HK1108723B - Rfid near field microstrip antenna - Google Patents

Description

RFID near field microstrip antenna

Background

Existing solutions for reading RFID tags use conventional antennas that provide a wide range of RFID tag readings. This scheme provides a large portion of the antenna energy to reach the far field. The far field region is defined as the distance d > λ/2 π, where λ is the wavelength. This value is about 5cm at a UHF frequency of 915 MHz. Thus, the 915MHz far field region is substantially outside 5cm, and similarly the near field region is substantially below 5 cm. For example, most RFID reading antennas are designed to read tags at a maximum distance of a few meters, which, of course, is in the far field region.

In some applications, namely RFID tag applicators (label applicators) and programmers, it is necessary to read and write only one RFID tag within a group of tags located in close proximity to each other. For example, on a label applicator, a plurality of labels are wound on a roll, which facilitates processing on the machine. On the roll, the labels are arranged side-by-side or end-to-end in close proximity. However, in conventional UHF antennas, directing energy onto only one tag at a time is difficult because conventional UHF antennas typically have a broad radiation pattern and are capable of directing energy into the far field. The broad radiation pattern may illuminate all RFID tags within range of the antenna. If we attempt to write a product code or serial number to one label, then all illuminated labels are programmed with the same code or serial number.

The conventional far field antenna used in such RFID UHF applications is a patch antenna. Typically, the radiating patch area is fed through a connector energized by the RFID circuit. Typically, the conductive plate is mounted on the back side a short distance from the patch area.

In these applications, it is desirable to read or write information to RFID tags that are in close proximity, for example, a tag applicator that requires programming, testing, and application of one tag at a time, which is not done by conventional far field antennas. Conventional radiating antennas require a sufficient distance between tagged items to prevent multiple items from being read or programmed simultaneously, or require the use of a metal window to block all tags other than those being programmed or read.

However, this technique does not adequately address the above-mentioned problems because if the individual labels are spaced far apart, the label applicator throughput can be reduced and the number of labels for a given roll size is limited. If a masking technique is used, a different mask is required for each different label shape and spacing. Therefore, handling different labels on the applicator line requires variations, which also significantly reduces throughput.

Disclosure of Invention

The present invention relates to a near field antenna configured to read out an RFID tag. The antenna is configured such that the localized electric field E emitted by the antenna at the operating wavelength resides substantially within the region defined by the near field. The localized electric field E directs the current distribution along the effective length of the antenna corresponding to a half-wave to a full-wave configuration. In one embodiment, the antenna is configured such that its near field region is defined by a distance from the antenna equal to λ/2 π, where λ is the operating wavelength of the antenna. In another embodiment, the near field antenna operates at a frequency of about 915MHz, and thus, the near field region distance is about 5 cm.

The invention also relates to a method of reading an RFID tag, comprising the steps of: a near-field antenna arrangement is provided that is configured such that a localized electric field E emitted by the antenna arrangement at an operating wavelength resides substantially within a region defined by the near-field, wherein the localized electric field E is such as to direct a current distribution along an effective length of the antenna corresponding to a half-wave to a full-wave configuration. The method further comprises the steps of: coupling the localized electric field E of the near field antenna arrangement to an RFID tag disposed within the near field region.

In one embodiment, the method further comprises the steps of: the antenna arrangement is configured such that the near field region is defined by a distance from the antenna arrangement equal to λ/2 π, where λ is the operating wavelength of the antenna arrangement. Further, the method may further comprise the steps of: the antenna arrangement operates at a frequency of about 915MHz, so that the near field region distance is about 5 cm.

The invention also relates to a near field antenna arrangement for reading out an RFID tag, comprising: antennas configured as single and continuous conductors. The antenna extends from one end forming a feed point to the other end forming a termination point. The termination point is connected to the ground plane through a resistor. The antenna device has a relative permittivity and is configured such that a localised electric field E emitted at an operating wavelength resides substantially within a region defined by the near field. The localized electric field E is a current distribution that is directed along the effective length of the antenna corresponding to a half-wave to a full-wave configuration.

In one embodiment, the effective length of the antenna arrangement is such that the distribution of current directed through the antenna produces a waveform having a wavelength proportional to nv/f, where v is the speed of the propagating wave, which is equal to the speed of light divided by the square root of the relative permittivity of the antenna arrangement, f is the frequency in Hz, and n ranges from about 0.5 for half a wavelength to 1.0 for the full wavelength. In another embodiment the antenna is a microstrip antenna and the near field antenna arrangement comprises a substrate having a first surface and a second surface, the thickness of which is defined by the distance between them, wherein the microstrip tracking antenna is arranged on the first surface of the substrate and the ground plane is arranged on the second surface of the substrate.

In another embodiment, the antenna arrangement is configured such that the propagating localized electric field E is coupled to an RFID tag having a longitudinal orientation along the effective length of the antenna arrangement.

The present invention provides a near field RFID antenna device having a linear element microstrip antenna, comprising: a substrate having a first surface, a second surface, and a thickness defined between the first surface and the second surface; a feed point on a first end region of the linear element microstrip antenna; a termination resistor on a second end region of the linear element microstrip antenna, the second end region being opposite the feed point; a linear microstrip disposed on the first surface of the substrate, the linear microstrip being electrically connected to the feed point and the terminating resistor; a ground plane disposed on the second surface of the substrate and partially disposed on the first surface of the substrate, the ground plane not being in contact with the linear microstrip line; the linear microstrip antenna is configured such that: the localized electric field E emitted by the linear element microstrip antenna at the operating wavelength resides substantially within the region defined by the near field; and the localized electric field E is to direct the current distribution along the effective length of the linear element microstrip antenna corresponding to a half-wave to full-wave configuration; the input impedance in ohms at the feed point is substantially equal to the impedance of the terminating resistor.

The invention also provides a method for communicating with an RFID tag, comprising the steps of: providing a near field RFID antenna assembly having a linear element microstrip antenna, said antenna assembly comprising: a substrate, the substrate having: a first surface; a second surface; and a thickness defined between the first surface and the second surface; a feed point on a first end region of the linear element microstrip antenna; a termination resistor on a second end region of the linear element microstrip antenna, the second end region being opposite the feed point; a linear microstrip disposed on the first surface of the substrate, the linear microstrip being electrically connected to the feed point and the terminating resistor; and a ground plane disposed on the second surface of the substrate and partially disposed on the first surface of the substrate, wherein the ground plane is not in contact with the linear microstrip line; disposing the RFID tag within a near field region defined by a distance equal to λ/2 π with respect to the near field antenna arrangement, wherein λ is an operating wavelength of the linear element microstrip antenna; providing an input impedance in ohms at the feed point substantially equal to an impedance of a terminating resistor; using the linear element microstrip antenna, a localized electric field E is generated that resides substantially within a region defined by the near field at the operating wavelength, where the localized electric field E is to direct a current distribution along an effective length of the linear element microstrip antenna corresponding to a half-wave to full-wave configuration.

Drawings

The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. These embodiments, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:

fig. 1 shows a perspective view of a patch radiating antenna device according to the prior art, with an RFID tag at a distance;

FIG. 2 illustrates a perspective view of a linear monopole microstrip antenna assembly having a large RFID tag thereon according to one embodiment of the present invention;

fig. 3 is a plan view of the linear antenna device shown in fig. 2;

[0007] FIG. 4 is a cross-sectional view taken along line 4-4 of FIG. 3;

fig. 5 is a graph of current flow along a linear microstrip tracking antenna of the antenna arrangement of fig. 3 and 4;

fig. 6 is a graph of a half-wave electric field E over the linear antenna arrangement shown in fig. 4;

FIG. 7 is a graph of the distribution of a full wave electric field E at 0 ° phase over the linear antenna arrangement of FIG. 4;

FIG. 8 is a graph of the distribution of a full wave electric field E at 90 ° phase over the linear antenna arrangement of FIG. 4;

FIG. 9 is a plan view of the linear antenna assembly of FIG. 4, wherein the RFID tags are oriented along the length of the linear antenna assembly and are separated by a gap;

figure 10 is a plan view of a linear monopole microstrip antenna assembly having an extended ground plane in accordance with one embodiment of the present invention;

FIG. 11 is a cross-sectional view taken along line 11-11 of FIG. 10;

FIG. 12 is an end view of the antenna assembly of FIG. 10 illustrating the distribution of the electric field;

FIG. 13 is a side view of the antenna assembly of FIG. 10 illustrating the distribution of the electric field;

figure 14 is a plan view of a linear monopole microstrip antenna assembly having a conductive enclosure according to one embodiment of the present invention;

FIG. 15 is a cross-sectional view taken along line 15-15 of FIG. 14;

figure 16 is a perspective view of a meander-line monopole microstrip antenna arrangement according to an embodiment of the invention;

fig. 17 is a plan view of the meander line antenna device of fig. 16;

FIG. 18 is a cross-sectional view taken along line 18-18 of FIG. 17;

FIG. 19 is a plan view of the meander line antenna arrangement of FIG. 17, wherein the RFID tags are oriented along the length of the meander line antenna arrangement and are separated by a gap;

figure 20 is a plan view of a meander-line monopole microstrip antenna arrangement having an extended ground plane according to an embodiment of the present invention;

FIG. 21 is a cross-sectional view taken along line 21-21 of FIG. 20; figure 22 is a plan view of a meander-line monopole microstrip antenna arrangement having a conductive housing according to an embodiment of the present invention; and

fig. 23 is a cross-sectional view taken along line 23-23 of fig. 22.

Detailed Description

The summary of the invention will be understood more fully from the detailed description given below and from the accompanying drawings of specific embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only.

Numerous specific details may be set forth herein in order to provide a thorough understanding of various possible embodiments of the invention. It will be apparent, however, to one skilled in the art that the embodiments may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the embodiments. It can be appreciated that the specific structural and functional details disclosed herein may be representative and do not necessarily limit the scope of the embodiments.

Some embodiments may be described using the terms "coupled" and "connected" along with their derivatives. For example, some embodiments may be described using the term "connected" to indicate that two or more elements are in direct physical or electrical contact. In another example, some embodiments may be described using the term "coupled" to indicate that two or more elements are in direct physical or electrical contact. The term "coupled," however, may also mean that two or more elements are not in direct contact with each other, but yet may still cooperate or interact with each other. Embodiments disclosed herein are not necessarily limited to such context.

It should be appreciated that reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment.

Turning now to the details of the present invention, FIG. 1 shows a patch radiating antenna assembly 10 comprising a patch antenna 12 having RFID tags 20 at spaced apart locations. The patch antenna electric field component oriented along the dipole of RFID tag 20 excites RFID tag 20 and allows the information on RFID tag 20 to be read at a distance d equal to Z1, Z1 being the distance from antenna device 10 where Z1 is much greater than λ/2 π, and λ is the wavelength.

In general, the patch antenna 12 is a radiation antenna designed such that the antenna impedance is substantially real impedance and mainly radiation is made up of radiation impedance. The value of the real impedance substantially matches the source impedance of the feed system, which is typically 50 ohms. The antenna impedance is mainly a real impedance and mainly a radiation resistance. The present invention relates to a near field antenna arrangement that intentionally reduces radiation in the far field and enhances a localized electric field E in the near field region. More specifically, such a near-field antenna apparatus can confine energy in a region close to the antenna, i.e., a near-field region, and prevent radiation in a far-field region. Thus, instead of interrogating RFID tags located outside the near field region, RFID tags in actual proximity to the near field antenna can be interrogated. At 915

At an operating frequency of MHz, the distance between the near field region and the antenna is about 5 cm. Tags outside 5cm cannot be read or written.

Although commonly referred to in the industry as an antenna, as used herein, an antenna assembly is a combination of various components, at least one of which contains an antenna that directly transmits or receives electromagnetic energy or signals.

In one embodiment of the present invention, FIG. 2 shows a near field antenna assembly 110 comprising a linear tracking element microstrip antenna 112 with a large RFID tag 120 in its vicinity. As shown in fig. 3 and 4, the near field antenna assembly 110 comprises a microstrip antenna 112 having a thickness "t" and is electrically coupled at a feed point 116 to a cable 114, the cable 114 typically being a coaxial cable, but not limited to a coaxial cable, and a terminating resistor "R1", typically 50 ohms, connected at an opposite or terminating end 118. The cable 114 has a first or signal terminal 114a and a second or ground reference terminal 114 b. Signals are fed from the feed point terminal 116 of the cable 114 through the feed system 124. This signal is typically a 50 ohm signal.

In one embodiment, a capacitive matching patch 122 (fig. 3) may be electrically coupled to the linear antenna 112 at the 50 ohm termination 118 for impedance matching, typically to minimize reflection losses.

As shown in fig. 3 and 4, the linear microstrip arrangement 110 includes a substantially rectangular microstrip tracking antenna 112 having a substrate 140, wherein the substrate 140 has a first surface 140a and an opposing second surface 140 b. The distance between the first surface 142 and the second surface 144 determines the thickness "H" of the substrate 140.

The microstrip arrangement 110 further comprises a ground plane 150 and has a configuration in which the microstrip line 112 is disposed on the first surface 140a of the substrate 140 and the ground plane 150 is disposed on the second surface 140b of the substrate 140. In one embodiment, the ground plane 150 is separated from the second surface 140b by a dielectric spacer 164, which may be an air gap (suitable structural support not shown) 164. The first terminal 114a of the cable 114 is electrically coupled to the microstrip antenna 112 and the second terminal 114b is electrically coupled to the ground plane 150.

In one embodiment, linear microstrip line 112 is substantially rectangular and has a width "W". The length "L" of the antenna device 110 extends from the feed point 116 to and includes a terminating resistor "R1". For example, linear microstrip line 112 is typically a thin conductor, but is not limited to copper. The thickness "t" is typically in the range of about 10 microns to about 30 microns at frequencies in the UHF range.

The substrate 140 is a dielectric material which may typically comprise a ceramic or FR-4 dielectric material having a thickness "H" and a total width "W_s", a ground plane 150 is provided below the substrate. At the termination point 118 of the linear microstrip 112, a termination resistor R1 circuit couples the termination point 118 of the linear microstrip line 112 to the ground plane 150.

The input impedance "Z" of the linear microstrip antenna 112 at the feed point 116 is designed to be approximately equal to the characteristic impedance of the cable 114 providing the feed signal in order to maximize the power coupled from the reader. (the reader is part of the feed system 124, which is an electronic system separate from the cable 114 or transmission network the antenna device 110 is coupled to the reader system via the cable 114.) the ratio W/H is typically greater than or equal to 1, with a specific range being approximately from 1 to 5.

In this case, the input impedance "Z" of the linear microstrip antenna device 110 expressed in ohms is given by the following equation:

formula (1)

Formula (2)

“ε_r"is the relative dielectric constant of the substrate 140. Thus, the width W of the microstrip and the height H of the substrate primarily determine the impedance "Z".

In one embodiment, the relative dielectric constant "ε of the substrate_r"generally ranges from 2 to 12. In another embodiment, the length "L" of the linear microstrip near field antenna apparatus 110 corresponds to the effective length of a half-wave to a full-wave, with the equivalent actual length being approximated by the formulaDetermine where "c" is the speed of light (approximately 3X 10)⁸m/s), "f" is the operating frequency in Hz, and "ε_r"is the relative dielectric constant of the substrate, and" n "ranges approximately from 0.5 for a comparable half-wave dipole antenna to 1.0 for a comparable full-wave dipole antenna.

In one embodiment, the terminating resistor "R1" may be adjusted such that its input impedance at the feed point 116 is approximately equal to 50 ohms or the characteristic impedance of the feed cable 114.

In another embodiment, the linear microstrip antenna 112 has a first longitudinal edge 112a and a second longitudinal edge 112B, and the microstrip antenna 112 is substantially centered on the substrate 140 and the ground plane 150, such that the longitudinal edges 142a and 142B of the substrate 140 and the longitudinal edges 152a and 152B of the ground plane 150 each extend a distance that is at least 2 times ("2W") the width "W" of the first and second longitudinal edges 112a and 112B. Therefore, the total width "W" of each of the substrate 140 and the ground plane 150_s"at least 5 times the width" W "(" 5W "). The substrate 140 also includes a lateral edge 142c at which the feed point 116 is disposed and a lateral edge 142d at which the terminating resistor R1 is disposed. Similarly, the ground plane 150 also includes a lateral edge 152c at which the feed point 116 is disposed and a lateral edge 152d at which the terminating resistor "R1" is disposed.

The near field antenna apparatus 110 intentionally reduces the far field and enhances the near field region. More specifically, the near field RFID antenna assembly 110 includes a unit antenna 112 configured such that the localized electric field E emitted by the antenna 112 resides substantially within an area defined by the near field, while the radiated field emitted by the antenna 112 resides substantially within an area defined by the far field relative to the antenna 112. Therefore, the near field antenna apparatus 110 has many advantages for adjustment. The real impedance of such a near field antenna arrangement 110 without a 50 ohm termination impedance is very low. Therefore, the radiation resistance is also low. A typical 50 ohm termination impedance R1 is added which makes the input impedance approximately equal to 50 ohms for matching the feed system 124 supplying power through the cable 114. This configuration and method of operation also results in a very low antenna "Q" factor, making the antenna a wideband antenna.

Ideally, microstrip antenna 112 is a half-wavelength "λ/2" antenna whose current profile is along the length of tracking microstrip antenna 112, as shown in FIG. 5.

At the feed point 116, the current peaks and is substantially in phase with the voltage applied by the feed system 124. The current decreases to zero at the midpoint of the microstrip antenna 112 and then continues to decrease and decreases to a negative peak at the terminating end 118.

Such a current-distributing linear microstrip antenna arrangement 110 operating in a half-wave dipole configuration may establish a positive electric field E at the feeding end 116 and a negative electric field E at the terminating end 118, as shown in fig. 5.

Fig. 6 shows the coupling of the near field electric field E over the near field microstrip antenna 112. More specifically, fig. 6 is a temporal plot of the normalized time varying electric field E over the microstrip antenna 112 for the half wavelength case. At the feed point 116, the electric field E has a maximum value. At the midpoint of the microstrip antenna 112, the electric field E decreases to zero. At the terminating end 118, the electric field E decreases to a negative peak or minimum. Because the RFID tag 120 is placed directly above such an antenna (see fig. 2), the differential electric field E of the microstrip antenna 112 is a current driven or directed along the length of the RFID tag 120 to energize the RFID tag 120 so that it can be read or written by an RFID reader, i.e., the near field antenna arrangement 112.

Accordingly, an RFID tag 120 placed above the microstrip antenna 112 and oriented along the length "L" of the microstrip antenna arrangement 110 transmits information to the microstrip antenna 112. It should be noted that depending on the material of the substrate 140, the substrate 140 effectively creates a slow wave structure, resulting in a total antenna length "L", which isWhere "c" is the speed of light in vacuum, "f" is the operating frequency, and "ε_r"is the relative permittivity or relative dielectric constant of the substrate material in a half-wave dipole antenna configuration. Therefore, when the relative permittivity or relative dielectric constant ε of the substrate 140_rAs the length "L" of the overall antenna arrangement is increased, the antenna arrangement is reduced and can therefore be used for smaller RFID tags. For example, a total microstrip length of 4.7cm, theoretical, can be achieved experimentally with a ceramic substrate having a dielectric constant of 12.5The length is 4.6 cm. Smaller antenna devices may be used to read or detect RFID tags of smaller items.

In one embodiment, the linear microstrip antenna arrangement 110 extends to a length corresponding to a full wave. Fig. 7 and 8 show the linear microstrip antenna arrangement 110 at some instant of time varying electric field E over the full-wave microstrip antenna arrangement, e.g., at 0 and 90 degrees phase, respectively.

At the feeding point 116, two momentary specific snap-shots of the differential electric field E can be observed when the feeding signal provided via the cable 114 is transmitted through a full 360 degree phase. There are two pairs of differential electric fields E at zero phase and only one pair at 90 phase. The actual differential electric field E coupled onto the RFID tag 120 is swept along the length "L" of the linear microstrip antenna 112. This is advantageous for alignment between the linear microstrip antenna 112 and the RFID tag 120. Increasing the dielectric strength (or relative permittivity ∈ of the substrate 140 material)_r") may at least partially compensate for the need to increase the overall antenna length" L ".

Referring to fig. 9, a series of RFID tags 120a through 120e are placed at a gap distance "d" apart, with one RFID tag 120c being above a single linear microstrip antenna arrangement 110. The RFID tags 120a through 120e have an orientation wherein the longitudinal orientation of the antenna dipoles of the RFID tags 120a through 120e is along the length "L" of the linear microstrip antenna arrangement 110.

To prevent the near field linear microstrip antenna arrangement 110 from reading or writing a tag 120b or 120d near the addressed tag 120c, the microstrip width "W", the length "L", and the total substrate width "W" may be adjusted accordingly_s". As the gap "d" between the RFID tags 120 a-120 e decreases, the microstrip width "W" and the total substrate width "W" of about "5W" must be reduced_s". The gap "d may be sized such that adjacent tags 120a, 120b, 120d, 120e extend far beyond the lateral edges 142a and 142b of the substrate 140 of the linear microstrip antenna 112, and therefore, the microstrip antenna arrangement 110 cannot detect the presence of adjacent RFID tags 120a, 120b, 120d, 120 e. Can be used forTo adjust the tracking width W, the length L and the parameters W/H and epsilon of the substrate_rThereby effectively realizing a current distribution corresponding to a half-wave to full-wave structure.

In one embodiment shown in fig. 10 and 11, the linear microstrip antenna arrangement 110' includes an extended or wrapped ground plane. More specifically, the linear microstrip antenna arrangement 110 'is the same as the linear microstrip 110 except that instead of the ground plane 150, the microstrip line 112 is disposed on the first surface 140a of the substrate 140 and the ground plane 150' is disposed on at least a part of the first surface 140a of the substrate 140, but it is not in contact with the microstrip line 112. The ground planes 150' are also disposed on the first and second edges 142a and 142b, respectively, of the substrate 140 and on the second surface 140b of the substrate 140. The ground plane 150' may also be separated from the second surface 140b by a dielectric spacer 164.

The ground plane 150' may also include tabs or ends 180a and 180b that overlap the first surface 140a and extend inward a distance "W", respectively_G"reach edges 112a and 112b, but do not contact tracking microstrip 112.

As shown in fig. 11, RFID tags 120a through 120e may be positioned immediately above antenna assembly 110 ', so that when one tag 120c rests above linear tracking microstrip antenna 112, the adjacent tags 120b and 120d rest substantially above flaps or ends 180a and 180b of ground plane 150'. As shown in fig. 12, the antenna assembly 110 'controls the position of the rf energy by propagating near field energy and the ground plane 150' wrapping around the sheet or ends 180a and 180b, which extend inward a distance W_GReaching edges 112a and 112b, respectively, but not contacting tracking microstrip antenna 112. Therefore, the electric field E extends substantially only from the tracking microstrip antenna 112 to the patches or ends 180a and 180b, effectively terminating the electric field E and preventing the antenna arrangement 110' from coupling to adjacent tags 120b and 120 d.

Fig. 13 illustrates a temporal coupling diagram of the time-varying near-field electric field E over the near-field microstrip antenna 112 of the antenna arrangement 110 ', as viewed from one side, e.g., side 152b of the ground plane 150 ' of the antenna arrangement 110 '. More specifically, fig. 13 is a graph of the normalized electric field E in the case of a half-wave length. In a similar manner to fig. 6, the electric field E has a maximum at the feeding point 116. At the midpoint of the microstrip antenna 112 along the length "L", the electric field E decreases to zero. At the termination point 118, the electric field E decreases to a negative peak or maximum.

When the RFID tag 120 is placed directly over the antenna assembly 110', as shown in fig. 12, the differential electric field E of the microstrip antenna 112 drives or directs a current along the length of the RFID tag antenna 112, thereby energizing the RFID tag 120 so that it can be read or written by an RFID reader, i.e., the near field antenna assembly 112. Accordingly, an RFID tag 120c positioned above the microstrip antenna 112 and oriented along the length L of the microstrip antenna arrangement 110' is also coupled to the microstrip antenna 112. Similarly, the tracking width W, length L, and substrate parameters W/H and ε may be adjusted_rThereby effectively achieving an effective current distribution corresponding to a half-wave to full-wave structure.

In one embodiment, referring to fig. 14 and 15, the linear microstrip antenna arrangement 110 (or 110') may be mounted within or on the conductive housing 160. The conductive housing 160 includes a base 162, and two longitudinal side walls 162a and 162b, and two lateral side walls 162c and 162d connected generally orthogonally thereto. The bottom surface 154 of the ground plane 150 is disposed on a base 162 for electrically coupling the conductive housing 160 to the ground plane 150. Therefore, the conductive housing 160 is grounded through the ground plane 150.

The sidewalls 162 a-162 d may be spaced apart from the edges 142 a-142 d of the substrate 140. The edges 142 a-142 d may contact the conductive shell 160, however, it may require a space capacity for the antenna device 110 (or 110') to fit into the conductive shell 160. The sidewalls 162 a-162 d may also be separated from the linear microstrip antenna 112 by a dielectric spacer material 170 so that the conductive enclosure 160 is electrically isolated from the linear microstrip antenna 112, the capacitive load 122 and the terminating resistor R1. The dielectric spacer material may include an air gap. The material of the conductive housing 160 may include aluminum, copper, brass, stainless steel, or similar metallic substances. It is envisioned that adding an extended-sided conductive housing 160, which is implemented with sidewalls 162 a-162 d adjacent to sides 142 a-142 d of substrate 140 in microstrip antenna arrangement 110, may further reduce unwanted coupling of adjacent RFID tags 120 to linear microstrip antenna arrangement 110.

In one embodiment of the present invention shown in fig. 16-18, the meander-line element microstrip antenna arrangement 210 may allow the apparent antenna length "L" to be longer for a given overall antenna size, e.g., for reading a very small RFID tag. The meander-line antenna arrangement 210 is similar in many respects to the linear element microstrip antenna arrangement 110 and, therefore, is described herein to identify differences in their structure and operation.

More specifically, fig. 16-18 illustrate a near field antenna assembly 210 that includes a meander line element microstrip antenna 212. The meander-line antenna 212 is at the width "W" of the substrate 140 as it proceeds from the feed point 116 along the length "L" to the terminating resistor R1 at the terminating end 118_s"upper" meanders. The meander-line microstrip tracking antenna 212 is "t" in thickness and is electrically coupled to the cable 114 at the feed point 116 and terminates at a terminating resistor R1 at a terminating end 118, the resistor R1 typically having a resistance value of 50 ohms.

The meander-line microstrip antenna 212 is different from the linear microstrip antenna 112 in that the meander-line microstrip antenna 212 guides current in two-dimensional directions. More specifically, in one embodiment, the meander-line microstrip device 210 comprises a plurality of alternating orthogonal contact conductive segments 214 and 216 configured as a square wave pattern of a meander-line microstrip tracking antenna 212. Conductive segment 214 is of length "L_MAre "linearly aligned with" and substantially parallel to at least one of the longitudinal side edges 142a and 142b of the base sheet 140. Conductive segment 216 is in lateral alignment with conductive segment 214 and in contact with conductive segment 214 in a straight alignment to form a square wave pattern. Each conductive segment 216 is oriented along a centerline axis C-C that is along length L of the conductive segment_sExtend and bisect the width thereof. The contacted conductive segments 214 and 216 may integrally form a single microstrip tracking antenna. The meander line antenna 212 may form other patterns that are not identical to square wave patterns in which the alternately contacting conductive segments 214 and 216 are not orthogonal. The embodiments are not limited in this context. The configuration of conductive segments 214 and 216 enables localized electric field E to drive or direct current in two dimensions.

At least one edge 142a, 142b of the substrate 140 has a length "L_M", and conductive segments 214 and 216 of the orthogonal contacts are disposed in alternating transverse and longitudinal orientations with respect to the at least one edge 142a, 142 b.

As shown in FIG. 17, conductive segments 214 are arranged in a longitudinal orientation that together define an overall length "L" of meander-line shaped microstrip tracking antenna 212_M", the tracking antenna 212 is a terminating resistor R1 that extends from the feed point to and is included at the termination point 118. Width "W" of meander-line-like tracking antenna 212_M"is defined as the width of one conductive segment 214 oriented in the longitudinal direction.

Similar to the linear microstrip antenna arrangement 110, the length "L" of the meander-line microstrip antenna arrangement 210_M"has an overall dimension ranging from a length substantially equal to an equivalent half-wave dipole antenna to an equivalent full-wave dipole antenna. The resulting electric field (electric field E) distribution is the same as that of the linear antenna device 110 shown in fig. 6 to 8.

In one embodiment, "W" of meander-line microstrip antenna arrangement 210_Mthe/H "ratio can be greater than or equal to 1, and specifically can range generally from about 1 to about 5. The relative dielectric constant of the substrate 140 may generally be in the range of about 2 to about 12. At least one edge 142a, 142b of substrate 140 may be configured such that it extends laterally from longitudinally oriented conductive segment 214 a distance substantially equal to or greater than width "W" of meander-line microstrip tracking antenna 212_M"double" 2W_M"or a plurality of times. In another embodiment, at least one edge 152a, 1 of the ground plane 15052b extend laterally from longitudinally disposed conductive segment 214 a distance equal to or greater than width "W" of meander-line tracking antenna 212_M". It is also envisioned that the meander-line antenna arrangement 210 may include a capacitive load 122 electrically coupled to the meander-line microstrip tracking antenna 212, which is typically adjacent to a terminating resistor R1.

As shown in fig. 17-19, (and described in a manner similar to the linear antenna arrangement 110 shown in fig. 9), a series of RFID tags 120 a-120 e are separated by a gap distance (d), with one RFID tag 120c positioned above a single meander-line microstrip antenna arrangement 210. The meander-line microstrip antenna arrangement 210 is configured such that the localized electric field E of the meander-line antenna 212 is coupled to an RFID tag 120, which is longitudinally oriented along the length of the meander-line microstrip antenna arrangement 210. The localized electric field E is a current driven or directed in the two-dimensional direction of the antenna 212.

To prevent the near field meander line shaped microstrip antenna arrangement 210 from reading or writing a tag 120b or 120d adjacent to the addressed tag 120c, the microstrip width "W" may be adjusted accordingly_M", length" L_M"and Total substrate Width" W_s". The microstrip width "W" as the gap "d" between the RFID tags 120 a-120 e decreases_M"and Total substrate Width" W_s"also decreases. The gap "d" is sized such that adjacent tags 120a, 120b, 120d, 120e are positioned beyond the lateral sides 142a and 142b of the meander-line microstrip antenna 212, and therefore, the microstrip antenna arrangement 210 cannot detect the presence of adjacent tags 120a, 120b, 120d, and 120 e. In the case of a meander-line microstrip antenna, the tracking width W is adjusted_MTotal effective length L_MAnd substrate parameters, effective current distribution corresponding to half-wave to full-wave structures can be achieved. This may be at a given fixed length L_MBy increasing the period L 'of the meander-line shaped tracking antenna'_MNumber of implementations.

In one embodiment, such as the embodiment shown in fig. 20 and 21, the meander-line microstrip antenna arrangement 210' comprises an extended or wrapped ground plane. More specifically, the meander-line-shaped microstrip antenna arrangement 210 'is identical to the meander-line-shaped microstrip antenna 210, except that, instead of the ground plane 150, a microstrip line 212 is provided on the first surface 140a of the substrate 140, and the ground plane 150' is provided on at least a portion of the first surface 140a of the substrate 140, but without contacting the microstrip line 212. In a manner similar to the linear microstrip antenna 110 ', a ground plane 150' is disposed on the first and second edges 142a, 142b of the substrate 140 and on the second surface 140b of the substrate 140. The ground plane 150' may be separated from the substrate by one or more dielectric spacers 164.

The ground plane 150' may include tabs or ends 180a and 180b that overlap the first surface 140a and extend inward a distance "W_G"then reach edges 212a and 212b, respectively, but do not make contact with tracking microstrip antenna 212.

As shown in fig. 21, RFID tags 120a through 120e may be positioned immediately above antenna assembly 210 ', so that when one tag 120c rests above meander-line tracking microstrip antenna 212, the adjacent tags 120b and 120d rest substantially above the flaps or ends 180a and 180b, respectively, of ground plane 150'.

Furthermore, as shown in fig. 22 and 23, and in a manner similar to the embodiments shown in fig. 14 and 15, the ground plane 150 of the meander-line microstrip antenna arrangement 210 (or 210') may be electrically coupled to the conductive enclosure 160. The sidewalls 162 a-162 d may be spaced apart from the edges 142 a-142 d of the substrate 140. The edges 142 a-142 d may contact the conductive shell 160, but may require a space capacity for the antenna device 110 (or 110') to fit into the conductive shell 160. The sidewalls 162 a-162 d may also be separated from the meanderline microstrip antenna 212 by the dielectric spacer material 170 so that the conductive enclosure 160 is electrically isolated from the meanderline microstrip antenna 212, the capacitive load 122 and the terminating resistor R1. The material of the conductive housing 160 may include aluminum, copper, brass, stainless steel, or similar metallic substances.

As discussed above, the tracking width W is adjusted_MTotal effective length L_MAnd the parameters of the substrate, an effective current distribution corresponding to a half-wave to full-wave structure can be achieved. This may be at a given fixed length L_MBy increasing the period L 'of the meander-line shaped tracking antenna'_MNumber of implementations.

We have disclosed the above embodiments of a near field antenna arrangement 110, 110 ', 210, 210' which is powered by a unit configuration of a cable 114 and a terminating resistor R1. The skilled person will appreciate that the near field antenna arrangement 110, 110 ', 210, 210' may also be powered by a dipole arrangement comprising a transformer. The embodiments are not limited in this context.

In view of the above description, embodiments of the present invention are directed to a near field antenna arrangement 110, 110 ', 210, 210' for reading RFID tags, wherein the antenna arrangement 110, 110 ', 210, 210' has a configuration such that the localized electric field E emitted by the antenna arrangement 110, 110 ', 210, 210' at the operating wavelength "λ" resides substantially within an area defined by the near field, while the radiated field emitted by the antenna arrangement 110, 110 ', 210, 210' at the operating wavelength "λ" resides substantially within an area defined by the far field relative to the antenna arrangement 110, 110 ', 210, 210'.

The various embodiments disclosed herein are designed such that the magnitude of the localized electric field E can be increased relative to the magnitude of the radiated field, and can be read by the antenna or antenna arrangement 110, 110 ', 210, 210' only when the RFID tag 120c is in the near field region (and cannot be read by the antenna arrangement 110, 110 ', 210, 210' when the RFID tag 120c is in the far field region). Furthermore, the magnitude of the radiation field may be reduced relative to the magnitude of the localized electric field E, and thus, may be read by the antenna or antenna arrangement 110, 110 ', 210, 210' only when the RFID tag 120c is in the near field region (and may not be read by the antenna arrangement 110, 110 ', 210, 210' when the RFID tag 120c is in the far field region). The relative dielectric constant of the antenna arrangement 110, 110 ', 210, 210' is ∈_r”。

The antenna or antenna arrangement 110, 110 ', 210, 210' is configured such that the near field region is defined by a distance from the antenna or antenna arrangement 110, 110 ', 210, 210' equal to "λ/2 π", where "λ" is the operating wavelength of the antenna or antenna arrangement 110, 110 '210, 210'. In one embodiment, the antenna or antenna arrangement 110, 110 ', 210, 210' operates at approximately 915MHz, and thus, the near field region distance is approximately 5 cm.

We also disclose a method for reading or writing to an RFID tag 120c, comprising the steps of: providing a near field antenna arrangement 110, 110 ', 210, 210' having a configuration such that a localized electric field E emitted by the antenna or antenna arrangement 110, 110 ', 210, 210' at an operating wavelength "λ" substantially resides within an area defined by the near field, and a radiation field emitted by the antenna or antenna arrangement 110, 110 ', 210, 210' at the operating wavelength "λ" substantially resides within an area defined by the far field relative to the antenna arrangement 110, 110 ', 210, 210'; and coupling the localized electric field E of the near field antenna apparatus 110, 110 ', 210, 210' to the RFID tag 120c disposed within the near field region.

The effective length L or L of the antenna arrangement 110, 110', 210, 210_MIt may be that the distribution of current directed through the antenna produces a waveform with a wavelength proportional to nv/f, where v is the propagation velocity of the wave, which is equal to the speed of light divided by the inverse of the relative dielectric constant of the antenna arrangement 110, 110 ', 210, 210', f is the frequency in Hz, and n ranges from about 0.5 for half wavelengths to about 1.0 for full wavelengths.

The method may further comprise the steps of: the magnitude of the localized electric field E relative to the magnitude of the radiated field is increased so that it can be read by the antenna arrangement 110, 110 ', 210, 210' only when the RFID tag 120c is in the near field region, and cannot be read by the antenna arrangement 110, 110 ', 210, 210' when the RFID tag 120c is in the far field region.

The method may further comprise the steps of: the amplitude of the radiation field is reduced with respect to the amplitude of the localised electric field E, and therefore onlyWhen the RFID tag 120c is in the near field region, it can be read by the antenna assembly 110, 110 ', 210, 210', and when the RFID tag 120c is in the far field region, it cannot be read by the antenna assembly 110, 110 ', 210, 210'. The method may further comprise the steps of: the antenna arrangement 110, 110 ', 210, 210' is configured such that the near field region is defined by a distance from the antenna arrangement 110, 110 ', 210, 210' equal to "λ/2 π", where "λ" is the operating wavelength of the antenna. The method may further comprise the steps of: the near field antenna operates at a frequency of about 915MHz, and therefore the near field region distance is about 5 cm. The effective length L or L of the antenna arrangement 110, 110', 210, 210_MIt may be that the distribution of current directed through the antenna produces a waveform with a wavelength proportional to nv/f, where v is the propagation velocity of the wave, which is equal to the speed of light divided by the inverse of the relative dielectric constant of the antenna arrangement 110, 110 ', 210, 210', f is the frequency in Hz, and n ranges from about 0.5 for half wavelengths to about 1.0 for full wavelengths.

It is envisioned that advantageous features of the near field antenna apparatus disclosed herein include:

(1) the range of reading/writing an RFID tag is limited by the near field distance d < lambda/2 pi;

(2) the largest portion of the electric field energy of the near field antenna 112 or 212 is lost to the terminating load resistor R1;

(3) the near field antenna arrangement has a low Q factor compared to the radiating far field antenna arrangement;

(4) the wide operating bandwidth derived from the low Q factor can be used for worldwide broadband UHF applications;

(5) the wide operating bandwidth and low Q factor may eliminate the need for simplified RFID reader circuitry for frequency hopping to prevent reader interference;

(6) the near-field antenna device has low radiation resistance and radiation efficiency compared to a radiating antenna device. Therefore, far-field radiation can be greatly reduced;

(7) the near field antenna apparatus is configured with a microstrip antenna of tracking size, substrate properties, and ground plane, which is designed to operate in a range from a half-wave antenna to a full-wave antenna;

(8) the element feed configuration provides a simple and more efficient feed configuration in which the electrical input or cable is connected directly to the beginning of the microstrip antenna and the ground of the connector is connected directly to the ground plane at the bottom of the substrate, as compared to other differential feed configurations that may require a transformer;

(9) the conductive housing has an open side for placement of the near field antenna assembly, the conductive housing being grounded to a ground plane of the antenna assembly. The conductive housing helps reduce stray electric fields attempting to couple to adjacent RFID tags that are adjacent to RFID tags disposed directly above the microstrip antenna;

(10) localization of the emitted electric field in the near field region facilitates compliance with regulatory requirements.

In view of the above, embodiments of the present invention allow RFID tags that are in close proximity to each other to be programmed. For example, RFID tags on a roll are characterized by a small separation distance between each tag. Embodiments of the present invention do not require the tags to be separated by a large distance and may prevent multiple tags from being read or programmed together. In addition, embodiments of the present invention facilitate identifying defective tags that are adjacent to normal functional tags.

Although the foregoing description contains many specifics, these should not be construed as limitations on the scope of the invention, but merely as exemplifications of preferred embodiments thereof. Many other possibilities are conceivable to the expert within the spirit and scope of the invention.

Claims (9)

1. A near field RFID antenna arrangement having a linear element microstrip antenna, the near field RFID antenna arrangement comprising:

a substrate having a first surface, a second surface, and a thickness defined between the first surface and the second surface;

a feed point on a first end region of the linear element microstrip antenna;

a termination resistor on a second end region of the linear element microstrip antenna, the second end region being opposite the feed point;

a linear microstrip disposed on the first surface of the substrate, the linear microstrip being electrically connected to the feed point and the terminating resistor;

a ground plane disposed on the second surface of the substrate and partially disposed on the first surface of the substrate, the ground plane not being in contact with the linear microstrip line;

the linear microstrip antenna is configured such that: the localized electric field E emitted by the linear element microstrip antenna at the operating wavelength resides substantially within the region defined by the near field; and the localized electric field E is to direct the current distribution along the effective length of the linear element microstrip antenna corresponding to a half-wave to full-wave configuration; the input impedance in ohms at the feed point is substantially equal to the impedance of the terminating resistor.

2. The near field RFID antenna assembly of claim 1 wherein the linear microstrip antenna is configured such that the near field region is defined by a distance from the antenna equal to λ/2 π, where λ is the operating wavelength of the antenna.

3. The near field RFID antenna assembly of claim 2 wherein said linear microstrip antenna is operated at a frequency of about 915MHz such that the near field region distance is about 5 cm.

4. The near field RFID antenna assembly of claim 1 wherein:

the linear element microstrip antenna is configured as a single and continuous conductor;

the resistor is connected to the ground plane and the antenna device has a relative dielectric constant.

5. The near field RFID antenna assembly of claim 4 wherein the effective length of the linear element microstrip antenna is such that the current distribution directed through the antenna produces a waveform having a wavelength proportional to nv/f, where v is the propagating wave speed equal to the speed of light divided by the square root of the relative dielectric constant of the antenna assembly, f is the frequency in Hz, and n ranges from 0.5 for half the wavelength to 1.0 for the full wavelength.

6. The near field RFID antenna assembly of claim 4 wherein the linear element microstrip antenna is a linear element microstrip tracking antenna.

7. The near field RFID antenna assembly of claim 4 wherein the localized electric field E is coupled to an RFID tag that is oriented longitudinally along the effective length of the linear element microstrip antenna.

8. A method of communicating with an RFID tag, comprising the steps of:

providing a near field RFID antenna assembly having a linear element microstrip antenna, said antenna assembly comprising:

a substrate, the substrate having:

a first surface;

a second surface; and

a thickness defined between the first surface and the second surface;

a feed point on a first end region of the linear element microstrip antenna;

a termination resistor on a second end region of the linear element microstrip antenna, the second end region being opposite the feed point;

a linear microstrip disposed on the first surface of the substrate, the linear microstrip being electrically connected to the feed point and the terminating resistor; and

a ground plane disposed on the second surface of the substrate and partially disposed on the first surface of the substrate, wherein the ground plane is not in contact with the linear microstrip line;

disposing the RFID tag within a near field region defined by a distance equal to λ/2 π with respect to the near field antenna arrangement, wherein λ is an operating wavelength of the linear element microstrip antenna;

providing an input impedance in ohms at the feed point substantially equal to an impedance of a terminating resistor;

using the linear element microstrip antenna, a localized electric field E is generated that resides substantially within a region defined by the near field at the operating wavelength, where the localized electric field E is to direct a current distribution along an effective length of the linear element microstrip antenna corresponding to a half-wave to full-wave configuration.

9. The method of claim 8, further comprising the steps of: the antenna arrangement is operated at a frequency of about 915MHz, so that the near field region distance is about 5 cm.