Pointe du Sable, a Haitian of African descent, who settled on the Chicago River in the 1770s and married a local Potawatomi woman. In 1795, following the War of the Wabash Confederacy, the area of Chicago was ceded by the Native Americans in the Treaty of Greenville to the United States for a military post. In 1803, Fort Dearborn was built and remained in use until 1837, except between 1812 and 1816 when it was destroyed in the Fort Dearborn Massacre during the War of 1812.

Incorporation and growth

War of 1812 On August 12, 1833, the Town of Chicago was incorporated with a population of 350. The first boundaries of the new town were Kinzie, Desplaines, Madison, and State streets, which included an area of about three-eighths of a square mile (1 km²). Within seven years the primarily French and Native American town had a population of over 4,000. Chicago was granted a city charter by Illinois on March 4, 1837. The opening of the Illinois and Michigan Canal in 1848 allowed shipping from the Great Lakes through Chicago to the Mississippi River and on to the Gulf of Mexico. The first rail line to Chicago, the Galena & Chicago Union Railroad, was completed the same year. These projects foreshadowed Chicago's eventual development into the transportation hub of the United States. Chicago also became home to national retailers, including Montgomery Ward and Sears, Roebuck and Company, offering catalog shopping using the city's expansive transportation connections. Sears, Roebuck and Company The geography of Chicago presented early citizens with many problems. The prairie bog nature of the area provided a fertile ground for disease-carrying insects. Early on, Chicago's population and commerce growth was stymied by lack of good transportation infrastructure. During spring, Chicago was so muddy from the high water that horses would be stuck past their legs in the street. One dirt road was so hazardous that it became known as the "Slough of Despond". Comical signs proclaiming "Fastest route to China" or "No Bottom Here" were placed to warn people of the mud. To address these transportation problems, the Board of Cook County Commissioners decided to improve two country roads toward the west and southwest. The first road crossed the "dismal Nine-mile swamp" and Des Plaines River to the west, then continued southwest to Walker's Grove, now known as Plainfield. The second road headed south, but its exact route is disputed. Early Chicago was also plagued by sewer and water problems. Many people described it as the filthiest city in America. To solve the problems, the city initiated the creation of a massive sewer system. In the first phase sewage pipes were laid across the city above-ground, with gravity moving the waste. The second phase, executed in 1855, involved raising the level of the city by four to seven feet (one to two meters); this was done by jacking up buildings and placing fill in order to raise streets above the swamp and the newly-laid sewer pipes. By 1857, Chicago was the largest city in what was then known as the Northwest. In a period of 20 years, Chicago's population grew from 4,000 to over 90,000 people. The 1860 Republican National Convention in Chicago nominated home-state candidate Abraham Lincoln for U.S. president. At the election of April 23, 1875 the voters of Chicago chose to operate under the Illi