Abstract:
In some embodiments, a microfluidic beam scanning focal plane array for a beam scanning antenna includes an elongated microfluidic channel that contains an electrically conductive antenna element suspended within a dielectric fluid that is provided within the channel and means for moving the position of the antenna element along a length of the channel to change a direction along which electromagnetic waves are transmitted or received.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to U.S. Provisional Application Ser. No. 61/843,363, filed Jul. 6, 2013, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     The increasing sampling capabilities of emerging surveillance, communication, and imaging systems (such as the wide area airborne-motion imaginary units, future satellite technologies, and millimeter-wave (mm-wave) imaging systems) necessitate high-gain antennas with wide-field-of-view (WFoV) beam steering capabilities. The antenna components of these systems are traditionally implemented with reflectors, lenses, and phased arrays. 
     Reflector antennas are typically not attractive as they are bulky and require a very precise mechanical elevation/azimuth scan over a WFoV at mm-waves. Microwave lenses, on the other hand, can be lightweight and compact. However, conventional designs generally suffer from low-scan volumes. More importantly, high-gain WFoV beam scanning typically requires a complicated radio frequency (RF) switch matrix and power divider implementations to accommodate tightly packed receive/transmit arrays at the focal plane. Phased antenna arrays offer important advantages over reflectors and lenses because they can potentially provide low-profile and high-efficiency apertures due to the absence of spill-over losses. However, for high-gain mm-wave apertures, their advantages are accompanied by high system complexity and cost. For example, a 30 GHz Ka-band ideal-phased array with 100% aperture efficiency can require an aperture size of 9×9 cm 2  to deliver 30 dB directivity. If realized from half-wavelength spaced antenna elements, the phased array may require 18×18 (i.e., 324) antennas and a substantial amount of hardware in the form of phase shifters and power dividers. 
     From the above discussion, it can be appreciated that it would be desirable to have practical and low-cost implementations of beam scanning antennas that meet the demanding needs of high gain and WFoV. Such implementations hold promise to transform the use of high data rate surveillance, communication, and imaging systems from specialized needs into mainstream technologies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may be better understood with reference to the following figures. Matching reference numerals designate corresponding parts throughout the figures, which are not necessarily drawn to scale. 
         FIG. 1A  is a side view of an embodiment of a beam scanning antenna that includes a microfluidic focal plane array. 
         FIG. 1B  is a schematic view of an embodiment of a microfluidic focal plane array that can be used to construct the beam scanning antenna of  FIG. 1A . 
         FIG. 2A  is a perspective view of an embodiment of a microfluidic focal plane array constructed from a stack comprising multiple layer of material. 
         FIG. 2B  is a schematic view of an embodiment of a chamber and a feed stub of the microfluidic focal plane array of  FIG. 2A . 
         FIGS. 3A and 3B  are graphs that show the simulated performance of a proposed microfluidic focal plane array design. More particularly,  FIG. 3A  shows the |S 11 | performance of antenna patches #1-#4 and  FIG. 3B  shows the normalized radiation pattern (φ=0°) when an antenna lens is excited by patches #1-#4. 
         FIGS. 4A-4D  are graphs that provide patch and array pattern comparisons when the antenna patch is located at the position of A: patch #1, B: patch #2, C: patch #3, and D: patch #4. 
         FIG. 5A  is a photograph of a switching speed test setup. 
         FIG. 5B  is a photograph of a mp6 piezoelectric micropump. 
         FIG. 6A  is a photograph of a fabricated feed network. 
         FIG. 6B  is a photograph of a dielectric extended hemispherical lens. 
         FIG. 6C  is a photograph of a top view of a prototype beam scanning antenna. 
         FIG. 6D  is a photograph of a bottom view of the prototype beam scanning antenna. 
         FIGS. 7A and 7B  are graphs that show the measured performance of the prototype beam scanning antenna. More particularly,  FIG. 3A  shows the |S 11 | performance and  FIG. 3B  shows the normalized gain pattern. 
         FIG. 8A  is a side view of a further embodiment of a beam scanning antenna that includes a microfluidic focal plane array. 
         FIG. 8B  is a schematic perspective view of an embodiment of a microfluidic focal plane array that can be used to construct the beam scanning antenna of  FIG. 8A . 
         FIG. 9  is a schematic view of an alternative embodiment of a microfluidic focal plane array that can be used to construct the beam scanning antenna of  FIG. 1A . 
         FIG. 10  is a schematic view of another alternative embodiment of a microfluidic focal plane array that can be used to construct the beam scanning antenna of  FIG. 1A . 
     
    
    
     DETAILED DESCRIPTION 
     As described above, it would be desirable to have practical and low-cost implementations of beam scanning antennas. Disclosed herein are microfluidic beam scanning focal plane arrays (FPAs) that can be used to form such antennas. The arrays comprise one or more microfluidic channels along which an antenna element can be positioned at different locations to scan a beam across a wide-field-of-view (WFoW). In some embodiments, the antenna element comprises a small volume of an electrically conductive liquid or a solid electrically conductive element that is suspended in a dielectric liquid. The position of the antenna element can be adjusted by urging the dielectric liquid through the channel(s) to cause the antenna element to move through the channel(s). In some embodiments, one or more microfluidic pumps are used to achieve this movement. 
     In the following disclosure, various specific embodiments are described. It is to be understood that those embodiments are example implementations of the disclosed inventions and that alternative embodiments are possible. All such embodiments are intended to fall within the scope of this disclosure. 
       FIG. 1A  schematically illustrates an example embodiment of beam scanning antenna  1  that includes a microfluidic focal plane array  10  and a hemispherical lens  12  that is positioned on top of the array. The lens  12  is made of an appropriate transparent or translucent material, such as optical glass or a polymer, and generally comprises a cylindrical base  16  and a hemispherical top  18 . Although a hemispherical lens is shown in  FIG. 1A , it is noted that other types of lenses could be used, such as Fresnel lenses and frequency selective surface-based synthesized lenses. 
     As is further shown in  FIG. 1A , the microfluidic focal plane array  10  has been manipulated to position an antenna element  20  in one of eight different possible positions along a microfluidic channel  22  that forms the array.  FIG. 1B  shows an example configuration for the array  10 . As is apparent in  FIG. 1B , the array  10  is formed as a one-dimensional array. It is noted, however, that, in other embodiments, the array can be two dimensional (see, e.g.,  FIG. 8B ). The microfluidic channel  22  comprises a narrow, elongated continuous channel along which the antenna element  20  can be driven. Spaced along the length of the channel  22  are multiple microfluidic chambers  24  (eight in this example) that are adapted to alternatively receive the antenna element  20 . In the illustrated embodiment, it is assumed that the antenna element  20  is an electrically conductive liquid, such as mercury (σ=1×106 S/m), that has a volume sufficient to more or less fill the chamber  24  to which it is driven. In other embodiments, however, the antenna element  20  can comprise a solid conductive element, such as a small metal plate or metalized plate. 
     As is further shown in  FIG. 1B , the microfluidic channel  22  is in fluid communication with microfluidic drive channels  26  and  28  that supply a dielectric fluid  30 , such as polytetrafluoroethylene (PTFE) fluid (e.g., AF1601S Teflon®), that is contained within the supply channels as well as the microfluidic channel  22 . The dielectric fluid  30  can be driven through the drive channels  26 ,  28  using a bi-directional micropump  32  that is also in fluid communication with the drive channels to drive the antenna element  20  to a desired chamber  24 . For example, if the dielectric fluid  30  is driven by the micropump  32  through the drive channel  28  (to the right in  FIG. 1B ), the dielectric fluid can drive the antenna element  20  toward the other drive channel  26  (to the left in  FIG. 1A ). By so driving the antenna element  20 , the antenna element can be, for instance, moved from the third position (as shown in  FIG. 1B ) to the second position from the left. 
     With further reference to  FIG. 1B , the focal plane array  10  further comprises a proximity-coupled microstrip line feed network  34  that includes an input line  36 , a feed line  38 , and multiple feed stubs  40  that extend from the feed line, one stub extending to each of the chambers  24  of the microfluidic channel  20 . The length of the feed stubs can be designed to be λ g /2 (λ g =6.62 mm at 30 GHz) to present an open circuit (OC) condition to the feed line  38  when the chambers  24  do not comprise the antenna element  20 . In addition, the separation between the stubs  40  (as well as the chambers  24 ) can be designed to be λ g  to provide the necessary OC conditions in order to direct the RF power to the antenna element  20  without needing any active RF switches. The feed network  34  is adapted to deliver radio frequency (RF) signals to or from the antenna element  20  when it resides in one of the chambers  24 . As an example, an RF signal can trace the path identified by line  42  in  FIG. 1B  from the input line  36  to the stub  40  associated with the antenna element  20 , which acts as a patch antenna. The antenna element  20  can then transmit the signal using the hemispherical lens  12 , as illustrated in  FIG. 1A . 
     The microfluidic focal plane array  10  can be formed in a variety of different ways. In some embodiments, the various channels of the array can be formed from microfluidic tubing. In other embodiments, the entire microfluidic focal plane array  10  can be fabricated from multiple layers of material using semiconductor fabrication techniques.  FIG. 2A  illustrates an embodiment of such a microfluidic focal plane array  50  in an exploded view. In the illustrated embodiment, the array  50  comprises a stack of layers that includes a ground plane  52 , a first layer  54  that overlies the ground plane on which a feed network  56  is formed, a second layer  58  that overlies the first layer, a third layer  60  that overlies the second layer in which a microfluidic channel  62  having multiple chambers  64  is formed, and a fourth layer  66  that overlies the third layer. In some embodiments, the ground plane  52  is made of a metal material such as copper, the first layer  54  is made of a dielectric material such as a fiber-reinforced polymeric material, the second layer  58  is made of another dielectric material such as a liquid crystal polymer (LCP) material, the third layer  60  is made of a further dielectric material such as a silicone material, and the fourth layer  66  is the hemispherical lens, which can be made of cross-linked polystyrene microwave polymer. 
     A microfluidic focal plane array was designed for evaluation and simulation purposes. The design had a construction similar to that shown in  FIG. 2A . In the design, the first layer  54  was a 127 μm thick RT5880 layer, the second layer  58  was a 50 μm LCP layer, the third layer  60  was a 2 mm thick polydimethylsiloxane (PDMS) layer, and the fourth layer  66  (lens) was made of Rexolite™.  FIG. 2B  shows the dimensions for one of the chambers  64  and one of the feed stubs  68  of the feed network  56  in the design. The microfluidic channel  20  and chambers  24  were 250 μm deep. The antenna element was assumed to be a small volume of liquid metal. In the simulation, a semi-infinite half-space was assumed over the PDMS layer to model the presence of the electrically large hemispherical lens. 
     The simulated |S 11 | performance as the liquid metal moved from the first (i.e., the leftmost) chamber to the fourth chamber over the passive proximity coupled feed network is depicted in  FIG. 3B  (due to the array symmetry, only the performances of patches #1 to #4 are presented). As can be seen in this figure, a resonant frequency of 30 GHz and similar |S 11 | performance is achieved when any of the chambers are filled with liquid metal. This implies the successful operation of the designed passive feed network in routing the RF power to the liquid metal patch element. An in-house ray tracing MATLAB® code that utilizes the ADS® simulated patterns was employed for computing the far field patterns generated by the extended hemispherical lens. As demonstrated in  FIG. 3B , moving the liquid metal among the chambers provides different excitation locations at the back surface of the extended hemispherical lens that, in turn, provides beam scanning capability. The focal plane array exhibits a half power beamwidth of 7° in the φ=0° elevation plane, implying 29 dB of directivity according to Krauss&#39; approximation. A ±30° FoV is accomplished over the φ=0° elevation plane with individual element patterns overlapping at their half power beamwidths. 
     It is important to note that the resonant nature of the utilized passive feed mechanism may result in performance degradation due to radiation leakage as realization of perfect open circuit stub terminations is not practically possible. In addition, a long microstrip input line was used to feed the antenna from the side of the lens (see  FIG. 6B ), which increases the network loss. Therefore, to quantify the loss associated with the feed network, computational studies were performed by comparing radiation pattern performance of the microfluidic based focal plane array to that of a stand-alone patch antenna excitation (i.e., without any feed line).  FIGS. 4A-4D  present these radiation pattern comparisons for all focal plane array elements. It is observed that the feed line loss accounts for 4.06 dB, 3.88 dB, 3.90 dB, and 3.82 dB reductions in realized gain for the patch antenna locations #1, #2, #3, and #4, respectively. The average feed network loss is therefore 3.92 dB. This is comparable to the performance of a conventional eight-element focal plane array implementation that would utilize a total of seven SP2T switches and activate a patch element through the series connection of three SP2T switches. Commercially-available, state-of-the-art Ka-band SP2T switches exhibit approximately 1 dB insertion loss and the conventional feed network will potentially exhibit greater than 3 dB insertion loss due to the additional interconnects, microstrip line sections, and bends. The radiation patterns presented in  FIG. 4  also reveal an enlargement in the sidelobe level for the microfluidic based beam scanning array within the ±30° scan range due to the feed network radiation. Specifically, the sidelobe level due to feed network radiation is relatively constant in the scan range and 20 dB below the main beam. 
     In addition to the radiation performance, the array beam scanning time was characterized using an mp6 piezoelectric micropump and mp-x controlling unit acquired from microComponents® as shown in  FIG. 5A . The micropumps are compact in size (30×1.5×3.8 mm 3 ) and two of them can be cascaded in series to form a bi-directional pumping unit. These pumps were able to move the liquid metal patch to the adjacent chamber in 70 msec. The switching time can be potentially reduced down to a few msec using mechanically faster pumps. 
     A prototype of the microfluidic channel was fabricated using a PDMS micromolding technique. To obtain the mold layer, negative photoresist (SU-8 2075) was spun onto a silicon wafer and then patterned with a UV light source. The PDMS oligomer and crosslinking prepolymer of the PDMS agent from a Sylgard™ 184 kit (Dow Corning) was mixed in a weight ratio of 10:1, poured onto the SU-8 mold, and then cured at room temperature for 24 hours to prevent PDMS shrinking due to heat. 
     Bonding the channels to a 50 μm LCP was accomplished by using APTES (3-aminopropyltriethoxysilane) functionalized SU-8 as an intermediate layer between the PDMS and LCP layers. SU-8 was spun on 50 μm thick LCP substrate and soft baked subsequently. The baked photoresist was then exposed to ultraviolet (UV) light and post baked again. After developing, the SU-8 was hard baked. The surface of the SU-8 coated LCP substrate was then activated by oxygen plasma treatment. Later, the substrate was placed in a 1% v/v APTES solution-heated to 800° C. for 20 minutes. Subsequently, the functionalized SU-8 and the fabricated PDMS micro channel mold were exposed to oxygen plasma. The two surfaces were placed in conformal contact for 1 hour. After this process, the two surfaces were irreversibly bonded to each other due to the formation of a strong Si—O—Si covalent bond. 
       FIGS. 6A and 6B  depict the lithographically-fabricated array feed network and machined dielectric lens. In order to obtain a robust structure, the lens was placed in custom built polystyrene foam holder as shown in  FIG. 6C . The feed network and channels were then flushed to their bottom surfaces using tape, as depicted in  FIG. 6D . 
     For measurement ease within the anechoic chamber, syringes were used to move the liquid metal antenna. The array |S 11 | shown in  FIG. 7A  was measured by an Agilent N5227A PNA. The array exhibited a matched impedance response at 30 GHz when chambers #1 to #4 were individually hosting the liquid metal antenna element. The beam scanning capability of the array was verified by measuring its realized gain patterns in the φ=0° elevation plane. As shown in  FIG. 7B , the array scanned the beam as the liquid metal antenna was moved among the chambers. Table I summarizes the expected and measured scan angles, measured maximum gain, and the array efficiency calculated relative to the 29 dB directivity. As it can be inferred from the table, the measured and expected scan angles are in a very good agreement. The small discrepancy is due to the possible misalignment while installing the array at the lens backside. A maximum realized gain of 24.8 dB was measured for patch #2, whereas the lowest gain was 21.5 dB when patch #4 was activated. On average, the array exhibited 5.7 dB loss. The measured loss values were comparable to the simulated 3.9 dB feed network loss described above. The additional 1.8 dB loss was identified to be due to the connector loss and lens-air interface reflection. Nevertheless, the obtained performance is promising as compared to a conventional switch based focal plane array implementation and holds potential for implementing mm-wave low-cost high gain beam scanning arrays. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Array Summarized Measured and Simulated Performance 
               
             
          
           
               
                   
                 Patch#1 
                 Patch#2 
                 Patch#3 
                 Patch#4 
               
               
                   
               
               
                 Scan Angle 
                 (32°, 30°) 
                 (25°, 22°) 
                 (14°, 13°) 
                 (7°, 4°) 
               
               
                 (measured, calculated) 
                   
                   
                   
                   
               
               
                 Measured Gain [dB] 
                 21.5 
                 23.2 
                 24.8 
                 23.9 
               
               
                 Calculated Efficiency 
                 17.8% 
                 26.3% 
                 38% 
                 30.9% 
               
               
                   
               
             
          
         
       
     
     The focal plane array described above comprises but one example focal plane array design that can be used.  FIG. 8  illustrates another embodiment.  FIG. 8A  illustrates a beam scanning antenna that comprises a microfluidic focal plane array  80  that is applied to hemispherical lens  82 . Unlike the hemispherical lens  12  of  FIG. 1A , however, the hemispherical lens  82  does not have a flat bottom (focal) surface. Instead, the lens  82  includes a cylindrical central portion  82 , a top hemispherical portion  82 , and a bottom hemispherical portion  88  that forms a spherically-curved bottom surface  90 . The microfluidic focal plane array  80  is flexible so that it can curve to conform to this bottom surface  90 . 
       FIG. 8B  illustrates an example embodiment for the microfluidic focal plane array  80 . As shown in this figure, the array  80  comprises a two-dimensional array of multiple microfluidic chambers  92  that are formed in a stack  94  of flexible material layers. An antenna element  96 , such as a small volume of conductive liquid or a metal or metallized microbead can be positioned within the chambers  92  to form a patch antenna by driving a dielectric fluid through microfluidic channels  98  that connect the chambers. The microfluidic channels  98  are arranged in rows and columns along the x and y directions and are in fluid communication with each other because they are in fluid communication with the chambers  92  to which they are connected. In such an embodiment, x and y direction movement of the antenna element  96  can be achieved using two bidirectional pumps (not shown), one for x direction movement and one for y direction movement. Not visible in  FIG. 8B  is the feed network used to address the chambers  92 . 
     As mentioned above, the antenna element need not be a conductive liquid. In alternative embodiments, the antenna element can comprise a metallized element that is suspended within the dielectric liquid inside a microfluidic channel. By way of example, the metallized element can comprise a glass plate that has been sputter-coated with a highly-conductive metal, such as copper. Greater performance may be achieved in such embodiments if the metal of the element has higher electrical conductivity than available liquid metals. Moreover, if hard substrates such as quartz or alumina substrates are used to fabricate the microfluidic channels, greater power handling than that achievable with the PDMS-LCP embodiments and prior art MEMS-switched embodiments may be possible. 
     In the above disclosure, the feed networks have been described and illustrated as comprising a resonant corporate feed network comprising a feed line and multiple feed stubs that extend from the feed line to chambers of the microfluidic channel. In some cases, the array pattern of a resonant corporate network fed microfluidic focal plane array can exhibit high side lobe level (SLL) due to the radiation leakage of the unloaded stubs. The SLL can be reduced by employing a feed network that exhibits a fewer number of open stub resonators. This can be accomplished by using a resonant straight feed network comprising a compact straight microstrip line that extends along the channel and terminates with an open circuit. Such an embodiment and its microstrip line  100  are schematically illustrated in  FIG. 9 . In this configuration, the antenna is loaded with a single straight open stub resonator regardless of its location. Therefore, the microfluidic focal plane array will radiate with a relatively lower SLL as compared to the corporate network fed case. 
     Although resonant straight feed networks improve the SLL, they still exhibit limited bandwidth, which may only be suitable for narrowband applications. For wideband operations, the bandwidth of the feed network can be significantly improved by resorting to a non-resonant layout. In such a layout, the feed network comprises a long straight microstrip line proximately coupled to a microfluidically repositionable patch antenna. Unlike the resonant feed network, the line is terminated with the characteristic impedance of the line, Z 0 . This can be achieved using a resistor placed at the open end of the line. Therefore, the feed network is non-resonant without bandwidth limitation at the expense of being lossy. In this layout, the antenna element can be positioned at any arbitrary location (not only discrete chambers) without losing its impedance matching. Hence, the feed network allows for continuous beam scanning.  FIG. 10  schematically illustrates such an embodiment. The embodiment includes a microstrip line  102  connected to a resistor  104  and a continuous microfluidic channel  106  that comprises no discrete chambers.