The range of radio waves is approximately 3 Hz, meaning three wavelengths per second, to 300 GHz, or 300,000,000,000 Hz, although the boundary isn't clearly defined, and waves with frequencies even smaller than these exist. This range is equivalent to wavelengths from 1 millimeter (.001 meter) to 100,000 kilometers (100,000,000 meters).
Radio waves are both man-made, and occur from natural sources, and have many uses, due to the fact that most of the radio spectrum pass through Earth's atmosphere intact. The percentage of waves that pass through the atmosphere are shown on the picture below, along with the rest of the spectrum.
Since radio waves can pass through the atmosphere, they are invaluable for communication purposes and information propagation takes place mainly in the radio spectrum. This portion of the spectrum is further divided into powers of 10. For example, the area from 3-30 Hz is known as Extremely Low Frequency (ELF), the area from 30-300 Hz is known as Super Low Frequency (SLF), the area from 300-3,000 Hz (3 kHz) is known as Ultra Low Frequency (ULF) and so on. The chart of frequencies above ULF are shown below.
The first radio waves to be used for communication are AM radio waves, which extend from 148.5 kHz (148,500 Hz) to 30 MHz (30,000,000 Hz). These waves are in the Low, Medium, and High Frequency ranges. However, waves of these frequencies are produced naturally by the ionosphere and magnetic storms, causing interference. As a result, FM radio, falling entirely under the Very High Frequency range, is now used for long distance broad casts. This area of the spectrum is the busiest of all, as television, FM radio and mobile communications all resides within 30-300 MHz.
The very low frequencies, (VLF and lower) are used for military (submarine) communications as they are useful in subaquatic environments.
The higher frequencies also have uses. Super High Frequency (3-30 GHz, or 3,000,000,000-30,000,000,000 Hz) is used for radar, and Extremely High Frequency (30-300 GHz) is used for radio astronomy.
The Very Large Array, located in New Mexico, USA, is used for radio astronomy. Large, parabolic satellite dishes concentrate the signals on a specific sources in the center of each individual telescope. Since many radio waves picked up from space have very long wavelengths, a very large apparatus is needed to capture a reasonable image. The Very Large Area has 27 radio telescope receivers, each 82 feet in diameter, and the images produced by each are compiled into a single one in the center.
This is an optical (visible light) image of the M87 elliptical galaxy. The active center and jet of radiation from it are visible, but in visible light, most of the galaxy is a haze of stars.
However, by use of radio astronomy, a much more specific picture can be taken. Unlike visible light, radio waves are not produced to a great extent by any part of the galaxy except the active center, and it can therefore be identified better in radio astronomy. The above image is taken by the Very Large Array (VLA). However, the bottom image, which is magnified tens of thousands of times more than the above one, is taken by the Very Long Baseline Array (VLBA). Since M87 is over 50 million light years away, a resolution of this magnitude can only be created by use of a planet sized radio telescope, which is essentially what VLBA is. Satellite dishes all over the world are synced together by a central computer in Europe, and therefore at least a small number of radio waves coming in over a Earth-sized area can be put together to get a very precise view, even if all the incoming waves aren't picked up.
Notable sources of radio waves from space include the Sun (only due to its proximity to Earth, because stars do not emit a majority of their energy in radio waves), galactic centers, such as the Milky Way itself, and active ones like M87, neutron stars, and very distant powerfully emitted objects such as quasars. The redshift effect of the quasars' movement away from us causes the waves to lengthen in wavelength as they approach Earth. As a result, many waves that start higher in frequency are radio waves when they reach Earth. This effect is most notable at very long distances, billions of light years away. Among objects at this distance, only quasars are bright enough to be visible, and are therefore excellent subjects of radio astronomy.
Sources: http://www.google.com/imgres?imgurl=http://www.electronics-radio.com/, http://en.wikipedia.org/wiki/Radio_astronomy, etc.
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