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The Foster scanner, or Variable Path scanner, is a type of radar system that produces a narrow beam that rapidly scans an area in front of it. Foster scanners were widely used in post-World War II radar systems used for artillery and mortar spotting. ==Description== The Foster scanner consists of two parts, a box-shape antenna and the "scanner" itself. The antenna consists of a thin rectangular section of a parabolic antenna, as if one cut the sides away on a conventional design leaving only the thin strip where the feed horn is mounted. Rectangular plates are placed on either side of the "cut", extending forward in the direction of broadcast. The resulting system has the basic size and shape of a large pizza box with the rear side rounded. The feed to the antenna is via a rectangular slot in a wave-guide running the length of the box in front of the parabolic section. The scanner itself consists of a section of rectangular waveguide formed into a hollow cone, like an ice cream cone. A second metal cone is placed within the first, leaving a gap between the two that forms a section of waveguide. Small metal "teeth" at the point where the input waveguide meets the cone causes the radar signal to be reflected 90 degrees into the slot between the two cones. Used alone, this system causes the radar signal to travel a different total path length at the narrow end of the cone as opposed to the wide end. This causes the wavefront of the signal to be rotated with respect to the antenna. The inner cone is not solid, but has a slot cut through the center to form another section of waveguide. Teeth on the outside of this cone pick up the signal from the slot between the cones, reflects it through its center slot, and then a second set of teeth sends it back into the slot between the two cones. The inner cone is allowed to rotate within the outer. As the inner cone rotates, the total signal length changes, more at the wide end than the narrow. When sent to the antenna, this causes the wavefront to scan across the area between the two horizontal plates, "snapping" to the initial position when the inner cone rotates back to the original point. The scanner allows scans at the rotational speed of the inner cone, which is easily built in a fashion allowing very rapid scanning. Typical angles are +/- 20 degrees. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Foster scanner」の詳細全文を読む スポンサード リンク
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