Multi-Beam Radio Frequency Array Antenna

This invention relates generally to radio frequency array antennas and more particularly to radio frequency array antennas adapted to form a plurality of simultaneously existing beams of radio frequency energy.

It is known in the art that a radio frequency array antenna may be arranged to produce a plurality of simultaneously existing beams of radio frequency energy. If such an array antenna is properly designed, each one of the beams has the gain and bandwidth of the entire antenna aperture. According to the art, a desired number of simultaneous beams may be obtained by connecting each antenna element in the array through a different constrained electrical path to a plurality of feed ports, the constrained electrical path being made up of an electromagnetic lens which equalizes the time delay of the electromagnetic energy between any given one of a number of feed ports and all points on corresponding planar wavefronts of either transmitted or received energy. One such antenna is described in U.S. Pat. No. 3,761,936, "Multi-Beam Array Antenna," inventors D. H. Archer et al, issued Sept. 25, 1973 and assigned to the same assignee as the present invention.

While such array antenna has been found quite satisfactory in many applications, it is sometimes necessary that such array antenna have sidelobes lower than those obtainable with a single electromagnetic lens. While such sidelobes are theoretically achievable by tapering the field amplitude across the array aperture, such levels are seldom achieved due to deviations in the aperture field amplitude and phase from the theoretically designed values. Such deviations are generally attributable to such things as mutual array element coupling and reflections within the electromagnetic lens. Conceptually, one method which might be used to correct the amplitude and phase deviations is through the insertion of a variable attenuator and phase shifter serially with each one of the array elements, such attenuators and phase shifters being adjusted to achieve the proper aperture distribution. However, the use of such arrangement would provide proper adjustment for only one beam at only a single frequency.

With this background of the invention in mind it is therefore an object of this invention to provide an improved multibeam array antenna having improved sidelobe characteristics over a relatively wide frequency bandwidth.

This and other objects of the invention are attained generally by providing an array antenna having: a plurality of spatially overlapping sets of N radiating elements; a like plurality of radio frequency lenses, each one having N input ports coupled to different ones of the N radiating elements in a corresponding one of the sets thereof, and a plurality of output ports; a second plurality of radio frequency lenses, each one having: a plurality of input ports coupled to a corresponding one of the output ports of a different one of the first plurality of radio frequency lenses; and, an output port, the electrical length from the output port of one of such second plurality of radio frequency lenses through the first plurality of radio frequency lenses and the radiating elements coupled thereto to all points on a corresponding wavefront being substantially equal.

With such an arrangement, sidelobes associated with any one of the radio frequency lenses in the first plurality thereof are reduced by the focusing effect of the second plurality of radio frequency lenses coupled thereto.

The above-mentioned and other features of the invention will become more apparent by reference to the following description taken together in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of a radio frequency array antenna system according to the invention; and

FIG. 2 is a block diagram of an alternative embodiment of a radio frequency array antenna system according to the invention.

Referring now to FIG. 1, a multibeam array antenna 10 is shown to include a plurality of, here four, spatially overlapping sets 12.sub.1 -12.sub.4 of radiating elements 14.sub.1 -14.sub.6. It should be noted that six radiating elements have been arranged in four sets for simplicity and it should be recognized that many more radiating elements and overlapping sets thereof will generally be used. Therefore, in the more general case when 2N radiating elements are used in the array, such 2N radiating elements are grouped in N+1 spatially overlapping sets. Each one of the sets 12.sub.1 -12.sub.4 of radiating elements is coupled to a corresponding one of a like plurality of radio frequency lenses 16.sub.1 -16.sub.4. Such radio frequency lens is described in the above referenced U.S. Pat. No. 3,761,936. Therefore, radio frequency lens 16.sub.1 is coupled to set 12.sub.1 (i.e. radiating elements 14.sub.1 -14.sub.3), radio frequency lens 16.sub.2 is coupled to set 12.sub.2 (i.e. radiating elements 14.sub.2 -14.sub.4 ), radio frequency lens 16.sub.3 is coupled to set 12.sub.3 (i.e. radiating elements 14.sub.3 -14.sub.5) and radio frequency lens 16.sub.4 is coupled to set 12.sub.4 (i.e. radiating elements 14.sub.4 -14.sub.6), as shown. More specifically, radio frequency lenses 16.sub.1 -16.sub.4 have here three input ports, 18.sub.1a, 18.sub.1b, 18.sub.1c . . . 18.sub.4a, 18.sub.4b, 18.sub.4c, respectively, as shown. The input ports are coupled via constrained electrical paths, here provided by coaxial cables (not numbered), to the radiating elements 14.sub.1 -14.sub.6 via 3:1 power dividers 20.sub.1 -20.sub.6. Each one of the power dividers has three output ports, 20.sub.1a, 20.sub.1b, 20.sub.1c . . . 20.sub.6a, 20.sub.6b, 20.sub.6c. Port 20.sub.1a is coupled to input port 18.sub.1a, ports 20.sub.1b and 20.sub.1c being terminated to ground through a suitable load, not numbered. The ports 20.sub.2a, 20.sub.2b and 20.sub.2c of power divider 20.sub.2 are coupled in input ports 18.sub.1b, 18.sub.2a, and to ground through a suitable terminating load, respectively, as shown. Output ports 20.sub.3a, 20.sub.3b and 20.sub.3c of power divider 20.sub.3 are coupled to input ports 18.sub.1c, 18.sub.2b and 18.sub.3a, respectively. Ports 20.sub.4a, 20.sub.4b and 20.sub.4c of power divider 20.sub.4 are coupled to input ports 18.sub.2c, 18.sub.3b, 18.sub.4a, respectively, as shown. Ports 20.sub.5a, 20.sub.5b and 20.sub.5c of power divider 20.sub.5 are coupled to ground through a suitable load, input port 18.sub.3c and input port 18.sub.4b, respectively, as shown. Output ports 20.sub.6a, 20.sub.6b of power divider 20.sub.6 are coupled to ground through suitable loads and output port 20.sub.6c is coupled to input port 18.sub.4c, as shown.

It should be noted that, in the general case, when 2N radiating elements are grouped in N+1 overlapping sets, each one of the N+1 radio frequency lenses will have N input ports.

A second plurality, here three, of radio frequency lenses 24.sub.1, 24.sub.2, 24.sub.3 are coupled to the first set of radio frequency lenses 16.sub.1, 16.sub.2, 16.sub.3, 16.sub.4. It should be noted that the number of radio frequency lenses in the second plurality thereof is here chosen as three for simplicity and the number of radio frequency lenses is equal to the number of independent simultaneous beams to be formed by the array antenna 10 and that in general case the number of radio frequency lenses in the second set thereof will generally be greater than three. Each one of such radio frequency lenses 24.sub.1, 24.sub.2, 24.sub.3 includes a number of input ports equal to the number of lenses in the first set thereof; hence, each one of such radio frequency lenses has four input ports, 26.sub.11, 26.sub.12, 26.sub.13, 26.sub.14 . . . 26.sub.31, 26.sub.32, 26.sub.33, 26.sub.34. Each one of the four input ports of each one of the radio frequency lenses in the second plurality thereof is coupled to an output port of a different one of the radio frequency lenses 16.sub.1 -16.sub.4 in the first plurality thereof through constrained electrical paths, here through coaxial cables. More specifically, input ports 26.sub.11, 26.sub.12, 26.sub.13, 26.sub.14 are coupled to output ports 28.sub.1a, 28.sub.2a, 28.sub.3a and 28.sub.4a of radio frequency lenses 16.sub.1, 16.sub.2, 16.sub.3, 16.sub.4, respectively, through coaxial cables 30.sub.11, 30.sub.12, 30.sub.13, 30.sub.14. Similarly, input ports 26.sub.21, 26.sub.22, 26.sub.23, 26.sub.24 are coupled to output ports 28.sub.1b, 28.sub.2b, 28.sub.3b and 28.sub.4b of radio frequency lenses 16.sub.1, 16.sub.2, 16.sub.3, 16.sub.4 through coaxial cables (not numbered). Likewise, input ports 26.sub.31, 26.sub.32, 26.sub.33, 26.sub.34 of radio frequency lens 24.sub.3 are coupled to output ports 28.sub.1c, 28.sub.2c, 28.sub.3c, 28.sub.4c of radio frequency lenses 16.sub.1, 16.sub.2, 16.sub.3, 16.sub.4 respectively through coaxial cables not numbered. Each one of the radio frequency lenses 24.sub.1 -24.sub.3 has an output port 32.sub.11, 32.sub.22, 32.sub.33 respectively, as shown. Output ports 32.sub.11, 32.sub.22, 32.sub.33 are connected to receivers 36.sub.1, 36.sub.2, and 36.sub.3, respectively, as shown. The outputs of receivers 36.sub.1 -36.sub.3 are fed to a utilization device 38 which detects which one or ones of such receivers 36.sub.1 -36.sub.3 are receiving radio frequency energy. One such arrangement is shown and described in the above referenced U.S. Pat. No. 3,761,936. It is here noted that the radio frequency lenses 16.sub.1 -16.sub.4, 24.sub.1 -24.sub.3, the interconnecting coaxial cables and the power dividers 20.sub.1 -20.sub.6 are arranged such that the electrical lengths from output port 32.sub.11 of radio frequency lens 24.sub.1 to all points on wavefront 34.sub.1 are equal. Likewise, the electrical lengths from output port 32.sub.22 of radio frequency lens 24.sub.2 to all points on wavefront 34.sub.2 are equal and the electrical lengths from output port 32.sub.33 of radio frequency lens 24.sub.3 to all points on wavefront 34.sub.3 are equal. It is also noted that output ports 32.sub.12, 32.sub.13, 32.sub.21, 32.sub.23, 32.sub.31, 32.sub.33 are coupled to ground through suitable loads (not numbered), as shown.

In order to understand the operation of the array antenna consider wavefront 34.sub.1. The electrical lengths between port 28.sub.1a and all points on wavefront 34.sub.1 are equal, the electrical lengths between port 28.sub.2a and all points on wavefront 34.sub.1 are equal, the electrical lengths between port 28.sub.3a and all points on wavefront 34.sub.1 are equal and the electrical lengths between port 28.sub.4a and all points on wavefront 34.sub.1 are equal. A portion of the radio frequency energy is received by the antenna elements 14.sub.1 -14.sub.3. One third of energy in set 12.sub.1 therefore is coupled through each one of such antenna elements 14.sub.1 -14.sub.3 to radio frequency lens 16.sub.1 the energy arriving at port 28.sub.1a "in phase" and the energy arriving at ports 28.sub.1b and 28.sub.1c "out of phase." That is, the vectorial addition of the "in phase" energy results in a maximum composite signal at port 28.sub.1a and the vectorial addition of the "out of phase" energy results in composite signals at ports 28.sub.1b, 28.sub.1c which are substantially less, say on the order of 14 db down from the maximum composite signal. This effect is described in the above referenced U.S. Pat. No. 3,761,936 and also U.S. Pat. No. 3,715,749, "Multi-Beam Radio Frequency System" inventors Archer et al issued Feb. 6, 1973 and assigned to the same assignee as the present invention. Likewise, the radio frequency energy associated with wavefront 34.sub.1 and received by the antenna elements 14.sub.2 -14.sub.4 in the set 12.sub.2 arrive "in phase" at port 28.sub.2a and arrive "out of phase" at ports 28.sub.2b and 28.sub.2c. Continuing: the radio frequency energy associated with wavefront 34.sub.1 and received by antenna elements 14.sub.3 -14.sub.5 in set 12.sub.3 arrive "in phase" at port 28.sub.3a ; and "out of phase" at ports 28.sub.3b and 28.sub.3c ; and the radio frequency energy associated with wavefront 34.sub.1 and received by antenna elements 14.sub.4 -14.sub.6 in set 12.sub.4 arrive "in phase" at ports 28.sub.4a and "out of phase" at ports 28.sub.4b and 28.sub.4c. The power dividers 20.sub.1 -20.sub.6 enable the "in phase" signals at ports 28.sub.1a , 28.sub.2a, 28.sub.3a and 28.sub.4a to be of equal level. However, because such wavefront 34.sub.1 is at an angle .theta. with respect to the face of the array, such face being represented by the dotted line 40, the "in phase" signals at such ports 28.sub.1a, 28.sub.2a, 28.sub.3a, 28.sub.4a differ in phase from one another by an amount related to the sin .theta.. The signals at ports 28.sub.1a, 28.sub.2a, 28.sub.3a, 28.sub.4a with this relative phase difference are applied to ports 26.sub.11, 26.sub.12, 26.sub.13, 26.sub.14 of lens 24.sub.1 and arrive "in phase" at port 32.sub.11 of lens 24.sub.1. The signals at ports 28.sub.1b, 28.sub.2b, 28.sub.3b, 28.sub.4b, having been reduced, here -14db, by lenses 16.sub.1 -16.sub.4, for reasons discusses above, arrive "out of phase" at port 32.sub.22 and therefore lens 24.sub.2 further reduces the sidelobes of wavefront 34.sub.1, here an additional -14db. Likewise, the signals at ports 28.sub.1c, 28.sub.2c, 28.sub.3c, 28.sub.4c, having been reduced, here -14db, by lenses 16.sub.1 -16.sub.4, for reasons discusses above, arrive "out of phase" at port 32.sub.33 and therefore lens 24.sub.3 further reduces the sidelobes of wavefront 34.sub.1, here an additional -14db. The effect of lenses 24.sub.1 -24.sub.3 then is to reduce the "effective sidelobes" of lenses 16.sub.1 -16.sub.4, here by -14db, therefore the sidelobe characteristics of the entire array antenna 10 is here -28db.

Likewise, for wavefront 34.sub.2, the energy associated therewith adds "in phase" at port 32.sub.22. Lenses 16.sub.1 -16.sub.4 reduce the sidelobes associated therewith -14db and lenses 24.sub.1, 24.sub.3 provide an additional -14db reduction so that a -28db sidelobe reduction is achieved by the array antenna 10. A similar situation results with respect to wavefront 34.sub.3, that is the energy associated therewith adds "in phase" at port 32.sub.33. Lenses 16.sub.1 -16.sub.4 reduce the sidelobes associated therewith -14db and lenses 24.sub.1, 24.sub.2 provide an additional -14db reduction so that a -28db sidelobe reduction is achieved by the array antenna 10.

Referring now to FIG. 2 array antenna 10' is shown. It is noted that such array antenna 10' is identical to array antenna 10 (FIG. 1) except that the power dividers have been eliminated. In particular, four sets 12.sub.1 ' -12.sub.4 ' of radiating elements are shown. Each one of the four sets 12.sub.1 ' -12.sub.4 ' includes a linear array of, here, three radiating elements, 14.sub.1 ' -14.sub.3 '; 14.sub.2 " -14.sub.4 "; 14.sub.3 '" -14.sub.5 '"; and 14.sub.4 "" -14.sub.6 "", respectively, as shown. Each linear array of radiating elements is disposed parallel to the X-axis as shown. The sets 12.sub.1 ' -12.sub.4 ' are disposed along the Z-axis, as shown. That is, the face 40' of the array antenna 10' is disposed in the X-Z plane, the Y-axis being orthogonal to the face of the array antenna 10'. It follows then that, looking along the Z-axis, the sets 12.sub.1 ' -12.sub.4 ' may be considered as four overlapping sets of radiating elements. With such an arrangement narrower elevation beamwidths are produced as compared to those produced using the array antenna 10 (FIG. 1) assuming array antennas 10, 10' each have radiating elements with identical characteristics. The elevation angle is defined as the angular deviation of the beam from the Z-axis. In this configuration, i.e., in the array antenna 10', statistical averaging of the aperture field error contributions from the radiating element outputs may be viewed as occurring in free space when considering transmit operation rather than occurring inside the power dividers shown in FIG. 1.

Having described preferred embodiments of the invention, it should now become evident to one of skill in the art that other embodiments incorporating these concepts may be used. For example, the array antenna may be used in a transmitter application, principals of reciprocity applying and therefore may be used in the multi-beam radio frequency system described in the above-referenced U.S. Pat. No. 3,715,749. Further other techniques for reducing sidelobes, such as attentuating the lens outputs, may also be incorporated into the described array antennas to further reduce the overall sidelobe levels of the array antennas. It is felt, therefore, that this invention should not be restricted to the disclosed embodiment but rather should be limited only by the spirit and scope of the appended claims.