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{{Short description, Type of radar system The Foster scanner, or Variable Path scanner, is a type of
radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
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. Modern radars in this role normally use electronic scanning in place of a Foster scanner for this purpose.


Description

The Foster scanner consists of two parts; a box-shaped antenna and the "scanner" itself. These are normally placed in front of a shaped reflector. The antenna consists of a thin rectangular section of a
parabolic antenna A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or pa ...
, as if one cut the sides away from a conventional parabolic antenna, leaving only the thin strip where the
feed horn A feed horn (or feedhorn) is a small horn antenna used to couple a waveguide to e.g. a parabolic dish antenna or offset dish antenna for reception or transmission of microwave. A typical application is the use for satellite television recept ...
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 The pizza box or pizza package is a folding packaging box made of cardboard in which hot pizzas are stored for take-out. The "pizza box" also makes home delivery and takeaway substantially easier. The pizza box has to be highly resistant, c ...
with the rear side rounded. In the UK, these are known as "cheese" antennas as they resemble a section cut from a wheel of cheese. The feed to the antenna is via a rectangular slot in a waveguide 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 An ice cream cone, poke (Ireland/Scotland) or cornet (England) is a brittle, cone-shaped pastry, usually made of a wafer similar in texture to a waffle, made so ice cream can be carried and eaten without a bowl or spoon, for example, the Hong Kon ...
. 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.


Typical use

The Foster scanner was used in a number of post-
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
counterbattery radar A counter-battery radar (alternatively weapon tracking radar or COBRA) is a radar system that detects artillery projectiles fired by one or more guns, howitzers, mortars or rocket launchers and, from their trajectories, locates the position on ...
systems, the first examples reaching service in the late 1950s and early 1960s. In these systems the scanner was mounted horizontally, in order to produce a signal that scanned back and forth horizontally in front of the radar set. Typical horizontal scanning ranges were between 40 and 50 degrees, with a 1 degree beamwidth. In typical systems, the scanner would be mounted so that it could be quickly pointed at two different vertical angles. This could be accomplished by moving the main antenna reflector behind the scanner, or through a variety of electronic means. The system would rapidly switch the scanner between these two angles, while the scanner itself continued to deflect the signal left and right. The location of the shell horizontally was revealed by the timing of the returned signal compared to the rotation of the scanner at that instant. Using traditional pulse timing methods the range of the shell could be determined at the same instant. Simple trigonometry using the range and the known fixed vertical angle of the beam revealed the shell's vertical location. The time it took to cross one beam and then the other revealed the shell's speed. Early systems included the US
AN/MPQ-4 The Radar Set AN/MPQ-4 was a US Army counter-battery radar primarily used to find the location of enemy mortars and larger artillery in a secondary role. Built by General Electric, it first entered service in 1958, replacing the earlier and much ...
, which used
lobe switching Lobe switching is a method used on early radar sets to improve tracking accuracy. It uses two slightly separated antenna elements to send the beam slightly to either side of the midline of the antenna. The radar signal switched between the two an ...
to switch between two angles about 35 mils apart (about 2 degrees). Reflected signals were displayed on two
B-scope A radar display is an electronic device to present radar data to the operator. The radar system transmits pulses or continuous waves of electromagnetic radiation, a small portion of which backscatter off targets (intended or otherwise) and retur ...
s. Measurements from the B-scopes were copied into a
ballistic calculator A ballistic table or ballistic chart is a tool which predicts the trajectory of a projectile, and is used to compensate for physical effects in order to increase the probability of the projectile reaching the intended target. Ballistic tables are ...
, which produced the X and Y location of the launcher. The UK Green Archer radar used a manual switch to flip between the vertical angles. The radar was normally left in the "lower" angle setting, while the operator waited for returns. When one was seen the switch was moved to the upper position, and the second return plotted. Ballistic calculation was performed by plotting the two points on an X-Y chart paper, and then moving mechanical cursors over the points. The rest of the calculation was carried out electronically.


History

The Foster scanner was originally designed by
John Stuart Foster John Stuart Foster (May 30, 1890 – September 9, 1964) was a Canadian physicist. Biography Born in Clarence, Nova Scotia, he completed his Ph.D. at Yale University with a dissertation on the first measurements of the Stark effect in Helium ...
while working at the
MIT Radiation Laboratory The Radiation Laboratory, commonly called the Rad Lab, was a microwave and radar research laboratory located at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts. It was first created in October 1940 and operated until 31 ...
as part of a technical exchange from the Canadian
National Research Council National Research Council may refer to: * National Research Council (Canada), sponsoring research and development * National Research Council (Italy), scientific and technological research, Rome * National Research Council (United States), part of ...
. It was quickly adapted for artillery spotting use, particularly for mortars. In this role the scanner would be used to make two or more rapid observations of the shell in flight, and then plotted to calculate the initial position.


References

* Edward George Bowen
"A Textbook of Radar"
CUP Archive, 1954, pp. 287-289


Further reading

* J. S. Foster
"Linear Electrical Scanner"
Radiation Laboratory, Massachusetts Institute of Technology, 1945
"Operator's Manual for Radar Set AN/MPQ-4A"
US Army, 1977 Radar Radar antennas