GB2257300A - Low radar echo structure - Google Patents

Abstract

To reduce the radar echo from a dihedral corner structure 50, the mutually orthogonal surfaces 51 and 52 comprise dielectric material. Equi-spaced wires 53 are embedded in the dielectric material and arranged parallel to the surfaces 51 and 52. Layers 64 and 65 of radiation absorbing material are located in the dielectric material parallel to the wires 53 and remote from the respective surfaces 51 and 52. The inductive susceptance of the wires 53 counteracts the dielectric material capacitance rendering it transparent to radar waves, which become transmitted and absorbed by the radiation absorbing material 64 and 65. <IMAGE>

Description

TITLE : LOW RADAR ECHO STRUCTURE The invention relates to a structure having low radar echo characteristics.

The detectability of a structure by a radar search beam depends on the strength of the radar echo from the structure as a fraction of the original beam intensity. Relevant structures include common radar targets such as aircraft, ships, tanks and other vehicles.

It is an object of the present invention to provide a structure having low radar echo characteristics.

The present invention provides a structure having at least one dihedral region of two non-coplanar surfaces of dielectric material, the material incorporating wires arranged parallel to both surfaces such that the surfaces are substantially dielectrically matched to free space in a given radar frequency band.

It has been found that dihedral surfaces constitute a major source of radar echoes, and that such echoes from dielectric surfaces incorporating parallel wires in accordance with the invention are greatly reduced over a frequency band determined by the permittivity of the material and the arrangement of wires.

The dielectric material preferably incorporates radar absorbing material arranged to absorb radar waves transmitted by the surfaces.

Each of the wires may be arranged at the same distance below a respective surface within the dielectric material. The wires may either be continuous, or continuous wires may be arranged alternately with periodically broken wires.

The dielectric material may be arranged in layer form over at least a part of the structure.

In order that the invention might be more fully understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows three forms of orthogonal dihedral structures capable of providing radar echoes, Figure 2 shows a dihedral region of a ship, Figure 3 shows a dihedral region of a road vehicle, Figure 4 shows a dihedral region of a missile, Figure 5 shows a dihedral region of an aircraft, Figure 6 shows a dihedral region of a vehicle wheel, Figure 7 shows a schematic three dimensional view of a dihedral structure incorporating wires in accordance with the invention, and Figures 8 and 9 show graphs of reflectivity versus radar frequency for dihedral surfaces with and without wires in accordance with the invention.

Referring to Figure 1, there are shown three general forms of orthogonal dihedral structures 10, 11 and 12 capable of acting as corner reflectors to produce radar echoes. The structures 10, 11 and 12 have respective pairs of orthogonal surfaces 13 and 14, 15 and 16, and 17 and 18. The surfaces 13 and 14 meet at a corner 19.

The surfaces 15 and 16 of structure 11 are non-intersecting, but are joined by a curved section 20. An open region 21 separates the two surfaces 17 and 18 of structure 12.

Referring now also to Figures 2 to 6 inclusive, examples of structure 10 include a corner 22 between a deck 23 and superstructure portion 24 of a ship 25, and corners 26 formed by an equipment cover 27 mounted on a road vehicle roof 28. Examples of structure 11 include fins 29 extending from the curved surface 30 of a missile 31, and corners 32 between an aircraft fin 33 and control surfaces 34, or corners 35 between aircraft wings 36 and fuselage 37. A tank or road vehicle wheel 38 of partly open construction provides an example of structure 12 from a combination of rim and hub surfaces 39 and 40.

Referring now to Figure 7, there is shown a dihedral corner structure 50 equivalent to structure 10 modified in accordance with the invention. The structure 50 incorporates two mutually orthogonal surfaces 51 and 52 of dielectric material in which continuous equispaced metal wires 53 are embedded. The wires 53 are arranged parallel to both surfaces 51 and 52 in two planes indicated by chain lines 54 and 55 a short distance into respective surfaces. A plane of incidence normal to both surfaces 51 and 52 for radar waves in a free space region 56 is indicated by propagation direction arrows 57, 58 and 59. Unit vector surface normals nl and n2 to surfaces 51 and 52 are indicated by arrows 60 and 61 respectively at points of incidence 62 and 63.Angles of incidence to surfaces 51 and 52 are indicated by 61 and 62 respectively, where 61 + 62 = 900. Layers indicated at 64 and 65 of radiation absorbing material are located parallel to the wires 53 remote from respective surfaces 51 and 52.

The arrangement of Figure 7 operates as follows. Radar waves propagating in the direction of arrow 57 are partially reflected at 62 in surface 51, and travel to 63 in surface 52 for further partial reflection in the direction of arrow 59. After the second reflection, the radar waves return for detection by their transmitting antenna (not shown). To reduce the radar cross section of a target incorporating the surfaces 51 and 52, it is necessary to reduce the doubly reflected signal. Radar waves polarized perpendicular to the plane of incidence (arrows 57 to 59) interact with the wires 53 in such a way that the surfaces 51 and 52 present a good dielectric match to free space over a wide frequency band.In effect, the inductive susceptance of the wires 53 counteracts the capacitance of the dielectric material of surfaces 51 and 52, and renders them transparent to radar waves in a particular frequency band.

A transmission line model may be employed to describe the reflection characteristics of dielectric-embedded wires, which appear as an inductive shunt across the line. The inductive susceptance B normalised to the dielectric is given by:

where: I = incidence angle ' permittivity of dielectric without wires F E+2sin2I P ~ pitch or spacing of wires C = wire circumference h = free space radar wavelength The wires 53 are placed close to the dielectric surfaces 51 and 52 to provide a dielectric match to free space for an incident radar wave in a broad frequency band.The frequency band may be selected by varying the wire parameters and their position below surfaces 51 and 52.

In view of the dielectric matching to free space provided by the wires 53, radiation polarized perpendicular to the plane of incidence is largely transmitted through the surfaces 51 and 52 with little reflected intensity, since the surfaces appear transparent to radar waves in the appropriate frequency band. The layers of absorbing material 64 and 65 absorb the transmitted intensity.

Radar waves polarized parallel to the plane of incidence are substantially unaffected by the wires 53. However, the Brewster angle phenomenon greatly reduces reflection of parallel polarised waves, so that radar returns of this polarization are much less significant than in the perpendicular case.

Referring now to Figures 8 and 9, there are shown computed graphs of theoretical percentage power reflectivity against radar wave frequency (GHz) for reflection at the two surfaces of a 900 dihedral structure of dielectric material with and without dielectric matching wires. The graphs are calculated for perpendicularly polarized radiation and on the basis that a matched load exists within the structure, ie that the structure incorporates radar wave absorbing material. In Figure 8, straight line graphs 80 to 83 correspond to a structure without wires with angles of incidence 61 at the first surface of 50 150, 250 and 450 respectively. Curves 84 to 88 correspond to reflectivity of the structure with wires, as illustrated in Figure 7, for angles of incidence 61 of. 50, 150, 250, 0 350 and 45 respectively, the wires 53 being 0.25 mm in diameter with a 6 mm spacing or pitch and located 1.78 mm below respective surfaces 51 and 52. Figure 9 shows similar straight line graphs 90 to 93 for incidence at 50, 150, 250 and 450 on an unwired structure, together with curves 94 to 98 for incidence at 50, 150, 250, 350 and 450 on a wired structure. In this case however, the wire pitch was 6.2 mm and the wires 53 were located 1.9 mm below surfaces 51 and 52.

By comparison of corresponding pairs of graphs for the same angle of incidence, eg graphs 80 and 84 or 90 and 93 for 5 incidence, it is apparent that the effect of the introduction of wires is to reduce the reflected intensity over a bandwidth of at least an octave.

In particular, for 5incidence curve 84 shows reduced reflectivity between about 10 GHz and 20.8 GHz as compared to graph 80. The corresponding interval for graphs 83 and 88 for 450 incidence is 9.8 GHz to 22.1 GHz. Curves 84 to 88 reach approximately zero t < 0.1%) reflectivity at about 13.8 GHz, whereas the minimum for curves 94 to 97 is about 12.9 GHz. This corresponds to movement of the minimum to lower frequencies with increasing wire depth and spacing, and is consistent with larger physical dimensions of radar apparatus corresponding to lower frequencies. Accordingly, the wire pitch and spacing may be arranged to conform to any frequency band in which low reflectivity characteristics are required.

From Figures 8 and 9 it is apparent that a structure having dihedral surfaces modified in accordance with Figure 7 will exhibit a reduced radar cross-section as compared to an unmodified structure over a specific frequency band dependent on the geometry of the incorporated wires. A surface may also be modified in accordance with the invention by attaching to it a dielectric layer incorporating radiation absorbing material and wires of appropriate geometry.

It has also been found that a mixture of continuous and periodically broken wires provides reduction in reflectivity over separate frequency bands.

The reduction in reflectivity obtained by means of the invention may be obtained with dielectrics having widely varying permittivities and loss targets.

Structures may be modified internally in accordance with the invention, in addition to external modification. A vehicle such as a helicopter may exhibit resonances from structures within its cabin, the dimensions of which may provide large echoing areas over certain frequencies and aspect angles. The modification of internal regions in accordance with the invention will break up reflections and damp resonances, thereby reducing undesirable radar echoes. Furthermore, grids of orthogonal or skew-symmetrically oriented wires may be incorporated in surfaces of arbitrary structures to change echoing characteristics. Different skew angles on different vehicles or structures of the same type would change echoing characteristics peculiar to that type.This may be beneficial in electronic warfare, where electronic countermeasures employ target recognition radars having for example range-gating, polarisation or spatial-phase recognition techniques.

It is well-known that rain, snow etc may produce precipitation clutter or radar returns sufficiently strong to obscure targets such as aircraft or tanks. Raindrops appear as symmetrical targets which reverse the direction and hence polarization of circularly polarized radiation. A circularly polarized antenna will not detect its image in symmetrical targets such as raindrops, since the radar return is circularly polarized in the opposite sense to that accepted by the antenna. Accordingly, the use of a circularly polarized antenna is a standard technique to cancel radar clutter from raindrops.

Asymmetric or dihedral-type reflections thus provide the major source of detectable radar returns. Moreover, as has been mentioned, in view of Brewster angle effects the radar return due to waves polarized in the plane of incidence is not appreciable. Accordingly, reduction of the radar return for waves polarized perpendicular to the plane of incidence in accordance with the invention provides a significant and important reduction in radar visibility.

Claims (7)

CLAItE

1. A structure having at least one dihedral region of two non-coplanar surfaces of dielectric material, the material incorporating wires arranged parallel to both surfaces such that the surfaces are substantially dielectrically matched to free space in a given radar frequency band.

2. A structure according to Claim 1 wherein the dielectric material incorporates radar wave absorbing material arranged to absorb radar waves transmitted by the surfaces.

3. A structure according to Claim 1 or 2 wherein the wires are arranged at a like distance below the respective surfaces.

4. A structure according to Claim 1, 2 or 3 wherein the wires comprise continuous and periodically broken wires.

5. A structure according to Claim 4 wherein the continuous and periodically broken wires are alternately arranged.

6. A structure according to any preceding claim wherein the dielectric material is arranged as a layer over at least part of the structure.

7. A structure incorporating surfaces substantially as herein described with reference to Figure 7.

7. A structure incorporating surfaces substantially as herein described with reference to Figure 7.

Amendments to the claims have been filed as follows 1. A structure having at least one dihedral region of two non-coplanar surfaces of dielectric material, the material incorporating wires underlying and arranged parallel to both surfaces such that the surfaces are substantially dielectrically matched to free space in a given radar frequency band as appropriate to reduce reflection of radar waves in that band.

2. h structure according to Claim 1 wherein the dielectric material incorporates radar wave absorbing arterial arranged to absorb radar waves transrltted by the surfaces.

3. A structure according to Claim 1 or 2 wherein the wires are arranged at a like distance below the respective surfaces.

4. A structure according to Claim 1, 2 or 3 wherein the wires comprise continuous and periodically broken wires.

5. A structure according to Claim 4 wherein the continuous and periodically broken wires are alternately arranged.

6. A structure according to any preceding claim wherein the dielectric material is arranged as a layer over at least part of the structure.