US2936453A - Passive reflector - Google Patents

Description

May 10, 1960 H. P. COLEMAN 2,936

I PASSIVE REFLECTOR I Filed July 2, 1957 2 Sheets-Sheet 1 INVENTOR H. PARIS COLEMAN I '7 ATTORNEYJ May 10, 1960 H. P. COLEMAN 2,936,453

PASSIVE REFLECTOR Filed July 1957 2 Sheets-Sheet 2 INVENI'OR H. PARIS COLEMAN ATTORNEY5 Pat I the.Na'vy' V I V i P qii i 11951; sfial 669,689

.6Claiins. (03343 -915) i I I transmissive. cede 1952 see. 266) hereinmay be manufactured used by or for theG overnment of the United States of Amerrcafor governmental purposes without the paymerit of 'any r oyalties thereon or therefor.

This invention relates in general to a lens antenna and m particularly to a passive microwave reflecting lens utilizing an artificial dielectrical medium.

'li lectrical signal reflectors :constructed of concentric rings of dielectricmaterial with a varying dielectricrcon stantaare well known in the art as adaptations of the lens principle pointed outin R. L. Lunebergs Mathematical. 'Iiheory o f opticsi, pages 208-213. Such structure af fogdg;a .;reflectorqwithexcellentfocusing abilities over a wide solid angle of scan and results in'max'imum antenna gain ,This apparatus has been limited in its applicationbecause f the'relatively large bulk and Weight oli;th,e:lens. ,Recentlypadvances have been made in de-.

creasing: thei.w eight of. such antenna designs by use of artificial 3dielectric, mediums but in many applications especially where size and weightare a critical consideration, the present.,lens apparatus is generally unsuitable.

. There areothertypesof antennas that do provide lightweightzandcompactness, for example, a corner reflector is..-a ,,device..which. r'eflectsa' fairly well focused signal wlienjthetdirectional alignment is'accurate. However, theficorner reflector. cannot even closely approach the uniform ;-response of a reflecting'lens over a wide angle since it does not reflect efliciently except at several rather limited angles, there being a marked deterioration of the response of a corner reflector at angular displacements of more than several degrees" from the optimum response angles/ I 'There'fofeg'it is an object of this invention to provide a -reflector antenna having the characteristics and advantag's of a solid dielectric lens without the inherent accompanyifig disadvanta e of great bulk and w i ht.

Another object is to provide such a lens in a structure capable of expansion from a compact initial volume to emerge hilly-expanded volume. I

*;no tlie"r object is to provide a method of creating an artificial dielectric medium of varying dielectric-constant er expanding such alens from an initial volume to a liii'ge 'fully-expaiidedvolume, such expansion to be unifoir'n "and accomplished Without the possibility of enringing the lens components.

.Fig. 4 is an illustration of the lens when it is partially :expanded Inbrief, the'lensdescribed herein comprises a'plurality of extensible filaments of dielectric material withv a plurality of conducting obstacles spaced in or on said extensible filaments at. predetermined longitudinal intervals,-sa'id extensible filaments positioned longitudinally between two end plates in a predetermined variation of density and radius configuration to form a dielectric me-v dium of suitable non-uniform dielectric constant. The aforementioned structure is enclosed by an envelope having a' plurality of bellows-likepleats in planes parale lel toj the? two-end plates, said bellows-like pleats facili tating efxpansionalong an axis normal to the two end plates and parallelto the extensible filaments. On one portion of the envelope is a reflector shield covering less than of the circumference, made suitable for, reflecting collimated rays for electromagnetic energy arriving at the lens.- surface of the reflector shield. Such a lens will provide a microwave energy reflecting assembly with excellent focusing characteristics through a large solid angle of scan.

Referring now to Fig. 1, the structure of the. cylindrical reflecting lens contains a plurality of extensible filaments 11 which are a dielectric made of plastic, paper or other 7 suitable material; Filaments 11 support a plurality of conducting obstacles 11(a) such as imbedded" metal pins, a metallic coating, or the like, which, are positioned lengthwise on' 01 therein. The extensible filaments -11 are attached longitudinally between the circular end plates 12 which may be of any lightweight non-conductor or'diele'c'tr'ic such as plastic, cardboard or other suitable material into which the extensible filaments 11 may preferably be molded. An envelope 13 of plastic,,paper or any other .niaterial of suitable weight and strength is:

spacers 15' may be of any lightweight substance such as plastic, paper or other suitable material and the. apertures 15(a) through which the filaments 11 are threaded perferably correspond to the filament spacing on the circula'r end plates 12. The spacers 15 also contain vents 15(b) to allow an expansion medium such as air, carbon dioxide, or other suitable material, to flow throughout the envelope 13. To retain the prescribed pattern of location of the extensible filaments 11, the spacers 15 are situated longitudinally throughout the envelope 13. The extensible filaments 11 are threaded through apers tures 15(a) in the spacers 15, thus maintaining proper location of the extensible filaments 11 in the expanded position and further serve the purpose of preventing snarling of the extensible filaments 11 when the envelope 13 is in the u'n'expanded position or being expanded. The spacers 15 also aid in giving the lens rigidity when such lens is fully expanded. The metallic reflector shield 16 may be achieved by a metallic coating or point on the surface of the envelope 13 and may be equal to or less than 180 in circumferential surface.

It is well known in the art that the normal energy propagational characteristic of a medium such as'air can be altered by the placement of conductors therein. By.

selecting the size and spacings of such conductors, the

2,936,455 j Patented May n aeo Such rays are focused on the inner dielectric constant may be eflectively altered substantially uniformly in a selected manner. To achieve suitable energy propagational characteristics in this invention the conductors 11(a) are spaced upon the extensible filaments 11 which are attached to the end plates 12 in the proper variation of density and radius configuration to simulate the alteration'of dielectric constant required fora selected end result.

When the lens is fully expanded, the extensible filaments 11 form a cylindrical dielectric medium of varying dielectric constant It satisfying the Luneberg formula center of the lens to the radial point where the dielectric constant is being determined. Then, as shown in Fig. 2,

collimated rays of electromagnetic energy entering the lens will be focused at a point S diametrically opposite the entrance of the ray w passing through the center of the lens. The reflector shield 16 should preferably cover somewhat less than 180 in circumferential surface to prevent undue shadowing of the collimated rays such as y and z, farthest away from the radius vector ray w, and still give the lens its greatest solid angle of reflecting coverage. As taught by the art in Modification of the Luneberg Lens to Perform as High-Gain Wide Angle Reflector, B. Gorr and F. S. Holt, Air Force Cambridge Research Center, Internal Memorandum, February 15, 1956, the shadow losses are negligible for a 90 reflector shield and increase as the reflector shield is enlarged in angular coverage. At 140 there is about 3 db loss in antenna gain and the shadow losses increase so rapidly thereafter as to make a larger angle reflector shield impractical. Therefore, 140 is recommended as a higher limit of the solid angle of scan and circumferential surface of reflector shield 16. Thus nearly all of the rays striking the reflector shield 16 at a point focus are reflected back through the lens to form a collimated beam of exceptional focus maximizing antenna gain.

The lens provides exceptional focus throughout a wide angular sector thereby eliminating the necessity of close directional alignment as is necessary with other apparatus. The lens further provides a radar marker or target for navigation or rescue signalling that requires no visual perception to precisely align such apparatus as a corner reflector since rotation of the lens through little more than half revolution will give a 360 coverage.

With reference to Figs. 3 and 4 it is apparent that the lens is made capable of expansion from a compact size Fig. 3 through partial expansion Fig. 4 to a large fully expanded volume of Fig. 1 by the elongation of the accordion-pleats of envelope 13. The support rings 14 typically used to form the accordion-pleats give the structure stability and control the uniform cylindrical shape while the envelope 13 is being expanded. The expansion of the antenna structure may be accomplished by applying a pressure differential within the envelope 13 either by chemical or physical means typically through a flexible tubular member 17 which can be closed off when desired. The lightweight of the structure makes normal respiratory pressure of the human body adequate for expansion. As the lens expands, the pressure difierential passes through the vents 15(b) provided in the spacers 15 to all sections of the envelope 13, expanding each section and pleat smoothly and uniformly as shown in Fig. 4. The spacers 15 and support rings 14 furnish rigidity to'the expanded envelope 13 and the spacers 15 also aid in maintaining the proper extensible filament 11 location. Further, by containing the extensible filaments 11 at several intervals throughout the envelope 13, the spacers 15 shorten the free lengths of the extensible filaments 11 in the unexpanded position thereby decreasing the 'possibility of their snarling or' entangling during collapsed storage or during expansion of the lens.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A radiant energy reflector comprising, first and second end members, a plurality of conducting obstacles, extensible means for spacing 'said plurality of conducting obstacles at predetermined locations between the first and second end members, a longitudinally extensible enclosure cooperative with said first and second end members for containing said plurality of conducting obstacles, said enclosure being of non-conducting material, means for establishing a fluid pressure within said enclosure, and means for reflecting radiant energy passing within said enclosure.

2. A radiant energy reflector comprising, first and second end members, a plurality of conducting obstacles, extensible means for spacing said plurality of conducting obstacles at predetermined locations between the first and second end members, spacer means for retaining said extensible means between said first and second end members and substantially perpendicular thereto, each of said spacers containing a plurality of apertures for maintaining selected spacing between adjacent extensible means and expansion pressure vent-means for restricting the passage of fluid through the spacers, a longitudinally extensible enclosure cooperative with said first and second end members for containing said plurality of conducting obstacles, and means for establishing a fluid pressure within said enclosure.

3. In a microwave reflector lens, a plurality of conducting obstacles, a plurality of extensible dielectric filaments, means for spacing said plurality of conducting obstacles at predetermined intervals along the plurality of extensible dielectric filaments, filament spacer means,- each of said filament spacer means having vent meansand a plurality of apertures, each of the plurality of extensible dielectric filaments being threaded through a respective one of the plurality of apertures in each of said filament spacer means such that the plurality of extensible dielectric filaments are parallel to one another and in the form of a cylindrical dielectric medium having a dielec-- tric constant whose magnitude varies along a radius through the center of the cylindrical dielectric medium,.

two end plates, the plurality of extensible dielectric filaments connected between the two end plates, means in-.

cluding bellows-like pleats cooperating with the two end plates for enclosing the space between the two end plates, said bellows-like pleats formed in planes parallel to the two end plates, means for reflecting radiant energy, said last mentioned means positioned between the two end plates such that the point of focus of any beam of rayswithin the solid angle of scan will fall on the inner surface of said last mentioned means.

4. A radiant energy reflector comprising first and second end members, a plurality of conducting obstacles,

extensible means for spacing said conducting obstacles:

between the first and second end members, spacer means for positioning said extensible means between the first and second end members and substantially perpendicular thereto, each of said spacer means containing a plurality of apertures for maintaining said extensible means in predetermined relation in the planes of said spacer means,

each of said spacer means containing vent means for per-- for positioning said extensible means between the first and second end members and substantially perpendicular thereto, each of said spacer means containing a plurality of apertures for maintaining saidextensible means in predetermined relation in the planes of said spacer means, each of said spacer means containing vent means for permitting passage of fluid therethrough, an extensible enclosure cooperative with said first and second end members, means for establishing a fluid pressure within said enclosure, and reflecting means associated with said enclosure for reflecting radiant energy, said reflecting means positioned between the first and second end members such that the. point of focus of any beam of rays within the solid angle of scan will fall on the inner surface of said reflecting means.

6. A radiant energy reflector comprising first and secend end members, a plurality of conducting obstacles, extensible means for spacing said conducting obstacles between the first and second end members, spacer means forpos-itioning said extensible means between the first vthreaded through a respective one of said plurality of apertures in each of said spacer means such that said extensible means are parallel to one another'and in the form of a cylindrical dielectric medium having a dielec tric constant whose magnitude varies along a radius through the center of the cylindrical dielectricv medium,

reflecting means tor reflecting radiant energy, said refleeting means positioned-between the first and second end members such that the point of focus of the beam,

of rays within the solid angle of scan will fall on the inner surface of the said reflecting means, and means cooperating with said first and second end members for maintaining said extensible means in an extended con-.

OTHER REFERENCES Designing Microwave Dielectric Lens, K. S. Kelleher et aL, vol. 29, June 1956, pp. 138-142.

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