Optical Limited Scan Antenna System

Abstract

This invention relates to an optical antenna system including an aperture lens, a feed lens and a feed array for scanning a pencil beam or multiple simultaneous beams over a limited angular sector with good sidelobe levels and minimum gain degradation. Both amplitude and phase distributions over the aperture lens are controlled for all scan angles. Also, the feed lens may be positioned in a manner such that virtually the entire aperture lens is illuminated for all scan angles thereby minimizing the size of the aperture lens, energy spillover and rate of gain decrease with angle from boresight.

Description

BACKGROUND OF THE INVENTION

One contemporary system of providing limited scanning or simultaneous beam clusters has been to employ a corporate fed array with subarray phase shifters. The corporate feed in this system, however, becomes large and complex and the number of phase shifters increases in proportion to the aperture size. An alternative technique is to employ a simple space fed array with element level phase shifter. This system has a simple feed structure with a wide field of view but requires a large number of phase shifters.

Still another technique is to employ a passive lens space fed by a "Sheleg" type feed which moves the feed aperture distribution intact by setting a uniform progressive phase in a number of phase shifters. Since the feed aperture scans, the size of the feed and the number of active elements increase as the scanning requirements increase. The feed distribution, however, does not correct for lens aberrations; therefore as the beam is scanned, the sidelobe level is limited by the growth of coma phase error. Further, a "Rudge-Witthers" technique for getting limited scan from a parabolic reflector has been employed. This system requires the feed to be physically moved to prevent spillover as the beam is scanned. If the feed is not physically moved, the parabolic reflector illumination will scan resulting in poor aperture utilization. None of the foregoing systems can provide satisfactory multiple simultaneous beams.

SUMMARY OF THE INVENTION

In accordance with the present invention, an aperture lens that is large in diameter compared with that of a feed lens is placed in confocal relationship therewith. Both the aperture and feed lens are entirely passive and are focused by means of fixed phase shifters or line lengths in the elements. A small phased array or other source is used to illuminate a portion of the feed lens with a plane wave segment. This wave passes through the feed lens, converges near the broadside focus at the focal plane, then spreads out again and is intercepted by the aperture lens which refocuses the energy to infinity. By changing the angle of the plane wave emanating from the small feed antenna, the beam is scanned in the far field. A significant reduction in total number of active elements, phase shifters and related control components is obtained over those used in the conventional phased array or subarray.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of the optical limited scan antenna system of the present invention showing the relative placement of the respective elements thereof;

FIGS. 2, 3 and 4 illustrate various species of aperture and feed lenses for use in the antenna system of FIG. 1;

FIG. 5 illustrates the operation of the antenna system of FIG. 1;

FIG. 6 shows radiation patterns calculated for the antenna system illustrated in FIG. 5; and

FIGS. 7 and 8 show alternative species of the feed array in the antenna system of FIG. 1.

DESCRIPTION

Referring to FIG. 1 of the drawings, the optical limited scan antenna system of the present invention includes an aperture lens 10 of focal length F, a feed lens 12 of focal length f, and a feed antenna 14 fed by a corporate feed 15 through phase shifters 16 which are controlled by a phase shift controller 17. The aperture lens 10 and feed lens 12 are confocal and are placed in back-to-back relationship with each other. The aperture lens 10 is large with a diameter D while the feed lens 12 is small by comparison with a diameter d. Both lenses 10, 12 are entirely passive and are focused by means of fixed phase shifters or line lengths between corresponding elements on opposite sides thereof. In order to maintain the feed lens 12 at a small size, the ratio d/D of the diameters of the feed lens 12 and the aperture lens 10 must be small such as one half or less. That is,

d/D .ltoreq. 1/2 (1)

In addition, the feed lens 12 must be in the far field of the common focal spot in order that geometrical optics may be used to design the lenses. Since the focal spot is of the order of .lambda.F/D in size where .lambda. is the wavelength, this requires that the focal lengths be much longer than .lambda. which is an insignificant restriction.

The optical limited scan antenna system of the invention operates as follows. The small feed antenna 14, such as the phased array shown in FIG. 1, is fed by means of the corporate feed 15 and phase shifters 16 controlled by a phase shift controller 17 in a manner to illuminate a section of the feed lens 12 with a plane wave segment. The feed array 14 is placed a distance h from the feed lens 12 and the angle of incidence of the plane wave thereon determined by phase shift controller 17 is designated .phi.. This plane wave passes through the feed lens 12 and converges near the broadside focus 18 at the focal plane, then spreads out again and is intercepted by the aperture lens 10 which refocuses the energy to infinity. The angle from normal at which the wave leaves the aperture lens is designated .theta.. By changing the angle .phi. of the plane wave emanating from the small feed antenna 14, by means of the phase shift controller 17, the beam emanating from the aperture lens 10 is scanned in the far field.

The number of active elements required for scanning in a certain angular sector is proportional to (f.sup.2/ F.sup.2) D.sup.2 as opposed to the conventional phased array or subarray wherein the number of active elements is proportional to D.sup.2. Thus, a significant reduction in total number of active elements, phase shifters and concomitant control components is obtained.

To maintain a high utilization of the large aperture lens 10, it is necessary to place the feed antenna 14 at the proper location -- to ensure that energy leaving the feed antenna 14 will be intercepted by the aperture lens 10 without spillover or under illumination for all scan angles. The proper location for the feed antenna 14 can be estimated by considering only a central ray on receive. For the angle of arrival .theta. as shown in FIG. 1, the ray intercepts the focal plane very close to the peak of the focal spot at a distance s = F.theta. from the axis, i.e., from the broadside focal point 18. Since parallel incident rays cross at the focal spot which is common to both the aperture lens 10 and feed lens 12, the rays will exit the feed lens 12 at the angle .phi. corresponding to the common focal spot displacement s whereby:

s = F.theta. (2)

s = f.phi. (3)

whereby

.phi. = F/f .theta. (4)

In addition, the distance X from the axis to the point 20 at which the central received ray intercepts the feed lens 12 is:

X = (F + f) .theta. (5)

This ray will cross the optical axis at the distance h from the feed lens 14 where h is related to X and .phi. as follows:

X = h.phi. (6)

Combining equations (4), (5) and (6) to eliminate X and .phi. produces the result that:

h = f(1 + f/F) (7)

provided that the scan angle .theta. is not large. This condition where .theta. in radians .apprxeq. sin .theta. is satisfied for limited scan. Since the central ray enters and leaves the system at points which are independent of scan angle, .theta. to a good approximation, the aperture lens 10 and feed array 14 are illuminated over fixed areas thereby achieving higher aperture utilization and low spillover.

The common focus and the similarity of the aperature and feed lenses 10, 12 ensures that a plane wave segment incident on either lens 10, 12 will produce a plane wave segment on the remaining lens with very little phase error despite phase errors introduced by either lens 10 or 12 alone. Similarity is designated as meaning that the feed lens 12 is a scaled replica of the aperture lens 10, the scale factor being the ratio of the focal lengths, f/F . Thus, the lenses 10, 12 are similar in shape and if focused with line lengths, these lengths at corresponding positions also have the ratio f/F . Examples of similar lenses are shown in FIGS. 2-4. Referring to FIG. 2, there is shown a flat constrained aperture lens 30 of focal length F with flat similar feed lens 32 with focal length f. Lens 30 has arrays of elements 33, 34 on opposite sides thereof and lens 32 has arrays of elements 35, 36 on opposite sides thereof. Corresponding elements of arrays 33, 34 and arrays 35, 36 may be connected together by line cable or fixed phase shifters with the length or phase shift at corresponding positions having the ratio f/F . Referring to FIG. 3 there is shown a doubly concave or parabolic aperture and feed lens 40, 42, respectively. In this case, the respective arrays on opposite sides of the lenses 40, 42 curve whereby all phase shifts or line lengths between corresponding elements remain constant. Referring to FIG. 4, there is illustrated the embodiment where aperture lens 44 and feed lens 46 are flat on the outside and concave on the confocal side. It is also possible to utilize unconstrained dielectric lenses or other lens designs provided that they preserve similarity as defined above.

Satisfactory operation of the scanning system of the present invention has been obtained for the embodiment shown in FIG. 5. Two doubly parabolic feedthrough aperture and feed lenses 50, 52 each having arrays with corresponding elements connected by equal line lengths are disposed confocally and fed by a planar phased array 54 arranged to reduce illumination scanning in the aperture lens 50. Aperture lens 50 has a diameter, D, of 80 wavelengths and an F/D = 0.563. Feed lens 52, on the other hand, has a diameter, d of 40 wavelengths and an f/d = 0.300. Calculated patterns are shown in FIG. 6. With a 0.8.degree. 3 db beamwidth and sidelobes below 25 db at broadside, it can be shown that the beam may be scanned 10 beamwidths with sidelobes below 20 db with good efficiency. The feed array 54 has 800 active elements and 20 wavelength diameter while a conventional corporate or spaced fed array requires approximately 4,000 active elements and phase shifters to achieve the same performance.

The embodiments of the optical limited scan antenna system of the present invention may be used with different feed antennas. Referring to FIG. 7, there is shown a Luneberg lens 60 for illuminating the feed lens 12. In this case the center of the Luneberg lens 60 is positioned a distance h from the feed lens 12. A feed horn 61 for a broadside beam is positioned at the diameter of the Luneberg lens 60 normal to the feed lens 12 on the side thereof opposite from lens 12. Other beams can be generated by using additional feed horns 62 rotated from the feed horn 61. Referring to FIG. 8, there is shown a spherical feedthrough lens 70 for illuminating the feed lens 12. In this case the center of the output sphere of lens 70 is positioned a distance h from the feed lens 12. A feed horn 71 for a broadside beam is disposed within the input hemisphere along a line connecting the respective centers of the hemispheres of lens 70. A deviation from this line by a feed horn 72, for example, scans the beam. The beam can be mechanically scanned by physically moving a feed horn 71 or 72 or multiple simultaneous beams can be provided by employing additional feed horns.

Claims

What is claimed is:

1. An antenna system comprising first and second similar lenses positioned back-to-back in confocal relationship relative to each other, the diameter of said first lens being large compared to the diameter of said second lens, and means for illuminating the side of said second lens opposite the side thereof facing said first lens with a plane wave at a predetermined angle in one direction from the axis of rotation of said first and second lenses thereby to transmit a plane wave in a direction opposite to said one direction at an angle from said axis of rotation equal to the ratio of the focal length of said second lens to the focal length of said first lens times said predetermined angle.

2. The antenna system as defined in claim 1 wherein said first and second lenses are of the flat constrained type with fixed foci.

3. The antenna system as defined in claim 1 wherein said first and second lenses are of the doubly concave type with fixed foci.

4. The antenna system as defined in claim 1 wherein said first and second lenses are concave on the sides facing each other and flat on the remaining sides.

5. The antenna system as defined in claim 1 wherein said means for illuminating the side of said second lens opposite the side thereof facing said first lens with a plane wave includes an array of radiating elements, a corporate feed having an output for each of said radiating elements, a controllable phase shifter connected between each output of said corporate feed and each respective radiating element, and means connected to said controllable phase shifters for controlling the phase shift imparted to electrical energy flowing therethrough thereby to direct a plane wave at a selected angle towards said second lens.

6. The antenna system as defined in claim 1 wherein said means for illuminating the side of said second lens opposite the side thereof facing said first lens with a plane wave constitutes a Luneberg lens.

7. The antenna system as defined in claim 1 wherein said means for illuminating the side of said second lens opposite the side thereof facing said first lens with a plane wave constitutes a spherical feedthrough lens.