US 3781896 A
An array of elements which provides improved directivity characteristics when engulfed in a low-velocity refractive medium having an effective array length in the direction of propagation not exceeding any of the corresponding transverse dimensions of the engulfment.
Description (OCR text may contain errors)
States Patent [1 1 [4 1 Dec. 25, 1973 ENGULFED SUPEIRECTIVE ARRAYS  Inventor: William .11. Toulis, 3920 Leland St.,
San Diego, Calif. 92106  Filed: Nov. 12, 1969  Appl. No.: 875,691
 US. Cl. 343/754, 343/911  Int. Cl. H01q 19/06  Field of Search 343/753, 754, 755, 343/908, 909, 911
 References Cited UNITED STATES PATENTS 2,433,924 1/1948 Riblet 343/873 2,611,869 9/1952 Willoughby... 343/753 3,230,536 l/l966 Cheston 343/754 3,321,763 5/1967 Ikrath et al. 343/754 3,487,413 12/1969 Shores 343/755 3,550,139 12/1970 McFarland.... 343/754 3,496,570 2/1970 Lewis 343/755 FOREIGN PATENTS OR APPLICATIONS 146,925 9/1954 Sweden 343/753 Primary ExaminerEli Lieberman Att0rneyRichard K. MacNeill  ABSTRACT 6 Claims, 5 Drawing Figures PATENTEUBECZS 1915 FIG. 4
INVENTOR. W/LL/AM J. TOUL/S ENG ULIFED SUPEIRDIRECTIVE ARRAYS RELATED APPLICATION This application relates to my previous US. Pat. No. 3,487,464, issued Dec. 30, 1969, for Superdirective Transmitting and Receiving Array.
BRIEF DESCRIPTION OF THE INVENTION The-present invention relates to engulfed superdirective arrays and more particularly to engulfed superdirective arrays utilizing a medium which is refractive and whose wave velocity is lower than that of the propagation medium or that provided by an additional focusing means, such as a Luneberg lens.
According to the invention, an array of elements is engulfed in a low-velocity refractive medium in a geometric curvilinear configuration, i.e., both the array and the engulfment are preferably curved. Since the engulfing medium is refractive, the waves to or from the elements will both refract and travel more slowly through the engulfment by an amount that varies with each element in the array. The basic intent is to create a situation wherein the phase interference effects occur more rapidly in radiation space due to the longer travel time in the low-velocity engulfment and prior to reaching the general outside medium; i.e., air or free space, in the case of electromagnetic elements, and water, in the case of sonar transducers. The physical shape of the array can take various forms, as dictated by the environmental necessity of associated equipment, etc. For the specific case of forming a multiplicity of beams in different directions in space, the one proportion which should be adhered to is that the effective array length in the direction of propagation should not exceed any of the corresponding transversed dimensions of the engulfment. This will insure a more uniform phasing capacity for the various preformed beams since the difference in path length through the engulfment is adirect measure of the attainable superdirective state; that is, an array in a spherical shell should yield uniformly superdirective beams; whereas, an elongated array might yield more superdirectivity for a few, but not all, of the possible beams.
The essential criterion for an engulfed antenna to be in a superdirective state is satisfied when the differences in path length occur within the engulfing lowvelocity medium. This requirement cannot be fulfilled with a plane array unless the boundary of the engulfing medium with free space is curved rather than planar; this condition assumes that delay lines are employed to form and steer the radiation pattern. An array in a spherical shell represents a practical configuration for phased arrays and especially if the refractive index of the engulfment in the shell is variable rather than homogeneous. However, total internal reflections limit the attainable supergain to only modest amounts unless the refractive index for each element of the array is designed to vary inversely with the radius, approximately. In addition to the attractive possibilities for superdirective phased arrays, the concept of engulfing an array may be used also to improve the aperture efficiency of a superdirective paraboloid and to provide complete preformed beam coverage for Luneberg and other spherical lenses. Thus, superdirective spherical lenses might prove to be simpler to design and construct than phased arrays.
An object of the present invention is the provision of engulfed arrays utilizing a low-velocity type of refractive medium for improved directivity.
A further object of the invention is the provision of engulfed superdirective arrays in which beam-steering may be simply accomplished through a proper phasing of the elements.
A third object of the invention is the provision of engulfed array elements to serve as feeds for continuous beam-steering in conjunction with focusing antennas such as paraboloids and Luneberg lenses.
Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the FIGS. thereof and wherein:
FIG. 1 is a schematic representation of one embodiment of the present invention;
FIG. 2 is a schematic representation of another embodiment of the present invention;
FIG. 3 is a schematic representation of an individually engulfed element of an array which is still another embodiment of the present invention;
FIG. 4 is a schematic representation of a closespaced array with elements engulfed in FIG. 3; and
FIG. 5 is a schematic representation of a modification of the embodiment of FIG. 4.
DETAILED DESCRIPTION OF THE DRAWINGS Referring to FIG. II, a curved homogeneous refractive medium is shown generally at 11 engulfing dipoles 12, 13,14,16,17, 18, 19, 21, 22 and 23. Solid lines 24, 26, 27, 28 and 29 indicate typical in-phase radiation paths when the boresight position of the main lobe is adjusted to be toward the zenith of the array. Dotted lines 31, 32, 33, 34, 36 and 37 indicate typical out-ofphase radiation paths, as seen from a typical azimuth position other than the specific boresight direction.
Referring to FIG. 2, a refractive engulfing medium of variable index n (r), is shown generally at 41 engulfing dipoles 42, 44, 46, 47, 48, 49, 50, 51, 52 and 43. Solid lines 53, 54, 56, 57 and 58 indicate typical in-phase radiation paths at an angle substantially toward the zenith of the array. Dotted lines 61, 62, 63, 64 and 66 indicate typical out-of-phase radiation paths, as seen from a typical angle other than the boresight direction.
Referring to FIG. 3, a dipole 72 is shown engulfed by a refractive low-velocity medium 73 which, in turn, is engulfed by low-velocity medium 71 whose refractive index n (r) is variable either continuously or stepwise.
Referring to FIG. 4, an engulfing refractive medium of variable refractive index n(r) is shown generally at 71 engulfing pre-engulfed dipoles 72 in the manner of truncating a portion of the engulfment in FIG. 3.
Referring to FIG. 5, a stepwise variable refractive medium is shown generally at 74, 75 and 76 engulfing pre-engulfed dipoles 77 which are carried by a reflective surface 78. Typical in-phase radiation paths in the direction of the zenith of the array are indicated by solid lines 79, $1, 82, 83 and 84. Typical out-of-phase radiation paths at an off-boresight position are illustrated by dotted lines 86, 87, 38, 89, and 91.
OPERATION Referring back to FIG. I, it can be seen that the inphase radiation paths represented by solid lines 24, 26, 27, 28 and 29 are characterized by symmetrical paths relative to the boresight direction toward the zenith; that is, the paths through the engulfing medium 111 for symmetrically opposite rays (such as 24 and 29, 26 and 28) are of equal length and, thus, the net difference is zero, as intended. On the other hand, the out-of-phase ray paths represented by dotted lines 32, 33, 34*, 36 and 37 have distinctively unequal paths through the engulfing medium which tend to enhance the resulting net differences in phase: rays 32 and 33 involve greater paths through the medium than in the in-phase condition while rays 36 and 37 involve shorter paths, thus, the resulting net differences in phase are greater than would have been with air alone as the only medium. It should be noted that the dotted line 31; is not transmitted through the engulfing medium ill, but reflected completely back into the medium in the manner predicted by Snells equation for the onset of total reflection. Either surface of the engulfing medium can be blocked or coated with a reflecting material, if desired, to avoid bilateral main lobes with the array. In this, and in all of the embodiments, the engulfing medium 11 can be homogeneous (i.e., have a uniform refractive index), or it can have a variable refractive index, as desired, or in the third alternative, can have a stepped refractive index (i.e., discrete layers of material of different refractive indices or tapered arrangements of one or more different materials).
Referring to FIG. 2, the medium 41 represents a spherical engulfment whose index n (r) varies with radius r. Here, again, the differences in path lengths through the medium between the solid lines 53, 54, 56, 57 and 58, and the dotted lines 61, 62, 63, 64 and 66 are self-evident, and, if dipoles 42-52 are properly phased, will result in an in-phase condition at a preferred azimuth, and a random-phase condition at another azimuth to the array.
The embodiment illustrated in FIGS. 3 and 4 relies on a distinctive rather than a general (as in FIGS. 1 and 2) layered engulfment of each element of an array in order to minimize, if not avoid, the design limits imposed by the onset of total internal reflection. The basic design structure of each element (for an array or feed for a focusing antenna) is shown in FIG. 3 to be spherical and to consist of a variable refractive index of radial symmetry with the dipole element at the center. Thus, this arrangement should be free of both refractive and total reflection effects and especially if the array element is small compared to the spherical engulfment. However, an array of such spherical elements is not likely to be an efficient beam-forming arrangement unless the spacing between them is considerably smaller than the approximate minimum diameter of one-half wavelength (in the proximate propagation space) of the spherical engulfment in FIG. 3. Consequently, a close-spaced array, as in FIG. 4, might be attained by a double truncation of each spherical engulfment. The truncated arrangement in FIG. 4 represents a compromise in the sense that the onset of total reflection is present, but to a lesser extent than in FIGS. 1 and 2. As a result, the ray paths through the engulfment will not be monotonically smooth, but will tend to nutate and possibly circumnavigate, partially, as the engulfment of each adjacent element is traversed.
Referring to FIG. 5, a modification of FIG. 4 is shown where individual dipoles 77 are engulfed in a medium 74 having one refractive index which, in turn, is engulfed in successive media 75 and 76 having different refractive indices. l-Iere, surface 78 is preferably reflective to avoid any back lobes. Again, it can be seen by reference to solid lines W, 81, 82, 83 and 84, and dotted lines 86, 87, 89, and 91, that the path lengths forming a radiation pattern at a normal azimuth to the array are different to the path lengths as seen in a direction other than the design normal azimuth allowing for beam-steering by phasing the individual dipoles, as desired.
It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that it is intended to cover all changes and modifications of the example of the invention herein chosen for the purposes of the disclosure which do not constitute departures from the spirit and scope of the invention.
The invention claimed is:
l. A superdirective array antenna comprising:
a plurality of phased radiation elements spatially disposed in a curved array forming one or more sharp radiation patterns in space; and
a refractive medium engulfing substantially only the effective extent of said array and operable for enhancing path-length differences and phaseinterference effects duplicating substantially the directivity of said array in infinite space of said refractive medium, said refractive medium comprising a lower wave velocity medium engulfing each said radiation element within an overall engulfing medium of higher wave velocity.
2. The superdirective array of claim 1 wherein:
said refractive medium has a curved geometrical shape.
3. The superdirective array of claim 1 wherein:
said separate engulfments are truncated.
4. The superdirective array of claim 1 wherein:
the engulfing medium has discrete layers of varying wave velocities.
5. The superdirective array of claim it wherein:
the wave velocity of said engulfing medium is continuously variable.
6. The superdirective array of claim I wherein:
an outside surface of said engulfing medium has curved sections corresponding to each radiation element.
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