Patent Publication Number: US-2007097009-A1

Title: Planar slot antenna design using optically transmissive materials

Description:
TECHNICAL FIELD OF THE INVENTION  
      The present invention relates to an antenna design using optically invisible materials. More specifically, the present invention relates to an optically transparent aperture antenna design with coplanar waveguide feed (CPW). The invention provides transparent antennas which do not require the use of vias and are properly “matched” with CPW structures.  
     BACKGROUND AND SUMMARY OF THE INVENTION:  
      The concept for designing optically transmissive (synonymous with transparent or visibly clear) antennas has been investigated by multiple organizations and individuals. The coupling of these apertures with simplistic feedlines has not been widely investigated. Very little work has been done in the development of optically transmissive apertures with low resistivity surfaces (&lt;15 Ω/square) and without the use of vias.  
      The present invention discloses such approaches as coplanar waveguide feeds (CPW) and the connection of those CPW feeds to miniature 50 ohm coaxial cables. The use of CPW provides an innovative approach to the design and fabrication of feedlines to optically transmissive apertures, regardless of type, provided they are “slot” type configurations. Without the use of CPW feedlines, the optically transmissive apertures visibility is severely compromised.  
      Two examples of antennas designed and developed using planar slot apertures fabricated with high transmission optically clear materials are presented herein. The antennas were fabricated both on Mylar and Glass substrates using both High Transmission Silver and Indium Tin Oxide (ITO). The apertures were slot type antennas. One antenna was configured as a slot bow tie antenna with tuning stubs and the other antenna was configured as a slot dipole. Both antennas were fed with a coplanar waveguide feeds in order to eliminate the use of vias when such electromagnetic waves were launched with micro-type coax cable (50 ohm based). The micro-coax used was basically a 32 mils outside diameter which made the total installation very low profile. Measurements indicate that it is possible to fabricate optically transmissive apertures with similar performance levels as apertures implemented using copper conductive materials. It was also discovered that coupling to these planar structures with coaxial cables became problematic for the flexible apertures; for solid materials (for example, glass), conductive bonds implementations were trouble free. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a schematic of a bow-tie antenna with tuning stubs embodiment;  
       FIG. 2  shows a schematic of a slot dipole antenna embodiment;  
       FIG. 3  shows a schematic of a coplanar waveguide portion of an antenna embodiment and its connection to a coaxial cable;  
       FIG. 4  shows a graph of return loss versus frequency for a Mylar bow-tie antenna using silver epoxy material to bond the coaxial cable feedlines directly to the coplanar waveguide feeds;  
       FIG. 5  shows a graph of return loss versus frequency for a Mylar bow-tie antenna using copper tape to bond the coaxial cable feedlines directly to the coplanar waveguide feeds;  
       FIG. 6  shows a graph of return loss versus frequency for a Mylar bow-tie antenna using copper tape soldered to the coaxial cable feedlines and bonding the copper tape to the coplanar waveguide feeds;  
       FIG. 7  shows a graph of return loss versus frequency for a glass substrate slot dipole antenna using silver epoxy material to bond the coaxial cable feedlines directly to the coplanar waveguide feeds; and  
       FIG. 8  shows a glass substrate slot dipole antenna using silver epoxy material to bond coaxial cable feedlines directly to the coplanar waveguide feeds. 
    
    
     DETAILED DISCLOSURE OF THE INVENTION  
      In the following description, two exemplary antenna embodiments are described. It will be obvious to those skilled in the art that the invention applies to many other possible embodiments as well. One embodiment described is a bow-tie antenna with tuning stubs and the other is a simple dipole antenna. Both antennas are implemented in the “slot” configuration. The main reason for deciding to use slot type configurations is based on the implementation and fabrication process. It is usually easier to “remove” material from a sputtered sheet of conductive surface over glass or Mylar than to deposit material of a certain geometrical configuration. The removal of the material was done with the use of laser ablation processes. An alternative removal technique involves the use of chemical baths. The process of laser ablation produces a resolution aperture without disturbing other parts of the sputtered conductive material. The results were very good using this fabrication process.  
       FIG. 1  shows an embodiment of the bow-tie antenna configuration  10 .  FIG. 2  shows an embodiment of the slot dipole antenna  20 . Both embodiments  10 ,  20  include a CPW feedline  18 ,  28 , respectively. Both antennas  10 ,  20  were designed for an operating center frequency of 2.0 GHz.  
      For the bow-tie antenna embodiment  10  shown in  FIG. 1 , a Mylar (polyester) substrate  12  was used. The substrate  12  was sputtered with a highly-transparent, highly conductive layer of High Transmission Silver (AgHT™) which was obtained from CP Films Inc. of Martinsville, Va. The AgHT surface has a resistivity of 8 ohms/square. The antenna configuration  10  is fed with CPW type feedline  18 , such configuration being done in a planar manner using the same sputtered conductive surface. The CPW feedline  18  was made to 50 ohms impedance, allowing easier interface to the micro-coaxial cable used. The design of the CPW feedline  18  is not described in detail since the design is straight forward and easily implementable by one skilled in the art with web downloadable software, one source of such information is www.rfcafe.com.  
      The fabrication of the aperture was made with the use of laser ablation. The drawing of the design of the bow-tie antenna  10  was made using AutoCad and such design was then provided to Laserod of Gardena, Calif., to provide laser ablation functions on multiple material surfaces. The laser ablation removed the AgHT material from the bow-tie portions  14  and the sides of the CPW feedline  18 , as shown by the cross-hatched portions of  FIG. 1 . Preparations were then made to connect the coaxial cables to the CPW feeds  18 .  
      For the slot dipole antenna embodiment  20  shown in  FIG. 2 , a glass substrate  22  was used. The glass substrate  22  was sputtered with a highly-transparent, quasi-metallic material of Indium Tin Oxide (ITO) which was obtained from Chomerics of Woburn, Mass., (CHO-ITO™). The ITO surface has a resistivity of 12 ohms/square. Modifications to transmission line program from Zeland Software, Inc. of Fremont, Calif., were made to account for the lower conductivity of the ITO material. The slot aperture  20  is fed with CPW type feedline  28 , such configuration being done in a planar manner using the same sputtered conductive surface. The CPW feedline  28  can be impedance matched with the slot aperture  20  at a feed point  26  and also impedance matched at a connection point for a cable. For example, the CPW feedline  28  was made to 50 ohms impedance at the connection point, allowing easier interface to the micro-coaxial cable used. The feed point  26  in this embodiment is located λ/20 from the slot dipole edge.  
      The fabrication of the aperture was made with the use of laser ablation. The drawing of the design of the slot dipole antenna  20  was made using AutoCad and such design was then provided to Laserod of Gardena, Calif., to provide laser ablation functions on multiple material surfaces. The laser ablation removed the ITO material from the slot aperture portion  24  and the sides of the CPW feedline  28 , as shown by the cross-hatched portions of  FIG. 2 . Preparations were then made to connect the coaxial cables to the CPW feedline  28  of the slot dipole antenna  20 .  
      Initially, the “bonding” of the center conductor and the shield of the coaxial cable to the planar sputtered surfaces and the CPW feedlines of the antennas shown in  FIGS. 1 and 2  was done using a silver epoxy material.  FIG. 3  illustrates the bonding of the conductors of a coaxial cable  30  to a CPW feedline  36 .  FIG. 3  shows a substrate covered with a sputtered conductive material  46  except for two sides  48  of the CPW feedline  36 . The sides  48  of the CPW feedline  36  define a center portion  38  of the CPW feedline  36  and an exterior portion  39 . The coaxial cable  30  comprises a shield  32  and a center conductor  34 . The shield  32  of the coaxial cable  30  is connected with silver epoxy material to the sputtered conductive material  46  at a location  42  in the exterior portion  39 , and the center conductor  34  of the coaxial cable  30  is connected to the center portion  38  of the CPW feedline  36  at a location  44 .  
      Measurements using the network analyzer were made to obtain S 11  parameters (return loss e.g. VSWR). The bonding to the sputtered material on the Mylar substrate produced poor results but the bonding to the sputtered material on the glass substrate produced excellent results. It was determined that because of the flexibility of the Mylar substrate, the bonding of the coaxial cable was “broken” thus producing very poor results. Alternative configurations, to be described below, were investigated for bonding the coaxial cable to the Mylar substrate.  
      One alternative, is to use conductive adhesive copper tape to bond the shield  32  of the coaxial cable  30  to the sputtered conductive material, and to bond the center conductor  34  of the coaxial cable  30  to the center portion  38  of the CPW feedline  36 .  
       FIG. 4  shows the voltage standing wave ratio (VSWR or SWR) when using the silver epoxy material to bond the coaxial conductors to the aperture on a Mylar surface and  FIG. 5  shows the SWR when using the copper tape to bond the coaxial conductors to the aperture on a Mylar surface. The apertures were identical and by bonding the coaxial feedline with the use of copper tape, an improvement was achieved in the VSWR. As can be observed by comparing  FIG. 5  with  FIG. 4 , the return loss function indicates better performance with the copper tape than with the silver epoxy material. However, other alternatives were also investigated.  
      The concept was advanced further by soldering the conductors of the coaxial cable to the copper tape and then bonding the copper tape to the CPW structure of the antenna. This implementation improved the results further.  FIG. 6  shows the return loss function for this implementation. The plot in  FIG. 6  shows good impedance matching from 1.5 GHz to about 5.8 GHz with a VSWR of less than 2. If we allow a VSWR of less than 3, then the aperture coverage is from 0.5 GHz to about 6 GHz.  
      As can be seen from the differences in the plots of  FIGS. 4, 5  and  6 , the mating of the conductors of the coaxial cable to the flexible antenna surface greatly impacts the results of the antenna. In the case of the rigid substrate like glass, the direct bonding method using silver conductive adhesive proved to be very good.  
       FIG. 7  shows the VSWR performance of the slot dipole antenna on glass. A VSWR of less than 2 was obtained between 0.7 GHz and 3.5 GHz. Above about 4 GHz, the slot dipole becomes non-resonant and the VSWR begins to rise accordingly. Similar results were obtained in the Mylar configuration (flexible aperture) when connections to the CPW feedline were made using the copper tape approach and the soldered coaxial cable and copper tape approach.  
      Optical transmission of the Mylar Silver aperture was 85% and the optical transmission of the Glass ITO aperture was 89%. The aperture of the bow-tie antenna is more visible than the aperture of the slot dipole antenna because the removed area for the bow-tie antenna is much greater as can be seen in  FIG. 1 .  
       FIG. 8  shows the slot dipole antenna with a glass substrate. The glass substrate was 47 mils thick of standard window glass (Borosilicate or Soda-Lime glass). As shown in the figure, the small coaxial cable is directly bonded to the ITO CPW feed. Results of this configuration are shown in  FIG. 7 . This configuration was well behaved electromagnetically using the silver epoxy bond.  
      While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are exemplary and not restrictive in character, it being understood that only exemplary embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention and the attached claims are desired to be protected.