Communications: radio wave antennas – Antennas – With coupling network or impedance in the leadin
Reexamination Certificate
1996-12-31
2001-02-06
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
With coupling network or impedance in the leadin
C343S850000, C342S044000, C455S041300
Reexamination Certificate
active
06184841
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to wireless communication systems and, more particularly, to antenna technology used in a radio frequency identification communication system.
2. Description of the Related Art
Radio Frequency Identification (RFID) systems are used for identification and/or tracking of equipment, inventory, or living things. RFID systems are radio communication systems that communicate between a radio transceiver, called an Interrogator, and a number of inexpensive devices called Tags or transponders. In RFID systems, the Interrogator communicates to the Tags using modulated radio signals, and the Tags respond with modulated radio signals.
FIG. 1
illustrates a Modulated Backscatter (MBS) system. In a MBS system, after transmitting a message to the Tag (called the Downlink), the Interrogator then transmits a Continuous-Wave (CW) radio signal to the Tag. The Tag then modulates the CW signal, using MBS, where the antenna is electrically switched, by the modulating signal, from being an absorber of RF radiation to being a reflector of RF radiation. Modulated backscatter allows communications from the Tag back to the Interrogator (called the Uplink). Another type of RFID system uses an Active Uplink (AU).
FIG. 2
illustrates an Active Uplink RFID system. In an AU system, the RFID Tag does not modulate and reflect an incoming CW signal, but rather synthesizes an RF carrier, modulates that RF carrier, and transmits that modulated carrier to the Interrogator. In some AU systems, the RF carrier used in the Uplink is at or near the same frequency as that used in the Downlink; while in other AU systems, the RF carrier used in the Uplink is at a different frequency than that used in the Downlink.
Conventional RFID systems are designed a) to identify an object passing into range of the Interrogator, and b) to store data onto the Tag and then retrieve that data from the Tag at a later time in order to manage inventory or perform some other useful application. In some RFID applications, directional antennas are used. For example, in an RFID-based electronic toll collection system, the Interrogator is overhung on top of the highway (see FIG.
3
). In this application, the transmit and receive antennas have the same beamwidth. In fact, transmit and receive frequently share the same antenna, using a circulator to separate the transmit and receive paths.
SUMMARY OF THE INVENTION
In accordance with an embodiment of the present invention, a general antenna system is disclosed suitable for applications in which an RFID Tag passes by an Interrogator. We then disclose an embodiment that uses a single planar antenna for transmit and a multi-element planar antenna array for receive. The multi-element planar antenna array is spaced such that each of the planar elements is four inches apart, center-to-center, thus defining a narrow 30° receive beamwidth in the horizontal plane. The vertical receive bandwidth is much greater than 30°, facilitating the Interrogator receiving signals at a variety of elevations. Furthermore, a multi-way microstrip combiner is used to sum the signals received from each of the planar antennas. To block interference from the transmit antenna and to improve receive sensitivity, this multi-way microstrip combiner is shielded using, in one embodiment, copper tape along its edges. In yet another specific embodiment, a four element receive antenna design is disclosed.
In this application, we disclose antenna technology suitable for a Cargo Tag system, which is an RFID-based system for tracking cargo containers. This application is used as a point of discussion, however the methods discussed here are not limited to a Cargo Tag system. The goal of the Cargo Tag system is to identify the contents of a Tag affixed to a cargo container when that cargo container comes within range of the Interrogator. The cargo container passes the gate of a warehouse at a certain speed, e.g. 10 meters/second, and the Interrogator, located behind and to the side of the passageway, is required to read the Tag. To save battery life in the Tag, the electronics, such as the microprocessor, of the Tag are “asleep” most of the time. Therefore, the Tag must be awakened by the Interrogator so that communications between the Interrogator and the Tag can begin. After the Tag is awakened, the antenna system must be designed for optimal communications.
In this disclosure, we describe a general antenna system that is suitable for applications in which an RFID Tag passes by an Interrogator. We then disclose a specific antenna system design, based upon the design of the general antenna system, that is well suited for Cargo Tag applications. This antenna system provides transmit and receive antennas that are small in size, light in weight, low in cost, and provides appropriate beam widths for these applications.
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Shober R. Anthony
Sweetman Eric
Wu You-Sun
Lucent Technologies - Inc.
Malvone Christopher N.
Phan Tho
Wong Don
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