Patch antenna for RFID tag

An antenna for use with a Radio Frequency Identification, RFID, tag. A well-known, simple “patch antenna” is formed by two metallic plates, one larger than the other, between which is sandwiched a dielectric sheet. Under the invention, the larger metallic plate is provided by the wall of a metallic shipping container. Thus, one form of the invention includes (1) the smaller metallic sheet, (2) the dielectric sheet, and (3) an RFID circuit. When the dielectric sheet is attached to the wall of the shipping container, a patch antenna is generated which the RFID circuit can use.

The invention concerns a patch antenna, sometimes called a microstrip or stripline antenna, used in connection with an RFID, Radio Frequency IDentification, tag.

BACKGROUND OF THE INVENTION

Numerous types of RFID tag are commercially available. A common type of RFID tag stores a small amount of data, such as an identifying number, and transmits the data to a nearby interrogating device, when the latter issues an interrogation signal.

In general, RFID tags can be viewed as containing four primary components. Three of the components are commonly fabricated in a single integrated circuit, IC, and they are: (1) a receiver, (2) a transmitter, both of which are sometimes termed a transceiver, and (3) memory to store data, such as the ID number stated above. The fourth component is an antenna, used to communicate with the interrogator.

In some designs, the antenna can be included in the IC, or fabricated on the same silicon wafer as the IC. The antenna can also be external to the IC.

In addition, other components can be present, to perform tasks such as (1) detecting an incoming interrogation signal, and in response launching a dormant tag into operation, (2) absorbing operating power from incoming rf radiation, (3) reading data in the memory and transmitting the data to the interrogator, and (4) discriminating an address in an incoming polling signal, to discern whether the interrogation signal is addressed to the RFID tag associated with the components, or to another RFID tag.

Some RFID tags are passive. They contain no power supply, and obtain operating power from rf energy delivered by the interrogator. Other RFID tags do contain power supplies, such as batteries of the size used in hearing aids. These latter RFID tags can not only transmit stored data, but they can also receive data from the interrogator, and can write the data received to memory in the RFID tag.

In general, passive devices do not receive and store incoming data but, of course, exceptions are possible.

The frequency of rf radiation used depends on the particular application of the RFID tags. For example, some tags use low-frequency radiation, in the AM or FM radio bands, which span roughly from 0.5 MHz to 150 MHz. Such radiation can pass through buildings and other structures. Using such radiation, one can read an RFID tag through a wall or building.

At higher frequencies, such as 1,000 MHz, the radiation begins to acquire the properties of visible light. Visible light will not penetrate walls and buildings. Using such high frequencies, one can only communicate with RFID tags which are within one's line-of-sight, with no intervening obstructions.

Further, at high frequencies, the presence of nearby conductive objects can interfere with operation of the RFID tags. While the detailed mechanism of the interference is complex, one can view the conductive objects as creating “echoes” of the rf signals. The echoes can jam communication. For example, the echoes can destructively add together, forming nulls where the net signal is zero. If the RFID tag or the interrogator is located at a null, no signal will be detected.

Therefore, when RFID tags using high-frequency radiation are used in the proximity of conductive objects, such as sea water or bodies of metal, problems can arise. As a specific example, problems are found when high-frequency RFID tags are used on steel shipping containers, particularly when multiple such containers are present.

OBJECTS OF THE INVENTION

An object of the invention is to provide an improved RFID tag, which can utilize high-frequency carrier frequencies, and operate in the presence of large conductive objects.

A further object of the invention is to use a patch antenna, also called a stripline antenna, in connection with an RFID tag.

A further object of the invention is to use a patch antenna in connection with an RFID tag, to allow the RFID tag to be attached to a steel shipping container.

SUMMARY OF THE INVENTION

A patch antenna is used in connection with an RFID tag, to accept incoming information, such as interrogation signals, and also to transmit data from the tag.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates a patch3, constructed of a conductive sheet, or film, such as aluminum. The patch need not be the shape shown, and patches are used which are square, rectangular, circular, triangular, linear (that is, a long thin rectangle), and hollow, such as a flat doughtnut shape.

A feed line6is connected to the patch3. Block9represents an RFID circuit, which contains the elements described in the Background of the Invention. Line12represents a ground line, which will be used to connect to a ground plane, later described.

Lines6and12are not connected together electrically, as shown by the schematic ofFIG. 2. Line12is connected to ground, GND, in the form of a ground plane, later described. A signal generator15, which corresponds to block9inFIG. 1, generates a signal, which is fed to an antenna ANT, through line6.

FIG. 3illustrates structures to which the apparatus ofFIG. 1is attached. The patch3is placed adjacent a sheet21of dielectric material, and is bonded to the sheet21in a known manner.

A film24of adhesive is placed adjacent the bottom side of the dielectric sheet21. This adhesive is used to attach the dielectric sheet21to a metallic ground plane27.

Ground plane27can take the form of a metallic sheet, or film, in which caseFIG. 3illustrates one embodiment of the invention, in exploded form. In another embodiment, ground plane27is provided by a metallic structure to which adhesive film24is attached. In this case,FIG. 3illustrates the invention in actual use. However, in this case, the part of the invention which is made or sold would not, in general, include the ground plane27.

Leg12A of line12forms a via, or layer-to-layer connection, between the RFID circuit9and the ground plane27. Window30, in sheet21, and window31, in layer24, allow the leg12A to pass through the respective layers, en route to the ground plane27.FIG. 4shows the windows in cut-away view. Block33inFIGS. 3 and 4indicates the attachment point of line12with the ground plane27.

FIG. 5illustrates a sequence of processing steps in one approach to fabricating the apparatus just described. The processing begins with what is conceptually a double-sided printed circuit board45, shown at the upper left part of the Figure. As discussed later in connection withFIG. 7, the dimensions required of the actual patch antenna may require (1) a different thickness of copper than is obtained with ordinary printed circuit boards, (2) a different thickness of dielectric substrate, and (3) a different dielectric material. If so, it is known in the art how to fabricate board45inFIG. 5conforming to those requirements.

The pattern, indicated by dashed lines48on board45, is etched in the copper on the upper side of the board45, producing the structure51, shown at the upper right part of the Figure. The ground plane52on the lower side of the board45is not etched.

A via54is formed, as indicated at the lower right of the Figure, which connects pad55to the ground plane52. The via54can be formed by drilling from the bottom of the board to the pad55, and filling the drilled hole with solder. The entire assembly can then be nickel-plated, to reduce corrosion, and to facilitate later soldering.

The RFID circuit9, at the lower left of the Figure, which can take the form of a surface-mount integrated circuit, is connected to the line6and pad55, as by soldering. Pad55corresponds to line12inFIG. 1.

The entire assembly57ofFIG. 5can be attached to a common steel shipping container59, as indicated inFIG. 6.

It may be desirable to make electrical contact between the ground plane52and the container59. This can be accomplished by, for example, abrading a spot (not shown) on the container59, to remove paint, corrosion, and other unwanted materials. Then the assembly57is attached to the container59, using a conductive adhesive, such as an epoxy containing a powdered metal, such as silver powder.

FIG. 7illustrates some generally accepted parameters used in the design of patch antennas, and various forms of the invention can be designed using some, or all, of those parameters. The symbol,r, that is epsilon-sub-r, refers to the relative dielectric constant of the DIELECTRIC inFIG. 7.

FIG. 8illustrates roughly the radiation pattern of a generic patch antenna. The Figure is a polar plot of electric field strength. It is emphasized that line160does not represent a boundary. That is, line160does not indicate that the electric field E is zero at point P1, outside the line160.

Rather, line160indicates the relative electric field strength E, at different angles. For example, the relative electric field strength at any point P2on line L2is represented by the length of arrow A2. The relative electric field strength at any point P3on line L3, a similar distance from the antenna as point P2, is represented by the length of arrow A3. The relative strengths of the two electric fields is represented by the relative sizes of the two arrows.

It is commonly accepted that the agency in a generic patch antenna which causes radiation is the fringing electric field between the patch203and the ground plane248. Line250represents the fringing field.

Consistent with this, one standard mode of feeding a signal to a patch antenna is shown inFIG. 9. Holes H1and H2, shown in the upper part of the Figure, are cut into the ground plane248and the dielectric221. A coaxial cable252, shown in the lower part of the Figure, is connected as indicated. The shell or sheath S is connected to the ground plane248. The signal wire W is connected to the patch203, as indicated by dot D.

The coaxial cable250is located at a position which is removed from the fringing field250ofFIG. 8, and does not interfere with the fringing field250.

In one form of the invention, the RFID circuit209occupies the position indicated inFIG. 10, on the patch203. That is, the RFID circuit209is located on the top of the patch203, and the dielectric221is located on the bottom of the patch203.

This positioning is justified by the argument just given with regard to the coaxial cable252inFIG. 9. Just as the coaxial cable252does not interfere with the fringing field250inFIG. 8, so will the RFID circuit209inFIG. 10refrain from interfering with the fringing field250inFIG. 8.

The RFID circuit209can be connected to the patch203and the ground plane248as indicated inFIGS. 11 and 12. InFIG. 12, it is pointed out that the line6ofFIGS. 11 and 1may largely be eliminated. Dot6A inFIG. 12represents the signal connection to the patch203, and can take the form of a direct connection between a pad (not shown) on the IC within RFID circuit209and the patch203. This type of direct connection is discussed again later.

Under this arrangement, the RFID circuit209inFIG. 10is located entirely within a boundary350defined by the periphery of the patch303. That is, if the dielectric221is made the same size as the patch203, then the entire apparatus is contained within the perimeter350, which bounds the patch203and dielectric221.

In another embodiment, the RFID circuit209can be embedded within the dielectric221, as shown inFIG. 13. Line6and line12are shown inFIG. 14.FIG. 15shows a cross-sectional view.

It is noted that RFID circuit209may take the form of a die cut from a silicon wafer. The RFID circuit is fabricated on that die. In general, the RFID circuit will be fabricated on the surface of the die. That is, the transistors, resistors, traces, and so on only penetrate one, or a few, microns into the die. Thus, a pad may be fabricated on the die, represented by dot D inFIG. 14, which can make direct contact with the patch203. Then line6would not be needed, but line12would be present, and would connect to the top of the die, where the integrated circuit has been fabricated. Conversely, the dot D could contact the ground plane248, in which case line12would not be needed, but line6would be present, running from the surface on the die where the integrated circuit is formed to the patch203.

In some situations, as explained above, the ground plane248inFIG. 13may not be associated with the RFID circuit as manufactured, but may be later provided by a metallic surface onto which the RFID circuit is affixed. For such a situation, the lower part ofFIG. 16shows a top view of RFID circuit209applied to patch203, which is affixed to dielectric221. Dielectric221may be manufactured the same size as patch203.

The upper part of the Figure shows a view of the bottom of the dielectric221, as seen by eye E. Patch203and RFID circuit209are drawn in phantom at the top, because the dielectric221blocks their view.

Affixed to the bottom surface of the dielectric221is a conductive pad290, which is connected to the RFID circuit209, by line12, which extends through the dielectric221. The overall assembly ofFIG. 16is attached to a metallic container, such as59shown inFIG. 6. The pad290ofFIG. 16makes contact with the wall of the container. The wall then acts as a ground plane.

That is, when the assembly is attached to the metallic container, the conductive wall of the container provides the function of the ground plane27inFIG. 3.

FIG. 17illustrates another embodiment. The RFID circuit209is fabricated onto a silicon substrate300(or other substrate if silicon technology is not used), which substrate300is a die cut from a larger silicon wafer. The Inventor points out that the RFID circuit209occupies the top surface T of the substrate300. Pads303connect with the active part of the IC, AIC, through traces306. The RFID circuit209is attached to the patch203, which is here shown below the dielectric221.

Several pads303are shown. Many of these can be used for testing purposes, during manufacture of the RFID circuit209. However, after manufacture, in one form of the invention, only two pads are used in the operative invention, namely, (1) a pad connecting to the signal lead, such as lead6inFIG. 1, and (2) a pad connecting either to the ground plane27inFIG. 3, or to pad290inFIG. 17which, in turn, will later connect to a ground plane, such as the wall of a shipping container. The other pads are insulated from the patch203.

The two pads which are used are labeled303A and303B inFIG. 18. Pad303B connects to pad290through via310.FIG. 19is a cross-sectional view. The connection between pad303A inFIG. 18and the patch203is indicated by rectangle315inFIG. 19.

One mode of operation of the invention is here emphasized. As explained above, one type of RFID tag obtains its operating power from incoming rf radiation, which is received by the tag's antenna. In the situation ofFIG. 8, that incoming radiation creates the fringing field250, causing power to flow into the RFID circuit.

One explanation for this power flow is the reciprocity theorem of antenna theory. That theorem states, in simple terms, that an antenna which radiates an electric field represented by arrow A2, is also a good absorber of a similar incoming electric field A2. That is, an antenna which generates a field when energized, also becomes energized when a similar field is generated by an external source.

The Inventor points out that, in the power-absorption mode, the RFID circuit can be electrically shielded by the patch203from the incoming radiation, yet can derive power from that radiation. For example, the RFID circuit209inFIG. 13would be shielded from incoming radiation entering from above, by patch203. Nevertheless, the RFID circuit209can absorb energy from the patch antenna.

Similarly, while RFID circuit209inFIG. 10may appear to be exposed to incoming radiation, it is submitted that such is not actually the case. The incoming electric field vector can be broken into two components, one parallel with the patch203, and one perpendicular to the patch203.

Since the patch203is conductive, the net field parallel to its surface, at the surface, must be zero. This is a standard boundary condition in electromagnetic theory. Thus, the RFID circuit209, in being adjacent to the patch203, sees a zero field component parallel with the patch203, at the surface of the patch203.

The component which is perpendicular to the patch203will be one of the fringing fields250inFIG. 8. It will extend from, for example, point PA to point PB, roughly indicated by dashed line E. However, the field strength will be equal to (1) the voltage difference between the patch203and the ground plane248, (2) divided by the length of dashed line E. That field strength will be less than that of fringing field250.

Further, this calculation of field strength presumed that the charge density across the patch203is uniform, or that charge is present at point PA. That is not necessarily so.

One model for the patch antenna states that the charge density is concentrated at the edges of the patch, and is zero, or nearly so, in the central region of the patch. Thus, under this model, the electric field along path E inFIG. 8will be substantially zero.

Therefore, it can be said that, while the RFID circuit does not directly receive incoming radiation, it nevertheless can derive power from the antenna immersed in the radiation.

Definitional matters. A distinction is drawn between an “RFID tag,” and an “RFID circuit.” The former is operative to receive and transmit rf signals, and includes an antenna. The latter, the “RFID circuit,” contains an electrical circuit, probably an integrated circuit, but lacks an antenna.

Of course, “antenna” is used in a practical sense. If an RFID circuit, lacking an antenna, is sufficiently close to a device transmitting data at the frequency for which the RFID circuit is designed, the RFID circuit will pick up that data, using its internal wiring as antenna. Nevertheless, the separate antenna used in normal operation is absent.

The RFID circuit stores data, and acts as a radio transceiver, which transmits the data. It may perform other functions. It may continually transmit the data, or may do so only when prompted by a polling signal.

One definition of “RFID tag” is the combination of (1) an RFID circuit with (2) an antenna. That is, the RFID circuit is largely inoperative, without the antenna. (Again, as stated above, the internal conductive traces on the RFID circuit can broadcast RF signals, which can be picked up at short range, without an antenna. However, in ordinary usage, an added antenna is used.)

Another definition of “RFID tag” is a small device, which is ordinarily portable until attached to an object, and which stores a code or number, and transmits that code/number. It may transmit the code/number periodically, or may do so only when prompted by an interrogation signal. It may perform other functions.

“Small” means (1) at least smaller than a brief case measuring 20×18×2 inches. “Small” can further mean smaller than 3×4×0.5 inches. “Small” can further mean smaller than 1×1×0.2 inches.

One specific type of RFID tag is the passive type, which derives operating power from incoming radiation, and which is not self-powered.

A specific type of passive RFID tag is the type which performs a single function, namely, transmitting a number stored within it when prompted to do so by an interrogation signal.

The term “patch antenna” is a term-of-art. One definition is a section of a strip line, wherein one conductor of the strip line forms a ground plane, and the other conductor, smaller in area than the ground plane, acts as an antenna.

It is known that patch antennas need not be flat.

A specific form of the invention utilizes a patch antenna in connection with a non-self-powered, passive, RFID circuit, operating at frequencies above 900 MHz. As explained above, the ground plane of the patch may take the form of the conductive wall of a shipping container. In such a case, it is expected that the problem of signal nulls discussed above, and other problems caused by nearby conductive objects, will be reduced.

A type of trade-off is seen here. Patch antennas, in general, are characterized by narrow bandwidth, low efficiency, and low gain, compared with antennas commonly used with RFID tags, such as dipole antennas. However, these disadvantages can be offset by the elimination of the problems otherwise caused by the metallic shipping container to which the RFID is attached.

From another point of view, a self-powered RFID circuit can transmit a stronger signal than a passive RFID circuit, which would imply a higher signal-to-noise ratio, which would imply less significant interference from nearby reflective objects, compared with a passive device. Under this reasoning, the passive RFID circuit would benefit from the patch antenna more than would a self-powered RFID circuit, because the passive RFID is more subject to noise problems.

RFID tags sometimes contain printed labels. Such a label can overlay part, or all, of the dielectric sheet21inFIG. 3. The label can be printed on the dielectric sheet21, or can be printed on a separate sheet which is attached to the dielectric sheet21.

Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.