Abstract:
A antenna ( 10 ) comprises a straight stem portion ( 11 ) and a spiral coiled portion ( 12 ) for providing the necessary length, but being compact enough for assembly in a pit tube ( 23 ) of a transmitter assembly ( 20 ) for subsurface enclosures for fluid meters.

Description:
CROSS-REFERENCE TO RELATION APPLICATION 
       [0001]    The benefit of priority based on U.S. Provisional Appl. No. 60/959,563, filed Jul. 13, 2008, is claimed herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to automatic meter reading (AMR) systems with radio transmitters or transceivers distributed in the field to receive metering signals and to transmit them to reader devices. In particular the invention is applied to systems where the meters and transmitters are disposed in subsurface enclosures, also know as “pit” enclosures. 
         [0003]    In automatic meter reading (AMR) systems, one embodiment of the prior art comprises: a printed circuit board, the battery and an antenna which are held together in a short plastic tube and encapsulated. The antenna assembly is then placed into a metal-walled pit with a metal or plastic cover. When installed in a subsurface pit installation, the antenna is at the level of the ground plane and projects slightly above the plastic or metal cover through a hole in the cover. Antenna radiation is strongly limited by the metal wall and especially a metal cover. In this situation, the antenna&#39;s size is strictly confined but high radiation efficiency and a high gain for the antenna are required. 
         [0004]    For electrically small antenna design, the uniform current distribution on the wire and the wire length meeting resonance condition are basic requirements to obtain high gain. Abrupt bending and zigzags decrease efficiency of small antennas as explained in W. L. Stutzman, and G. A. Thiele,  Antenna Theory and Design,  New York: J. Wiley &amp; Sons, 2003 and R. S. Elliott,  Antenna Theory and Design,  New York: J. Wiley &amp; Sons, 1998. 
       SUMMARY OF THE INVENTION 
       [0005]    The invention relates to an antenna and to an antenna assembly for particular use in meter reading networks. 
         [0006]    The antenna includes an elongated member having a total length of approximately 0.277λ, where λ corresponds to a frequency, f, in a range from 450 Mhz to 470 Mhz, the elonagated member having a straight stem part comprising about 20% of the total length and a coiled upper part of about 80% of the total length, the upper part being coiled in a spiral pattern. 
         [0007]    The antenna is assembled with a ground plane and a portion of conductor connecting the antenna through the ground plane to circuitry beneath the ground plane, preferably providing at least a transmitter for radiating radio signals in the selected frequency band through the antenna. The radio signals carry meter data information to a receiver in an AMR (automatic meter reading) network. 
         [0008]    The antenna, the ground plane and transmitter circuitry are disposed in a hollow stem of a housing of a type used in subsurface meter pit installations. 
         [0009]    Other objects and advantages 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 
         [0010]      FIG. 1  is a schematic perspective diagram of an antenna of the present invention; 
           [0011]      FIG. 2  is a plan view schematic diagram of a feedthrough connection of the antenna of  FIG. 1  through a ground element; and 
           [0012]      FIG. 3  is a schematic view in elevation of an assembly that incorporates the components of  FIGS. 1 and 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The present antenna  10  of  FIG. 1  is formed of a wire slightly longer than a quarter-wave length to be the total length of the antenna to meet the resonance condition. The main radiation straight wire  11  is approximately 1/20λ long, on which the current distribution is the top of the sinusoidal distribution and close to flat. All other “spare” wire is spiraled into a coil  12  to save space. It is interesting that the “spare wire” is also a part of the antenna  10 . A difference from the straight wire radiator  11 , which is vertically polarized, is that the spiral coil  12  is vertically polarized for magnetic field, or horizontally polarized for electrical field. Thus, this antenna has dual polarization. Without too many bends and zigzags, this antenna  10  has relatively high efficiency and gain. 
         [0014]    This antenna is preferred for use in a frequency band from f=450 MHz to f=470 MHz, and thus f=460 MHz is the center frequency, although it might be applied to other frequency bands as well.  FIG. 1  shows the profile of the antenna  10 . 
         [0015]    The total length of the wire in  FIG. 1  is 181 mm and is approximately 0.277λ. As the main radiator, the straight wire part  11  is 33 mm long, the “spare” part of the 148 mm long wire is then spiraled into a coil  12 , which is the load of the wire antenna and the other part of the antenna itself. 
         [0016]      FIG. 2  is the feed arrangement. The RF feedthrough opening  16  is located above the top side of the PCB  24 , and the antenna is fed by a full circle coil  15  (or any length of arc wire or even a straight wire) above a disk  13  acting as a ground plane. In  FIG. 2 , the diameter of the conductive disk  13  (ground plane) is 34 mm. The height of the feed arc wire  15  above the ground disk  13  is approximately 2 mm. The distance from the hole  16  in the ground disk  13  to the center  14  is 14 mm. 
         [0017]    Using NEC wire antenna simulation software, a 3.16 dB gain from spiral-coil loaded wire antenna and a 2.64 dB gain from a quarter wave monopole antenna without ground plane were obtained, 2.64 dB gain is also close to 2.15 dB for a half-wave dipole antenna according to W. L. Stutzman, and G. A. Thiele,  Antenna Theory and Design,  New York: J. Wiley &amp; Sons, 2003. 
         [0018]    The feed coil  15  in  FIG. 2  can be treated as a piece of microstrip transmission line. Different coil lengths result in different input impedance, therefore, the length of the coil can meet various antenna environments (printed circuit board, box etc.). To fully understand the matching impedance characteristics, three different feed coils were designed: A represents straight wire feed (no coil at all); B, quarter circle feed coil; C, full feed coil as seen in  FIG. 2 . Although these three examples are given, it is possible to have the feed coil extend any length between zero and a full circle around the ground plane. In testing, the power radiates from this spiral antenna. Another monopole was used to pick up signal from about 45 cm away from the spiral antenna. The receiving monopole is parallel to the short straight wire of the spiral antenna to have them polarization matched. Since all these antennas have a donut-shaped pattern and a frequency range that is narrow band, the received power may represent relative antenna gain and matching property. VSWR and S 21  parameters were obtained by using a HP analyzer. 
         [0019]    Table I presents some test results for a straight through (no loop) wire feed (A), a quarter loop feed coil (B) and a full loop feed coil (C) arrangement. From the results, it seems that the antenna (C) with a full circle feed coil inherently matches the 50Ω line. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 MEASUREMENT RESULTS 
               
             
          
           
               
                   
                   
                   
                 Power 
               
               
                   
                 VSWR 
                 Zin (Ω) 
                 (dBm) 
               
               
                   
                   
               
             
          
           
               
                   
                 A 
                 11.5 
                 22 - j 
                 −35.0 
               
               
                   
                   
                   
                 105 
               
               
                   
                 B 
                 7.0 
                 20 - j 66 
                 −31.0 
               
               
                   
                 C 
                 1.5 
                 35 - j 8.5 
                 −28.0 
               
               
                   
                 Monopole 
                 4.8 
                 13 - j 24 
                 −30.0 
               
               
                   
                   
               
             
          
         
       
     
         [0020]    When this antenna part of a water meter transmitter assembly unit, the antenna assembly  20  (PCB  24 , dielectric tube  23 , etc) becomes part of the new antenna  21 , and therefore the antenna gain and the input impedance will be changed. The entire unit  20  including antenna  21  is shown in  FIG. 3 . 
         [0021]    It was noticed that when the antenna  21  and PCB  24  are placed into the dielectric tube  23 , the tube  23  adds an equivalent inductance to the antenna input impedance. The cap  22 , which is made by the same dielectric material as the tube  23 , increases the inductance to antenna&#39;s input impedance. The tube  23  is suspended from the cap  22  into the pit enclosure cavity. The cap  22  rests on top of a lid of the pit enclosure and the components  22 ,  23  can be connected by a threaded connection. In this situation, the full feed coil structure impedance is non-matching and the received power drops. By measurement, the quarter circle feed coil inherently matches the unit at this situation. However, the unit must be encapsulated in dielectric material. The unit will become solid and combine antenna, PCB and tube firmly. Thus, the quarter circle feed arrangement mismatches again in impedance. It is interesting that the potting material adds to the antenna&#39;s inductance too. Consequently, a straight wire feed arrangement matches the unit inherently. Test results are shown in Table II. It should be noted that in table I, all results are from bald antennas and in table II, A and B represent antenna with realistic units, which can be treated as part of the antenna. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 MEASUREMENT RESULTS FOR ANTENNA IN THE UNIT 
               
             
          
           
               
                   
                 Not Potted 
                   
                 Potted 
                   
               
             
          
           
               
                   
                   
                 Power 
                   
                 Power 
               
               
                   
                 VSWR 
                 (dBm) 
                 VSWR 
                 (dBm) 
               
               
                   
                   
               
             
          
           
               
                 A 
                 1.8 
                 −27.0 
                 1.3 
                 −25.0 
               
               
                 B 
                 4.3 
                 −31.0 
                 4.5 
                 −27.0 
               
               
                   
               
             
          
         
       
     
         [0022]    Theoretically, the antenna must still be matched in impedance to the final environment (metal wall, earth, plastic cover etc.). However, since the existing pit products use different-shaped walls and covers, the matching to all types of surrounding apparatus would require a further type of adjustment feature. By measuring, we found that various surrounding mismatch VSWR of the antenna to about 2.0 and power decreases approximately 1 dBm. 
         [0023]    Field tests were conducted. We put the antenna with the entire unit into a metal-walled pit with plastic cover. A receiver with a 2-element array dipole antenna was mounted on the top of a 24 foot pole. In urban areas, approximately −100 dBm power was received at a location 0.5 mile away from the unit, −105 dBm power was received 1.0 mile apart from the transmitter. In an abandoned airfield area, −110 dBm power from 1 mile apart and −113 dBm power from 1.5 miles away from the unit were received. Replacing the plastic cover by metal cover, −110 dBm power was received 1 mile away from the unit in the airfield environment. The signal disappeared 1.5 miles away from the transmitter. The data obtained above are only reference points due to fading and complicated environmental conditions. 
         [0024]    In summary, a spiral coil loaded short wire antenna for AMR transmitters has been developed. The arrangement utilizes the maximum current part of the wire to be the main radiating part. The top part of the wire is spiraled to save space and to be the load of the straight wire radiator, while the load itself is the other part of the antenna. The antenna has a relatively high gain and can meet various matching situations. Its gain is at the same level of a quarter-wavelength monopole without ground plane. It provides a dual polarized radiation and a donut-shaped pattern with different feed arrangements. The antenna&#39;s structure is very simple and easy to manufacture. The cost is low because the antenna is made of a piece of wire and a small metal disk. The antenna is applicable to AMR systems and other wireless communication systems. It can also be utilized as elements of array antennas because of its small and simple structure and good performance.