US 3787856 A
A spherical microwave lens and reflector arrangement in which the reflector is part-spherical and is mounted so as to be movable towards and away from the lens in order that variations in the phase or amplitude of a reflected signal may be produced.
Description (OCR text may contain errors)
United States Patent 1191 Pyrah Jan. 22, 1974 SPHERICAL MICROWAVE LENSES  References Cited  Inventor: Patrick Frank Pyrah, Knebworth, UNITED STATES PATENTS England 3,116,486 12/1963 Johnson et al 343/755 2,716,746 8/1955 Howery  Asslgnee' fg 'gg gg sgg 2,866,971 12/1958 Kelleher 343/755  Filed: July 18, 1972 Primary Examiner-Eli Lieberman [21 1 p No 272 804 Attorney, Agent, or Firm-Cushman, Darby &
' H Cushman  Foreign Application Priority Data  ABSTRACT July 26, 1971 Great Br1ta1n 1. 34990/71 A spherical microwave lens and reflector arrangement in which the reflector is part-spherical and is mounted 2? 343/18 hfi 3g g so as to be movable towards and away from the lens in E 1 g l 8 18/1) order that variations in the phase or amplitude of a reflected signal may be produced.
6 Claims, 2 Drawing Figures SPHERICAL MICROWAVE LENSES This invention relates to spherical microwave lenses which are provided with reflectors for reflecting electromagnetic waves used in radar systems.
It is known to provide a spherical microwave lens with a fixed metallic reflector by means of which incoming electromagnetic waves, after being brough to a focus on or close to the surface of the lens, may be reflected back along a path parallel to the path of the incoming waves. Such reflectors generally consist of a part-spherical band or cap of sheet metal fixed on or close to the outer surface of the lens. Such an arrangement of spherical lens and fixed reflector provides a reflected wave signal of constant amplitude and phase.
The object of the present invention is to provide a spherical lens and reflector arrangement, the reflected wave signal from which may be varied in terms of phase and/or amplitude.
According to the present invention, a spherical microwave lens arrangement comprises a spherical microwave lens with a part-spherical reflector movable towards and away from the lens.
In one arrangement of the invention the lens has a focus outside the sphere, and the reflector is movable about a position co-incident with the focus point of the lens, towards and away from the surface of the lens on either side of the focus point whereby movement of the reflector produces a variation in the phase of a reflected wave.
In another arrangement of the invention the lens has a focus approximately on its surface, and a coating of an electrically resistive material extends over part of the outer surface of the lens, the reflector being mov able between a position where it lies against the resistive coating, and a position where it is spaced from the resistive coating by a distance of approximately one quarter of a wavelength, whereby movement of the reflector produces a variation in the amplitude of a reflected wave.
The electrically resistive material may be carbon loaded cloth, carbon loaded rubber or colloidal graphrte.
In order that the invention may be better understood, two preferred embodiments will now be described with reference to the accompanying drawings of which FIG. 1 shows in part-section one form of the invention in which two lenses and two reflectors are used, one of the reflectors being fixed relative to its associated lens and the other reflector being movable about the focus point of its associated lens so as to produce a variation in the phase of a reflected wave, and
FIG. 2 shows in part-section another form of the invention in which a single reflector is movable towards and away from a single lens which is provided with a re sistive coating over part ofits outer surface so as to produce a variation in the amplitude of a reflected wave.
Referring to FIG. 1, a spherical microwave lens 1, 2 of dielectric material is mounted in each nose cone 3, 4 of a pair of fairings mounted on the wing tips of a tar get aircraft. The lenses 1, 2 are designed to have a focus F at a distance of half of the wavelength away from the surface of the lens.
A reflector 5 in the nose cone 3 is of part spherical form and has an inside curvature approximately equal to the outside curvature of the lens 1. It is mounted on the end of a movable ram 6 extending from an electric actuator 7 which is thus able to move the reflector 5 towards and away from the lens 1 about the focus 7. The actuator 7 is controlled by means of signals from a programme unit (not shown) in the aircraft.
A reflector 8 in the nose cone 4 is also of part spherical form and has an inside curvature approximately equal to the outside curvature of the lens 2. This reflector is fixed to a bulkhead 9 at the rear of the nose cone. It is shown as being positioned with the focus point F of the lens 2 falling on its forward surface. This position may be adjusted to be forward of the focus point or to the rear of the focus point. The position would, however, be determined and fixed before use of the lenses.
Although only one of the reflectors shown in FIG. 1 is movable in use, both reflectors may be mounted so as to be movable in use, the extent of the movement being determined as follows:
When the reflector is within a distance of one eighth of the wavelength on either side of the focal point of the lenses 1 and 2 the radar cross-section is reasonably constant. Thus, the reflector 5, when placed at the focal point F and moved towardsand away from the lens 1 by a distance of one eighth of the wavelength is able to reflect substantially all of the received wave energy but, in so moving the reflector, the phase of the wave is changed. Movement of the reflector over the total range of one quarter of a wavelength produces a change in phase of the reflected wave energy of half a wavelength.
The distance over which the radar cross-section of a lens remains substantially constant is equal to a distance of approximately one quarter of a wavelength for most lenses, but this distance need not be equally spaced on either side of the focal point of the lens. For example, three quarters of the distance may be from the focal point towards the lens, the remaining quarter of the distance being from the focal point away from the lens. Thus whilst the range of movement of a reflector is approximately one quarter of a wavelength, the range may be divided into movement either side of the focal point in many ways.
By using the apparatus shown in FIG. 1, simple glint may be simulated. However, one use of this form of in vention lies in the testing of a radar system where variation of the phases of waves from a number of reflectors may be used to simulate glint from a target, i.e. where the reflected wave appears to come from a position other than from the target. In such. a case, two or more spherical lenses would be employed spaced some distance apart and the phase change characteristics of each of them (i.e. the spacing of the reflectors from their respective lenses) is varied by means of a programme controlling the actuator so as to cause the resultant, combined reflected wave to appear to come from a position other than that of any of the lenses.
In the arrangement shown in FIG. 2, in which it is desired to vary the amplitude of the reflected wave signal, a resistive coating 11 such as a film, card or sheet of a material of suitable resistive value is placed upon the surface of a spherical lens 12 and a reflector 13 is mounted for movement towards and away from the coated surface of the lens along an axis through the centre of the lens. The lens and reflector are mounted within a nose cone 10 of a target and the reflector is movable by means of an electric actuator 7 as in the arrangement shown in FIG. 1. The focal point F of the lens in this case is on the surface of the lens. The amount of energy reflected by the lens is at a maximum when the reflector is positioned against the resistivecoated lenssurface but the amount of wave energy reflected is reduced as the reflector is moved away from the focal point of the lens, as the resistive coating absorbs the energy, and the amount of movement may be as much as one quarter of a wavelength away from the focal point of the lens. Thusthe amplitude of the reflected wave may be varied by movement of the reflector.
The reflector may be moved from a position where it lies adjacent to the resistive coating, to a position where it lies at a distance of one quarter of the wavelength in use away from the resistive coating.
suitable materials for the resistive coating which may be in the form of a sprayed coating, a film or sheet, are colloidal graphite, carbon loaded cloth, laminate or carbon loaded rubber sheet.
Typical uses for this form of the invention where only a single lens and reflector are required lie in the testing or radar systems where controlled fading of the reflected signal is required or where one target may be used to simulate several different echoing areas.
In both of the examples described, a reflector may be moved manually or remotely by any of various means such as a nut and sore mechanism, a solenoid operated mechanism etc., but such mechanisms are well known and to not form a part of the invention.
1. A microwave lens arrangement comprising a spherical microwave lens with a focus approximately on its surface and an electrically resistive material over part of its outer surface, and a part spherical reflector movable towards and away from the lens between posi tions where it is against the resistive coating or spaced from it by approximately one quarter of a wavelength, whereby movement of the reflector produces a variation in the amplitude of a reflected wave.
2. A microwave lens arrangement as in claim 1 wherein the electrically resistive material is carbon loaded cloth.
3. A microwave lens arrangement as in claim 1 wherein the electrically resistive material is carbon loaded rubber.
4. A microwave lens arrangement as in claim 1 wherein the electrically resistive material is colloidal graphite.
5. An apparatus for simulating the glint of an object illuminated by a radar beam including, in combination, two spherical microwave lenses spaced transversely to the radar beam, a fixed reflector associated with one lens, a movable reflector associated with the other lens, and means for moving the movable reflector towards and away from its associated lens thereby varying the phase difference between the beams reflected from the fixed and movable reflectors.
6. An apparatus for simulating the glint of an object illuminated by a radar beam including in combination two transversely spaced spherical microwave lenses, a separate reflector associated with each lens and movable towards and away from its associated lens and means for moving the reflectors independently of each other whereby movement of at least one of the reflectors produces a variation of the phase difference be tween the beams reflected by said reflectors.