Abstract:
A reconfigurable radio frequency aperture including a substrate, a plurality of reconfigurable patches on the substrate, and a plurality of reconfigurable coupling elements on the substrate, wherein at least one reconfigurable coupling element is coupled between a reconfigurable patch and another reconfigurable patch, and wherein the reconfigurable coupling elements affect the mutual coupling between reconfigurable patches.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to and claims the benefit of U.S. Provisional Patent Application Ser. No. 61/940,070, filed Feb. 14, 2014, and is related to U.S. patent application Ser. No. 14/617,361, filed Feb. 9, 2015, and U.S. patent application Ser. No. 13/737,441, filed Jan. 9, 2013, which are incorporated herein as though set forth in full. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to antennas and in particular to active phased array antenna and radio frequency apertures. 
       BACKGROUND 
       [0003]    Reconfigurability of a radio frequency (RF) aperture, such as a phased array antenna, is a highly desirable feature so that the radiation characteristics can be changed by modifying the physical and electrical configuration of the array to provide a desired performance metric, such as a desired frequency, scan angle, or impedance. 
         [0004]    Prior art phased arrays typically use transmit/receive (TR) modules with phase shifters, amplifiers in each radiation element. A spacing of TR modules that is close to λ/2 or less than λ/2 is generally used to prevent grating lobes, where λ is the wavelength of the center frequency of a transmitted or received signal. A λ/2 or less spacing between the TR modules together with the size or aperture of the phased array antenna determines the number of TR modules required in the phased array antenna. For a given size or aperture of a phased array antenna, it is desirable to have fewer TR modules, because the number of TR modules drives the cost of the phased array antenna. 
         [0005]    It is also desirable to be able to reconfigure phased array antenna to achieve different beam patterns. In the prior art this requires reconfiguring the RF feed to the TR modules, and therefore these prior art phased arrays have quite limited reconfigurability. 
         [0006]    In the prior art, J. Luther, S. Ebadi, and X. Gong in “A Microstrip Patch Electronically Steerable Parasitic Array Radiator (ESPAR) Antenna with Reactance-Tuned Coupling and Maintained Resonance”  IEEE Trans. Antenna Propag ., Vol. 60, No. 4, April 2012, pp. 1803-1813 describe using varactors and coupling capacitors between the driven and parasitic patches as means of controlling the coupling for a parasitic phased array. The array elements are fixed and the tuning of the varactors switches the beam. P. W. Hannan, D. S. Lerner, and G. H. Knittel in “Impedance Matching a Phased-array Antenna over Wide Scan Angles by Connecting Circuits”,  IEEE Trans. Antenna Propag ., Vol. AP-13, January 1965, pp. 28-34 describe the use of connecting circuits between transmission lines to improve the scan impedance and scan performance of a phased array. Phase shifters are used for beam-steering, and an array is described made of wideband elements and using lumped element capacitors/inductors for changing the phase of the signals between the radiating elements. 
         [0007]    What is needed is an RF aperture and active phased array antenna that has improved reconfigurability, and that can have a fewer number of TR modules. The embodiments of the present disclosure address these and other needs. 
       SUMMARY 
       [0008]    In a first embodiment disclosed herein, a reconfigurable radio frequency aperture comprises a substrate, a plurality of reconfigurable patches on the substrate, and a plurality of reconfigurable coupling elements on the substrate, wherein at least one reconfigurable coupling element is coupled between a reconfigurable patch and another reconfigurable patch, and wherein the reconfigurable coupling elements affect the mutual coupling between reconfigurable patches. 
         [0009]    In another embodiment disclosed herein, a reconfigurable radio frequency aperture comprises a plurality of reconfigurable patches on the substrate, and a plurality of reconfigurable parasitic elements on the substrate, wherein at least one reconfigurable parasitic element is between a reconfigurable patch and another reconfigurable patch, wherein at least one reconfigurable coupling element is coupled between a reconfigurable patch and a reconfigurable parasitic element, or between one reconfigurable parasitic element and another reconfigurable parasitic element, and wherein the reconfigurable coupling elements and the reconfigurable parasitic elements affect the mutual coupling between reconfigurable patches a substrate. 
         [0010]    These and other features and advantages will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features, like numerals referring to like features throughout both the drawings and the description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  shows an RF aperture with driven patches spaced λ apart with parasitic patches and reconfigurable coupling elements in accordance with the present disclosure; 
           [0012]      FIG. 2A  shows a portion of an RF aperture with coupling elements having phase change material (PCM) switches to provide reconfigurability of the coupling elements, and  FIGS. 2B and 2C  show metal patches with PCM switches between them to provide reconfigurability of patch size in accordance with the present disclosure; 
           [0013]      FIG. 3A  shows an RF aperture with patches spaced λ apart, and  FIG. 3B  shows a plot of the scanned radiation pattern where the main beam is scanned to 30° in accordance with the prior art; 
           [0014]      FIG. 4A  shows an RF aperture with patches spaced λ apart with a coupling element or network between them, and  FIG. 4B  shows patches spaced λ apart with parasitic patches in accordance with the present disclosure; 
           [0015]      FIGS. 5A and 5B  show plots comparing the gain patterns of the configurations shown in  FIGS. 4A and 4B , respectively, in accordance with the present disclosure; 
           [0016]      FIGS. 6A and 6B  show plots of return-loss for a configuration with driven patches connected with high impedance lines, and driven patches connected with parasitic patches or elements, respectively, in accordance with the present disclosure; 
           [0017]      FIG. 7A  shows a network representation of a phased array antenna system, and  FIG. 7B  shows an electro-magnetic (EM) simulation model of a single patch with two parasitically coupled elements reactively loaded in accordance with the present disclosure; 
           [0018]      FIG. 8  shows an example of beam scanning with reactive loads on the parasitic elements in accordance with the present disclosure; and 
           [0019]      FIG. 9  shows an example of beams formed by reconfiguring parasitic elements and coupling elements in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    In the following description, numerous specific details are set forth to clearly describe various specific embodiments disclosed herein. One skilled in the art, however, will understand that the presently claimed invention may be practiced without all of the specific details discussed below. In other instances, well known features have not been described so as not to obscure the invention. 
         [0021]    The present disclosure describes an active phased array system with a reduced number of TR feed module that has a pixelated reconfigurable electro-magnetic (EM) surface  10 , as shown in  FIG. 2B . The pixelated reconfigurable electro-magnetic (EM) surface  10  may be a substrate with reconfigurable patches  12 . The sizes of the reconfigurable patches  12  may be changed by connecting adjacent patches with switches  14  as shown in  FIG. 2C . The switches  14  may be phase change material that can be switched to an ON conducting state, or to an OFF non-conducting state. To connect adjacent patches  12  the PCM switches are put in an ON conducting state. The patches  12  may be metal patches. 
         [0022]    The pixelated reconfigurable electro-magnetic (EM) surface  10  may also have reconfigurable coupling lines  16 , as shown in  FIG. 2A . The reconfigurable coupling lines  16  may be metal. The coupling lines  16  may be configured to be in various configurations by switches  18 , as shown in  FIG. 2A , which may also be a phase change material that can be put in an ON conducting state, or in an OFF non-conducting state.  FIG. 1 , which is an example detail of one row the pixelated reconfigurable electro-magnetic (EM) surface  10  of  FIG. 2B , shows examples of how the coupling lines  16  may be switched into various configurations by turning ON and OFF switches  18 . As can be seen in  FIG. 1 , the coupling lines  16  may be configured to be straight lines or serpentine lines between adjacent patches  12  or parasitic elements  20 . 
         [0023]    Further, the pixelated reconfigurable electro-magnetic (EM) surface  10  may have reconfigurable parasitic elements  20  that are not driven, for example, by a transmit/receive (TR) module  30 . The parasitic elements  20  may be metal and be parasitic patches of various sizes and shapes. The parasitic elements  20  may be reactively loaded by reactive loads  70 , as shown in  FIG. 7B . The reactive loads  70  may include capacitive and inductive loads. By reconfiguring the size of patches  12 , the coupling lines  16 , and the size, shape and reactive loading of the parasitic elements  20 , a desired performance metric, such as a desired frequency, scan angle, or impedance may be attained. 
         [0024]    As discussed above, the pixelated EM surface  10  shown in  FIG. 2B  is formed by a two dimensional periodic array of metal patches  12  separated by small gaps with 14 switches between gaps that can be activated and deactivated. In addition, as discussed above, the pixelated EM surface has coupling elements  16 , and parasitic elements or patches  20 , as shown in  FIGS. 1 and 2A . The patches  12  may be driven with TR modules  30  for transmit and receive applications. 
         [0025]    The array spacing between patches  12  may be greater than λ/2 at the center frequency. Controlled coupling between patches  12  is achieved by configuring the coupling lines  16  and/or the parasitic patches  20  with the goal being to suppress any grating lobes at large scan angles and also to maintain a low constant voltage standing wave ratio (VSWR) over the scan angle. 
         [0026]    As discussed above with reference to  FIGS. 2B and 2C , an embodiment of this invention uses phase change (PCM) for the switches  14  in the gaps between the metal patches  12  to change the effective patch sizes. The details of the use of PCM for switches for a reconfigurable EM surface is further described in U.S. patent application Ser. No. 14/617,361, filed Feb. 9, 2015, which is incorporated herein as though set forth in full. 
         [0027]    The present disclosure has the following advantages over the prior art: a reduction in the number of TR modules  30  required, and a corresponding reduced number of phase shifter bits for controlling beam steering in a phased array. Conventional phased arrays use a TR module with monolithic microwave integrated circuits (MMICs), which have phase shifters and amplifiers in each radiation element. These MMICs are the largest part of the total antenna cost. A spacing less than λ/2 is typically used in the prior art to prevent grating lobes, and antenna reconfiguration requires changing the antenna feeds. These factors drive the cost and complexity for a conventional phased array antenna. 
         [0028]    In the present disclosure, with reference to  FIGS. 1 and 2A , the RF feed lines  32  from the TR modules  30  to the patches  12  are fixed and need not be reconfigured. Patches  12  have dimensions less than the desired wavelength, and parasitic elements and coupling lines  16  are configured on the top surface of the pixelated EM surface  10  to maintain beam scanning and impedance match over a scan angle. The spacing between patches  12  may be greater than λ/2 at the operating center frequency, which makes it possible to decrease the number of radiating elements and hence the cost. This is accomplished by suppressing the grating wave power and keeping the reflected power to a minimum using controlled coupling provided by the reconfigurable coupling lines  16  and the configurable parasitic patches  20 , which suppress grating lobes by changing the mutual coupling between the radiating patches  12 . 
         [0029]      FIG. 1  shows an RF aperture with metallic patches  12  spaced λ apart with feed lines  32  from TR modules  30  to drive the patches  12 , and reconfigurable coupling lines  16  between the patches  12  and between parasitic patches  20 . In the embodiment of  FIG. 1 , which shows a linear array, the reduction in number of TR modules is 50% due to spacing being λ between driven patches  12  rather than having a λ/2 spacing between the driven patches  12 . For a two dimensional array, λ spacing results in a 4 to 1 reduction in the number of TR modules compared to having a λ/2 spacing between the driven patches  12 . The TR modules  30  and the controlled mutual coupling between each patch  12  can provide beam steering. 
         [0030]      FIG. 2A  shows a detail of a reconfigurable coupling line  16  between a patch  12  and a passive parasitic patch  20 . The reconfigurable coupling line  16  includes PCM switches  18 , which provides low resistance connections between portions of the coupling line when the PCM  18  is in an ON state, or separates portions of the coupling line  16  when the PCM  18  is in an OFF state. By switching the PCM switches  18  ON or OFF, many configurations of the coupling lines  16  may be provided. For example,  FIG. 1  shows a number of different coupling line  16  configurations. By switching all of the PCM switches  18  in a coupling line  16  to an OFF position, a coupling line  16  between patches may be set to an open position, so that there is no coupling between patches. For example, in  FIG. 1  the switches  18  are set so that a break  34  or open  34  is in one of the coupling lines  16 , so that there is no connection between the adjacent patch  12  and parasitic patch  20 . 
         [0031]      FIG. 2B  and  FIG. 2C  which is a detail of  FIG. 2B , show an RF aperture  10  with a pixelated array of metallic patches  12  with phase change material (PCM) switches  14  between the metallic patches  12 . The PCM material  14  lies in the gaps between the metallic patches  12  such that when actuated into an ON state, the PCM switch provides a low resistance bridge between two patches  12 , thus effectively connecting them electrically and therefore changing the effective size of the patch  12 . The same method of changing the effective size of a patch  12  may also be used to change the effective size and shape of parasitic patches  20 , such as for example parasitic patches  20  shown in  FIGS. 1 and 4A . PCM material  14  may be placed in gaps between smaller parasitic patches  20  and switched on and off to change the size of the parasitic patches  20  in the same manner as shown in  FIGS. 2B and 2C  for patches  12 . 
         [0032]    The PCM switches  14  and  18  may have an insertion loss of about 0.1 dB and an on-state resistance (R on ) of less than 0.5Ω. The R off /R on  ratio for the PCM switch may be greater than or equal to 10 4 , which provides an RF isolation that is greater than 25 dB. Actuation of particular patterns of PCM switches  14  and  18  may be used to reconfigure the metallic patches  12  and coupling lines  16  on the top surface of the RF aperture  10 . 
         [0033]      FIG. 3A  shows a prior art two element metallic patch  40  array with a λ 0 , the wavelength of center frequency f 0 , spacing of 150 mm at 2 GHz, rather than a λ 0 /2 spacing and with a beam scan angle of 30° from the broadside. When the two patches  41  are excited with equal amplitude and uniform progressive phase difference between them, and with the main beam  42  scanned to ˜30° from boresight, a grating lobe  44  appears at ˜−20°, as shown in  FIG. 3B . In general, using a spacing between λ/2 and λ reduces the number of TR elements and hence the cost of a phased array system; however, results in such grating lobes. 
         [0034]    As discussed above, the patches  12 , the reconfigurable coupling lines  16 , and the parasitic patches  20  can all be reconfigured. In order to suppress the grating lobes, two methods may be used. The first method, as shown in  FIG. 4A , employs reconfigurable coupling lines  16  between two driven patch elements  12 . In the second method, as shown in  FIG. 4B , parasitic patches  20  between driven patches  12  are used to control the phase between driven patches  12 . The parasitic patches may or may not be connected with reconfigurable coupling lines  16  to the driven patches  12 . The two methods may also be combined so that the patches  12 , the reconfigurable coupling lines  16 , and parasitic patches  20  are all reconfigured in order to suppress the grating lobes. 
         [0035]    Electromagnetic simulations show that both approaches effectively suppress the grating lobe level of a λ 0  spaced two element array, as shown in  FIGS. 4A and 4B , to be approximately the same as the grating lobe level for a λ 0 /2 spaced array.  FIGS. 5A and 5B  show beam pattern plots comparing the configurations shown in  FIGS. 4A and 4B , respectively. For the configuration of  FIG. 4A  with coupling lines  16 , the plot in  FIG. 5A  shows that the gain pattern  50  has a grating lobe that is less than the grating lobe of the gain pattern  52  for the same configuration as  FIG. 4A  without coupling lines  16 . For the configuration of  FIG. 4B  with parasitic patches  20 , the plot in  FIG. 5B  shows that the gain pattern  54  has a grating lobe that is less than the grating lobe of the gain pattern  56  for the same configuration as  FIG. 4B  without the parasitic patches  20 . Full wave electro-magnetic (EM) simulations and multi-objective based optimization may be used for design of the coupling/parasitic elements. Both methods also maintain return-loss/VSWR characteristics of a λ 0 /2 spaced array, as shown in  FIGS. 6A and 6B , for the configurations of  FIGS. 4A and 4B , respectively, at a center frequency of 2 GHz. S 11  and S 22  are essentially the same for the configuration of  FIG. 4A , as shown in  FIG. 6A . For the configuration of  FIG. 4B , curve  57  plots S 11  and curve  59  plots S 22 , as shown in  FIG. 6B . 
         [0036]    Those familiar with the art of phased arrays know that a phased array system can be treated as a multiport antenna system, as shown in  FIG. 7A , which shows a network representation of a phased array antenna system with two ports  60  and  62 . The coupling lines  16  can be represented in terms of equivalent circuits. Lumped element models can be derived to calculate the coupling coefficients and coupling pattern of the array and the parameters can be varied with the scan angle and frequency. Parasitic patches  20  themselves can be represented as resonant circuits with mainly capacitive coupling between them to change the radiation characteristics. 
         [0037]      FIG. 7B  is an electro-magnetic (EM) simulation model of a single driven patch  12  with two parasitic patches  20  reactively loaded with reactive loads  70 . The reactive loads may be switched in or out, or the reactive loads changed by controlling switches  72 , which may be PCM material. The resonant antenna elements can also be represented by a parallel resistor, inductor, capacitor (RLC) circuit with reactive loading. The matching network may be required for wide scans and is an effective way to compensate for the variation of the element impedance with scan angle. 
         [0038]      FIG. 8  is a simulation example showing beam scanning at 0 degrees  80 , +10 degrees  82 , and −10 degrees  84  with reactive loads on the parasitic elements that can be used for developing the equivalent circuit models for the reconfigurable array. 
         [0039]      FIG. 9  shows another embodiment of the present disclosure. In this embodiment a source  90  radiates to the RF aperture  92 , which produces a radiated beam pattern with far field beams, such as far field beam patterns  94  and  96 . The far field beam patterns  94  and  96  vary depending on how the RF aperture  92  has been configured by switching PCM switches  14  and  18  either ON or OFF to reconfigure driven patches  12 , parasitic patches  20 , and reconfigurable coupling lines  16  as discussed above. 
         [0040]    The embodiments of the present disclosure have the following advantages. The TR module count in phased arrays may be reduced without the disadvantage of prior art methods that use sub-arraying or sparse arrays, which cannot achieve wide angle scans and low-VSWR. The antenna characteristics may be changed using the reconfigurable parasitic elements. Controlled coupling with the reconfigurable coupling lines allows grating lobe free beam scans using an array spacing of greater than λ/2 at the design frequency. Also, reconfiguration occurs only on one surface of the RF aperture, which avoids the complication of reconfigurable RF feed lines. 
         [0041]    Having now described the invention in accordance with the requirements of the patent statutes, those skilled in this art will understand how to make changes and modifications to the present invention to meet their specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention as disclosed herein. 
         [0042]    The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the Claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the step(s) of . . . . ”