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# What Is a Fan Beam?

Article Details
• Written By: M.J. Casey
• Edited By: Daniel Lindley
2003-2019
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Fan beam describes a pattern of matter or energy emitted from a transmitter, particularly a radio wave transmitter. The shape of the beam from various medical imaging instruments may determine the clarity and resolutions of the images. Fan beams, applied in radio astronomy, are more effective than tight pencil beams in scanning deep space for radio signals.

The cross-sectional shape of a fan beam is an ellipse. An ellipse, or oval, has a major axis and a minor axis. Both axes cut the oval in two equal halves. In a fan beam, the major axis is at least three times longer than the minor axis, resulting in an oval that is quite squashed. If the beam could be seen from the side, it would look like a fan with rays pointing from the origin and spreading out in a radial direction.

Paint sprayers often have a fan beam attachment to create a wide beam with narrow height. Lawn sprinklers are also configured in this shape. Nozzles with this shape create a wide beam, making uniform application more likely. The distance between the nozzle and the object determines the width of the beam and the density of the applied material.

Medical imaging and radio astronomy applications prefer the fan beam transmitter. It is effective because more data are accumulated in a single pass, yet the data density is uniform. This density is critical when scanning space, as variations in density could be misinterpreted as meaningful radio signals. The data returned from the scans must be manipulated to create useful images. Mathematicians have developed a mathematical routine, the fan beam function, which accounts for the geometry of the beam.

Computer tomography (CT), a medical imaging device, demonstrates the complexity of these calculations. In this machine, X-rays bounce off the tissues of the body area under examination and are collected by detectors. The machine circles the patient, taking over a thousand cross-sectional images. A computer manipulates the data and recreates a two-dimensional image or a three-dimension model of the scanned area.

The wireless industry is also investigating fan beam technology. In the range of radio frequencies assigned to wireless local communications, 60 gigahertz (60 GHz), radio waves diffract or bend around obstructions poorly. A person standing between the transmitter and a person using a laptop causes a substantial loss in the signal received by the computer. Fan beam antennas significantly reduce this problem.