Patent Grant
7403158
The present invention relates to Radio Frequency (RF) antennas, and more particularly, this invention relates to an antenna featuring an annular shape with thin metal conductor traversing the central opening thereof.
Newer designs and manufacturing techniques have driven electronic components to small dimensions and miniaturized many communication devices and systems. Unfortunately, antennas have not been reduced in size at a comparative level and often are one of the larger components used in a smaller communications device. For instance, directional RF antennas use plates as the radiating conductor. However, to achieve good performance, such devices tend to be very large, as the antenna diameter depends on the operating wavelength. The wavelengths of 46-49 MHz signals are 18 feet, while 900 MHz signals are about one foot long. Modern technologies such as Radio Frequency Identification (RFID) would benefit from smaller antenna size.
In current, everyday communications devices, many different types of patch antennas, loaded whips, copper springs (coils and pancakes) and dipoles are used in a variety of different ways. These antennas, however, are sometimes large and impractical for a specific application. For instance, a 900 MHz directional antenna requires a 1 foot diameter footprint. Current attempts to produce smaller directional antennas are often referred to as patch antennas. Conventional patch antennas typically have a ground plane diameter about equal to, or larger than, the operating wavelength, e.g., typically have a diameter of about 6-8 inches at 900 MHz. Such antennas provide a gain on the order of +8 dBic or +6 dBi, where dBic refers to antenna gain, decibels referenced to a circularly polarized, theoretical isotropic radiator and dBi refers to antenna gain, decibels referenced to a theoretical isotropic radiator.
However, to achieve the reduced size, patch antennas have heretofore been constructed with high dielectric constant materials, e.g., ceramics, metal oxides, etc., making them both expensive, and very heavy. The additional weight makes such antennas impractical for implementation in portable devices, cost more to ship, etc. A further drawback of antennas implementing the high dielectric constant materials is that they only operate in a narrow bandwidth.
Thus, it would be desirable to not only reduce antenna size and weight, but also to do so without significant degradation of gain and bandwidth.
To provide the aforementioned desirable advantages, a compact, low weight, high gain, circular polarized antenna is disclosed that provides high gain for a minimal amount of area.
A circular polarized antenna according to one embodiment includes an electrically conductive element having a generally annular outer portion and first and second elongate inner members coupled to the outer portion. A ground shield is spaced from the element, the ground shield providing an effective ground plane, the effective ground plane having a maximum width in a direction parallel to a plane of the element of less than about one half and in some embodiments less than about one-third, of an operating wavelength. A dielectric material is positioned between the element and at least a portion of the ground shield.
A circular polarized antenna according to another embodiment includes a substantially square-shaped electrically conductive element having a plurality of voids defined therein, edges of the element along the voids defining an outer portion of the element and at least two elongate inner members of the element. A ground shield is spaced from the element and having a substantially square-shaped outer periphery, the ground shield providing an effective ground plane, the effective ground plane having widths in a direction parallel to a plane of the element and perpendicular to each other and perpendicular to straight sections of the outer periphery of less than about one-third of an operating wavelength. A dielectric material is positioned between the element and at least a portion of the ground shield.
A circular polarized antenna according to yet another embodiment includes an electrically conductive element having a generally annular outer portion and inner members extending from an inner periphery of the outer portion and lying in about the same plane as the outer portion. A ground shield is spaced from the element. A dielectric material is positioned between the element and at least a portion of the ground shield, the dielectric material having a dielectric constant less than about 2 at 0° C., ideally less than about 1.1 at 0° C.
System implementations are also presented, including RFID systems. RFID systems typically include a plurality of RFID tags and an RFID interrogator in communication with the RFID tags.
Other aspects and advantages of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention.
For a fuller understanding of the nature and advantages of the present invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings.
The following description is the best mode presently contemplated for carrying out the present invention. This description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each and any of the various possible combinations and permutations.
In the drawings, like and equivalent elements are numbered the same throughout the various figures.
The following specification describes a compact circular polarized antenna that provides high gain for a minimal amount of area and weight. The antenna is annular in geometry to maximize surface area for the minimum dimensions. Antennas constructed as described herein exhibit a gain of greater than about 2 dBi, and in most instances, greater than about 6 dBic or 3 dBic within a similar frequency band as conventional patch antennas. The reduction in size is achieved by implementing a unique radiating element design, low dielectric constant material, and a ground shield.
Many types of devices can take advantage of the embodiments disclosed herein, including but not limited to Radio Frequency Identification (RFID) systems (all Classes) and other wireless devices/systems; portable electronic devices such as portable telephones and other audio/video communications devices; and virtually any type of electronic device where an antenna is utilized. To provide a context, and to aid in understanding the embodiments of the invention, much of the present description shall be presented in terms of an RFID system such as that shown in
As shown in
Communication begins with a reader 104 sending out signals via an antenna 110 to find the tag 102. When the radio wave hits the tag 102 and the tag 102 recognizes and responds to the reader's signal, the reader 104 decodes the data programmed into the tag 102 and sent back in the tag+ reply. The information can then be passed to the optional server 106 for processing, storage, and/or propagation to another computing device. By tagging a variety of items, information about the nature and location of goods can be known instantly and automatically.
RFID systems may use reflected or “backscattered” radio frequency (RF) waves to transmit information from the tag 102 to the reader 104. Since passive (Class-1 and Class-2) tags get all of their power from the reader signal, the tags are only powered when in the beam of the reader 104. Class-3 and higher tags may include an on-board power source, e.g., a battery.
The element 202 has a generally annular outer portion 204 and first and second elongate inner members 206, 208 coupled to the outer portion 204. The outer portion 204 is a preferably continuous layer of conductive material. The outer portion 204 preferably has a generally rectangular inner periphery 210 and outer periphery 212 that approximates the shape of the ground shield 220. In the embodiment shown, the outer portion 204 has square shaped peripheries 210, 212.
The inner members 206, 208 preferably lie along a common plane 214 with the outer portion 204. In some embodiments, the inner members 206, 208 are continuous with the outer portion 204 and/or each other, i.e., formed in the same processing step such that there are no seams between the outer portion 204 and the inner members 206, 208. For instance, the outer portion 204 and inner members 206, 208 can be formed simultaneously by deposition on a substrate such as a printed circuit board. Alternatively, the element 202 can be formed as a large continuous sheet, and voids created therein, e.g., by cutting or stamping, to define the inner members 206, 208. In other embodiments, the inner members 206, 208 are coupled to the outer portion 204 and/or each other, e.g., by welding, soldering, riveting, etc. The cross-sectional shape of the inner members 206, 208 is not critical, and can be rectangular, round, oval-shaped, etc. The length to width ratio of the inner members 206, 208 may be in a range of 2:1 to 1000:1. An illustrative embodiment has inner members 206, 208 with an axial length to cross sectional width ratio of 10:1.
The inner members 206, 208 may be long and thin relative to the width Wo of the outer portion 204. The width Wo of the outer portion 204 is preferably at least 2× the width Wi of the inner members 206, 208. In other embodiments, the width Wo of the outer portion 204 is in the range of between 2× and 100× the width Wi of each inner member, e.g., 4×, 5×, 6×, 7×, 8×, 10×, 20×, 100×, etc.
The width Wg of the ground plane is preferably at least 2.5× the width Wo of the outer portion 204. In other embodiments, the width Wg of the ground plane is in the range of between 2.5× and 10× the width Wo of the outer portion 204, e.g., 3×, 4×, 5×, etc.
In a preferred embodiment, the size of the conductive area of the element 202 is sufficient to maintain a high surface current and associated magnetic field, and the gap between the element 202 and the ground shield 220 is sufficient to produce an electric field in any phase of the stimulus source at the port 250 of the antenna 110.
As shown in
In the embodiment shown in
Any electrically conductive material can be used to form the element 202, with metals being preferred. Illustrative materials from which to form the element 202 include copper, aluminum, etc. The elements described above, and especially those shown in
With continued reference to
Preferably, the ground shield 220 is positioned closest to the element 202 in an area where the ground shield 220 is in the same plane 214 as the element 202. In other words, a distance between the element 202 and the portion of the ground shield 220 lying along the same plane 214 as the element 202 is less than a distance between the ground shield 220 and the element 202 as measured in a direction perpendicular to the plane 214 of the element 202.
The effective ground plane created by the ground shield 220 preferably has a width Wg in a direction parallel to a plane 214 of the element 202 of less than about ½ of an operating wavelength of the antenna 110. Thus, for example, for an antenna transmitting a 900 MHz signal, the operating wavelength would be about 1 foot, and the width Wg of the effective ground plane would be about 6 inches or less. Preferably, the width Wg of the effective ground plane is less than about one-third of the operating wavelength. Thus, for example, for an antenna transmitting a 900 MHz signal, the operating wavelength would be about 1 foot, and the width Wg of the effective ground plane would be about 4 inches or less. The width Wg of the effective ground plane in one embodiment is greater than about one-ninth of the operating wavelength, but can be smaller.
Note that the width Wg of the effective ground plane as shown is a width between the straight sections of the ground shield 220 sidewall. The width of the effective ground plane can also be measured from corner to corner of the ground shield 220, which will then be the maximum width of the ground shield 220. The same constraints as defined above can be applied to the maximum width. Further, a second width Wg2 can be defined perpendicular to the first width Wg, and Wg2 may or may not equal Wg.
In an illustrative embodiment, the widths Wg, Wg2 of the ground shield 220 are each between about 1 and about 5 inches. An illustrative height Hs of the peripheral sidewall 224 of the ground shield 220 (if present) is between about 0.25 inches and about 2 inches. One experimental embodiment has a ground shield 220 with Wg=4 inches, Wg2=4 inches, and Hs=0.88 inches. The distance between the plane 214 of the element 202 and the bottom 222 of the ground shield 220 is in the range of between about 0.25 inches and about 0.85 inches.
Any electrically conductive material can be used to form the ground shield 220, with metals being preferred. Illustrative materials from which to form the element 202 include copper, aluminum, etc.
A dielectric material 240 is positioned between the element 202 and at least a portion of the ground shield 220. A dielectric material is a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic fields. To reduce the overall weight of the antenna 110, the dielectric material 240 preferably has a low dielectric constant, e.g., a dielectric constant of less than about 2 at 0° C., ideally less than about 1.1 at 0° C. Substances with a low dielectric constant include a vacuum, air, and most gases such as helium and nitrogen. Accordingly, one preferred dielectric material is a gas such as air. Air has a dielectric constant of 1 at 0° C. and 1 atmosphere. Accordingly, the element 202 may be supported above the bottom of the ground shield 220, e.g., by a printed circuit board or other substantially RF transparent substrate, thereby sandwiching a layer of air therebetween. If a definable layer of dielectric material is desired, a material having air in voids thereof, such as STYROFOAM, sponges, etc. may also be used. A container or bladder encapsulating the dielectric material 240 can also or alternatively be provided between the element 202 and the ground shield 220. The latter embodiments may provide the additional benefit of giving additional support to the element 202.
With continued reference to
Coupling capacitors 258, 260 are preferably formed between the feeding pins 252, 254 and the inner members 206, 208. Note that a distributor plate can be used instead of the coupling capacitors 258, 260.
While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
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