US 3496570 A
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' Feb. 17, 1970 B. Ew.s 3,496,570
' VAN AT'I'A ARRAY Filed March 28, 196'? INVENTOR BERNARD L. LEWIS ATTORNEYS United States Patent US. Cl. 343754 Claims ABSTRACT OF THE DISCLOSURE A dielectric slab is placed or disposed in adjoining relationship to the line or plane of antenna elements of an otherwise conventional Van Atta array, the dielectric constant and thickness of the slab selected to produce reflection and refraction of portions of the wave incident on the array to cancel waves specularly reflected by the antenna elements of the array, without deleteriously affecting the retrodirective reradiation to be performed by the array. Alternatively, the surface of the array 1s curved or stepped, without use of a dielectric slab, thus increasing the path lengths of certain of the Wave portions in air, which is compensated for by concomitant reduction inlengths of the respective transmission lines interconnecting the antenna elements of the array. Accordingly, there is no substantial eifect on the retrod1rective reradiation of incident signal by the array, but What would otherwise be specular reflection is diifused by the array surface.
I 7 Background of the invention The present invention relates generally to methods and means for eliminating or substantially reducing interference from specular reflection in Van Atta arrays. V I
Generally speaking, the Van Atta array or reflector 1s a form of electromagnetic wave reflector comprismg a plurality of antenna elements disposed in a symmetrical array relative to the geometric center, symmetrically arranged pairs of the antenna elements beingcoupled or interconnected by appropriate transmission lines in terms of the type of antennas employed, of equal electr cal length, to produce retrodirective reradiation (or backreflection) of electromagnetic waves incident on the array, with relatively broad-angle coverage in comparison to a conductive reflecting sheet of the same area. In essence, wave portions or wavelets along a phase front of an incident electromagnetic wave within the angle of coverage of the array are received or absorbed bythe antenna elements in their respective paths, converted to or guided as electrical energy which is fed through the associated transmission lines, and reradiated as a reflected wave from the connected antenna elements of the pairs back in the direction from whence it originated. The equality of the path lengths of the wave portions between incident and reflected fronts of the wave is such that the reflected energy is reinforced along the phase front.
The Van Atta principle iswell known and thoroughly discussed in the literature; hence, further discussion of basic concepts is unnecessary here. For a broader treatment of the underlying theory of operation and exemplary construction, reference is made to United States Letters Patent 2,908,002, entitled Electromagnetic Reflector," granted Oct. 6, 1959 to Lester Van Atta.
Like other receiving antenna arrays, however, Van Atta reflectors produce some scattering of the incident wave or signal back into space. Referring for example to a paper entitled A Van Atta Reflector Consisting of Half Wave Dipoles by J. Appel-Hansen in IEEE Transactions of Antennas and Propagation, vol. AP-l4, No. 6, November 1966, pp. 694-700, it is observed by the author that presently known Van Atta reflectors suffer from interference between the signal scattered from the array and that which is reradiated from the array via the cable couplings between the antennas. Considering the effect of a plane wave incident from an arbitrary direction on a linear Van Atta reflector consisting of four parallel half- Wave dipoles (which, incidentally, are the most often selected antenna elements for a Van Atta array) arranged in two pairs, the author notes that each dipole is influenced by the plane wave and, in addition, by an excitation attributable to energy absorbed from the wave by the connected dipole and transmitted via the transmission line therebetween. From this he points out that the current distribution in each dipole is composed of two parts, one of which is that current induced by the incident wave, and the other part the current owing to the presence of the connected dipole. The latter current is that producing the fields responsible for the wave reradiated in the retrodirective direction, i.e., the Van Atta eifect. The firstmentioned current component, however, produces 'a scattered field, which in the case of half-wave dipoles is of substantially the same magnitude as the retrodirective wave field, causing specular reflection interfering with the 'retrodirectively reradiated wave, most strongly when the plane wave is incident from the direction normal to the line or plane of the array. More particularly, the scattered signal can cause substantial cancellation of the retrodirective signal. Another generally deleterious eifect is the result of coupling between the antenna elements. The author concludes that considerable desirable backreradiation is produced where proper selectionof (but still equal) lengths of transmission lines connecting the antenna elements is made.
As previously stated, all receiving arrays scatter some of the incident radation back into space, and the Van Atta array produces scattering irrespective of the particular type of antenna element employed, although where halfwave dipoles are used the effect is more pronounced. Accordingly, it is a principal object of the present invention to provide means for preventing specular reflection from any Van Atta array.
It is another important object of the invention to prevent propagation of specularly reflected signal or of a component thereof, in the retrodirective direction from a Van Atta reflector.
Still another significant object of my invention is to increase the angle of coverage of Van Atta arrays.
Summary of the invention Briefly, according to the basic concepts and principles of my invention means are provided in the otherwise conventional Van Atta array for eliminating or reducing the undesirable specular reflection from the array.
In one general form of the invention, the aforementioned means comprises a dielectric medium having two parallel surfaces one of which abuts or is placed in adjoining or adjacent relationship to the surface or line of the array either coincident therewith or generally parallel thereto, preferably along the plane containing the specular phase center of the array. The dielectric constant of the medium is selected to produce reflections of incident signal of equal magnitude and angle of reflection from the interface or boundary between the dielectric contiguous with the array and the medium in which the array is immersed, generally air. One of the reflected rays is a direct result of the air-dielectric interface while the other ray is one which has penetrated the dielectric and has thus undergone refraction and specular reflection from the surface of the array. The thickness of the dielectric between the opposite surfaces thereof is selected to render these two reflected rays of opposite phase, so that one cancels or substantially cancels the other at the air-dielectric interface.
In another form of my invention the aforementioned specular deflection reducing means comprises an arrangement of the antenna elements of the array in stepped or curved configuration so that the surface of the array deviates progressively, on either side of its geometric center, from a flat planar or straight line surface, and a reduction in length of interconnecting transmission lines corresponding to the increase in path length through the air. Accordingly, the array surface reflectively diffuses signals which would otherwise be specularly reflected at least in part in the retrodirective direction, and hence substantially reduces interference with and cancellation of desired reradiation in the back direction, while acting as a flat array to the incoming wave producing energy fed through the transmission lines interconnecting the antenna elements.
Brief description of the drawings The above and still further objects, features and attendant advantages of the present invention will become more apparent from a consideration of the following detailed description of certain preferred embodiments thereof, especially when taken in conjunction with the accompanying drawings, in which:
FIGURE 1 is a symbolic schematic diagram of one general form of my invention;
FIGURE 2 is a perspective view of an embodiment of the general form of my invention depicted in FIGURE 1; and
FIGURE 3 is a plan view of a Van Atta array incorporating a further form of my invention.
Description of the preferred embodiments Referring now to FIGURE 1, the principles contributing to one aspect of my invention are based, at least in part, upon difference in index of refraction between materials of different dielectric constant. In particular, the desired result of preventing the existence of a specularly reflected signal depends on reflection of rays (alternative referred to as waves or beams) from the interface or boundary between different dielectric media and on refraction of a ray within a dielectric medium.
As shown in FIGURE 1, the Van Atta array, which may be linear or planar, for example, or may be of other known or proposed simple or complex geometrical configurations such as circular, cylindrical, spherical, tetrahedral, cubic, rectangular, and so forth, and which is symbolically depicted by the box appropriately labeled and designated by reference numeral 10, is covered by a slab 12 of dielectric material. For the sake of simplicity array is assumed to be linear and the dielectric slab 12 is contiguous with the line of antenna elements of the array (or surface of the array), adjoining the elements along the plane 15 containing the specular phase center.
The antenna elements of Van Atta array or reflector 10 are arranged in symmetrical pairs connected by equal lengths of appropriate transmission line, in conventional or known fashion; the entire configuration differing from any typical Van Atta array only in the placement of dielectric slab 12 against the antenna elements such that incoming electromagnetic waves are incident first on the slab. The dielectric constant k of the slab or plate is preselected in accordance with known principles of electromagnetic and optical theory to produce a reflected wave or wave portions from the air-dielectric interface (designated by reference numeral 18) whose strength or magnitude is at least approximately equal to the wave or wave portions reflected from the array itself.
With reference to the figure, for a ray 20 incident at an angle i relative to the normal to the array and which is to undergo refraction in the dielectric medium 12 at an angle 1' to the normal, and thence is specularly reflected from the array, again at the angle r to the normal, and is finally refracted at the air-dielectric interface at the angle 1', the relationship between these angles and the dielectric constant k of slab 12 is sin r i The thickness 1 of slab 12 is also preselected, such that the phase of ray 22, which penetrates the air-dielectric interface 18 and is reflected from the array 10, is opposite that of ray 21 which is directly reflected from the airdielectric interface. Accordingly, since the ultimate direction of the two rays (21 and 22) is the same, and they are of opposite phase, they tend to cancel each other out. To this end, the thickness 1? of the dielectric plate is preferably one-quarter wavelength of the incident wave in the dielectric of which the plate is composed. Mathematically,
where A is the free space wavelength of the wave. Alternatively, t is chosen to have as small a value as possible under the particular conditions that may be encountered in practice, to obtain the greatest bandwidth for the resultant improved array.
As an added advantage of the technique employed in the system of FIGURE 1, the refraction of the incoming wave at the air-dielectric interface effectively reduces the angle of incidence i on the elements of the array to the angle of refraction 2', thereby increasing or broadening the agle of coverage of the array. It will be observed, for example, from Expression 1, above, that r is less than i for values of k exceeding unity (the dielectric constant of air), and that in such instances the dielectric-covered array is capable of reradiating incident signal over a greater angle than that of which a conventional Van Atta array is capable. Moreover, the cancellation of specular reflection transfers the power of the cancelled wave to the normal back-reradiated signal of the array, thus increasing array efficiency.
An exemplary embodiment of the improved array of FIGURE 1 is shown in somewhat greater detail in FIG- URE 2. In the configuration illustrated in FIGURE 2 the dipole antennas are placed in Van Atta array on the surface of (or are embedded in the surface of) dielectric slab 33, such as by conventional deposition techniques. A ground plane (conductive reflecting sheet) is disposed on the opposite face of the slab and pairs of symmetrically disposed dipoles (relative to the geometric center of the array) are connected by equal lengths of transmission line 37, 38, 39.
By way of example only, without any intent to restrict the scope of the invention, the dipoles are selected to be A (2 /k) in length and are spaced A /2 apart. The dielectric constant of slab 33 is selected as k=2.5 (suitable materials being Plexiglas, rutile, pyrite, amorphous selenium, germanium, or strontium titanate, to name a few),
and the thickness to be t== (4 /2.5).
The wave specularly reflected from the array is cancelled by wave portions of substantially equal magnitude and opposite phase which have penetrated and emerged from the dielectric. That is, the wave portions subsequently cancelling the specular reflections are refracted upon entry into dielectric plate 33, reflected from ground plane 35 and refracted again at the dielectric-air interface.
Referring again to the more general case of FIGURE 1, when t is chosen to be of small magnitude (e.g., A /(4 /k) and kzZ, the specified technique of matching to produce cancellation of specular reflection is effective over a large range of angles of incidence because of refraction effects. If, for example, k has the value 2 and i is less than 45 degrees, then r is less than 30 degrees (from Expression 1 and the maximum variation in path length through the dielectric with changes in i is AL 2V5 1)- 0.134 (3) In that event the maximum phase error (i.e., deviation from opposite phase) for ray 22 is 24 degrees when it combines with ray 21, so that partial (substantially complete) cancellation is still achieved, since the resultant specularly reflected ray has less than one-quarter the power of either of the combined rays alone.
Referring now to FIGURE 3, an alternative embodiment of an improved Van Atta array capable of eliminating or substantially reducing specular reflection in the direction from which the incident wave emanated, includes antenna elements 50, such as dipoles, which are disposed in relatively displaced configuration to produce a curved or stepped array surface. Accordingly, the array surface deviates from that of a linear or flat planar array as indicated by dotted line 52, so that extra path lengths 55, 56, for example, in air are presented to the incoming wave portions.
The continuous curvature or discretely stepped character of the array surface is compensated for, insofar as the retrodirective reradiation response of the array is concerned, by reduction of the respective transmission line length by an amount equal to the extra path length in air. Thus, for example, transmission line 58 is reduced in length by an amount equal or approximately equal to twice the length of path 55. It follows, of course, that the transmission line lengths interconnecting pairs of symmetrically disposed antenna elements are no longer equal. The result is that the array is operative as a flat plate for normal retrodirective reflection of signal (since equal path lengths for signal wave portions are encountered just as in the standard Van Atta array), but specularly reflects in the manner of a diffusing surface, thereby preventing or reducing retrodirective specular reflection.
A ground plane 62 may be disposed behind the surface of the array, if desired, to ensure complete diffusion of specular reflection of incident waves.
1. A passive electromagnetic reflector for retrodirectively reradiating incident electromagnetic waves while eliminating or substantially reducing interference from scattered fields accompanying incidence of said waves on said reflector, comprising a conventional Van Atta array of antenna elements which are symmetrically disposed in pairs about a selected geometric center of the array, each of said symmetrically disposed pairs of elements interconnected by a separate transmission line; and means including a dielectric member for varying at least one conventional physical characteristic of the surface of said array and the electromagnetic path lengths for incoming wave components which would otherwise result in said scattering fields.
2. The invention according to claim 1 wherein. said separate transmission lines are of equal electromagnetic path length, and said dielectric member is of dielectric constant exceeding that of the surrounding medium, said dielectric member having a pair of parallel surfaces one of which is contiguous with the surface of said array and having a predetermined dielectric constant to produce specular reflected components attributable to the direct reflection of incident wave components from said interface and to reflection of incident wave components from the array surface after penetration of said dielectric member, which are of substantially equal magnitude and direction, the thickness of said member between said parallel surfaces being preselected to produce opposite phasing of said reflected components.
3. The invention according to claim 1 wherein said dielectric member is contiguous with the surface of said array and has substantially uniform thickness in the direction normal to said surface of said array, said thickness selected to be approximately equal to or less than a quarter wavelength of the electromagnetic waves to be reradiated, in the dielectric medium of said member.
4. The invention according to claim 3 wherein said dielectric member is disposed behind the surface of said array relative to incoming electromagnetic waves to be reradiated by said array.
5. The invention according to claim 4 wherein at least some of said antenna elements are disposed on a surface of said dielectric member, and a conductive reflecting sheet is disposed against the opposite surface of said dielectric member.
6. A passive electromagnetic reflector for retrodirectively reradiating incident electromagnetic waves while eliminating or substantially reducing interference from scattered fields accompanying incidence of said waves on said reflector, comprising a conventional Van Atta array of antenna elements which are symmetrically disposed in pairs about a selected geometric center of the array, each of said symmetrically disposed pairs of elements interconnected by a separate transmission line; and means varying at least one conventional physical characteristic of the surface of said array and the electromagnetic path lengths for incoming wave components which would otherwise result in said scattering fields; wherein said means comprises an arrangement of said antenna elements such that the surface of said array deviates progressively from a line perpendicular to the normal to the array surface at said geometric center to either side of said center, said separate transmission lines deviating from equal electromagnetic path length by respective amounts equal to the distance in said surrounding medium from the antenna elements interconnected thereby to said perpendicular line.
7. In a Van Atta array in which pairs of antenna elements symmetrically disposed about a geometric center of the array are interconnected by respective transmission lines, for retrodirective reflection of an electromagnetic wave from incident on the array, the improvement comprising means altering the electromagnetic path lengths of suflicient components of the incident wave front to produce oppositely phased substantially equal amplitude components relative to specularly reflected components of the incident wave front to substantially cancel said specularly reflected components while maintaining equal path lengths, and hence, substantial reflection without cancellation, for those components of the incident wave front that would normally undergo retrodirective reflection.
8. The invention according to claim 7 wherein said means comprises a dielectric member having a dielectric constant exceeding that of air, said member positioned adjacent the surface of said array and having a thickness selected to produce said opposite phasing by refraction of componentsof said wave front in conjunction with reflection of components of said Wave front from the surface of said array.
9. The invention according to claim 8 wherein said dielectric member has a thickness of substantially one quarter wavelength of the incident wave.
10. The invention according to claim 7 wherein said means comprises a progressive rearward deviation of said antenna elements from a planar array to either side of said geometric center, said transmission lines deviating from equal lengths by respective amounts equal to said progressive deviation of each of the antenna elements connected by the respective transmission line.
References Cited UNITED STATES PATENTS Iarns 343-777 Van Atta 343776 Hannan 343776 X Malech 343-754 US. Cl. X.R.