Abstract:
An electronic price label which is suitable for use in both small EPLs having lengths of about 1.5 to 2 inches and large size EPLs having lengths of about 3 to 4 inches. The antenna includes a folded planar conductor including a generally planar radiator portion coupled to a antenna port on a printed circuit board, a generally planar ground plane portion coupled to a ground on the printed circuit board having a width and oriented generally parallel to the radiator portion, and a dielectric layer between the radiator portion and the ground plane portion. The width of the ground plane portion is larger than the width of the radiator portion by a distance sufficient to produce an electric field which is linearly polarized in the width direction, thereby eliminating a need for shielding.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to electronic price label (EPL) systems, and more specifically to an electronic price label antenna for electronic price labels of different sizes. 
     EPL systems typically include a plurality of EPLs for merchandise items in a transaction establishment. EPLs typically display the prices of corresponding merchandise items on store shelves and are typically attached to a rail along the leading edge of the shelves. A transaction establishment may contain thousands of EPLs to display the prices of the merchandise items. The EPLs are coupled to a central server from where information about the EPLs is typically maintained in an EPL data file. Price information displayed by the EPLs is obtained from a price look-up (PLU) data file and stored within an EPL price change record. 
     EPLs are produced in different sizes. Two sizes are typically mounted to shelf rails or edges. A first or large size EPL is typically about three to four inches in length and is used in connection with most items in a store. An example of the a large size EPL is disclosed in U.S. Pat. No. 5,668,560, which is assigned to the assignee of the present invention. This patent is hereby incorporated by reference. 
     A second or small size EPL has been created for use in connection with small items which occupy very little shelf space. Use of the small size EPL allows many different types of small items to placed adjacent one another. Use of a large size EPL in connection with these small items would waste shelf space between small items. The small size EPL is about one and a half inches to two inches in length and has a smaller display than the large size EPL. The small size EPL also takes a smaller adhesive overlay for displaying item information which rarely changes. 
     One problem with the small size EPL is that it includes a different antenna than the large size EPL, since the antenna in the large size EPL is too large. 
     Therefore, it would be desirable to provide a common antenna and communication circuitry design for large and small size EPLs in order to reduce overall cost. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, an electronic price label (EPL) antenna for electronic price labels of different sizes is provided. 
     The electronic price label is suitable for use in both small EPLs having lengths of about 1.5 to 2 inches and large size EPLs having lengths of about 3 to 4 inches. 
     The antenna includes a folded planar conductor including a generally planar radiator portion coupled to a antenna port on a printed circuit board, a generally planar ground plane portion coupled to a ground on the printed circuit board having a width and oriented generally parallel to the radiator portion, and a dielectric layer between the radiator portion and the ground plane portion. The width of the ground plane portion is larger than the width of the radiator portion by a distance sufficient to produce an electric field which is linearly polarized in the width direction, thereby eliminating a need for shielding. 
     It is accordingly an object of the present invention to provide an EPL antenna for EPLs of different sizes. 
     It is another object of the present invention to provide an improved EPL antenna design. 
     It is another object of the present invention to provide an EPL antenna which requires fewer solder points. 
     It is another object of the present invention to provide an EPL antenna which is small enough in height to reduce EPL housing sizes to no more than about 7 mm to 9 mm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of the EPL antenna and a circuit board with communication circuitry; 
     FIG. 2 is a cross sectional view of the EPL antenna and circuit board of FIG.  1  through plane  1 — 1 . 
     FIG. 3 is a perspective view of the EPL antenna, the circuit board, and a small size display; 
     FIG. 4 is a perspective view of a small size EPL containing the components of FIG. 3; 
     FIG. 5 is a perspective view of the EPL antenna, the circuit board, and a large size display; 
     FIG. 6 is a perspective view of a large size EPL containing the components of FIG. 5; 
     FIG. 7 is a block diagram of the large and small size EPLs; and 
     FIG. 8 is a diagram illustrating a capacitance used in the block diagram of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, antenna  12  and printed circuit board  16  of electronic price label  10  are shown. 
     Antenna  12  is an air dielectric quarter-wave antenna, implemented as a folded planar conductor  14 . Antenna  12  may also be referred to as a microstrip patch antenna. Antenna  12  includes strip  18 , top planar conductor  20 , first side planar conductor  22 , bottom planar conductor  24 , and second side planar conductor  26 . 
     Strip  18  is generally centered along top planar conductor  20  and is oriented substantially perpendicularly to top planar conductor  20 . Strip  18  and contacts printed circuit board  16 . Strip  18  maintains antenna  12  at a predetermined height above bottom planar conductor  24 . The bottom end of strip  18  forms an antenna port  36  for passing a received signal to communication circuitry  30 . Strip  18  is about 3 mm in height. Printed circuit board  16  has a thickness of about 1 mm. Communication circuitry  30  is shown as a single element to simplify the drawings, but actually employs much of the area of printed circuit board  16 . 
     Top planar conductor  20  acts as a radiator of antenna  12 . Top planar conductor  20  is about 20 mm long by about 15 mm wide. Top planar conductor  20  is oriented substantially perpendicularly to first side planar conductor  22 . 
     First side planar conductor  22  is electrically connected or shorted to bottom planar conductor  24  and is oriented substantially perpendicularly and centered along bottom planar conductor  24 . First side planar conductor  22  maintains antenna  12  at a predetermined height above bottom planar conductor  24 . First side planar conductor  22  is about 20 mm long by about 4 mm high. 
     Bottom planar conductor  24  acts as a ground plane of antenna  12  and is electrically connected to the common ground of communication circuitry  30  through electrical trace  37  on printed circuit board  16 . Bottom planar conductor  24  is substantially parallel to top planar conductor  20 . Bottom planar conductor  24  is about 27 mm long by about 30 mm wide. 
     Second side planar conductor  26  extends bottom planar conductor  24  and also acts as corner reflector which enhances the performance of antenna  12 . This additional ground plane area provides better antenna performance. Second side planar conductor  26  is about 27 mm long by about 5 mm high. 
     Antenna  12  has an operating frequency of about 2.45 GHz. Antenna  12  uses the air gap between top planar conductor  20  and bottom planar conductor  24  as a dielectric. The width of top planar conductor  20  (about 15 mm) together with the height of first side planar conductor  22  (about 4 mm) determine the resonant frequency of antenna  12 . The length of top planar conductor  20  (about 20 mm), the length of first side planar conductor  22  (about 20 mm) and the length and height of strip  18  (about 1 mm and 3 mm, respectively) determine the impedance of antenna port  36 . 
     Advantageously, bandwidth is optimal. The length of top planar conductor  20  (about 20 mm) is as large as possible given the EPL size limitation and the balancing of resonance frequency and impedance. Also, the height of top planar conductor member  20  from bottom planar conductor  24  is increased to a maximum height given EPL size limitations. The difference between ground plane area provided by bottom planar conductor  24  and second side planar conductor  26  (about 27×35 mm) and radiator area provided by top planar conductor  20  (about 20×15 mm) makes antenna  12  relatively directive and gives antenna  12  a good antenna gain and high efficiency. 
     Antenna  12  is linearly polarized. The distance between top planar conductor  20  and second side planar conductor  26  results in an electric field which radiates mainly in the width axis (FIG.  1 ). Therefore, the close proximity of displays  42  or  52  along the length axis (to the left or right of antenna  12 ) has very little affect on performance. 
     Planar conductor  14  may be fabricated from a stainless or cold-rolled steel to a thickness of about 0.2 mm. Planar conductor  14  is then bent, resulting in antenna  12  having the shape illustrated in FIG.  1 . 
     Printed circuit board  16  sits on antenna  12 . Since bottom planar conductor  24  acts as the ground plane, printed circuit board  16  area is minimized. 
     Printed circuit board  16  includes communication circuitry  30  and power supply  34 . 
     Communication circuitry  30  is preferably wireless communication circuitry which operates using wireless backscatter communication techniques. 
     Power supply  34  is preferably a battery. 
     Strip  18  connects to communication circuitry  30  through electrical traces  38  to form an antenna port  36  for antenna  12 . With reference to FIG. 2, antenna port  36  is a soldering pad on printed circuit board  16  and connects to electrical traces  38 . Advantageously, printed circuit board  16  includes only two solder points  80  and  82 , point  80  at antenna port  36  and point  82  at common ground electrical trace  37 . 
     A signal incident on antenna  12  generates an RF voltage between antenna port  36  and bottom planar conductor  24 . This RF voltage is resonated and detected by detector  60  (FIG. 7) in communication circuitry  30 . Antenna  12  is designed to have a port impedance that is inductive to conjugately match the capacitive impedance of detector  60 . 
     Turning now to FIG. 3, antenna  12 , printed circuit board  16 , and small size display  42  of small size electronic price label  40  are shown. 
     Small size display  42  is typically capable of displaying 2×4 or 2×5 characters and is about 30 mm(length)×25 mm(width)×2.5 mm(height). Small size display  42  is mounted on top of printed circuit board  16  in a compact fashion. Given that display  42  is located along the length axis, antenna performance was optimized along the width axis instead. Thus, the presence of display  42  has a minimal affect on antenna performance along the length axis and no shielding is required. 
     Turning now to FIG. 4, small size electronic price label  40  is shown. 
     Small size electronic price label  40  further includes housing  44  and overlay  45 . Housing  44  can be made smaller in height, to no more than about 7 mm to 9 mm, using antenna  12 . Housing  44  fits within a suitable mount on the edge of shelf  46  where small items are arranged. 
     Overlay  45  typically uses as much frontal surface area of housing  44  as is practical for displaying static information. 
     Turning now to FIG. 5, antenna  12 , printed circuit board  16 , and large size display  52  of large size electronic price label  50  are shown. 
     Large size display  52  is typically capable of displaying typically, 2×4 or 2×6 characters. Large size display  52  is about 70 mm(length)×20 mm(width)×2.5 mm(height) in size and is mounted on top of printed circuit board  16 . Given that display  42  is located along the length axis, antenna performance was optimized along the width axis instead. Thus, the presence of display  42  has a minimal affect on antenna performance along the length axis and no shielding is required. 
     Turning now to FIG. 6, large size electronic price label  50  is shown. 
     Large size electronic price label  50  further includes housing  54  and overlay  55 . Housing  54  can be made smaller in height, to no more than about 7 mm to 9 mm, using antenna  12 . Housing  54  fits within a suitable mount on the edge of shelf  56  where items  58  larger than small items  48  are arranged. 
     Overlay  55  typically uses as much frontal surface area of housing  54  as is practical for displaying static information. Thus, overlay  55  is larger than overlay  45 . 
     Referring now to FIG. 7, communication circuitry  30  includes detector  60 , signal processing circuitry  62 , microcontroller  64 , and bias control circuit  66 . 
     Detector  60  is preferably a Schottky barrier-type silicon diode. Detector  60  couples to data port  88  at the junction of capacitance  68  and capacitance  70 . 
     Capacitance  68  is a quarter-wave transmission line or RF open-circuit stub, implemented as a curved trace  90  on printed circuit board  16  (FIG.  8 ). The load impedance at the open end of trace  90  is effectively an open circuit which is transformed to an RF short circuit at data port  88  during operation at the carrier frequency of 2.45 GHz. 
     Capacitance  70  is a discrete component between detector  60  and signal processing circuitry  62 . 
     Operation is similar to the EPL disclosed in the previously incorporated U.S. Pat. No. 5,668,560. A modulated RF signal is received by antenna  12 . Because of the series resonance of antenna  12  and diode  60 , all of the detected RF signal appears across capacitance  68 . Capacitance  70  couples the resulting signal to signal processing circuitry  62 . Signal processing circuitry  62  amplifies the detected RF signal and detects on/off keyed data bits in the amplified RF signal. Microcontroller  64  processes the data bits to generate data for display by display  42  or  52 . Microcontroller  64  also controls bias circuit  66  which controls a bias current and impedance of detector  60 . 
     Advantageously, antenna  12  reduces cost because it can be used in different sizes of EPLs. It also allows housing heights to be reduced. 
     Although the present invention has been described with particular reference to certain preferred embodiments thereof, variations and modifications of the present invention can be effected within the spirit and scope of the following claims.