US 3703723 A
A portable passive reflector that utilizes a Luneberg lens to focus upon a diode array positioned about the focal region of the lens, an interrogating signal eminating from a predetermined direction. A reflecting plate is mounted behind the diode array a distance equivalent to multiples of a quarter wavelength of the interrogating signal. By selectively pulsing the diode array, the effective cross section of the reflector as seen by the interrogating radar is varied thus modulating the reflected signal.
Description (OCR text may contain errors)
United States Patent Albanese et al.
[451 Nov.21, 1972  PORTABLE PASSIVE REFLECTOR  Inventors: Victor Albanese; Arthur E. Johnson, both of Valley Stream; Charles L. Dietz, Williston Park, all of NY.
 Assignee: Grumman Aerospace Corporation,
Bethpage, L.I., NY.
221 Filed: 1811.9,1970
211 Appl.No.: 1,610
V [1.5. C! ..343/18 D, 343/911 L  Int. Cl. ..H01q 15/08  Field of Search ..343/l8 D, 911 L  References Cited UNITED STATES PATENTS 3,308,464 3/1967 Lewis ..343/l8 D Kelleher ..343/l8 D Cole et al. ..343/9ll L Primary Examiner-Malcolm F. Hubler Attorney-Morgan, Finnegan, Durham & Pine  ABSTRACT A portable passive reflector that utilizes a Luneberg lens to focus upon a diode array positioned about the focal region of the lens, an interrogating signal eminating from a predetermined direction. A reflecting plate is mounted behind the diode array a distance equivalent to multiples of a quarter wavelength of the interrogating signal. By selectively pulsing the diode array, the effective cross section of the reflector as seen by the interrogating radar is varied thus modulating the reflected signal.
4 Claims, 5 Drawing Figures I I I l I I I I IIIIIIIIIIIIIIII I l I I PKTENTED 21 3. 7 O3, 723
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INVENTORS VICTOR JALBANESE ARTHUR1-'.'E.JOHN$ON BY CHARLES LDIETZ ATTORNEYS PORTABLE PASSIVE REFLECTOR BACKGROUND AND OBJECTS OF THE INVENTION angle coverage. Additionally, passive reflectors have been constructed which were capable of modulating the reflected signal. The advantages of combining the above features into one passive reflector embodiment, in addition to having that embodiment be light in weight, portable, reliable and inexpensive, are readily apparent. 7
It is, accordingly, an object of this invention to provide a new and improved passive reflector that incorporates the foregoing advantages into a single passive reflector embodiment in a simple and efflcacious manner.
Another object of this invention is to provide a new and improved passive reflector that enables the modulation of a reflected signal by the selective pulsing of a diode matrix.
Another object of this invention is to provide a new and improved passive reflector that utilizes general purpose diodes to comprise the diode matrix.
Another object of this invention is to provide a new and improved passive reflector that is capable of reflecting a narrow beam reply signal.
Another object of this invention is to provide a new and improved passive reflector wherein the reflected signal is reinforced by energy reflected from a reflecting plate appearing behind a diode matrix that encompasses a range of focal points of the lens.
Another object of this invention is to provide a new and improved passive reflector wherein an interrogating pulse is reflected without imparting significant delay between the interrogating pulse and the reflected pulse.
Another object of this invention is to provide a new and, improved passive reflector wherein interrogating pulses from more than one source are capable of being reflected.
These and other objects and advantages of the invention will be set forth in part hereinafter or will be obvious from the description which follows or from practicing the invention. The invention consists in the novel parts, constructions, arrangements, combinations and improvements herein shown and described.
SUMMARY OF THE INVENTION Briefly described, the present invention is directed to a new and improved passive reflector for utilization as a portable beacon to identify uncharted positions to specific types of interrogating radar.
A Luneberg lens is utilized to focus r-f incident energy at a peripheral point diametrically opposite its center point of entry into the lens. The electrical path length from the aperture plane to the focal point is the same for each ray thus permitting a plane wave intersecting the lens to be focused upon the focal point with a maximum of amplitude.
A diode matrix is affixed to the lens so as to encompass focal points generated by a radiating source that can vary over a 120 azimuth and over a 40 elevation.
A reflecting plate, having a configuration similar to that of the diode matrix, is affixed behind the diode matrix and is spaced from the diode matrix a distance equal to a multiple of a quarter of a wavelength of the interrogating wave.
R-F signal modulation for the passive reflector is provided by the two-stage switching of the diode matrix. When the diodes are unbiased, a signal of maximum in tensity is reflected by the passive reflector. This is caused by a free space grid resonance which represents a low impedance thus reflecting most of the r-f energy impinging upon the matrix. In this active mode, the reflector plate behind the diodes reinforces the series resonance.
Applying a d-c positive bias to the diodes, results in a shunt resonant condition. That is, most of the r-f energy now passes through the diode matrix, defocuses, reflects from the reflector plate and is scattered. The resulting reflected signal received by the interrogating radar is thus at its minimum intensity. By successively biasing and unbiasing the diode matrix, the reflected r-f energy becomes amplitude modulated. Coding may be accomplished by selectively biasing and unbiasing the diode matrix according to a preconceived sequence.
It will be understood that the foregoing general description and the following detailed description as well are exemplary and explanatory of the invention but are not restrictive thereof. Thus, while the passive reflector of this invention is particularly adapted to and was primarily designed for use as a portable Forward Air Controller Beacon (FACB), the principles underlying the objects of the invention are not limited to such usage. However, since the invention is particularly adaptable to such usage, reference will be made hereinafter thereto in order to provide an example of a practical and useful embodiment of the invention.
The accompanying drawings, referred to herein and constituting a part hereof, illustrate the preferred embodiment of the invention, and together with the description, serve to explain the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view of a passive reflector embodying the present invention;
FIG. 2 is an enlarged partial front elevational view depicting the face plate of the modulator.
FIG. 3 is an enlarged front elevational view of the lens assembly detached from the remainder of the passive reflector structure and sectioned so as to expose to view the diode matrix and the reflector plate.
FIG. 4 is a perspective view of the lens assembly illustrating the basic principles of the invention.
FIG. 5 is a top view of the lens assembly of FIG. 4 de picting the reflection of two signals eminating from separate radiating sources.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now more particularly to the embodiment of the invention shown in the accompanying drawings, there is illustrated in FIG. I a Forward Air Controller Beacon (FACB), indicated generally by reference numeral 10 comprising a lens assembly 9 mounted upon a portable support structure 11. Additionally mounted upon support structure 11 is modulator unit 12. Modulator unit 12 is electrically coupled over conductors 13 and 14 to a d-c source of potential, namely, battery 15. Additionally, modulator unit 12 is coupled electrically to lens assembly 9 over electrical conductors 16.
Lens assembly 9 and modulator unit 12 are both detachably affixed to support structure 11 thus enabling the PACE to be dismantled into a number of components for ease of transport. Electrical conductors 13 and 14 detachably couple battery 15 to modulator 12 by way of plug-in socket 24. Similarly, electrical connectors 16 detachably couple lens assembly 9 to modulator 12 by way of plug-in socket 17. Modulator 12 may also be detached from support structure 11 by the loosening of retaining bolts 18.
Support structure 11 may be any type of portable supporting structure. As here preferably embodied, support structure 11 takes the form. of a tripod, whose legs are individually adjustable as to length so as to enable the level positioning of the lens assembly upon any terrain feature.
In accordance with this invention, lens assembly 9 (FIG. 3) comprises a Luneberg lens 19, a lens radome 20, a diode matrix 21, a reflector plate 22 and a foam layer 23.
Luneberg lens 19 may be any of a number of commercially available lenses. As herein preferably embodied, an 18 inch lens is utilized which is constructed of a solid foam sphere whose dielectric is smoothly graded from FLO to e= 2.0. The lens will provide a radar cross section of 200 square meters at 16.5 Gl-l over 70 percent of a 90 circular cap with a minimum radar cross section of 150 square meters over the remaining 30 percent.
Lens radome 20 is designed to protectively encase lens 19, diode matrix 21, reflecting plate 22 and foam layer 23. Radome 20 can be fashioned from any material evidencing structural strength and low loss passive electrical properties. As herein preferably embodied, a radome fabricated of fiberglass is utilized.
Diode matrix 21 comprises an array of one-half wavelength spaced, horizontally arranged, alternately positioned, serially connected, general purpose diodes. As here preferably embodied, a matrix array of approximately 1,100 general purpose diodes is utilized. The diodes 27 are horizontally placed and are spaced in rows separated by a distance of one-half of a wavelength with the rows alternately positioned so as to create a diode matrix that has 120 azimuth and 40 elevational coverage. The diode matrix 21 can either be physically affixed to or be an integral part of lens 19 with the diode matrix positioned on the lens so as to coincide with the focal points generated by a radiating source whose position can vary 120 in azimuth and 40 in elevation.
In accordance with the invention, reflecting plate 22 As previously described, electrical connectors 16 electrically couple diode matrix 21 to modulator unit 12 (FIG. 2). Modulator unit 12 may be of any known design and, in the herein preferred embodiment, modulator unit 12 is capable of generating an output signal that either is a periodic dot-dash signal, or if desired, is an interrupted dot-dash signal.
FIG. 2 illustrates the face plate of modulator unit 12. When power switch 28 is switched to its on position and blinker switch 25 is in its off position, a periodic time modulated dot-dash type signal is supplied to diode matrix 21. By varying the settings of code switches 26, various dot-dash type codes are created. As here preferably embodied, three code switches 26 are utilized so as to enable the generation of eight distinct coded signals.
If blinker switch 25 is switched to its on position when power switch 28 is in its on position, a time delay is imposed between each coded sequence of signals. Such a delay aids in the detection of a coded sequence by separating the sequences from each other by a time pause.
In accordance with the invention and with particular reference to FIG. 4, an aircraft is depicted emitting interrogating signals from its search radar. Should the aircraft approach lens assembly 9 from a point within the window defined by A B D "and C that is, as herein preferably embodied, a window having a azimuth and a 40 elevation with respect to the horizon, the signals emitted from the search radar on the aircraft will strike lens 19 and will be focused upon a peripheral point E on the back surface of lens 19 which is diametrically opposite to the points of entry into the lens of the energy signals. By having the emitting source of energy fall within the window A B D and C the focused energy directed to the back of the lens is focused upon a portion of the lens that has affixed to it the diode matrix 21 and reflector plate 22. The result is that when the diode array is not pulsed, signals of maximum intensity are reflected back to the aircraft along a path identical to the path followed by the interrogating signals when they approached the lens. Maximum reflection occurs in this mode due to the fact that the unbiased diode matrix 21 creates a free space grid resonance which represents a low impedance to the interrogating signal thus causing most of the energy impinging upon the matrix to be reflected. The reflector plate 22 reinforces the series resonance by adding shunt capacitance at the plane of the grid. The reflected signal does not experience a delay other than a delay due to free space transit time, and since the signal is reflected back along the same path that its corresponding interrogating signal followed, a narrow beam reply signal is generated which reduces the chances of outside detection.
When diode array 21 is pulsed, a major portion of the energy is no longer reflected back to its source as a focused signal. Rather, the energy passes through diode matrix 21, defocuses, reflects off of reflecting plate 22 and is then scattered. Although some of the scattered reflected energy is detected by the search radar on the initiating aircraft, a noticeable difference in signal intensities will exist between the energy level of the signal reflected when diode'matrix 21 was not pulsed and the energy level of the signal reflected from the pulsed diode matrix. In keeping with the invention, a variation of from 25 to 50 in the energy level of these signals results in acceptable operation of the system.
This difference in energy levels between the return signals results in amplitude modulated return signals as viewed by the airborne search radar, the modulation frequency of such signals being determined by the frequency of the diode matrix pulsing signal. In keeping with the invention, a modulated periodic diode matrix pulsing signal can be used rather than an unmodulated periodic diode matrix pulsing signal so as to impart distinguishable characteristics to one passive reflector beacon as opposed to another.
In accordance with the invention, FIG. 5 depicts the situation wherein lens assembly 9 reflects signals that eminate from more than one radiating source. As long as a radiating source falls within the elevational and azimuth limits of coverage of lens assembly 9, reflection of such a signal will result.
The preceding description and accompanying drawings relate primarily to the use of the portable passive reflector as a Forward Air Controller Beacon. However, as previously mentioned, it should be understood that the concepts of the invention are not limited to the specific embodiments herein shown and described but departures may be made therefrom within the scope of the accompanying claims, without departing from the principles of the invention and without sacrificing its chief advantages.
What is claimed is:
l. A portable passive reflector beacon for the reflection of an interrogating signal comprising:
a. a source of d-c potential;
b. a modulating network electrically coupled to said source for providing a modulated output signal;
c. a dielectric lens structure capable of focusing energy from a point of entry into said lens to a focal point appearing at the peripheral edge of said lens and diametrically opposite to said point of entry;
d. a diode matrix affixed to the periphery of said dielectric lens so as to encompass an area that is defined by the focal points of said lens which would respond to an r-f energy source eminating from a point above the horizon;
e. reflecting means affixed to said lens and behind said diode matrix a distance equal to a distance defined in multiples of a quarter of a wavelength of said interrogating wave; and
f. means for electrically coupling said diode matrix to said modulated output signal so as to conductively bias said diode matrix according to a predetermined sequence.
2. A portable passive reflector beacon as described in claim 1 wherein said diode matrix is affixed to said lens so as to encompass focal points generated by a radiating source that can vary over a azimuth and a 40 elevation with respect to said lens.
3. A portable passive reflector beacon as described in claim 1 wherein said diode matrix utilizes general purpose diodes horizontally positioned with respect to the vertical axis of said lens.
4. A portable passive reflector beacon as described in claim 1 wherein means having a low dielectric constant is positioned between said diode matrix and said reflecting means.