Abstract:
A global navigation satellite system (GNSS) antenna system includes interference mitigation and multipath canceling. Multiple ports or phased arrays of antennas can be provided. Antennas can comprise controlled radiation pattern antennas (CRPA). Crossed dipole and patch antenna configurations can be utilized.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND PATENT 
       [0001]    This application is related to and claims priority in U.S. patent application Ser. No. 61/720,915, filed Oct. 31, 2012; Ser. No. 61/720,891, filed Oct. 31, 2012; Ser. No. 61/720,905, filed Oct. 31, 2012; and Ser. No. 61/732,787, filed Dec. 3, 2012, all of which are incorporated herein by reference. U.S. Pat. No. 8,102,325 is also incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to antennas, and in particular, to broadband antennas which are particularly well-suited for GNSS applications and which include antenna components formed of polytetrafluoroethelyne (PTFE) materials. 
         [0004]    2. Description of the Related Art 
         [0005]    Various antenna designs and configurations have been produced for transmitting and receiving electromagnetic (wireless) signals. Antenna design criteria include the signal characteristics and the applications of the associated equipment, i.e., transmitters and receivers. For example, stationary, fixed applications involve different antenna design configurations from mobile equipment. 
         [0006]    Global navigation satellite systems (GNSS) have progressed within the last few decades to their present state-of-the-art, which accommodates a wide range of positioning, navigating, and informational functions and activities. GNSS applications are found in many industries and fields of activity. For example, navigational and guidance applications involve portable GNSS receivers ranging from relatively simple, consumer-oriented, handheld units to highly sophisticated airborne and marine vessel equipment. 
         [0007]    Vehicle-mounted antennas are designed to accommodate vehicle motion, which can include movement in six degrees of freedom, i.e., pitch, roll and yaw corresponding to vehicle rotation about X, Y and Z axes in positive and negative directions respectively, as well as translations along such axes. Moreover, variable and dynamic vehicle attitudes and orientations necessitate antenna gain patterns which provide GNSS ranging signal strengths throughout three-dimensional ranges of motion corresponding to the vehicles&#39; operating environments, for example, aircraft in banking maneuvers that the require below-horizon signal reception. Ships and other large marine vessels, on the other hand, tend to operate relatively level and therefore normally do not require below-horizon signal acquisition. Terrestrial vehicles have varying optimum antenna gain patterns dependent upon their operating conditions. Agricultural vehicles and equipment, for example, often require signal reception in various attitudes in order to accommodate operations over uneven terrain. Modern precision agricultural GNSS guidance equipment, e.g., sub-centimeter accuracy, requires highly efficient antennas which are adaptable to a variety of conditions. 
         [0008]    Another antenna/receiver design consideration in the GNSS field relates to multipath interference, which is caused by reflected signals that arrive at the antenna out of phase with the direct signal. Multipath interference is most pronounced at low elevation angles of reception, e.g., from about 10 to 20 degrees above the horizon. They are typically reflected from the ground and ground-based objects. Antennas with strong gain patterns at or near the horizon are particularly susceptible to multipath signals, which can significantly interfere with receiver performance based on direct line-of-sight (LOS) reception of satellite ranging signals and differential correction signals (e.g., DGPS). Therefore, important GNSS antenna design objectives include achieving the optimum gain pattern, balancing rejecting multipath signals, and receiving desired ranging signals from sources, e.g., satellites and pseudolites, at or near the horizon. 
         [0009]    Because it is desirable to improve the accuracy, reliability, and confidence level of an attitude or position determined through use of a GNSS, a Satellite-Based Augmentation System (SBAS) may be incorporated if one that is suitable is available. There are several public SBASs that work with GPS. These include the Wide Area Augmentation System (WAAS), developed by the United States Federal Aviation Authority, European Geostationary Navigation Overlay Service (EGNOS), developed by the European Community, as well as other public and private pay-for-service systems such as OmniSTAR®. 
         [0010]    Conventional GPS antennas include ceramic patch, cross dipole, and microstrip patch configurations. Ceramic patch designs are of compact size and have the benefit of low cost, but their bandwidths tend to be narrow and they are not generally suitable in high accuracy applications. The cross dipole antenna has a high gain at low elevation angles and consequently exhibits less desirable multipath performance. It also has complicated assembly issues. There are numerous microstrip patch antennas in the art including commonly assigned U.S. Pat. No. 5,200,756 issued to Feller. This three dimensional microstrip patch antenna has relatively high gain at low elevation angles. U.S. Pat. No. 6,252,553, issued to Solomon, is a multi-mode patch antenna system and method of forming and steering a spatial null. This antenna uses four feed probes and geometrical non-symmetry, and the radiating patch is assembled over the ground plane. The active circuit employed also requires an additional circuit card. U.S. Pat. No. 6,445,354, issued to Kunysz, is termed a pinwheel antenna design. The pinwheel antenna has generally good performance including the ability to reduce multipath interference, but it is difficult to manufacture compared to other antenna configurations. This antenna also employs two circuit cards, an RF absorber, and a cable connection between both cards. U.S. Pat. No. 6,597,316, issued to Rao et al., is a spatial null steering microstrip antenna array. This antenna also exhibits good multipath reducing properties and accuracy but its feed circuit is comparatively complicated, consisting of four coaxial probes and three combiners. U.S. Pat. Nos. 5,200,756; 6,252,553; 6,445,354; and 6,597,316 are incorporated herein by reference. 
         [0011]    Conventional patch antennas are typically formed of a patch radiation element positioned in relation to a ground plane, and electrically referenced thereto, and separated from the ground plane by a dielectric material. The dielectric material most commonly used is an FR-4 composite which is a common printed circuit board (PCB) material formed of glass fiber reinforced epoxy resin. Commonly assigned U.S. Pat. No. 7,429,952, issued to Sun and incorporated herein by reference, is directed to a patch antenna configuration including a patch radiation element formed on an upper PC board and a ground plane PCB separated from the patch board by dielectric layers formed of a ceramic/PTFE composite. There are problems with the use of composite materials as dielectrics including indeterminate homogeneity and consistency. Material inconsistencies which would not be a problem at HF or VHF frequencies become a concern at L-Band and higher frequencies because of the proportionately shorter wavelengths involved at such frequencies. Additionally, the relatively high dielectric constant of materials like FR-4 is a factor in the narrow bandwidth of patch antennas formed therefrom, and a narrow bandwidth is desirable in some applications for reducing interference with desired signals. In some GNSS applications, an increased bandwidth is desirable to receive various GNSS ranging signals and additionally SBAS augmentation signals. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention is directed to GNSS antenna configurations including a radiating structure positioned in spaced relation to a ground plane with one or more intervening dielectric layers formed of polytetrafluoroethelyne (PTFE) materials. The use of PTFE materials in the dielectric layer results in lower loss compared to FR-4 composites and other materials and moderate bandwidth in the antenna unit to accommodate multiple GNSS frequencies and augmentation signals. 
         [0013]    An embodiment of the GNSS antenna is a patch antenna configuration including a circular upper patch antenna PC board, a circular PTFE dielectric layer, and a circular ground plan PC board with a low noise amplifier (LNA) and other components fabricated thereon. The patch antenna board may be a copper clad FR-4 board etched to form a circular patch antenna radiator on a top surface. The antenna board is preferably of a very thin dimension to minimize signal losses. In an embodiment of the patch antenna board, the radiator element and the supporting board are drilled to form a cross-pattern of four lines of holes radiating at 90° intervals from a center point. The dielectric layer can be formed by one or more circular sheets of PTFE to achieve a desired thickness. The PTFE dielectric boards are provided with a pair of crossed slots which intersect at the center of the sheets. The ground plane board can be formed by a circular FR-4 board which is foil clad to form a ground plane for the antenna unit. The ground plane cladding can be formed on the upper side of the ground plane board with microstrip conductors on the lower surface to form or connect circuit elements of the LNA and a four port hybrid combiner. Alternatively, it is foreseen that the ground plane cladding may be formed on the lower surface of the ground plane board with etched openings receiving the elements of the LNA and the hybrid. 
         [0014]    The antenna patch board, the dielectric boards, and the ground plane board are provided with aligned holes to receive fasteners, such as nylon screws and nuts. The boards are assembled with the crossed slots in the dielectric boards aligned with the lines of holes in the antenna patch board. Selected feed holes of the lines of holes are aligned with port terminals of the hybrid. Tinned copper conductors are soldered between the feed holes and the port terminals of the hybrid and extend through the slots in the dielectric boards to form feed lines to the hybrid. The patch antenna unit may be housed in an enclosure including a base support and a top cover or radome to seal the antenna unit therein. The enclosure may include one or more external line feeds for connection to GNSS processing circuitry, such as a GNSS receiver and circuitry controlling displays, controlled equipment, or the like. The enclosure may also include mounting hardware for mounting the antenna unit, as on the roof of a vehicle. 
         [0015]    The GNSS antenna system of the present invention, using a PTFE dielectric layer above a ground plane, can also be applied to antenna radiator configurations other than the circular patch configuration described above. The antenna configurations can include a dual frequency circular patch configuration with a capacitor-tuned etched slot, a crossed dipole configuration with dipole arms supported by a mast or vertical member, a low profile crossed dipole configuration with dipole arms formed by etching a PC board which is shaped to a desired profile, and the like. In each configuration, the radiating element or structure is spatially and electrically referenced to a ground plane through a dielectric layer formed by one or more layers of PTFE material. 
         [0016]    The present invention is directed to GNSS antenna configurations including a crossed loop GNSS antenna system with loop conductors formed on printed circuit boards (PCBs) with a substrate formed of polytetrafluoroethelyne (PTFE) materials. A radiating assembly of the antenna is formed of a pair of the circuit boards which are joined in an intersecting manner to position two loop antenna components in a 90° angular relationship. Each of the loop boards includes a rectangular section, with a pair of outer support legs depending therefrom. In an embodiment of the crossed loop antenna system, the loop boards are sized to accommodate a full wave sized square loop antenna element at the desired operating frequency. Thus, each side of the loop is approximately a quarter wavelength long. 
         [0017]    One of the loop boards is a top slotted loop board and has a top slot formed therein which extends from a center of the rectangular section to the top edge of the top slotted loop board. The other loop board is a bottom slotted loop board and has a bottom slot extending from the center of the rectangular section to the bottom edge of the bottom slotted loop board. The loop boards are joined in an intersecting relationship by aligning the top slot with the bottom slot and sliding the boards along the slots until the center ends of the slots meet. In some embodiments of the crossed loop antenna system, edges of the slots may be secured to the other loop board by the use of an adhesive, glue, cement, welding, or the like. Lower ends of the support legs may be provided with mounting tabs which may be provided with tab solder pads, as will be described further below. 
         [0018]    The loop boards are formed of foil covered PC boards of which a substrate is a polytetrafluoroethelyne or PTFE material. The foil is etched away to leave the loop conductors of the boards. On the top slotted loop board, there is a gap in a top conductor section where the top conductor intersects the top slot. The separated ends of the top conductor are provided with gap solder pads. The center of the top conductor of the bottom slotted loop board is provided with an elongated solder pad on both sides which are interconnected, as by a plated-through hole. Ends of the elongated solder pads are soldered to the gap solder pads when the loop boards are joined to bridge the top conductor gap of the top slotted loop board. Bottom ends of the loop conductor of each loop board are provided with feed terminal solder pads at the bottom edges of the square section of the loop boards. Although the loop boards described above are of a single layer of substrate, it is foreseen that the loop boards could be formed as two layer laminates with the loop conductors sandwiched between the substrates of the laminate. 
         [0019]    An embodiment of the crossed loop antenna system includes a ground plane board on which the intersected loop boards are mounted. The ground plane board may be of a conventional PC board configuration, such as of a foil cladded FR-4 construction. Preferably, foil cladding an upper surface of the ground plane board is substantially complete, except in areas through which conductors are required to pass. The ground plane board is provided with loop board mounting slots which receive the tabs at the ends of the support legs. The tabs may be secured to the ground plane board by soldering the tab solder pads to the foil cladding on the top surface, and possibly the lower surface, of the ground plane board. On the lower side of the ground plane board, low noise amplifier or LNA circuitry may be provided. Preferably, a separate LNA board is provided which has components of the LNA circuitry positioned on a bottom surface. The LNA board can be separated from the ground plane board by one or more layers, such as layers of PTFE or other dielectric material. The LNA circuitry may be formed by a combination of surface mount elements and microstrip components etched from foil cladding on the lower surface of the LNA board. 
         [0020]    In an embodiment of the crossed loop antenna system, the loop conductors are connected to a combiner board positioned at the lower edges of the rectangular sections of the loop boards. The combiner board may be of a generally square shape and has conductors thereon which form a hybrid combiner to receive signals from the loop conductors in the proper phases. The combiner may be connected to the LNA circuitry by means of a short section of coaxial cable. The combiner board may be supported by non-conductive stand-off legs and non-conductive screws. 
         [0021]    The crossed loop antenna unit may be housed in an enclosure including a base support and a top cover or radome to seal the antenna unit therein. The enclosure may include one or more external antenna line feeds for connection to GNSS processing circuitry, such as a GNSS receiver and circuitry controlling displays, controlled equipment, or the like. The enclosure may also include mounting hardware for mounting the antenna unit, as on the roof of a vehicle. 
         [0022]    Various objects and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. 
         [0023]    Various objects and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The drawings constitute a part of this specification, include exemplary embodiments of the present invention, and illustrate various objects and features thereof. 
           [0025]      FIG. 1  is a schematic diagram of a typical high precision GPS (GNSS) arrangement. 
           [0026]      FIG. 2  is a schematic diagram of another typical arrangement. 
           [0027]      FIG. 3  is a schematic diagram of an arrangement with low noise amplifiers (LNAs) connected to antennas or ports. 
           [0028]      FIG. 4  is a schematic diagram of a general arrangement for a simplified narrow bandwidth (CW) controlled radiation pattern antenna (CRPA). 
           [0029]      FIG. 5  is a schematic diagram of a general arrangement for a phased array with four antennas or ports. 
           [0030]      FIG. 6  shows a phased antenna array with ceramic patch antennas. 
           [0031]      FIG. 7  is an enlarged, side elevational view of an embodiment of an antenna system with PTFE components according to the present invention in the form of a patch type antenna unit, with a part shown in cross section to illustrate components of the antenna unit. 
           [0032]      FIG. 8  is an exploded perspective view of the components of the patch type antenna unit. 
           [0033]      FIG. 9  is a top plan view of a patch antenna assembly of the patch type antenna unit. 
           [0034]      FIG. 10  is a top plan view of a pair of PTFE layers of the antenna unit. 
           [0035]      FIG. 11  is a bottom plan view of a low noise amplifier (LNA) assembly mounted on a lower side of a ground plane board of the patch type antenna unit. 
           [0036]      FIG. 12  is an enlarged, side elevational view of a modified embodiment of an antenna system with a total of four PTFE layers, with a part shown in cross section to illustrate components of the antenna unit. 
           [0037]      FIG. 13  is an enlarged perspective view of a modified patch embodiment of a dual frequency antenna unit of the present invention which incorporates an etched slot element tuned to a second frequency. 
           [0038]      FIG. 13A  is an enlarged detail of the antenna unit embodiment shown in  FIG. 13  with a single-capacitor patch tuning point. 
           [0039]      FIG. 13B  is an enlarged detail of the antenna unit embodiment shown in  FIG. 13  with a double-capacitor patch tuning point. 
           [0040]      FIG. 14  is a perspective view of a radiating element of a low profile crossed dipole antenna unit according to the present invention. 
           [0041]      FIG. 15  is a cross sectional view of the low profile crossed dipole antenna taken on line  9 - 9  of  FIG. 14  and illustrating further details thereof. 
           [0042]      FIG. 16  is an enlarged perspective view of a vertically extended crossed dipole antenna unit according to the present invention. 
           [0043]      FIG. 17  is a side elevational view of an embodiment of a crossed loop antenna system of the present invention with portions broken away to illustrate components thereof. 
           [0044]      FIG. 18  is a perspective view of the crossed loop antenna system with a radome removed to illustrate loop antenna boards of the system. 
           [0045]      FIG. 19  is a top plan view of a ground plane board of the crossed loop antenna system. 
           [0046]      FIG. 20  is a bottom plan view of an LNA board of the crossed loop antenna system with LNA circuitry mounted on the lower side thereof. 
           [0047]      FIG. 21  is an elevational view of the loop antenna boards being fitted together in a crossing configuration. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     I. Introduction and Environment 
       [0048]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. 
         [0049]    Certain terminology will be used in the following description for convenience in reference only and will not be limiting. For example, up, down, front, back, right and left refer to the invention as orientated in the view being referred to. The words, “inwardly” and “outwardly” refer to directions toward and away from, respectively, the geometric center of the aspect being described and designated parts thereof. Forwardly and rearwardly are generally in reference to the direction of travel, if appropriate. Said terminology will include the words specifically mentioned, derivatives thereof and words of similar meaning 
       II. Automatic Signal Maximization for GPS Antennas 
       [0050]    In typical high precision GPS antenna systems (e.g.,  FIG. 1 ) one of the principal methods of ensuring high quality RHCP polarization is to utilize a passive phasing network or hybrid component. This is essential in environments with high levels of multipath interference. Difficulty can arise in multiband antennas due to signal loss in this device, which may be exasperated by use of additional front end filtering. In addition passive devices are only available for some ports numbering (i.e., a 2 port hybrid or a 4 port combiner). 
         [0051]    As a universal alternative for that arrangement this application proposes that an analog or digital control network may be fitted as a replacement. This control network may consist of a a) Phase Shifter and/or b) Attenuator. The advantage of this arrangement is that these phase shifters may not need to be connected prior to the low noise amplifiers due to phasing adjustment and may be microprocessor controlled to adjust for maximum signal level response. This then allows the use of a low cost combiner. The general arrangement is displayed in  FIG. 2 . An alternative is displayed in  FIG. 3  with that advantage of improved noise figure response. The intelligent control may either be built into the antenna (given receiver feedback) or may be controlled directly from the receiver. These arrangements may be configured for any number of ports above 2 with the benefit of saving PCB real estate and cost for high number of ports or antennas. The antenna array for any of the embodiments described herein can comprise any of the GNSS antenna constructions shown in the patent applications incorporated herein by reference above; Feller et al. U.S. Pat. No. 8,102,325 for GNSS Antenna with Selectable Gain Pattern, Method of Receiving GNSS Signals and Antenna Manufacturing Method; and various other antenna constructions, including patch antennas, crossed-dipole, etc. 
       III. Alternative Embodiment with Simplified CW only Controlled Radiation Pattern Antenna (CRPA) for GNSS 
       [0052]    The typical method for resolving interference problems in GPS units consists usually of either a) an adaptive filter to remove an in-band jammer or b) a Controlled Radiation Pattern Antenna (CRPA). The adaptive filter is limited to the extent that if the signal is not narrowband (CW) or if it is of sufficient strength it will overload the analog sections of the GNSS receiver. 
         [0053]    CRPAs overcome the overload problem by consisting of a number of antennas (an array) and receivers (usually fewer than four) whose outputs are monitored by a controller which adjusts phase shifters and/or attenuators to control the effective radiation pattern of the array in such a fashion as to null out the interferer. These are typically used in high cost applications where the purchase of multiple antennas and receivers can be justified (often military applications). 
         [0054]    This application consists of a simplified arrangement for a CRPA which does not require multiple receivers, and which may be self-contained in a single antenna enclosure, however does contain an adaptive control algorithm that functions for CW jamming. 
         [0055]    By producing a solution only for CW jamming, it simplifies detection of the jammer by allowing the use of a log detector instead of multiple receivers. It therefore allows the use of a number of low cost antennas to be housed in a single enclosure.  FIG. 4  shows the general arrangement: 
         [0056]    Simplified detection may be applied either to each antenna channel individually and/or to the combined channel as shown in  FIG. 4 . Selection of arrangement will be dependent on the exact algorithm chosen. In addition nulling may often be achieved by phase only minimization rather than phase+attenuation minimization. This may then reduce costs further. 
       IV. Enhanced Low Cost Ceramic Phased Array Antennas for GPS 
       [0057]    The typical low cost antenna that has found its way into most consumer applications is the ceramic patch antenna. Almost universal, these antennas have a single feedpoint and beveled corner in order to promote RHCP polarization. In actuality these antennas have severely elliptical polarizations which results in a high susceptibility to LHCP and hence multipath interference. 
         [0058]    In this application it is proposed to make use of the reasonable efficiency of these antennas, and to repair the polarization characteristics by configuring them in a circular array and combining them using appropriate combining and phase networks. Multiple elements (preferably more than two) may be used in this configuration.  FIG. 5  shows the general arrangement. 
         [0059]    In addition to repairing the RCHP characteristics, additionally it is possible to control the elevation radiation pattern by adjusting the placement of these antenna&#39;s (distance from center of array) and by rotation of each one of these elements. Each element type will be uniquely adjusted depending on its elliptical or linear polarization characteristics.  FIG. 6  shows the prototype device. 
       V. Alternative Embodiment with Antenna Unit Incorporating PTFE Components 
       [0060]    Referring to  FIGS. 7-16  in more detail, the reference numeral  101  generally designates an embodiment of an antenna unit incorporating PTFE components according to the present invention. The antenna unit  101  generally includes a radiating element  104 , a ground plane element  106  positioned in spaced relation to the radiating element  104 , and a dielectric element  108  positioned between the radiating element  104  and the ground plane element  106 . 
         [0061]    Referring to  FIGS. 7 and 8 , the illustrated radiating element  104  includes a circular patch antenna board  114 , the ground plane element  106  includes a ground plane board  116 , and the dielectric element  108  includes a pair of layers  118  of PTFE. Referring to  FIGS. 7 and 9 , the illustrated patch antenna board  114  is formed by a foil clad FR-4 PC board forming a substrate  122  on which a circular antenna radiator patch  124  remains from a process such as etching. The illustrated patch  124  has arrays or lines  126  of holes  128  drilled therethrough and through the substrate  122 . The illustrated lines  126  are straight, equal in length, and radiate from a center  130  of the patch  124  at 90° angular intervals. A middle hole  132  of each line  126  is provided with a soldering pad and may be plated through (not shown). The diameter of the patch  124  provides coarse tuning of the antenna unit  101 . The lines  126  of holes  128  form a finer tuning structure for the patch  124  and provide a means of coupling signals gathered by the patch  124  to subsequent circuitry. The substrate  122  has a plurality of assembly holes  134  and notches  136  spaced circumferentially about the periphery thereof. Preferably, the substrate  122  has a minimal thickness to minimize signal losses and may have a thickness on the order of 0.6 mm (24 mil). 
         [0062]    Referring to  FIGS. 7 and 11 , the illustrated ground plane board  116  is formed by a circular foil clad FR-4 PC board having an upper surface  140  and a lower surface  142 . As illustrated, the board  116  has ground plane cladding  143  covering most of the upper surface  140 , with openings (not shown) etched for a purpose described below. The lower surface  142  has conductors forming or connecting components of a low noise amplifier or LNA circuit  144  and a four port hybrid combiner  146  having terminals  148 . The terminals  148  include holes which include solder pads (not shown). The LNA  144  may include a combination of microstrip segments, surface mount components, and discrete components (not shown). Further, the LNA  144  may include one or more antenna line feed connectors  150  for connection of the antenna unit  101  to subsequent circuitry, such as a GNSS receiver (not shown). The ground plane board  116  has an external shape which is similar to the shape of the patch antenna board  114  and is provided with circumferentially spaced assembly holes  152 . It is foreseen that the ground plane board  116  may also be provided with assembly notches (not shown) similar to the assembly notches  136  of the patch antenna board  114 . As with the patch antenna board  114 , the ground plane  116  has a minimal thickness to minimize signal losses and may have a thickness on the order of 0.6 mm (24 mil). It is foreseen that the ground plane board  116  could alternatively be formed with ground plane cladding on the lower surface  142  with openings in the cladding for conductors of the LNA  144  and hybrid  146  isolated from the ground plane cladding. 
         [0063]    Referring to  FIGS. 7 and 10 , the illustrated dielectric element  108  is formed by a pair of circular PTFE layers  118 . PTFE or polytetrafluoroethelyne is the generic name of a polymer material also known by the proprietary name of Teflon®. The illustrated PTFE layers  118  have an external shape which is similar to the shape of the patch antenna board  114  and include pluralities of circumferentially spaced assembly holes  156  and notches  158 . Each of the illustrated PTFE layers  118  has a pair of elongated slots  160  formed therein which intersect at 90° at a center  162  of the layer  118 . The center  162  of the layer  118  may be provided with a bore  164  at the intersection of the slots  160 . The illustrated PTFE layers  118  have a thickness of 0.125 in (approximately 3.0 mm), although it is foreseen that other thicknesses may be appropriate for a given application. It is also foreseen that a single PTFE layer  118  of an appropriate thickness could be employed. Moreover, additional PTFE layers  118  may be utilized, as shown in the alternative embodiment antenna unit shown in  FIG. 12  and described below. 
         [0064]    The antenna unit  101  is formed by sandwiching the PTFE layers  118  between the patch antenna board  114  and the ground plane board  116 . The slots  160  in the PTFE layers  118  are aligned with the lines  126  of holes  128  in the patch antenna board  114 . Additionally, the middle holes  132  of the lines  126  are aligned with the terminals  148  of the hybrid  146  on the ground plane board  116 . The assembly holes  134 ,  156 , and  152  are aligned, as are the assembly notches  136  and  158 . The boards  114  and  116  and the PTFE layers  118  are held together by sets of fasteners  166 , such as nylon screws and nuts. In the illustrated antenna unit  101 , signal feeds  168  ( FIG. 7 ) from the antenna patch  124  are provided by conductors soldered between middle holes  132  and hybrid terminals  148  through the slots  160  of the PTFE layers  118 . The signal feeds  168  may be in the form of tinned copper wires. 
         [0065]    The illustrated antenna unit  101  is mounted in a weatherproof enclosure  170  ( FIGS. 7 and 8 ) formed by an enclosure base  172  and a cover or radome  174 . The enclosure  170  may be provided with one or more external antenna line feeds  176  coupled to the LNA antenna feeds  150  and providing for connection of the LNA circuitry  144  with subsequent signal processing circuitry (not shown). The enclosure  170  may also be provided with mounting hardware (not shown) for mounting the antenna unit  101  on a vehicle (not shown). 
         [0066]      FIG. 12  shows an antenna unit  181  comprising a modified embodiment or alternative aspect of the present invention. The antenna unit  181  includes four PTFE layers  118 , which can be used for increasing the spacing between the radiating element  104  and the ground plane board  116  for optimizing the performance of the antenna unit  181 . Otherwise the antenna unit  181  can be constructed similar to the primary embodiment antenna unit  101 . Functionally the antenna units  101  and  181  have similar operating characteristics. 
         [0067]    Features of the antenna units  101  and  181  can be applied to antenna configurations employing radiating elements other than the patch antenna board  114 .  FIGS. 13-16  illustrate additional exemplary embodiments and alternative aspects of the antenna unit  101  employing representative types of radiating elements in combination with ground plane elements and intervening dielectric layers formed of PTFE materials. 
         [0068]      FIG. 13  illustrates a dual frequency or dual band patch antenna unit  215  of the present invention employing a patch radiating element  217  positioned in spaced relation to a ground plane element  219  with an intervening dielectric element  221  formed of a PTFE material. The illustrated radiating element  217  is formed from an FR-4 type PC board material with a circular foil antenna patch  223  formed by etching a foil clad substrate  225 . The antenna patch  223  includes an elongated slot  227  formed by etching or, alternatively, by a machining operation. The illustrated slot  227  is rectangular and is centered on a diameter of the circular patch  223 . The slot  227  may include a reactive element  229 , such as a capacitor, which bridges the side edges of the slot  227  to tune the antenna patch  223  to a particular frequency or range of frequencies of interest. In the illustrated dual frequency antenna unit  215 , the patch  223  can, for example, be tuned to the L2 frequency (1227.60 MHz) while the slot  227  is tuned to the L1 frequency (1575.42 MHz). The antenna unit  215  may include a feed structure (not shown) to couple a signal from the antenna patch  223  to LNA circuitry (not shown) on the ground plane element  219 . One or more antenna feed line connectors  231  may connect to the LNA circuitry  144  to output a signal to subsequent processing circuitry (not shown). The antenna unit  215  may be housed in an enclosure (not shown) somewhat similar to the enclosure  170 . 
         [0069]      FIG. 13A  is an enlarged detail of the antenna unit embodiment shown in  FIG. 13  with a single-capacitor patch tuning point  233  with a shunt-to-ground conductor  234  and a conductor extension  236  providing capacitance with the patch  223 . 
         [0070]      FIG. 13B  is an enlarged detail of the antenna unit embodiment shown in  FIG. 13  with a double-capacitor patch tuning point  235  with a shunt-to-ground conductor  234  and a conductor extension  236 . An intermediate conductor  237  provides additional capacitance in series with the patch  223  and the conductor extension  236 . It will be appreciated that one or both of the patch  223  and the slot  227  can be tuned independently. The slotted-patch antenna unit  215  can be provided with any combination of slot-tuning (e.g., with the capacitor  229 ) and/or patch-tuning (e.g., with the shunt-to-ground tuning points  233  and  235 ). Alternatively, the antenna unit  215  can be constructed and operated with other tuning components, or without tuning components. 
         [0071]      FIGS. 14 and 15  illustrate a low profile crossed dipole antenna unit  240  according to the present invention. The antenna unit  240  includes a radiating element  242  positioned in spaced relation to a ground plane element  244  with an intervening dielectric element  246  formed of a PTFE material. The illustrated radiating element  242  is of a molded PC board configuration with dipole elements  248  formed on a substrate  250  of a clad PC board material. The shape of the dipole elements  248  determines the beam pattern of the antenna unit  240  to balance an effective angle of use of the unit  240  with rejection of multipath signals. The radiating element  242  may be of a relatively rigid nature or may, alternatively, be flexible. The radiating element  242  is secured to the dielectric element  246  and the ground plane element  244  by fasteners  252 , such as sets of nylon screws and nuts. The shape of the radiating element  242  may be maintained by dielectric spacer posts  254  positioned between the substrate  250  and the dielectric element  246 . The illustrated antenna unit  240  may include hybrid combiner circuitry  256  which is coupled to the dipole elements  248  and which feeds signals therefrom to LNA circuitry (not shown) positioned on a lower side of the ground plane element  244  by way of a transmission line  258  such as a short length of coaxial cable. The low profile crossed dipole antenna unit  240  may be housed in a weatherproof enclosure  260  similar to the enclosure  170 . External antenna line feeds  262  can be provided on the bottom of the enclosure  260 . 
         [0072]      FIG. 16  illustrates a vertically extended crossed dipole antenna unit  265  including a crossed dipole radiating element  267 , a ground plane element  269 , and a dielectric element  271  formed of a PTFE material. The radiating element  267  is in the form of a crossed dipole radiating arm assembly  273  including pairs of opposing dipole arms  275  secured to a hub  277 . The arm assembly  273  is supported in spaced relation to the ground plane element  269  and the dielectric element  271  by a vertical support  279  formed by a PC board. The vertical support  279  may include matching circuitry  281  and LNA circuitry  283 . The beam pattern of the antenna unit  265  can be controlled by the droop of the dipole arms  275 , with a deeper droop increasing the angular response of the antenna unit  265  and a shallower droop decreasing the angle of response and additionally decreasing the response of the unit  265  to multipath interference. Additional features of crossed dipole type antennas can be found in commonly assigned U.S. Pat. No. 8,102,325, which is incorporated herein by reference. The antenna unit  265  can be housed in an enclosure (not shown) similar in some respects to the enclosure  170 . 
       VI. Alternative Embodiment with Crossed Loop Antenna System 
       [0073]    Referring to  FIGS. 17-21  in more detail, the reference numeral  301  generally designates an embodiment of a crossed loop antenna system incorporating PTFE components according to the present invention. The illustrated antenna system  301  generally includes an enclosure assembly  303 , a ground plane assembly  305  including a ground plane board  306  and an LNA board  307 , and a radiating assembly  309  including a pair of loop antenna boards  310  and  311  joined in a 90° relationship. The enclosure assembly  303  generally includes an enclosure base  314  and a radiotransparent weather cover or radome  315  sealingly joined with the base  314 . The ground plane assembly  305  is supported on the enclosure base  314  and has the radiating assembly  309  secured thereto in an upright relation. 
         [0074]    Referring to  FIGS. 17 and 18 , the illustrated ground plane assembly  305  includes the ground plane board  306  and the LNA board  307  which are separated by one or more dielectric boards  317 . The boards  317  can be formed of PTFE or other materials, such as an FR-304 composite. The ground plane board  306  has a foil cladding  319 , such as a copper foil cladding, on most of its top surface which forms an electrical ground plane for the system  301 . The foil cladding  319  may be coated with a material such as a lacquer  320  or the like to seal the cladding  319  from corrosion. The illustrated ground plane board  306  is provided with aligned sets of slots  321  in a 90° pattern which are sized and spaced to receive ends of the loop boards  310  and  311 , as will be described further. The board  306  may be provided with bores  322  for supporting a combiner board  324  ( FIGS. 17 and 18 ), as will be described further. A feed bore  325  may be provided, as will be described further. Finally, a plurality of assembly holes  326  are provided in circumferentially spaced relation about the periphery of the ground plane board  306 . 
         [0075]    The illustrated LNA board  307  has components (not detailed) of a low noise amplifier or LNA circuit or assembly  328  on a bottom surface  329  thereof. The LNA circuitry  328  may be formed of a combination of surface mount components and microstrip elements (not shown). The LNA circuitry  328  may include one or more feed connectors  330  which provide for connection of the LNA circuitry  328  to further processing stages of a GNSS receiver or the like (not shown). The LNA board  307  is provided with a plurality of circumferentially spaced assembly holes  331  about its periphery which may be aligned with the assembly holes  326  of the ground plane board  306  and with similar holes (not shown) formed in the dielectric boards  317 . The ground plane board  306 , the dielectric boards  317 , and the LNA board  307  may have their assembly holes  326  and  331  aligned to receive fasteners  333  ( FIG. 17 ) to assemble the ground plane assembly  305 . The fasteners  333  may be sets of nylon screws and nuts or the like. The enclosure base  314  may have one or more external feed connectors  335  ( FIG. 17 ) which connect with the feed connectors  330  of the LNA board  307  to connect the LNA circuitry  328  with further stages. 
         [0076]    Referring to  FIGS. 17 ,  21 , and  6 , the radiating assembly  309  includes the loop antenna boards  310  and  311 . Each of the boards  310  and  311  includes a rectangular upper section  338  with a pair of support legs  339  extending from a lower end thereof. Each of the illustrated legs  339  has a mounting tab  340  at a lower end which is sized to be received in one of the slots  321  of the ground plane board  306 . The tabs  340  may be secured in the slots  321  by gluing, welding, or soldering of solder pads (not shown) on the tabs  340  and near the slots  321 . One of the boards, such as board  310 , is a top slotted board, having a top opening slot  342 . The other board, such as board  311 , is a bottom slotted board, having a bottom opening slot  343 . When the boards  310  and  311  are assembled, the slots  342  and  343  are aligned and the boards are slid until ends  344  of the slots meet. The boards  310  and  311  are secured together in a 90° relation with the slot ends  344  meeting by gluing, welding, or the like. 
         [0077]    Each of the illustrated loop antenna boards  310  and  311  is formed of a foil cladded substrate of polytetrafluoroethelyne or PTFE material. The copper foil cladding is etched to leave conductors  346  forming a square loop  348 . Each of the illustrated loops  348  is a full wave loop at the frequency of operation of the antenna system  301 . Thus, each side of the loops  348  is a quarter wavelength long, as shown in  FIG. 17 . In the illustrated antenna system  301 , the loop conductors  346  are formed on only one side of each board  310  and  311 . It is foreseen that each of the boards  310  and  311  could be formed as a dual layer laminated board (not shown) with the loop conductors  346  formed on one of the surfaces within such a laminated board. 
         [0078]    On the top slotted loop antenna board, illustrated as board  310 , the slot  342  requires a gap in the upper loop conductor  346 . In order to complete the circuit of the loop  348  on the board  310 , a pair of gap solder pads  350  is provided. The bottom slotted board  311  is provided with somewhat elongated solder pads  351  at a center of the top loop conductor  346 . The solder pads  348  on opposite sides of the board  311  are interconnected, as by a plated through hole  352 . When the boards  310  and  311  are joined, the gap solder pads  350  of the board  310  are soldered to the elongated solder pads  351  to complete the circuit of the loop  348  on the top slotted board  310 . This also interconnects the loops  348  of the boards  310  and  311 . However, the center of the top conductor  346  of the loops  348  is at a voltage null. This is a typical interconnection of crossed loop antennas. Each of the loops  348  has a set of feed conductors  354  which terminate in feed solder pads  356 . 
         [0079]    When the antenna loop boards  310  and  311  are joined, the feed conductors  354  are coupled to conductors of combiner or hybrid circuitry (not shown) on the combiner board  324 . The feed solder pads  356  are soldered to combiner solder pads (not shown) to couple the antenna loops  348  with the combiner circuitry. The combiner board  324  is supported by combiner support posts  360  which join with the bores  322  provided on the ground plane board  306 . The illustrated loop board legs  339  and posts  360  have lengths to position the lower conductors  346  of the antenna loops  348  at a quarter wavelength from the ground plane conductor  319  of the ground plane board  306  at the frequency of operation of the antenna system  301 , as shown in  FIG. 17 . The combiner circuitry may be coupled to the LNA circuitry  328  by means of a short length of coaxial cable  362  which extends from the combiner board  324 , through the feed bore  325  ( FIG. 19 ) of the ground plane board  306  and through the dielectric layers  317 , if present, to the LNA circuitry  328  on the LNA board  307 . 
       VII. Alternative Embodiment Multipath Cancelling Antenna 
       [0080]    What is proposed is a multipath cancelling antenna which will subtract any left hand circular portion of signals from tracking in a GNSS receiver. This can be accomplished with an antenna which has both left hand circular polarization (LHCP) and right hand circular polarization (RHCP) ports available. This is common as many antennas use quadrature hybrids to generate the phasing and normally the LHCP port is simply terminated. What is proposed is to use the signals received from the LHCP port to determine which satellites have very high multipath and remove them from the solution. Use a GNSS receiver to track satellites on the LHCP port of the antenna. Any satellites whose CNo is within 10 dB of the RHCP receiver CNo, should be removed from the navigation calculation of the main receiver using the RHCP signal path. This can be accomplished using an inexpensive GPS module to simply determine the signal strength and PRN of satellites with poor polarization. 
         [0081]    A further alternative is to make a perfect RHCp antenna across both bands by using the LHCP port of the hybrid and phasing it and recombining it to cancel on the RHCP side. This is required because due to tolerances and repeatability antennas usually end up with +/−2 dB of axial ratio not the perfect 0 dB. Axial ratio is the major to minor axis ratio for an ellipse defined by the polarization. A perfect circle has equal axis and so the ratio is 1 or 0 dB. A further issue is GNSS signals occupy two major bands 1.165 to 1.26 GHz and 1.54 to 1.61 GHz. It is possible to make perfect polarization at one band or the other but to achieve this on both is very difficult. This technique of recombining a sample of the LHCP out of phase can be achieved separately on each band. A test setup with a linear polarized transmit signal is required with a tuning Voltage adjusted on the phase shifter until both the horizontal and vertical orientations have the exact same level. 
         [0082]    Another implementation to achieve the cancellation of reflections is to use a second antenna which does not see the upper hemisphere of the gain pattern, but is downward looking. The GNSS antennas are upward pointing to receive the satellites signals. Using a downward pointing antenna will only receive reflections which need to be cancelled. These can either be removed from the solution or cancelled using a phase shifter and tracking algorithm. 
       VIII. Conclusion 
       [0083]    It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.