Patent Grant
7786948
Array antennas may have subapertures, i.e. small groups of elements, embedded in the main aperture which have some of their received energy sent to a separate (from the main) receiver channel for a special use such as a guard channel. In many cases, an RF coupler in the feed network diverts a small amount of the subaperture receive energy to the special use channel without significantly affecting the main channel. The subaperture may have too little receive gain and too high a noise figure for some purposes.
An array antenna with an embedded subaperture includes an array of radiator elements. The array includes a subaperture of one or a group of the radiator elements. A main receive channel is coupled to at least some of the radiator elements by a feed network. An RF power dividing network is connected in a signal path between the subaperture and the special use receive channel, and is adapted to allow at least most of the RF energy from the subaperture to pass to the special use receiver channel while diverting a small amount of energy to the main receive channel. The array includes circuitry for introducing an amplitude taper to signals received from the array of radiator elements, so that some of the signals from the radiator elements are attenuated to achieve the amplitude taper. The circuitry includes the RF power dividing network, wherein the small amount of energy diverted to the main receive channel from the subaperture is substantially equal to an attenuated signal level for the subaperture to achieve an amplitude taper attenuation.
Features and advantages of the disclosure will readily be appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawing wherein:
In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals. The figures are not to scale, and relative feature sizes may be exaggerated for illustrative purposes.
An exemplary embodiment of an antenna architecture optimizes system performance by embedding a subaperture in a way that allows the subaperture to remain a part of the active area of the main antenna, while still acting as an independent aperture. The result is that the main aperture retains all of its active area and the subaperture sends most of its RF energy to the special use receiver channel without significant reduction in its independence.
An exemplary embodiment of this architecture capitalizes on the fact that most active arrays utilize some form of attenuation on the elements in receive to achieve an amplitude taper across the array. Whether achieved by T/R element commands to set variable attenuators in the T/R element, or in the receive RF feed, this attenuation may be relocated to a strategic location in the feed. At an appropriate level in the RF feed, the energy from the selected element or group of elements can pass through an RF power dividing network or device which diverts a small amount of energy equal to the expected attenuated signal level. A power dividing device suitable for the purpose is a directional RF coupler. The RF power dividing network or device allows most of the RF energy to pass to the special use receiver channel.
The radiator elements 60B, 60C . . . 60O are connected to a main feed network 70, which has a plurality of ports 70-1, 70-2 . . . 70-14 connected to the output (or I/O in the case of an active array) ports of each of these radiator elements. The network 70 combines the contributions from these radiator elements at a single port 70-15. In an exemplary embodiment, the feed network 70 may be constructed, e.g., as a corporate feed network. The feed network 70 includes a plurality of combiners 72-1, 72-2 . . . 72-13 which combine the signal contributions from radiator elements 60B-600 at port 70-15.
In an exemplary embodiment, the array may have an amplitude taper, indicated generally as 102, applied to the signal contributions from the radiators of the respective radiating elements to the main receive channel. In the example depicted in
In this exemplary embodiment, the edge radiator elements 60A and 60P have output ports connected through lines 76-1 and 76-2 to ports of RF directional couplers 90 and 96, respectively. The directional couplers respectively divert a small amount of energy equal to the expected attenuated signal level for the respective edge radiator elements for the selected or desired array amplitude taper for the main receive channel. Thus, the power split ratios of the RF couplers 90 and 96 are set to match the main aperture amplitude taper for elements 60A and 60P, respectively. The RF directional couplers 96, 90 allow most of the RF energy to pass to the special use receiver channels 98 and 92, respectively. The RF directional coupler in an exemplary embodiment prevents energy from coupler 72-11 from being diverted to the special use receive channel 98, for example; the isolation from a directional coupler should be enough to prevent degraded performance. Say, for example, that the main amplitude taper for the main receive channel would apply an attenuation level of 12 dB to the signals received at the edge radiator elements 60A and 60P relative to the signal level received at the center of the array. Conventionally, the attenuation of 12 dB would be applied by setting attenuators in the radiator element or by design of the feed network. In the embodiment of
An alternate embodiment of an array system 150 is depicted in
The array system 150 further includes a special use receive channel 192, which receives signal contributions from a subaperture including radiator elements 160G, 160H, through an RF coupler 190 which has an input 190-1 connected to the output of signal combiner 172-4, an output 190-2 connected to the special use receive channel 190, and another output 190-3 connected to the signal combiner 172-6 of the feed network 170. The coupling ratio between the two outputs is selected to provide the attenuation for elements 160G and 160H to achieve a desired main aperture amplitude taper 202. For radiator elements 160A-160F, any attenuation needed to achieve the desired amplitude taper may be provided by dynamic settings of attenuators in the radiator elements, by built-in feed taper, or both. In this exemplary embodiment, no dynamic or feed taper is applied to the signal contributions from the radiator elements 160G, 160H, and the attenuation is instead applied by the RF coupler 190. Hence the amplitude taper will not exactly match the ideal amplitude taper, but this is typically acceptable in an exemplary application. There are several reasons why this may be acceptable for some applications. First, the embedded apertures typically may be closer to an edge of the array because the center of the aperture generally requires full power to the main receive channel. Second, amplitude tapers (whether in the feed or achieved by T/R element attenuation) generally have a gradual slope with steps between elements being small, especially near the edges of the main aperture where the embedded apertures would be typically be placed. Third, errors at the edges of the array have significantly less impact on the performance of the system. So at the edges of the array, where the average attenuation might be around 16 dB or more, an error of 1 or 2 dB (representing 160H having the same attenuation as 160G, instead of having perhaps 1 or 2 dB more attenuation) would not have a noticeable affect.
The independent steering capability of the embedded subapertures may be limited only by the acceptable perturbation of the main channel performance. This may be explained through the analogy of a pair of bi-focal glasses. For an ESA with an embedded aperture, if the embedded aperture is independently steered, it would cause that group of elements to no longer be in focus with the rest of the main aperture. The embedded aperture would not be as significant a portion of the main as the bifocal analogy, so it might be better to think of it as a small distortion near the edge of one's sunglasses. The light is already dimmed so the effect of the distortion is reduced. Since the contribution of the embedded subaperture to the main is attenuated, the independent steering has only a small impact of the main channel sidelobes.
For the elements in the subaperture(s), the beam steering computer may be programmed to control those elements differently from the main aperture elements. Any T/R element level dynamic tapering attenuation that would be applied to those elements may already be accounted for in the feed design. Consequently, tapers loaded into the BSC may have attenuation for subaperture elements zeroed out. The only attenuation applied to subaperture elements may be from calibration. T/R elements are generally not designed to retain their own calibration data (settings that align them in phase and gain with their neighbors.) These calibration offsets get sent to each T/R element as part of the beam steering command from the beam steering computer. In effect, in one exemplary embodiment, the beam steering computer uses the desired beam position as input to the phase slope equations to calculate the ideal phase and gain settings for each T/R element, “adds” any desired taper settings, then “adds” the calibration to the ideal settings to obtain the actual commands that will make the T/R elements achieve the true phase and gain needed to properly point the beam. In the exemplary embodiments illustrated in
Although the foregoing has been a description and illustration of specific embodiments of the subject matter, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope and spirit of the invention as defined by the following claims.
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| Number | Date | Country | |
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| 20090058754 A1 | Mar 2009 | US |
