Abstract:
A directive monopole antenna element with good RF performance (e.g., directivity and cross-polarization) and a low assembly cost is provided. The directive monopole antenna includes a dielectric support structure and one or more conductive directors coupled to the support structure. Each of the conductive directors is disposed parallel to every other conductive director and in a first plane of the support structure. The directive monopole antenna further includes a conductor coupled to an end of the support structure. The conductor has a feed probe section disposed in the first plane perpendicular to the one or more conductive directors and extending beyond the end of the support structure. The conductor further has a bent section disposed in the first plane parallel to the one or more conductive directors. The feed probe section and the bent section are electrically coupled. The directive monopole antenna element may be fed by a waveguide or a coaxial feed line.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   FIELD OF THE INVENTION 
   The present invention generally relates to monopole antennas and, in particular, relates to directive linearly polarized monopole antennas for use in phased arrays. 
   BACKGROUND OF THE INVENTION 
   In many antenna arrays, it is desirable to use antenna elements that are both highly directive and simple to assemble. One type of endfire antenna element used in various antenna arrays is a “Yagi” element. While Yagi elements exhibit good directivity, the cost and complexity of their assembly (e.g., one half of the driver dipole must be connected to ground, increasing the over-life risk and number of manufacturing steps) leave much to be desired. Another endfire antenna element used in various antenna arrays is a “zigzag” element. A zigzag element can be probe-fed into the waveguide of an array with low assembly cost (e.g., not requiring a connection to ground), but the RF performance of this kind of element is unsuitable for many applications (e.g., having poor cross-polarization and directivity/bandwidth). 
   SUMMARY OF THE INVENTION 
   The present invention solves the foregoing problems by providing a directive monopole antenna element with good RF performance (e.g., directivity and cross-polarization) and low assembly cost and complexity. 
   According to one embodiment of the present invention, a directive monopole antenna comprises a dielectric support structure and a conductor coupled to an end of the support structure. The conductor has a feed probe section disposed in a first plane of the support structure and extending beyond the end of the support structure. The conductor further has a bent section disposed in the first plane perpendicular to the feed probe section. The feed probe section and the bent section are electrically coupled. The directive monopole antenna further comprises one or more conductive directors coupled to the support structure, each of the one or more conductive directors being disposed in the first plane of the support structure and parallel to the bent section of the conductor 
   According to another embodiment of the present invention, an antenna array comprises a plurality of bent directive monopole antenna elements, each of which includes a dielectric support structure and one or more conductive directors coupled to the support structure. Each of the one or more conductive directors is disposed parallel to every other one of the one or more conductive directors and in a first plane of the support structure. Each of the plurality of bent directive monopole antenna elements further includes a conductor coupled to an end of the support structure. The conductor has a feed probe section disposed in the first plane perpendicular to the one or more conductive directors. The conductor further has a bent section disposed in the first plane parallel to the one or more conductive directors. The feed probe section and the bent section are electrically coupled. The antenna array further comprises a ground plane with a plurality of openings corresponding to the plurality of bent directive monopole antenna elements. Each of the plurality of bent directive monopole antenna elements is disposed in one of the plurality of openings in the ground plane. The antenna array further comprises a plurality of waveguides corresponding to the plurality of bent directive monopole antenna elements. Each of the plurality of bent directive monopole antenna elements is fed by a corresponding waveguide. The antenna array further comprises one or more amplifiers operatively coupled to the plurality of bent directive monopole antenna elements. 
   It is to be understood that both the foregoing summary of the invention and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
       FIGS. 1A to 1D  illustrate directive monopole antennas according to various embodiments of the present invention; 
       FIG. 2  illustrates a directive monopole antenna according to one embodiment of the present invention; 
       FIG. 3  illustrates a directive monopole antenna according to one embodiment of the present invention; 
       FIG. 4  illustrates a side view of an antenna array including a plurality of directive monopole antennas according to one embodiment of the present invention; 
       FIG. 5  illustrates an antenna array including a plurality of directive monopole antennas according to one embodiment of the present invention; 
       FIGS. 6A to 6C  illustrate waveguide assemblies for an antenna array according to one embodiment of the present invention; and 
       FIGS. 7 to 9  are graphs illustrating various advantages in performance of directive monopole antennas according to various aspects of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following detailed description, numerous specific details are set forth to provide a full understanding of the present invention. It will be apparent, however, to one ordinarily skilled in the art that the present invention may be practiced without some of these specific details. In other instances, well-known structures and techniques have not been shown in detail to avoid unnecessarily obscuring the present invention. 
   According to various embodiments of the present invention, a highly directive endfire antenna with excellent RF characteristics (e.g., cross-polarization) can be inexpensively manufactured and easily mounted in a ground plane, or in an antenna array with a shared ground plane, without experiencing any of the drawbacks of the Yagi (e.g., over-life risk, manufacturing complexity, etc.) or the zigzag (e.g., poor RF performance) antenna element designs. 
   In  FIG. 1A , a highly directive bent monopole antenna  100  is illustrated according to one embodiment of the present invention. Directive monopole antenna  100  includes support structure  101 , which in this exemplary embodiment is composed of a low-loss dielectric material such as an Ultem® resin. Lying in a plane of support structure  101  and coupled thereto are a number of parallel conductive directors  102 . In the present exemplary embodiment, directors  102  are composed of a conductive material, such as copper. Coupled to one end  103  of support structure  101  is a conductor  104 , which includes a bent section  104   a  parallel to directors  102 , and a feed section  104   b  extending from end  103  and perpendicular to directors  102 . Bent section  104   a  and feed probe section  104   b  are electrically coupled by intermediate section  104   c . In the present exemplary embodiment, conductor  104  is formed by bending a single length of conductive material, such as copper, into the illustrated shape. In alternative embodiments, a conductor such as conductor  104  may be formed by bonding multiple discrete pieces of conductive material, and may comprise numerous different conductive materials. Directive monopole antenna  100  further includes a dielectric plug  105 , which is disposed at end  103  of support structure  101 , surrounding feed probe section  104   b . Dielectric plug  105  allows directive monopole antenna  100  to be coupled with a ground plane without allowing feed probe section  104   b  to come into electrical contact with the ground plane, as is illustrated in greater detail below. 
     FIG. 1B  illustrates another directive monopole antenna  110  according to one embodiment of the present invention. Directive monopole antenna  110  differs from directive monopole antenna  100  illustrated in  FIG. 1A  in the arrangement of conductor  114 . In the present exemplary embodiment, the feed probe section  114   b  and the bent section  114   a  of conductor  114  are electrically coupled by a number of intermediate sections  114   c.    
   In the foregoing exemplary embodiments, directive monopole antennas  100  and  110  are illustrated as being “center-fed,” in that the feed probe sections thereof are disposed approximately in the middle of the ends of the directive monopole antennas. While this arrangement renders the directive monopole antennas relatively insensitive to rotation around the feed probes (with respect to the endfire position of the antennas, but not, obviously, with respect to the polarization thereof), it will be readily apparent to one of skill in the art that the scope of the present invention is not limited to such an arrangement. Indeed, as is illustrated in  FIG. 1C , a directive monopole antenna  120  is configured in an “offset-fed” arrangement, in which the feed probe section  124   b  is disposed closer to one side of the end  123  of the support structure  121 . In such an arrangement, the dielectric plug  125  is similarly disposed nearer to one side of end  123 . As is apparent from  FIG. 1C , in such an arrangement, an intermediate section may not be necessary to electrically couple feed probe section  124   b  to the bent section  124   a  of conductor  124 . 
   Turning to  FIG. 1D , a directive monopole antenna  130  is illustrated in an “electrically center-fed” arrangement, according to one embodiment of the present invention. In this arrangement, the support structure  131  of directive monopole antenna is wider than directors  132 , which are offset to one side of support structure  131 . Conductor  134  includes a feed probe section  134   b , surrounded by a dielectric plug  135 , near the middle of end  133 . The bent section  134   a  of conductor  134  is offset closer to the opposite side of support structure  131  than directors  132 . Such a physical configuration can provide an “electric center” of directive monopole antenna  130  directly above feed probe section  134   b , rendering directive monopole antenna  130  relatively insensitive to rotation about feed probe section  134   b.    
   According to one aspect of the present invention, computer optimization is used to select the dimensions of the conductor and the directors, together with the spacing between them, based upon the desired operating frequencies and performance characteristics of the directive monopole antenna. Turning to  FIG. 2 , an exemplary experimental embodiment of a directive monopole antenna of the present invention is illustrated, together with the dimensions and arrangement of the components thereof. As can be seen with reference to  FIG. 2 , directive monopole antenna  200  includes six conductive directors  202 - 207  with varying dimensions and varying space between them. According to one aspect of the present invention, all of the conductive directors, together with the bent section  208   a  of conductor  208 , are about λ/2 long (i.e., within 20% of λ/2), where λ is an operational frequency of directive monopole antenna  200 . For example, in the present exemplary embodiment optimized for use between about 11.7 GHz and 12.2 GHz (where λ/2 is approximately 0.5″), bent section  208   a  and each one of conductive directors  202 - 207  are within 20% of 0.5″. This is the result of the computer optimization process beginning with a value of λ/2 for the bent section and each one of the conductive directors of the directive monopole antenna, and then iteratively adjusting the length of each component (a process intimately familiar to those of skill in the art), together with the vertical position of each component (i.e., the spacing between each) until the antenna exhibits, in computer simulation, the desired RF performance characteristics (e.g., directivity greater than 15 dB, cross-polarization better than −35 dB, etc.). The “Y” values indicated in  FIG. 2  are a measurement of the distance of each component above a ground plane, a feature included in some embodiments of the present invention, which is illustrated in greater detail with respect to  FIG. 3 , below. 
     FIG. 3  illustrates a directive monopole antenna  300  according to another embodiment of the present invention, in which a ground plane  301  is provided. Ground plane  301  is disposed perpendicular to both support structure  302  and feed probe section  303 . An opening  304  is provided in ground plane  301 , for mounting support structure  302  and exciting the conductor (via feed probe section  303 ). Waveguide plug  305 , which surrounds feed probe section  303 , has an outside diameter and shape approximately equal to the inside diameter and shape of opening  304 , to facilitate the easy mounting of support structure  302  onto ground plane  301 , and to prevent feed probe section  303  of the conductor from coming into electrical contact with ground plane  301 . In various embodiments of the present invention, opening  304  and waveguide plug  305  may be circular, polygonal, or even irregular, as may be required by the design constraints of directive monopole antenna  300 . 
   While the foregoing exemplary embodiments have been described with reference to directive monopole antennas having four, five or six conductive directors, the scope of the present invention is not limited to such arrangements. Rather, as will be readily apparent to one of skill in the art, the present invention has application to directive monopole antennas with any number of directors greater than or equal to one. 
   Turning to  FIG. 4 , an antenna array is illustrated according to one embodiment of the present invention. Antenna array  400  includes a plurality of bent directive monopole antenna elements  401  (similar to those described in greater detail above with respect to  FIGS. 1A to 2 ) mounted in corresponding openings  402  of ground plane  403 . According to one aspect of the present invention, the spacing between adjacent elements  401  is greater than about λ, where λ is an operating wavelength of antenna array  400 . When the antenna array  400  of the present exemplary embodiment is operated in a transmit mode, a signal  408  passes through controllable phase shifter  407   a  and a controllable attenuator  407   b  to amplifier  406  (e.g., a solid state power amplifier “SSPA”), and from amplifier  406  to waveguide filter  405 , which provides the signal in turn to each of the waveguides  404 . In each waveguide  404 , the signal excites a corresponding bent directive monopole antenna element  401 . While the foregoing exemplary embodiment has been described with reference to a transmit mode of operation (e.g., in which amplifier  406  is a SSPA), it will be apparent to one of ordinary skill in the art that the antennas and antenna arrays of the various embodiments of the present invention may be configured to operate in a receive mode (e.g., in which amplifier  406  is a low noise amplifier “LNA”). 
   While the exemplary embodiment illustrated in  FIG. 4  has been illustrated with elements  401  arranged linearly, the scope of the present invention is not limited to such an arrangement. For example, in another embodiment, a 2×2 array of elements may be used. As will be apparent to one of skill in the art, the present invention has application to arrays of any number of antenna elements, disposed in any arrangement. 
     FIG. 5  illustrates an antenna array according to another embodiment of the present invention. Antenna array  500  includes a plurality of bent directive monopole antenna elements  501  mounted in corresponding openings  502  of ground plane  503 . Antenna array  500  includes a waveguide assembly  504  for directing a signal to each element  501 . Waveguide assembly  504  is illustrated in greater detail with respect to  FIGS. 6A to 6C , below. 
   According to the present exemplary embodiment, waveguide assembly  504  includes three stacked plates  504   a ,  504   b  and  504   c , illustrated in  FIGS. 6A ,  6 B and  6 C, respectively. Plate  504   a  illustrated in  FIG. 6A  includes waveguides  505 , each of which feeds a single antenna element  501 . Waveguides  505  are arranged in pairs, such that when plate  504   a  is stacked upon plate  504   b  (illustrated in  FIG. 6B ), each pair of waveguides  505  is fed through one of two ports  506   a  at the end of a single waveguide filter  506 . When plate  504   b  is stacked upon plate  504   c  (illustrated in  FIG. 6C ), each waveguide filter  506  is fed a signal from amplifier port  507 . In this manner, a signal from an amplifier passes through amplifier port  507 , through waveguide filter  506 , through two ports  506   a  to four waveguides  505 , each of which corresponds to a single antenna element  501 . 
     FIG. 7  is a graph illustrating theoretically predicted (e.g., using WIPL-D and NEC) advantages of a directive monopole antenna in directivity and both co-polar and cross-polar isolation according to one embodiment of the present invention. As can be seen with reference to  FIG. 7 , a directivity of 15.1 dB (reference no.  701 ) can be achieved over a receive band of 14.0 GHz to 14.7 GHz, while enjoying better than −30 dB of cross-polarization (reference no.  702 ) relative to peak co-polar over a range of scan angles from about −10° to about 10° (e.g., inside the scan angle of the Earth disk from geostationary orbit). 
     FIG. 8  is a graph illustrating theoretically predicted (e.g., using WIPL-D and NEC) advantages of a directive monopole antenna in directivity and both co-polar and cross-polar isolation according to one embodiment of the present invention. As can be seen with reference to  FIG. 8 , a directivity of 15.1 dB (reference no.  801 ) can be achieved over a transmit band of 11.7 GHz to 12.7 GHz, while enjoying better than −30 dB of cross-polarization (reference no.  802 ) relative to peak co-polar over a range of scan angles from about −10° to about 10° (e.g., inside the scan angle of the Earth disk from geostationary orbit). 
     FIG. 9  is a graph illustrating experimentally confirmed advantages of a directive monopole antenna in directivity and both co-polar and cross-polar antenna patterns according to one embodiment of the present invention. As can be seen with reference to  FIG. 9 , the computer optimized directive monopole antenna  200  of  FIG. 2  enjoys better than −30 dB of cross-polarization (reference no.  901 ) relative to peak co-polar over a range of scan angles from about −10° to about 10° (e.g., inside the scan angle of the Earth disk from geostationary orbit). 
   While the present invention has been particularly described with reference to the various figures and embodiments, it should be understood that these are for illustration purposes only and should not be taken as limiting the scope of the invention. There may be many other ways to implement the invention. Many changes and modifications may be made to the invention, by one having ordinary skill in the art, without departing from the spirit and scope of the invention.