Abstract:
A marine radar antenna has a single dielectric plate mounted in front of the waveguide polarization grid between two horn plates. A strip of dielectric material is secured to the upper and lower surfaces of the plate to form a forwardly and rearwardly facing step on each surface. The steps are located forwardly of the ends of the horn plates and are positioned to produce reflections substantially 180° out of phase with extraneous energy within the antenna. The dielectric plate is supported by a foamed plastics material within an outer radome.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to antennas. 
     The invention is more particularly concerned with radar antennas, such as for ships. 
     Conventional marine radar antennas are of bar shape and are mounted horizontally to rotate about a vertical axis. A slotted waveguide extends horizontally across the width of the antenna, the slots opening along a side of the waveguide into a horn. In order to achieve a beam with a relatively narrow width in elevation, the aperture of the horn in a vertical direction has to be relatively large. This results in an antenna having a relatively large size in the vertical direction. This is a disadvantage because it increases the wind resistance of the antenna so that it must be made relatively robust, have bearings of a heavy construction and be driven by a high power motor. 
     It has long been known that the dimensions of a radar antenna can be reduced by using a dielectric material. The dielectric has the effect of constraining the microwave energy as it emerges from the antenna and can enable the use of a lower profile antenna shape (“Gain enhancement of microwave antennas by dielectric-filled radomes”, James et al, Proc. IEE, vol 122, no 12, December 1975, pp 1353-1358). WO95/29518 describes an antenna with several plates of dielectric material extending parallel to the direction of the main energy beam. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an alternative antenna. 
     According to one aspect of the present invention there is provided an antenna including a waveguide extending along a first direction and arranged to propagate energy from a face of the guide in a second direction at right angles to the first direction, the antenna including a dielectric member of generally plate shape having an edge extending parallel to the face of the guide and having opposite surfaces facing in directions orthogonal to the first and second directions, and the dielectric member having at least one discontinuity on at least one of the surfaces arranged to scatter energy and enhance the properties of the energy radiated from the antenna. 
     The discontinuity preferably includes a step extending along the length of the dielectric member. The dielectric member may have two steps facing in opposite directions. The dielectric member may have a step on both surfaces and preferably has two steps facing in opposite directions on both surfaces. The or each discontinuity may be provided by a strip secured to each surface of the dielectric member to extend along its length. The antenna preferably has a single dielectric member, the thickness of the dielectric member being substantially less than the height of the antenna. The dielectric member is preferably of a foamed plastics material. The antenna preferably includes a polarisation grid located forwardly of the face of the waveguide, the antenna including two horn plates extending forwardly of the polarisation grid and a rear edge of the dielectric member being located between the horn plates. The or each discontinuity may be located forwardly of the horn plates. The location of the or each discontinuity is preferably selected to produce reflections that are substantially 180° out of phase with extraneous energy produced within the antenna. The location of the or each discontinuity is preferably selected to control sidelobes of a beam of the energy and to enhance peak gain. The dielectric member may be supported by an expanded foam material, which may be contained within an outer radome that extends rearwardly along the waveguide. 
     According to another aspect of the present invention there is provided a marine radar antenna including a waveguide extending along a first, horizontal direction for rotation about a vertical axis and arranged to propagate energy forwardly in a second, horizontal direction from a face of the guide at right angles to the first direction, the antenna including a dielectric member of generally plate shape having an edge extending parallel to the face of the guide and having opposite surfaces facing vertically up and down, and the dielectric member having at least one discontinuity on at least one of the surfaces arranged to scatter energy, to control sidelobes of a beam of the energy and to enhance peak gain. 
     A radar antenna for a ship, according to the present invention, will now be described, by way of example, with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional side elevation view of the antenna; and 
     FIG. 2 is a perspective view of parts of the antenna. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The antenna extends in a horizontal direction  1  and directs a beam of radiation in a second horizontal direction  2  at right angles. The antenna is supported by a mount (not shown) for rotation about a vertical axis  3  so that the radiation beam is swept in azimuth. 
     A waveguide  4  extends across the width of the antenna at its rear side. The waveguide  4  is of hollow metal construction and rectangular section. The forward-facing vertical face  5  of the waveguide  4  is slotted in the usual way so that energy is propagated from this face. Energy is supplied to one end of the waveguide  4  from a conventional source (not shown). The waveguide  4  is supported within an intermediate housing  6  of sheet metal and rectangular section having an open rear end  7  and a forward end  8  that is closed by a wall cut with parallel vertical slots  9  to form a polarisation grid  10 . The polarisation grid  10  is 94.1 mm high, is 1 mm thick and it is spaced from the slotted face  5  of the waveguide  4  by 57.4 mm. Two choke bars  11  and  12  extend along the waveguide  4  within the intermediate housing  6 . Two metal horn plates  13  and  14  attached to the upper and lower surfaces of the intermediate housing  6  project forward of the polarisation grid  10  by a distance of 77 mm. 
     The antenna also includes a single dielectric member  20  having a plate  21 , which is 13 mm thick, that is, substantially less than the height of the polarisation grid  10  and of the antenna itself. The plate  21  is of a foamed plastics, such as PVC, sold under the name Forex, and is rectangular in section, being 339 mm long, that is, in the direction  2  of beam propagation. The rear edge  22  of the plate  21  extends parallel to the waveguide  4  and the polarisation grid  10  and is spaced from the grid by 55.5 mm so that it is located between the horn plates  13  and  14 . The forward edge  23  of the plate  21  extends parallel to the rear edge  22 . Two strips  24  and  25  of the same material are bonded to the upper surface  26  and lower surface  27  respectively of the plate  21 . The strips  24  and  25  are each 6 mm thick and 71 mm wide extending across the width of the plate  21 . The strips  24  and  25  are spaced from the rear edge  22  of the plate  21  by 49.4 mm. The strips  24  and  25  each have a rear-facing vertical edge  28  and a forward-facing vertical edge  29  forming discontinuities in the surface of the dielectric member  20 . Instead of using separate strips bonded to the plate, the plate could be formed integrally with the side strips, such as by moulding or by machining. 
     The dielectric member  20  is enclosed within a radome  30 , which has an open rear end  31  sealed to the outside of the horn plates  13  and  14 , and a domed, closed forward end  32 . The radome  30  is 1 mm thick and is made of foamed PVC, such as Forex. Internally, the radome  30  has a height of 98.1 mm and is spaced from the forward edge  23  of the dielectric member  20  by 6 mm. The radome  30  provides environmental protection for the antenna on its forward-facing side; there is also some form of protective cover (not shown) along its rear-facing side. The dielectric member  20  is supported within the radome  30  by an expanded polystyrene foam material  34  filling the forward end of the radome and the space within the horn plates  13  and  14  forwardly of the polarisation grid  10 . 
     In operation, a major part of the energy propagated from the waveguide  4  is loosely confined along the dielectric member  20  in the direction of the axis  2 . Energy is also scattered from discontinuities within the antenna, such as the forward end of the horn plates  13  and  14 . This other, extraneous, energy adversely affects the transmitted beam. The positioning of the discontinuities introduced by the steps  28  and  29  is selected to enhance the properties of the transmitted beam by producing reflections that are approximately 180° out of phase with this extraneous energy. It has been found that these discontinuities  28  and  29  can be used to control the sidelobes of the beam and to enhance the peak gain. The material  34  filling the radome  30  and the material of the radome itself do not have any appreciable effect on the transmitted beam. 
     The antenna of the present invention has a relatively small profile with a height of just over 100 mm but can produce a beam with characteristics similar to that of a conventional antenna having a height of around 300 mm. The reduced height reduces wind resistance of the antenna and reduces loading on the antenna bearings and the motor drive. 
     The strips  24  and  25  introduce two discontinuities on each side of the plate  21  but in other arrangements it may only be necessary to have one discontinuity and this may be provided on one side only. A single discontinuity could be provided by a strip that tapers across its width so that it produces a step along one edge and merges smoothly with the surface of the plate on the other edge. Discontinuities could be produced in other ways such as by narrow ribs or by slots or other indentations in the plate. The plate need not have a constant thickness along its length but could, for example, taper to a reduced thickness away from the waveguide. It will be appreciated that the dimensions given above are for a particular construction and are for an antenna operating in the S-Band at 3.05 GHz. The dimensions for different constructions and different frequency antenna can readily be determined by scaling the dimensions in proportion to the frequency and by further experimentation.