A REFLECTOR FOR ELECTROMAGNETIC ENERGY

Abstract: A lightweight radar reflector comprising two converging lenses (21 and 22) with a reflecting surface (26) applied to the outer convex surface of one of the lenses. The lenses are preferably meniscus lenses provided with peripheral mating flanges (24 and 25) for assembly. In one form the lenses are moulded from a mixture of silica powder and polyester resin to give a dielectric constant of 3.414 for each lens. In one arrangement there may be provided means to allow the lenses to be set to a predetermined separation so that the radar reflectance of the combination can be adjusted. In a further form the lenses may comprise thin shells of polycarbonate filled with silica powder to produce the desired dielectric constant.

Description: A Reflector for Electromagnetic Energy The invention relates to reflectors for electromagnetic radiation and in particular, though to exclusively, to radar reflectors for enhancing the radar cross-section of an object. GB Patent Application No 2194391A describes a reflector comprising a spherical lens, having adielectricconstant of substantiallyequal to3.414 with a reflective coating formed over apart of the spherical surface of the lens. Electromagnetic radiation, eg radar, is focussedbythelensontothe reflector and then reflected back towards the radar source. When suitably designed with the reflector covering about one half of the lens a highly uniform radar cross section covering substantiallyahemisphere of anglesof incidenceresulted. Thismeant that twolenses, back-to-back, couldprovide a substantiallyuniform radar cross-section, independentofthedirectionof incidence of the radiation. Such reflectors provide a simpler and cheaper alternativetoLuneberglensesandtheiruniformresponsemakesthemsuitable foruse, forexample, ontopofyachtmaststoprovidesuitablylargeechoeson ships' radar scanners to mimimise the liklihood of collisions. The invention provided a material with the correct dielectric constant and low loss transmission characteristics. However theweight of the reflector is a critical factor in various applications and a compromise was needed between maximising the radar cross-section and mimi iεing the weight. Theobjectofthepresent invention istoprovidean improvedreflectorhaving a lightweight structure compared with the prior art arrangementr. The invention provides a reflector for electromagnetic radiation having: a first input converging lens; a secondconverging lenscoaxial withsaidfirstlensandhavingaconvex rear surface; and a reflective coating applied to the convex surface of the second lens; the lenses being arranged such that electromagnetic energy fran a source incident on the first lens is refracted onto the second lens then reflected from the reflective coating back towards the source of the energy. In a particularly advantageous arrangement for use as a radar reflector the lenses have a dielectric constant of 1.414. Advantageously the lenses aremoulded fromsilica flour in a polyester resin binder. In a convenient arrangement the lenses are meniscus lenses, the secondlensbeingprovidedaround itsperipherywithanannular flangeandthe first lensbeing generallyhemispherical andprovidedwith a rebatedportion around the periphery thereof for matingwith a complementary portion of the periphery of the flange. The reflecting surface ispreferably a zinc spray coating. The angular response of the reflector may be adjusted by providing means to adjust the separation of the two lenses. The invention will now be described byway of example onlywith reference to the accompanying Drawings of which: Figure 1 is a schematic view of a reflector according to the invention, shewing various radar ray tracings; and Figures 2a and 2b are sectional and planviewsof one reflector arrangement. As can be seen in Figure 1 a passive radar target comprises a first hemisphericalmeniscus lens10 which focusesmicrowaveenergy11-13 towards a secondlens14. Microwave energy incident on the second lens14 isfocused on the rear convex surface 15 of the second lens which is coatedwith a zinc spray radar reflective coating. The twolensesaremadefromsilicaflour in a polyester resin binder. The rear convex surface 15 of the lens 14 is part spherical while the front surface 16 is a convex axially .symmetric surfacewhich is flattenednear the lensaxis17. Thedielectricconstantofthesilicaflour/polyester resinis close to 3.414, which was described inGB Patent Application No 2194391Aas the optijnumvalue for a radar reflector usinga single solid lens/reflector. The spacing, dimensions and surface curvatures of the lens are design variables selected for the desired radar cross-section and polar response (including monostatic or bistatic operation). The incident radar beams 11, 12 and 13 illustrate computer generated ray tracings for angles of incidence of respectively Oo, 300 and 6O0 and the respective reflected beams are represented by the references 18, 19 and 20. Figure 2 shows a practical arrangement of the radar reflector in which the first and second lenses 21 and 22 are both moulded from silca flour in polyester resin. The second lens 22 is formed with an integral annular flange 23 which serves to provide a means to secure the two lenses together with correct spacing therebetween. She outer peripheries of the first lens 22 and flange 23 are provided with complementary rebated surfaces24, 25 for correct assembly of the two lenses. Ihe part spherical outer surface26 of the second lens 22 is coated by means of a zinc spray. In one arrangement having the following approximate dimensions: diameter of first hemispherical lens = 19 cm diameter of first lens = 13 cm the measured radar cross-sections were: X band : 4.1m2 J band : 7.3m2 Comparedwith theprior art solidlensarrangement, thepresent inventionhas a considerable advantage in reduced weight for the same radar cross-section perforamce. In addition themouldings are considerably easier tomakesince a large single sphere of 19 cm diameter, for example, would produce considerable exothermic heat on curing which would lead to cracking and consequently increase the energy loss in the lens. In one arrangement an adjustment means has been provided so that the two separate lens portions shown in Figure 2 can be set to an adjustable saeparationwithinpredetermined1imits. Bythismeanstheangular response of the reflector can be adjusted in dependence on the selected application.

Claims: £l__j_τ__ 1. A reflector for electromagnetic radiation comprising a lens of uniform dielectricconstantandareflector characterisedinthatthereisprovided: a first input converging lens (10,21); a second converging lens (14,22) coaxial with said first lens and having a convex rear surface; and areflectivecoating (15,26) appliedtotheconvexsurfaceofthesecondlens; thelensesbeingarrangedsuch thatelectromagneticenergy (11) fromasource incident on the first lens is refracted ontothe secondlens then reflected (18) from the reflective coating back towards the source of the energy. 2. A reflector for electromagnetic radiation as claimed in claim 1 characterised in that the lenses have a dielectric constant of 3.414. 3. A reflector for electromagnetic radiation as claimed in claim 1 or 2 characterisedin thatthelensesaremouldedfromsilicaflourinapolyester resin binder. 4. A reflector for electromagnetic radiationas claimed in claim 1 or 2 characterised in that the lenses comprise thin shells filled with a dielectric material. 5. A reflector for electromagnetic radiation as claimed in claim 4 characterised in that the shell is a polycarbonate material. 6. A reflector for electromagnetic radiation as claimed in claim 4 or 5 characterised in that the dielectric material is powdered silica. 7. Areflectorfor electromagneticradiationasclaimedinanyonepreceding claim characterised in that the lenses are meniscus lenses. 8. A reflector for electromagnetic radiation as claimed in claim 4 characterised in that the secondlens (22) isprovidedaround its periphery withanannularflange (23) andthefirsflens (21) isgenerallyhemispherical and provided with a rebated portion (24) around the periphery thereof for matingwithacomplementaryportion (25) of theperipheryof theflange (23). 9. Areflectorforelectromagneticradiationasclaimedinanyonepreceding claim characterised in that the reflecting surface (15,26) is a zinc spray coating. 10. Areflectorfor electromagnetic radiationasclaimedinanyonepreceding claimcharacterised inthatthereisprovidedmeanstosetthespacing ofthe two lenses to a predetermined value.

Inventors: GILBERT MARK TIMOTHY, RIX CLIFFORD, GILBERT, MARK, TIMOTHY, RIX, CLIFFORD
Applicants: SECR DEFENCE BRIT, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1990
Main classification: H01Q15
IPC codes: H01Q15