Abstract:
An inexpensive, easy to assemble patch antenna is disclosed where unwanted polarizations in the transmitted RF energy are minimized. A feedboard, spacer and resonator are held in a compressed relationship by two halves of the antenna housing. The spacer is a thermo-formed sheet with semi-spherical spacers. The spacers have a height that provides the desired spacing between the feedboard and the resonator.

Description:
CROSS REFERENCE TO RELATED INVENTION 
     This application is related to the following commonly assigned an concurrently filed U.S. patent applications entitled “Patch Antenna Using Non-Conductive Thermo Form Frame”, Ser. No. 09/425,373, filed Oct. 22, 1999; and “Patch Antenna Using Non-Conductive Frame, Ser. No. 09/425,374, filed Oct. 22, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to antennas; more particularly, patch antennas. 
     2. Description of the Prior Art 
     FIG. 1 illustrates an exploded view of a prior art patch antenna assembly. Front housing  10  and rear housing  12  form the outer surfaces of the antenna assembly. The two sections of the housing enclose multi-layered feedboard  14 , resonators  16  and  18  and spacers  20 . Spacers  20  are attached to front side  22  of feedboard  14  by screws  24 . Screws  24  mate with threads on the inside of spacers  20  by passing through holes  26  in feedboard  14 . Resonators  16  and  18  are attached to spacers  20  in a similar fashion. Screws  28  mate with threads on the inside of spacers  20  by passing through holes  30  in resonators  16  and  18 . The spacers are chosen so that they provide a space of approximately {fraction (1/10)} of a wavelength between feedboard  14  and resonators  16  and  18 . The assembled feedboard, spacers and resonators are mounted inside of the enclosure formed by front housing  10  and rear housing  12 . A signal to be transmitted by the antenna assembly is provided to conductor  40  of multi-layered feedboard  14 . Conductor  40  is typically positioned on one layer of feedboard  14  such as on top layer  42 . An insulating layer is typically provided between conductor  40  and a ground plane layer of feedboard  14 . The ground plane layer normally has openings or slots  44  which allow the signal from conductor  40  to couple to resonators  16  and  18  so that the signal can be transmitted through front housing  10 . 
     FIG. 2 provides a more detailed illustration of the assembled feedboard  14 , spacers  20  and resonators  16  and  18 . Screws  24  pass through holes in feedboard  14  to mate with the threaded inside portion of spacer  20 . Similarly, screws  28  pass through holes in resonators  16  and  18  to mate with the threaded inside portion of spacers  20 . 
     This prior art patch antenna assembly suffers from several shortcomings. The assembly is expensive to assemble because of the many individual parts such as eight spacers and 16 screws. The spacers are expensive to mass produce because they include threaded inner portions. Additionally, the holes made through resonators  16  and  18  to allow screws  28  to mate with spacers  20  create unwanted patterns in the radio frequency energy radiated by the antenna assembly. For example, if the antenna is being used for a horizontally polarized transmission, the holes introduce additional non-horizontal polarizations in the transmitted signal. 
     SUMMARY OF THE INVENTION 
     The present invention provides an inexpensive, easy to assemble patch antenna that does not introduce unwanted polarizations in the transmitted radio-frequency (RF) energy. A feedboard, spacer and resonator are held in a compressed relationship by two halves of the antenna housing. The spacer is a thermo-formed sheet with semi-spherical spacers. The spacers have a height that provides the desired spacing between the feedboard and the resonator. 
     In one embodiment, spherical spacers are positioned between the feedboard and resonator using an adhesive and then the feedboard, spacers and resonators are held in position by compression provided by the antenna housing assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates a prior art patch antenna assembly; 
     FIG. 2 illustrates a prior art feedboard, spacer and resonator assembly; 
     FIG. 3 illustrates an exploded view of a patch antenna assembly having semi-spherical spacers; 
     FIG. 4 illustrates a cross section of an assembled patch antenna system having semi-spherical spacers; 
     FIGS. 5A,  5 B, and  5 C illustrate a sheet having semi-spherical spacers; 
     FIG. 6 illustrates the relationship between a feedboard, semi-spherical spacers, a resonator and the front portion of the antenna housing; 
     FIG. 7 illustrates a semi-spherical spacer with resonator locator tabs; and 
     FIG. 8 illustrates spherical spacers. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 illustrates patch antenna assembly  100 . The assembly is enclosed by rear housing section  112  and front housing section  114 . Resonator elements  116  and  118  are positioned in front housing section  114  by placing them within the outline of guide ribs  120  and  122 . Semi-spherical spacer sheets  124  and  126  are then placed on top of resonators  116  and  118  respectively. It should be noted that ribs  120  and  122  are high enough to also aid in the positioning of spacer sheets  124  and  126 . Spacer sheets  124  and  126  include semi-spherical spacers  128  which provide the desired spacing between the resonators and feedboard  130  (typically, 0.1 wavelengths of the signal to be transmitted). It should be noted that guide ribs  120  and  122  do not extend higher than semi-spherical spacers  128  so that ribs  120  and  122  do not interfere with the spacing provided by semi-spherical spacers  128 . Multi-layer feedboard  130  is then positioned upon semi-spherical spacers  128 . Feedboard  130  is positioned in front housing section  114  by positioning tabs  132 . Multi-layer feedboard  130  is a board containing a ground plane, a plane containing conductor  134  and insulating layers on the top and bottom surfaces and between conductor  134  and the ground plane. Slots  136  and  138  in the ground plane permit a radio frequency (RF) signal on conductor  134  to couple to resonators  116  and  118  so that RF energy may be transmitted through front housing section  114 . Metallic cover  140  is positioned over multi-layer feedboard  130  using guide tabs  132  to aid in positioning. Rear housing section  112  then mates with front housing section  114  and locks in place by interacting with locking tabs  142 . Rear section  112  and metallic cover  140  contain openings  144  and  146  respectively which provide a passage through which a conductor can pass for attachment to point  148  on conductor  134 . 
     FIG. 4 illustrates a cross section of an assembled patch antenna having semi-spherical spacers. Front housing section  114  interlocks with rear housing section  112  by the action of tabs  142  and  150 . Resonators  116  and  118  are positioned on the inside surface of front section  114  and are positioned between guide ribs  120  and  122 . Each resonator typically measures 0.6 by 0.6 wavelengths of the signal to be transmitted. Spacer sheets  124  and  126  with semi-spherical projections are also positioned between guide ribs  120  and  122  respectively and on top of resonators  116  and  118 . It should be noted that a slight space is shown between the spacer sheets and the patch elements. This spacing is only included to aid in illustrating the components of the assembly; in actuality, the spacers are held tightly against the patch elements by feedboard  130 . Feedboard  130  is positioned on top of semi-spherical spacers  128  of spacer sheets  124  and  126 . Positioning tabs  132  aid in correctly positioning feedboard  130  in front housing section  114 . Metallic cover  140  is placed over feedboard  130  and tabs  132  provide guidance in positioning metallic cover  140 . Rear cover  112  is pressed on to front section  114  so that tabs  142  and  150  interact to hold sections  114  and  112  together. Rear section  112  includes a series of parallel ribs  152  that form an interference fit of approximately 0.0005 mils or more between the ribs and metallic cover  140 , feedboard  130 , spacers  124  and  126 , resonators  116  and  118 , and the inside surface of front section  114 . This results in a compression force of approximately 0.25 pounds being placed on the feedboard, spacers and resonators when the locking tabs of sections  112  and  114  engage. This compression force holds the feedboard, spacers and resonators in position. 
     FIGS. 5A,  5 B, and  5 C illustrate a semi-spherical spacer sheet. Semi-spherical spacer sheet  170  includes semi-spherical spacers  172 . Spacer sheet  170  is formed using a thermo-formed or heat pressed low radio frequency loss plastic that is on the order of 5 or 6 mils thick. Such materials include polycarbonate (PC) polymethyl methacrylate (PMMA) or polypropylene (PP) among others. PC is available from General Plastics, PMMA is from Rehm-Haas, whereas PP is from a number of plastic material vendors such as DuPont and Phillips 66. In one embodiment as illustrated in FIG. 5B, the semi-spherical spacers  172  are approximately 2.5 millimeters high for a 1.9 GHz antenna. For a frequency of x, 2 millimeters is suggested to provide a {fraction (1/10)} wavelength spacing between the feedboard and resonator; however, an extra 0.5 millimeters may be included to provide for a small amount of compression in the semi-spherical spacers. In this embodiment the spacers are approximately 7 millimeters in diameter where the inner spacers are positioned 14 millimeters apart and the outer spacers are positioned 35 millimeters apart. It should be noted that sheet  170  essentially contains spacers or bubbles  172  that are formed in a normally flat sheet. For example, if a cross section of sheet  170  is taken along line  174  as illustrated in FIG. 5C, it can be seen that semi-spherical spacers  172  are essentially semi-spherical depressions formed in relatively flat sheet  170 . This thermoforming process is relatively inexpensive and does not require special or expensive machine tools. For these reasons, the development time is relatively short. 
     FIG. 6 illustrates a detailed view of the relationship between front housing section  114 , resonator  116  or  118 , a semi-spherical spacer sheet such as spacer sheet  170  and feedboard  130 . Feedboard  130 , spacer sheet  170 , resonator  116  and front section  114  are held in a compressed relationship when the tabs of rear section  112  and front section  114  engage. It should be noted that the compressed relationship results in the spacing between feedboard  130  and resonator  116  being controlled by the height of semi-spherical spacers  172 . The shape of semi-spherical spacers  172  provide a rigid structure that withstands the compression provided by housing sections  112  and  114 . 
     FIG. 7 illustrates semi-spherical spacer sheet  190  having locator tabs  192 . Locator tabs  192  are used to position resonator  116  relative to spacer sheet  190 . This provides the advantage of eliminating guides ribs  120  and  122 . Additionally, slots  194  are included in front section  114  so that tabs  192  do not interfere with resonator  116  lying flat against the inside surface of front section  114 . In addition, slots  194  aid in positioning spacer sheet  190  and resonator  116  on the inside surface of section  114 . As described earlier, feedboard  130  is positioned on semi-spherical spacers  195  of spacer sheet  190 . Feedboard  130 , spacers  194  and resonator  116  are held in a compressed relationship by rear section  112  and front section  114  when interlocking tabs  142  and  150  are engaged. 
     FIG. 8 illustrates the use of spherical spacers  200 . Resonator  116  is positioned on the inside surface of front housing section  114  using guide ribs  120 . Spherical spacers  200  are then placed on the exposed surface of resonator  116 . It may be desirable to use an adhesive to position spherical spacers  200  so that they do not move into undesired locations during assembly. It is also possible to include spherical spacers  200  in a liquid or foam that will hold the spheres in position during assembly. It should be noted that any adhesive, liquid or foam used to aid in the positioning of spheres  200  should be transparent or very low loss with respect to the RF signal being transmitted. Feedboard  130  is then placed on top of spherical spacers  200 . Feedboard  130 , spherical spacers  200  and resonator  116  are held in a compressed relationship when the locking tabs of rear section  112  and front section  114  are engaged.