US3234556A

US3234556A - Broadband biconical wire-grid lens antenna comprising a central beam shaping portion

Info

Publication number: US3234556A
Application number: US175369A
Authority: US
Inventors: Robert L Tanner
Original assignee: Individual
Current assignee: Individual
Priority date: 1962-02-23
Filing date: 1962-02-23
Publication date: 1966-02-08
Anticipated expiration: 1983-02-08
Also published as: NL288228A; US3234557A; DE1466432A1; GB1025182A

Description

Feb. 8, 1966 R. TANNER 3,234,556

BROADBAND BICONICAL WIRE-GRID LENS ANTENNA COMPRISING A CENTRAL BEAM SHAPING PORTION Filed Feb. 23, 1962 3 Sheets-Sheet l INVENTOR.

ROBERT L. TANNER ATTORNEY Feb. 8, 1966 R. L. TANNER 3,234,556

BROADBAND BICONICAL WIRE-GRID LENS ANTENNA COMPRISING A CENTRAL BEAM SHAPING PORTION Filed Feb. 25, 1962 3 Sheets-Sheet 2 53 47 aw; 5 55 lL 1| 4%? 1L 49 INVENTOR.

ROBERT L. TANNER ATTORNEY Feb. 8, 1966 L TANNER NA 3,234,556

R. BRO AND B NICA IRE- ID LENS ANTEN CO R SING ENTR BEA HAPING POR N Filed Feb. 23, 1962 A Sheets-Sheet 5 IOO INVENTOR ROBERT L. TANNER J um- 316mm? ATTORNEY United States Patent 3,234,556 BROADBAND BICONICAL WIRE-GRID LENS ANTENNA COMPRISING A CENTRAL BEAltl SHAPING PORTION Robert L. Tanner, 4780 Alpine Road, Menlo Park, Calif. Filed Feb. 23, 1962, Ser. No. 175,369 16 Claims. (Cl. 343-753) This invention relates to lens-type antennas and more particularly to a broad band uniform grid-wire antenna for operating :over bands lying within the frequency range extending from below 1 to above 1,000 megacycles per second.

Present day attennas for operating Within the above stated frequency range are of many types. In one very important portion of the range, the so-called HF (highfrequency) range, which embraces frequencies from approximately 3 mc. (megacycles per second) to 30 mc., and which is used for long distance radio communication, one type of antenna is the rhombic antenna. The rhombic antenna consists of four current carrying conductors, each several wavelengths long, in the form of a diamond or rhombus. The structure is fed at one end by a transmission line and terminated at the other end in a resistance. The two sides of the rhombus are, in effect, a continuation of the feeding transmission line in which the conductors diverge from the feed to the middle of the rhombus and then converge again to the terminating resistance. The rhombic antenna transmits energy in the direction of the termination. As a receiving antenna it receives from that direction.

Another Class of antennas is .the linear array. This antenna is composed of many elementary antennas usually resonant dipole antennasarrayed along a line or in a plane. The spacing between the elements of the array is usually about one-half wavelength. The purpose of arranging the elements in an array is that it permits the electromagnetic energy to be concentrated in a relatively narrow beam. The beam may be in the line of the arrayin which case the array is known as an end fire array-or at right angles to itwhen it is called a broadside array.

Another type of array which is finding increasing use is the circular or Wullenweber array, in which the elementary antennas are arranged along the periphery of a circle. In this antenna a beam is formed by feeding the elements in a segment of the array equal to approximately half of the elements of the array. Each element is fed through a separate individual phasing network. By means of these networks the currents in the different elements are adjusted so that their radiations add to form a relatively sharp beam. The beam is in a direction which bisects the are formed by the excited elements. The useful feature of the Wullenweber array is that the direction of the beam can be changed by switching the feeds to a corresponding group of elements at a different angular position in the array.

The antennas just described are examples of antenna types that have proved useful in different applications. The rhombic antenna, for example, has for many years been the most commonly used antenna in long distance radio communication service. It has the advantages of simplicity, relatively low cost of construction and reasonably good performance. Linear array antennas have been frequently used for short wave broadcasting applications for long distance communications and as radar 3,234,556 Patented Feb. 8, 1966 antennas. The Wullenweber array, because of its scanning capability, has been most frequently used as a highresolution direction finding antenna.

Although the antennas described and other existing antennas have proved useful, they all have certain inherent limitations. For example, the rhombic antenna, although inexpensive to build it land costs are ignored, makes very inefficient use of land area. A single large rhombic antenna for operation over a 4 to 10 mc. band might require a land area of 10 acres or more. Another limitation is the useful operating bandwidth. Rhombic antennas cannot be operated over frequency bandwidth greater than 2 or 2 /2 to 1 without excessive deterioration in radiation patterns. Thus to cover the band from 4 to 30 rnc. used in long distance radio communications at least two separate antennas, with a total land requirement of 15 to 20 arces, will be needed for each circuit. Thus in a communication station serving 20 different circuits the total land area required for the installation of rhombic antennas could be as high as 400 acres.

A further limitation of rhombic antennas is the high level of side lobes and back lobes. Principal side lobes may be only 6 db below the main lobe. When used as a receiving antenna the high side lobes of the rhombic make it susceptible to interfering signals arriving at angles different from the principal direction. When it is used as transmitting antenna, the high side lobes cause energy to be radiated in other than the desired direction, giving rise to signals interfering with other circuits, either nearby or in other parts of the world. Still another limitation is that the beam of the rhombic cannot be scanned.

The linear array is also subject to certain limitations. The useful operating bandwidth is even narrower than that of the rhombic antenna. In addition, such antennas may be substantially more expensive to construct. Their beams can be steered over relatively small angles, by varying the phases of the currents in the different elements of the array, but this is accomplished only at the expense of substantial complication in the feed system, and a consequent increase in cost.

The Wullenweber array can be scanned over the entire 360 degree range of azimuth. Because of the many separate phasing circuits and switches required, however, the Wullenweber is very expensive to build. Power handling limitations in the phasing and switching networks tend to limit the use of the Wullenweber to receiving applications. In addition, although the Wullenweber can be scanned to any azimuth angle, it is limited to operation at a single azimuth angle at any given time.

It is therefore a primary object of this invention to provide a broadband, low side-lobe and low back-lobe antenna for an operating frequency lying within the frequency range from below 1 mc. to substantially above 1,000 mc. The antenna of this invention also is to be capable of operation over a frequency bandwidth greater than 10 to 1.

It is a further object of this invention to provide an antenna particularly suitable for use in the HF and VHF frequency ranges, which relative to its performance, is small in physical size, inexpensive to construct and install, and light in weight.

It is a still further object of this invention to provide a broadband antenna of the stationary type which is capable of scanning a beam, either by moving the feed to a different point on the perimeter of the antenna or by switching from one to another of many feeds spaced around the perimeter.

It is another object of this invention to provide an antenna which is capable of forming a plurality of beams in a plurality of directions by simultaneously exciting a plurality of feeds positioned at a plurality of points on the perimeter of the antenna.

It is still another object of this invention to provide a broadband lens-type antenna which can, more simply and efiiciently than devices heretofore known, shape the electromagnetic energy from simple feed antenna into a beam of any desired sharpness, allow substantial flexibility in the shape of the beam, scan a narrow beam through a large arc, and which has adjustable features so that changes of radiation pattern can be made in the field with relative simplicity.

Briefly, the lens-type antenna of this invention is constructed of a pair of opposed wire mesh grids. The size of the individual meshes is dimensioned to be small compared with the shortest operating wavelength to provide a propagation characteristic which is substantially independent of the operating frequency and isotropic.

In one embodiment of this invention the equivalent dielectric constant of wire mesh grid structure is changed by changing the distance between opposite grids from a distance of separation which is small compared with the mesh size to a distance of separation which is large compared with the mesh size. By properly varying the distance of separation between different portions of the grids, and thereby the velocity of propagation, an antenna having circularly symmetric beam forming properties can be constructed.

In another embodiment of this invention the equivalent dielectric constant of the wire mesh structure is changed either by inductively loading the meshes or capacitively loading across the wire-grids. The velocity of propagation varies with the degree of inductive and capacitive loading, and by judicious choice of loading a lens having the desired characteristics is provided.

The antenna of this invention will be described in connection with an antenna system for converting a point source of HF energy from a simple feed antenna into a cophasal wave front. Furthermore, the antenna of this invention is constructed to be circularly symmetric, the point source being located at a peripheral portion of the antenna while the main beam is provided by the antenna from the diametrically opposed peripheral portion. A hypothesized optical structure having a similar circularly symmetric focusing property is known as a Luneburg lens. To obtain this symmetric focusing property, the Luneburg lens is fashioned from a disc or sphere of dielectric whose permittivity varies parabolically from a value of 2.0 at the center to a value of 1.0 at the periphery.

Other objects and a better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of a wire-grid lens antenna constructed in accordance with this invention in which a desired change in the propagation characteristic is obtained by a change in the spacing between opposite wiregrids;

FIG. 2 is a sectional view taken along lines 2-2 of FIG. 1;

FIG. 3 is a schematic top view of a Luneburg-type lens for converting a peripherally located point source into diametrically opposed cophasal plane wave;

FIG. 4 is a sectional view similar to the view shown in FIG. 2 of a further embodiment of the wire-grid lens antenna;

FIG. 5 is an enlarged detail view in perspective of a pie-shaped portion of the wire-grid lens antenna of FIG. 4;

FIG. 6 is an illustrative diagram of wave propagation parallel to one of the wires of a square Wit m 115?" ful in explaining the theory of operation of this invention;

FIG. 7 is an illustrative diagram of wave propagation along a diagonal of the wires of a square wire mesh useful in explaining the theory of operation of this invention;

FIG. 8 is a top View of a lens portion constructed of hexagon mesh wire-grids;

FIG. 9 is a perspective view of a further embodiment of a wire-grid lens portion of the antenna of this invention in which the change in the propagation characteristic is obtained by selective capacitive loading across opposing wire meshes;

FIG. 10 is a perspective view of a still further embodiment of a wire-grid lens portion of the antenna of this invention in which the change in the propagation characteristic is obtained by selective inductive loading of the meshes of the wire meshes; and

FIG. 11 is a top view of a lens portion of a further embodiment of this invention.

Referring now to the drawings, and particularly to FIGURES l and 2 thereof, there is shown a wire-grid lens antenna 20 constructed in accordance with this invention and comprising an upper wire-grid 21 and a lower wiregrid 22. Wire-

grids

21 and 22 are circular and are suspended in opposite and overlying relation from a plurality of non-conductive peripheral support members 23 such as wooden, plastic or fiber glass poles.

Antenna 20 actually comprises a center portion 24 which forms a wire-grid lens for azimuthal beam shaping and a peripheral portion 25 which forms a radiating structure for elevational beam shaping and for matching the impedance between lens 24 and the surrounding space. In the embodiment of the invention shown in FIGURES 1 and 2 radiating structure 25 is formed as a biconical, or radially flared horn.

As a practical matter, upper wire-grid 21 may include an upper lens wire-grid fastened securely to an aluminum ring 31 which is light in weight and which may also be supported upon a plurality of circularly spaced nonconductive poles 32 as best shown in FIG. 2. Smoothly joined to ring 31 is the inner edge of an upper horn wiregrid 33 in the form of an annulus which has its outer edge supported by peripheral poles 23. Similarly, lower wiregrid 22 may be made in two parts, a lower lens wire-grid 34 fastened to an aluminum ring 35 supported upon poles 32 below ring 31 and a lower horn wire-grid 36 fastened along its inner edge to ring 35 and supported along its outer edge by peripheral poles 23.

Even though the radiating structure 25 of antenna 20 is shown in the form of a biconical horn of wire mesh similar to that used for the construction of lens 24, other types of radiators or wave launching devices may be used. These would include horns utilizing a different type of wire-grid. Horns formed of solid conductive sheet may likewise be utilized. For example, a wire-grid having a different mesh structure from that employed in forming lens 24 may be used as shown in FIG. 5.

As will be explained in greater detail hereinafter, lens wire-grids 30 and 34 are formed of metallic wires and may have a variety of different mesh structures. For example, the mesh structure may be square, triangular or hexagonal. However, even though diflerent mesh structures may be employed, the two wire-grids 30 and 34 forming a lens must be substantially identical to one another and must be oriented with respect to one another in such a manner that corresponding sides of opposite wire meshes lie in a common plane which is vertical to both wire-grids, at least in those lens portions in which wiregrid separation is equal to or less than the mesh size. The reason therefore is that when grid separation is small, corresponding sides of opposite meshes form two-wire transmission lines.

Lens 24 having been formed with proper alignment of correspondingly opposed meshes, has been found to have certain very important properties. As long as the operating wavelength is large compared to the mesh size, the transmission characteristic of the space between Wire grids 30 and 34 is substantially independent of the operating frequency and depends solely on the distance of their separation. It has been determined that for all practical purposes good independence is obtained by selecting the mesh to be equal to or less than one-sixth of the shortest operating wavelength. Accordingly, the mesh size determines the highest frequency of operation of the lens of this invention.

It has also been found that as long as the distance between opposite wire-grids 30 and 34 is small compared to the mesh size, say one-tenth, the velocity of propagation is substantially /2 of the velocity of light. Also, as long as the distance between opposite wire-grids 30 and 34 is large compared to the mesh size, say four times, the velocity of propagation is substantially equal to the velocity of light.

For the two-dimensional circularly symmetric wire-grid lens antenna, such as antenna 20, to provide a sufficiently narrow azimuthal beam, it has been found desirable to make the diameter of grid-wire lens 24 several times larger than the longest operating wavelength. For example, if the lens diameter is selected to be three wavelengths at the lowest operating frequency, the half-power beam width at this frequency will be approximately degrees. Accordingly, the diameter of the grid-wire lens determines the lowest frequency of operation of this invention.

By way of summary then, the bandwidth of the wiregrid lens antenna is determined at the high frequency end by the mesh size and at the low frequency end by the lens diameter.

An example, which will best show the physical dimension of an antenna constructed in accordance with this invention, will now be given for the case in which the desirable variation of the transmission characteristic is obtainable by varying the effective dielectric constant between 1 and 2. Assume a desired frequency range from 4 to megacycles per second. The desired mesh size is about one-sixth of the shortest wavelength, or about 1.7 meters. The diameter for good resolution is about three times the longest wavelength or 225 meters. Variation of grid spacing to obtain a change of efiective dielectric constant from 1 to 2 is from a minimum spacing of about one-tenth of the mesh size, or 17 centimeters to a maximum spacing four times the mesh size, or about 6.7 meters.

These properties of lens 24 may be utilized to construct an infinite variety of lenses, each having a desirable transmission characteristic which varies in accordance with grid spacing. By way of example, a particularly useful lens 36 is shown in FIG. 3 which is formed to convert a point source 37 located at its periphery into a cophasal line front 38 diametrically opposite thereto. The path of energy is designated by reference numeral 39. A lens having the above stated property is known as the microwave equivalent of the optical Luneburg lens. As can be seen from the plurality of paths 39 depicted, the bending, and therefore the velocity of propagation increases with increasing angular deviation from the diametrical bisector passing through source 37. The required relation of the transmission characteristic of a Luneburg lens is given by the relation Where:

It is the equivalent index of refraction for a wave propagating between the wire grids;

a is the diameter of the wire-grid lens; and

r is the distance from the center of the wire-grid lens.

For a Luneburg type wire-grid lens constructed in accordance with this invention which will operate over a frequency range of 4 to 30 me. using a 5 ft. square mesh wire grid of No. 8 wire (Radius-.064 inch) and having a diameter a of 600 ft. the following table provides the distances of grid separation s as a function of distance from the lens center r.

r, feet s," inches Feet.

Generally speaking, to design a wire-grid lens in accordance with this invention, it is only necessary to derive an expression for the transmission characteristic in terms of distance of separation of the wire-grids and the coordinates of the lens and thereafter determine s for every point of the lens. The equivalent index of refraction is a function of size and shape of the mesh, the diameter of the wire and the spacing between wire-grids. A formula for the index of refraction of a square mesh is given in Properties of a Pair of Wire-Grids for Use in Lens-Type HF Antennas by Andreasen and Tanner, 1961 Western Electronic Show and Convention, paper No. l/ 3.

Referring now to FIGS. 4 and 5, there is shown a further embodiment of the wire-grid lens antenna in which the lower wire-grid is planar and in which a sectional construction is utilized, each section being pie-shaped. An tenna 45 comprises a plurality of pie-shaped sections 45, each one of which includes a planarly suspended pieshaped lower wire-grid 46 and a curvedly suspended pieshaped upper wire-grid 47. The center of antenna 44 is formed by a non-conductive support pole 48 to which the pointed end of the individual pie-shaped wire-grids are attached. Additional non-conductive support poles 49 are provided to assure a good durable suspension without undue sag.

Wire-grids 4-6 and 47 are of triangular mesh and are spaced in such a manner that the wires are substantially parallel to form individual transmission lines. The peripheral edge of each pie-shaped wire-grid 47 may be affixed to an aluminum ring 50 which is supported on non-conductive support poles 49 and which provides a convenient edge support for a flared horn radiating structure 51.

Radiating structure 51 likewise includes a plurality of a lower and an upper annular wire-

grid sectors

52 and 53, but instead of having a triangular mesh structure they are formed of square mesh. The outer peripheral edge of wire-grids sectors 53 are supported by nonconductive poles 54 which may be anchored by means of guy wires 55. For proper functioning, the guy wires must be broken at intervals by means of insulators. The inner peripheral edge of wire-grid sectors 53 are fastened to ring 50. Wire-grid sectors 52 are shown to form planar extension of lens wire-grids 46 to allow antenna 44 to be as close to the ground as possible and to provide a slightly upwardly directed beam. Of course radiating structure 51 may be formed to have both its upper and its lower annular wall flared at any angle to provide the desired elevational beam angle and beam pattern.

To explain the operation of the wire-grid lens of this invention, reference is made to FIGURES 6 and 7 which both show a portion of a square-mesh wire-grid lens. FIGURE 6 shows a wire-grid lens portion 60, in which the upper wire grid is spatially positioned exactly over the lower wire grid of the pair of grids forming the lens. The lower grid is consequently hidden from view by the upper wire grid. Representative wires forming the wire grids are shown as

wires

61 and 62. FIGURE 7 similarly shows a wire-grid lens portion 63 in which the upper wire grid is spatially positioned so as to hide the lower wire grid from view. Representative wires of the visible wire grid are 64 and 65.

Consider now a circumstance in which a wave is traveling toward the right through lens portion 63. The Wave advances along a broad front and therefore may be considered to travel along many parallel corridors, each of which will have identical currents and fields. A representative corridor is shown in FIGURE 7 contained between

lines

74 and 75. Consider first the case in which the upper and lower grids are spaced a distance which is small compared to the distance between nodes of the mesh. For this case, opposing wires in the upper and lower grids constitute, in effect, the two wires of a twowire transmission line, the plane of which is perpendicular to the plane of the grids. That portion of the wave which travels down the corridor defined by

lines

74 and 75 includes

currents

72 and 73. Because the grids are so close together that the opposing wire members have the properties of two-wire transmission lines, the wave energy is constrained to follow the direction of the wires. In advancing between successive mesh nodes along the center of the corridor between 74 and 75, the wave advances a distance equal to the diagonal of the mesh square. Because it was constrained to follow the Wires, however, the actual distance travelled by the Wave is equal to two times the mesh side. The distance actually traveled is therefore greater than the distance of effective advance by the ratio VT, which is the ratio of twice the side of a square to its diagonal. As a consequence, the wave has been effectively slowed by the factor equal to /2.

Consider now the case illustrated in FIGURE 6 where the wave travels parallel to wire 62. Again, because it advances on a broad front it can be considered as traveling down many parallel corridors. In FIGURE 6 such a corridor is defined by lines 66 and 67. Because of the symmetry of currents in adjacent corridors, no current flows across these lines, which is equivalent to saying, in considering the portion of the wave traveling down this corridor, that lines 66 and 67 could be replaced by a perfect open circuit, and the remainder of the grid structure on either side of the corridor removed.

Because of the fact that the grids are spaced so closely together that opposing wires in the upper and lower grids constitute two-wire transmission lines, the structure contained in the corridor defined by 66 and 67 is now equivalent to a central two-wire transmission line carrying current 70 loaded at intervals equal to the mesh side length by the short sections or stubs of open-circuited two-wire transmission line into which the cross wires degenerate. The impedance presented by the small open-circuited stubs is capactive, and their effect is to apply to the transmission line formed by the central wires of the corridor a periodic capactive loading which in effect doubles the capacitance per unit length of that line, but leaves its inductance unchanged.

It is well known that the wave velocity on an open two-wire transmission line is equal to the velocity of light in free space and is given by the formula where C is the capacitance per unit length of the transmission line and L is the inductance per unit length. Because the effect of the cross wires of the grid is to apply a capactive loading which doubles the effective capacitance per unit length of the line, the velocity of the wave in the grid is seen to be It is therefore evident that a wave traveling parallel to one set of grid wires is slowed by a factor equal to /2, just as was the case with a wave traveling in a direction diagonal to the grid wires. The same slowing factor holds true for a wave traveling in any direction in the space between the grids provided that the mesh size of the grids is small compared with the wavelength and the spacing between the grids is small compared with the mesh size. Therefore, the wire grid pair for this condition is also substantially isotropic.

Now consider the situation if the spacing between the grids is increased to a distance that is large compared with the mesh size. It is well known that under most circumstances a Wire grid having a mesh size small compared with the wavelength behaves electrically the same as if it were a solid metal plate. This is the condition that prevails when the grids are separated a distance which is large relative to the mesh size. It is also well known that a wave propagating between parallel metal plates in the so-called TEM mode (transverse electro-magnetic), which has its electric field vector polarized perpendicular to the plane of the metal plates, travels at free space velocity.

Thus it is evident that when the grids are spaced a distance large compared with the mesh size so that they simulate metal plates a quasi-TEM wave propagating in the space between them travels at free space velocity. When the spacing between the grids is reduced, the Wave is slowed. Maximum slowing is achieved when the gridto-grid spacing is made small compared with the mesh size, in which case the wave is slowed by a factor equal to /2. From the foregoing discussion it is also evident that the wave propagating between the grids has its major component of electric field polarized perpendicular to the plane of the grids. All wave velocities between free space velocity and l/ /2 times free space velocity can be obtained by an appropriate spacing of the grids relative to the mesh size. The relation between wave velocity and grid parameters is given in the previouslyreference paper by Andreasen and Tanner.

Although the preceding discussion is carried out in terms of square mesh grids, other mesh shapes also work. Specifically, grids composed of meshes having the form of equalateral triangles and regular hexagons are both suitable. An analysis similar to that carried out for square mesh grids shows that in the case of grids spaced a distance small compared with the mesh size,

a slowing factor of /2 is obtained with both triangular mesh grids and hexagonal mesh grids.

As already mentioned, the square-mesh and the triangular-mesh wire-grids are not the only grid structures useful in practicing this invention. In fact, since both the triangular-mesh and hexagonal-mesh have more planes of symmetry, they are less anisotropic. Also, the hexagonal mesh has been found to show the least frequency dependence.

FIG. 8 shows a hexagonal-mesh wire-grid which has been found useful in practicing the invention.

FIGS. 9, l0 and 11 show three further embodiments of wire-grid lenses. In the wire-grid lens of FIG. 9 the Wave velocity of a wave propagating between an upper wire-grid 91 and a lower wire-grid 92 is slowed down by selective capacitive loading. Generally,

Wiregrids

91 and 92 may be suspended in overlying relation and a number of capacitive loads such as 93 connected across junction points 94. The advantage of a capacitive loading is that the velocity may be decreased below /Z of the velocity of light, the lowest velocity obtainable with unloaded Wire-grid lenses. In other words, capacitive loads provide a means for increasing the range through which the velocity may be changed.

FIG. 10 shows a wire-grid lens comprising an upper wire-grid 101 overlying a lower wire-gird 102. The wave velocity is decreased by inserting inductive loading into the wires between junction points such as inductive loads 103. Just as before, the velocity can be decreased by a value below /2 of that of the free space velocity by utilizing lumped inductors 103, thereby overcoming the range limitation of changing the distance of separation of overlying spaced wire-grids.

In certain instances, a wire-grid lens antenna may be constructed incorporating a change of wire-grid spacing in combination with capacitive and/ or inductive loading. In this manner the wave velocity may be changed from that of free space velocity to /2 of free space velocity by utilizing the rather convenient method of varying the separation between wire-grids and in those places where a further decrease of velocity is desired, capacitive or inductive loading is added.

FIG. 11 shows a wire-grid lens 110 comprising a pair of overlying wire-grids, the upper one being designated by reference character 111 and the lower one being hidden from view by grid 111. The wave velocity is decreased in lens 116* by physically lengthening the path from one mesh junction point, say 112, to the adjacent junction point, say 113. This lengthening of path is accomplished by including a zig-zag portion 114 in the wire forming the side of a mesh.

By way of summary, the wave velocity in a wire-grid lens is equal to the velocity of light when the grid spacing is large in comparison with the mesh size. The wave velocity can be decreased by decreasing the spacing between wire-grids. In this manner the slowest wave velocity obtainable is /2 the velocity of light. The wave velocity can also be decreased by inductive and capacitive loading which produces a phase shift. Lastly, instead of changing the phase by electric impedance means, the velocity may also be decreased by increasing the physical path length of the transmission line as shown in FIG. 11.

Feeding of the grid-wire lens antenna may be accomplished by conventional means such as a feed horn mounted for rotation along the peripheral portion of the lens antenna. In case of directional antennas which radiate along different azimuthal directions, a single lens structure with fixed feed horns facing the azimuthal direction of radiation provide a very useful structure.

There has been described a wire-grid lens antenna utilizing a pair of overlying wire-grids to form the lens portion and a radiating structure mounted to the end of the lens portion. The wave velocity may be changed by (l) changing in spacing between grids or (2) capacitive loading of the grid or (3) inductive loading of the grid or (4) increasing the transmission line geometrically between mesh junctions. The antenna of this invention remains relatively insensitive to frequency as long as the mesh size is small compared with the shortest operating wavelength.

What is claimed is:

1. A wire-grid lens antenna comprising a pair of spaced, overlying, conductive wire-grids having substantially the same mesh structure, the mesh openings of said wire-grids having a size which is small in comparison with the shortest operating wavelength and being arranged in substantial spaced alignment with one another, said spaced Wire grids defining therebetween a Wave propagating region for a wave polarized with its electric field perpendicular to the plane of the Wire grid, means for supporting said overlying wire-grids at each point of said antenna with a spacing varying from a distance which is large in comparison with the mesh opening size to a distance which is small in comparison with the mesh opening size to provide a selected variation of the wave propagation velocity from that substantially equal to the velocity of light to that substantially equal to 1/2 the velocity of light.

2. A circularly symmetric wire-grid lens antenna for converting a peripherally located point source of radiation to a cophasally extended source directing a beam along the diametrical bisector of said antenna which passes through said point source, said antenna comprising a pair of circular, spaced, overlying, conductive wire-grids arranged to form a surface of revolution about the axis of said antenna, the said spaced wire grids defining therebetween a wave propagating region for a wave polarized with its electric field perpendicular to the plane of the wire grid, mesh opening size of said wire-grids being less than one-fourth of the shortest operating wavelength and the spacing of said wire-grids increasing in a radially outwardly going direction from a distance which is small compared with the mesh opening size to a distance which is large compared with the mesh opening size, and non-conductive means for supporting and spacing said wire-grids.

3. A circularly symmetric wire-grid lens antenna for converting a peripherally located point source of radiation to a cophasally extended source directing a beam along the diametrical bisector of said antenna which passes through said point source, said antenna comprising a pair of circular, spaced, overlying conductive wiregrids arranged to form a surface of revolution about the axis of said antenna, said spaced Wire grids defining therebetween a wave propagating region for a wave polarized with its electric field perpendicular to the plane of the wire grid, the mesh opening size of said wire-grids being less than one-fourth of the shortest operating wavelength, the diameter of said wire-grids being greater than twice the longest operating wavelength, and non-conductive means supporting said wire-grids with a spacing which increases in a radially outwardly going direction from a distance which is small compared with the mesh opening size to a distance which is large compared with the mesh opening size.

4. A circularly symmetric wire-grid lens antenna for converting a peripherally located point source of radiation to a cophasally extended source directing a beam along the diametrical bisector of said antenna which passes through said point source, said antenna comprising a pair of circular, spaced, overlying conductive wire-grids arranged to form a surface of revolution about the axis of said antenna, said spaced wire grids defining therebetween a wave propagating region for a wave polarized with its electric field perpendicular to the plane of the wire grid, the mesh opening size of said Wire-grids being less than one-fourth of the shortest operating wavelength, the diameter of said wire-grids being greater than twice the largest operating wavelength and the spacing of said wiregrids increasing in a radially outwardly going direction from a distance which is less than one-half the mesh opening size to a distance which is greater than twice the mesh opening size, a radiating structure afiixed to the peripheral edge of said pair of wire-grids, and non-conductive means for supporting and spacing said Wire-grids.

5. A wire-grid lens antenna in accordance with claim 4 in which said variations in wire-grid spacing is such that the effective dielectric constant varies parabolically.

6. A wire-grid lens antenna comprising a central radiation beam shaping structure with a beam radiating structure on the periphery, said central beam shaping structure including a pair of conductive, spaced, overlying wire-grids whose mesh opening size is less than onefourth of the shortest operating wavelength, said spaced wire grids defining therebetween a Wave propagating region for a wave polarized with its electric field perpendicular to the plane of the wire grid, the spacing between opposite points on said pair of wire-grids being selected to provide a desired predetermined relative effective permittivity.

7. A wire-grid lens antenna comprising a central radiation beam shaping structure with a beam radiating structure on the periphery, said central beam shaping structure including a pair of conductive, spaced, overlying wiregrids each having a mesh opening size of less than onefourth of the shortest operating Wavelength and having a maximum wire-grid separation of less than five times the mesh size, said spaced wire grids defining therebetween a wave propagating region for a Wave polarized with its electric field perpendicular to the plane of the wire grid, and lumped impedance means associated with certain meshes for decreasing the relative efiective permittivity between said certain meshes.

8. A wire-grid lens in accordance with claim 7 in which said lumped impedance are inductors connected in series with the wires forming the sides of said certain meshes.

9. A wire-grid lens in accordance with claim 7 in which said lumped impedance are capacitors connected between opposite junctions forming the corners of said certain meshes.

10. A Wire-grid lens antenna comprising a central radiation beam shaping structure with a beam radiating structure on the periphery, said central beam shaping structure including a pair of conductive, spaced, overlying wiregrids having a mesh opening size of less than one-fourth of the shortest operating wavelength, said spaced wire grids defining therebetween a Wave propagating region for a wave polarized with itselectric field perpendicular to the plane of the wire grid, certain pairs of overlying meshes being formed of Wires which include a zig-zag portion for increasing the mesh size, the increase in electrical path length being selected to provide a predetermined decrease in the relative elfective permittivity in the region of said certain meshes.

11. A wire-grid lens antenna comprising a substantially circular central beam shaping portion surrounded by a beam radiating portion, said central beam shaping portion comprising a pair of conductive, spaced, overlying wire-grids arranged to form a surface of revolution about the axis of said antenna, said spaced Wire grids defining therebetween a wave propagating region for a wave polarized with its electric field perpendicular to the plane of the wire grid, the mesh size of said wire-grids being less than one-fourth of the shortest operating wavelength, said beam radiating portion extending from said pair of circular wire-grids in the form of a biconical horn.

12. A wire-grid lens antenna comprising a central beam shaping structure surrounded by a beam radiating structure, said central beam shaping structure including a pair of spaced, overlying conductive wire-grids of the same mesh structure, said spaced wire grids defining therebetween a wave propagating region for a wave polarized with its electric field perpendicular to the plane of the wire grid, the mesh openings of said wire-grids having a size which is small in comparison with the shortest operating wavelength and being arranged in substantial spaced alignment with one another, the spacing between overlying 113. In a wire-grid lens antenna the improvement in the A lens comprising a pair of spaced, overlying, conductive wire-grid meshes having a mesh opening size which is smaller than one-fourth of the shortest operating wavelength, and means for supporting said grid meshes relative to one another to flare outwardly in all directions from their center at a rate increasing with the radial distance from their center whereby a plane Wave applied to one edge of said pair of grid meshes with a polarization such that its electric field is perpendicular to the plane of said grid meshes is focused substantially at a point at the opposite edge.

14. In a wire-grid lens antenna as recited in claim 13 wherein the geometrical configuration of the individual meshes in said wire-grid are in the shape of an equilateral triangle.

15. In a wire-grid lens antenna as recited in claim 13 wherein the geometrical configuration of the individual meshes in said Wire-grid are in the shape of a square.

16. In a wire-grid lens antenna as recited in claim 13 wherein the geometrical configuration of the individual meshes in said wire-grid are in the shape of a hexagon.

References Eited by the Examiner UNITED STATES PATENTS 2,485,138 10/1949 Carter 343-915 2,511,916 6/ 1950 Hollingsworth 333- 2,576,181 11/1951 Iams 343754 2,576,182 11/1951 Wilkinson 343754 2,596,251 5/1952 Kock 343-909 2,720,588 10/1955 Jones 343754 2,756,424 7/1956 Lewis 343-909 2,884,629 4/1959 Mason 343780 3,047,860 7/ 1962 Swallow 343897 3,116,485 12/1963 Garson 343754 FOREIGN PATENTS 402,834 12/ 1933 Great Britain.

OTHER REFERENCES Silver: Microwave Antenna Theory MIT Rad. Lab. Series, vol. 12, page 449 relied upon.

HERMAN KARL SAALBACH, Primary Examiner.

Claims

1. A WIRE-GRID LENS ANTENNA COMPRISING A PAIR OF SPACED, OVERLYING, CONDUCTIVE WIRE-GRIDS HAVING SUBSTANTIALLY THE SAME MESH STRUCTURE, THE MESH OPENINGS OF SAID WIRE-GRIDS HAVING A SIZE WHICH IS SMALL IN COMPARISON WITH THE SHORTEST OPERATING WAVELENGTH AND BEING ARRANGED IN SUBSTANTIAL SPACED ALIGNMENT WITH ONE ANOTHER, SAID SPACED WIRE GRIDS DEFINING THEREBETWEEN A WAVE PROPAGATING REGION FOR A WAVE POLARIZED WITH ITS ELECTRIC FIELD PERPENDICULAR TO THE PLANE OF THE WIRE GRID, MEANS FOR SUPPORTING SAID OVERLYING WIRE-GRIDS AT EACH POINT OF SAID ANTENNA