HK1155558A - Broadband antenna with multiple associated patches and coplanar grounding for rfid applications - Google Patents

Description

Broadband antenna with multiple associated patches and coplanar ground for RFID applications

RELATED APPLICATIONS

This application claims benefit of U.S. provisional patent application No.61/059,665 filed 6/2008 and is a continuation of the portion of U.S. patent application No.12/247,994 filed 8/2008 entitled "RFID Patch Antenna with coplanar reference group and Floating groups," U.S. patent application No.12/247,994 claims benefit of U.S. provisional patent application No.60/978,389 filed 8/10/2007, the disclosure of which is incorporated herein by reference in its entirety.

Technical Field

The present invention relates generally to low cost, low thickness, compact broadband patch antennas having two or more connected radiating elements and a reference ground conductor, all in the same geometric plane or in closely spaced parallel planes, and optionally including a direct electrical short connection (DC closed short circuit) between the patch system and the ground conductor for dissipation of electrostatic charges. Such patch antennas or arrays of such patch antennas have utility in Radio Frequency Identification (RFID) applications, where UHF band signals are communicated between a reader (transceiver) and a tag (transponder) via the patch antenna. The present invention is particularly useful in RFID applications where it is desirable to create a space on a surface such as an RFID smart shelf, smart counter top or other RFID enabled surface that has well controlled directional UHF signal emissions, which contains many RFID tag items and enables items in the space to be reliably read using UHF signals from an RFID reader attached to an antenna, while reducing the extent and severity of the impact of null zones (null zones) or locations in the space where UHF signals are too weak to communicate with RFID tags. The present invention with its improved bandwidth is particularly useful in situations where items and materials in the environment surrounding the antenna greatly affect (i.e., detune, or cause significant shifts in) the resonant frequency of conventional narrow band prior art antennas away from the operating frequency of the tag and reader (transceiver).

Background

Radio Frequency Identification (RFID) systems and other forms of electronic article surveillance are increasingly being used to track their location or to arrange items of some economic, security or other significance. In these applications, typically, transponders or tags are attached to or placed inside the items to be tracked and these transponders or tags are in at least intermittent communication with a transceiver or reader that reports the tag (and item (from inference)) location to a person or software application via a network to which the reader is directly or indirectly attached. Examples of RFID applications include the tracking of retail items offered for auction in stores, inventory management of those items in back rooms of stores, on store shelf fixtures, displays, counters, housings, cabinets, closets, or other fixtures, and the tracking of items to and through points of sale and store exits. There are also article tracking applications involving warehouses, distribution centers, trucks, vans, shipping containers, and other article storage or delivery points as the articles pass through the retail supply chain. Another application area of RFID technology relates to asset tracking, where valuable items (not necessarily sold to the public) are tracked in certain environments to prevent theft, loss, or misplacement, or to maintain the integrity of an asset chain of custody. These applications of RFID technology are given by way of example only and it should be understood that many other technology applications exist.

RFID systems typically use a reader antenna to transmit an electromagnetic carrier wave encoded with a digital signal to the RFID tag. Thus, the reader antenna is a critical component that facilitates communication between the tag and the reader, and affects the quality of the communication. The reader antenna may be considered a transducer that converts the signal-carrying alternating current from the reader to a signal-carrying oscillating electromagnetic field or wave that fits an adjacent second antenna located in the tag, or alternatively, that also facilitates the conversion of the signal-carrying oscillating electromagnetic field or wave (transmitted from or modified by the tag) to a signal-carrying alternating current for demodulation by the reader and the reverse process of communicating with the reader. In general, the resonant characteristics and tuning (and tunability) of a reader antenna are determined not only by the antenna geometry and dimensions, stacking (material layering) and construction, and manufacturing materials, but also by the characteristics of the environment surrounding the antenna. Ideally, an RFID antenna will have a large bandwidth-i.e., will effectively transmit and receive signals over a relatively wide frequency range centered at or near the frequency at which the RFID tag and reader are designed to operate. For antennas with sufficient bandwidth, small changes in the resonant characteristics of the system caused by occasional and largely uncontrolled changes in the antenna environment will not result in severe detuning of the antenna away from the design frequency. The types of antennas used in RFID systems include patch antennas, slot antennas, dipole antennas, loop antennas, and many other types and variations of these types.

In the case of a passive RFID system, the RFID tag is powered by an electromagnetic carrier wave. Once powered, passive tags interpret Radio Frequency (RF) signals and generally provide an appropriate response by generating timed intermittent perturbations in an electromagnetic carrier wave. These perturbations of the coded tag response are sensed by the reader through the reader's antenna. In the case of active RFID systems, the tag contains its own power source (such as a battery) that can be used to initiate RF communication with the reader by generating its own carrier and encoding RF signals, or by increasing the data processing rate of the tag, or by increasing the power in the response of the tag, and thus the maximum communication distance between the tag and the reader.

In particular for passive RFID systems, it is often convenient to distinguish the performance of the RFID system and its antenna in terms of near field versus far field performance. The terms "near field" and "far field" are relative terms and relate to the wavelength of the carrier in which the terms "near" and "far" are meaningful. When the distances involved in an application are much larger than the length of the carrier and the antenna size, the application is a far field application and often the antenna (at a large distance from the antenna) can be considered as a point source (as in most telecommunication applications). On the other hand, when the distances involved in an application are much shorter than the wavelength and antenna size, the relevant electromagnetic interaction between the antennas (e.g., reader antenna and tag antenna) is a near-field interaction. In this case, the reactive electric or magnetic component dominates the EM field, and the interaction between the two coupled antennas occurs through perturbations in the field. When the application of interest involves distances in the order of the carrier wavelength, the situation is more complex and cannot be considered as a simple near field or a simple far field. In this patent application, this situation is referred to as "intermediate field".

The two common frequency bands used by commercial RFID systems are HF (13.56MHz) and UHF (approximately 850 to 950MHz, the particular band depending on the country being targeted). Since tags on RFID tagged consumer items are typically used in many applications throughout the supply chain (from manufacturing and distribution to the final retail store location), the functional requirements of the retail store shelf are only one of several sets of factors that affect tag frequency selection. There are many factors and requirements in the supply chain that are of interest to various trading partners, and in this complex situation, both HF and UHF1 are widely used to track tagged items on and in smart shelves, racks, cabinets, and other retail stores, warehouses, and other commercial fixtures. U.S. patents 7,268,742, 6,989,796, 6,943,688, 6,861,993, 6,696,954, 6,600,420, and 6,335,686 all relate to the use of RFID antennas for smart shelves, cabinets, and related fixtures. The 13.56MHz wave has a wavelength just above 22 meters (72 feet), while the wavelength of UHF radiation used in RFID applications is about one third of a meter, or about 12 inches. Since the distance characteristics of RFID applications at the item level, which involve tracking and surveillance of tagged items on or in shelves, cabinets, racks, counters, and other such fixtures, are on the order of a few feet (e.g., 0.5 feet to a few feet), it is apparent that when UHF technology is used, the antenna interaction is near, far, and intermediate field. In such a case, poor selection of the reader antenna type, or a poor design of the appropriate type, may result in poor performance and application failure of the overall RFID system. One of the reasons for this is that in the mid-field case, the electric and magnetic fields emitted from the reader antenna vary significantly over the relevant surface (e.g., the surface of a retail store shelf holding the tagged item). This field may be strong at one location and much weaker at another location a few inches away (because the wavelength of UHF radiation is only a few inches) and the general nature of UHF systems is much more complex than observed in 13.56MHz applications. Thus, in situations where UHF is used in RFID item tracking on shelves and other storage fixtures, the design of the reader antenna becomes critical.

The detection range of passive RFID systems is typically limited by signal strength at close range (e.g., typically less than a few feet for UHFRFID systems). Because of this read range limitation in passive UHFRFID systems, many applications utilize portable reader units that can be manually moved around a group of tagged items in order to detect all tags, particularly if the tagged items are kept in a space that is significantly larger than the detection range of a stationary or fixed reader equipped with one fixed antenna. However, portable UHF reader units suffer from a number of disadvantages. The first involves the cost of human labor associated with the scanning activity. Once paid for, fixed infrastructure is much cheaper to operate than human powered systems, which have ongoing labor costs associated with them. In addition, portable units often cause uncertainty as to the exact location of the tag being read. For example, a user may record the reader location, but may not know the tag location during a read event well enough for a given application. That is, the use of portable RFID readers often results in spatial resolution certainty of only a few feet, and many applications require knowledge about the location of tagged items within a spatial resolution of a few inches. Portable RFID readers may also be more susceptible to loss or theft than is the case for fixed readers and antenna systems.

As an alternative to portable UHF RFID readers, large fixed reader antennas driven with sufficient power to detect a greater number of tagged items may be used. However, such antennas may be awkward, aesthetically displeasing, and the radiated power may exceed allowable legal or regulatory limits. Furthermore, these reader antennas are often located in stores or other locations where space is at a premium and using such large reader antennas is expensive and inconvenient. Additionally, it should be noted that when a single large antenna is used to measure a large area (e.g., a group of retail store shelves, or an entire cabinet, or an entire counter, etc.), it is not possible to resolve the location of a tagged item to a particular point on the shelf fixture or a small sub-section thereof. In some applications, it may be desirable to know the location of a tagged item with a spatial resolution of a few inches (e.g., if there are many small items on the shelf and it is desirable to minimize the time for manual searching and picking). In this case, the use of a single large reader antenna is undesirable because it is often not possible to locate the item with the desired spatial resolution.

Alternatively, a fully automated mobile antenna system may be used. Us patent 7,132,945 describes a shelf system employing a moving or scanning antenna. This method makes it possible to measure a relatively large area and also eliminates the need for manual labor. However, as is often observed in machines incorporating moving parts, the introduction of moving parts into commercial shelving systems may prove impractical due to the high cost of the system, the complexity of installation, and maintenance costs, as well as the inconvenience of system downtime. A smart antenna that forms a beam can scan a space with a narrow beam and without moving parts. However, if compared to passive antennas, as active devices, they are typically large and expensive.

To overcome the disadvantages of the methods described above, fixed arrays of small antennas are used in some UHF RFID applications. In this approach, many reader antennas spanning a large area are connected to a single reader or group of readers via some switching network, as described, for example, in U.S. patent 7,084,769. Smart shelves and other similar applications involving the tracking and inventory review of small tagged items in or on RFID-enabled shelves, cabinets, housings, racks, or other stationary devices may use a fixed array of small antennas. In tracking tagged fixed items in smart shelves and similar applications, fixed arrays of small antennas have several advantages over portable readers, systems with a single large fixed antenna, and mobile antenna systems. First, the antennas themselves are small and therefore require relatively little power to measure the space around each antenna. Thus, in a system that interrogates these antennas one at a time, the system itself requires relatively little power (typically much less than 1 watt). Furthermore, by interrogating each small antenna in a large array, the system can therefore measure a large area with relatively little power. Also, since UHF antennas used in antenna arrays are typically small and measure a small space (due to its limited power and range of less than 1-3 feet) with a particular known spatial location, tagged items that must be read by a given antenna in the array are also located to the same spatial resolution of 1-12 inches. Thus, a system using a fixed array of small antennas may determine the location of a tagged item more accurately than a portable RFID reader and a system using a small number of relatively large antennas. Also, since each antenna in the array is relatively small, it is easier to hide the antenna inside a shelf or other storage fixture, thus improving aesthetics and minimizing damage from external vandalism events (e.g., curiosity-driven operations by children, or malicious activities by the average person). Also, the fixed antenna array does not involve moving parts and therefore does not suffer from the disadvantages associated with moving parts as described above. Also, small antennas similar to those used in such antenna arrays are cheaper to replace when a single antenna element fails (relative to the cost of replacing a single large antenna). Also, the fixed antenna array does not require special manual labor to perform the scanning of the tagged items, and therefore does not have the high cost of manual labor associated therewith associated with portable reader and antenna systems, or with the mobile cart approach.

In smart shelving and similar applications, it is often important for economic and aesthetic reasons: the antennas used in antenna arrays are simple, low cost, easily retrofitted into existing infrastructure, easily hidden from view by people in the vicinity of the antennas, and can be quickly installed and connected. These application requirements are more easily met with an antenna structure that minimizes the total antenna thickness. That is, thin or unobtrusive (low profile) antennas are easier to hide and are easier to fit into existing infrastructure without requiring special modifications to the existing infrastructure. Also, reducing the thickness of the antenna tends to reduce the cost of the antenna because less material is used in thinner antennas. For reasons of cost and ease of installation, it is also desirable to have the simplest possible method of attaching the RF feed cable or wire to the antenna. Preferably, the attachment should be made at one location, on one surface, without the need for holes or special channels, wires, or conductive vias through the antenna substrate. Importantly, the design of UHF antennas allows for the reading of RFID tags in the space near the antenna without a "dead zone" (or small area) between and around the antennas where the transmitted field is too weak to facilitate communication between the antenna and the reader. Another requirement for antennas used in smart shelves and similar applications is their ability to read items at multiple tag antenna orientations (i.e., tag orientation independence, or at least close to the nature of the ideal case).

Conventional patch antennas, slot antennas, dipole antennas, and other common UHF antenna types that may be used in antenna systems such as those described above often contain multiple layers. Us patent 6,639,556 shows a patch antenna with this layered structure and a central hole for RF feeding. Us patent 6,480,170 also shows a patch antenna with a reference ground and a radiating element on opposite sides of an intermediate dielectric. Multi-layer antenna designs can result in excessive manufacturing costs and excessive antenna thickness (complicating retrofitting of existing infrastructure during antenna installation and making it more difficult to hide the antenna from view). Multi-layer antenna designs also tend to complicate the form of attachment of connecting wires, such as coaxial cables between the antenna and the reader, because the connection of the signal carrier and reference ground occurs on different layers.

For UHF smart shelf applications, patch antennas are a good choice of antenna type because the field emitted from the patch antenna is primarily in a direction orthogonal to the plane of the antenna, so the antenna can be located on or inside the shelf surface and create RFID real estate in the area just above the shelf and relatively easily read tagged items located on the shelf surface. Of course, it is a prerequisite that a particular patch antenna design provides a sufficiently large bandwidth and radiation efficiency for a given convenient and practical power input to create a sufficiently large space around the antenna that a tagged item can be reliably and consistently read. Conventional patch antennas described in the prior art have a main radiating element of conductive material fabricated on top of a dielectric material. A reference ground element, which is a planar layer of conductive material that is electrically grounded with respect to signals transmitted or received by the antenna, is typically disposed beneath (i.e., on the opposite side of) the dielectric material. In typical patch antenna designs, which are well known in the art, the antenna main radiating element and the reference ground element are in parallel planes separated by a dielectric material, which in some cases is simply an air-isolation layer (spacer). Also, in the usual case, the main radiating element and the reference ground element are made in such a way that one is directly above the other or in such a way that one substantially overlaps the other in their respective parallel planes. A disadvantage of conventional multilayer patch antenna designs is that the connection of the signal carrying shielded cable or twisted pair wire between the antenna and the RFID reader must be attached to the antenna on two separate layers separated by a dielectric material, thus requiring a connection hole or via in the dielectric layer.

The size of the gap between the radiating element and the reference ground conductor (i.e. the dielectric layer thickness) is a critical design parameter in conventional patch antennas, since the thickness of this gap determines to a large extent the bandwidth of the antenna for a given dielectric material. As the gap decreases, the bandwidth narrows. Tuning of the antenna in a given application becomes very difficult if the bandwidth of the antenna is too narrow, and uncontrollable variations in the environment (e.g., metal objects, human hands, or other items or materials being introduced into the area being monitored by the antenna unexpectedly and randomly) during normal operation can result in shifts in the resonant frequency combined with the too narrow bandwidth, causing significant detuning of the antenna and malfunctions in the RFID tag detection and reading. Thus, for practical reasons, there is a lower limit to the distance between the ground plane and the radiating element in conventional patch antenna designs for a given application, and this constrains the overall thickness of the antenna.

Another constraint on the thickness of conventional patch antennas stems from the radiation efficiency (fraction of the total electrical energy entering the antenna as electromagnetic radiation emissions). If the dielectric thickness or gap between the reference ground and the radiating element is too small, the radiation efficiency will be too low, since too much energy to the antenna is wasted as heat flowing into the dielectric and the surrounding environment.

The above discussion shows that: (1) patch antenna designs can be used effectively in UHF smart shelves and similar applications, and (2) the use of patch type antennas can be even more advantageous and more fully meet the aforementioned practical requirements of smart shelves if there is some way to overcome the constraints on antenna thickness resulting from high bandwidth and radiation efficiency requirements. Also, it would be advantageous to find a new design of patch antenna that simplifies the attachment of feed cables or wires. In addition, it would be advantageous to find a new antenna design that spreads UHF radiation more evenly and over a larger area of the surface of the shelf containing the antenna (i.e., in the area above the plane of the radiating element) than is possible with conventional patch antenna designs. As noted above, the relatively short wavelength (about 12 inches) of UHF emissions can present challenges to designers of UHF smart shelves that wish to be able to effectively and consistently read tags at any location on the shelf. A better UHF antenna design will minimize this problem and allow better "field spreading" or "field shaping" in the area directly above and around the edges of the antenna. Such an improved Patch Antenna design is described in U.S. patent application No.12/247,994, entitled "RFID Patch Antenna with a Coplanar Reference group and a floating group," filed on 8.10.2008. The embodiments described in this patent application relate to patch antenna designs having radiating elements and ground reference elements in the same plane or in closely spaced parallel planes such that there is no substantial overlap between the radiating elements of the first plane and the ground reference elements of the second plane. That is, the radiating element and the ground element may be side-by-side rather than stacked with the radiating element on top of the ground element. This invention, in its various embodiments as described in the aforementioned patent applications, overcomes the above-mentioned limitations of conventional patch antenna designs and results in a new patch antenna that is much thinner without sacrificing bandwidth and radiation efficiency. Also, the present invention enables simpler antenna feed cable attachment than conventional patch antenna methods. Also, the present invention enables a more evenly distributed UHF field around the antenna, which makes it easier to avoid dead zones and enables smart shelf designers to spread or shape the field evenly around the antenna. In RFID smart shelf applications and similar applications where it is desirable to interrogate adjacently located RFID tags with low power RFID signals located in a small physical space that often causes tuning difficulties for conventional patch antennas, the invention results in excellent antenna gain, bandwidth, and tuning robustness in its various embodiments, particularly those that implement a floating ground element in addition to a reference ground element and located below a plane that holds the main radiating element and the reference ground.

The present invention extends the concept of U.S. patent application No.12/247,994, entitled "RFID Patch Antenna with a Coplanar Reference group and Floating groups," filed on 8/10 of 2008 by adding an additional, auxiliary radiating element that is directly electrically connected to the main radiating element. These one or more auxiliary radiating elements lie in the same plane containing the main radiating element and are attached to the main radiating element at one or more edges by conductive traces. These auxiliary radiating elements may be of similar size and shape to the main radiating element, or they may be slightly narrower and have a higher aspect ratio. The addition of the one or more auxiliary radiating elements to the coplanar antenna design described in the aforementioned patent application provides several advantages. First, the bandwidth increases. Second, thinner antennas can be designed to meet a given bandwidth requirement. Finally, the antenna radiation gain increases (i.e., less power is required to scan the space).

Disclosure of Invention

According to an embodiment, a reader antenna is provided for placement within a storage fixture (e.g., a shelf, cabinet, drawer, or rack) for transmitting and receiving RF signals between, for example, an RFID reader and an RFID tag or transponder.

In one particular aspect, an antenna for a radio frequency identification system is described, the antenna comprising a reference ground; an antenna feeding section; a primary patch antenna element for intermediate field transmission and reception of one of HP and UHF signals, wherein the primary patch antenna element is electrically coupled to the antenna feed; and one or more additional patch antenna elements for intermediate field transmission and reception of one of HF and UHF broadband signals, wherein each of the one or more additional patch antenna elements is electrically connected to an edge of the primary patch antenna element for transmission and reception of one of the HF and UHF signals. The one or more additional patch antenna elements provide gain enhancement of one of the HF and UHF signals.

In another aspect, a DC connection for ESD protection is provided.

The reader antenna may be placed in a variety of configurations, including but not limited to configurations in which, for each antenna, the main radiating antenna element and the reference ground element of the antenna lie in the same physical or geometric plane, or in two parallel closely spaced planes separated by a dielectric layer, with little or no overlap between the radiating antenna element and the reference ground element, and in which one or more auxiliary radiating antenna elements are placed adjacent to and in the same plane containing the main radiating element, or in one or more closely spaced parallel planes, with no or little overlap between the main and auxiliary radiating elements, and such that each auxiliary radiating element is electrically connected by a conductive trace to an edge of the main radiating element.

Also, one or more floating ground planes may be included, optionally in a plane parallel to or in the same plane as the geometric plane of the radiating antenna element, to improve, control or optimize the electric or magnetic field strength or shape around the antenna.

In a preferred embodiment, the RFID-enabled storage fixture is equipped with a plurality of patch antennas, each having its own reference ground element that is coplanar or substantially coplanar with the primary and auxiliary radiating elements of the respective patch antenna.

Further, in another embodiment, these RFID-enabled fixtures are implemented using an intelligent network in which antennas are selected, activated, and otherwise managed by a supervisory control system.

Drawings

These and other aspects and features of the present invention will become apparent to those skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, in which:

figure 1 shows a typical patch antenna design of the prior art.

Fig. 2 shows a patch antenna with a coplanar reference ground as described in the previous patent application.

Fig. 3 shows an example of a patch antenna in which an auxiliary radiating antenna element has been placed near a main radiating antenna element and connected to the main radiating antenna element.

Fig. 4 shows an example of a patch antenna with a coplanar reference ground and a coplanar auxiliary radiating antenna element with a high aspect ratio rectangular shape.

Fig. 5 shows an example of a patch antenna with a coplanar reference ground and two coplanar auxiliary radiating antenna elements with a high aspect ratio rectangular shape.

Fig. 6 shows a detailed view of the coaxial cable connections to the antenna patch and the reference ground plane.

Fig. 7 shows an example of an alternative radiating antenna element shape.

Fig. 8 shows a conventional patch antenna used in computer simulations with an antenna size tag.

Fig. 9 shows a return loss plot for the conventional patch antenna design shown in the dimensions shown in fig. 8.

Fig. 10 shows the results of a computer simulation regarding the bandwidth as a function of the antenna substrate (dielectric layer) thickness of a conventional patch antenna.

Fig. 11 shows a dual patch antenna with coplanar reference ground for use in computer simulations with an antenna size tag.

Fig. 12 shows a return loss plot for the dual patch antenna shown in fig. 11.

Fig. 13 shows an example of a patch antenna with radiating element antennas oriented in different directions.

Fig. 14 shows an example of a patch antenna array with antenna elements oriented in different directions.

Fig. 15 shows an example of a patch antenna array with an alternative arrangement of radiating antenna elements.

Detailed Description

Embodiments and applications of the present invention will now be described. Other embodiments may be realized and changes may be made to the disclosed embodiments without departing from the spirit or scope of the present invention. While the embodiments disclosed herein have been described with particular application to the field of RFID systems, it should be apparent that the invention can be implemented in any technology having the same or similar problems.

In the following description, reference is made to the accompanying drawings which form a part hereof and which illustrate several embodiments. It is to be understood that other embodiments may be utilized and structural and operational changes may be made without departing from the scope of the description provided.

Fig. 1 is a diagram showing a patch antenna from the prior art. In this design, the supporting dielectric material 100 separates the radiating wire element 110 (the top surface of the dielectric) from the reference ground element 120 (the bottom surface of the dielectric). The feed point 135 requires a hole in the dielectric so that the ground element of the feed cable (not shown) can be attached to the reference ground 120. Each radiating antenna element described herein is also referred to as a patch antenna element because it is formed using a patch antenna configuration and is used for transmission and reception of HF and UHF signals, particularly for intermediate field transmission on the order of wavelengths as discussed herein. As further described herein, these antenna elements are formed from a thin sheet of copper or some other conductive metal or ink printed, etched, or otherwise fabricated on a thin insulating sheet.

Fig. 2 is a diagram illustrating an exemplary patch antenna assembly according to an embodiment of the patent application incorporated by reference and claiming priority hereto above. In this embodiment of the invention, a first supporting dielectric material 100 similar to that typically used in printed circuit boards is used to support the radiating antenna element 110 and the reference ground element 120. In another embodiment, the radiating antenna element may be copper or some other conductive metal or ink (e.g., copper for patch antennas and reference grounds and trace elements formed on mylar) printed, etched, or otherwise fabricated on a thin insulating substrate, such as a thin plastic sheet (not shown in fig. 2) that is itself placed on top of a suitable dielectric material 100, such as foam of a particular design thickness (e.g., 2 or 3mm), if desired, or also for mounting on a shelf with a suitable dielectric material. In fig. 2, floating ground 130 is a solid metal sheet, metal foil, laminated to the top or bottom surface of the dielectric material or to some other convenient carrier surface (not shown in fig. 2) above or below dielectric layer 100. Alternatively, the floating ground 130 may be a printed or etched conductor on the underside of the same circuit board or other dielectric material layer 100, the dielectric material layer 100 supporting the radiating antenna element 110 and the reference ground element 120. In an embodiment of the present invention, there is an air-filled space or gap between the dielectric material supporting the floating ground 130 and the dielectric material supporting the radiating antenna element 110 and the reference ground element 120. In an embodiment, the size of the air space or gap is maintained by a non-conductive support that holds the edges of the two printed circuit boards at a fixed separation distance. In another embodiment, elements 110, 120, and 130 are all fabricated on both sides of a single dielectric material, such as foam. Antenna patch 110, reference ground 120, and floating ground 130 are typically formed from a solid copper metal plating, but it should be immediately apparent to those skilled in the art that other types of conductive materials may be used for these elements of the antenna assembly. The antenna is fed with a signal at point 150 and at point 150, in an embodiment, the coaxial cable 140 has been attached with the cable core conductor soldered to the radiating antenna element and the cable screen soldered to the reference ground element as shown. In an embodiment, the total spacing between the antenna patch 110 and the floating ground 130 is between 2 and 3mm, although larger or smaller spacings may also be used. Easy feeding is a clear advantage of this structure, since the radiating antenna element 110 and the reference ground element 120 are arranged in the same plane and close to each other.

Fig. 3 is a diagram illustrating an exemplary patch antenna assembly in accordance with an embodiment. Here, an auxiliary radiating antenna element 360 has been added and physically attached to the main radiating antenna element 110 via a conductive trace 370. In an embodiment, the auxiliary radiating antenna element is disposed adjacent to and coplanar with the main radiating element. In other embodiments, the auxiliary radiating elements may be disposed in one or more closely spaced parallel planes with little or no overlap between the main radiating element and the auxiliary radiating elements. It should be appreciated that in alternative embodiments, the trace 370 may not necessarily be connected to an edge of the radiating element. The connection may be internal to the radiating element and its position may be changed.

In the present invention, the auxiliary antenna element may be placed on any one of the three open sides of the main antenna element. Fig. 3 shows one of these positions, but it should not be construed as limiting the invention in any way. The auxiliary radiating element may be similar in shape and scale to the main radiating element (as shown in fig. 3), or it may have a different shape or scale, as shown in fig. 4, where the auxiliary radiating antenna element 460 has been connected to the main radiating antenna element 110 via trace 470. Although the radiation bandwidth is slightly reduced relative to the design of fig. 3, an advantage of an auxiliary radiating antenna element with this high aspect ratio is that the antenna design is more compact.

According to various embodiments, the patch antenna may be relatively small and formed on a thin substrate. As a result, it is susceptible to electrostatic charge buildup that can damage circuit components. For example, touching the patch antenna may generate electrostatic discharge (ESD) pulses that can damage RF components coupled to the patch antenna. Fig. 4 illustrates a DC short connection 410 that provides ESD protection by safely dissipating accumulated charge. In particular, DC short circuit connection 410 reduces the accumulation of electrostatic charge between main radiating antenna element 110 and reference ground element 120, and also improves antenna performance by strongly suppressing third harmonic signals that affect in-band and out-of-band communications.

The DC short connection 410 may be provided in the same plane as the main radiating antenna element 110 and the reference ground element 120. In contrast, a conventional patch antenna, such as a microstrip antenna, may connect ground through a PCB substrate to the center of the patch, which is virtual ground due to the symmetry of the patch. According to various embodiments, it is advantageous to provide a DC shorting connection 410 that is coplanar with the patch and ground, as it may not be possible or desirable to route ground connections through the substrate.

In an embodiment, additional auxiliary radiating antenna elements may be added to further increase the radiating bandwidth. Fig. 5 shows two auxiliary radiating antenna elements 460 and 560 connected to main radiating antenna element 110 via traces 470 and 570. It should be appreciated that other embodiments exist in which additional auxiliary antenna elements are added. Fig. 5 is provided by way of example only and is not intended to limit the scope and application of the present invention.

Fig. 6 shows in more detail the connection point 150 of the coaxial cable 140 to the antenna patch 110 and the reference ground 120. Coaxial cable 140 includes a core conductor 664 and a shield 662 separated by an insulator 663. In cabling 150, core conductor 664 is connected to main radiating antenna 110 with solder 668. Also, the shield 662 is connected to the reference ground element 120 with solder 666. It will be apparent to those skilled in the art that the coaxial cable 140 shown in the figures of the present invention may be replaced by any other suitable transmission line, including microstrip lines, coplanar lines, or conductor sets capable of carrying signals and reference voltages as required in the application to which the present invention is directed, and that such replacement may be made without departing from the spirit of the present invention.

The main and auxiliary radiating antenna elements of the present invention may be implemented in any pattern or geometry (e.g., square, rectangular, circular, free-flow, etc.). Many of these alternative shapes are shown in fig. 7, including a rectangular shape 710, a rectangular shape 720 with trimmed corners along one diagonal, a rectangular shape 730 with slits, a rectangular shape 740 with two orthogonal slits, a circular shape 750, a circular shape 760 with slits, and a circular shape 770 with two orthogonal slits. These alternatives are shown by way of example only and are not intended to limit the scope and application of the present invention. Trimmed or angled corners (such as those used for antenna shape 720) and slots (such as those used in antenna shapes 730, 740, 760, and 770 of fig. 7) result in a more circularly polarized field around the antenna and improve label readability. The main and auxiliary radiating antenna elements of the present invention may be constructed of metal plates, metal foils, printed or sputtered conductive inks or paints, conductive polymer materials, metal wire mesh, or other functionally equivalent materials (e.g., films, plates, foils, etc.), or any other homogeneous or composite material with appropriate conductivity. The material of the antenna substrate 100 is a dielectric material (e.g., a material typically used for printed circuit boards) or any other material with negligible conductivity (including combinations of two or more different types of such materials with negligible conductivity that may be used in laminated or layered structures).

The transmission line, shown as cable 140, may have tuning components (not shown), such as capacitors or inductors, at either end or along its length. The size (e.g., capacitance or inductance) of these tuning components is selected based on the desired matching and bandwidth characteristics of the antenna according to conventions well known to those skilled in the art.

Each of the following may be adjusted individually or together to optimize antenna radiation bandwidth, radiation gain, radiation pattern, radiation efficiency, and antenna polarization: the shape of the main and auxiliary radiating antenna elements and the reference ground element, the relative position between the main and auxiliary radiating antenna elements and the reference ground element, the position and width of the conductive traces, the feed position of the main and reference ground elements, the size and position of the slots, slits, or other voids in the main and auxiliary radiating antenna elements and/or the reference ground element, and the presence or absence of a floating ground element, its size and shape, the dielectric material between the radiating antenna elements and the floating ground element and its thickness, and the location or presence of an electrical connection or "short circuit" between the main radiating antenna elements and the floating ground. Also, the above-described characteristics of the antenna and its various components, particularly the characteristics of the antenna element shape, slots, slits, and cut corners, may be adjusted to achieve the desired antenna dimensions. For example, the details of the slot or slot, or the nature of the cut corner, also have a significant effect on the frequency response of the antenna and can be used to increase the bandwidth of the antenna. It can be observed that the addition of one auxiliary square radiating element with diagonal corner cuts contributes two natural resonant frequencies to the antenna characteristics, while the addition of one auxiliary radiating element with a high aspect ratio can contribute one resonant frequency. As a result, the introduction of the auxiliary radiating element extends the radiation bandwidth of the antenna. It is well known that the bandwidth of a patch antenna decreases as the thickness of the dielectric substrate decreases. The addition of one or more auxiliary radiating antenna elements in accordance with the present invention allows the use of thinner substrates without sacrificing antenna bandwidth. Another advantage of the auxiliary radiating element is the increased gain due to the combined effect (constructive and destructive interference) of the radiated electromagnetic field from the antenna element.

For typical antenna designs of the prior art, placing a metal object under the antenna changes the resonant frequency of the antenna and can cause severe detuning. The present invention has greatly alleviated this problem. The antenna structure of embodiments of the present invention performs well even when a metal plate or other conductive object, such as a metal retail item or storage shelf, is placed closely beneath the antenna structure due to the confined electromagnetic field. Since the floating ground introduced for the metal shelf acts as a reflector, radiation can only occur in one direction. Thus, the antenna has a higher gain, but generally has a reduced bandwidth.

Detailed computer simulations were performed to demonstrate certain advantages of the present invention over the prior art.

Fig. 8 shows a particular embodiment of a prior art patch antenna with square radiating antenna elements with cut corners (for generating a circular polarization field and enhancing bandwidth) and square reference ground elements in a plane below the plane of the radiating antenna elements. Distance a in fig. 8 is 4.65 inches and distance B is 1.3 inches. Note that the corner cut is implemented at a 45 degree angle. The distance C (referenced to the edge length of the ground element) is 8 inches. The distance D between the two planes in fig. 8 is 0.5 inches. The feed point of the antenna in fig. 8 is located 2.975 inches (distance E) from the side edge of the radiating element and 0.415 inches (distance F) from the leading edge of the radiating element. In the simulation, air was used as the dielectric substrate between the two planes. Copper was used for the radiating element and the reference ground. It is assumed that the material surrounding the antenna is air. Figure 9 shows the return loss in dB as a function of frequency for the antenna.

Fig. 10 shows that the bandwidth (at the center frequency of 915 MHz) decreases with decreasing dielectric substrate thickness, where an air substrate is used. For example, for a 0.5 inch air substrate, the bandwidth is about 14.8% at-7 dB and about 10.5% at-10 dB. Note from fig. 10 that when the dielectric substrate thickness is reduced to 1/8 inches, the bandwidth is only about 4% at a return loss of-10 dB and about 5% at a return loss of-7 dB.

Fig. 11 shows a specific example of a patch antenna (divided into upper and lower halves) having a coplanar reference ground 120, a coplanar main radiating antenna element 110, and a single coplanar auxiliary radiating antenna element 360 of the same size and shape as the main radiating antenna element. This antenna was simulated by a computer using the following element dimensions (see fig. 11): a (length of the main radiating element and the auxiliary radiating element) is 5.96 in; b is 0.85 in; g is 0.75 in; c (offset of trace 370 from the centerline) is 0.1 in; d is 0.44 in; e (width of the feed trace of reference ground 120) ═ 0.24 in; f (width of the feed trace of main radiating element 110) ═ 0.24 in; h is 1.14 in; i-0.1 in, and J-5.60 in. Note that the main and auxiliary radiating antenna elements are square (i.e., their length is equal to their width). Also, note that the separation distance between the traces feeding the reference ground and the two portions of the main radiating element is 0.01 in. Simulations were performed assuming a floating ground plane (not shown in fig. 11) 0.125 inches below the plane containing the radiating and reference ground elements. The 0.125in gap is assumed to have the dielectric properties of a cross-linked foam.

Fig. 12 shows the results of a computer simulation of the antenna of fig. 11. The introduction of the auxiliary radiating element results in an additional resonance peak. That is, fig. 12 shows three peaks, while fig. 11 of the aforementioned patent application (U.S. patent application No.60/978,389) corresponding to a coplanar antenna without an auxiliary radiating element shows only two peaks. The bandwidth of the dual radiating element antenna of figure 11 can be calculated using the data shown in figure 12. At a return loss of-7 dB, the bandwidth extends from 890MHz to 970MHz, which corresponds to a bandwidth of 8.5% (with reference to the center frequency of 930 MHz). At a return loss of-10 dB, the bandwidth is 7%. Thus, for a dual radiating antenna element of the present invention (e.g., as shown in fig. 11), the bandwidth is about 75% greater than the prior art antenna shown in fig. 8 when compared at equal dielectric substrate thicknesses.

In another embodiment of the invention, the metal of the merchandiser itself may be used as a floating ground, or, alternatively, the merchandiser may be constructed such that a common piece of metal serves as a floating ground plane and physical support for the antenna assembly or array of antenna assemblies, and an object that may be placed on a fixture, such as a retail item that owns the RFID tag.

Fig. 13 shows an example of a patch antenna with radiating antenna elements oriented in different directions to improve polarization. In particular, due to the presence of the DC short-circuit connection 410, the relative phase shift between the radiating antenna element 110 and the auxiliary radiating antenna element 460 has changed. Rotating the auxiliary radiating antenna element 460 by, for example, 90 degrees with respect to the radiating antenna element 110 can provide an antenna having a good circular polarization ratio throughout the entire frequency band.

Fig. 13 shows only one structure with a large bandwidth of circular polarization, of course, there are other structures available by changing the circular polarization orientation of a single patch (left or right handed), by changing the relative phase shift between two patches (orientation of a single patch, and the position and length of the connection 470).

Fig. 13 also shows a DC short 410 connected to an auxiliary radiating antenna element 460 via 1-0117 in a manner similar to that described in fig. 4 with DC short connection 410, but also including a recessed connection 117. The recessed connection 117 is used to provide a slot at the edge of the patch antenna element 110 so that the DC short connection is connected to the patch antenna element 110 at a location closer to the center of the patch antenna element 110, thereby enabling the virtual ground to be connected to ground without affecting antenna performance due to the addition of the trace 410, since radiation is primarily along the edge of the patch. Furthermore, fig. 13 shows the antenna feed 115 supplying RF signals and ground, and the antenna feed 115 can be seen in more detail with respect to fig. 11.

Fig. 14 shows an example of a patch antenna array. In an embodiment, each antenna in the array may be oriented along the same direction. For example, the first antenna (i.e., comprising elements 110A and 460A) and the other antenna (i.e., comprising elements 110B and 460B) are oriented along the same direction. Further, each antenna may include radiating antenna elements oriented in different directions (as shown in fig. 13). Each antenna may also radiate with left-hand circular polarization or right-hand circular polarization.

Fig. 15 shows an example of a patch antenna array with an alternative arrangement of radiating antenna elements. In an embodiment, a first antenna (i.e., comprising elements 110A and 460A) may be rotated 180 degrees relative to another antenna (i.e., comprising elements 110B and 460B). Of course, the antenna array may include more than the two antennas shown. The physical orientation of the antenna and the circular polarization orientation are determined by application schemes that can be varied for different situations.

According to various embodiments, each antenna includes its own path to ground 120 via a DC short connection 410.

The present invention expressly encompasses and encompasses all embodiments that may be contemplated by varying one or more features of the embodiments described herein, including the radiating antenna element size, shape, thickness, void, slit, or slot shape, the reference ground element size, shape, placement in two-dimensional space of the plane occupied by the radiating antenna element, the distance separating the radiating antenna element from the reference ground element, the location and manner of signal feed line or cable attachment to the radiating antenna element and the reference ground element, the presence or absence of one or more floating ground elements, the size, shape, or thickness of the floating ground plane, the separation distance between the floating ground and the radiating antenna element, the dielectric material used to separate the radiating antenna element from the reference ground and floating ground, the dielectric material used to fabricate the radiating antenna element, the dielectric material used to separate the radiating antenna element from the reference ground, the floating ground, the dielectric material, The conductive material of the reference and floating grounds, the number of antenna assemblies used in the array, or the material and structure used to house and protect the antenna assemblies or the array of antenna assemblies.

The present invention also encompasses all embodiments in which a single antenna assembly (i.e., having a single patch antenna) is substituted for the array of antenna assemblies.

It should also be noted that various antenna assembly arrays may be constructed in which the antenna assemblies occupy two different planes. For example, an array of antenna elements may be constructed in which some of the elements lie within a first geometric plane and the remainder of the elements lie within a second geometric plane orthogonal to the first geometric plane. This embodiment is given by way of example only and it should be noted that the two planes need not necessarily be orthogonal. Also, it is contemplated that more than two geometric planes may be used in the arrangement of the antenna assembly. In certain applications where, for example, the orientation of an RFID tag to be interrogated by an antenna is unknown, or known to be random or varying, a multi-planar array of such antenna assemblies may improve the robustness of the array. In addition, the application may require specific electric or magnetic field polarizations that may be produced by arranging the antenna assembly in multiple planes. All embodiments that may be envisaged for arranging a plurality of antenna components in a plurality of planes are explicitly included in the present invention.

Although specific circuits, components, modules or dimensions thereof have been disclosed herein in connection with exemplary embodiments of the invention, it should be apparent that any other structurally or functionally equivalent circuit, component, module or dimension may be used in practicing the various embodiments of the invention. Therefore, it is to be understood that the invention is not limited to the specific embodiments disclosed herein (or apparent from the disclosure).

Claims (21)

1. An antenna for use in a radio frequency identification system having a Radio Frequency (RF) signal, comprising:

a reference ground section;

an antenna feeding section;

a primary patch antenna element for intermediate field transmission and reception of RF signals, wherein the primary patch antenna element is electrically coupled to the antenna feed; and

one or more additional patch antenna elements for intermediate field transmission and reception of RF signals, wherein each of the one or more additional patch antenna elements is electrically connected to an edge of the main patch antenna element for transmission and reception of RF signals, and wherein the one or more additional patch antenna elements provide gain enhancement of the RF signals.

2. The antenna of claim 1, further comprising a thin dielectric substrate on which the primary patch antenna element and the one or more additional patch antenna elements are formed.

3. The antenna of claim 2, wherein the thin insulating substrate is a mylar.

4. The antenna of claim 2, wherein the RF signal is one of an HF and UHF broadband signal, and wherein the thin insulating substrate in combination with the one or more additional patch antenna elements provides sufficient bandwidth for the one of an HF and UHF broadband signal.

5. The antenna of claim 1, wherein one or more additional patch antenna elements are rotated to have different orientations with respect to the main patch antenna element.

6. The antenna of claim 5, wherein the different orientation is 90 degrees between the primary patch antenna element and the one or more additional patch antenna elements.

7. The antenna of claim 1, further comprising a thin insulating substrate on which the reference ground, the main patch antenna element, and the one or more additional patch antenna elements are formed as coplanar electrical conductors.

8. The antenna of claim 1, wherein the primary patch antenna element and the one or more additional patch antenna elements are arranged in a plurality of closely spaced parallel planes and do not substantially overlap.

9. The antenna of claim 1, wherein the primary patch antenna element has a different aspect ratio than the one or more additional patch antenna elements.

10. The antenna of claim 1, wherein the one or more additional patch antenna elements comprise at least two patch antenna elements, each patch antenna element being electrically connected to an edge of the main patch antenna element.

11. The antenna of claim 1, further comprising a floating ground that is substantially parallel to and overlaps each of the primary patch antenna element and the one or more additional patch antenna elements.

12. The antenna of claim 1, further comprising a DC short connection between an edge of the primary antenna element and the reference ground, wherein the DC short connection between the primary patch antenna element and the reference ground does not substantially interfere with the RF signal, wherein the DC short connection provides an electrostatic discharge path for electrostatic pulses from the primary patch antenna element to the reference ground.

13. The antenna of claim 12, further comprising a floating ground that is substantially parallel to and overlaps each of the primary patch antenna element and the one or more additional patch antenna elements.

14. The antenna of claim 1, wherein the reference ground, the antenna feed, the primary patch antenna element, and the one or more additional patch antenna elements together form an antenna assembly, and further comprising a plurality of antenna assemblies, thereby providing an antenna array.

15. The antenna of claim 14, wherein the plurality of antenna components are formed on a common thin insulating substrate.

16. The antenna of claim 14, wherein one of the antenna assemblies is rotated to have a different orientation relative to the other of the plurality of antenna assemblies.

17. An antenna for use in a radio frequency identification system having a Radio Frequency (RF) signal, comprising:

a reference ground section;

an antenna feeding section;

a patch antenna element for intermediate field transmission and reception of RF signals, wherein the patch antenna element is electrically coupled to the antenna feed; and

a DC short connection between an edge of the patch antenna element and the reference ground, wherein the DC short connection between the patch antenna element and the reference ground does not substantially interfere with RF signals, wherein the DC short connection provides an electrostatic discharge path for electrostatic pulses from the patch antenna to the reference ground.

18. The antenna of claim 17, wherein the DC short connection comprises a notched connection, wherein edges of the patch antenna element are notched such that the DC short connection connects to the patch antenna element at a location closer to a center of the patch antenna element.

19. The antenna of claim 18, wherein the reference ground and the primary patch antenna element are coplanar conductors.

20. The antenna of claim 19, further comprising a floating ground that is substantially parallel to and overlaps the patch antenna element.

21. The antenna of claim 17, further comprising a thin dielectric substrate on which said patch antenna elements are formed.