CA1297970C - Passive radar target - Google Patents
Passive radar targetInfo
- Publication number
- CA1297970C CA1297970C CA000540228A CA540228A CA1297970C CA 1297970 C CA1297970 C CA 1297970C CA 000540228 A CA000540228 A CA 000540228A CA 540228 A CA540228 A CA 540228A CA 1297970 C CA1297970 C CA 1297970C
- Authority
- CA
- Canada
- Prior art keywords
- radar target
- passive radar
- lens
- shell
- rearward
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01Q—ANTENNAS, i.e. RADIO AERIALS
- H01Q15/00—Devices for reflection, refraction, diffraction or polarisation of waves radiated from an antenna, e.g. quasi-optical devices
- H01Q15/23—Combinations of reflecting surfaces with refracting or diffracting devices
Landscapes
- Aerials With Secondary Devices (AREA)
Abstract
Abstract A passive radar target comprises a solid lens (50) of substantially uniform dielectric constant, having a reflecting surface (51) integrally formed therewith, the lens being constructed of particulate material having a dielectric constant selected such that radar waves striking the surface of the lens are focussed on the reflecting surfac.e In one form the particulate material comprises silica flour (91,101) contained within a thin radar transparant polycarbonate shell. The shell is formed of two similar halves (102,103) with a pressed aluminium reflective lining (92)106) in one half. By making the lens axially symmetrical such that the forward and rearward surfaces have a radius of curvature that decreases with distance from the transmitter. Two such lens-reflectors back-to-back will provide substantially omnidirectional operation. In an alternative arrangement back-to-back concave reflecting surfaces (114,115) are centrally located within a single spherical shell (117) filled with particulate material (116).
In some applications a particulate filler such as powdered slate is held by means of a polyurethane foam.
In some applications a particulate filler such as powdered slate is held by means of a polyurethane foam.
Description
~ PASSI~T~ ~ T~F~
Tlleinvention relatesto radarreflectors ortargets and in particular tolens arrangements for enhancing the ra~arcross section of a target.
Radar reflectors sueh as trihedral corner reflectors are frequently earried on the masts of yachts to enhanee the radar cross section for the yachts' safety by making them moLe visible to scanning radars of nearby ships. These refleetors are also usecl in tar~ets for weapon practice where radar si~natures are tailored to simulate practical targets.
Enh~need radar cross seetions ean also beachieved using lens-reflector assemblies The best known assem~ly is the Lune~erglenswith reflector. Thisis a fairly expensivedevice to make, especially when intendecl for use at higher microwave fre~uencies, ie above I band. This rnainly arises as a conse~luenceof the construction requiring a nulnber of concentrie eonti~uoushollow shellswith dielectric constantsa function of their radius. The material of the shells has also to be of low loss at the freclueneies at which it is to be usecl. The radar microwaves are focussed by the lens on to a eoneave reflector and thence thro~yh the lens and back towards the radar emitter.
This system is passive, involving no moving parts~, and whel~used with missiles or projectiles it is ~enerally made symn;etrical about the lonyitudinal axis of the missile or projectile to produce an axially symmetrie response whieh is independent oE
any spin. In a projeetile application it is necessary for the lens-reflector assembly to withstand hiyh g acceleration and high spin rates and thus careful attention has to be given to the design of this assembly. In this application a metallic reflector is yenerally held against a portion of the surface of the Lunebery lens by elamping or by adhesive. In addition to ruygedrless lens-reflector assemblies are suitable for use with linearly polariseclradars ~vertical orhorizontal polarisation) ~79'7~
and also by inserting a suitably spaced grid bekween the lens and the reflector, correct rotation of the reflected wave can be achieved as required for circularly polarised radars.
One possible solution to the current expensive and complex lens-reflector arrangements is to replace the Luneberg lens by a single spherical lens (ie of uniform dielectric constant), with very li~tle penalty in performance and weight.
The performance penalty is almost negligible when compared with crude versions of the Luneberg lens which may have only 3 or 4 shells. The difference in performance stems largely from the fact that the wave front o~ the reflected wave is not plane, but curved with the solid dielectric lens arrangemen~, whereas wlth a true Luneberg lens (ie with an infinite number of shells of differing dielectric constant), the wavefront is plane. The solid dielectric lens can be made more slmply than the Luneberg lens, however the focussing of such lenses depends upon the dielectric constant of the material and unless a suitable material is available to focus the mlcrowave energy on the back surface o~ the lens it ls necessary ~or there to be an air gap between the lens and the reflector. Thi3 add~ a constructlonal di~iculty particularly where a robust lens-re~leator assembly is necessary.
The object of the present invention is to provide a robust lens-re~lector a~sembly whlch can be simply constructed. A
secondary ob~ect ls to provlde such an assembly which can be made more cheaply than previously posslble.
~ he inventlon provides, a passive radar target wherein there is provided a lens ~'~ 2 ~29P7~
comprising a radar transparent shell filled with a particulate material of substantially uniform dielectric constant equal. to 3.4, the shell having outwardly convex forward and rearward por~ions and being providecl with a reflecting surface integrally formed with the rearward portion the.reof and the particulate material being finely powdered and compacted inside the shell such that there are substantially no voids; the arranyement being such that radar waves incident on the forward portion of the shell are focused on the re~lecting surface.
The part.iculate material may be held together within a constraining envelope, it may be bound together by means of an adhesive or it may be held together by means of a foam plastics material. The reflecting surface may be applied to the outside of the lens or may preferably be inside where it is free from environmental contamination or damage.
In one form the re~lecting surface may in contact with one portion of the surface of the lens such that radar waves striking other portions of the surface of the lens are focussed on the reflecting portion. The extent of the reflecting surface will depend upon the required angular response. It will thus be possible to tailor the response according to the application. In a small projectile where maximum enhancement is required in a ~iven direction the lens may be spherical and the reflecting æurface preferably covers a hemisphere of the lens and may be a coating applied by sprayln~.
Preferably the lens is axially symmetrical havin~
forward and rearward surfa~es, the re1ectiny surfAce beiny in contact wlth the rearward surface. :~n one form the lens may be ~L2979701 22762-5~1 spherical with the reflectlve surface preferably covering the whole of the rearward surface. In one arrangement the forwarcl and rearward surfaces have a rad.tus of curvature that decreases with distance from the axis of symmetry. In this latter arrangement it has been found preferable for the forward and rearward surfaces to have differing radii of curvature at the axis o~ symmetry. In such an arrangement the radar tar~et can be made to have a high reflectivity throughout a solid angle of substantially 2~.
The particulate material of the lens may be quartz (fused silicon dioxide)~ Quartz has a dielectric constant close to the ideal for use in a spherical lens. In addition quartz has good low loss properties, making it a very suitable material. A
cheaper materlal with a close dielectric constant is sulphur. In an alternative arrangement therefore the lens 'i~ 3a ~7~
ma~/ include particulate sulph~r. This may be in ti-,e forrl)of a sul~hur composition bonded in vacuum with an eI~oxy resin. ~he dieleclric constant of the bonde~ sulpilur however is too laroe for ~ perfect spherical lens. By forming the lens of two hemispheres such that the reflecting portion orhemispherehasa smaller radius of curvature than the other portion or ~,emisphere, the waves can be brought to focus at the reflecting surface. Sulphur bonded lenses have relatively high losses at microwave fre~uencies due to the dielectric losses in the epoxy resin binder. This choice ofbinderis determined by theneed to minimise deleterious heating effects on the sulphur. A
polyester resin binder when used did not cure.
An advantageous arrangement uses a spherical lens n~ade of silica glass beads or silica flour with a polyester resin binder. By varyiny the relative weights of binder and particulate silica the dielectric constant can be adjusted to the required value for focussing on the reflectin~ surface. In addition the lens can be made usinc, inexpensive materials in a convenient moulding process. The presence of the binder can be used to aclvantage to maintain a relatively constant radar cross section over a range of microwave frec~uencies: since the binder losses probably increase with fre~uency as also does the radar cross section normally, these two frequency dependent effects will to some extent balance each other.
In a particularly advantageous arrangemerlt the particulate material is contained within a thin, radar transparent shell, such as polycarbonate or ABS. This obviates the need for a binder. In this arrangement the reflecting surface can be provideclon the inside of the shell and in contact therewith. This provides a particularly cheap and robustlens-reflector assembly. The shell may be conveniently made in two identical forward and rearward hemispherical portions and the reflector may be a metal pressing inserted inside the rearwarcl portion before assemblin~ the portions together.
Advantageously an aperture is provic,eci in the shell for filling 12~7g7~
the s~here with the particulate material. Alternatively the shell may ~e fille~ with a plastics foam such as a poiyurethane loaciec~ with a partic~late filler. ~ne filler which has ~een used with a polyurethane foam is powdered slate.
In the lens-reflector assemblies of the present invention the performance efficiency is not significantly dependent upon the smoothness of the lens and thus polishing of the surface is not necessary. In addition to being cheap.
lenses according to the invention: produce radarcross-sections comparablewith the simpleconventional Luneberglens-reflector assemblies; produce substantially uniform response over a wide includec~ angle cone (in the case of a spherical lens of substantially 120); and can operate up to theJ, Kand L bands of frequencies and higher~
The reflecting surface may be formed as a vaned grid on the surface so that the radar target can be used for circularly polarised radar.
The invention will now be described by way of example only with reference to the attached drawings of which:
Figure 1 shows a known passive radar enhancing lens-reflector assem~ly;
Figure 2 shows a second known solid lens-reflector assembly within a protectiny housing;
Figure 3 illustrates the parameters used incalculating the optimum dielectric constant of a spherical dielectric lens.
Figure 4 shows a Figure 2 lens modified to allow use of a material having larger than optimum dielectric constant;
Figure S shows a lens/reflector arrangement where the lens material has optimum dielectric constant;
Figures6-8 arethe polarresponsecurves of theFigure5 radar target measured at 9 GHzr 13.5 GHz and 35 GHz;
Fiyure 9 shows an alternative spherical lens design;
Fig~re 10 shows a more complex shaped lens design similar in construction to Figure 9; ancl s ~L2~70 Figure 11 shows a sectional view of a lens-reflector assembly civing substantially omnidirectiorlal performance.
Figure 1 shows a known radar enhancing lens-reflector combination comprising a Luneberg lens 11 and a reflector 12.
The Luneberg lens 11 comprises a plurality of conti~uous thin spherical shells 13 arranged such that the dielectric constant of each successive outer shell is greater than the next inner shell. The lens is designed to focus microwave energy of a desired frequency band on to the rear surface of the lens. The reflector 12 comprises a plastic part-spherical shell formed by moulding and provided with a metallised reflecting layer on its inner concave surface.
One application of the use of radar enhancement is a mast-head target reflector for a yacht as illustrated in Figure 2. Housed within a radar-transparent housing 20 is a known lens-reflector assembly 21 of alter~ative design to Figure 1.
(~Q ~
The lens 22 is a solid perspeY~lens which focuses incident microwave energy behind the lens. Because the dielectric constant of perspex is non-optimum it is supported such that there is a fixed gap 23 between the lens 22 and the reflector 2 whereby the microwave energy is focussed on to the reflector.
The gap 23 is filled by a suitable filler material to provide mechanical support for the assembly. A radar absorbingannular ring 25 is provided to seal the yap 23 to prevent incident radar waves from entering the gap 23 directly without traversing the len~.
These types of lens-reflector assembly may also be used as practice targets and may be carried by one type of projectile to simulate another. When carried in a projectile the len~-reflector system must be capable of achieving the required radar cross section, and must be robust enough to withstand the severe environment experienced during ~iring of the projectile, subsequent high speed rotation as it travels through the ~29'797~3 atmosphere, and also heating by means of friction of the air on the projectile ~urface.
In its simplest form the present invention em~loys a generally spherical lens with a reflecting coating applied directly to a portion of the s~rface of the lens to produce a mechanically simple and robust arrangement which can be used in the above applications.
The focussing of rays by a solid lens of dielectric constant ~s is illustrated with reference to Figure 3.
At the front surface of the lens the rays are refracted accoræing to Snell's law such that:
sin ~ = k sin 0 (1) where G is the angle of incidence of the ray 30 with respect to the normal 31, d is the angle of the refracted ray 32 to the normal 31 and k is the refractive index.
The reEractive index is given by:
k= J~ (2) The radius m at which focus will occur is given by:
m = R [sin e tan (90-~ * 0) - cos e] (3) where R is the radius of the lens.
1'he rays are focussed on to the rear surface of the lens when e~uationsl to 3 are satisfied form= Rwith e = 45when the dielectric constant s = 3 . 414 . The point of focus 33 will then be on the rear $urface of thelens at point34. The angle of incidence ~ = 4S leading to a dielectric constant of 3.414 is found to achieve the mean orbestfocus for rays of all incidence.
Materials with a dielectric constant of 3.414 and also with a low loss (tan ~) at microwave frequencies, however, are ~2~
22762-5~1 either not readily available or cannot readily be engineered into the correct shape. An example of a material having about -the correct dielectric constant is quartz (silicon diox.ide when Eired~. Quartz spheres are made from single crystals and are expensive -to produce. There are, however, readily available materials which have low loss and are easily machineable but have lower dielec~ric constants than the optimum value ey polystyrene and perspex. The use of such materials however causes the microwaves to be focussed behind the rear surface of the lens and therefore requires the provision of an air gap as shown in Figure 2. This arrangement leads to complexity of construction since the gap between the lens and reflector must be accurately maintained.
In addition damaye and deterioration of the reflecting surface mus~ be avoided.
The inventors realised that by using particulate material held in a generally spherical form of lens, wave reflection can take place at a refleator on the rear surface of the lens. In one form shown in Figure 4 a lens has been made usiny a moulded, generally spherical lens of sulphur bound in an epoxy resin. This has a dielectric aonstant of about 4.0 at a ~requency of lKHz and ~o rays would be focussed within a true spherical lens as shown in Figure 5. However by modifyiny the lens moulding as shown in Figure 4 the 45 rays 41 are brought to a focus on the rear reflectiny coated surface 43 of the lens. The composition of the sulphur/epoxy resin lens aomprised:
lOOg sulphur lOOy epoxy resin 50g hardener 2y accelerator The fron~ hemisphere 42 was made 86mm in diame~er and the rear refleatlng hemlsphere 43 was 68mm in diameter. The estimated 10BB tan ~ was 0.03. Tlle measured radar enhanaed area (REA) results were as ~2~7~ 22762-50L
follows:
= 0.028m2 at 9 GHz = 0.022m2 at 13.5 GHz = Om2 at 35 GHz The effect of the improved focussing plane in the sulphur/epoxy resin lens is masked by the large losses due to the epoxy resin. However this approach may be used with other mate-rials whose properties may be satisfactory in all respects e~cept for a dielectric constant which is too high. An alternative lens is described with reference to Figure 5. The lens 50 comprises an 86mm diameter moulded sphere made from silica glass beads bonded with polyester resin. A zinc spray radar reflecting coating 51 is applied to a thickness of at least 120 microns over a hemisphere.
The dielectric constant of the sphere is adjusted by appropriate choice of material proportions to give the optimum value substan-tially equal to 3.414 at which microwave rays 52 incident at an angle of incidence 53 equal to 45 are brought to a focus 54 at the reflecting surface of the lens.
Figures 6 to 8 respectively show the polar response 20 curves 60, 70 and 80 for an 86mm diameter lens at 9.0 GHz, 13.5 GHz and 35 GHz. The lens was made up from equal proportions by weight of siLica glass beads (Grade 3) and polyester resin (Strand-glass Crystic). The dielectric constant was measured as 3.29 at 1 KHz. The measurements shown were taken in an anechoic chamber.
Assuming that the loss tan ~=0.003 then the calculated REA is:
= O.l~m2 at 9.0 GHz = 0~37m2 at 13.5 GHz = 1.56m2 at 35 GHz ~9~
These are to be cor,-lpared with the measured values:
- O.llm2 at 9.0 GHz = 0.22m2 at 13.5 G~æ
=0.017m2 at 35 GH~
The measured value at 35 GHz is low due to the hi~h dielectric loss in the polyester resin at this frequency.
Thelens as described above may be improvedby replaciny the silica beads with silica flour. The measured values of the REA are shown in the Table. Measurements are also shown at the same fre~uencies for a slightly smaller quartz lens.
~able Radar Cross Sectional Areas (m2) Frequency Silica Flour/Polyester resin ~uartz (86mm diameter) (76mm) 12 GH2 0.807 0.3~
18 GHz 1.3 0.51 38 GHz 1.63 2.3 These results show that the silica flour/polyester resin lens has better performance than the quartz lens over most of the measured frequency range. The impaired performance of the silica flour compared to quartæ at the highest measured frequerlcy is due to the higher diel~ectric loss.
Figure 9 shows an alternative, particularly robust lens-reflector arrangement. A hollow moulded plastics ball 90 is filled with silica powder 91. A hemispherical aluminium reflector 92 con~orming to the inner surface of the ball 90 is provided. This produces a particularly robust arrangement.
The ball is preferably polypropylene or ~BS or other plastics material having lo~ radar absorption. The ball is ma~e in two halves and into one is fitted a hemispherical pressed alur,iniurn ln ~ 2~7~317~
reflector. The two halves of the b~ll are then adhered to~ether and tl,e ball filled with the silica powder throu~h a hole 93.
Duriny fillin~, the ball 90 is a~itate~ to ensure that the filling is complete. After filling the hole 93 is seale~.
Althougll the lenses described thus far have had spherical forward and rearward reflecting surfaces, it has beel-found that for particular design applications the lens surfaces may be optimised analytically. One such arrangement is shown in Figure 10. A hollow plastics shell 100 is filled with silica powder 101 as in the Figure 9 arrangement, The shell is formed from two similar, but different, halves, a forward half 102 and a rearward half 103 provided with a metallic lining 106. The lens is symmetrical about the axis 104 and the shell surfaces are 60 formed that the radius OL' curvature at a surface position 105 decreases as the distance d of the point from the axis of symmetry 104. The radii of curvature of the two halves differ on the axis 104 but become the same as d increases. This arrangement is lighter than a spherical lens-reflector of similar radar cross-section and also its specific shape can be tailored to produce a broader angular response than the arrangements described previously. The radar cross-section is such that a high reflectivity is obtained for substantially all incidence angles when two such lens-reflectors are arranged back-to-back: each lens-reflector having a substantially uniform reflection respon~se throughout a solid angle of about 2ll. When using a spherical lens-reflector, as with the prior art I.uneberg lenses, reflection takes place only within about an angle oE 60. With the non-spherical arrangenlent shown in Figure 10 the front-back asymmetry has been found to give a slightly divergent return beam spreading through a contained angle of about 30 back along the line of incidence. This has been found to give better performance than for a lens-reflector giving parallel reflection and is particularly of benefit when source and receiver are not colocated. Then, providing source and ~2~7~
receiver subtend an angle at the target of less than 15G the receiver will receive reflected radiation from the target.
One of the advantages of the powder packed shell lens compared with the lens made using an adhesive binder is that there is no non-uniformity of performance due to trapped air within the lens.
An alternative approach to the described use of solid lenses ~ould be to use plastics material appropriately foamed with an inert gas and loaded with a particulate filler selected to achieve the required dielectric constant. A useful combination has been found to be a polyurethane foam with a powdered slate filler. This lens could be provided with a reflecting metallic coating but preferably the foamed material is formed within a polypropylene shell provided with a reflector as in the arrangements of Fiyures 9 and 10.
In an application such as a radar reflector for the masthead of a boat, two back to back lenses of the type shown in Figure 10 will provide substantially omnidirectional reflectance. An alternative arrangement involving a single lens is shown in Figure 11. Shown in dashed lines are two back-to-back spherical lenses 111 and 112 with a solid double reflector 113 between the two lenses. The reflector 113 is cylindrical with a radiused reflecting surface 114rll5 at each end. Such an arrangement of lenses and reflector is e~uivalent to the combination of lens-reflectors described above to give substantially omnidirectional perormance. In the Figure 11 arrangement. however, the spherical lenses 111 and 112 are replaced by a single enveloping spherical lens 116. A
polypropylenè spherical shell 117 has attached diametrically opposed spigots 118,119 which support the double reflector 113 centrall~ wlthi~ the spherical lens 116. The remaining cavity within the shell 117 is filled as before with a suitable dielectric particulate material such as silica flour. The structural integrity of this arranger,lent can be improved by ~2~7~
using a foamed resin lens with a particulate filler~ replacir.g the silica flour. The lens-reflector assembly could then be used without an ecapsulating shell altl-ough in practice a polypropylene shell will provide protection for the foam lens.
The foamed plastics lens arrangements are lighter than the other embodiments of the invention described and thus are advantageous for applications where weight limitation is an important criterion.
Other variations and modifications of the lens-reflectors described above will be apparent to those skilled in the art, all falling within the scope of the invention claimed.
Tlleinvention relatesto radarreflectors ortargets and in particular tolens arrangements for enhancing the ra~arcross section of a target.
Radar reflectors sueh as trihedral corner reflectors are frequently earried on the masts of yachts to enhanee the radar cross section for the yachts' safety by making them moLe visible to scanning radars of nearby ships. These refleetors are also usecl in tar~ets for weapon practice where radar si~natures are tailored to simulate practical targets.
Enh~need radar cross seetions ean also beachieved using lens-reflector assemblies The best known assem~ly is the Lune~erglenswith reflector. Thisis a fairly expensivedevice to make, especially when intendecl for use at higher microwave fre~uencies, ie above I band. This rnainly arises as a conse~luenceof the construction requiring a nulnber of concentrie eonti~uoushollow shellswith dielectric constantsa function of their radius. The material of the shells has also to be of low loss at the freclueneies at which it is to be usecl. The radar microwaves are focussed by the lens on to a eoneave reflector and thence thro~yh the lens and back towards the radar emitter.
This system is passive, involving no moving parts~, and whel~used with missiles or projectiles it is ~enerally made symn;etrical about the lonyitudinal axis of the missile or projectile to produce an axially symmetrie response whieh is independent oE
any spin. In a projeetile application it is necessary for the lens-reflector assembly to withstand hiyh g acceleration and high spin rates and thus careful attention has to be given to the design of this assembly. In this application a metallic reflector is yenerally held against a portion of the surface of the Lunebery lens by elamping or by adhesive. In addition to ruygedrless lens-reflector assemblies are suitable for use with linearly polariseclradars ~vertical orhorizontal polarisation) ~79'7~
and also by inserting a suitably spaced grid bekween the lens and the reflector, correct rotation of the reflected wave can be achieved as required for circularly polarised radars.
One possible solution to the current expensive and complex lens-reflector arrangements is to replace the Luneberg lens by a single spherical lens (ie of uniform dielectric constant), with very li~tle penalty in performance and weight.
The performance penalty is almost negligible when compared with crude versions of the Luneberg lens which may have only 3 or 4 shells. The difference in performance stems largely from the fact that the wave front o~ the reflected wave is not plane, but curved with the solid dielectric lens arrangemen~, whereas wlth a true Luneberg lens (ie with an infinite number of shells of differing dielectric constant), the wavefront is plane. The solid dielectric lens can be made more slmply than the Luneberg lens, however the focussing of such lenses depends upon the dielectric constant of the material and unless a suitable material is available to focus the mlcrowave energy on the back surface o~ the lens it ls necessary ~or there to be an air gap between the lens and the reflector. Thi3 add~ a constructlonal di~iculty particularly where a robust lens-re~leator assembly is necessary.
The object of the present invention is to provide a robust lens-re~lector a~sembly whlch can be simply constructed. A
secondary ob~ect ls to provlde such an assembly which can be made more cheaply than previously posslble.
~ he inventlon provides, a passive radar target wherein there is provided a lens ~'~ 2 ~29P7~
comprising a radar transparent shell filled with a particulate material of substantially uniform dielectric constant equal. to 3.4, the shell having outwardly convex forward and rearward por~ions and being providecl with a reflecting surface integrally formed with the rearward portion the.reof and the particulate material being finely powdered and compacted inside the shell such that there are substantially no voids; the arranyement being such that radar waves incident on the forward portion of the shell are focused on the re~lecting surface.
The part.iculate material may be held together within a constraining envelope, it may be bound together by means of an adhesive or it may be held together by means of a foam plastics material. The reflecting surface may be applied to the outside of the lens or may preferably be inside where it is free from environmental contamination or damage.
In one form the re~lecting surface may in contact with one portion of the surface of the lens such that radar waves striking other portions of the surface of the lens are focussed on the reflecting portion. The extent of the reflecting surface will depend upon the required angular response. It will thus be possible to tailor the response according to the application. In a small projectile where maximum enhancement is required in a ~iven direction the lens may be spherical and the reflecting æurface preferably covers a hemisphere of the lens and may be a coating applied by sprayln~.
Preferably the lens is axially symmetrical havin~
forward and rearward surfa~es, the re1ectiny surfAce beiny in contact wlth the rearward surface. :~n one form the lens may be ~L2979701 22762-5~1 spherical with the reflectlve surface preferably covering the whole of the rearward surface. In one arrangement the forwarcl and rearward surfaces have a rad.tus of curvature that decreases with distance from the axis of symmetry. In this latter arrangement it has been found preferable for the forward and rearward surfaces to have differing radii of curvature at the axis o~ symmetry. In such an arrangement the radar tar~et can be made to have a high reflectivity throughout a solid angle of substantially 2~.
The particulate material of the lens may be quartz (fused silicon dioxide)~ Quartz has a dielectric constant close to the ideal for use in a spherical lens. In addition quartz has good low loss properties, making it a very suitable material. A
cheaper materlal with a close dielectric constant is sulphur. In an alternative arrangement therefore the lens 'i~ 3a ~7~
ma~/ include particulate sulph~r. This may be in ti-,e forrl)of a sul~hur composition bonded in vacuum with an eI~oxy resin. ~he dieleclric constant of the bonde~ sulpilur however is too laroe for ~ perfect spherical lens. By forming the lens of two hemispheres such that the reflecting portion orhemispherehasa smaller radius of curvature than the other portion or ~,emisphere, the waves can be brought to focus at the reflecting surface. Sulphur bonded lenses have relatively high losses at microwave fre~uencies due to the dielectric losses in the epoxy resin binder. This choice ofbinderis determined by theneed to minimise deleterious heating effects on the sulphur. A
polyester resin binder when used did not cure.
An advantageous arrangement uses a spherical lens n~ade of silica glass beads or silica flour with a polyester resin binder. By varyiny the relative weights of binder and particulate silica the dielectric constant can be adjusted to the required value for focussing on the reflectin~ surface. In addition the lens can be made usinc, inexpensive materials in a convenient moulding process. The presence of the binder can be used to aclvantage to maintain a relatively constant radar cross section over a range of microwave frec~uencies: since the binder losses probably increase with fre~uency as also does the radar cross section normally, these two frequency dependent effects will to some extent balance each other.
In a particularly advantageous arrangemerlt the particulate material is contained within a thin, radar transparent shell, such as polycarbonate or ABS. This obviates the need for a binder. In this arrangement the reflecting surface can be provideclon the inside of the shell and in contact therewith. This provides a particularly cheap and robustlens-reflector assembly. The shell may be conveniently made in two identical forward and rearward hemispherical portions and the reflector may be a metal pressing inserted inside the rearwarcl portion before assemblin~ the portions together.
Advantageously an aperture is provic,eci in the shell for filling 12~7g7~
the s~here with the particulate material. Alternatively the shell may ~e fille~ with a plastics foam such as a poiyurethane loaciec~ with a partic~late filler. ~ne filler which has ~een used with a polyurethane foam is powdered slate.
In the lens-reflector assemblies of the present invention the performance efficiency is not significantly dependent upon the smoothness of the lens and thus polishing of the surface is not necessary. In addition to being cheap.
lenses according to the invention: produce radarcross-sections comparablewith the simpleconventional Luneberglens-reflector assemblies; produce substantially uniform response over a wide includec~ angle cone (in the case of a spherical lens of substantially 120); and can operate up to theJ, Kand L bands of frequencies and higher~
The reflecting surface may be formed as a vaned grid on the surface so that the radar target can be used for circularly polarised radar.
The invention will now be described by way of example only with reference to the attached drawings of which:
Figure 1 shows a known passive radar enhancing lens-reflector assem~ly;
Figure 2 shows a second known solid lens-reflector assembly within a protectiny housing;
Figure 3 illustrates the parameters used incalculating the optimum dielectric constant of a spherical dielectric lens.
Figure 4 shows a Figure 2 lens modified to allow use of a material having larger than optimum dielectric constant;
Figure S shows a lens/reflector arrangement where the lens material has optimum dielectric constant;
Figures6-8 arethe polarresponsecurves of theFigure5 radar target measured at 9 GHzr 13.5 GHz and 35 GHz;
Fiyure 9 shows an alternative spherical lens design;
Fig~re 10 shows a more complex shaped lens design similar in construction to Figure 9; ancl s ~L2~70 Figure 11 shows a sectional view of a lens-reflector assembly civing substantially omnidirectiorlal performance.
Figure 1 shows a known radar enhancing lens-reflector combination comprising a Luneberg lens 11 and a reflector 12.
The Luneberg lens 11 comprises a plurality of conti~uous thin spherical shells 13 arranged such that the dielectric constant of each successive outer shell is greater than the next inner shell. The lens is designed to focus microwave energy of a desired frequency band on to the rear surface of the lens. The reflector 12 comprises a plastic part-spherical shell formed by moulding and provided with a metallised reflecting layer on its inner concave surface.
One application of the use of radar enhancement is a mast-head target reflector for a yacht as illustrated in Figure 2. Housed within a radar-transparent housing 20 is a known lens-reflector assembly 21 of alter~ative design to Figure 1.
(~Q ~
The lens 22 is a solid perspeY~lens which focuses incident microwave energy behind the lens. Because the dielectric constant of perspex is non-optimum it is supported such that there is a fixed gap 23 between the lens 22 and the reflector 2 whereby the microwave energy is focussed on to the reflector.
The gap 23 is filled by a suitable filler material to provide mechanical support for the assembly. A radar absorbingannular ring 25 is provided to seal the yap 23 to prevent incident radar waves from entering the gap 23 directly without traversing the len~.
These types of lens-reflector assembly may also be used as practice targets and may be carried by one type of projectile to simulate another. When carried in a projectile the len~-reflector system must be capable of achieving the required radar cross section, and must be robust enough to withstand the severe environment experienced during ~iring of the projectile, subsequent high speed rotation as it travels through the ~29'797~3 atmosphere, and also heating by means of friction of the air on the projectile ~urface.
In its simplest form the present invention em~loys a generally spherical lens with a reflecting coating applied directly to a portion of the s~rface of the lens to produce a mechanically simple and robust arrangement which can be used in the above applications.
The focussing of rays by a solid lens of dielectric constant ~s is illustrated with reference to Figure 3.
At the front surface of the lens the rays are refracted accoræing to Snell's law such that:
sin ~ = k sin 0 (1) where G is the angle of incidence of the ray 30 with respect to the normal 31, d is the angle of the refracted ray 32 to the normal 31 and k is the refractive index.
The reEractive index is given by:
k= J~ (2) The radius m at which focus will occur is given by:
m = R [sin e tan (90-~ * 0) - cos e] (3) where R is the radius of the lens.
1'he rays are focussed on to the rear surface of the lens when e~uationsl to 3 are satisfied form= Rwith e = 45when the dielectric constant s = 3 . 414 . The point of focus 33 will then be on the rear $urface of thelens at point34. The angle of incidence ~ = 4S leading to a dielectric constant of 3.414 is found to achieve the mean orbestfocus for rays of all incidence.
Materials with a dielectric constant of 3.414 and also with a low loss (tan ~) at microwave frequencies, however, are ~2~
22762-5~1 either not readily available or cannot readily be engineered into the correct shape. An example of a material having about -the correct dielectric constant is quartz (silicon diox.ide when Eired~. Quartz spheres are made from single crystals and are expensive -to produce. There are, however, readily available materials which have low loss and are easily machineable but have lower dielec~ric constants than the optimum value ey polystyrene and perspex. The use of such materials however causes the microwaves to be focussed behind the rear surface of the lens and therefore requires the provision of an air gap as shown in Figure 2. This arrangement leads to complexity of construction since the gap between the lens and reflector must be accurately maintained.
In addition damaye and deterioration of the reflecting surface mus~ be avoided.
The inventors realised that by using particulate material held in a generally spherical form of lens, wave reflection can take place at a refleator on the rear surface of the lens. In one form shown in Figure 4 a lens has been made usiny a moulded, generally spherical lens of sulphur bound in an epoxy resin. This has a dielectric aonstant of about 4.0 at a ~requency of lKHz and ~o rays would be focussed within a true spherical lens as shown in Figure 5. However by modifyiny the lens moulding as shown in Figure 4 the 45 rays 41 are brought to a focus on the rear reflectiny coated surface 43 of the lens. The composition of the sulphur/epoxy resin lens aomprised:
lOOg sulphur lOOy epoxy resin 50g hardener 2y accelerator The fron~ hemisphere 42 was made 86mm in diame~er and the rear refleatlng hemlsphere 43 was 68mm in diameter. The estimated 10BB tan ~ was 0.03. Tlle measured radar enhanaed area (REA) results were as ~2~7~ 22762-50L
follows:
= 0.028m2 at 9 GHz = 0.022m2 at 13.5 GHz = Om2 at 35 GHz The effect of the improved focussing plane in the sulphur/epoxy resin lens is masked by the large losses due to the epoxy resin. However this approach may be used with other mate-rials whose properties may be satisfactory in all respects e~cept for a dielectric constant which is too high. An alternative lens is described with reference to Figure 5. The lens 50 comprises an 86mm diameter moulded sphere made from silica glass beads bonded with polyester resin. A zinc spray radar reflecting coating 51 is applied to a thickness of at least 120 microns over a hemisphere.
The dielectric constant of the sphere is adjusted by appropriate choice of material proportions to give the optimum value substan-tially equal to 3.414 at which microwave rays 52 incident at an angle of incidence 53 equal to 45 are brought to a focus 54 at the reflecting surface of the lens.
Figures 6 to 8 respectively show the polar response 20 curves 60, 70 and 80 for an 86mm diameter lens at 9.0 GHz, 13.5 GHz and 35 GHz. The lens was made up from equal proportions by weight of siLica glass beads (Grade 3) and polyester resin (Strand-glass Crystic). The dielectric constant was measured as 3.29 at 1 KHz. The measurements shown were taken in an anechoic chamber.
Assuming that the loss tan ~=0.003 then the calculated REA is:
= O.l~m2 at 9.0 GHz = 0~37m2 at 13.5 GHz = 1.56m2 at 35 GHz ~9~
These are to be cor,-lpared with the measured values:
- O.llm2 at 9.0 GHz = 0.22m2 at 13.5 G~æ
=0.017m2 at 35 GH~
The measured value at 35 GHz is low due to the hi~h dielectric loss in the polyester resin at this frequency.
Thelens as described above may be improvedby replaciny the silica beads with silica flour. The measured values of the REA are shown in the Table. Measurements are also shown at the same fre~uencies for a slightly smaller quartz lens.
~able Radar Cross Sectional Areas (m2) Frequency Silica Flour/Polyester resin ~uartz (86mm diameter) (76mm) 12 GH2 0.807 0.3~
18 GHz 1.3 0.51 38 GHz 1.63 2.3 These results show that the silica flour/polyester resin lens has better performance than the quartz lens over most of the measured frequency range. The impaired performance of the silica flour compared to quartæ at the highest measured frequerlcy is due to the higher diel~ectric loss.
Figure 9 shows an alternative, particularly robust lens-reflector arrangement. A hollow moulded plastics ball 90 is filled with silica powder 91. A hemispherical aluminium reflector 92 con~orming to the inner surface of the ball 90 is provided. This produces a particularly robust arrangement.
The ball is preferably polypropylene or ~BS or other plastics material having lo~ radar absorption. The ball is ma~e in two halves and into one is fitted a hemispherical pressed alur,iniurn ln ~ 2~7~317~
reflector. The two halves of the b~ll are then adhered to~ether and tl,e ball filled with the silica powder throu~h a hole 93.
Duriny fillin~, the ball 90 is a~itate~ to ensure that the filling is complete. After filling the hole 93 is seale~.
Althougll the lenses described thus far have had spherical forward and rearward reflecting surfaces, it has beel-found that for particular design applications the lens surfaces may be optimised analytically. One such arrangement is shown in Figure 10. A hollow plastics shell 100 is filled with silica powder 101 as in the Figure 9 arrangement, The shell is formed from two similar, but different, halves, a forward half 102 and a rearward half 103 provided with a metallic lining 106. The lens is symmetrical about the axis 104 and the shell surfaces are 60 formed that the radius OL' curvature at a surface position 105 decreases as the distance d of the point from the axis of symmetry 104. The radii of curvature of the two halves differ on the axis 104 but become the same as d increases. This arrangement is lighter than a spherical lens-reflector of similar radar cross-section and also its specific shape can be tailored to produce a broader angular response than the arrangements described previously. The radar cross-section is such that a high reflectivity is obtained for substantially all incidence angles when two such lens-reflectors are arranged back-to-back: each lens-reflector having a substantially uniform reflection respon~se throughout a solid angle of about 2ll. When using a spherical lens-reflector, as with the prior art I.uneberg lenses, reflection takes place only within about an angle oE 60. With the non-spherical arrangenlent shown in Figure 10 the front-back asymmetry has been found to give a slightly divergent return beam spreading through a contained angle of about 30 back along the line of incidence. This has been found to give better performance than for a lens-reflector giving parallel reflection and is particularly of benefit when source and receiver are not colocated. Then, providing source and ~2~7~
receiver subtend an angle at the target of less than 15G the receiver will receive reflected radiation from the target.
One of the advantages of the powder packed shell lens compared with the lens made using an adhesive binder is that there is no non-uniformity of performance due to trapped air within the lens.
An alternative approach to the described use of solid lenses ~ould be to use plastics material appropriately foamed with an inert gas and loaded with a particulate filler selected to achieve the required dielectric constant. A useful combination has been found to be a polyurethane foam with a powdered slate filler. This lens could be provided with a reflecting metallic coating but preferably the foamed material is formed within a polypropylene shell provided with a reflector as in the arrangements of Fiyures 9 and 10.
In an application such as a radar reflector for the masthead of a boat, two back to back lenses of the type shown in Figure 10 will provide substantially omnidirectional reflectance. An alternative arrangement involving a single lens is shown in Figure 11. Shown in dashed lines are two back-to-back spherical lenses 111 and 112 with a solid double reflector 113 between the two lenses. The reflector 113 is cylindrical with a radiused reflecting surface 114rll5 at each end. Such an arrangement of lenses and reflector is e~uivalent to the combination of lens-reflectors described above to give substantially omnidirectional perormance. In the Figure 11 arrangement. however, the spherical lenses 111 and 112 are replaced by a single enveloping spherical lens 116. A
polypropylenè spherical shell 117 has attached diametrically opposed spigots 118,119 which support the double reflector 113 centrall~ wlthi~ the spherical lens 116. The remaining cavity within the shell 117 is filled as before with a suitable dielectric particulate material such as silica flour. The structural integrity of this arranger,lent can be improved by ~2~7~
using a foamed resin lens with a particulate filler~ replacir.g the silica flour. The lens-reflector assembly could then be used without an ecapsulating shell altl-ough in practice a polypropylene shell will provide protection for the foam lens.
The foamed plastics lens arrangements are lighter than the other embodiments of the invention described and thus are advantageous for applications where weight limitation is an important criterion.
Other variations and modifications of the lens-reflectors described above will be apparent to those skilled in the art, all falling within the scope of the invention claimed.
Claims (29)
1. A passive radar target wherein there is provided a lens comprising a radar transparent shell filled with a particulate material of substantially uniform dielectric constant equal to 3.4, the shell having outwardly convex forward and rearward portions and being provided with a reflecting surface integrally formed with the rearward portion thereof and the particulate material being finely powdered and compacted inside the shell such that there are substantially no voids; the arrangement being such that radar waves incident on the forward portion of the shell are focused on the reflecting surface.
2. A passive radar target as claimed in claim 1 wherein the reflecting surface is provided on the inner surface of the rearward portion of the lens shell.
3. A passive radar target as claimed in claim 2 wherein the lens is axially symmetrical having forward and rearward surfaces, the reflecting surface being in contact with the rearward surface and wherein the forward and rearward surfaces have a radius of curvature that decreases with distance from the axis of symmetry.
4. A passive radar target as claimed in claim 3 wherein the forward and rearward surfaces have differing radii of curvature at the axis of symmetry.
5. A passive radar target as claimed in claim 4 wherein the lens is spherical and the particulate material is silica in the form of glass beads or silica flour.
6. A passive radar target as claimed in claim 5 wherein the shell material is polycarbonate.
7. A passive radar target as claimed in claim 6 wherein the shell is made in two portions, a forward and a rearward portion, and the reflecting surface is a metal pressing inserted inside the rearward portion.
8. A passive radar target as claimed in claim 2 wherein the lens is spherical and the particulate material is silica in the form of glass beads or silica flour.
9. A passive radar target as claimed in claim 8 wherein the particulate material is hound together by means of an adhesive.
10. A passive radar target as claimed in claim 9 wherein a polyester resin binder is used.
11. A passive radar target as claimed in claim 10 wherein the shell material is polycarbonate.
12. A passive radar target as claimed in claim 11 wherein the shell is made in two portions, a forward and a rearward portion, and the reflecting surface is a metal pressing inserted inside the rearward portion.
13. A passive radar target as claimed in claim 2 wherein the particulate material is held together by means of a foam plastics material.
14. A passive radar target as claimed in claim 13 wherein the shell is filled with a polyurethane plastics foam loaded with a particulate filler.
15. A passive radar target as claimed in claim 14 wherein the particulate filler is powdered slate.
16. A passive radar target as claimed in claim 15 wherein the lens is axially symmetrical having forward and rearward surfaces, the reflecting surface being in contact with the rearward surface and wherein the forward and rearward surfaces have a radius of curvature that decreases with distance from the axis of symmetry.
17. A passive radar target as claimed in claim 16 wherein the forward and rearward surfaces have differing radii of curvature at the axis of symmetry.
18. A passive radar target as claimed in claim 17 wherein the shell material is polycarbonate.
19. A passive radar target as claimed in claim 18 wherein the shell is made in two portions, a forward and a rearward portion, and the reflecting surface is a metal pressing inserted inside the rearward portion.
20. A passive radar target as claimed in claim 15 wherein the shell material is polycarbonate.
21. A passive radar target as claimed in claim 20 wherein the shell is made in two portions, a forward and a rearward portion, and the reflecting surface is a metal pressing inserted inside the rearward portion.
22. A passive radar target as claimed in claim 2 wherein the shell material is polycarbonate.
23. A passive radar target as claimed in claim 22 wherein the shell is made in two portions, a forward and a rearward portion, and the reflecting surface is a metal pressing inserted inside the rearward portion.
24. A passive radar target as claimed in claim 4 wherein the shell material is polycarbonate.
25. A passive radar target as claimed in claim 24 wherein the shell is made in two portions, a forward and a rearward portion, and the reflecting surface is a metal pressing inserted inside the rearward portion.
26. A passive radar target as claimed in claim 5 wherein the particulate material is bound together by means of an adhesive.
27. A passive radar target as claimed in claim 26 wherein a polyester resin binder is used.
28. A passive radar target as claimed in claim 27 wherein the shell material is polycarbonate.
29. A passive radar target as claimed in claim 28 wherein the shell is made in two portions, a forward and a rearward portion, and the reflecting surface is a metal pressing inserted inside the rearward portion.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8615317A GB2194391B (en) | 1986-06-23 | 1986-06-23 | A passive radar target |
| GB8615317 | 1986-06-23 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1297970C true CA1297970C (en) | 1992-03-24 |
Family
ID=10599963
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000540228A Expired - Lifetime CA1297970C (en) | 1986-06-23 | 1987-06-22 | Passive radar target |
Country Status (3)
| Country | Link |
|---|---|
| CA (1) | CA1297970C (en) |
| GB (1) | GB2194391B (en) |
| NZ (1) | NZ220784A (en) |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1989000773A1 (en) * | 1987-07-10 | 1989-01-26 | The Secretary Of State For Defence In Her Britanni | A passive radar target |
| GB9118041D0 (en) * | 1991-08-21 | 1991-10-09 | Secr Defence | Radar reflectors |
| AU2000273299A1 (en) * | 2000-07-31 | 2002-02-13 | The Boeing Company | Method for producing a spatially stratified optical system for use in the micronand sub-micron wavelength regime |
| US6433936B1 (en) * | 2001-08-15 | 2002-08-13 | Emerson & Cuming Microwave Products | Lens of gradient dielectric constant and methods of production |
| US20070002922A1 (en) * | 2005-06-30 | 2007-01-04 | Intel Corporation | Retro-reflecting lens for external cavity optics |
| DE102019209508B4 (en) | 2019-06-28 | 2024-05-23 | Deutsches Zentrum für Luft- und Raumfahrt e.V. | Method and device for detecting a spatial area using radar waves |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1056854A (en) * | 1962-09-24 | 1967-02-01 | Elliott Brothers London Ltd | Improvements in or relating to electromagnetic energy reflectors |
| GB1348672A (en) * | 1971-07-26 | 1974-03-20 | British Aircraft Corp Ltd | Microwave lenses |
| FR2197247B1 (en) * | 1972-07-13 | 1978-07-21 | Tokyo Keiri Kk |
-
1986
- 1986-06-23 GB GB8615317A patent/GB2194391B/en not_active Expired - Lifetime
-
1987
- 1987-06-22 CA CA000540228A patent/CA1297970C/en not_active Expired - Lifetime
- 1987-06-22 NZ NZ22078487A patent/NZ220784A/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| GB2194391B (en) | 1991-02-27 |
| GB8615317D0 (en) | 1986-07-30 |
| NZ220784A (en) | 1990-03-27 |
| GB2194391A (en) | 1988-03-02 |
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