Abstract:
In one embodiment, an antenna has two annular conductive elements separated by a slot, and a third conductive element connecting the annular conductive elements together at at least one end of the slot. In another embodiment, an antenna has two generally parallel conductive elements of different heights separated by a slot, and a third conductive element connecting the conductive members together at at least one end of the slot.

Description:
BACKGROUND 
     The invention relates to a slot antenna. 
     Wireless radio systems are used in remote metering (e.g., utility metering) applications in which electronic components must be placed in spaces not originally designed for such components. In water metering applications, for example, a transceiver and an antenna typically must fit within a small underground housing originally intended only for a mechanical water meter. In such an application, antenna performance is impeded because the antenna must transmit through the walls and lid of the underground housing and through the ground itself. 
     SUMMARY 
     In one aspect, the invention features an antenna having two annular conductive elements separated by a slot, and a third conductive element connecting the annular conductive elements together at at least one end of the slot. In some embodiments the conductive elements may be of similar diameters; they may be of different heights; they may consist of conductive tape or conductive wire; they may be less than two inches in diameter. The antenna also may include a dielectric insulator, and it may include a feed point to which a feed element may connect. The antenna also may be less than 0.5″ in total height. 
     In another aspect, the invention features an antenna having two generally parallel conductive elements of different heights separated by a slot, and a third conductive element connecting the conductive members together at at least one end of the slot. In some embodiments, the generally parallel conductive members may extend along a dielectric material; and the third conductive element may connect the generally parallel conductive elements together at two ends of the slot. 
     In other aspects, the invention features methods of making an antenna. One method includes providing a generally straight slot antenna, and securing two ends of the generally straight slot antenna to form an annular slot structure. Another method includes positioning two annular conductive elements to form a slot between them, and connecting a third conductive element to each of the annular conductive elements to form at least one end of the slot. Another method includes positioning two conductive elements of similar lengths and of different widths to form a slot between them, and connecting a third conductive element between the other two conductive elements to form at least one end of the slot. Still another method includes providing a stamp having two annular conductive elements in the same plane connected together by a third conductive element, and bending the stamp so that the two annular conductive elements are no longer in the same plane but are essentially parallel to each other. 
     In another aspect, the invention features a stamp for use in forming an antenna. The stamp includes two annular conductive elements connected by a third conductive element, all three of which lie in substantially the same plane. 
     Advantages of the invention may include one or more of the following. An antenna may be made small enough to fit entirely or partially within a pre-drilled hole formed in a standard underground housing lid. The antenna also may be housed within a protective structure that passes through such a pre-drilled hole and that positions the antenna above the ground. 
     Vertical polarization of an antenna may be achieved with a very small vertical dimension (e.g., 0.5″ or less). A simple slot structure may be used to create an antenna having an omnidirectional radiation pattern. The conductors used to form the slot structure may have different heights (an “offset slot” structure), which allows, e.g., more clearance between the radiating slot and an underground housing lid. Furthermore, the antenna may be fed at a position offset from the center of the slot, which provides a simple way to match the input impedance of the antenna with the characteristic impedance of the conductor feeding the antenna. The antenna may include a dielectric other than air to reduce the wavelength of a transmitted or received signal in the antenna, which in turn allows, e.g., reduction of the slot length and therefore the antenna&#39;s overall dimensions. 
     The antenna may be fabricated easily and inexpensively from, e.g., a conventional straight slot antenna, a die-cut stamp, or conductive wires or strips. 
     Other advantages of the invention will become apparent from the following description and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is perspective view of a vertically-polarized, omnidirectional antenna. 
     FIG. 2 is a perspective view of an alternative configuration of a vertically-polarized, omnidirectional antenna. 
     FIG. 3 is a view of a straight slot antenna that may be used to form a vertically-polarized, omnidirectional antenna. 
     FIGS. 4A and 4B are views of a die-cut stamp that may be used to form a vertically-polarized, omnidirectional antenna. 
     FIG. 5 is a schematic view of a vertically-polarized, omnidirectional antenna connected to a radio transceiver in an underground water meter. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a vertically-polarized, omnidirectional slot antenna  10  consists of two annular (or ring-shaped) conductors  12 ,  14  centered along a common longitudinal axis  16  and joined by a conductive shorting post  18 . The annular conductors are separated by a slot  20 , the circumferential dimension L 1 (“length”) of which equals the length L 2 , L 3  (circumference) of each annular conductor  12 ,  14  less the length L 4  of the conductive shorting post  18 . The vertical dimension H 1  (“height”) of the slot  20  defines the distance separating the annular conductors  12 ,  14 . The annular conductors  12 ,  14  and the conductive shorting post  18  may consist of virtually any conductive material, but highly conductive metals, such as copper, silver, or aluminum, are especially suited for use in the antenna  10 . The annular conductors  12 ,  14  may be conductive strips with height dimensions H 2 , H 3 , as shown in FIG. 1, but other structures, such as conductive wires, also may be used. 
     The antenna is driven by signals from a bipolar signal feed element  24 , such as a coaxial cable or a balanced two-wire line, the conductors  26 ,  28  of which each connect to one of the annular conductors  12 ,  14 . Because the conductors  26 ,  28  of the signal feed element  24  connect across the slot  20 , the annular conductors  12 ,  14  are driven at opposite polarities, creating a vertically-polarized electric field. Unlike a standard center-fed slot antenna (i.e., an antenna fed at a position equidistant from the slot&#39;s ends), antenna  10  may be fed at any point along the length L 1  of the slot  20  (i.e., the signal feed element  24  may be connected at any point along the periphery of the annular conductors). Typically, the position of the signal feed element  24  is selected so that the input impedance of the antenna  10 , as seen by the signal feed element  24 , matches the characteristic impedance of the feed element  24 . The antenna&#39;s input impedance is approximately zero if the feed element  24  is connected at the shorting post  18  and increases as the feed position moves away from the shorting post  18  toward the center of the slot  20 . When a typical fifty ohm coaxial cable is used as the feed element  24 , the feed position is selected to yield an input impedance of 50+j0 ohms. In practice, the appropriate feed position for a particular antenna may be determined by measuring continuously the antenna&#39;s input impedance as the position of the feed element  24  is varied. 
     The annular conductors  12 ,  14  typically wrap around a cylindrically-shaped dielectric insulator  22 . In general, any dielectric material may be used, including inexpensive materials such as Styrofoam®, Teflon®, or plastics having relatively low dielectric losses. In some applications, air may serve as the dielectric, eliminating the need for the insulator  22 , in which case a non-conductive support member could be positioned opposite the shorting post  18  to support the annular conductors  12 ,  14 . 
     The diameter of the dielectric insulator  22 , and therefore the lengths of the annular conductors  12 ,  14  and the slot  20 , are determined by several factors, including the frequency at which the antenna  10  is to operate and the dielectric constant (K) of the insulator  22 . In general, the length L 1  of the slot  20  should be less than but approximately equal to ½-wavelength in the dielectric at the desired frequency of operation, which allows the antenna  10  to operate with no phase reversals in the RF currents created in the antenna  10 . The exact length of the slot  20  is determined by adjusting its length until the antenna is near resonance at the desired operating frequency. Since the wavelength of a transmitted or received signal in the antenna  10  is inversely proportional to the square-root of the effective dielectric constant of the insulator  22  and surrounding air, the diameter of the insulator  22  declines as the dielectric constant of the material increases. 
     The height H of the antenna is limited only by the spacial constraints of the application in which it is to be used and by the minimum heights of the annular conductors  12 ,  14  and the slot  20  required for proper operation. The antenna  10  therefore is vertically-polarized with a very small minimum vertical dimension, and because the antenna  10  is annular and has no phase reversals in the RF currents, its radiation pattern is omnidirectional (i.e., the antenna radiates a full 360° around the longitudinal axis  16 ). 
     The annular conductors  12 ,  14  and the shorting post  18  may be fastened to the dielectric insulator  22  in many ways. For example, the annular conductors  12 ,  14  and the shorting post  18  may consist of a conductive strip with an adhesive backing (e.g., copper tape) that adheres to the dielectric insulator  22 . A conductive material, such as a metallic wire or solder connection, may be used to bridge any gaps that may exist between the shorting post  18  and either of the annular conductors  12 ,  14 . Alternatively, the annular conductors  12 ,  14  and the shorting post  18  may be set into grooves formed in the outer surface  30  of the dielectric insulator  22 . 
     In FIG. 1, the annular conductors  12 ,  14  are of approximately equal height and have height dimensions H 2  and H 3  that are approximately twice as large as the height dimension H 1  of the slot  20 . This configuration produces a radiation pattern that travels in a direction generally perpendicular to the longitudinal axis  16  of the antenna and that is centered at the middle of the antenna&#39;s overall height dimension H. Referring also to FIG. 2, the height dimension H 3  of the lower conductor  14  may be greater than that of (H 2 ) of the upper conductor  12 . This places the slot  20  nearer the top of the antenna  10 , which in turn causes the antenna  10  to radiate energy at points higher than those emitting energy in the configuration of FIG.  1 . The configuration of FIG. 2 is useful, e.g., when the antenna  10  is to operate close to the ground, such as in the underground metering application described below. 
     Referring to FIG. 3, an annular slot antenna may be formed from a straight slot antenna  50  having two conductors  52 ,  54  of similar lengths L 2 , L 3 . The conductors are separated by a slot  56  and connected at their ends  58 ,  60  by shorting posts  62 ,  64 . An annular slot antenna is formed by bending the straight slot antenna  50  until its ends  58 ,  60  meet and then securing (e.g., soldering) the ends  58 ,  60  together. When the ends  58 ,  60  are connected, the shorting posts  62 ,  64  join to form a single shorting post like that shown in FIG.  1  and FIG.  2 . The straight slot antenna  50  may or may not be wrapped around a dielectric insulator. 
     Referring to FIGS. 4A and 4B, the antenna also may be formed from a die-cut stamp  70  created from a conductive (e.g., aluminum) sheet. The stamp  70  includes two annular sections  74 ,  76  connected together by a conductive post  78 . The annular sections  74 ,  76  intersect the post  78  at two “bend points”  72   a ,  72   b , respectively. Two conductive stems  80 ,  82  extend from the inner surfaces  84 ,  86  of the annular sections, intersecting the annular sections at two additional “bend points”  72   c ,  72   d , respectively. The die-cut stamp  70  is inexpensive and easy to create in mass production. 
     To form the antenna  10 , the stamp  70  is bent by 90 degrees at each of the four bend points  72   a-d . Each of the annular sections  74 ,  76  of the stamp  70  forms one of the annular conductors  12 ,  14  of the antenna  10 , and the conductive post  78  forms the antenna&#39;s shorting post  18 . Likewise, the two conductive stems  80 ,  82  form the conductors  26 ,  28  of the signal feed element. A non-conductive support (not shown) may be placed between the annular conductors  12 ,  14  to preserve the shape and dimensions of the antenna  10 . Also, a dielectric insulator (not shown here) may be placed within and/or between the annular conductors  12 ,  14 . 
     Referring now to FIG. 5, a vertically-polarized, omnidirectional slot antenna  10  is suited for use in remote metering applications in which an underground device, such as a water meter  32 , must exchange information over a wireless channel with a control center (not shown). In a typical situation, the water meter  32  and an electronic transceiver  34  are located underground  35  in a housing  36  covered by a lid  38 , which typically is made from metal, fiberglass, or some other rigid and durable material. The antenna  10  is positioned either within or just above a standard sized hole  40  (usually less than two inches, and often approximately 1¾″, in diameter) formed in the lid  38 . A protective housing  42  made, e.g., of durable plastic protects the antenna  10  and secures it to the lid  38 . 
     In operation, the antenna  10  transmits signals provided to it by the transceiver  34  and receives signals transmitted by the control center at an assigned frequency, e.g., a frequency in the “Industrial, Scientific, and Medical” (ISM) band (902 MHZ to 928 MHZ). For a typical antenna operating, e.g., at 920 MHZ (λ air  =12.8″) and having an effective dielectric constant of about two, the length of the slot is approximately 4.5″, which is approximately ½-wavelength at the effective dielectric constant. The diameter of the antenna is about 1.5″, which allows the antenna to fit into a structure passing through the 1¾″ hole formed in the housing lid. The height of the antenna  10  in such an application typically is less than 1.0″ and often will be 0.5″ or less. The height dimension of the lower conductor typically is two to three times greater than the height dimensions of the slot and the upper conductor. 
     Other embodiments of the invention are within the scope of the following claims. For example, the annular conductors may take on any one of numerous shapes, including circular, ovular, hexagonal, etc. Also, the antenna may, in some applications, be mounted within the underground housing, e.g., to the underside of the housing lid. Furthermore, the antenna may be used in a wide variety of applications other than the underground metering application described above.