in diameter. Telescopes working at wavelengths shorter than 30 cm (above 1 GHz) range in size from 3 to 90 meters in diameter. Frequencies The increasing use of radio frequencies for communication makes astronomical observations more and more difficult (see Open spectrum). Negotiations to defend the frequency allocation for parts of the spectrum most useful for observing the universe are coordinated in the Scientific Committee on Frequency Allocations for Radio Astronomy and Space Science. Some of the more notable frequency bands used by radio telescopes include: Every frequency in the United States National Radio Quiet Zone Channel 37: 608 to 614 MHz The "Hydrogen line", also known as the "21 centimeter line": 1,420.40575177 MHz, used by many radio telescopes including The Big Ear in its discovery of the Wow! signal 1,406 MHz and 430 MHz The Waterhole: 1,420 to 1,666 MHz The Arecibo Observatory had several receivers that together covered the whole 1–10 GHz range. The Wilkinson Microwave Anisotropy Probe mapped the Cosmic microwave background radiation in 5 different frequency bands, centered on 23 GHz, 33 GHz, 41 GHz, 61 GHz, and 94 GHz. Big dishes The world's largest filled-aperture (i.e. full dish) radio telescope is the Five-hundred-meter Aperture Spherical Telescope (FAST) completed in 2016 by China. The 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 4,450 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. Although the dish is 500 meters in diameter, only a 300-meter circular area on the dish is illuminated by the feed antenna at any given time, so the actual effective aperture is 300 meters. 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 was the Arecibo radio telescope located in Arecibo, Puerto Rico, though it suffered catastrophic collapse on 1 December 2020. Arecibo was one of the world's few radio telescope also capable of active (i.e., transmitting) radar imaging of near-Earth objects (see: radar astronomy); most other telescopes employ passive detection, i.e.,