Patent Publication Number: US-6911939-B2

Title: Patch and cavity for producing dual polarization states with controlled RF beamwidths

Description:
This application is a continuation of U.S. application Ser. No. 09/785,033, filed Feb. 16, 2001, entitled, “Method and System for Producing Dual Polarization states with Controlled RF Beamwidths”, the entire contents of which are hereby incorporated by reference. 

   TECHNICAL FIELD 
   The present invention is generally directed to an antenna for communicating electromagnetic signals, and relates more particularly to a planar array antenna having patch radiators exhibiting dual polarization states and producing substantially rotationally symmetric radiation patterns with controlled beamwidths. 
   BACKGROUND OF THE INVENTION 
   Diversity techniques at the receiving end of a wireless communication link can improve signal reception without additional interference. One such diversity technique is generating dual simultaneous polarization states. The term “dual simultaneous polarization states” typically means that an antenna has at least two different radiators, where each radiator simultaneously generates or receives RF energy according to a separate and unique polarization relative to an opposing active radiator. Therefore, unlike circular polarization which employs phasing between respective radiators, dual simultaneous polarization states requires respective radiators to be fed in phase. Those skilled in the art recognize that an antenna&#39;s polarization is defined to be that of its electric field, in the direction where field strength is maximum. 
   Dual polarization states can increase performance of a base station antenna that is designed to communicate with portable communications units having mobile antennas. The effectiveness of dual polarization for a base station antenna relies on the premise that transmit polarization of a typically linearly polarized mobile or portable communications unit will not always be aligned with a vertical linear polarization for the antenna at a base station site nor will it necessarily be in a linearly polarized state. Further, depolarization, which is the conversion of power from a reference polarization into the cross polarization, can occur along the multi-path propagation between the mobile user and a base station. 
   In order to compensate for the effects of depolarization, dual polarization can be employed at a base station antenna in order to communicate with mobile or portable communication units. However, dual polarization or polarization diversity typically requires a significant amount of hardware that can be rather complex to manufacture. Further, conventional dual polarized antennas typically cannot provide symmetrical radiation patterns where respective electric field (E) and magnetic field (H) plane beamwidths are substantially equal. Additionally, conventional antenna systems usually cannot provide for a wide range of magnetic field (H) plane beamwidths from a compact antenna system. In other words, the conventional art typically requires costly and bulky hardware in order to provide for a wide range of operational beamwidths, where beamwidth is measured from the half-power points (−3 dB to −3 dB) of a respective RF beam. 
   Another draw back of the conventional art relates to the manufacturing of an antenna system and the potential for passive intermodulation (PIM) that can result because of the material used in conventional manufacturing techniques. More specifically, with conventional antenna systems, dissimilar materials, ferrous materials, metal-to-metal contacts, and deformed or soldered junctions are used in order to assemble a respective antenna system. Such manufacturing techniques can make an antenna system more susceptible to PIM and therefore, performance of a conventional antenna system can be substantially reduced. 
   A further problem in the conventional art is the ability to effectively control the beamwidth of the resulting radiation patterns of a dual polarized antenna system. The conventional art typically does not provide for any simple techniques for controlling beamwidth of a dual polarized antenna system. 
   Unrelated to the problems discussed above, antenna designers are often forced to design antennas in a backward fashion. For example, because of the increasing public concern over aesthetics and the “environment”, antenna designers are typically required to build an antenna in accordance with a radome that has been approved by the general public, land owners, government organizations, or neighborhood associations that will reside in close proximity to the antenna. Radomes are typically enclosures that protect antennas from environmental conditions such as rain, sleet, snow, dirt, wind, etc. Requiring antenna designers to build an antenna to fit within a radome as opposed to designing or sizing a radome after an antenna is constructed creates many problems for antenna designers. Stated differently, the antenna designer must build an antenna with enhanced functionality within spatial limits that define an antenna volume within a radome. Such a requirement is counterproductive to antenna design since antenna designers recognize that the size of antennas are typically a function of their operating frequency. Therefore, antenna designers need to develop high performance antennas that must fit within volumes that cut against the ability to size antenna structures relative to their operating frequency. 
   Accordingly, there is a need in the art for a substantially compact antenna system that can fit within a predefined volume and that can exhibit dual polarization states while also providing for adjustable beamwidths. There is a further need in the art for a compact dual polarization antenna system that can provide radiation fields having substantially rotationally symmetric radiation patterns. There is also a need in the art for a compact antenna system that can generate RF radiation patterns where the beamwidth of respective RF fields for respective radiating elements are substantially equal and are relatively large despite the compact, physical size of the antenna system. There is a further need in the art for a compact antenna system exhibiting dual polarization states that can also provide for adjustable beamwidths in a fairly simple manner. Further, there is another need in the art for a compact antenna system that can be manufactured with ease and that can utilize manufacturing techniques which substantially reduce passive intermodulation. There is an additional need in the art for a substantially compact antenna system that can handle the power characteristics of conventional antenna systems without degrading the performance of the antenna system. 
   SUMMARY OF THE INVENTION 
   The present invention solves the aforementioned problems with an antenna system that can generate RF radiation fields having dual simultaneous polarization states and having substantially rotationally symmetric radiation patterns. The term “rotationally symmetric” typically means that radiation patterns of respective radiators having different polarizations are substantially symmetrical and substantially equal. In other words, the present invention can generate RF radiation patterns where the beamwidths of respective RF fields for respective radiating elements are substantially equal and are relatively large despite the compact, physical size of the antenna system. For example, the present invention can produce radiation patterns where each RF polarization produced by an individual radiating element is substantially equal to a corresponding orthogonal RF polarization produced by another individual radiating element. For example, the beamwidths produced by each radiating element can be adjusted from widths of approximately sixty-five (65) to ninety (90) degrees, where beamwidth is measured from the half-power points (−3 dB to −3 db) of a respective RF beam. Other beamwidths are not beyond the scope of the present invention. 
   This enhanced functionality can be achieved with a compact antenna system, where the antenna system (without a radome) can typically have a height of approximately less than one seventh ({fraction (1/7)}) of a wavelength and a width that is less than or equal to six-tenths (0.6) of a wavelength. With a radome, the antenna system can have a height of approximately one-fifth (⅕) of a wavelength. The antenna system can comprise one or more patch radiators and a non-resonant patch separated from each other by an air dielectric and by relatively small spacer elements. The patch radiators and non-resonant patch can have predefined shapes for increasing polarization discrimination. 
   In one exemplary embodiment, the patch radiators and non-resonant patch can have a substantially circular shape. The circular shape can enable the patch radiators and non-resonant patch to maintain orthogonality of two polarizations over a given angular region to ensure that any two RF signals are highly de-correlated. The circular shape of the patch radiators can also keep E (electric field) and H (magnetic field) plane beamwidths of individual radiating elements substantially equal and symmetrical. 
   The beamwidth of RF energy generated by one or more lower resonant patch radiators can be controlled by an upper non-resonant patch. The upper non-resonant patch is typically spaced at a non-resonant distance relative to the lower patch radiators to prevent resonance while controlling the beamwidth of the resultant RF radiation patterns. 
   The lower patch radiators can be mounted to a printed circuit board that can comprise an RF feed network and a ground plane which defines a plurality of symmetrically, shaped slots. In one exemplary embodiment, the slots can comprise a double-H shape that has an electrical path length that is less than or equal to a half wavelength. 
   The slots within the ground plane of the printed circuit board can be excited by stubs that are part of the feed network of the printed circuit board. The slots, in turn, can establish a transverse magnetic mode of RF radiation in a cavity which is disposed adjacent to the ground plane of the printed circuit board and a ground plane of the antenna system. 
   The slots can be aligned along a diagonal of a cavity while the cavity can be concentrically aligned with geometric centers of the patch radiators. The feed network of the printed circuit board can be aligned with portions of the cavity such that the portions of the cavity function as a heat sink for absorbing or receiving thermal energy produced by the feed network. Because of this efficient heat transfer function, the printed circuit board can comprise a relatively thin dielectric material that is typically inexpensive. 
   The cavity disposed between the printed circuit board and the ground plane of the antenna system can function electrically as a closed boundary when mechanically, the cavity has open corners. The open corner design facilitates ease in manufacturing the cavity. The open corners of the cavity can also have dimensions that permit resonance while substantially reducing Passive Intermodulation (PIM). 
   PIM can be further reduced by planar fasteners used to attach respective flanges and a planar center of a respective cavity to the ground plane of the printed circuit board and the ground plane of the antenna system. The planar fasteners can comprise a dielectric adhesive. In addition to the dielectric adhesive, the present invention can also employ other types of fasteners that reduce the use of dissimilar materials, ferrous materials, metal to metal contacts, deformed or soldered junctions and other similar materials in order to reduce PIM. 
   For example, the patch radiators and non-resonant patch can be spaced apart by plastic fasteners that permanently “snap” into place. Such fasteners not only reduce PIM, but such fasteners substantially reduce labor and material costs associated with the manufacturing of the antenna system. 
   While providing a product that can be manufactured efficiently, the present invention also provides an efficient RF antenna system. The RF energy produced by the cavity, slots, and stubs can then be coupled to one or more lower patch radiators. The one or more lower patch radiators can then resonate and propagate RF energy with a wide range of H-plane beamwidths that can extend between approximately sixty-five (65) and ninety (90) degrees. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration showing an elevational view of the construction of an exemplary embodiment of the present invention. 
       FIG. 2  is an illustration showing a side view of the exemplary embodiment shown in FIG.  1 . 
       FIG. 3  is an illustration showing an isometric view of the exemplary embodiment shown in  FIGS. 1 and 2 . 
       FIG. 4  is an illustration showing an isometric view of some core components of an exemplary embodiment of the present invention. 
       FIG. 5  is a cross-sectional view of the exemplary embodiment illustrated in  FIG. 4  taken along the cut line  5 — 5 . 
       FIG. 6  is a block diagram illustrating some of the core components of the exemplary embodiment illustrated in FIG.  5 . 
       FIG. 7A  is an illustration showing an elevational view of the exemplary embodiment illustrated in  FIG. 4  while also showing hidden views of the slots which feed the cavity and one or more radiating elements. 
       FIG. 7B  is an illustration showing an exemplary slot according to the present invention. 
       FIG. 8  is an illustration showing an exploded view of an exemplary embodiment of the present invention. 
       FIG. 9  is an illustration showing a bottom or rear view of a ground plane of the printed circuit board comprising the feed network as illustrated in FIG.  8 . 
       FIG. 10A  is an illustration showing an isometric view of an exemplary resonant cavity for the present invention. 
       FIG. 10B  is an illustration showing an enlarged area focused on an exemplary corner structure of the resonant cavity shown in FIG.  10 A. 
       FIG. 11  is an illustration showing a typical mounting arrangement for an antenna provided by an exemplary embodiment of the present invention. 
       FIG. 12A  is a graph illustrating the beamwidths of (E) and (H) plane radiation patterns according to one exemplary embodiment of the present invention. 
       FIG. 12B  is a radiation pattern in terms of voltage illustrating the beamwidths of (E) and (H) planes according to the exemplary embodiment illustrated in FIG.  12 A. 
       FIG. 13A  is a graph illustrating beamwidths of another (E) and (H) plane radiation pattern according to an alternative exemplary embodiment of the present invention. 
       FIG. 13B  is a radiation pattern in terms of voltage illustrating the beamwidths of (E) and (H) planes according to the alternative exemplary embodiment illustrated in FIG.  13 A. 
       FIG. 14  is an exemplary logical flow diagram describing a method for producing dual simultaneous polarization states and a rotationally symmetric radiation pattern where the electric field and magnetic field beamwidths of individual radiating elements are substantially equal and symmetrical. 
       FIG. 15  is a logical flow diagram illustrating an exemplary slot excitation routine of FIG.  14 . 
       FIG. 16  is another logical flow diagram illustrating an exemplary beamwidth adjustment routine of FIG.  14 . 
   

   DETAILED DESCRIPTION OF THE PRESENT INVENTION 
   The antenna of the present invention can solve the aforementioned problems and is useful for wireless communications applications, such as personal communication services (PCS) and cellular mobile radio telephone (CMR) service. The antenna of the present invention can use polarization diversity to mitigate the deleterious effects of fading and cancellation resulting from a complex propagation environment. The antenna system can include a patch radiator, a printed circuit board disposed adjacent to the patch radiator, and plurality of slots disposed within a ground plane of the printed circuit board. The antenna further includes a cavity disposed adjacent to the ground plane of the printed circuit board and a second ground plane disposed adjacent to the cavity. The antenna system radiates RF energy having dual simultaneous polarization states and having substantially rotationally symmetric radiation patterns. 
   Turning now to the drawings, in which like reference numerals refer to like elements,  FIG. 1  is an illustration showing an elevational view of one exemplary embodiment of the present invention. Referring now to  FIG. 1 , an antenna system  100  is shown for communicating electromagnetic signals with the high frequency spectrums associated with conventional wireless communication systems. An antenna system  100  can be implemented as a planar array of radiating elements  110 , known as wave generators or radiators, wherein the array is positioned along a vertical plane of the antenna as viewed normal to the antenna site. 
   The antenna system  100 , which can transmit and receive electromagnetic signals, includes radiating elements  110 , a ground plane  120 , and a feed network  130 . The antenna system  100  further includes beam shaping elements  140 , a printed circuit board  150  and ports  160 A and  160 B. 
   Referring now to  FIG. 2  which illustrates the side view of the antenna system  100  of  FIG. 1 , the spatial relationship between beam forming elements  140  and the radiating elements  110  are more clearly shown. On a side of the printed circuit board  150  opposite to the radiating elements  110  and beam forming elements  140  are a plurality of cavities  200  which will be discussed in further detail below. The ports  160 A and  160 B can comprise coaxial cable type connectors. 
     FIG. 3  further illustrates an isometric view of the antenna system  100  which can comprise one or more radiating elements  110  and beam forming elements  140 . The antenna system  100  as illustrated in  FIG. 3  is very compact yet high performance product that can be placed or positioned in a very narrow or small volume such as a radome. For example, in one exemplary embodiment, the length L can be approximately 48 inches while the width W can be approximately 8 inches. The height H of the antenna system  100  (including a radome) can be 2.75 inches. In this exemplary embodiment the operating frequency range is approximately from 806 MHz to 896 MHz. In terms of wavelength, this means that the width W can be less than or equal to a six-tenths (0.6) of a wavelength. Similarly, the height H can be less than or equal to one-seventh ({fraction (1/7)}) of a wavelength without a radome. With a radome, the antenna system can have a height of approximately one-fifth (⅕) of a wavelength. The length L can be varied depending upon the number of radiating elements  110  desired to be in the antenna system  100 . 
   Referring now to  FIG. 4 , this figure illustrates some of the core components of antenna system  100  in more enlarged detail.  FIG. 4  illustrates how ground plane  120  further includes groves  400  that can support a radome (as will be discussed in further detail below). As mentioned above, the present invention can include one or more radiating elements  110  while, typically (in an exemplary embodiment), only one beam forming element  140  is employed. 
   Referring now to  FIG. 5 , this figure illustrates a cross-section of the antenna system  100  illustrated in FIG.  4 . This particular cross-section is taken along the cut line  5 — 5  as illustrated in FIG.  4 .  FIG. 5  provides further details of the mechanical elements which form the inventive antenna system  100 . The sizes of materials illustrated in  FIG. 5  are not shown to scale. In other words, some of the materials have been exaggerated in size so that these materials can be seen easily. A more accurate depiction of the relative sizes of materials will be illustrated below with respect to FIG.  11 . 
   The beam forming element  140  is spaced from the radiating element  110  by a spacing S 1 . Spacing S 1  is typically a nonresonant dimension. That is, the parameter S 1  relative sizes is typically neither a resonant dimension nor a dimension that promotes resonance of the beam forming element  140 . The beamwidth of the present invention can be controlled by adjusting the spacing parameter S 1  and by adjusting the diameter D 140  of the beam forming element  140 . The diameter D 140  is also typically a non-resonant dimension. 
   By increasing the spacing parameter S 1  (the space between the beam forming element  140  and the radiating element  110 ) the beamwidth of the electromagnetic radiation emitted by the antenna system  100  can be increased. Conversely, beamwidth can be decreased by lowering the S 1  parameter (decreasing the spacing between the upper and lower patches) and by increasing the diameter D 140  of the beam forming element  140 . 
   The radiating antenna element  110  can be spaced from the printed circuit board  150  by a spacing parameter S 2  which is typically a resonant value. In other words, the parameter S 2  is one that typically promotes resonance of the radiating patch element  110 . In terms of wavelength, the parameter S 2  is typically between 0.03 to 0.04 wavelengths (or 0.42 to 0.55 inches at the exemplary operating frequency range). The diameter D 110  of the radiating element is typically between 0.40 to 0.47 wavelengths. However, the present invention is not limited to these values. Other resonant dimensions are not beyond the scope of the present invention. 
   The beam forming element  140  is typically held in place relative to the radiating element  110  by spacer elements/fasteners  500  which can comprise dielectric stand-offs. The radiating element  110  is similarly spaced from the printed circuit board  150  by a plurality of spacers/fasteners  500 . The spacers/fasteners  500  are typically designed to permanently “snap” into place in order to eliminate or reduce the use of soldering points of the present invention. This, in turn, also substantially reduces work in the manufacturing process of the Antenna System  100 . Further, by using such spacers/fasteners passive intermodulation (PIM) can also be substantially reduced or eliminated. However, the present invention is not limited to “snap” type fasteners. Other fasteners or dielectric supports that can reduce PIM are not beyond the scope of the present invention. For example, slim or narrow blocks of dielectric foams could be used to support the radiating elements  110  and beam forming elements  140 . Also, the fasteners  500  do not need to permanently fix these elements. That is, releasable fasteners  500  could be employed and not depart from the scope and spirit of the present invention. 
   As illustrated in  FIGS. 4 and 5 , the beam forming element  140  and the radiating element  110  typically comprise patch elements. The beam forming element  140  and radiating element  110  are typically made from conductive materials such as aluminum. Specifically, both elements can be made from aluminum  5052 . Similarly, the cavity  200  can also be constructed from aluminum. However, other conductive materials are not beyond the scope of the present invention for the resonating structures. Further, the radiating element  110  and beam forming element  140  can also be constructed with combinations of materials such as dielectric materials coated with a metal. Those skilled in the art will appreciate the various ways in which radiating elements can be constructed without departing from the scope and spirit of the present invention. 
   In one preferred exemplary embodiment, both the beam forming element  140  and Radiating Element  110  are substantially circular in shape. The circular shape of the patches  140 ,  110  in combination with the apertures or slots  700  (as will be discussed below) and resonating cavity  200  increase polarization discrimination by the antenna system  100 . The circular shape of the Patches  140 ,  110  can also contribute to maintaining the orthogonality of two polarizations over a given angular region to ensure that any two RF signals are highly de-correlated. 
   The circular shape of the beam forming element  140  and radiating element  110  can also maximize the performance of the polarization by keeping the electric (E) and magnetic (H) plane beamwidth substantially equal. The circular shape of the beam forming element  140  and radiating element  110  also permits the antenna system  100  to keep radiation patterns symmetrical. However, the present invention is not limited to circularly shaped elements. Other shapes include, but are not limited to, square, rectangular, and other similar shapes that maximize the performance of dual polarization by keeping electric (E) and magnetic (H) plane beamwidth substantially equal. 
     FIG. 5  illustrates further details of the antenna system  100  that are not shown in the previous figures. For example, portions of the feed network  130  are substantially aligned over portions of the cavity  200 . By aligning portions of the feed network  130  over portions of the cavity  200 , such as flanges  520  (as will be discussed in further detail below) the present invention can dissipate heat energy formed within the feed network  130  more efficiently and rapidly. The flanges  520  can serve as a heat sink to portions of the feed network  130 . 
   By using portions of the resonating  200  cavity as a heat sink, a relatively thin printed circuit board  150  can be used. The cavity  200  can be fastened to the printed circuit board  150  (and more specifically, the ground plane  530  of the printed circuit board  150 ) by using a planar fastener  540  such as a dielectric adhesive. This planar fastener  540  can then reduce the thermal resistance between the feed network  130  and the flange  520 . 
   The cavity  200  can also be attached to the ground plane  120  with a similar planar fastener  540  such as a dielectric adhesive discussed above. Using such fasteners not only reduces the thermal resistance between the feed network  130  and the cavity, it also substantially reduces passive intermodulation (PIM). With portions of the cavity  200  functioning as a heat sink for the feed network  130  disposed upon a printed circuit board  150 , a relatively thin substrate of material can be used as the printed circuit board  150 . The cavity  200  is attached to the ground plane  530  of the printed circuit board  150  with a planar fastener  540 . Similarly, the cavity  200  is attached to the radome supporting ground plane  120  by a planar fastener  540 . 
   The cavity  200  typically propagates a transverse magnetic (TM 01 ) mode of RF energy for the two polarizations supported by the antenna system  100 . Since cavity  200  resonates, the height or spacing S 3  of the cavity has a resonant dimension of 0.027 wavelengths (or a dimension of 0.375 inches at the exemplary operating frequency). The width W 1  of the resonant cavity  200  can have a resonant dimension of 0.433 wavelengths. However, the present invention is not limited to these values. Other resonant dimensions are not beyond the scope of the present invention. While propagating a transverse magnetic mode of RF energy, cavity  200  can also substantially increase the front to back ratio of the antenna system  100 . The cavity  200  is excited by two or more slots  700  as will be discussed in further detail below. 
     FIG. 6  is a functional block diagram illustrating the various components which make up the compact antenna system  100 . This figure highlights one exemplary and preferred arrangement of the components of the antenna system  100 . Of the components illustrated in  FIG. 6 , there are a select few which may be considered the core components of the Antenna System  100  that provide the enhanced functionality in such a compact antenna volume. The core components may be considered as the beam forming element  140 , the radiating element  110 , the printed circuit board  150 , the ground plane  530  with slots  700 , and the cavity  200 . 
   Referring now to  FIG. 7A , further details of the slots  700 A-C disposed within the ground plane  530  are shown. The slots  700 A-C are excited by a corresponding number of stubs  710 A-C that are positioned within the feed network  130  disposed on one side of the Printed Circuit Board  150 . The slots  700  are typically symmetrically-shaped in order to reduce cross-polarization between respective slot. The slots  700 A,  710 C are oriented perpendicular to the central slot  700 B. Such an orientation of the slots  700  sets up or establishes dual polarization states. 
   Further, it is desirable to orient the slots  700  along geometric diagonals  720 A and  720 B in order to maintain slant forty-five polarizations over an intended region of operation while improving port-to-port isolation. Placing the slots  700  along the geometric diagonals  720 A and  720 B can also reduce cross-polarization between the two dual polarization states established by the antenna system  100 . The slots  700  are also designed to be narrow and symmetrical in order to increase port-to-port isolation. The spacing and orientation of the slots  700  relative to the radiating element  110  can optimize the desired transverse magnetic TM 01  mode of operation within the resonating cavity  200  for the two polarizations. In this embodiment, two orthogonal TM 01  modes are generated in the cavity  200 . 
   Optimization can be accomplished by placing these slots  700  along the geometric diagonals  720 A,  720 B and using the center of the cavity  200  as the origin for the radiating patches  110 . That is, the geometric centers of the radiating element  110 , beam forming element  140 , and cavity  200  can be substantially aligned. However, the present invention is not limited to this number and combination of slots. For example, instead of three separate slots the present invention could employ a cross-shaped slot (not shown) to feed the antenna patches. But with this cross-shaped design, two soldering connections would be required for a respective crossed-slot. And soldering connections could degrade antenna performance somewhat because of the resulting PIM. 
   Referring now to  FIG. 7B , the slots  700  can also have a predefined shape. For example, in one exemplary embodiment, each Slot  700  have the substantially double-h shape. However, the present invention is not limited to this shape. Other shapes include, but are not limited to, shapes that have an electrical length that is less than or equal to one-half the wavelength. The electrical length of a slot is typically found by measuring the one-half of the perimeter of the opening, starting at one far end of the slot to another far end. An electrical length of less than or equal to one-half of a wavelength facilitates efficient coupling of RF energy to the cavity  200  and patch radiating element  110 . The orientation and placement of the slot  700  should be designed for equal beamwidths of the polarizations so that the polarization factor can be maintained at a value of 45. 
   Referring now to  FIG. 8 , this figure illustrates an exploded view of the components of the antenna system  100 . A protective radome  800  comprising a PVC material can be used to cover the antenna system  100 . A radome  800  preferably comprises a PVC material manufactured in the desired form by an extrusion process. The radome  800  is attached to the grooves  400  formed in the ground plane  120 . A pair of end caps  810 A and  810 B are positioned along a minor dimension at an end of the ground plane  120  and cover the remaining openings formed at the end of the combination of the ground plane  120  and the radome  800 . Encapsulation of the antenna system  100  within the sealed enclosure formed by the ground plane  120 , a radome  800 , and the end caps  810 A-B protects the antenna system  100  from environmental elements, such as direct sunlight, water, dust, dirt and moisture. 
   The printed circuit board  150  is a relatively thin sheet of dielectric material and can be one of many low-loss dielectric materials used for the purpose of radio circuitry. In one preferred and exemplary embodiment, the material used can have a relative dielectric constant values of d k =3.38 (and ε r =2.7—when substrate is used as microstrip). In the preferred exemplary environment, teflon-based substrate materials are typically not used in order reduce cost. However, TEFLON-based and other dielectric materials are not beyond the scope of the present invention. Disposed adjacent to the printed circuit board  150  is the ground plane  530  which is illustrated with further detail in FIG.  9 . 
   Referring now to  FIG. 9 , the ground plane  530  contains the slots  700  used to excite the cavity  200 . These slots  700  can be preferably etched out of the ground plane  530  by photolithography techniques. 
   Referring now to  FIG. 10A , this figure further illustrates the details of the resonant cavity  200 . The cavity  200  is preferably made from aluminum and has a design which promotes accurate repeatability while substantially reducing passive intermodulation (PIM). However, other conductive materials are not beyond the scope of the present invention. The cavity  200  comprises walls  1000 A-D that are spaced apart from each other by a predetermined distance d (See FIG.  10 B). This predetermined distance d between the walls  1000  at the corners allows for reasonable tolerances in manufacturing, but is typically small enough such that the cavity  200  electrically operates as a closed boundary for RF energy propagating within the cavity  200 . In other words, the cavity  200  can function electrically as a closed boundary when mechanically the cavity has open corners. The open corners of the cavity typically have dimensions that permit resonance while substantially reducing passive intermodulation (PIM). The open corners of the cavity also function as drainage holes for any condensation that may form within a respective cavity  200 . 
   Referring now to  FIG. 10B , a distance d exists between cavity walls  1000 C and  1000 D. As mentioned above, distance d is sized such that the cavity can resonate while at the same time it can substantially reduce passive intermodulation since there is no metal-to-metal contact between the respective walls  1000 C and  1000 D. PIM is further reduced by the present invention because dissimilar materials, ferrous materials, metal-to-metal contacts, and deformed or soldered junctions are preferably not used in order to substantially reduce or eliminate this physical phenomenon. 
   For example, in addition to the open corners of the cavity  200 , the present invention employs (as discussed above) planar fasteners  540  to attach the Flanges  520  of the cavity  200  to the ground plane  530  of the printed circuit board  120 . Meanwhile, the base of the cavity  200  can be attached to the radome-supporting ground plane  120  by another dielectric planar fastener. Similarly, the radiating element  110  is supported by non-soldered spacers/fasteners  500 , and also supports additional spacers/fasteners  500  to support the beam forming element  140 . 
   Referring now to  FIG. 11 , this figure further illustrates a more accurate depiction of the relative sizes (thickness) of materials which make up the antenna system  100 . Further mechanical details of the spacers/fasteners  500  are shown. As mentioned previously, these spacers/fasteners are preferably constructed from dielectric materials to reduce (PIM) while also permitting ease of manufacturing of the antenna system  100 . That is, the spacers/fasteners  500  can be permanently “snapped” into place without the use of any deformed or soldered junctions. 
   Referring now to  FIG. 12A , this figure illustrates a linear plot of antenna gain versus the angular position of a radiation pattern for a ninety (90) degree beamwidth embodiment of antenna system  100 . That is, this graph illustrates the gain for an antenna system  100  designed to have 90 degrees of coverage between respective three (3) dB or half power points in a radiation pattern. This graph demonstrates that the (E) and (H) beamwidth of an independent polarization are substantially equal. Substantially equal (E) and (H) plane beamwidths will maintain the orthogonality of the two polarization states over a given angular region to insure that two received signals are highly decorrelated. Two polarization states are not shown in  FIG. 12A , only one polarization state with substantially equal E and H planes is illustrated. For this particular exemplary embodiment, the angular region has been designed for 90 degrees. 
   To obtain a 90 degree beamwidth the diameter and spacing S 1  of the gain forming element  140  can be adjusted. As noted above, to increase the (E) and (H) plane beamwidth, the spacing between the beam forming element  140  and the radiating element  110  is increased, while the diameter of the Beam Forming Element  140  can be reduced. Conversely, to decrease the (E) and (H) plane beamwidth, the separation S 1  between the beam forming element  140  and the radiating element  110  can be decreased while the diameter D 140  of the beam forming element can be increased. With the present invention, it is possible to maintain about five degrees of difference between 3 dB beamwidths of respective (E) and (H) plane radiation patterns of a particular polarization. 
   Referring to  FIG. 12B , this figure is a radiation pattern in polar coordinates and in terms of voltage illustrating the ninety (90) degree beamwidth embodiment discussed in FIG.  12 A. The pattern illustrates the (E) plane radiation pattern with a solid line and the (H) plane pattern with a dashed or dotted line. 
   Referring now to  FIG. 13A , this figure illustrates a plot of antenna gain versus the angular position of a radiation pattern for a sixty-five (65) degree beamwidth embodiment of antenna system  100 . That is, this graph illustrates the gain for an antenna system  100  designed to have 65 degrees of coverage between respective three (3) dB or half power points in a radiation pattern. This graph also demonstrates that the (E) and (H) beamwidth of an independent polarization are substantially equal. Substantially equal (E) and (H) plane beamwidths will maintain the orthogonality of the two polarization states over a given angular region to insure that two received signals are highly decorrelated. Two polarization states are not shown in  FIG. 13A , only one polarization state with substantially equal E and H planes is illustrated. 
   Referring to  FIG. 13B , this figure is a radiation pattern in polar coordinates and in terms of voltage illustrating the sixty-five (65) degree beamwidth embodiment discussed in FIG.  13 A. The pattern illustrates the (E) plane radiation pattern with a solid line and the (H) plane pattern with a dashed or dotted line. 
     FIG. 14  illustrates a logical flow diagram  1400  for a method of generating RF radiation fields having dual, simultaneous polarization states and having substantially rotationally symmetric radiation patterns. The logical flow diagram  1400  highlights some key functions of the antenna system  100 . 
   Step  1410  is the first step of the inventive process  1400  in which the slot  700  disposed within the ground plane  530  are oriented orthogonal to one another. By orienting the slots orthogonal to one another in step  1410 , isolation between separate RF polarizations can be maintained while cross-polarization can be reduced. 
   Next, in step  1420 , the antenna system  100  is assembled without metal-to-metal contacts and soldering. More specifically, in this step, the antenna system  100  can be manufactured in a way to substantially reduce passive intermodulation (PIM). Dissimilar materials, ferrous materials, metal-to-metal contacts, and deformed or soldered junctions are typically not employed or are limited in the antenna system  100  in order to substantially reduce or eliminate PIM. One way in which PIM is substantially reduced or eliminated is the use of dielectric planer fasteners  540  in order to connect portions of the cavity  200  to the slotted ground plane  530  and the ground plane  120 . Another way in which PIM is reduced or substantially eliminated is by employing open corners in the cavity  200  where respective walls, such as walls  1000 C and  1000 D of  FIG. 10B , are spaced apart by the predetermined distance d. 
   Next, in step  1430  RF energy is propagated along the feed network  130  of the printed circuit board  150 . In step  1440 , heat is dissipated from the feed network  130  into flanges  520  of the cavity  200 . 
   In routine  1450 , the slots disposed in ground plane  530  set up or establish a transverse magnetic (TM) mode of RF energy in the cavity  200 . Further details of routine  1450  will be discussed in further detail below with respect to FIG.  15 . 
   In step  1460 , the radiating elements such as the lower patch radiators  110  are excited with RF energy emitted from the slot  700  or the stubs  710  or both. Next, in step  1470 , RF radiation is produced with nearly equal dual polarizations by the substantially compact antenna system  100 . In routine  1480 , the nearly equal dual polarizations are maintained and beamwidths can be adjusted with the beam shaping element  140 . Further details of routine  1480  will be discussed below with respect to FIG.  16 . 
     FIG. 15  illustrates an exemplary slot excitation routine  1450  of FIG.  14 . Routine  1450  begins with step  1500 . In step  1500 , the slots  700  are aligned along geometric diagonals  720  of the cavity  200 , as illustrated in FIG.  7 . This alignment of the slots  700  produces a desired transverse magnetic mode of RF energy in the cavity  200  while substantially reducing cross-polarization and increasing isolation between respective ports  160 A and  160 B. 
   Next, in step  1510 , the slots  700  are shaped to be symmetrical and sized such that each slot  700  has an effective electrical length of less than or equal to a half wavelength for efficient RF coupling to or from the feed network  130  and the cavity  200  or radiating patch  110 . The routine then returns to step  1460  of FIG.  14 . 
     FIG. 16  illustrates an exemplary beam width adjustment routine  1480  of FIG.  14 . Routine  1480  begins with step  1600 , in which it is determined whether the beamwidth of the antenna system  100  needs adjustment. If the inquiry to decision step  1600  is positive, then the “yes” branch is followed to step  1610 . In step  1610 , the beamwidth of the antenna system  100  can be adjusted by changing the spacing between the beam forming element  140  and the radiating element  110 . Typically, the spacing is of a non-resonant dimension since in one exemplary feature of the present invention, the beam forming element  140  does not resonate RF energy. If the inquiry to decision step  1600  is negative, then the “no” branch is followed to step  1640 . 
   In step  1620 , it is determined whether further beamwidth adjustment is needed. If the inquiry to decision step  1620  is positive, then the “yes” branch is followed to step  1630 , in which the beamwidth of the antenna system  100  can be adjusted by changing the diameter of the beam forming element  140 . It is noted that the present invention is not limited to the sequence or chronology of steps illustrated in these logic flow diagrams. Therefore, one of ordinary skill in the art recognizes that the beamwidth of antenna system  100  can be first adjusted by changing the diameter of the beam forming element  140  instead of first changing the spacing between the beam forming element  140  and radiating element  110 . Further, those skilled in the art will also recognize that adjustments to beamwidth can also be made by changing both the spacing between the beam forming element  140  and the radiating element  110 , as well as changing the size of the beam forming element  140 . In step  1640 , the routine returns to FIG.  14 . 
   The present invention provides an aperture or slot coupled patch elements that generate dual slant 45 degree polarization in addition to substantially rotationally symmetric radiation patterns. The present invention generates RF radiation patterns where the beamwidths of respective RF fields for respective radiating elements are substantially equal and are relatively large despite the compact, physical size of the antenna system. For example, the present invention produces radiation patterns where each RF polarization produced by an individual radiating element is substantially equal to a corresponding orthogonal RF polarization produced by another individual radiating element. 
   The present invention provides a compact antenna system that has a height (without radome) of less than one-seventh ({fraction (1/7)}) of a wavelength and a width that is less than or equal to one-half of a wavelength. With a radome, the height can be one-fifth (⅕) of a wavelength. While being compact, the present invention is power efficient. The present invention incorporates an efficient heat transfer design such that a feed network transfers its heat to a resonating cavity used to set up desired transverse magnetic modes of RF energy. The efficient heat transfer permits the present invention to utilize relatively thin dielectric materials for the printed circuit board supporting the feed network. 
   The present invention employs circular metallic radiating elements for the purpose of obtaining circular and symmetric (E) and (H) plane 3 dB beamwidths having simultaneous slant  45  dual polarization states. The spacing S 2  of the radiating element  110  relative to the printed circuit board  150  and the diameter of the radiating element  110  is used to improve the impedance beamwidths of the antenna system  100 . The beam forming element  140  is used to vary the 3 dB beamwidths to obtain desired values by adjusting its diameter and varying its spacing S 1  between the radiating element  110  and the beam forming element  140 . The present invention further incorporates a low PIM design approach by utilizing capacitive coupling of all potential metal-to-metal junctions through employing non-conductive planar fasteners and open corners for the resonant cavity  200 . The low PIM design approach also yields efficient and low cost manufacturing methods. For example, the planar fasteners  540  eliminate any need for soldering the resonant cavity  200  to the ground plane  530 . The use of dielectric spacers  500  further eliminates any need for costly dielectric spacer sheets while also reducing assembly time. 
   The present invention also employs two orthogonal forty-five degree slanted slots that are non-collocated along perpendicular lines of symmetry at forty-five degrees from an array axis. Such slots eliminate a need for a feed line to cross over to provide improved cross-polarization and port-to-port isolation. 
   Alternative embodiments will become apparent to those skilled in the art to which the present invention pertains without departing from its spirit and scope. Thus, although this invention has been described in exemplary form with a certain degree of particularity, it should be understood that the present disclosure has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description.