The present disclosure relates generally to a polarization-insensitive antenna for a radio frequency (RF) identification (RFID) reader operative for transmitting or receiving electromagnetic waves in mutually orthogonal polarization planes, and to an RFID system for, and a method of, scanning item tags oriented at different orientations and associated with items contained in a controlled area, advantageously for inventory control of the RFID-tagged items, by using one or more such antennas.
RFID systems are well known and are commonly utilized for item tracking, item identification, and inventory control in manufacturing, warehouse, and retail environments. Briefly, an RFID system includes two primary constituents: a reader (also known as an interrogator), and a tag (also known as a transponder). The RFID system may have multiple readers, and multiple tags may be read by each reader. Each tag is a miniature device attached or associated with an item to be monitored, or with a package for the item, or with a container for multiple items, and typically includes an item tag antenna, a power management component, a radio or transceiver component, a microprocessor, and a memory. Each item tag is capable of responding, via its item tag antenna, to an electromagnetic wave, i.e., an RF interrogating signal, wirelessly radiated by a reader antenna of the reader. Some item tags are deemed active, because they contain their own battery power source. Other item tags do not contain their own power source and are deemed passive. Each passive tag receives the radiated wave, and its power management component locally generates a direct current (DC) voltage that is used to power all the electrical components on the item tag. Each item tag responsively modulates the interrogating signal, and generates and wirelessly radiates a return electromagnetic wave, i.e., an RF information response signal, back to the reader in a backscatter process. The return information response signal is modulated in a manner that conveys identification data (also known as a payload) from each item tag back to the reader. The identification data can then be demodulated, decoded, stored, processed, displayed, or transmitted by the reader as needed. The identification data can denote a serial number, a price, a date, a destination, a true bearing, other attribute(s), or any combination of attributes, and so on.
The RFID system is often used in an inventory monitoring and tracking application. For example, in order to take inventory of RFID-tagged items in a retail store, it is known to fixedly position at least one stationary RFID reader either overhead in a controlled area or inventory location, or at doorways, loading docks, and assembly lines, and then, to operate each such stationary reader to automatically read whatever tags are in the coverage range of each reader. For superior RF coverage, it is known to provide each stationary reader with multiple reader antennas that radiate a multitude of scan beams that are steered both in azimuth, over an angle of 360 degrees around a vertical axis, and in elevation, over an angle of about 180 degrees away from the vertical axis.
A conventional item tag antenna is typically a dipole, which radiates an electromagnetic wave entirely in one plane of polarization, e.g., either in a horizontal plane (horizontal polarization), or in a vertical plane (vertical polarization). Thus, the item tag antenna is polarized in only one direction or plane: vertical or horizontal. To obtain the best reading performance, the reader antenna and the item tag antenna should be matched in polarization. However, the orientation of the tags in the controlled area is typically unknown. Therefore, a horizontally polarized reader antenna is unable to accurately and quickly read an item tag with a vertically polarized item tag antenna. Likewise, a vertically polarized reader antenna is unable to accurately and quickly read an item tag with a horizontally polarized item tag antenna.
The art has proposed various RFID reader antenna designs, such as a circularly polarized patch antenna that simultaneously radiates electromagnetic waves in mutually orthogonal polarizations to obtain polarization insensitivity, but at a cost in antenna gain of at least about 3 dB as compared to a linearly polarized dipole antenna. This results in half as much power radiated in each polarization. Although the gain could be increased, the resulting radiation pattern of the circularly polarized antenna is narrower as compared to the linearly polarized dipole antenna. In addition, the circularly polarized antenna is more prone to multipath reflections and path-induced losses, with concomitant unpredictable changes in the polarization angle at each reflection, as compared to the linearly polarized dipole antenna.
The art has further proposed a switched cross-polarized antenna comprising a horizontally-oriented dipole with a horizontal polarization, a vertically-oriented dipole with a vertical polarization, and a powered switch for sequentially switching between the dipoles and the horizontal and vertical polarizations. Such a switched antenna has full power radiated in each polarization, a broader radiation pattern, and little or no change in the polarization angle in a multipath environment. However, as advantageous as such a switched antenna has been, an extra DC power cable needs to be routed between a DC power source in the reader and the powered switch in the switched antenna. When there are multiple antennas for each reader as described above, there is a corresponding number of extra DC power cables, one for each reader antenna, that are required to be deployed. Depending on the venue and/or whether the RFID system is being installed in an existing venue, it may be difficult, expensive, and unaesthetic to install and run these extra DC power cables. Rather than routing extra DC power cables, the art has also proposed running DC power along the RF cable between the reader and each antenna. This requires DC couplers and decouplers at the reader and each antenna. In other words, the reader and each antenna are matched in a customized design, which is difficult to retrofit without changing the reader and the matched antennas.
Accordingly, there remains a need for a polarization-insensitive, less costly, easy-to-install, and unobtrusive, reader antenna that is suitable for an RFID reader, without requiring the deployment of extra DC power cables, or the complete retrofit of the reader and each matched antenna, for use in an RFID system for, and a method of, scanning RFID item tags oriented at various orientations and associated with items located in a controlled area, especially for inventory control of the RFID-tagged items.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and locations of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The method and structural components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.
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