Circularly Polarized Antenna

Abstract

A circularly polarized antenna is provided using a planar array of at least two perpendicular pairs of linearly polarized elements (slot antennas or dipoles). The distance D between a given pair of parallel elements is selected to provide in a plane perpendicular to the elements of the pair, a field pattern very similar to that in a plane parallel to the elements, where both principal planes pass through the center of the array and are perpendicular to the plane of the array. Elements of a given pair are excited in phase, and the two pairs are excited in phase quadrature, whereby circularly polarized radiation is produced over a very wide sector of space in each principal plane, and therefore in all planes passing through the axis of the array.

Description

ORIGIN OF THE INVENTION

The invention described herein was made by employees of the United States Government and may be manufactured and used by or for The Government for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

This invention relates to antennas and more particularly to circularly polarized antennas. The prior art has attempted to provide circularly polarized antennas and a large number of quasi omnidirectional circularly polarized antennas have been designed and developed. These antennas vary from simple crossed-slot antennas to equi-angular spiral antennas. Each end of this spectrum of antennas has certain disadvantages. For example, crossed-slot antennas cannot provide circularly polarized antenna beams over wide angles. Equi-angular spiral antennas have the disadvantage that their sector of good polarization (effective beamwidth) cannot be varied over a fairly wide range by simple design changes. That is, complex design changes must be made to an equi-angular spiral antenna to vary its sector of good circular polarization over a wide range.

Therefore, it is an object of this invention to provide a circularly polarized antenna.

It is also an object of this invention to provide a circularly polarized antenna that provides a circularly polarized beam over a wide sector of space, which sector can be varied by simple changes to the antenna configuration.

It will be appreciated by those skilled in the art and others that it is particularly desirable to provide an antenna having a circularly polarized beam that can be flush mounted on a vehicle such as an airplane or a space vehicle.

Therefore, it is yet another object of the invention to provide an antenna having a circularly polarized beam that is suitable for flush mounting.

It is a still further object of this invention to provide an antenna having a circularly polarized beam that is formed of slot antenna elements to provide a relatively narrow beam.

SUMMARY OF THE INVENTION

In accordance with a principle of this invention, a circularly polarized antenna is provided. The antenna comprises two sets of linearly polarized antenna elements. The sets of antenna elements are arrayed in similar patterns in two planes with the first set being positioned orthogonal to the second set. The two sets are driven in phase quadrature.

In accordance with another principle of this invention, each set of antenna elements comprises a plurality of slots in a parallel array. In addition, the two sets of plural slots are mounted on the end of a waveguide structure which provides the driving phase quadrature signals.

In accordance with a still further principle of this invention, the waveguide structure includes a flared pyramidal horn and an end cover in which cover is formed the two sets of plural slots. In accordance with yet another principle of this invention, the two sets of plural slots each include only two slots.

In accordance with an alternative embodiment of the invention, the two sets of antenna elements are formed of dipole elements -- one set of dipole elements being arrayed orthogonal to the other set of dipole elements.

It will be appreciated from the foregoing brief summary of the invention that the invention provides a circularly polarized antenna which can be formed of either slot or dipole elements. When the antenna is formed of slot antenna elements, it is suitable for flush mounting on an aircraft or space vehicle. The antenna is structurally uncomplicated in that it requires only two sets of slots or dipoles arrayed in parallel -- one set being arrayed orthogonal to the other set. In addition, the antenna of the invention requires that the first set be driven in phase quadrature with respect to the second set. By changing the length of the slots or their separation, the antenna's beam-width, and therefore its sector of good polarization, can be easily varied over a wide range.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a pictorial diagram illustrating one antenna slot configuration for broad beamwidth formed in accordance with the invention;

FIG. 2 is a graph of normalized field patterns v. angle (.theta.) from the antenna beam axis for the antenna configuration illustrated in FIG. 1;

FIG. 3 is a pictorial diagram of an alternate antenna slot configuration for narrow beamwidth formed in accordance with the invention;

FIGS. 4A and 4B are diagrams of alternate waveguide configurations having the slot configuration illustrated in FIG. 1 mounted on the ends of the waveguides;

FIG. 5 is a pictorial diagram of an antenna dipole configuration of the invention;

FIGS. 6A and 6B illustrate further alternative slot configurations formed in accordance with the invention;

FIGS. 7A and 7B illustrate further waveguide systems for applying quadrature signals to the slot configurations formed in accordance with the invention; and

FIG. 8 illustrates a still further waveguide system formed in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior to discussing the preferred embodiments of the invention, the theory of operation of the invention as best understood is presented. The antenna element patterns of simple linearly polarized antenna elements (either slots or dipoles) are different in the two principal planes of operation of the elements. The invention combines a set formed of two or more of such antenna elements in a specific array so as to make the array pattern of the combined antenna elements nearly similar in the two principal planes of operation. In addition, this first set is combined with a second set also formed of two or more antenna elements. The second set is positioned orthogonal to the first set and its elements are driven in phase quadrature with respect to the elements of the first set. When this overall antenna structure is driven in this manner, a nearly circularly polarized antenna beam operating over a wide sector of space is produced. Because the antenna can be formed of slot elements, it is suitable for flush mounting on an aircraft or space vehicle.

FIG. 1 is a pictorial diagram illustrating one embodiment of the invention formed of antenna slot elements. Four antenna slot elements 11, 12, 13 and 14 are illustrated in FIG. 1, geometrically arrayed in an X-Y plane as two parallel pairs 11 and 12, and 13 and 14, with one pair being orthogonal to the other pair. The Z axis of the coordinate system intersects the geometric center of the antenna slot configuration.

The distance between a parallel pair of antenna elements 11 and 12 or 13 and 14 is designated D, and the length of each antenna slot is designated 2h. The angle .phi., illustrated in FIG. 1, is the angle from the X-axis of a particular beam sector, and the angle .theta. is the angle from the Z-axis of the beam sector.

In operation, the four slot elements 11, 12, 13 and 14 formed in a plate 15 are excited in pairs for broadside radiation; that is, the first pair at elements 11 and 12 are excited in phase with each other, and the second pair of elements 13 and 14 are excited in phase with each other. The first pair of elements is excited in phase quadrature with respect to the second pair of elements so as to produce a circularly polarized beam at broadside.

The field pattern in the plane containing the axis located between either pair of antenna elements (for example, the X-Z plane for the first pair 11 and 12) is determined by the element field pattern of the pair, and, hence, by the electric field (current) on the pair of slot elements. This field pattern is zero in the far field at Z=0. The field pattern of the same pair in the plane normal to the antenna axis between the pair (in this case the Y-Z plane) is determined entirely by the array factor of the two antenna elements forming the pair. Hence, this distribution or pattern can be considerably modified by varying the distance D. Thus, it is possible to make the field pattern in this plane (Y-Z plane) very similar to the field pattern in the axis plane (X-Z) over a wide range of angles near broadside. When this is done for both pairs of antenna elements and when the pairs of antenna elements are excited in phase quadrature, a circularly polarized antenna beam extending over a wide range of angles in the X-Z and Y-Z plane is provided. A typical far field pattern showing the horizontally polarized (E.theta. cos .phi.) and the vertically polarized (Ey) fields at .phi.=0 are illustrated in FIG. 2. E is the normalized (horizontally or vertically) polarized field pattern.

The field of the embodiment of the invention illustrated in FIG. 1 is approximately circularly polarized even in the planes determined by setting .phi. = .+-. 45.degree.. Hence, this combination of antenna elements assures extremely good circular polarization for all angles of .phi., even for large angles of .theta.. In one actual embodiment of the invention, the antenna element slots illustrated in FIG. 1 were designed so that with 2h/.lambda. = 0.50 (.lambda. being the free space wavelength at the frequency of operation) the orthogonal polarizations were matched at the half power point. The parameters resulted in D/.lambda. = 0.397 and a half power beamwidth of 78.degree. .

It should be noted that for the foregoing example, 2h is greater than D; hence, the actual slot configuration for that example was not exactly as illustrated in FIG. 1; rather, it was as illustrated in FIG. 3 -- that is, the slots actually intersect. The slot configuration illustrated in FIG. 3 produces a fairly narrow beam with good circular polarizatization; whereas, the slot configuration illustrated in FIG. 1 produces a broader beam. However, in both cases, the distance 2h (the antenna element length) determines the basic pattern width. The distance D is thereafter chosen to provide a nearly equal pattern in the two orthogonal planes as previously described.

FIGS. 4A and 4B illustrated two types of apparatus for exciting the slot elements in phase quadrature. FIG. 4A illustrates an inner square waveguide 21. Located on each of the four sides of the inner square waveguide 21 are four side waveguides 23, each one-half wavelength long. Four transverse slots 25 (two of which are shown dotted in FIG. 4A) are located in the sides of the square waveguide one-quarter wavelength from the end to allow signals moving down the inner square waveguide 21 to pass into the four side waveguides 23. The outer end of the inner square waveguide is enclosed to short circuit signals moving down the inner square waveguide 21. Slots are formed in the ends of the side waveguides 23. As illustrated in FIG. 4A, the slots are in the configuration illustrated in FIG. 1; however, any slot configuration suitable for carrying out the broad concept of the invention as previously described can be formed in the ends of the side waveguides.

One method of establishing equal phase quadrature signals in the square waveguides is also shown in FIG. 4A. This figure shows the square waveguide excited by a crossed slot pair 29 located one-quarter wavelength from the shorted end of a feed (lower) waveguide 31. The slots are inclined at 45.degree. from the lower guide axis. This geometry couples equal, in-phase, orthogonally polarized waves into the square waveguide 21. A dielectric slab 33 delays the relative phase of one of the polarizations and establishes a circularly polarized wave. Many other methods of accomplishing this same result are known in the art and several are described in the "Antenna Engineering Handbook," Henry Jasik, Editor, McGraw-Hill Book Co., 1961, Ch. 17. In general, any suitable prior art method can be used to supply incident orthogonal modes in phase quadrature to the slots 25. A circuit for performing this function for the coaxial line geometry is illustrated in FIG. 5.

If the distance D is less than one-half wavelength, the waveguide of FIG. 4A may be dielectrically loaded in order to propagate the frequency chosen. The structure illustrated in FIG. 4A operates as follows: two orthogonal mode signals travel down the waveguide 21 in phase quadrature. As previously described, the waveguide is short circuited at its outer end and the transverse slots are cut one-quarter wavelength from the end on all four sides. These slots are excited by means of the interrupted longitudinal current and couple into the four side waveguides in the same manner. Each of the side waveguides supports a single mode which then excites the slots at the ground plane resulting in a circularly polarized antenna beam being formed in the manner previously described. As with the inner square waveguide, the side waveguides may be dielectrically loaded.

FIG. 4B illustrates an alternative waveguide apparatus for exciting slots in phase quadrature. FIG. 4B comprises a square waveguide 35 with the end of the waveguide enclosed by a suitable plate in which the antenna slot configuration is formed. In accordance with well known waveguide principles, two orthogonal modes travel down the waveguide and excite the slots in phase quadrature. While the waveguide structure illustrated in FIG. 4B is less complex than the waveguide structure illustrated in FIG. 4A, care must be taken to assure that cross polarized components are not excited at each of the individual slots. The FIG. 4B waveguide configuration may or may not be dielectrically loaded to improve performance, as desired.

It will be appreciated by those skilled in the art that the broad beam configuration of FIG. 1 can be implemented using the waveguide arrangements of either FIGS. 4A or 4B. The waveguide arrangement of 4B can also be used to implement the narrower beam four slot configuration of FIG. 3.

It will also be appreciated by those skilled in the art that the basic concept of the invention can be carried out by other apparatus. For example, circularly cylindrical waveguides can be used to excite the slot configuration. The techniques necessary for the excitation of the slots using circularly cylindrical waveguide are the same as previously described for square waveguides.

FIG. 5 illustrates an embodiment of the invention wherein dipole antenna elements, as opposed to slots, are used to form the circularly polarized antenna beam. The embodiment of the invention illustrated in FIG. 5 comprises: a 90.degree. hybrid 41; first and second power dividers 43 and 45; four baluns 47, 49, 51 and 53; and four pairs of antenna dipole elements 55, 57, 59 and 61. One pair of dipole elements are attached to and project outwardly from opposite sides of one balun in any well known manner. More specifically, the first pair of dipole elements 55 project out of opposite sides of the first balun 47. In a similar manner, the second pair of dipole elements 57 project from the second balun 49; the third pair of dipole elements 59 from the third balun 51; and the fourth pair of dipole elements 61 from the fourth balun 53.

In operation, the incoming signal is applied via an incoming terminal 63 to the input of the 90.degree. hybrid 41. The 90.degree. hybrid splits the incoming signal into phase quadrature signals that are applied to the first and second power dividers 43 and 45. The power dividers divide the signals equally. The first power divider 43 is connected to the first and third baluns 47 and 51. The second power divider 45 is connected to the second and fourth baluns 49 and 53. In this manner, phase quadrature signals are applied via the baluns to the antenna dipole elements. That is, one signal mode is applied to the first and third baluns and a phase quadrature signal mode is applied to the second and fourth baluns. The first and third baluns are mounted (by means not shown) so that their respective dipole elements are arrayed in parallel in a manner similar to the slot array of one pair of slots in the FIG. 1 embodiment. The second and fourth baluns are mounted (by means also not shown) so that their respective dipole elements are also arrayed in parallel, but orthogonal to the direction of the dipole elements connected to the first and third baluns. Hence, a dipole array similar to the slot array of the previously described embodiments is formed. FIG. 5 illustrates that the dipoles are mounted at a distance D.sub.z above a ground plane 65. While this mounting is not essential to the operation of the invention, it is illustrated because it is appropriate for many applications, such as aerospace applications, for example.

The embodiment of the invention illustrated in FIG. 5 operates in the manner identical to the embodiments of the invention previously described. That is, phase quadrature signals are applied through the baluns to the dipole elements. Due to the configuration of the dipole element array, a circularly polarized beam is formed. As with the previously described slot embodiments, the length of the dipole elements (2h) , from end to end, and the distance between the pairs (D) determines the exact configuration of the circularly polarized antenna beam that is formed.

The foregoing description has described the basic structural embodiments of the invention and their operation. However, the previously described embodiments are useful only when relatively wide beamwidths are desired. More specifically, because the minor lobe level of the array factor increases when D exceeds .lambda./2 until D reaches a value equal to the major lobe at D=.lambda., the number of these undesirable "grating lobes" increases with a further increase in D. As opposed to this, when the slot length is increased, only a single beam is formed in the element pattern, because there is no periodicity to give rise to grating lobes. Therefore, even though it is possible to match the main lobe structure of a long slot with the main lobe of the two elements array factor, the array factor side lobes are large and the side lobe structure is, in general, very poorly matched. Hence, the previously described four slot or dipole configurations are not satisfactory when a very narrow beam is desired.

The slot configurations illustrated in FIGS. 6A and 6B and hereinafter described avoid the difficulty just described. FIG. 6A illustrates a slot configuration wherein three parallel slots 67 or 69 make up each antenna set. Each set of slots is formed orthogonal to the other set, as previously described. The slots are formed in a plate 70 such that each slot of one set intersects all three slots of the other set; hence, 2h>2D. FIG. 6B illustrates an alternative three parallel slot arrangement wherein only the center slots of the pairs intersect; that is, 2h<2D. It will be appreciated by those skilled in the art that while the FIGS. 6A and 6B embodiments illustrate three parallel slots making up each of the orthogonal sets, this is merely by way of example, and that more parallel slots could be included in each set, if desired. In general, the slot arrays shown illustrated in FIG. 6A and 6B illustrate that the antenna elements must be invarient under a rotation of any multiple times 90.degree. .

Whether the geometry of FIG. 6A or FIG. 6B is used in a particular antenna structure depends upon how the slots are excited. If the amplitude of the orthogonally polarized fields is constant, the orientation of FIG. 6A is required, because the field distribution across each slot will be tapered (in fact, the field distribution will be zero at the ends of the slots), but each of the slots must be excited with the same amplitude as every other slot. The array factor beamwidth is narrower than the element factor beamwidth if the total slot array length (L) is set equal to 2h. L is defined as (N-1)D, where N is the number of slots. For this reason, the slots are preferably somewhat longer than the total slot array length L. For other choices of design geometry, the FIG. 6B embodiment is preferable to the FIG. 6A embodiment.

Various types of apparatus can be used to excite the embodiments of the invention illustrated in FIGS. 6A and 6B. For example, a multi-mode field can be established within a horn feeding the structure in order to improve the element pattern side lobes. Alternatively, a similar improvement in the array pattern can be effected by symmetrically varying the slot thickness or the slot spacing. Further, the array pattern can be improved by using a multi-mode excitation for each polarization in the plane perpendicular to the slot axes.

In principle, the waveguide geometries of FIGS. 4A and 4B can also be used to excite the N-slot configuration illustrated in FIGS. 6A and 6B. The horn arrangements of FIG. 7 are an extension of the basic feeding system of FIG. 4B, and the special embodiment illustrated in FIG. 8 is a hybrid of the two excitation schemes illustrated in FIGS. 4A and 4B. It can be used to excite a relatively narrow beam set of six slots, and it avoids the problem sometime encountered with the geometry of FIG. 4A with spacings for which it is necessary to dielectrically load the center waveguide.

FIGS. 7A and 7B illustrate waveguide systems for exciting the general N-slot configurations illustrated in the FIGS. 6A and 6B. The FIG. 7A system comprises a square waveguide 71 feeding a small flare, pyramidal horn 73. An end plate or cover 75 including the particular slot configuration being used fits over the end of the horn 73. The dual mode signals move down the waveguide and the horn and excite the slots in phase quadrature in accordance with well known waveguide principles. A dielectric lens can be added to the embodiment illustrated in FIG. 7A if larger flare angles are desired.

The FIG. 7B waveguide system includes the square waveguide and the pyramidal horn 73 illustrated in FIG. 7A. In addition, mounted between the end cover 75 and the end of the pyramidal horn 73, is a large square waveguide 77. This embodiment provides a higher order mode aperture field distribution with a more suitable amplitude distribution for pattern shaping than does the FIG. 7A embodiment. Many other types of apparatus can be utilized by the invention to feed the various slot configurations will be apparent to those skilled in the art. Hence, the invention should not be construed as limited to the signal feeding apparatus illustrated in the drawings and described herein.

The horn arrangements of FIGS. 7A and 7B can in principle be used to excite any of the slot combinations, although it is best to have the cover plate approximately the same area as the extended area of the array of slots. Otherwise, it becomes difficult to couple energy out through the slots efficiently. For this reason the geometries of 4A and 4B are preferable for the four slot configurations.

As with the two slot configurations previously described, the slot length of the FIG. 6A and 6B configurations is determined by the desired beamwidth. The number of slots is determined by the criteria that the slots spacing should be kept less than about 0.7.lambda.. The actual slot separation D is determined by matching the main beam to that of the slot element pattern so that the desired circularly polarized beam is formed.

In conclusion, it should be pointed out that the beam-width is determined by the slot length 2h. Once this length is chosen, the number of slots and the required slot spacing to set the array factor equal to the element factor must be determined. The array length L should be nearly the same as the slot length 2h, and at the same time the slots should be no more than 0.6 or 0.7.lambda. apart. This determines the minimum number of slots. The remaining step is to equate beamwidths by choosing the slot separation between about 0.4.lambda. and 0.7.lambda., and to do this in a way consistent with the design sidelobe requirements of a particular embodiment of the invention.

It will be appreciated by those skilled in the art and others that the foregoing description has described a novel apparatus for providing a circularly polarized antenna beam. Preferred embodiments of the invention have been illustrated and described. However, various other antenna configurations and feed apparatus such as cylindrical waveguides can be utilized to carry out the basic concept of the invention. For example, the embodiment shown in FIG. 5 assumes that coaxial lines are used to carry signals to and from the dipoles and therefore baluns are required to convert the field geometry to the balanced configuration required for exciting the dipoles. It is to be understood that a two wire line arrangement could also be used; in this case without baluns.

Claims

What is claimed is:

1. A circularly polarized antenna comprising

a planar array of linearly polarized antenna elements selected from a group consisting of antenna slots and dipoles, said elements being of the same size and arranged in two orthogonally oriented sets of parallel elements, both sets having the same geometric center, elements of a given set being spaced a given distance apart such that a field produced by a given pair of adjacent elements in a set when excited in phase will produce in a first plane perpendicular to the plane of said array, and perpendicular to said pair of elements, at their centers, a pattern very similar to the field pattern of said given pair of elements in a second plane perpendicular to said first plane and to said plane of the array when excited in phase, where said second plane intersects said first plane midway between said given pair of elements, and

means for exciting said elements of the other set in phase quadrature with said one set.

2. A circularly polarized antenna as defined in claim 1 wherein each set of elements is comprised of one pair of elements.

3. A circularly polarized antenna as defined in claim 1 wherein said elements are selected to be antenna slots, said excitation means comprises a square waveguide, and said elements are arranged in a plate covering one end of said square waveguide.

4. A circularly polarized antenna as defined in claim 3 wherein said excitation means includes four side waveguide sections, each side waveguide section being coupled to said square waveguide section through a separate transverse slot, and each sidewave guide section feeding a separate element.

5. A circularly polarized antenna as defined in claim 1 wherein each set of elements is comprised of three parallel slots, said excitation means comprises a waveguide, and said elements are arranged in a plate covering one end of said waveguide.

6. A circularly polarized antenna as defined in claim 5 wherein said waveguide comprises a square waveguide section and a flared pyramidal horn waveguide section attached to the end of said square waveguide section, and said plate is attached to the outer end of said flared pyramidal horn waveguide section.

7. A circularly polarized antenna as defined in claim 6 further including a large square waveguide section located between the outer end of said flared pyramidal horn waveguide section and said plate.

8. A circularly polarized antenna as defined in claim 1 wherein said first set of antenna elements is comprised of parallel slots in a plate covering the end of a waveguide and wherein said second set of antenna elements is comprised of N/2 parallel slots in said plate, where N is an even integer greater than four.

9. A circularly polarized antenna as defined in claim 8 wherein said waveguide comprises a square waveguide section and a flared pyramidal horn waveguide section attached to the end of said square waveguide section, and said plate is attached to the outer end of said flared pyramidal horn waveguide section.

10. A circularly polarized antenna as defined in claim 9, wherein said plate is attached to the outer end of said flared pyramidal horn waveguide section by a large square waveguide section.

11. A circularly polarized antenna as defined in claim 1 wherein each of said antenna elements is selected to be a dipole.

12. A circularly polarized antenna as defined in claim 11 wherein each of said two sets of antenna elements includes one pair of dipoles.

13. A circularly polarized antenna as defined in claim 12 wherein said excitation means comprises:

a 90.degree. hybrid adapted to receive a signal;

a first power divider having its input connected to one output of said 90.degree. hybrid and its outputs connected to both dipoles to one pair of said antenna elements; and

a second power divider having its input connected to a second output of said 90.degree. hybrid and its outputs connected to both dipoles of the other pair of said antenna elements.