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Distributed rfid antenna array utilizing circular polarized helical antennasDistributed rfid antenna array utilizing circular polarized helical antennas description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070146230, Distributed rfid antenna array utilizing circular polarized helical antennas. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation of U.S. patent application Ser. No. 11/417,768, filed on May 4, 2006, which is a continuation-in-part of International Patent Application No. PCT/US05/37138, filed on Oct. 18, 2005, which claims priority from U.S. Provisional Application No. 60/625,273, filed on Nov. 5, 2004. These prior applications are incorporated herein by reference in their entirety. FIELD [0002] The technology described in this patent document relates generally to radio frequency identification (RFID) systems. More particularly, the patent document describes a distributed RFID antenna array that utilizes one or more circular polarized helical antennas. BACKGROUND [0003] The RFID system described herein is related to the inventions described in commonly assigned U.S. Patent Application Pub. No. 2004/0056091, which is incorporated herein by reference in its entirety. In that patent application, it was pointed out that a need exists for an advertising compliance monitoring system that provides versatility and flexibility by providing an RFID tag, associated with a specific sign or product display, that communicates tag data to an external reader. [0004] U.S. Patent Application Pub. No. 2004/0056091 describes an RFID system that may include RFID tags of various types (e.g., passive, semi-passive or active), backscatter reader transmitters (BRT), and hubs. Typically, each BRT is a fully self-contained, battery operated unit, and utilizes three antennas. Two medium-gain patch antennas are used to read the tags, and a whip antenna is used to report the received data over a wireless link to the hub. This system functions well and is capable of detecting and reporting tags in a variety of retail environments and at different frequencies. It is desirable, however, to provide an even more economical RFID system by centralizing some or all of the electronics that have been distributed across areas or sub-areas in a given facility, thereby reducing redundancy and cost. It is also desirable to increase the read range of tags by the system to reduce the number of antennas required and to increase the reliability of tags being read under marginal conditions. SUMMARY [0005] In accordance with the teachings described herein, RFID systems are provided that include a distributed RFID antenna array utilizing one or more circular polarized helical antennas. A plurality of RFID tags may be used, with each RFID tag including a linear polarized antenna for communicating RFID tag signals. One or more receiver antennas may be used for receiving the RFID tag signals from the RFID tags. An RFID tag signal reader may be used to process RFID tag signals received by the receiver antennas. In one example, the receiver antennas may include a circular polarized helical antenna element. One or more transmitter antennas may be used for transmitting an RF signal to the plurality of RFID tags, the transmitter antennas including a circular polarized helical antenna element. A transmitter may be used to generate the RF signal for transmission by the transmitter antennas. In one example, the RFID tag signal reader and the transmitter may be included in a single reader/transmitter unit. BRIEF DESCRIPTION OF THE DRAWINGS [0006] FIG. 1 depicts an example RFID system that includes a BRT hub that covers a designated area such as an entire commercial sales facility. [0007] FIG. 2 depicts an example RFID system that includes a plurality of BRT hubs that are used in a plurality of designated areas to cover a larger facility. [0008] FIG. 3 depicts an example RF transmitter with a high power amplifier and a band-pass filter. [0009] FIG. 4 depicts an object having an RFID tag associated therewith. [0010] FIG. 5 is a graph illustrating example quadrifiler helix antenna gain patterns to show that the antenna has a low gain on the axis and a high gain on the sides. [0011] FIG. 6 depicts an example switched backscatter tag (SBT) illustrating the manner in which the switch is opened and closed to accept or reject a BRT carrier signal. [0012] FIGS. 7A and 7B depict an example transmitter antenna having a circular polarized quadrifiler helix antenna element. [0013] FIG. 8 depicts the example quadrifiler helix antenna attached to an amplifier circuit. [0014] FIGS. 9A and 9B depict an example receiver antenna having a single turn helix antenna element. [0015] FIG. 10 depicts the example single turn helix antenna attached to an amplifier circuit. [0016] FIGS. 11-13 depict another receiver antenna embodiment that includes a single turn helix antenna element. DETAILED DESCRIPTION [0017] FIG. 1 depicts an example RFID system that includes a backscatter reader/transmitter (BRT) hub (called a "Spider") that covers a designated area of a facility. The RFID system may, for example, be used to detect and report the presence and location of radio frequency (RF) tags across selected zones in a retail environment. The RFID system may also be used to centralize RF transmission and receiving functions to reduce the expense of recurring components. A single BRT hub ("Spider") may be used that includes antennas attached to multiple transmit and receive ports to cover a designated area of a facility. In small facilities, a single BRT hub may be used to cover the entire facility as the designated area. The Spider may, for example, be connected to AC power to eliminate the cost and maintenance of batteries, as well as allowing more read cycles, if desired. This also may permit higher wattage to be used in the transmit function, potentially increasing the size and reliability of detection zones. [0018] In FIG. 1, a small facility 10 is shown in which the designated area 12 to be covered by a BRT hub 14 includes the entire facility. The BRT hub 14 is coupled to a plurality of transmitters (TX 1, 2) 16-18 and a plurality of receivers (RX 1-10) 20-38, for example using coaxial cable. The plurality of receivers 20-38 are positioned to provide coverage of the entire designated area 12 (the entire facility 10). Preferably, only one TX and one RX are active at a time. It will be noted that RX 22 is able to receive data from RFID tags 60, 62, and 64 at different distances in the sub-area covered by RX 22, as illustrated by concentric circles 54, 56, and 58. Also it will be noted that the transmitter TX 16 has concentric rings 48, 50, and 52 that illustrate the transmitter-to-tag zones covered by the range of transmitter TX 16, thus showing that the transmitting antenna TX 16 is positioned to illuminate at least a portion of the RFID tags (in the RX zones covered by RX 20, 22, 26, 30, 34, 36, and 38) in the designated area. In like manner, TX 18 shows corresponding concentric rings illustrating illumination coverage ranges and representing transmitter-to-tag zones covering at least a portion of the RFID tags. Between the two transmitters TX 16 and 18, all of the RFID tags in the designated area (the facility 12) are capable of illumination. Continue reading about Distributed rfid antenna array utilizing circular polarized helical antennas... 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