EP0271778A2

EP0271778A2 - Half circular 360o scanning radar array

Info

Publication number: EP0271778A2
Application number: EP87117917A
Authority: EP
Inventors: Jeffrey Thomas Nemit
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1986-12-04
Filing date: 1987-12-03
Publication date: 1988-06-22
Also published as: EP0271778A3

Abstract

An improved radar scanning antenna employing a phased array of directional radiating elements (82,90,92) which are aligned in a segment of a curved line, such as a half circular array (80), and which employs means (94) for exciting the elements in selected directions over a 360° scan.

Description

BACKGROUND OF THE INVENTION

This invention relates to a radar scanning antenna apparatus which employs a phased array of directional radiating elements aligned in a segment of a curved line such as a cylinder and preferably a circular cylinder. The antenna system employs means for exciting the elements in selected directions over a 360° scan.
In the prior art it is well known that a single face phased array can scan most of the hemisphere with plus or minus 45° of azimuthal coverage being typical. Four faces can then be used to provide 360° of azimuth coverage. Circular or cylindrical arrays can provide 360° of azimuth coverage with relatively uniform characteristics of beam width and gain with azimuth scan angle. The problem with both these approaches is the excessive number of elements and physical size necessary to realize required performance.
Further in the prior art, U.S. Patent 4,491,845, Rothenberg, discloses a wide angle phased array dome lens antenna with a reflection transmission switch. An antenna feed harness is located at the zenith of a dome antenna such that it radiates into a phased array antenna situated at the opening of the dome. Each radiating element of the phased array has both an electronic phase shifter and a reflection/transmission switch. For wide angle scanning the switches are set for the reflection mode, causing the phased array to radiate into the dome with the antenna beam propagating through the dome at wide angles. With the switch set for the transmission mode the phased array operates as a conventional lens array operating in the transmission mode for plus and minus 60° coverage or in the reflection mode with the dome lens to provide as much as plus or minus 120° of coverage in the opposite field of view. Rothenberg functionally provides a similar result to applicants concept with a more complex mechanism.
U.S. Patent 4,489,325, Bauck et al., discloses an electronically scanned antenna system. An N element excitation array 16 is coupled to a L element radiation array 10 by means of a parallel plate lens 15. The radia tion array is a linear array while the excitation array lies on a curve which is locally approximately circular but with decreasing radius at the outer edges. A M element subset 18 of the excitation array is coupled with transmit received apparatus through a switching matrix digitally controlled amplitude settings and phase shifters in the power distribution network. A lookout table receives an indication of the desired scan angle and provides input to the switching matrix in amplitude/phase shifters to select the proper subset of the excitation array and the optimum complex way for each element. This is fundamentally a linear array which only scans 80°, that is plus or minus 40°, and is not really relevant to wide angle scanning.
U.S. Patent 4,085,404, Gallant, describes a method of smoothly commutating or moving a beam in space using a lens and a plurality of probes. It does not address very wide angle scanning.
Patent 4,580,140, Cheston, discloses a twin aperture phased array lens antenna. If it is desired to scan 360° in azimuth two lens antennas 11 and 29 are positioned back to back. This approach uses complex routing of the signal.
U.S. Patent 4,044,360, Wolfson et al., discloses a two mode rf phase shifter, particularly for a phase scanner array, of a type which may be employed in the subject invention and is assigned to the assignee of the present invention.

SUMMARY OF THE INVENTION

Accordingly it is one object of this invention to provide a radar scanning antenna employing a phased array of directional radiating elements aligned in a segment of a curved line and employing means for exciting the elements in selected directions over a 360° scan.
Another object of the invention is to provide such an antenna array in which the curved line is a cylinder and preferrably a circular cylinder.
Still another object of the invention is to provide a radar apparatus for scanning 360° employing an array formed of a segment of a cylinder having bidirectional radiators selectively excitable in selected directions over a 360° scan.
These and other objects are achieved by providing a radar scanning antenna employing a phased array of directional radiation elements which are aligned in a segment of a curved line, or surface of a circular cylinder, and employ a feed subarray for exciting the elements in selected directions over a 360° scan.
The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention and further objects and advantages thereof can best be understood by reference to the following description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the invention showing a beam scan of 0°.
FIG. 2 is a schematic diagram illustrating a beam scan of 45°.
FIG. 3 is a schematic diagram showing a beam scan of 90°.
FIG. 4 is a schematic diagram of the invention showing a beam scan of 180°.
FIG. 5 is a schematic diagram of the invention showing a beam scan of 135°.
FIG. 6 is a schematic diagram of the invention showing a beam scan of 120°.
FIG. 7 is a schematic diagram of an element which can be used to transmit in either the exterior or interior direction employed in the invention.
FIG. 8 is a schematic diagram of an alternate element employing an elemental phase shifter which could be used to transmit in either the exterior or interior direction.
FIG. 9 illustrates still another alternate approach for providing a bidirectionally transmitting element.
FIG. 10 is a schematic diagram of a multiple feed arrangement for feeding a half cylinder such as used in the subject invention showing one of the feeds in blown up form.
FIG. 11 A & B are schematic diagrams illustrating how the F/D ratio may be increased by conventional sub reflector techniques.
FIG. 12 is a schematic diagram illustrating the different sectors covered by the antenna of the subject invention.
FIG. 13 is a three dimensional diagram showing a transport configuration of the antenna of the subject invention.
FIG. 14 is a schematic diagram showing the antenna array main aperature and a sub array feed with one radiating element shown in blown up form.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, there is illustrated a preferred embodiment which describes a half circular array which can be used to provide 360° of azimuthal coverage. When the array is extended in the vertical plane a half cylinder array is formed with 360° of azimuthal coverage and elevation scan coverage limited in angle by existing well known design constraints. The technique to be discussed is also applicable to truncated half cones and segments of a sphere. FIG.'s 1-6 show how a half cylinder array can be ultilized to provide 360° of azimuth coverage. To achieve this the illumination is first moved around the exterior of the cylinder. This provides slightly over a hundred and eighty degrees of coverage. Then the illumination is moved around the interior of the cylinder to provide the remainder of the coverage to complete the 360°. Note that in Fig. 3, for example, there is a smaller active aperature on the exterior. The half cylinder concept requires an element which can transmit in either the exterior or interior direction. Turning to FIG. 7 this can be accomplished with an active element array module 20 comprising a power amplifier, a receiver ampli fier, a phase and other well known devices plus means 22 and 24 for switching the interior/ exterior ports 26 and 28 respectively.
An alternate approach illustrated in FIG. 8 would use an elemental phase shifter such as that disclosed in U.S. patent 4,044,360 Wolfson. The diode switch 30 switched back and forth between the bits below 180° 32 and those from 0° to 180°, 34 which are transmitted from the interior element 36 and the exterior element 38 respectively to provide a 360° reflection phaser. FIG. 9 shows an alternate dual state phaser in which the interior element 40 transmits from the bits below 180° 42 and is controlled in the following manner. When pin diode 44 is shorted and pin diode 46 is open there is a phase state 0. When pin diode 44 is open and diode 46 shorted a phase state of 180° is achieved due to current reversal in coupling loop 48. When the pin diode 44 and 46 are in the same state, shorted or open, a reflection without added components is achieved. This dual state phaser can be used in a half cylinder lens, angle scanning being provided by the phase shifters.
The illumination could be moved about the cylinder by switching the input signal between horn feed such as illustrated in figure 10, where horn 50 would achieve angles about 0° degrees or 180° degrees, horn 52 angles about 45° or 180° plus 45° degrees, horn 54 angles about 60° degrees or 180° plus 60° degrees, horn 56 angles about -45° or -225° and horn 58 angles about -60° or -240°. Note that the cylinder 60 can be slightly larger or smaller than 180°. More feeds can be used for smaller movement of the aperture illumination over the phased array surface. Less feeds are used for coarser aperture illumination control. Feed 50 is shown in blown up form to illustrate multi-element feed for aperture distribution control as well as multiple clusters of beams including use for adaptive nulling.
In some applications the required ratio of the distance of the feed from the aperture to the radiating aperture width f/d may be larger than is physically practical due to unique required design parameters. The ef fective f/d can be substantially increased by using the well known principle of reflection as shown in FIG. 11A and 11B. Here the feed elements are located near the radiating arc. They direct energy toward a subreflector 62 shown at the left of FIG. 11B. The energy is reflected from the flat subreflector 62 to the cylindrical array 64. The apparent or effective feed location is shown in FIG. 11A on the left arc 66. The focal length 68 shown is approximately equal to the diameter of the cylinder but the feed elements 70 occupy a much smaller physical space within the half cylinder.
The antenna array of the subject invention is characterized by a semi cylinder array which is space fed and operates in both transfer and reflective modes. This provides full performance in the 120° forward sector as illustrated in FIG. 12 as well as rearward coverage with some reduction in performance in a 90° secondary sector. In addition the aperture affords additional peripheral coverage in side sectors labeled peripheral sector in FIG. 12, where operational demands can be greatly reduced but where continuous search coverage is desirable. The peripheral coverage as shown in FIG. 12 can be good enough for search in these sectors of lesser hostility and can provide continuity of radar burst communications capability as well in these otherwise unavailable regions.
Monopulse can be implemented in fore and aft sectors, but assymetries are likely to occur in the regions of peripheral coverage. A space feed is shown to efficiently generate a single beam or formed clusters of beams including adaptive nulling in the primary sector of coverage.
A partial disadvantage of the subject invention is that full performance can not be provided over the full 360° of azimuth. However it seems less of an issue since generally the peripheral regions of coverage where operational demands are the lowest are the regions of coverage where jamming influence is the least and where performance increases beyond the immediate requirement. Accordingly if the peripheral regions can be covered with greater haste and lesser accuracy the more threatened regions of cover age can achieve proportionally more attention while still adequately covering the rest. A typical transportable arrangement for an antenna such as employed in the subject invention is illustrated in FIG. 13 which shows the antenna 72 stowed for transport on the top of a conventional S-280 shelter 74. Cut out rail 76 on the side of the shelter allows a lower profile in transport. When erected as shown in the dashed line 78 the antenna can be trained plus or minus 60° for optimization of threat coverage without moving the equipment.
Turning now to FIG. 14 there is illustrated an antenna configuration in accordance with the subject invention having a space fed shaped array. This configuration has a space fed main aperture 80 which uses a forward/reverse phased and curved surface employing elements 82 illustrated in blown up section 84 and employing a phase shifter 86 a wave guide 88 and two radiators 90 and 92. This configuration is similar to that shown in the referenced patent to Wolfson 4,044,360. The subarray 94, used to space feed the main aperture, is a curved azimuth scannable array. The secotr beams generated by the subarray are scanned to different sections of the main aperture depending on the azimuth angular sector which is being scanned in the far field. Forward coverage is designed to generate the optimum performance, and, for example, close to one hundred percent of the main aperture will be used. The scan range can be plus or minus 45° in the azimuth far field. The phase shifters in the main aperture determine the main aperature scan. Specific sectors of the main aperture can be illuminated and used in the processing of the main aperature illumination.
Some of the major performance atrributes of the curved space fed array are its limited 360° azimuth performance, its sectored feed capability and its scanned monopulse beams in azimuth and elevation planes. Forward coverage is optimum for the array while reverse coverage will be slightly degraded by aperture blockage and side coverage will be degraded by a limited aperture size in the azimuth plane. However, the limited coverage will make available several but not all of the important radar functions. The sectored feed introduces considerable flexibility into the synthesis of the main aperture illumination. Several sectors of the main aperture may be illuminated in order to optimize coverage in the side area of the far field. The monopulse capability in azimuth and elevation has low sidelobe prformance particularly in the forward direction.
Phase shifters in the present invention present an impact on system costs due to the relatively large numbers required. Low cost low loss phase shifters are required and may be of the diode type, ferrite type, or active element type. Diode phase shifters for instance have the advantage of low cost. Ferrite phase shifters involve lower losses and faster switching speeds.

Claims

1. A radar scanning antenna characterized by a phased array of directional radiating elements aligned in a segment of a curved line and means for exciting said elements in selected directions over 360°.

2. The scanning antenna of claim 1 characterized in that said curved line is a cylinder.

3. The scanning antenna of claim 2 characterized in that said cylinder is circular.

4. Radar Apparatus for scanning 360° characterized by an array formed of a segment of a cylinder having bidirectional radiators selectively excitable in selected directions over 360°.

5. The Radar Apparatus of claim 4 characterized in that said segment of a cylinder is a segment of a circular cylinder.