Abstract:
An antenna with an FR-4 dielectric material layer includes at least one metallization layer having metallic dipoles organized into two clusters. Each of the two clusters includes metallic dipoles generally elongated along a common axis to produce signals of specific polarization. Each of the two clusters is oriented orthogonal to the other to produce two separate, orthogonally polarized signals. Each of the two clusters is associated with a dedicated stripline feed, positioned and oriented to maximize gain of the radiating element. Power from each stripline planar feed couples to the metallic dipoles through a dedicated aperture in the stripline ground plane.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND INCORPORATION BY REFERENCE 
     The present application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 61/934,381, filed Jan. 31, 2014, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed generally toward antennas, and more particularly to electronically scanned antennas. 
     BACKGROUND 
     Current planar radiating element technology using high dielectric constant FR-4 materials cannot provide a radiating element with relatively wide frequency band and scan volume and good polarization performance. Conventional aperture coupled patch antennas have gain and polarization limitations. Electronically scanned antenna systems often employ a circularly polarized field. Linear polarization imposes inherent limitations on Active Electronically Scanned Antenna (AESA) systems. A circularly polarized radiating element with relatively wide frequency band and scan volume comprised of FR-4 material, would be preferable, if it could be achieved in a single unit cell. 
     Electronically scanned antennas generally include a manifold layer for distributing power to a feed layer. The feed layer feeds power to a patch layer that transitions the power to free space. The patch layer, for a conventional aperture coupled patch antenna, typically requires low dielectric constant materials that are unsuitable for FR-4 manufacturing processes. Further, the patch layer is substantially thicker than the manifold or feed layers, creating an unbalanced printed circuit board. 
     Aperture coupled patch antennas generally have a low dielectric constant substrate and two printed circuit board patches. Patches tend to scatter into lower order Floquet modes. In order for the patch to operate effectively, the lower order Floquet modes must be relatively constant over scan and frequency necessitating a small unit cell size and a low dielectric constant polytetrafluoroethylene (PTFE) based substrate. The small unit cell size means that the module density is high, significantly increasing the cost of the antenna and the thermal cooling requirements of the antenna. The use of low dielectric constant PTFE based material also significantly increases the cost of the antenna. Further, scan performance in the H plane scan is poor. Current planar radiating element technology cannot provide a low cost relatively broadband (˜30%) aperture coupled dual polarized radiating element comprised exclusively of FR-4 materials, manufactured using standard printed circuit board (PCB) processes, and with a built in radome. Furthermore, the PTFE based aperture layers are substantially thicker than the manifold or feed layers creating an unbalanced printed circuit board. 
     Consequently, it would be advantageous if an apparatus existed that is a balanced FR-4 printed circuit board, and suitable for use as an aperture coupled dual polarized radiating element with moderately wide frequency bandwidth and scan volume. 
     SUMMARY 
     Accordingly, embodiments of the present invention are directed to a novel apparatus that is a balanced FR-4 printed circuit board and suitable for use as an aperture coupled dual polarized radiating element with moderately wide frequency bandwidth and scan volume. 
     In one aspect, embodiments of the present invention are directed to a radiating element including two linearly polarized feeds, each associated with a plurality of metal elements in a metal layer. An antenna including apertures as described herein produces two polarized signals, 90° out of phase, per-unit-cell. 
     In a further aspect, embodiments of the present invention are directed to a radiating element configured for dual polarization that utilizes higher order dielectric constant materials. In preferred embodiments, an antenna according to the present invention is implemented as a balanced printed circuit board manufactured using standard FR-4 manufacturing processes. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  shows a block diagram of an antenna system according to embodiments of the present invention; 
         FIG. 2  shows a cross-sectional side view of a radiating element according to embodiments of the present invention with two stripline fed apertures; 
         FIG. 3  shows a top view of a ground plane of a radiating element with two apertures to couple the dual polarized signals from the feed layer to the lower metallization layer; 
         FIG. 4  shows a top view of a lower metallization layer of a radiating element according to embodiments of the present invention; 
         FIG. 5  shows a top view of a mid-metallization layer of a radiating element according to embodiments of the present invention; 
         FIG. 6  shows a top view of a upper metallization layer of a radiating element according to embodiments of the present invention; 
         FIG. 7  shows a graphical representation of the performance of a radiating element according to at least one embodiment of the present invention; 
         FIG. 8  shows a graphical representation of the performance of a radiating element according to at least one embodiment of the present invention; 
         FIG. 9  shows a graphical representation of the performance of a radiating element according to at least one embodiment of the present invention; 
         FIG. 10  shows a graphical representation of the performance of a radiating element according to at least one embodiment of the present invention; 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the subject matter disclosed herein, which is illustrated in the accompanying drawings. The scope of the invention is limited only by the claims; numerous alternatives, modifications and equivalents are encompassed. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
     Referring to  FIG. 1 , an antenna system according to embodiments of the present invention includes a feed layer  100 , a manifold layer  104  connected to the feed layer  100 , a module layer  102  connected to the feed layer  100  and the manifold layer  104 , and an array of radiating element unit cells  106  connected to the feed layer  100 . Each radiating element unit cell  106  generates two orthogonally polarized fields. The radiating element unit cells  106  include layers of FR-4 material with printed circuit patterns. The coupling from the radiating element unit cell  106  to the feed layer  100  is through apertures cut in the feed layer&#39;s  100  top ground plane. 
     At least one embodiment of the present invention may include an active electronically scanned antenna. In at least one embodiment, radiating element unit cells  106  in the antenna include two stripline fed apertures. Two orthogonally oriented stripline fed apertures allow for dual polarized excitation of dipole elements in a metal layer of the radiating element. The feed layer  100  and aperture layer may comprise a high dielectric constant material such as FR-4 or similar material. In the context of the present invention, high dielectric constant may be understood to refer generally to a dielectric greater than 3.3. Embodiments of the present invention are directed specifically toward materials with a dielectric constant of between 3.3 and 3.7, though a person of ordinary skill in the art having the benefit of the disclosure may appreciate that other dielectric constants are envisioned. In at least embodiment, the feed layer  100  comprises two 20 mil (0.508 millimeters) FR-4 layers and the aperture layer comprises a 20 mil FR-4 layer, a 20 mil FR-4 layer and another 20 mil FR-4 layer. Such an embodiment scans well over a moderately wide frequency band and offers low cross-polar radiation. 
     Referring to  FIG. 2 , a radiating element according to embodiments of the present invention with two stripline fed apertures is shown. The radiating element  210  has a number of printed circuit board layers; all printed circuit board layers include a high dielectric material suitable for FR-4 manufacturing processes. The printed circuit board is balanced to reduce warping. 
     The radiating element  210  may include a built in radome layer  200 , an upper metallization layer  204 , a mid-metallization layer  214 , a lower metallization layer  206 , and an aperture feed layer  202  as described more fully herein. Two stripline fed apertures  216 ,  218  are configured to receive electronic signals from the aperture feed layer  202  and excite the lower metallization layer  206 . 
     In at least one embodiment, a low profile radiating element substrate has a height of 60 mil (1.524 millimeters) (0.074 free space wavelengths at 14.5 GHz). In at least one embodiment, the substrate material is Rogers 4003, having a dielectric constant of 3.55, and loss tangent of 0.0278. In one embodiment, the unit cell size is 0.25λ 2  at 14.5 GHz. Radiating elements according to the present invention may have scan performance of approximately −10 dB return loss out to 30° half conical scan angle for arbitrary phi angle. 
     In at least one embodiment, the radome layer  200  may include a layer of FR-4 applied at the end of the manufacturing process to protect the underlying metal layers. FR-4 may be applied without “potato-chipping” the board because the underlying printed circuit board is balanced. 
     Referring to  FIG. 3 , a radiating element  300  according to the present invention has a ground plane  302  layer. The ground plane layer  302  defines two apertures openings  308 ,  312  each corresponding to a stripline  306 ,  310 . In at least one embodiment, a first aperture  308  stripline  306  structure is offset by 90° from a second aperture  312  stripline  310  structure in the ground plane  302 . The location and orientation of each apertures opening  308 ,  312  and corresponding stripline  306 ,  310  is determined in relation to the gain of the radiating element with respect to the metallic layers and the final geometry of the metallic dipoles of the radiating element as described herein. 
     Referring to  FIG. 4 , a radiating element  400  according to the present invention may include a lower metallization layer  402 . The lower metallization layer  402  includes a plurality of metallic squares  404 , organized to tune the radiating element in a particular frequency range and balance additional metal layers as described herein. 
     Referring to  FIG. 5 , a radiating element  500  according to the present invention may include a mid-metallization layer  502 . The mid-metallization layer  502  includes a plurality of metallic squares  504 , organized for wide angle scan. The dipole metallic squares may be excited by signals from a lower metallization layer such as in  FIG. 4 . 
     Referring to  FIG. 6 , a radiating element  600  according to the present invention may include an upper metallization layer  602 . The upper metallization layer  602  includes a plurality of metallic dipole strips  604 , organized for wide angle scan. The dipole metallic strips may be excited by signals from a lower metallization layer such as in  FIG. 4 . 
     A person skilled in the art having the benefit of the disclosure may appreciate that while the exemplary embodiment described herein is specifically directed toward metallic dipole strips  604  organized into dipole strip clusters  614 ,  616  in an upper metallization layer  602 , metallic dipole strips organized into dipole strip clusters in other metallization layers may also be effective in producing per-unit-cell dual polarization. Radiating elements according to embodiments of the present invention may produce a per-unit-cell dual polarized signal suitable for active electronically scanned array systems. 
     Referring to  FIG. 7 , in at least one exemplary embodiment of the present invention, the performance of a radiating element operating with θ (theta) of 30 degrees and φ (phi) of 0 degrees is shown in a frequency range of 10.7 to 14.5 GHz. Performance is measured as return loss in decibels. Return loss is shown for a horizontal polarization  700  and a vertical polarization  702 . 
     Referring to  FIG. 8 , in at least one exemplary embodiment of the present invention, the performance of a radiating element operating with θ (theta) of 30 degrees and φ (phi) of 29.97 degrees is shown in a frequency range of 10.7 to 14.5 GHz. Performance is measured as return loss in decibels. Return loss is shown for a horizontal polarization  800  and a vertical polarization  802 . 
     Referring to  FIG. 9 , in at least one exemplary embodiment of the present invention, the performance of a radiating element operating with θ (theta) of 30 degrees and φ (phi) of 60.03 degrees is shown in a frequency range of 10.7 to 14.5 GHz. Performance is measured as return loss in decibels. Return loss is shown for a horizontal polarization  900  and a vertical polarization  902 . 
     Referring to  FIG. 10 , in at least one exemplary embodiment of the present invention, the performance of a radiating element operating with θ (theta) of 30 degrees and φ (phi) of 90 degrees is shown in a frequency range of 10.7 to 14.5 GHz. Performance is measured as return loss in decibels. Return loss is shown for a horizontal polarization  1000  and a vertical polarization  1002 . 
     It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description of embodiments of the present invention, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.