radio stations

A radio station is an audio (sound) broadcasting service, traditionally broadcast through the air as radio waves (a form of electromagnetic radiation) from a transmitter to an antenna and a thus to a receiving device. Stations can be linked in radio networks to broadcast common programming, either in syndication or simulcast or both. Today, stations also broadcast via cable FM, local wire networks, satellite and the Internet.

History

see also main article History of radio

The earliest radio stations were simply radio telegraph systems and did not carry audio. The first claimed audio transmission that could be termed a broadcast occurred on Christmas Eve in 1906, and was made by Reginald Fessenden. While many early experimenters attempted to create systems similar to radiotelephone devices where only two parties were meant to communicate, there were others who intended to transmit to larger audiences. Charles Herrold started broadcasting in California in 1909 and was carrying audio by the next year.

For the next decade, radio tinkerers had to build their own radio receivers. KDKA AM of Pittsburgh, Pennsylvania (owned by Westinghouse) started broadcasting as the first licensed "commercial" radio station on November 2, 1920. The commercial designation came from the type of license—they didn't start airing advertisements until a few years later. The first broadcast was the results of the U.S. presidential election, 1920. The Montreal station that became CFCF-AM began program broadcasts on May 20, 1920, and the Detroit station that became WWJ began program broadcasts beginning on August 20, 1920, although neither held a license at the time.

Radio Argentina began regularly scheduled transmissions from the Teatro Coliseo in Buenos Aires on August 27, 1920, making its own priority claim. The station got its license on November 19, 1923. The delay was due to the lack of official Argentine licensing procedures before that date. This station continued regular broadcasting of entertainment and cultural fare for several decades. [1][2]

Types

Radio stations are of several types. The best known are the AM and FM stations; these include both commercial, public and nonprofit varieties as well as student-run campus radio stations found throughout the developed world.

Although now being eclipsed by internet-distributed radio, there are many stations that broadcast on shortwave bands using AM technology that can be received over thousands of miles (especially at night). For example, the BBC has a full schedule transmitted via shortwave. These broadcasts are very sensitive to atmospheric conditions and sunspots.

AM

AM stations were the earliest broadcasting stations to be developed. AM refers to amplitude modulation, a mode of broadcasting radio waves by varying the amplitude of the carrier signal in response to the amplitude of the signal to be transmitted.

One of the advantages of AM is that its unsophisticated signal can be detected (turned into sound) with simple equipment. If a signal is strong enough, not even a power source is needed; building an unpowered crystal radio receiver was a common childhood project in the early years of radio.

AM broadcasts occur on North American airwaves in the mediumwave frequency range of 530 to 1700 kHz (known as the "standard broadcast band"). The band was expanded in the 1990s by adding nine channels from 1620 to 1700 kHz. Channels are spaced every 10 kHz in the Americas, and generally every 9 kHz everywhere else.

Many countries outside of the U.S. use a similar frequency band for AM transmissions. Europe also uses the longwave band. In response to the growing popularity of FM radio stations in the late 1980s and early 1990s, some North American stations began broadcasting in AM stereo, though this never really gained acceptance.

AM radio has some serious shortcomings.

• The signal is subject to interference from electrical storms (lightning) and other EMI.
• Fading of the signal can be severe at night.

AM signals exhibit diurnal variation, travelling much longer distances at night. In a crowded channel environment this means that the power of regional channels which share a frequency must be reduced at night or directionally beamed in order to avoid interference, which reduces the potential nighttime audience. Some stations have frequencies unshared with other stations in the U.S.; these are called clear channel stations. Many of them can be heard across much of the country at night. (This is not to be confused with Clear Channel Communications, which currently owns many U.S. radio stations.)

• AM radio transmitters transmit audio frequencies up to 15 kHz, but most receivers are only capable of reproducing frequencies up to 5 kHz. At the time that AM broadcasting began in the 1920s, this provided adequate fidelity for existing microphones, 78 rpm recordings, and loudspeakers. The fidelity of sound equipment subsequently improved considerably but the receivers did not. Reducing the bandwidth of the receivers reduces the cost of manufacturing and makes them less prone to interference. In the United States, AM stations are never assigned adjacent channels in the same service area. This prevents the sideband energy generated by two stations from interfereing with each other.Bob Carver created an AM tuner employing notch filtering that demonstrated an AM broadcast can exceed the 15 kHz bandwidth of FM stations without objectionable interference. His expensive tuner was never widely adopted, and became obsolete with the advent of HD Radio.

FM

FM refers to frequency modulation, and occurs on VHF airwaves everywhere (except Japan) in the frequency range of 88 to 108 MHz. Japan uses the 76 to 90 MHz band. FM stations are much more popular in economically developed regions, such as Europe and the United States, especially since higher sound fidelity and stereo broadcasting became common in this format.

FM radio was invented by Edwin H. Armstrong in the 1930s for the specific purpose of overcoming the interference (static) problem of AM radio, to which it is immune. At the same time, greater fidelity was made possible by spacing stations further apart. Instead of 10 kHz apart, they are 200 kHz apart—the difference between the lowest current FM frequency in the U.S., 88.1 MHz and the next lowest, 88.3 MHz. This was far in advance of the audio equipment of the 1940s, but wide interchannel spacing was chosen to reduce interference problems that existed with AM.

In fact 200 kHz is not needed to accommodate an audio signal — 20 kHz to 30 kHz is all that is necessary for a narrowband FM signal. The 200 kHz bandwidth allowed room for ±75 kHz signal deviation from the assigned frequency plus a 50 kHz guardband to eliminate adjacent channel interference. The larger bandwidth allows for broadcasting a 15 kHz bandwidth audio signal plus a 38 kHz stereo "subcarrier" — a piggyback signal that rides on the main signal. Additional unused capacity is used by some broadcasters to transmit utility functions such as background music for public areas, GPS auxiliary signals, or financial market data.

The AM radio problem of interference at night was addressed in a different way. At the time FM was set up, the only available frequencies were far higher in the spectrum than those used for AM radio. Using these frequencies meant that even at far higher power, the range of a given FM signal was much lower, thus its market was more local than for AM radio. Reception range at night was the same as daytime, and while the problem of interference between stations has not disappeared, it is far less.

The original FM radio service in the U.S. was the Yankee Network, located in New England. See [3] [4] [5]. Broadcasting began in the early 1940s but did not pose a significant threat to the AM broadcasting industry. It required purchase of a special receiver. The frequencies used were not those used today: 42 to 50 megahertz. The change to the current frequencies, 88 to 108 megahertz, began at the end of World War II and was to some extent imposed by AM radio owners so as to cripple what was by now realized to be a potentially serious threat.

FM radio on the new band had to begin from step one. As a commercial venture it remained a little used audio enthusiast's medium until the 1960s. The more prosperous AM stations, or their owners, acquired FM licenses and often broadcast the same programming on the FM station as on the AM station (simulcasting). The FCC limited this practice in the 1970s. By the 1980s, since almost all new radios included both AM and FM tuners (without any government mandate), FM became the dominant medium, especially in cities. Because of its greater range, AM remained more common in rural environments.

Digital

Digital radio broadcasting has emerged, first in Europe (the UK in 1995 and Germany in 1999), and later in the United States. The European system is named DAB, for Digital Audio Broadcasting, and uses the public domain EUREKA 147 system. In the United States, the IBOC system is named HD Radio and owned by a consortium of private companies called iBiquity. An international non-profit consortium Digital Radio Mondiale (DRM), has introduced the public domain DRM system.

It is expected that for the next 10 to 20 years, all these systems will co-exist, while by 2015 to 2020 digital radio may predominate, at least in the developed countries.

Satellite

Satellite radio broadcasters are slowly emerging, but the enormous entry costs of space-based satellite transmitters, and restrictions on available radio spectrum licenses has restricted growth of this market. In the USA and Canada, just two services, XM Satellite Radio and Sirius Satellite Radio exist.

Other

Many other non-broadcast types of radio stations exist. These include:

• base stations for police, fire and ambulance networks
• military base stations
• dispatch base stations for taxis, trucks, and couriers
• emergency broadcast systems
• amateur radio stations

Program formats

For a list of common formats, see the main article Radio format

Radio program formats differ by country, regulation and markets. For instance, the U.S. Federal Communications Commission designates the 88–92 megahertz band in the U.S. for non-profit or educational programming, with advertising prohibited.

In addition, formats change in popularity as time passes and technology improves. Early radio equipment only allowed program material to be broadcast in real time, known as live broadcasting. As technology for sound recording improved, an increasing proportion of broadcast programming used pre-recorded material. A current trend is the automation of radio stations. Some stations now operate without direct human intervention by using entirely pre-recorded material sequenced by computer control (see Voice-tracking).

See also

• History of broadcasting
• History of radio
• Television station
• List of radio stations
• International broadcasting
• Callsign (radio)
• Low power radio station
• Radio format
• Construction permit

External articles

Patents

• U.S. Patent 1082221,Georg Graf Von Arco, "Radiotelegraphic station" (December 1913)
• U.S. Patent 1116111, Richard Pfund, "Station for the transmission and reception of electromagnetic wave energy". (November 1914)
• U.S. Patent 1214591, Gustav Reuthe, "Antenna for radiotelegraph station" (February 1917)

General

• Federal Communications Commission website
• DXing.info - Information about radio stations worldwide
• Radiomusications from Transdiffusion
• The MIT radio-locator
• BBC reception advice
• Dominica's Radio Stations
• SWDXER ¨The SWDXER¨ - with general SWL information and radio antenna tips.
• Online radio stations