Passive radar target

Details Passive radar target Passive radar target

1990-03-12

1990-11-27

Tarcza, Thomas H.

Communications: directive radio wave systems and devices (e.g.,

Radar reflector

With spherical lens

343911R, 343911L, H01Q 1523

Patent

active

049739654

DESCRIPTION:

BRIEF SUMMARY
The invention relates to radar reflectors or targets and in particular to lens arrangements for enhancing the radar cross-section of a target.
Radar reflectors such as trihedral corner reflectors are frequently carried on the masts of yachts to enhance the. radar cross section for the yachts' safety by making them more visible to scanning radars of nearby ships. These reflectors are also used in targets for weapon practice where radar signatures are tailored to simulate practical targets.
Enhanced radar cross sections can also be achieved using lens-reflector assemblies. The best known assembly is the Luneberg lens with reflector. This is a fairly expensive device to make, especially when intended for use at higher microwave frequencies, i.e. above I band. This mainly arises as a consequence of the construction requiring a number of concentric contiguous hollow shells with dielectric constants a function of their radius. The material of the shells has also to be of low loss at the frequencies at which it is to be used. The radar microwaves are focussed by the lens on to a concave reflector and thence through the lens and back towards the radar emitter. This system is passive, involving no moving parts, and when used with missiles or projectiles it is generally made symmetrical about the longitudinal axis of the missile or projectile to produce an axially symmetric response which is independent of any spin. In a projectile application it is necessary for the lens-reflector assembly to withstand high 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 generally held against a portion of the surface of the Luneberg lens by clamping or by adhesive. In addition to ruggedness lens-reflector assemblies are suitable for use with linearly polarised radars (vertical or horizontal polarisation) and also by inserting a suitably spaced grid between 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 little 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 of the reflected wave is not plane, but curved with the solid dielectric lens arrangement, whereas with 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 simply 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 microwave energy on the back surface of the lens it is necessary for there to be an air gap between the lens and the reflector. This adds a constructional difficulty particularly where a robust lens-reflector assembly is necessary.
The object of the present invention is to provide a robust lens-reflector assembly which can be simply constructed. A secondary object is to provide such an assembly which can be made more cheaply than previously possible.
The invention provides:
a passive radar target comprising a solid lens of substantially uniform dielectric constant, having a reflecting surface 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 surface. The particulate 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 i

REFERENCES:
patent: 3079289 (1963-02-01), George, Jr. et al.
patent: 3145382 (1964-08-01), Cuming et al.
patent: 3307187 (1967-02-01), Horst
patent: 4224626 (1980-09-01), Sternberg
patent: 4288337 (1981-09-01), Ota et al.
patent: 4482513 (1984-11-01), Auletti

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