Abstract:
An electromagnetic bandgap device is mountable on a RF disturbing structure of an antenna to minimize signal field disturbance imparted thereby. The RF disturbing structure is oriented in a direction substantially parallel to a path of travel of an antenna signal and located within a field covered by the signal transmitted by a feed. The bandgap device comprises a plurality of RF perturbing elements connected to the RF disturbing structure and spaced apart from one another in the direction substantially parallel to the signal path. The perturbing elements are positioned, configured and sized to direct a disturbed portion of the signal away therefrom to reduce field disturbance generated by the disturbed signal portion interacting with an undisturbed portion of the antenna signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Priority of U.S. Provisional Application No. 60/614,986, filed on Oct. 4, 2004, is hereby claimed. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of antennas and is more particularly concerned with an electromagnetic bandgap device to reduce the antenna field disturbance. 
   BACKGROUND OF THE INVENTION 
   It is well known in the art to use dielectric stiffeners in the manufacturing of antennas, especially between reflector shells of dual-gridded reflectors (DGRs), to minimize the RF (radio frequency) impact of such stiffeners on the overall antenna RF performance. Although dielectric materials such as Kevlar™, glass fibers and the like are used, the stiffeners are not ideal RF transparent structural posts and result in antenna field disturbance with typical increased sidelobe degradation of the signal. 
   Photonic bandgaps (PBGs) have been recently developed and used in microwave based applications such as in transmission lines with enclosed or channeled fields, including closed and open wave guides and the like, in which all the RF signal gets transmitted through. PBG structures include periodically disposed electrically reflective elements and exhibit RF properties that prevent propagation of electromagnetic waves in a specific direction at pre-determined frequency bands. 
   Known PBG technology is not applicable to open field antennas because of the relatively large signal cross-sectional path they have at any location between the feed and the reflector of the antennas, as opposed to transmission lines. 
   Accordingly, there is a need for an electromagnetic bandgap antenna structural element that improves the overall antenna performance. 
   SUMMARY OF THE INVENTION 
   It is therefore a general object of the present invention to provide an electromagnetic bandgap device for antenna structures. 
   An advantage of the present invention is that the electromagnetic bandgap device reduces the field disturbance of the antenna signal. 
   Another advantage of the present invention is that the electromagnetic bandgap device redirects (or reflects) the disturbed portion of the antenna signal away, typically orthogonally, from the signal path direction to limit its impact on the undisturbed portion of the signal, and avoid further reflection thereof back into the undisturbed portion of the signal. 
   A further advantage of the present invention is that the electromagnetic bandgap device can be used to obviate mechanical defects and/or non-uniformity of structural members of an antenna that would disturb the field of the RF antenna signal. 
   According to a first aspect of the present invention, there is provided an electromagnetic bandgap device for mounting on a RF disturbing structure of an antenna to minimize signal field disturbance imparted thereby, the RF disturbing structure being oriented in a direction substantially parallel to a path of travel of an antenna signal and located within a field covered by the signal, the bandgap device comprises: a plurality of RF perturbing elements connectable to the RF disturbing structure and spaced apart from one another in the direction substantially parallel to the signal path, said plurality of perturbing elements being positioned, configured and sized to direct a disturbed portion of the signal away therefrom so as to reduce field disturbance generated by the disturbed signal portion interacting with an undisturbed portion of the antenna signal. 
   Typically, the RF perturbing elements are substantially periodically spaced from one another; and preferably equally spaced from one another. 
   Alternatively, the RF perturbing elements are un-equally spaced from one another following a predetermined trend. 
   In one embodiment, the RF perturbing elements direct the disturbed portion of the signal substantially away from the signal path so as to allow loss of the disturbed signal portion. 
   In one embodiment, the RF perturbing elements direct the disturbed portion of the signal away therefrom in a direction generally perpendicular from the signal path. 
   In one embodiment, the RF perturbing elements are made out of RF reflective materials; and typically metallic materials. 
   Typically, the RF perturbing elements are positioned around, bonded or etched on at least a portion of the RF disturbing structure. 
   Alternatively, the RF perturbing elements are inserts locatable inside on at least a portion of the RF disturbing structure. 
   In one embodiment, the RF perturbing elements are spaced from one another by a spacing substantially equals to about three quarter of an average wavelength of the signal over a predetermined frequency range. 
   Other objects and advantages of the present invention will become apparent from a careful reading of the detailed description provided herein, with appropriate reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further aspects and advantages of the present invention will become better understood with reference to the description in association with the following FIGS., in which similar references used in different FIGS. denote similar components, wherein: 
       FIG. 1  is a simplified perspective view of an electromagnetic bandgap device in accordance with an embodiment of the present invention mounted on structural members of an antenna; 
       FIG. 2  is a simplified enlarged and broken view taken along line  2  of  FIG. 1 , showing the bandgap device on the structural members maintaining the two reflectors spaced apart from one another; 
       FIG. 3  is a simplified enlarged and broken section view taken along line  3 – 3  of  FIG. 2 , showing the RF perturbing elements of the bandgap device on the structural post between the two reflectors; 
       FIG. 4  is a simplified elevation view of a dual gridded reflector (DGR) used for testing, showing the location of seven stiffeners (non-illustrated inter-costal rings were also used at the periphery between the two reflectors); 
       FIGS. 5 and 6  are graphical antenna test results, showing the DGR aperture magnitude of the antenna of  FIG. 4  with nominal stiffeners without and with bandgap devices of the present invention respectively; 
       FIGS. 7 and 8  are graphical antenna test results similar to  FIGS. 5 and 6  respectively, showing the DGR aperture phase of the antenna of  FIG. 4  without and with bandgap devices of the present invention mounted on the stiffeners respectively; 
       FIG. 9  is a graphical antenna test result, showing the measured side lobe performances of the rear shell of the DGR of  FIG. 4  with nominal stiffeners (without the bandgap device of the present invention); and 
       FIG. 10  is a graphical antenna test result similar to  FIG. 9 , showing the measured side lobe performances of the rear shell of the DGR of  FIG. 4  with bandgap devices of the present invention mounted on the stiffeners. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to the annexed drawings, the preferred embodiments of the present invention will be herein described for indicative purpose and by no means as of limitation. 
   Referring to  FIG. 1 , there is schematically shown an electromagnetic bandgap device  10  in accordance with an embodiment of the present invention. The bandgap device  10  is typically connected to structural members  14  of an antenna  12  that are within or adjacent the field of view  18  of the electromagnetic signal  16  transmitted through the antenna  12  from the feed  20 . 
   Typically, such structural members  14  are stiffeners or posts  22  and intercostals rings (or walls) or portions thereof  24  used as structural reinforcements between the two front and rear shells  26 ,  28  of dual-gridded reflector (DGR) assemblies. In the design of the antenna  12  shown in  FIG. 1 , these RF disturbing structural members  14  are located either within or adjacent to the field of view  18  of the signal reflected by the front surface  30  of the rear shell  28 . 
   The RF disturbing structural members  14  are usually partially Radio-Frequency (RF) transparent to limit their electrical impact on the antenna performance, but the latter is not mandatory. Accordingly, they typically include RF transparent materials such as, but not limited to, Kevlar™, glass fibers and thermoplastic materials including commonly known polyester or polyethylene terephthalate (PET) (including Mylar™), polyimide (including Kapton™), fluorinated ethylene propylene (FEP) (including polytetrafluoroethylene (PTFE) Teflon™) and the like materials. The structural members  14  are typically oriented between the two shells  26 ,  28  in a direction  32  substantially parallel or acute to an average direction of travel  34  of the antenna signal between the incoming signal  16  and the signal  16   b  reflected by the reflector surface  30 . 
   Referring more specifically to  FIGS. 2 and 3 , the electromagnetic bandgap device  10  is a plurality of (at least one) RF perturbing elements  40 . The perturbing elements  40  are typically, but not limited to, metallic rings wrapped around at least a portion of, preferably all along, the structural members  14 , or metallic or dielectric insert(s)  41  (shown in dotted lines) placed inside the structural members  14  and in general are made out of materials with forms or shapes that can be used to create a perturbation of the electromagnetic fields inside and/or in the vicinity of the structural members  14 , or at least a portion thereof. The perturbing elements  40  are typically periodically, preferably equally, spaced from one another by a pre-determined spacing  42  in the direction  32  substantially parallel or acute to the direction  34  of the antenna incident and/or reflected signal  16 ,  16   b . The closest perturbing elements  44  to the signal reflecting surface  30  is typically spaced therefrom by the same or a multiple of the pre-determined spacing  42 . In some case, the perturbing elements  40  can also be selectively un-equally spaced from one another, such as following a logarithmic, an exponential or the like predetermined trend, to obtain the desired bandgap improvement over a larger frequency bandwidth and/or over a larger angular range of both incident and reflected RF signals  16 ,  16   b.    
   Each perturbing elements  40  is typically made out of an electrically reflective material such as, but not limited to, dielectrics and metallic materials. 
   The pre-determined spacing  42  typically depends on the frequency range of the electromagnetic signal being transmitted by the antenna  12 . Typically, the spacing  42  is a multiplier of a quarter of the wavelength (λ/4) of the signal, preferably about three quarter of the wavelength (3λ/4) and is optimized for the reasons explained further down below. As it would be obvious to one skilled in the art, the larger the spacing  42  the smaller the RF blockage of the incoming signal  16 ′ from the feed  20  to the rear shell surface  30  due to the rings  40  is. 
   Since the direction of the signal  16  varies between the incoming signal  16 ′ from the feed  20  and the reflected signal  16 ″ away from the rear shell surface  30 , the direction of the spacing  42  is typically anywhere from about the incoming direction  16 ′ and about the reflected direction  16 ″, and preferably about halfway there between in the average direction  34 , as shown in  FIG. 3  and also called the signal path. The direction of the bandgap device  10  may vary depending on the location of the structural member  14  relative to the field of view  18  of the signal  16 . 
   OPERATION 
   During transmission of the antenna  12 , a portion of the RF signal  16 ′,  16 ″ hits the bandgap device  10  or perturbing elements  40  and is directed away therefrom in a reflected direction  50 . The pre-determined spacing  42  helps determining this reflected direction  50  of the disturbed portion  16   a  of the signal  16 . It is therefore highly desirable that the reflected direction  50  be generally away from both the feed source  20  and the rear shell surface  30  such that the disturbed portion  16   a  of the signal  16  has a minimized impact on the undisturbed portion  16   b  of the signal  16  and on the pattern performances of the antenna  12 . 
   Accordingly, the disturbed portion  16   a  of the signal  16 , including the disturbed portion  16   a ′ of the incoming signal  16 ′ and the disturbed portion  16   a ″ of the reflected signal  16 ″, is typically reflected away from the signal path  34  or off-axis, toward a direction free of reflective surfaces (not shown) around the antenna  12 , such that it is substantially entirely lost, as shown in  FIGS. 2 and 3 . Moreover, the spacing  42  is pre-determined and optimized to ensure the disturbed signal portion  16   a  is reflected in the desired direction, typically substantially perpendicularly from the signal path  34 . 
   EXAMPLE 
   An exemplary test was performed on a DGR composed of a solid graphite back shell  28  and a polarization sensitive (i.e. gridded) kevlar front shell  26 . The rear shell antenna operates at about 14.0 to 14.5 GHz. To maintain the structural integrity of the DGR, seven stiffeners  22  and inter-coastal walls  24  are used as structural reinforcements between the two shells  26 ,  28 , as shown in  FIG. 4 . The DGR was tested in an antenna near field test range with and without (nominal configuration) the electromagnetic bandgap device  10  of the present invention located on the seven stiffeners  22 . The results of the DGR aperture planar magnitude field distribution of the antenna without and with the bandgap devices  10 , shown in  FIGS. 5 and 6  respectively, and of the DGR aperture planar phase distributions of the antenna without and with the bandgap devices  10 , presented in  FIGS. 7 and 8  respectively, clearly show that the bandgap devices  10  of the present invention significantly reduces the impact of the stiffeners  22  on the antenna performances. 
   Similarly, the antenna was tested in a compact antenna test range for the side lobe performances of the rear shell  28  at about 14.0 GHz. With the nominal stiffeners  22 , the measured side lobe directivity was as high as +10 dBi inside an isolated coverage area, indicated by a dotted closed line, as shown in  FIG. 9 . From the same test performed with the stiffeners  22  including the bandgap devices  10  of the present invention, the measured side lobe directivity was reduced by more than 4 dB with respect to the nominal case in the isolation coverage area, indicated by a dotted closed line, as shown in  FIG. 10 . 
   Although the present electromagnetic bandgap device has been described with a certain degree of particularity, it is to be understood that the disclosure has been made by way of example only and that the present invention is not limited to the features of the embodiments described and illustrated herein, but includes all variations and modifications within the scope and spirit of the invention as hereinafter claimed.