Distributed RFID antenna array utilizing circular polarized helical antennas
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.
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.
FIELDThe 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.
BACKGROUNDThe 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.
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.
SUMMARYIn 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
In
Each of the transmitters TX 16 and 18 is coupled to the BRT hub 14, for example with coaxial cable. In like manner, each of the receiver antennas in each sub-area is coupled to the BRT hub 14, for example using coaxial cable. Of course, wireless connections, or other well-known known types of connections could be used instead of coaxial cable.
When the transmitting antenna 16 illuminates RFID tags within its range, one of the RF signal receiving antennas, such as RX 22, receives the modulated tag signals and conveys them to the BRT hub 14 over coaxial cable (such as 42) for transmission to a remote server. A modulated RFID tag signal may be received by more than one RX antenna when read sequentially (for example RX 26 and RX 28). In such cases, the BRT hub (Spider 14) may forward both RX events to the server, and may ascertain a location within a store using closest zone readings, received signal strength indicator (RSSI) readings, antenna intersection, or other algorithms. One preferred method is disclosed in commonly assigned copending application Ser. No. 11/418,319, entitled “Systems and Methods for Approximating the Location of an RFID Tag,” filed on even date herewith, the subject matter of which is incorporated herein in full.
The transmitting antennas 44 and 46 associated with respective transmitters TX 16 and 18 should be omni-directional in order to illuminate tags over a large area. A shaped beam with low gain on axis and a high gain to the sides is ideal. For example, a quadrifiler helix antenna, as illustrated in
Typically, the transmit beam gain from TX 16 to RX 38 would be lower than the transmit beam gain from TX 16 to RX 22. Quadrifiler helix antennas are range compensating. The gain of the antenna is higher for objects farther away, which compensates for free-space power loss due to distance. This is illustrated in
Further, quadrifiler helix antennas are typically inexpensive. The antennas 44 and 46 shown in
Under FCC rules, part 15, a conducted RF output power of 1 Watt is allowed. The BRT's that are used in the system disclosed in commonly assigned U.S. Patent Application Publication No. 2004/0056091 are battery powered and have a maximum output power of 200 mW to conserve battery life while “illuminating” tags (e.g., reflect and receive backscatter modulated signals produced by the tags). Increasing conducted transmitter power will illuminate tags in a larger area and better illuminate tags marginally located in existing zones. The use of the quadrifiler helix antenna enables a gain of approximately 6 dbic translating into an effective isotropic radiated power (EIRP) of +36 dBm or 4 W. This is an increase of approximately 9 dB over the BRT patch antenna disclosed in the above identified published and commonly assigned co-pending patent application. This translates into an increase of 8 times the power.
The performance of an RF reader may be affected by transmitter power being coupled into the BRT receiver through the receiver antenna. The backscattered signal from the RFID tag is extremely small, and its detection can easily be overwhelmed by the backscatter transmitter carrier wave signal. Therefore, the separation of the TX antenna and the RX antenna, as shown in
Also, the use of the switched backscatter RFID tag (SBT) 102 shown in
When the switch 108 is in the open position, as shown, each antenna side is ¼ of the wavelength of the carrier frequency, which makes it a good receiver, and therefore absorbs more of the reader carrier frequency so it is not reflected back to the reader. This combination results in a substantial increase in the ratio of a “mark” (a 1 in binary state monitoring) to “space” (a 0 in binary state monitoring) signal received by the BRT. The increased ratio results in a dramatic improvement in the reader's ability to track the modulated signal containing the tag data across much larger distances. It also allows tags to be read more easily under marginal conditions, such as when they are close to liquid or metal (conditions well known in the art to be quite challenging for tags in the UHF band). In one example, the tag has improved performance because the antenna is T-shaped, with the antenna elements across the top of the tag, pointing out and away from other circuitry on the printed circuit board. This increases the effectiveness of the available frequency aperture and reduces antenna de-tuning.
The clean switching between “on” and “off” of a resonant aperture increases the mark-to-space ratio of the backscatter data as received by the BRT. It is this increased ratio that improves the BRT's ability to detect tags in a specific area of the store area being monitored using a carrier frequency, thereby allowing tags with a cleanly-switched resonant aperture to be detected at a much greater distance than tags without a cleanly-switched resonant aperture.
The system shown in
Note in
Multiple Web antennae are connected to a single backscatter transmitter/receiver in the Spider, for example through coaxial cables. These coaxial cables pass through a switch matrix. This matrix and the long coaxial cables combine to create additional attenuation, thereby lowering the received signal level. To overcome this loss, a low noise amplifier (LNA) is positioned at each RX antenna. These amplifiers draw small amount of current (≈15 mA) through the coaxial cable using bias tees. Locations in retail environments that are difficult or expensive to monitor via coaxial cable, such as external fuel pump signage, could still be served by the previously-designed BRT's with distributed reader/transmitter electronics by forwarding their data wirelessly to the master Spider.
Using a linear polarized tag in an RFID system is typically more economical than using a tag with circular polarization. A linear polarized tag can typically be made smaller than a tag using circular polarization because a linear polarized antenna needs to operate in only one axis. However, from a system standpoint the radiation patterns of the antennas in the transmitter, receiver and tag should all be aligned or coplanar to achieve the most robust link and the best performance. This is most easily achieved in a retail environment using circular polarized antennas because maintaining coplanar antenna alignment between linear antennas in a retail environment is often impractical. A good compromise is the use of circular polarized antennas for the receivers and transmitters and linear polarized antennas for the RFID tags. In this manner, a high level of overall system performance may be maintained, while reducing the cost of the RFID tags.
In the illustrated example, the antenna structure 202 is attached to the dielectric core 204 using a plurality of holes 208 in the dielectric core 204. As illustrated in
The amplifier circuit 220 may, for example, be attached to the ceiling of a retail environment such that the antenna 200 extends downwardly from the ceiling. In addition, the amplifier circuit 220 may be coupled to other components in the RFID system via an external connector 224, such as a coaxial cable connector. In one example, the amplifier circuit 220 may include two or more gain settings that may be used to tune the amplifier circuit 220 for use in different sized retail environments. For example, a higher gain setting for the amplifier 220 may be used for a larger retail environment.
The amplifier circuit 250 may, for example, be located in the ceiling of a retail environment, for example above the ceiling tiles. In addition, the amplifier circuit 250 may be coupled to other components in the RFID system via an external connector 254, such as a coaxial cable connector.
This written description uses examples to disclose the invention, including the best mode, and also to enable a person skilled in the art to make and use the invention. The patentable scope of the invention may include other examples that occur to those skilled in the art.
Claims
1. A coreless helical antenna structure, comprising:
- a dielectric base structure;
- an antenna element extending away from the dielectric base structure in a spiral pattern and having a circular polarized radiation pattern; and
- a plurality of support structures that attach the antenna element to the dielectric base structure.
2. The coreless helical antenna structure of claim 1, wherein the plurality of support structures are configured to maintain the spiral pattern of the antenna element.
3. The coreless helical antenna structure of claim 1, wherein the antenna element includes a single radiating arm that extends away from the dielectric base structure in the spiral pattern.
4. The coreless helical antenna structure of claim 3, wherein the single radiating arm of the antenna element forms a single turn helix antenna.
5. The coreless helical antenna structure of claim 3, wherein the single radiating arm is formed from a single antenna wire.
6. The coreless helical antenna structure of claim 1, further comprising:
- a metallic antenna backplane that adds directivity to the circular polarized radiation pattern of the antenna element.
7. The coreless helical antenna structure of claim 6, wherein the metallic antenna backplane is integral to the dielectric base structure.
8. The coreless helical antenna structure of claim 1, wherein the plurality of support structures include openings and the antenna element is supported within the openings.
9. The coreless helical antenna structure of claim 3, wherein the plurality of support structures maintain a desired pitch of the antenna element with respect to the dielectric base structure.
10. The coreless helical antenna structure of claim 9, wherein the antenna element has a total length and the pitch is about equal to ⅕ the total length of the antenna element.
11. The coreless helical antenna structure of claim 1, wherein the plurality of support structures are made of a dielectric material.
12. The coreless helical antenna structure of claim 11, wherein the plurality of support structures are plastic.
13. The coreless helical antenna structure of claim 1, further comprising an amplifier circuit coupled to the antenna element and operable to amplify a signal received by the antenna element.
14. The coreless helical antenna structure of claim 1, wherein the coreless helical antenna structure is configured as a receiver antenna for an RFID system.
15. The coreless helical antenna structure of claim 1, wherein the coreless helical antenna structure is supported within an enclosure.
16. A coreless helical antenna structure, comprising:
- a dielectric base structure;
- an antenna element that includes a single radiating arm extending away from the dielectric base structure in a spiral pattern and having a circular polarized radiation pattern; and
- a support structure that attaches the antenna element to the dielectric base structure and that is configured to maintain a desired pitch of the antenna element with respect to the dielectric base structure.
Type: Application
Filed: Jan 3, 2007
Publication Date: Jun 28, 2007
Inventors: Gary Overhultz (River Forest, IL), Gordon Hardman (Boulder, CO), John Pyne (Erie, CO), Edward Strazdes (Lafayette, CO)
Application Number: 11/649,046
International Classification: H01Q 1/36 (20060101);