Abstract:
The present invention provides a high-performance and small sized helical antenna element and array thereof for use in an aircraft communication system or the like, where stringent spatial restrictions and gain requirements generally apply. The performance of the array is enhanced by increasing the lateral distances between the antenna elements over a portion of the elements where the windings thereof have high current amplitude. The sweeping envelope of the array is maintained small by recovering the initial distancing over a portion of the elements where the windings thereof have low current amplitude.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention pertains to the field of antennas, and in particular, to helical antenna elements and arrays thereof. 
       BACKGROUND 
       [0002]    A helical antenna array generally comprises a series of helical antenna elements, each one of which comprising a conductor, such as a wire, tape, moulded conductor, stamped conductor, extrusion, or printed circuit, having a nominally helical geometry that, when energized, generates a circularly or substantially circularly polarized beam. In some realisations the helices may have more than one winding, where the windings may have the same or different pitches and the same or different starting positions. To ensure structural integrity, the helical winding is usually supported by a dielectric former consisting of a cylinder or the like, and as such has a substantially circular helix cross-section. Helical antenna arrays may further comprise a ground plane, which provides a signal return or ground connection for the RF source of the antenna elements, and can further reflect that part of the electromagnetic wave generated by the antenna elements that propagates in the rearward direction, i.e. the ground plane effectively re-directs this radiation forwards. The live terminal of the RF source, on the other hand, connects to the starting point of the antenna&#39;s helical winding, which in some cases lies proximal to or almost immediately above the ground plane. Thus, the ground plane may provide circuit continuity for the input transmission line, usually a coaxial cable, which excites the antenna. For example, the center conductor of the coaxial line connects to the end of the helical winding, whereas the outer conductor of the coaxial line connects to the ground plane. The ground plane may have a planar surface, or alternatively, may consist of a cup, as shown in U.S. Pat. No. 6,664,938. In some realisations there may be no ground plane with the wave being launched either between adjacent windings or at a point along one or more windings. 
         [0003]    The performance of relatively small helical antenna elements can be characterized, at least in part, by a gain parameter, which usually ranges from 5 to 12 dBIc. While in some cases, higher gain levels in excess of 12 dBIc can be achieved by using longer helices, significantly large length increments are often required to achieve relatively small gain increments. Therefore, a helix antenna is generally considered to be more efficient in terms of gain achieved as related to structural volume, when it is relatively short. For many purposes, a more expedient solution to achieving higher gains is to assemble an array of moderately sized helices. 
         [0004]    In some applications, such as those shown in U.S. Patent Application Publication No. 2008/0012787, a helical antenna element may have a conical shape, where the winding diameter at the feed end of the winding may be greater than the diameter at the radiating end. Conical helix structures may be advantageous when a helix antenna is to be operated over a wide frequency band. In other applications, such as the ones shown in U.S. Pat. No. 6,172,655 and U.S. Patent Application Publication No. 2004/0135732, helices are wound about formers of varying cross-section diameters, increasing linearly toward a central maximum, and reducing linearly thereafter. Antenna elements of this type are commonly known in the art to provide for increased broadband performance. These examples may further comprise varying helix winding densities, wherein a winding has smaller pitches at the feed end and larger pitches at the radiating end. 
         [0005]    As will be appreciated by the person of ordinary skill in the art, a helix is generally excited by connecting the lower extremity of its winding to an RF source. An electromagnetic wave then travels around the winding. This wave ultimately launches radiated fields when it arrives at the top the radiating or terminal end of the winding. A major portion of the radiated fields then propagates forwards, following a direction that is dictated predominantly by the phase distribution of the wave along the helix winding. In the design of high gain, fixed beam arrays, it is generally desirable to design the individual helices for maximum gain along the axis of the helix winding. 
         [0006]    Many factors may contribute to the reduction of the gain of a helical antenna: the termination of the antenna, if open-circuited, carries no current; the dielectric material of the support structure may introduce dissipative losses and stored energy with related mismatch losses; mutual coupling between adjacent helices can broaden the beam; the axial design of conventional helices makes inefficient use of the volume within which the antenna may be rotated; and the high launching impedance resulting from small winding diameters can result in an inferior matching structure. 
         [0007]    When several helices are assembled together so as to form an array, electromagnetic couplings may occur between neighbouring helices. Conventional excitation of the array with uniform helix orientations exacerbates this problem by maximising the coupling between the elements. One impact of the coupling is to progressively pull the patterns of the individual elements towards the centre of the array. The individual elements of the array then radiate in different directions, thereby reducing the gain of the array. Additionally, the coupling narrows the impedance bandwidth, and may increase mismatch loss. For example, in a four-element array comprising non-helical elements, a power gain of roughly 5 dB can be achieved using the array, over the gain of a single element. Given the electromagnetic couplings between helix elements, however, a four-element helix array is more likely to have a power gain of only 4 dB higher than that of a single helix element. 
         [0008]    U.S. Pat. No. 5,874,927 provides one approach to improving the performance of a helical antenna array by tilting the otherwise linear helical antenna elements away from one another, whereby such tilting is reported to broaden the effective aperture of the array. This approach, while providing some advantages over parallel implementations, also has the effect of increasing the overall sweeping radius of the array, which, in some embodiments where spatial limitations are of crucial importance, can limit the applicability of such design. 
         [0009]    For example, helical antenna arrays are commonly used for satellite communications in aircrafts or the like. Examples of satellite communications may include, but are not limited to, airborne and/or ground based communications for receiving weather reports and/or air traffic control information, or for communicating status and emergency messages, to name a few. Furthermore, such satellite communication systems may also be useful in providing such services as telephone communications, Internet services, and/or other forms of data exchange to the aircraft passengers. In the context of aircraft communications, helical antenna arrays are commonly mounted at the tail section of an airplane or the like, which tends to be very narrow and may limit the size of the antenna array that can be deployed. Consequently, a person of ordinary skill in the art would appreciate that the installation and operation of a helical antenna array for aircraft communications may impose certain operational and structural limitations to the type of antenna suitable for such applications. 
         [0010]    Furthermore, as aircraft communication systems generally relay communications via a link from the aircraft to a communications satellite, which communications are then relayed to grounded resources via a separate link, and since such systems are generally expected to function independently of the position of the aircraft around the globe, the associated aircraft communications antenna should generally be capable of pointing its radiation towards a selected satellite at all times. Accordingly, the antenna beam should be steered by appropriate means depending on the local latitude and longitude of the aircraft, the attitude of the aircraft, and the heading of the aircraft. In some applications, an electronic steering method is used to reduce the number of mechanically moving or turning parts of the antenna structure. However, such steering methods generally are not applied to single helix implementations. Rather, mechanical steering methods may be used alone or in combination with electronic steering. As noted above, however, the aircraft may impose certain limitations relating to the available spaces within which the antenna can be installed and operated (i.e. steered). These limitations place very demanding constraints on the size of the antenna assembly, and the scan envelope volume that the antenna assembly requires. For instance, in order to mechanically steer the antenna within the tail section of the aircraft to scan a desired coverage area, spatial limitations should generally be respected irrespective of antenna orientation, namely, the antenna should operate freely within a scan radius or volume as prescribed by a radome covering a top portion of the aircraft tail section and the antenna in operation. Similarly, radomes on top of trucks, trains, ships, fuselages and other vehicles are compact and may limit the sweeping volume of the antenna installed. 
         [0011]    Accordingly, solutions as provided by U.S. Pat. No. 5,874,927, while providing some operational advantages over standard arrays, may be of limited suitability in the above context where spatial limitation applies, or where an increase to an array sweep radius cannot generally be accommodated in standard installations. 
         [0012]    Therefore there is a need for a new helical antenna element and array thereof that overcomes some of the drawbacks of known antenna arrays, or that provides the public with a useful alternative. 
         [0013]    This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention. 
       SUMMARY 
       [0014]    An object of the invention is to provide a helical antenna element and array thereof. In accordance with one aspect of the invention, there is provided an antenna comprising: a ground plane; and an array of helical antenna elements, each helical antenna element comprising a support structure and a conductor helically supported thereby, geometric centers of cross-sections of a helix formed by the conductor defining a respective axis of the helical antenna element, the axis extending from the ground plane in a direction substantially perpendicular thereto, the helical antenna element having a terminal end and having a base end mounted to the ground plane; wherein at least one lateral distance from the axis of a first helical antenna element between the base and terminal ends thereof to the axis of a second helical antenna element is greater than lateral distances from the axis of the first helical antenna element at the base and terminal ends thereof to the axis of the second helical antenna element at respective base and terminal ends thereof. According to this aspect of the invention, at least one said helical antenna element bulges near its mid-point such that the array of elements more fully fills a spherical volume and consequently achieves a higher gain from an available spherical volume. A variety of embodiments of this bulging can be envisaged including: Incorporation of loading disks or other metallic structures, offset from the helix axis, at some point along the helix length; modifying the helix support to define a non-linear axis resulting in a distancing between at least a portion of said non-linear axis relative to the axis of another element as a function of distancing from said ground plane; incorporation of additional windings or winding segments that are not uniform about the helix axis; or incorporation of dielectric materials that are not uniformly disposed about the helix axis. 
         [0015]    In accordance with another aspect the invention provides for an antenna comprising: a ground plane; and an array of nominally helical antenna elements, each one of which comprising a support structure and a conductor helically supported thereby defining respective element axes extending from said ground plane in a direction substantially perpendicular thereto; wherein one or more of the nominally helical elements bulges such that the array more uniformly fills a spherical volume, and such that the projected aperture and projected area of the array are increased while the top and bottom footprints are not increased. 
         [0016]    In some embodiments of the invention, at least one lateral distance from the axis of a first helical antenna element between the base and terminal ends thereof to the axis of a second helical antenna element is greater than lateral distances from the axis of the first helical antenna element at the base and terminal ends thereof to the axis of the second helical antenna element at respective base and terminal ends thereof. 
         [0017]    In some embodiments of the invention, the bulging is introduced by conductive plates ohmically or capacitively coupled to the helical winding at one or more points between the terminal and base ends of one or more helices; these conductive plates being offset from the axes of the helices towards the outside of the array such that the array shape becomes increasingly spherical. 
         [0018]    In some embodiments of the invention, the bulging is introduced by means of asymmetric dielectric loading at one or more points between the terminal and base ends of one or more helices. 
         [0019]    In accordance with another aspect of the invention, there is provided an antenna comprising: a ground plane; and an array of helical antenna elements, each one of which comprising a support structure and a conductor helically supported thereby defining a respective element axis extending from said ground plane in a direction substantially perpendicular thereto; wherein each said helically supported conductor is fed by means of connection to a printed circuit board attached to, or forming, said ground plane; wherein said printed circuit board forms part of a power divider; and wherein said power divider incorporates one or more short-circuited and/or open-circuited loading stubs for dispersion compensation. 
         [0020]    In accordance with another aspect of the invention, any one of the above antennae may be used in an aircraft communication system. 
         [0021]    In accordance with another aspect of the invention, any one of the above helical antenna elements may be used in the manufacture of a helical antenna array. 
         [0022]    Other aims, objects, advantages and features of the invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0023]      FIG. 1  is a perspective view of a helical antenna array, in accordance with one embodiment of the invention. 
           [0024]      FIG. 2  is an exploded view of the antenna array of  FIG. 1 , showing a top down perspective of components thereof, and an optional off-axis conductive loading plate shown in relation to an antenna element thereof. 
           [0025]      FIG. 3  is an exploded view of the antenna array of  FIG. 1 , showing a bottom up perspective of components thereof. 
           [0026]      FIG. 4  is a perspective view of an antenna element of the antenna array of  FIG. 1 . 
           [0027]      FIG. 5  is a diagrammatic representation of antenna element cross-sections in a quadrilateral antenna array of four helical antenna elements defining respective non-linear axes, in accordance with one embodiment of the invention, showing laterally overlapping base and terminal end element cross-sections in hard lines and increased intermediate cross-sections in dashed lines displaced laterally along their respective non-linear axes and thereby distanced relative to one another along diagonal axes of the array. 
           [0028]      FIG. 6  is a diagrammatic representation of antenna element cross-sections in a dual antenna array of two helical antenna elements defining respective non-linear axes, in accordance with one embodiment of the invention, showing laterally overlapping base and terminal end element cross-sections in hard lines and increased intermediate cross-sections in dashed lines displaced laterally along their respective non-linear axes and thereby distanced relative to one another. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. 
         [0030]    The following provides a description of a helical antenna array, and antenna elements thereof, in accordance with different embodiments of the invention. In general, the array will comprise a ground plane and an array of helical antenna elements, each one of which comprising a support structure and a conductor helically supported thereby defining respective element axes extending from said ground plane in a direction substantially perpendicular thereto. For example, different embodiments may comprise two, four or more helical antenna elements, which, depending on the embodiment and the application for which the array is intended, may be substantially identical elements, or structurally or operationally different elements. 
         [0031]    As will be appreciated by the person of skill in the art, different embodiments may be designed and used for different applications. For instance, and as introduced above, helical antenna arrays are commonly used for satellite communications, which may include but are not limited to ground and/or airborne satellite communications, such as described above in the context of aircraft communications. Clearly, while some of the embodiments described below may be particularly amenable for use in aircraft communication systems, these embodiments are not intended to be limited as such, as the features of these embodiments, and the operational improvements and/or advantages provided thereby, may be equally applicable in other contexts where helical antenna arrays are commonly used, as will be appreciated by the person of ordinary skill in the art. For the purpose of the following description, however, the embodiments of the invention will be described within the context of aircraft communications, and particularly, wherein an antenna array is generally mounted for operation within the limited spatial confines of a radome or the like, as commonly found at the tail end of an aircraft, and wherein operation of the antenna array requires a certain level of spatial freedom in allowing the array to sweep a suitable scan area to provide suitable coverage. Accordingly, in accordance with some embodiments, improvements in the performance of the antenna array are provided in comparison with traditional arrays having similar spatial dimensions or profiles, thereby providing a potential replacement for traditional arrays without imposing changes to existing spatial restrictions for such antennas. 
         [0032]    For instance, and in accordance with some embodiments of the invention, the antenna array may incorporate one or more of the below-described modifications, which, alone or in different combinations, may increase the overall gain in the array, reduce dissipative losses in the array, mitigate mutual couplings between antenna elements, or correct the squinting effect commonly found in such arrays due to electromagnetic couplings between elements. In the context of a steerable antenna in aircraft communication systems, where a helix array may be subject to continuous reorientation by tilting the array and its beam so that it can be pointed in different directions, these modifications may, in accordance with different embodiments, allow for maintaining an overall sweeping volume of the antenna array while achieving higher gains. Further, the antenna structure can generally be rotated about each of two orthogonal axes in order to synthesize volumetric coverage. In some embodiments, each axis passes through the centre of the antenna structure, thereby reducing the scan envelope of the array, i.e. the single envelope that contains the antenna assembly in all its various different scan orientations; this scan envelope will thus fix the minimum size of the radome structure within which the antenna components can be housed. On an aircraft, there are generally many hard limitations relating to the available spaces within which the antenna can be installed; therefore, achieving significant operational gains without significantly increasing the overall antenna structure can provide significant advantages in this field. As indicated above, however, the operational gains achieved by the embodiments of the invention herein described are equally applicable in other contexts where structural size limitations are not as strictly applicable. 
         [0033]    It will be appreciated that the examples provided below describe, in accordance with different embodiments of the invention, different features, which, alone or in combination, can allow for an improved helical antenna array performance. Accordingly, the person of skill in the art will appreciate that while different features are combined in describing a same exemplary embodiment, these features may be equally considered alone or in different combinations to provide different desirable effects without departing from the general scope and nature of the present disclosure. 
         [0034]    Referring now to  FIGS. 1 to 4 , and in accordance with one exemplary embodiment of the invention, a helical antenna array, generally referred to using the numeral  100 , will now be described. As shown in these Figures, the array  100  generally comprises a ground plane  102  and four substantially identical antenna elements  104 , each one of which extending substantially perpendicularly from the ground plane and comprising a support structure  106  and a conductor  108  (e.g. conductive wire) helically supported thereby. It will be appreciated that while four antenna elements are depicted herein, different numbers of antenna elements may be considered herein without departing from the general scope and nature of the present disclosure. Namely the four-element examples depicted herein are meant as exemplary only, as the features described herein may be equally applicable to other arrays comprising two, three, four or more antenna elements. 
         [0035]    In this particular embodiment, each support structure  106  is shaped such that respective conductors  108  are wound thereabout to define respective non-linear axes (not explicitly shown) which results in a mutual distancing between element axes over at least a portion of these axes. Namely, antenna elements  104  are shown to diverge laterally from one another over a base portion thereof (i.e. a portion of the elements near the ground plane  102 ). In particular, a non-linear axis distancing is maximized along the diagonal axes of this array, namely maximizing their effect with respect to a geometrical centre of the array. This initial distancing, in operating the array  100 , will have the effect of substantially redressing respective beams generated by the antenna elements  104 , thereby at least partially mitigating the mutual coupling or squinting effect that is otherwise common with linear antenna elements, and increasing the operable gain of the array. 
         [0036]    Furthermore, the support structures  106  are shaped such that, while non-linear axes allow for an initial distancing between elements, these axes are brought back together toward the terminal or radiating end  110  of the elements, providing for an intermediate bulging  112  in the antenna elements. In this particular embodiment, the helix radius is also increased toward the center portion of the helix, as will be described in greater detail below, thereby participating in the creation of the intermediate bulging  112 . Accordingly, while the initial distancing/bulging is provided to induce a redressing of respective element beams, this distancing is not maintained for the length of the antenna elements, but rather, it is brought back toward or even to its original configuration, thereby reducing the effect this distancing may otherwise have on the sweeping envelope of the array. A similar bulging effect can be obtained by a variety of other means including dielectric loading, introduction of offset resonators or additional winding segments that are offset from the helix axis and distortion of the winding dimensions such that the outer portions of the windings are fattened. 
         [0037]    Referring to  FIGS. 1 to 4 , near the lower half of the structure where the winding&#39;s current amplitude is generally high, the perturbed or bent geometry has the effect of tilting the wave front of each individual helix outwards, or away from the geometrical centre of the array. This effect can compensate for the inward tilt angles brought about by couplings between helices. Towards the top or radiating ends of the array, the tilt angle perturbations are of the reverse sign, but the winding current has a much reduced amplitude, and in consequence the reverse sign tilt angles have little effect on the additional gain achieved by the outward inclination of the antenna elements near the base end thereof. In other words, the gain is not fully or even significantly reduced by the subsequent inward inclination of the elements. 
         [0038]    Therefore, depending on the parameters selected in defining and forming these non-linear axes, the performance of the antenna array can be increased without necessarily increasing its sweeping envelope. For example, in the illustrative embodiment of  FIGS. 1 to 3 , the non-linear axes are defined by respective non-linear perturbations extending along one or more of the vertices of the otherwise octagonal helix structures. Accordingly, the centre of the octagonal section for a given winding turn is displaced laterally, and the radius of its octagonal shape is also increased. However, the magnitudes of each of these two perturbations vary along the length of the winding. Specifically, the perturbations reduce to zero at either winding end (i.e., at the terminal and base ends of each element), that is, a winding cross section at the terminal end of a given antenna element substantially overlaps a winding cross section at the base end thereof. 
         [0039]    For the purpose of illustration,  FIGS. 5 and 6  provide different examples of antenna element cross sections taken both at the base and terminal ends (e.g. overlapping cross sections shown in hard line with geometrical center thereof identified by the ‘+’ symbol), and at an intermediate level along the antenna elements&#39; respective non-linear axes (shown in dashed lines with laterally displaced geometrical centers thereof identified by the ‘×’ symbol). In these examples, the intermediate cross-sections are shown as both laterally displaced and increased in size, but of a same shape (i.e. circular). In the example provided in  FIG. 5  for a quadrilateral array, the respective geometrical centers  500  are displaced symmetrically with respect to a geometrical center of the array  550 , i.e. along diagonal axes thereof. In  FIG. 6 , respective displacements for a dual array are shown with respect to a lateral axis joining the two antenna elements. It will be appreciated by the person of ordinary skill in the art that these embodiments are meant as examples only, as different perturbations and/or variations in element cross sections and alignment may be considered within the present context to define non-linear element axes and achieve similar results. 
         [0040]    Referring now to  FIGS. 1 to 4 , the antenna array  100 , in accordance with one embodiment of the invention, further comprises a number of additional features, which, alone or in combination, may allow for an improvement in array performance. 
         [0041]    For example, the ground plane  102  generally comprises a conductive sheet  130  or the like upon which the antenna elements  104  are mounted. As depicted in  FIGS. 1 to 4 , the ground sheet  130  extends laterally to define the base of the array, and terminates along its edges in a raised lip  132 . The ground plane  102  may be shaped to define a notch  134  through which a suitable dielectric spar  136  may be introduced for cooperative coupling to an array mounting structure  138  provided on the ground plane  102 . The spar may allow for operative coupling of the array to a drive mechanism configured for rotating the array about an axis thereof. For example, the present embodiment allows for the array to rotate about a lateral axis located through a geometrical centerline of the array such that the rotation thereabout does not outwardly extend the sweeping envelope of the array. The present embodiment also allows for the array to longitudinally rotate about a perpendicular axis defined by a corresponding geometrical centerline of the array. The longitudinal rotation may be implemented through a rotation platform  140  upon which the spar  136  is mounted. Accordingly, the combined mechanism allows for a reorientation of the antenna array  100  about orthogonal axes within a prescribed sweeping envelope substantially defined by the diameter of the base plane  102  and the diameter of the array at the terminal end of the helical antenna elements  104 . For this purpose, the outer edge of the ground plane may be appropriately shaped to allow for the rotation of the four-helix array without mechanical interference with the scanning mechanism. 
         [0042]    In another embodiment, one or more ground cups, rather than a single ground plane, may be used to provide, in some implementations, for greater efficiency and gain. 
         [0043]    In another embodiment, the spar  136  is manufactured of a dielectric material incorporating one or more air pockets as a means for reducing the amount of dielectric material within the array volume and thus reducing the potential impact that the spar may have on array performance. 
         [0044]    Still referring to  FIGS. 1 to 4 , the nominal helix axes may further be rotated relative to each other such that the space between their respective feed points is increased for reduced coupling and increased array gain. 
         [0045]    It is apparent that the foregoing embodiments of the invention are exemplary and can be varied in many ways. Such present or future variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.