Radio frequency (RF) is any of the electromagnetic wave frequencies that lie in the range extending from around 3 kHz to 300 GHz, which include those frequencies used for communications or radar signals. RF usually refers to electrical rather than mechanical oscillations. However, mechanical RF systems do exist (see mechanical filter and RF MEMS).
Although radio frequency is a rate of oscillation, the term “radio frequency” or its abbreviation “RF” are used as a synonym for radio – i.e., to describe the use of wireless communication, as opposed to communication via electric wires.
Radio waves have the longest wavelengths in the electromagnetic spectrum. These waves can be longer than a football field or as short as a football. Radio waves do more than just bring music to your radio. They also carry signals for your television and cellular phones.
How do we “see” using Radio Waves?
Objects in space, such as planets and comets, giant clouds of gas and dust, and stars and galaxies, emit light at many different wavelengths. Some of the light they emit has very large wavelengths – sometimes as long as a mile!. These long waves are in the radio region of the electromagnetic spectrum.
Because radio waves are larger than optical waves, radio telescopes work differently than telescopes that we use for visible > light (optical telescopes). Radio telescopes are dishes made out of conducting metal that reflect radio waves to a focus point. Because the wavelengths of radio light are so large, a radio telescope must be physically larger than an optical telescope to be able to make images of comparable clarity. For example, the Parkes radio telescope, which has a dish 64 meters wide, cannot give us any clearer an image than a small backyard telescope!
In order to make better and more clear (or higher resolution) radio images, radio astronomers often combine several smaller telescopes, or receiving dishes, into an array. Together, the dishes can act as one large telescope whose size equals the total area occupied by the array.
The Very Large Array (VLA) is one of the world’s premier astronomical radio observatories. The VLA consists of 27 antennas arranged in a huge “Y” pattern up to 36 km (22 miles) across — roughly one and a half times the size of Washington, DC.
The VLA, located in New Mexico, is an interferometer; this means that it operates by multiplying the data from each pair of telescopes together to form interference patterns. The structure of those interference patterns, and how they change with time as the earth rotates, reflect the structure of radio sources in the sky.
The Very Large Array, one of the world’s premier astronomical radio observatories, consists of 27 radio antennas in a Y-shaped configuration on the Plains of San Agustin fifty miles west of Socorro, New Mexico. Each antenna is 25 meters (82 feet) in diameter. The data from the antennas is combined electronically to give the resolution of an antenna 36km (22 miles) across, with the sensitivity of a dish 130 meters (422 feet) in diameter. http://www.vla.nrao.edu/
A radio telescope is a specialized antenna and radio receiver used to receive radio waves from astronomical radio sources in the sky in radio astronomy. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency portion of the electromagnetic spectrum emitted by astronomical objects, just as optical telescopes are the main observing instrument used in traditional optical astronomy which studies the light wave portion of the spectrum coming from astronomical objects.
Radio telescopes are typically large parabolic (“dish”) antennas similar to those employed in tracking and communicating with satellites and space probes. They may be used singly, or linked together electronically in an array. Unlike optical telescopes, radio telescopes can be used in the daytime as well as at night. Since astronomical radio sources such as stars, nebulas and galaxies are very far away, the radio waves coming from them are extremely weak, so radio telescopes require very large antennas to collect enough radio energy to study them, and extremely sensitive receiving equipment. Radio observatories are preferentially located far from major centers of population to avoid electromagnetic interference (EMI) from radio, television, radar, motor vehicles, and other EMI emitting devices.
The range of frequencies in the electromagnetic spectrum that makes up the radio spectrum is very large. This means that the types of antennas that are used as radio telescopes vary widely in design, size, and configuration. At wavelengths of 30 meters to 3 meters (10 MHz – 100 MHz), they are generally either directional antenna arrays similar to “TV antennas” or large stationary reflectors with moveable focal points. Since the wavelengths being observed with these types of antennas are so long, the “reflector” surfaces can be constructed from coarse wire mesh such as chicken wire. At shorter wavelengths parabolic “dish” antennas predominate. The angular resolution of a dish antenna is determined by the ratio of the diameter of the dish to the wavelength of the radio waves being observed. This dictates the dish size a radio telescope needs for a useful resolution. Radio telescopes that operate at wavelengths of 3 meters to 30 cm (100 MHz to 1 GHz) are usually well over 100 meters in diameter. Telescopes working at wavelengths shorter than 30 cm (above 1 GHz) range in size from 3 to 90 meters in diameter.
World’s second-largest [above] radio-telescope “dish” and second-largest single-aperture radio telescope at Arecibo Observatory in Puerto Rico
The Green Bank Radio Telescope, at Green Bank, West Virginia, USA, the largest fully steerable radio telescope dish in the world.
The world’s largest filled-aperture (i.e. full dish) radio telescope is the recently completed Five hundred meter Aperture Spherical Telescope (FAST) built by China. The 500-meter-diameter (1,600 ft) dish with an area as large as 30 football fields is built into a natural Karst depression in the landscape in Guizhou province and cannot move; the feed antenna is in a cabin suspended above the dish on cables. The active dish is composed of 4450 moveable panels controlled by a computer. By changing the shape of the dish and moving the feed cabin on its cables, the telescope can be steered to point to any region of the sky up to 40° from the zenith. Construction was begun in 2007 and completed July 2016 and the telescope became operational September 25, 2016.
The world’s second largest filled-aperture telescope is the Arecibo radio telescope located in Arecibo, Puerto Rico. Another stationary dish telescope like FAST, whose 305 m (1,001 ft) dish is built into a natural depression in the landscape, the antenna is steerable within an angle of about 20° of the zenith by moving the suspended feed antenna. The largest individual radio telescope of any kind is the RATAN-600 located near Nizhny Arkhyz, Russia, which consists of a 576-meter circle of rectangular radio reflectors, each of which can be pointed towards a central conical receiver.
The above stationary dishes are not fully “steerable”; they can only be aimed at points in an area of the sky near the zenith, and cannot receive from sources near the horizon. The largest fully steerable dish radio telescope is the 100 meter Green Bank Telescope in West Virginia, United States, constructed in 2000. The largest fully steerable radio telescope in Europe is the Effelsberg 100-m Radio Telescope near Bonn, Germany, operated by the Max Planck Institute for Radio Astronomy, which also was the world’s largest fully steerable telescope for 30 years until the Green Bank antenna was constructed. The third-largest fully steerable radio telescope is the 76-meter Lovell Telescope at Jodrell Bank Observatory in Cheshire, England, completed in 1957. The fourth-largest fully steerable radio telescopes are six 70-meter dishes: three Russian RT-70, and three in the Goldstone network.
A typical size of the single antenna of a radio telescope is 25 meters. Dozens of radio telescopes with comparable sizes are operated in radio observatories all over the world.
Radiotelescopes in space
Since 1965, humans have launched three space-based radio telescopes. In 1965, the Soviet Union sent the first one called Zond 3. In 1997, Japan sent the second, HALCA. The last one was sent by Russia in 2011 called Spektr-R.
Main article: Astronomical interferometer
See also: Radio astronomy § Radio interferometry
Detail view of an antenna in ALMA, Chile.
The Very Large Array, an interferometric array formed from many smaller telescopes, like many larger radio telescopes.
One of the most notable developments came in 1946 with the introduction of the technique called astronomical interferometry. Astronomical radio interferometers usually consist either of arrays of parabolic dishes (e.g., the One-Mile Telescope), arrays of one-dimensional antennas (e.g., the Molonglo Observatory Synthesis Telescope) or two-dimensional arrays of omnidirectional dipoles (e.g., Tony Hewish’s Pulsar Array). All of the telescopes in the array are widely separated and are usually connected using coaxial cable, waveguide, optical fiber, or other type of transmission line.
Recent advances in the stability of electronic oscillators also now permit interferometry to be carried out by independent recording of the signals at the various antennas, and then later correlating the recordings at some central processing facility. This process is known as Very Long Baseline Interferometry (VLBI). Interferometry does increase the total signal collected, but its primary purpose is to vastly increase the resolution through a process called Aperture synthesis. This technique works by superposing (interfering) the signal waves from the different telescopes on the principle that waves that coincide with the same phase will add to each other while two waves that have opposite phases will cancel each other out. This creates a combined telescope that is equivalent in resolution (though not in sensitivity) to a single antenna whose diameter is equal to the spacing of the antennas furthest apart in the array.
Atacama Large Millimeter Array [below] in the Atacama desert consisting of 66 12-metre (39 ft), and 7-metre (23 ft) diameter radio telescopes designed to work at sub-millimeter wavelengths
A high quality image requires a large number of different separations between telescopes. Projected separation between any two telescopes, as seen from the radio source, is called a baseline. For example, the Very Large Array (VLA) near Socorro, New Mexico has 27 telescopes with 351 independent baselines at once, which achieves a resolution of 0.2 arc seconds at 3 cm wavelengths. Martin Ryle’s group in Cambridge obtained a Nobel Prize for interferometry and aperture synthesis. The Lloyd’s mirror interferometer was also developed independently in 1946 by Joseph Pawsey’s group at the University of Sydney. In the early 1950s, the Cambridge Interferometer mapped the radio sky to produce the famous 2C and 3C surveys of radio sources. An example of a large physically connected radio telescope array is the Giant Metrewave Radio Telescope, located in Pune, India.
The largest array, LOFAR (the ‘LOw Frequency ARray’), is currently being constructed in western Europe, consisting of about 20,000 small antennas in 48 stations distributed over an area several hundreds of kilometers in diameter, and operates between 1.25 and 30 m wavelengths. VLBI systems using post-observation processing have been constructed with antennas thousands of miles apart. Radio interferometers have also been used to obtain detailed images of the anisotropies and the polarization of the Cosmic Microwave Background, like the CBI interferometer in 2004.
The world’s largest physically connected telescopes, the SKA (Square Kilometre Array), are planned to start operation in 2024.