Abstract:
An antenna ( 14, 15 ) for installation in a subsurface ground enclosure has an F-shaped radiating element ( 15 ) having a rectangular strip ( 15   c ) disposed on edge and two spaced apart bars ( 15   a,    15   b ) disposed substantially perpendicular to the rectangular strip ( 15   c ) for connection to a first edge of a circuit board ( 31 ) and an L-shaped ground plane extension element ( 14 ) extending from a second edge ( 31   b ) of the circuit board ( 31 ) and then turning substantially perpendicular downward to provide a longer ground plane within a confined rectangular space. The antenna ( 14, 15 ) is dimensioned so as to be tuned to a resonant frequency in a range from 450 Mhz to 470 Mhz.

Description:
TECHNICAL FIELD 
     This invention relates to automatic meter reading (AMR) systems for collecting meter data signals over a geographical area, such as a municipality or municipal utility district, and more particularly to transmitter assemblies for location in subsurface utility enclosures installed in the ground. 
     DESCRIPTION OF THE BACKGROUND ART 
     In moderate climate zones, utility meters, particularly water meters, are located in subsurface enclosures in areas near residences or other dwellings. Such enclosures are referred to as “pits.” An example of such enclosure is illustrated in Cerny et al., U.S. Pat. No. 5,298,894, issued Mar. 29, 1994, and assigned to the assignee of the present invention. In these systems, a transmitter or transceiver, and an associated antenna, are enclosed in one or more sealed enclosures which are located in a larger pit for the water meter. The antenna must be assembled in a housing in which the electronics are encapsulated for moisture protection. The assemblies for the transmitter and antenna must be fairly compact to be mounted inside the pit enclosure. However, generally, the encapsulant should not contact the antenna and it may alter its performance due to capacitive effects of a dielectric material. 
     The invention provides an antenna for radiating at a specified frequency and sufficient gain to transmit signals to receivers in fixed networks as far away as possible for the available power. Typically, the transmitter assembly is powered by one or more batteries. Prior antennas have been able to transmit satisfactorily up to about 0.5 mile. With the antenna of the present invention, it is to increase this distance up to about 1.0 mile. 
     Therefore, the invention will enable one to provide an improved antenna in a transmitter assembly for installation in a subsurface enclosure. 
     SUMMARY OF THE INVENTION 
     The invention provides an antenna for a transmitter assembly for installation in a subsurface utility enclosure. 
     The antenna has an F-shaped element with two cross bars extending to one orthogonal edge of a circuit board, and a stem portion that is bent at approximately ninety degrees from the plane of the circuit board. The antenna also has an L-shaped ground plane extending from a second edge of the circuit board orthogonal to the first edge of the circuit board, the ground plane also having a portion bent at approximately ninety degrees from the plane of the circuit board to provide a compact lateral area for the assembly without increasing a height of the assembly. The F-shaped element and the L-shaped element are preferably made of a conductive metallic sheet material. 
     The antenna is dimensioned such that it is tuned to a resonant frequency in a preferred range from 450 Mhz to 470 Mhz. 
     The antenna provides the necessary range for a transmitter for reaching receivers in a fixed network while keeping the size of the assembly very compact. 
     Other aspects of the invention, besides those discussed above, will be apparent to those of ordinary skill in the art from the description of the preferred embodiments which follows. In the description, reference is made to the accompanying drawings, which form a part hereof, and which illustrate examples of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of the assembly including the antenna of the present invention; and 
         FIG. 2  is detail perspective view of the circuit board portion of the assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , the assembly  10  of the present invention has a housing with a bottom portion  11  and a lid  12  of non-metallic, non-RF interfering material. The bottom housing portion  11  has an L-shaped antenna compartment  13  for receiving the antenna  14 ,  15  and a rectangular battery compartment  16  for receiving at least one battery  17  and an upstanding L-shaped interior barrier  18  of two upstanding spaced apart and parallel walls  18   a ,  18   b  separating the antenna compartment  13  and the battery compartment  16 . The spacing in the antenna compartment  13  between the walls  18   a ,  18   b  and the outer wall of the housing bottom portion  11  has been slightly exaggerated in  FIG. 1 . Inside the housing bottom portion is an interior ledge  19  for supporting a circuit board assembly  30  and an L-shaped cover  20  seen in  FIG. 1 . In addition a post  21  is situated in the battery compartment  16  with a projection  22  for receipt in a hole  31   a  in the circuit board  31 . A cable  33  enters the housing through a slot opening  12   a  in the lid  12  and connects to the circuit board  3  near edge  31   e . The circuit board  31  receives signals from a meter register or meter encoder through the cable  33  representing units of consumption of a utility, and these are converted to radio frequency signals for transmission through the antenna  14 ,  15 . 
     A circuit board assembly  30  includes a circuit board  31  with circuitry for a radio transmitter. Although the invention is disclosed in the context of a transmitter, it is also applicable to a transmitter combined with a receiver (“transceiver”) for two-way communication, the term “transmitter should be understood to include a part of a transceiver. An L-shaped metal ground plane element  14  of conductive material extends from one edge  31   b  of the circuit board  31  and has a portion bent at a 90-degree angle to the extending portion. The antenna  14 ,  15  is designed to operate in a range from 450 Mhz to 470 Mhz. The L-shaped element  14  has some cut-out portions  14   a  near the edge  31   b  of the circuit board  31 . The antenna  14 ,  15  is designed to obtain resonant operation at 460 Mhz, at a highest available power by controlling the dimensions of the ground plane  14  and the radiating element  15 . The ground plane  14  and the circuit board  31  together have a length of 105 mm. If one part is shortened the other part must be lengthened to retain this dimension. 
     An F-shaped metal radiating element  15  extends from another edge  31   c  of the circuit board  31  that is orthogonal to the first edge  31   b . The F-shaped element  15  includes a wider top bar  15   a , which serves as a shorting member, and a narrower middle bar  15   b  that serves as a conductive member for the radiating energy to a broad, flat, F-stem strip  15   c  that is bent at a 90-degree angle to the two cross bars  15   a ,  15   b . The F-stem also extends for 105 mm. The minimum dimension for the F-shaped element  15  and the L-shaped ground plane would be 165 mm, which is ¼ of a 660 mm wavelength provided at 460 Mhz. The F-shaped element  15  and the L-shaped element were made longer to obtain resonant operation and higher gain. The antenna  14 ,  15  extends from two orthogonal edges  31   b ,  31   c  of the circuit board  31  in a plane defined by the circuit board with two portions bent at approximately ninety degrees from the plane of the circuit board  31  to provide a compact area-to-height aspect ratio. Although gain is reduced by the bending the two portions, it is more than made up for by the added length of the ground plane  14 . The F-shaped element and the L-shaped element are preferably made of a conductive metallic sheet material. 
     The battery  17  is encapsulated with a sealing material (not shown) in the battery compartment  16  and the interior barrier  18  forms a support for the printed circuit board  31  as well as a barrier against the intrusion of sealant into the antenna compartment  16  in which the antenna  14 ,  15  is disposed when the unit  10  is assembled. An internal cover element  20  is disposed around the printed circuit board  31  and over the antenna compartment  13  and the antenna  14 ,  15  to provide a second barrier against the entry of sealing material into the antenna compartment  13 . 
     A sealing material (not shown) is disposed in the battery compartment to protect the battery  17  from moisture. Sealing material is also disposed on both sides of the printed circuit board and at the location where an edge of the cover element  20  meets an inner wall of the housing bottom portion  11 . The sealing material is not disposed in the antenna compartment  13  or in contact with the antenna  14 ,  15 , except along the edges of the circuit board  31 , so as not to affect the operation of the antenna  14 ,  15 , due to a capacitive effect that the material would have on the electrical properties of the antenna  14 ,  15 . 
     Encapsulation of the electronics is necessary, because, the outer housing of thermoplastic material  11 ,  12 , is not impervious to water. 
     In a method of assembly, the circuit board  31  is positioned over the barrier  18  within a housing bottom portion  11 . The portions of the antenna  14 ,  15  extending from edges  31   b ,  31   c  of the circuit board are placed into corresponding portions of the antenna compartment  13 . The cover  20  is positioned around the circuit board  31  and over the antenna compartment  13  to shield the antenna compartment  13  from most of the encapsulating material. Encapsulating material (not shown) is injected into the space above the circuit board  31  and down around an edges  31   d ,  31   e  of the circuit board  31  separated by a gap from the outer wall of the housing bottom portion  11  such that encapsulating material flows into the battery compartment  16 , and fills the battery compartment  16  to cover the battery  17  and encapsulate the bottom side of the circuit board  31 . For a drawing of the encapsulant, reference is made to a copending application of the assignee filed on even date herewith and entitled “Sealed Transmitter Assembly for Subsurface Utility Installations.” The encapsulant, also referred herein to as sealing material, then seals both top and bottom sides of the circuit board  31 , the battery  17  or batteries and the cable  33  in the battery compartment  16  and any seam between the sealing cover  20  and inside wall of the bottom housing portion  11  and any gap between the circuit board  31  and the inside wall of the housing bottom portion  11 . The housing lid  12  is then placed over the housing bottom portion  11  to enclose the assembly  10 . It may be secured to the housing bottom  11  by a snap fit or other known methods. 
     Once the assembly  10  is assembled it may be installed in a pit enclosure, with the use of a bracket of the type disclosed in Bublitz et al., U.S. Pat. No. 6,378,817, that mounts the assembly  10  some distance under the pit lid or by fastening the assembly directly underneath the pit lid. It is assumed in this instance that the pit lid is made of a non-metallic material that does not interfere with radio signals to any great extent. 
     This has been a description of a preferred embodiment, but it will be apparent from the above description that variations of a type that are apparent to one of ordinary skill in the art may be made in the details of other specific embodiments without departing from the scope and spirit of the present invention, and that such variations are intended to be encompassed by the following claims.