Abstract:
An antenna is disclosed that comprises a pair of conductive, orthogonal arches and a pair of conductive annular sector plates, wherein adjacent legs of each arch are fastened to one of the annular sector plates and the opposite adjacent pair of legs is fastened to the remaining annular sector plate. The entire antenna structure is spaced apart from a conductive ground plane by a thin dielectric medium. The antenna is driven by a feed conduit passing through the conductive ground plane and dielectric medium and attached to one of the annular sector plates, wherein the two orthogonal arched act as a pair of crossed dipole elements. This arrangement of elements provides a radiation pattern that is largely omni-directional above the horizon.

Description:
STATEMENT OF GOVERNMENT SUPPORT 
   The United States Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. 

   CROSS REFERENCE TO RELATED APPLICATIONS 
   None 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a radio antenna. More particularly, the present invention relates to an improved radio antenna that is compact, mountable to a conductive surface, and having nearly constant gain over a hemisphere of solid angle so that it is essentially omni-directional when located near the surface of the earth. 
   2. Related Art 
   It is generally known that antenna performance is dependent upon the size and shape of the constituent antenna elements as well as the relationship between various antenna physical parameters (e.g., the length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal. These relationships determine several antenna operational parameters, including input impedance, gain, and radiation pattern. In general, the minimum physical dimension for an operable antenna is on the order of a quarter wavelength of the operating frequency or some multiple thereof. 
   The rapid and wide spread growth and utilization of GPS and wireless communications and the evolution of the devices that support these systems has created a continued need for physically smaller, more efficient antennae that are capable of wide bandwidth operation, and multiple frequency-band operation. As the size of these devices shrink, the antennae used by the devices must shrink correspondingly. Thus physically small antennae operating in the frequency bands of interest and providing properties such as high gain and omni-directionality continue to be sought after. 
   One antenna commonly used in many applications today is the half-wavelength dipole antenna. The radiation pattern of this device is the familiar toroidal donut shape with most of the energy radiated uniformly in 360° of rotation perpendicular to the longitudinal axis of the dipole with energy decreasing with increasing angular elevation from the horizon. Antenna gain, therefore, is highest for a vertical dipole in a plane of the horizon and decreases with increasing angular elevation from the horizon. In order to efficiently detect systems such as GPS and cellular signals, it is desirable to have an antenna whose gain is nearly constant gain over a hemisphere of solid angle so that it is essentially omni-directional above the horizon for antennae located near the surface of the earth. 
   SUMMARY 
   It is therefore an object of this invention to provide an improved antenna having an essentially omni-directional above the antenna horizon. 
   Another object of the invention is to provide an improved antenna that is easily tunable with simple circuit elements such as capacitors. 
   Yet another object of the invention is to provide an antenna designed to use a metallic surface under it as a ground-plane. 
   A further object of the invention is to provide an antenna that can provide a circularly polarized signal. 
   To achieve these and other objects, there is provided an antenna structure having hemispherical orthogonally crossed elements that may be electrically fed together or separately. Moreover, these and other objects, advantages, and features of the invention will become apparent to those skilled in the art after reading the following description of the various embodiments when considered with the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating one or more preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings: 
       FIGS. 1A-1C  show the derivation of the antenna according to the present embodiment from a loop antenna. 
       FIG. 2A  is a top view of an antenna according to a hemispheric embodiment of the present invention showing the two gaps between the horizontal arcs. 
       FIG. 2B  is a cross-sectional side view of the antenna according to the embodiment shown in  FIG. 2A , the relationship between the cross-like members and the semi-circular ring segments, and the dielectric layer and ground plane. 
       FIGS. 3A-3C  show various embodiments of conductive plates that may be used to practice the invention. 
       FIGS. 3D-3F  show various radiation structures comprising two or more dipole elements illustratively joined with different conductive plate configurations. 
       FIGS. 3G-3I  show cross-sectional views of a variety of different radiation structure geometries which may be used to practice the invention. 
       FIG. 4  is a cross-sectional view of the antenna according to the embodiment of  FIG. 2A  showing the electrical field during antenna operation. 
       FIG. 5  shows an electrical circuit which models the electrical behavior of the antenna described herein. 
       FIG. 6  shows a photographic image of an antenna constructed in accordance with the embodiment of  FIG. 2A . 
       FIG. 7  shows the return loss of the antenna measured on a network analyzer. 
       FIG. 8  shows a simulated radiation pattern of the antenna. 
       FIG. 9  shows a cross-sectional view of the embodiment shown in  FIG. 2A  showing the interior of the antenna filled with a dielectric material other than air. 
       FIG. 10A  shows a top view of an antenna according to the present embodiment modified to provide circularly polarized transmission. 
       FIG. 10B  shows a cross-sectional side view of an antenna according to the present embodiment modified to provide circularly polarized transmission. 
       FIGS. 11A and 11B  show cross-sectional side views of two different embodiments of an antenna modified to provide circularly polarized transmission, wherein the crossed elements have the same diameter and where a portion of the center one element is deformed to allow access to the crossing arm. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following disclosure and in the appended claims, terms such as “normal” and “right angles,” are used which relate one structure to another or to the environment. These terms are intended to mean “generally,” or “substantially” normal, etc., to allow for some reasonable degree of tolerance that does not preclude the substantial attainment of the objects and benefits of the invention, and are not intended to mean “exactly” 90°. 
   In general, the size of an antenna should be an integer fraction of the wavelength transmission/reception. However, in addition to changing its size, the resonant frequency of an antenna can be altered also by simple changes in its physical structure. The following design shows an antenna that can be small compared to the wavelength. The design being advanced is a derivative of a loop antenna. In general,  FIGS. 1A-1C  show the design progression as follows: 
   The starting point for the design is a conventional vertical loop antenna  1 , such as in  FIG. 1A ; 
   A second vertical loop antenna  2  whose axis is perpendicular to the first loop  1  is added as shown in  FIG. 1B ; 
   The upper half of the combined structure, obtained by slicing midway with a horizontal plane, is attached to a circular ring that is split into two arcs  3  and  4 , as shown in  FIG. 1C ; 
   The resultant cross-shaped dome-like structure is then placed above a ground plane  5  with an intervening dielectric layer  6  to prevent the structure from directly contacting the ground plane; and 
   An electrical feed-point  7  to the antenna is placed between the ground plane and one of the horizontal arc segments. 
   The antenna of one embodiment of the invention, therefore, is shown in  FIGS. 2A and 2B  and comprises a split horizontal annular plate combined with two semi-circular arch-like structures all of which are conductive and in electrical communication with one another such that when the arch-like structures are electrically driven with a radio frequency (RF) signal they function as crossed dipole radiator elements. However, while a hemispherical structure is shown in  FIG. 2A , many other geometries are possible. These may include, but are not limited to, a hemi-ellipsoid or oblate hemisphere; a cube; an orthorhombic prism; and a polyhedral pyramidal structure, wherein the structures may comprise single straight segments, multiple-straight segments, single curving segments, or a combination of straight and curving segments. Moreover, the number of dipole radiators (i.e., a mirror-image pair of oppositely directed elements) may be any number greater than 2. Examples of these structures and various combinations thereof are shown in  FIGS. 3A-3I  and while not all may be practical they are shown for illustrative purposes as delineating the scope of the embodiments described herein. 
   The simplest of these embodiments is shown in  FIGS. 2A and 2B  and forms the basis for describing the present invention. However, other structures are possible such as those shown in  FIGS. 3A and 3B . 
   The antenna, in accordance with the embodiment illustrated by  FIGS. 2A and 2B , is described as follows. Antenna  10  comprises a pair of conductive plates, in this case semi-circular ring segments  11  and  12  cut from a flat plate each forming a portion of an annulus. Ring segments  11  and  12  rest on dielectric layer  13  above conductive ground plate  14  and are located opposite each other at a mirror-plane and on a common diameter such that opposite ends of each ring form a gap  15  of equal size at either side of the sector sections. In addition, antenna  10  further comprises an electrically conductive radiation structure  20 , shown in  FIG. 2B  as a hemispherical dome having four adjacent and equally sized sector wedges cut through the thickness of the dome to form a cross-like structure comprising two wide, semicircular arches  16  and  18  crossing each other at right angles. Semicircular arches  16  and  18 , therefore, comprise two pairs of oppositely directed radiator elements. Moreover, structure  20  is joined to ring segments  11  and  12  in such a way that the inside edge of each of the legs  21 ,  22 ,  23  and  24  of structure  20  are located along a common diameter between the inside and outside diameters of ring segments  11  and  12 . In addition, legs adjacent one another across the gaps  15  are disposed about equidistant from each other. 
   In general, antenna  10  is electrically excited on one of the two ring segments  11  and  12  at feed point  19 . The opposite side of the horizontal ring segments  11  and  12  are optionally connected using an electrical element such as a capacitor to provide additional tuning flexibility. Finally, antenna  10  is physically secured above the ground plane using a set of fasteners such as screws or bolts (not shown). However, care must be taken to ensure that the fasteners do not provide an electrical path between the ground plane and the antenna structure since the dielectric insulating layer is intended to act as a capacitor from the antenna to ground. That is, the fasteners must be either electrically insulating (e.g. nylon screws) or electrically isolated from the horizontal ring by using a heavy plastic bushing or insert sleeve, for instance, around each of the bolts or screws. Alternatively, the major parts of the antenna may be fastened to the dielectric and the dielectric to the ground plane by the use of an adhesive layer.  FIG. 4  shows the circuit of the electric current as the antenna, according to the present embodiment, is driven with a radio frequency signal. 
   The antenna has a narrow bandwidth and must be tuned to the desired frequency. As seen in the electrical model of the antenna shown in  FIG. 5 , the thickness of the dielectric insulating plate and the gaps between the horizontal annular plates substantially affect the capacitance of the antenna. Z r  encapsulates the radiation resistance and inductance of the antenna. C d  is the capacitance between each horizontal arc of the antenna structure and the ground plane. If extra capacitance is added between the “free” end of the horizontal ring and the ground plane, it will contribute to C d . C g  is the capacitance of the gap  15  between each of the two arcs. 
   Therefore, dielectric insulator  13  acts as a capacitor from the antenna to ground as do gaps  15  between the conductive plate segments shown in  FIG. 2A . Both provide a means for adding capacitance from the primary “feed” arm of the antenna to the secondary arm and both of these features can be adjusted to tune the antenna for the desired frequency. In particular, the antenna frequency can be changed by i) altering the thickness of the dielectric insulator; ii) by changing the width of the gap between the horizontal arcs; iii) by adding additional capacitance between the “free” end of the horizontal ring and the ground plane; or iv) by changing a combination of these parameters. By adjusting these parameters, the antenna can be forcibly tuned to a frequency much smaller than the resonant frequency of a simple loop antenna of similar dimensions. Furthermore, this antenna is designed to use the metallic surface under it as a ground plane and is not negatively impacted by it. 
   The design described herein can be fabricated in many ways. The ground plane underneath the antenna must be conductive; and while this requirement may be met in many ways, a piece of metal sheet stock or a metal-coated surface will suffice. The dielectric layer above the ground plane can be made from any electrically insulating materials such as plastics, plastic resins, epoxy resins, mica, glass, and the like. In particular, acetal (e.g. DELRIN®) or polycarbonate (e.g. LEXAN®) resins, or filled, epoxy resins such as fiberglass are useful in this regard since they are relatively inexpensive, and can be purchased as sheet stock readily available in a variety of thicknesses. The dome structure of the embodiment of  FIG. 2A  can be made from any useful electric conductor but is best fabricated from a common metal or metal alloy such as aluminum, copper, or steel. Alternatively, the dome structure may be cast or molded from a polymer resin, a thermoplastic, or a thermosetting is plastic and then coated with a conducting layer either by electrical or electroless plating, vapor spraying, sputtering, particle vapor deposition, chemical vapor deposition. The thickness of the conductive coating affects antenna losses. 
     FIG. 6  shows a prototype of a finished antenna that was machined out of aluminum, anodized, and coated with nylon. A 2.4 mm thick sheet of polycarbonate plastic was used as the dielectric insulator. To measure the performance of the prototype antenna, the antenna was attached to the ground plane as described above and then connected to a network analyzer and the return loss was measured. As is shown in  FIG. 7 , the present antenna exhibits a modest return loss of −13.1 dB at 287.5 MHz and a much better return loss of −27.3 dB at 299 MHz. In order to estimate the radiation behavior of the prototype antenna, a simulation was run using simulation software available from Ansoft Corporation (Pittsburg, Pa.).  FIG. 8  provides a graphical representation of the antenna simulated radiation pattern showing it is indeed essentially omni-directional in azimuth and in elevation from the horizon to zenith. 
   Alternative Embodiments 
   The antenna can be operated at other frequencies by adjusting the parameters previously described. Scaling the physical size of the antenna will also result in a corresponding change in operational frequency, e.g. reducing the size of the antenna will allow it to operate at higher frequencies. 
   Another embodiment comprises filling the interior space beneath the crossed elements of the antenna and the ground plane with a dielectric medium  90 , other than air, such as is shown in  FIG. 9 . Moldable materials such rubbers, foams, and curable resins are useful. In particular, natural and synthetic dielectric material such as mica, wood, glass, gypsum, chalk, ceramic, various oxides and carbonates, rubbers, phenolics, urea and maleimide resins, polymers, polymer resins, epoxy resins, acetal resins, acrylics, polyvinyl chlorides, polyurethanes, polyisocyanurates, polytetrafluoroethylenes, thermoplastic plastics, thermosetting plastics, and combinations thereof, are particularly useful. This approach has the effect of making the antenna electrically “smaller” and therefore able to operate at lower frequencies by changing the dielectric value of the interior volume of space beneath the crossed elements of the antenna and the ground plane. It is to be understood, of course, that while  FIG. 9  illustrates an embodiment having a particular radiation structure any of the other structure described above are equally useful. 
   Another embodiment comprises an antenna structure that provides circularly polarized radiation. As shown in  FIGS. 10A and 10B , a simple modification to the preferred embodiment can be made which amounts to replacing the cross-like structure of  FIGS. 2A and 2B  with two separate semicircular arch elements  104  and  106 , wherein one arch extends over the other, and wherein a dielectric pad  102  separating the two where the two members cross each other as is shown in  FIG. 10A . This embodiment also includes replacing the two semi-circular, annular ring segments with four equivalent smaller ring segments by bisecting each of the former annular ring segments such that each of the two ends of each arch rests on two separate segments. An equivalent structure is shown in  FIGS. 3C and 3F  which uses quarter segments of a square conductive plate. First and second “feed” lines (not shown) are connected to electrical feed points  103  and  105  attached to two adjacent ring segments such that each of the separate arch elements can be separately is driven by an electrical signal. Again, it is understood that the radiation structures described in this embodiment may be replaced with any of the structure described above and illustrated in  FIGS. 3D through 3I . 
   Furthermore, this alternative embodiment may be deployed in two different configurations. The first comprises a structure wherein the two semicircular arches have different diameters. The second comprises the structure shown in  FIGS. 11A and 11B  wherein both of the two arches have the same diameter but wherein one of them includes either an intermediate rise or dip in its diameter along a short distance at the center of its length depending on whether the one arch passes over or under the second arch. Both of these alternative embodiments allow each of the two arch elements to be driven separately allowing an operator to control the signal phase fed into each element and, therefore, the polarity of each element. Moreover, the structures illustrated in  FIGS. 3G through 3I  can be similarly modified and applied to this embodiment. 
   Finally, to the extent necessary to understand or complete the disclosure of the present invention, all publications, patents, and patent applications mentioned herein are expressly incorporated by reference therein to the same extent as though each were individually so incorporated. 
   Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the disclosures herein are exemplary only and that various other alternatives, adaptations, and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited to the specific embodiments as illustrated herein, but is only limited by the following claims.