Limited scan antenna array

[57] ABSTRACT

An electronically steerable antenna array comprising phased element antennas has essentially uniform gain for small scan angles. An idealized shape of element pattern in the array environment is closely approached by controlling the complex mutual coupling among the element antennas for pattern shaping. The resulting element-in-array pattern has a nearly flat top, steep sides, and also has low sidelobes in the angular regions where grating lobes of the array factor can appear upon steering. Consequently, the antenna array has high and almost constant gain over a limited range of steering angles in all directions from the principal mechanical axis of the planar array, and has low sidelobes and low grating lobes.

Claims, 12 Drawing Figures

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PATENTED 0EC3H974

3.858,221

SHEET 1 OF 4

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ΓΑΤΕΝΤΕ00Ε03 11974

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SHEET 2 OF 4

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PATENTED DEC 3 11974

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SHEET 3 OF 4

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ATENTED 0EC311974 3,858,221

SHEET 4 OF 4

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3,858

LIMITED SCAN ANTENNA ARRAY

BACKGROUND OF THE INVENTION

The far-field radiation pattern of an antenna array is the sum of contributions of each of the element anten- 5 nas which constitute the array, with the relative phases of the contributions in the far field being taken into account in the summation. Under certain circumstances, an antenna array pattern may be represented as a product of an array factor and an element pattern. The re- 10 quired circumstances are that the elements be identical, that their generator impedances be identical, that the mutal energy interaction between each element and its neighboring elements extend over a region which is small compared with the over-all size of the array so 15 that edge effects are relatively small, and that the elements other than those at the edges all see almost the same environment of neighboring elements, as would be the case when the elements are uniformly spaced on a plane. 20

The array factor describes an antenna pattern which would be produced by a hypothetical antenna array consisting of isotropically radiating antenna elements whose phase centers are located at the phase centers of the elements of the actual array. 25

The element pattern to be employed in representing the actual array pattern as a product of an array factor and an element pattern, is the element pattern which would be exhibited by an actual element antenna when it is located in its array environment. Such an element ³⁰pattern may be produced by exciting one element antenna when it is mounted in the array, with the other element antennas not directly excited, but instead loaded at their terminals by impedances equal to the internal impedance of their inactive generators. It is ³³well known that the element pattern which is thus obtained, hereinafter referred to as the element-in-array pattern, is in general quite different from a pattern which would be produced by the same element antenna mounted alone on a ground plane. The differences are ⁴⁰due to parasitic excitation of elements which surround the directly excited element. The neighboring elements are excited by energy coupled to them through mutual impedances and paths which exist among them and the directly excited element. A part of the energy thus cou- ⁴⁵pled to the neighboring elements is radiated to the far field and combines with energy radiated from the directly excited element to produce the element-in-array pattern. This pattern is, of course, dependent upon the coupling parameters between the element antennas as ⁵⁰they are located in the array as well as upon the characteristics which a single element would have if standing alone.

When the principle beam of an array is steered off the array’s normal mechanical axis by changing the rel- ⁵⁵ative phases of excitation of the element antennas, only the array factor is steered in space. The element-inarray pattern is not steered nor altered appreciably when the steering covers relatively small angles. Thus the element-in-array pattern is stationary in space during steering, and the array factor, by which it is multiplied to arrive at the resultant actual array pattern, is offset angularly so that the tip of the central lobe of the array pattern appears to describe approximately the ₆₅contour of the element-in-array pattern when the array factor is steered progressively farther off the mechanical axis. The amplitude of the central or principal lobe ,221 of the array factor is relatively constant as the antenna beam is steered off axis by small angles; it is diminished only in proportion to the consine of the steering angle, the cosine effect resulting from a foreshortening of the array when projected at the small angle steered off axis.

For many applications in which the beam must be steered over an angular range centered at the mechanical axis, an ideal element-in-array pattern would be one whose shape exactly compensates for the minor cosine reduction of the array factor so that the resulting pattern would have uniform sensitivity for all steering angles within the range. Beyond the steering range, the element-in-array pattern would ideally be zero so that sidelobes and grating lobes would be suppressed. Each sidelobe of the array pattern is created mainly by only a small percentage of the elements of the array. The grating lobes are those other than the principal lobe which are produced by concerted phase action of a great number of the elements. Grating lobes may arise at angles of the array pattern for which the path lengths of energy contributions from neighboring elements differ by an integral multiple of a wavelength.

SUMMARY OF THE INVENTION

In the present invention, antenna elements are arrayed on a uniformly spaced grid in a plane. The dimensions of the antenna elements themselves are selected in such a way as to control the mutal electromagnetic coupling between elements so that the elementin-array pattern approaches the ideal element-in-array pattern for antenna arrays which are to be steered electronically over small angles. That is, the mutual coupling is controlled to provide an element-in-array pattern having a main lobe with a nearly flat top across the scanning range and having rapidly decreasing sides outside of that range and having low sidelobes and low pattern values at the angles where grating lobes could occur.

One or more structural elements such as a thin dielectric sheet may be used also to control mutual energy transfer laterally among elements of the array.

Accordingly, one object of the present invention is to provide an antenna array which has nearly equal gain everywhere within an angular region over which it is capable of being electronically steered.

Another object of the invention is to provide an antenna array whose characteristics are controlled to have relatively high and uniform gain over a steering range, low side lobes, and low grating lobes, by controlling the electromagnetic energy coupling among element antennas of the array.

Another object of the invention is to provide an array whose elements have an element-in-array pattern with a substantially constant sensitive over a central angular range and a very rapid reduction in sensitivity from the central range to more remote angular ranges and very low sensitivity in more remote ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a high-gain broadside antenna array.

FIG. 2 shows a principal lobe of the array in polar coordinates, and its beam steering range.

FIG. 3 is a block diagram of a conventional electronic beam steering system.

FIG. 4a is a diagram of the construction of an element antenna for a preferred embodiment of the invention for linear wave polarization.

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FIG. 4b shows the construction of an element antenna for circular wave polarization.

FIG. 5 shows an arrangement for measuring an element-in-array pattern of an element antenna.

FIG. 6 depicts two element-in-array patterns of an element antenna in which a disk member has a diameter which is correct.

FIG. 7 shows two element-in-array patterns of the element antenna of FIG. 6, but in which the disk member has a diameter which is too small.

FIG. 8 shows two element-in-array patterns of the element antenna of FIG. 6, but in which the disk member has a diameter which is too large.

FIG. 9a shows a side view of an embodiment in which the element antennas are dielectric rods.

FIG. 9b is a front view of a portion of the array of FIG. 9a.

FIG. 9c is a side view of yet another embodiment in which the element antennas are dipoles whose mutal energy coupling is determined in part by a dielectric sheet that covers them.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In a preferred embodiment of the invention, a number of element antennas 10 are arrayed over a ground plane 12 at the intersections of a uniformly spaced grid as shown in FIG. 1. The spacing between rows and columns of elements 10 is in the range of 0.75 to 1.10 wavelengths such as 0.85 wavelengths for the frequency of excitation employed. A principal mechanical axis 14 of the antenna is perpendicular to the plane of the array 12 as shown in FIG. 2, and defines the direction of the center of a principal lobe 16 of a resultant antenna array pattern when all of the element antennas 10 are excited with equal phase. The principal lobe 16 of the resultant array pattern may be steered over an angular range 18 centered on the mechanical axis 14, by linear phasing of the element antennas 10 relative to each other depending upon their positions. In the preferred embodiment described herein, steering may be accomplished over a range 18 extending to an outer limit in any direction from the mechanical axis 14, the outer limit being in a range 0° to at least 15°. In FIG. 2, the principal lobe 16 of the resultant array pattern is shown in polar coordinate form for the condition when the principal lobe is located on the mechanical axis, and is shown as a dotted line 20 for a steering position 15° off the axis.

Each element antenna 10 is mounted on the common ground plane 12 and in this embodiment is fed from behind the ground plane 12 at element antenna terminals 22 through any suitable baluns known in the prior art. To simplify the description of this invention, a conventional electronic beam steering control system is employed, as shown in FIG. 3. Each balun 19 is driven by a phase shifter 21 which can be typically of a conventional diode switching type controllable by DC potentials applied to the diodes of the phase shifter 21 by a beam steering control circuit 23. The phase shifters 21 are excited by a corporate feed system from a source 25.

All of the element antennas 10 of the array are identical. The element antennas 10 in the preferred embodiment are similar to short backfire antennas except for the selection of their dimensions. The dimensions in the present invention are determined empirically. In FIG.

4a an element antenna 10 is shown for radiating an electromagnetic wave of one linear polarization.

Each element antenna 10 comprises two poles or transmission members 24 connected to the input terminal 22 and mounted perpendicularly to the ground plane 12. An insulator 26 prevents the ground plane 12 from short-circuiting the element. The transmission members 24 are between 0.4 and 0.6 wavelength long and are connected to a conductive disk 28 at their ends remote from the ground plane. The disk 28 has a diameter in the range of 0.38 to 0.62 wavelength. If desired, the transmission members 24 may be electrically connected together either instead or in addition by a capacitor whose position on the transmission members 24 with respect to the terminating disk 28 may be adjusted. Approximately midway between the ground plane 12 and the terminating disk 28 on the transmission members 24, a radiating dipole 30 is mounted. One arm of the dipole 30 is mounted on each of the transmission members 24, each arm 32 being approximately one-fourth wavelength long. When the antenna element 10 is excited, current flows from the terminals 22 along the transmission members 24 to the dipole 30 and out along the dipole arm 32 to their ends where it is continuously reflected as is well known in the prior art. The current in the dipole arms 32 together with the voltage established between the two dipole arms creates standing waves of electromagnetic fields between the ground plane 12 and the terminating disk 28 of the element antenna 10. The ground plane 12 and the terminating disk 28 serve as ends of a rather loosely bounded cavity, centered on the transmission members 24, which resonates under the stimulation of the dipole 30. Electromagnetic energy easily escapes from the cavity and is radiated past the edges of the terminating disk 28 in a direction generally broadside to the ground plane 12. At the same time, the rapidly alternating electric and magnetic fields in the region around the element antenna 10 excite current flow in the other element antennas 10 near the directly excited element antenna principally by capacitive and magnetic induction, it is believed. Because all of the elements 10 of the array are excited directly at their terminals during normal operation, the particular element whose structure and operation are being described in parasitically excited in like manner by the neighboring elements so that the far-field radiation from every element has a parasitically excited component as well as a component due to direct excitation.

Shown in FIG. 4b is an alternative form 33 of element antenna suitable for radiating circularly polarized waves. Element 33 has two pairs of dipole arms 34, 36 which can be excited with 90° phase displacement between their exciting currents to produce waves of circular, elliptical or linear polarization. The phase displacement required between their currents for circular or elliptical polarization can be achieved by selecting the lengths of the two pairs of dipole elements so that they have reactive impedances of opposite signs.

All of the elements 10 contribute to the resultant array pattern in the far field; the contribution which is due to excitation energy originally entering the array at the terminals 22 of one particular element antenna 10 produces an element-in-array pattern of one element. In order that the nature of the element-in-array pattern be clear, a circuit for measuring it approximately is shown in FIG. 5. In FIG. 5, nine element antennas 10

3,858,221 are shown arranged broadside on a grid which represents a portion 38 of the entire ground plane as marked with dotted lines on FIG. 1. The center element of the group of elements 10 in FIG. 5 is shown being excited by a generator 40 which has an internal impedance Z. 5 Each of the other eight elements 10 which are the nearest neighbors of the directly excited element, is terminated in the same impedance Z, which represents the internal impedance of its own presently inactive generator. The exciting voltage is zero for all except the cen- 10 ter element whose element-in-array pattern is to be measured. Radiation patterns measured in the far field under these conditions are the element-in-array patterns of an element antenna 10 if the element-in-array pattern is significantly influenced only by the nearest 15 neighbors. A more precise element-in-array pattern could be measured by including more element antennas farther out in the array, also passively terminated in impedance Z.

A set of measured element-in-array patterns of an el- 20 ement 10 employed in the preferred embodiment is shown in FIG. 6 on rectangular coordinates. The element that produced the patterns of FIG. 6 was in a 7 X 7 array over a ground screen. Curves 42,44, are principal E-plane and H-plane element-in-array patterns 25 respectively. It should be noted that portions of the Eplane and H-plane patterns 42, 44 within ± 15° about the 0° mechanical axis have almost constant strength. The patterns decline slowly from 15° to 30° off axis and thereafter fall more rapidly to levels of less than 18 dB ³θ below the crest of the main lobe.

The desired flat-top shapes, well within ± 0.5 decibels, of the element-in-array patterns 42, 44 are achieved in the preferred embodiment with lengths of 0.5 wavelength for the transmission members 24, a ter- ³⁵minating disk 28 diameter of 0.405 wavelength, a dipole 30 length from the unsupported end of one dipole arm 32 to the unsupported end of the other arm 32 of 0.45 wavelength, and with the dipole arms located 0.25 wavelength from the screen and from the disk 28. The ⁴⁰spacing between element antennas is 0.85 wavelength, on a square grid. These dimensions produce appropriate values of partial capacitance from the terminating disk to the terminating disks of neighboring antennas and to the ground plane, and produce appropriate self ⁴³and mutual inductances, capacitances, and energy transmission parallel to the ground plane to shape the element-in-array pattern to a flat-topped, steep-sided shape which is desired, as shown in FIG. 6.

The proper dimensions of various structural mem- ⁵⁰bers of the element antennas depends upon the dimensions selected for other structural members of the same element. The disk 28 can have diameters between 0.25 and 0.60 wavelength. The disk can be located between 0.4 and 0.6 wavelengths from the main reflector. Spac- ⁵⁵ings between elements ordinarily range from 0.75 to 1.1 wavelength.

FIGS. 7 and 8 show element-in-array patterns obtained under conditions similar to those of FIG. 6, except with incorrect diameters of the terminating disks ⁶⁰28. The patterns of FIG. 7 are for a terminating disk diameter of 0.275, which is too small, and the patterns are not flat-topped. The patterns of FIG. 8 are for a terminating disk diameter of 0.475, which is too large, and the patterns therefrom are not flat-topped either.

To obtain the resultant far-field pattern of the antenna array, the element-in-array pattern 42 is multi6 plied at every value of angle on the abscissa of FIG. 6 by the value of an array factor at the same angle, as was discussed above. The array factor employed must correspond to the steering angle for which a pattern is sought.

While the energy transferred between elements of the array is relatively low for the type of antenna element described in the preferred embodiment, it is nevertheless significant and great enough to be manipulated as was described herein to shape the element-inarray pattern to achieve the desired array performance.

It can be seen in FIG. 6 that resultant patterns of the array will have a principal lobe whose maximum value is almost as great for a beam steering angle of 15° as it is for a steering angle of 0°; the maximum value is smooth between those angles. This is a very desirable result for many applications of steerable arrays; one advantage is that it minimizes amplitude-modulation of the sensitivity of the array as a function of beam steering angle.

While the antenna array has been described in terms of antenna elements of a particular type resembling a short backfire antenna, and although specific dimensions of the element antenna of the preferred embodiment have been recited in the interest of providing a specific example, the invention may be practiced by any of a variety of element antennas in array, provided only that their structure makes them amenable to appropriate tailoring of the complex mutual energy coupling from an element to its neighbors when in array, and provided that a particular pattern shape which is desired is one of the element-in-array patterns which are achievable by manipulation of the dimensional parameters of the element antenna and of spacing between element centers.

An example of art embodiment employing dielectric rod element antennas is shown in FIGS. 9a and 9b. Dielectric rods 50 are arranged on a grid spacing over a ground screen 52, and are fed by waveguides 54 from an array feed network. The diameter and length of a rod 50 affects the energy coupled transversely to neighboring rods 50, to shape an element-in-array pattern.

Another embodiment, shown in FIG. 9c, utilizes halfwave dipole element antennas 56 supported near a ground plane 58, and has a thin dielectric sheet 60 covering the entire array. The dielectric sheet has a relative dielectric constant of about 10, and is strongly affects the amount and phase of energy coupled laterally among the dipole element antennas, to shape the element-in-array pattern.

It is not necessary that the element antennas be located at the intersections of a square grid as was described in the preferred embodiment. The principal requirement upon the placement of element antennas is that all of the antennas except the ones near the edge should have the same values of complex mutual coupling with respect to the element antennas surrounding them.

Although the invention has been described above in transmitting-antenna terms it. is also suitable by reciprocity for use as a receiving antenna, or for both transmitting and receiving.