Abstract:
Various examples are provided for spherical monopole antennas. In one example, among others, a spherical monopole antenna includes a spherical conductor on a first side of a substrate and a ground plane disposed on the substrate. The spherical conductor is electrically coupled to a connector via a tapered feeding line and the ground plane surrounds at least a portion of the connector on the second side of the substrate. In another example, among others, a method includes forming a tapered mold in a die layer disposed on a first side of a substrate, filling the tapered mold with a conductive paste, and disposing a spherical conductor on a large end of the tapered mold. The conductive paste is in contact with a signal line extending through the substrate into a small end of the tapered mold and in contact with the spherical conductor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to co-pending U.S. provisional application entitled “SPHERICAL MONOPOLE ANTENNA” having Ser. No. 61/842,631, filed Jul. 3, 2013, the entirety of which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Ultra-wideband (UWB) is a technology for transmitting data over a large bandwidth greater than 500 MHz. Super-wideband (SWB) is one providing at least a bandwidth ratio of 10:1 for high-resolution. UWB and SWB are used for high-data-rate wireless communication, long-range radar and imaging systems. UWB/SWB antennas are key components for such wireless communication, radar, and imaging systems. Antenna characteristics include input impedance, radiation pattern, gain, efficiency, etc. Because of their use in portable wireless devices, the antenna designs are affected by many factors such as space limitations, geometry, multi antenna interference, etc. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0004]      FIGS. 1A and 1B  are examples of perspective and cross-sectional views, respectively, of a spherical super wideband (SWB) antenna in accordance with various embodiments of the present disclosure. 
           [0005]      FIG. 2  is a plot of an example of return loss of a spherical SWB antenna of  FIGS. 1A and 1B  in accordance with various embodiments of the present disclosure. 
           [0006]      FIGS. 3A and 3B  are graphical representations of examples of a spherical SWB antenna including coplanar waveguide feeding in accordance with various embodiments of the present disclosure. 
           [0007]      FIG. 4  is a graphical representation illustrating an example of a fabrication process of a spherical SWB antenna of  FIGS. 1A and 1B  in accordance with various embodiments of the present disclosure. 
           [0008]      FIG. 5  is a plot of an example of return loss of a fabricated spherical SWB antenna of  FIGS. 1A and 1B  in accordance with various embodiments of the present disclosure. 
           [0009]      FIGS. 6A-6D  are plots of examples of radiation patterns of the fabricated spherical SWB antenna of  FIGS. 1A and 1B  in accordance with various embodiments of the present disclosure. 
           [0010]      FIG. 7  is a plot of examples of gain and group delay of the fabricated spherical SWB antenna of  FIGS. 1A and 1B  in accordance with various embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Disclosed herein are various examples related to embodiments of spherical monopole antennas. In this disclosure, the design, fabrication, and characterization of spherical monopole antennas using a super wideband technique with a tapered feeding line is discussed. Reference will now be made in detail to the description of the embodiments as illustrated in the drawings, wherein like reference numbers indicate like parts throughout the several views. 
         [0012]    Using a super wideband (SWB) technique can provide at least a ratio bandwidth of 10:1 for high-resolution sensing through, e.g., wall radar and surveillance systems. The extremely wide bandwidth may be achieved by accommodating smooth antenna geometries such as, e.g., a tapered feed line, a rounded ground plane and/or a circular/elliptical patch. While showing good bandwidth performance, planar monopole antennas can suffer from substrate dielectric loss and distortion in the omni-directional radiation pattern. Three dimensional (3D) SWB antennas can provide better omni-directionality. 
         [0013]    A 3D SWB monopole antenna such as, e.g., a spherical SWB antenna can be designed, fabricated and characterized as will be described. For example, a separate conductive sphere (e.g., a steel ball) may be adopted as a main radiator. A 3D tapered feeding line can be implemented by, e.g., a layer of photopatternable polyurethane (e.g., D50, MacDermid Inc. or other appropriate patternable material), multidirectional ultraviolet (UV) lithography, and molded conductive paste. 
         [0014]    The low frequency cutoff of the spherical SWB antenna may be mainly determined by the diameter of the conductive sphere of the spherical SWB antenna at its quarter wavelength, where the conductive sphere serves as a main radiator. The upper cutoff can be greatly enlarged by using a tapered feeding line between a coaxial connection and the conductive sphere, which can be fabricated using thick photopatternable polyurethane (e.g., D50, MacDermid Inc.) and 3D multidirectional UV lithography. In some implementations, the spherical SWB antenna can have a 10 dB bandwidth between about 2.4 GHz and about 23.2 GHz (a ratio bandwidth of 9.7:1), and an omni-directional radiation pattern with a maximum gain of approximately 2.9 dBi at 10 GHz. 
       Antenna Configuration 
       [0015]    Referring to  FIGS. 1A and 1B , shown are perspective and cross-sectional views, respectively, of an example of a spherical SWB antenna  100 . In the example of  FIG. 1 , the spherical SWB antenna  100  includes a conductive sphere  103 , a tapered feeding line  106 , and a circular ground plane  109 . In some embodiments, the conductive sphere  103  can be, e.g., a steel ball, copper ball, or other appropriate hollow conductive shell or solid conductive ball. As shown in the cross-sectional view of  FIG. 1B , the tapered feeding line  106  electrically couples the conductive sphere  103  and a coaxial connection  112  that extends through the ground plane  109 . In the example of  FIGS. 1A and 1B , a patternable die layer  115  such as, e.g., a photopatternable polyurethane (PU) layer surrounds the tapered feeding line  106 . The patternable die layer  115  may be circular or other appropriate geometrical pattern such as, e.g., a polygon. As illustrated in  FIGS. 1A and 1B , the circular ground plane may be located underneath a laminate layer  118  of, e.g., a printed circuit board (PCB). For example, the laminate layer (or substrate)  118  may be a layer of polytetrafluoroethylene (PTFE) such as, e.g., RT-duroid 5880LZ. The ground plane  109  is located on a side of the laminate layer  118  opposite the conductive sphere  103  and tapered feeding line  106  as shown in  FIG. 1B . The geometry of the ground plane  109  may be, e.g., circular, hexagonal, octagonal, or other appropriate pattern. 
         [0016]    The height of the spherical SWB antenna  100  is approximately the sum of the ball (or sphere) diameter (B d ) and the height of the die layer  115  (D h ), which determines the lowest resonant frequency corresponding to approximately a quarter wavelength at the lowest frequency. The operating bandwidth of the spherical SWB antenna  100  depends on the dimensions of the tapered feeding line  106 . Dimensions of the spherical SWB antenna  100  can be designed and optimized using a commercial 3D electromagnetic simulator such as, e.g., CST Microwave Studio or ANSYS High Frequency Structure Simulator. 
         [0017]    An example of a spherical SWB antenna  100  was implemented to test the operational characteristics. The geometry of the fabricated spherical SWB antenna  100  of  FIGS. 1A and 1B  can be: ball diameter B d =24 mm of the conductive sphere  103 ; diameter D d =30 mm of the die layer  115 ; height D h =5 mm of the die layer  115 ; diameter G d =70 mm of the circular ground plane  109 ; upper diameter T u =6 mm of the tapered feeding line  106 ; and bottom diameter T b =1 mm of the tapered feeding line  106 . The laminate layer  118  is RT-duroid 5880LZ (ε r =1.96) with a thickness of 0.508 mm. Other thicknesses of the laminate layer  118  may be used. 
         [0018]      FIG. 2  is a plot  200  illustrating the effect on the return loss for variations in the upper diameter T u  of the tapered feeding line  106 .  FIG. 2  provides simulated results for upper diameters of T u  =1 mm (curve  203 ), T u =3 mm (curve  206 ), and T u =6 mm (curve  209 ). The bottom diameter of the tapered feeding line  106  remained constant at T b =1 mm. 
         [0019]    Referring to  FIGS. 3A and 3B , shown is an example of a spherical SWB antenna  300  including coplanar waveguide feeding.  FIG. 3A  is an exploded view illustrating the relationship between the conductive sphere  103 , the tapered feeding line  106  and a coplanar waveguide  312  located on a side of a substrate  318  adjacent to the conductive sphere  103 .  FIG. 3B  provides a cross-sectional view of the spherical SWB antenna  300  including coplanar waveguide feeding. The spherical SWB antenna  300  includes a conductive sphere  103  coupled to a coplanar waveguide  312  via a tapered feeding line  106 . In the example of  FIGS. 3A and 3B , the coplanar waveguide  312  and a ground plane  309  are disposed on the same side of the substrate  318  as the conductive sphere  103  and the tapered feeding line  106 . In other implementations, the coplanar waveguide  312  and ground plane  309  may be disposed on the side of the substrate  318  that is opposite the conductive sphere  103  and the tapered feeding line  106 . 
         [0020]    In some embodiments, the conductive sphere  103  can be, e.g., a steel ball, copper ball, or other appropriate hollow conductive shell or solid conductive ball. In the example of  FIG. 1B , the conductive sphere  103  includes a hollow conductive shell with a central void  303  that may be filled with air, a dielectric, a polymer (e.g., Styrofoam), a metal, or other appropriate material. The thickness of the hollow conductive shell may be, e.g., about 10 μm to about 20 μm thick for use at about 1 GHz. The tapered feeding line  106  electrically couples the conductive sphere  103  and the coplanar waveguide  312  that extends through the ground plane  309  as shown in  FIG. 3A . When the coplanar waveguide  312  and ground plane  309  are disposed on the side of the substrate  318  that is opposite the conductive sphere  103  and the tapered feeding line  106 , a via (or other appropriate connection) that extends through the substrate may be used to couple the tapered feeding line  106  to the coplanar waveguide  312 . As illustrated in  FIG. 3B , a patternable die layer  115  such as, e.g., a photopatternable polyurethane (PU) layer surrounds the tapered feeding line  106 . 
       Antenna Fabrication 
       [0021]    Referring to  FIG. 4 , shown is an example of fabrication of a spherical SWB antenna  100  of  FIGS. 1A and 1B  with a tapered feeding line  106  using micro-fabrication processes. The process begins with a substrate  403  (e.g., a planar substrate) clad on a single side with copper  406  in  FIG. 4( a ) . A circular ground plane  109  may be formed in the copper layer  406 . On the single side copper clad substrate  403 , a circular cavity  409  having a diameter D d  and height D h  for the die layer  115  is defined on a side of the substrate  403  opposite the copper  406  in  FIG. 4( b )  and a liquid-state negative photopatternable PU  412  (e.g., D 50  or other appropriate patternable material) is poured into the circular cavity  409 . In  FIG. 4( c ) , a photomask  415  is placed over the photopatternable PU  412  with a thin protection film  418  placed on top. Lithographic exposure using 3D multidirectional UV radiation  421  is performed to crosslink the liquid-state negative photopatternable PU  412 . The direction of the UV radiation  421  forms a tapered mold  424  in the die layer  115  for the tapered feeding line  106 . For example, unexposed D 50  can be washed away in water to form the tapered mold  424  and a feeding hole  427  may then be drilled through the substrate  403  (e.g., using a CNC (computer numerical controlled) lathe) as shown in  FIG. 4( d ) . 
         [0022]    Moving to  FIG. 4( e ) , the tapered mold  424  is filled with conductive paste (e.g., a gel-state silver paste), followed by assembling a coaxial connection  112  such as, e.g., a SMA (SubMiniature version A) connector through the feeding hole  427 . In this way, the signal line  430  of the coaxial connection  112  is electrically connected to the tapered feeding line  106 . A second connection of the coaxial connection can be coupled to the copper layer  406 . After removing the form from around the circular cavity  409  and placing the conductive sphere  103  on the conductive paste filled tapered feeding cavity  424  in  FIG. 4( f ) , the spherical SWB antenna  100  can be left at the room temperature for about 12 hours to solidify the conductive paste and complete the electrical connection with the conductive sphere  103 . Other methods for solidifying the conductive paste may also be utilized to secure the conductive sphere  103  and/or signal line  430  in position.  FIGS. 4( g ) and ( h )  show perspective and cross sectional views of the fabricated tapered mold  424  in a die layer  115  of D 50 .  FIG. 5  includes an image of a fabricated spherical SWB antenna  100 . 
         [0023]    The spherical SWB antenna  300  of  FIGS. 3A and 3B , including coplanar waveguide feeding, may be fabricated in a similar fashion. The coplanar waveguide  312  and the ground plane  309  may be formed on a side of the substrate  318 . A cavity may be defined over the coplanar waveguide  312  and the ground plane  309  and a liquid-state negative photopatternable PU (e.g., D 50  or other appropriate patternable material) can be poured into the cavity. The cavity may be on the same side of the substrate  318  or the opposite side of the substrate  318  as the coplanar waveguide  312  and ground plane  309 . A photomask is placed over the photopatternable PU with a thin protection film placed on top. Lithographic exposure using 3D multidirectional UV radiation is performed to crosslink the liquid-state negative photopatternable PU and form a tapered mold in the die layer  115  for the tapered feeding line  106 . The tapered mold extends through the die layer  115  providing access to a contact area of the coplanar waveguide  312 . The tapered mold may be filled with conductive paste (e.g., a gel-state silver paste) to form the tapered feeding line  106 , which is electrically connected to the coplanar waveguide  312 . The contact area may be at the end of the coplanar waveguide  312  and, in some implementations, may extend through the substrate  318 . For example, the contact area may include a via that extends through the substrate  318  from an end of the coplanar waveguide  312  for connection with the tapered feeding line  106 . The conductive sphere  103  may then be disposed on the conductive paste filled tapered feeding cavity and the conductive paste allowed to solidify to complete the electrical connection with the conductive sphere  103 . 
       Test Results 
       [0024]    The fabricated spherical SWB antenna  100  of  FIG. 5  was characterized using a vector network analyzer (HP E8361A) after one port calibration from 1 to 40 GHz and standard horn antenna (JXTXLB-10180, AINFO Inc.).  FIG. 5  shows a plot  500  of the simulated and measured return loss of the fabricated spherical SWB antenna  100  as curves  503  and  506 , respectively. The simulated and measured 10 dB-bandwidths of the antenna were 166% (2.5 GHz-26.8 GHz, 10.7:1 ratio bandwidth) and 163% (2.45 GHz-23.2 GHz, 9.7:1 ratio bandwidth), respectively. The slight deviation between the measured bandwidth and the simulated one may be due to the fabrication tolerance. 
         [0025]    Referring to  FIGS. 6A-6D , shown are the simulated and measured radiation patterns at 3 GHz, 5 GHz, 7.5 GHz and 10 GHz, respectively. The plots of  FIGS. 6A-6D  include simulated and measured radiation patterns in both the E-plane and H-plane.  FIGS. 6A-6D  illustrate monopole-like radiation patterns at each frequency for the spherical SWB antenna  100 . Also,  FIGS. 6A-6D  show good omni-directional radiation patterns. 
         [0026]    Referring next to  FIG. 7 , shown is a plot  700  of the simulated and measured maximum gain (curves  703  and  706 , respectively) and group delay (curves  709  and  712 , respectively). Although there is a small discrepancy between the simulated and measured maximum gain and group delay, they show similar trends. The decreased gain at 12 GHz may be attributed to the contribution of self-resonance of the D 50  layer with a finite size. Changing the dimension of the die layer  112  may alleviate this. The simulated and measured group delay (curves  709  and  712 , respectively) of the spherical SWB antenna  100  is less than ±1 ns, which is excellent for pulse communication. 
         [0027]    A 3D spherical SWB antenna  100  was designed, fabricated and characterized. As seen by  FIGS. 5, 6A-6D, and 7  measured results were well matched with the simulated results. The spherical SWB antenna  100  has a 10 dB-bandwidth of 163% (ratio bandwidth of 9.7:1) and a maximum gain of about 2.9 dBi at 10 GHz. The spherical SWB antenna  100  exhibits good manufacturability, low cost, and a good omni-directional radiation pattern. Also, the lowest resonant frequency is easily tunable by assembling a different size of conductive sphere  103 , and therefore the design and process can be scalable. 
         [0028]    It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims. 
         [0029]    It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of “about 0.1% to about 5%” should be interpreted to include not only the explicitly recited concentration of about 0.1 wt % to about 5 wt %, but also include individual concentrations (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within the indicated range. The term “about” can include traditional rounding according to significant figures of numerical values. In addition, the phrase “about ‘x’ to ‘y’” includes “about ‘x’ to about ‘y’”.