CN101076644B

CN101076644B - Radio-frequency identification near field linear microband antenna

Info

Publication number: CN101076644B
Application number: CN2005800427114A
Authority: CN
Inventors: 理查德·L.·科佩兰德; 加里·M.·沙弗尔
Original assignee: Sensormatic Electronics Corp
Current assignee: Adt Services LLC; Tyco Fire and Security GmbH; Sensormatic Electronics LLC
Priority date: 2004-11-02
Filing date: 2005-11-02
Publication date: 2012-03-21
Anticipated expiration: 2025-11-02
Also published as: CN101103165B; CN101084357A; CN101052775B; CN101052775A; CN101076644A; CN101103165A; CN101084602A; CN101084602B

Abstract

A near field linear element microstrip antenna is disclosed which is configured to read an RFID label such that a localized electric E field emitted by the antenna at an operating wavelength resides substantially within a zone defined by the near field. The localized E field directs a current distribution along an effective length of the antenna corresponding to a half-wave to a full-wave structure.

Description

Radio frequency identification near-field linear microstrip antenna

Cross Reference to Related Applications

The benefit of priority from U.S. provisional patent application No. 60/624,402, entitled "NEAR FIELD PROBE FOR READING RFID TAGS AND LABELSAT CLOSE RANGE," filed on 2.11.2004 by Shafer et al, and U.S. provisional patent application No. 60/659,289, entitled "LINEARONOPOLE MICROROSTRIP RFID NEAR FIELD ANTENNA," filed on 7.3.2005 by Copeland et al, are claimed by this application and are incorporated herein by reference in their entirety.

Background

Existing means of reading RFID (radio frequency identification) tags apply conventional antennas that provide a large read range for RFID tags. This approach provides a large portion of the antenna energy for use in 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 for UHF (ultra high frequency) frequencies of 915 MHz. Thus, the far field region at 915MHz is substantially over 5cm, and similarly, the near field region is substantially within 5 cm. Most RFID reader antennas are designed to read tags, for example, a few meters furthest away, and of course this distance is well within the far field region.

In some applications, i.e., RFID tag applicators and programmers, it is desirable to read only one RFID tag within a group of tags that are in close proximity to each other. For example, on a label applicator machine, labels are packaged on a spool for processing on the machine. On the reel, the labels abut side by side or end to end. However, since conventional UHF antennas generally have a wide radiation pattern and properly direct energy to the far field, it is difficult for conventional UHF antennas to direct energy to only one tag at a time. The wide radiation pattern illuminates all RFID tags within range of the antenna. If an attempt is made to write a product code or serial number to one of the labels, all of the illuminated labels are programmed with the same code or serial number.

The conventional far-field radiating antenna used in such RFID UHF applications is a patch antenna. Typically, the radiating patch is fed through a connector that is energized by the RFID electronics. Typically, the conductive plate is mounted on the back side and spaced a small distance from the die.

Conventional far field antennas do not perform well for applications such as those described above where a tag applicator that needs to program, test, and apply one tag at a time wishes to read information from or write information to an RFID tag at a very close distance. Conventional radiating antennas require marked items to be separated by a substantial distance to prevent multiple items from being read or programmed simultaneously, or require a metal window to shield all tags except those being programmed or read.

However, this technique does not completely solve the problem because if the labels are spaced further apart, the applicator throughput is reduced and the number of labels in a given spool size is limited. If shielding techniques are used, it is desirable to use different shielding for each different label shape and spacing. Therefore, various changes are required to handle different labels on the application line, thereby also significantly reducing throughput.

Disclosure of Invention

The present disclosure relates to a near field RFID antenna assembly comprising a substantially linear element microstrip antenna configured such that a localized E-field emitted by the antenna is substantially within a region defined by the near field. The localized E-field directs the current distribution along the effective length of the antenna corresponding to a half-wave to full-wave structure.

The substantially linear microstrip antenna may include: a substantially rectangular microstrip; a substrate having a first surface and a second surface and a thickness defined therebetween; and a ground plane. The microstrip may be disposed on a first surface of the substrate and the ground plane may be disposed on a second surface of the substrate. The antenna assembly may include a feed point at one end of the linear microstrip and a terminating resistor at the other end of the linear microstrip, the resistor being electrically coupled to the ground plane.

In one embodiment, the linear microstrip has a width W and the substrate has a thickness H such that an input impedance Z of the antenna assembly, in ohms, is substantially equal to the following equation (1):

wherein,

and epsilon_rIs the relative dielectric constant of the substrate.

The ratio of W/H may be greater than or equal to 1. The substrate and the ground plane may each have a width at least five times the width W (5W). The linear microstrip may have first and second longitudinal edges, and the microstrip may be substantially centered on the substrate such that each of the edges of the substrate and the edges of the ground plane extend from the first and second longitudinal edges a distance at least twice the width W (2W). Relative dielectric constant ε of substrate_rAnd may be from about 2 to about 12.

The linear microstrip may have a length L extending from the feed point to the termination resistance and including the termination resistance, the length L being given by the following equation (2):

where c is the speed of light in m/s (about 3X 10)⁸m/s), f is the operating frequency in Hz, ε_reIs that

And n is from about 0.5 for an equivalent half-wave dipole antenna to about 1.0 for an equivalent full-wave dipole antenna.

The input impedance of the antenna at the feed point may be approximately equal to the characteristic impedance of a cable supplying the feed signal at the feed point. The linear microstrip trace may have a thickness from about 10 microns to about 30 microns.

In one embodiment, the substrate has first and second edges along a length of the substrate, and the ground plane is disposed on at least a portion of the first surface of the substrate and is free of contact with the microstrip. The ground plane is disposed on the first and second edges of the substrate and on the second surface of the substrate.

In one embodiment, the ground plane of the antenna assembly is electrically coupled with the conductive housing. The conductive housing may be separated from the microstrip antenna by at least one dielectric spacer. The dielectric spacer may comprise an air gap.

The antenna assembly is configured such that the localized E-electric field of the antenna assembly couples with an RFID tag oriented longitudinally along the length of the antenna assembly.

Drawings

The subject matter regarded as the embodiments is particularly pointed out and distinctly claimed in the concluding portion of the specification. However, the embodiments of the invention, 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 illustrates a perspective view of a patch radiating antenna assembly spaced a distance from an RFID tag, according to the prior art;

figure 2 illustrates a top perspective view of one embodiment of a linear monopole microstrip antenna assembly according to the present disclosure with a large RFID tag present above;

FIG. 3 is a plan view of the linear antenna assembly of FIG. 2;

FIG. 4 is a cross-sectional elevation view taken along line 4-4 of FIG. 3;

FIG. 5 is a graphical representation of current flow along a linear microstrip antenna trace of the antenna assembly of FIGS. 3 and 4;

FIG. 6 is a graphical representation of a half-wave electric field (E-field) distribution across the linear antenna assembly of FIG. 4;

FIG. 7 is a graphical representation of a full wave E-field distribution over the 0 phase linear antenna assembly of FIG. 4;

FIG. 8 is a graphical representation of a full wave E-field distribution over the 90 phase linear antenna assembly of FIG. 4;

FIG. 9 is a plan view of the linear antenna assembly of FIG. 4 and an RFID tag oriented along the length of the linear antenna assembly and separated by a void;

figure 10 is a plan view of one embodiment of a linear monopole microstrip antenna assembly having an extended ground plane according to the present disclosure;

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

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

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

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

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

figure 16 is a top perspective view of one embodiment of a meanderline monopole microstrip antenna assembly according to the present disclosure;

FIG. 17 is a top plan view of the meander line antenna assembly of FIG. 16;

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

FIG. 19 is a plan view of the meander antenna assembly of FIG. 17 and an RFID tag oriented along the length of the meander antenna assembly and separated by a void;

figure 20 is a plan view of one embodiment of a meanderline monopole microstrip antenna assembly with an extended ground plane according to the present disclosure;

FIG. 21 is a cross-sectional end elevational view taken along line 21-21 of FIG. 20;

figure 22 is a plan view of one embodiment of a meanderline monopole microstrip antenna assembly having a conductive housing according to the present disclosure; and

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

Detailed Description

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

Numerous specific details are set forth in order to provide a thorough understanding of the many possible embodiments of the disclosure. However, it will be understood by those of ordinary skill in the art that the embodiments may be practiced without these specific details and that 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 with each other. 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. However, the term "coupled" may also mean that two or more elements are not in direct contact with each other, but yet still co-operate or interact with each other. Embodiments disclosed herein are not necessarily limited in this regard.

It is noted that any reference in the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included as valuable 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 disclosure. Fig. 1 shows a patch radiating antenna assembly 10 including a patch antenna 12, with RFID tags 20 depicted at a distance apart. The patch antenna E field component along the dipole orientation of RFID tag 20 excites RFID tag 20 and allows the information on RFID tag 20 to be read at a distance d from antenna assembly 10 of Z1, where Z1 is much greater than λ/2 π, λ being the wavelength.

In general, the patch antenna 12 as a radiation antenna is designed such that the antenna impedance is substantially real and mainly consists of radiation impedance. The value of the real impedance substantially matches the signal source impedance from the feed system, which is typically 50 ohms. The antenna impedance is mainly real and mainly radiation resistance. The present disclosure relates to near-field antenna assemblies that intentionally attenuate far-field radiation and enhance localized E-fields in the near-field region. More specifically, such a near-field antenna assembly confines energy to a region close to the antenna, i.e., a near-field region, and prevents radiation in a far-field region. Thus, RFID tags that are physically close to the near field antenna are interrogated, but those that are outside the near field region are not. At an operating frequency of 915MHz, the near field region is about 5cm from the antenna. Tags outside the 5cm range are not read or written.

Although often referred to in the art as an antenna, as used herein, an antenna assembly is defined as an assembly of parts, at least one of which includes an antenna that directly transmits or receives electromagnetic energy or signals.

In one embodiment of the present disclosure, fig. 2 shows a near field antenna assembly 110 including a trace linear element microstrip antenna 112 above which a large RFID tag 120 is present in close proximity. In addition, as shown in fig. 3 and 4, the near-field antenna assembly 110 includes a microstrip antenna 112 having a thickness "t", the microstrip antenna 112 being coupled to a cable 114 at a feed-in point end 116, and terminated at an opposite or terminating end 118 to a terminating resistor "R1" of typically 50 ohms, the cable 114 typically being, but not limited to, a coaxial cable. The cable 114 has a first or signal end 114a and a second or ground reference end 114 b. A signal is fed from the cable 114 at the feed point 116 through a feed system 124. The signal is typically 50 ohms.

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 to achieve impedance matching, typically to minimize reflections.

As best illustrated in fig. 3 and 4, the linear microstrip assembly 110 includes a substantially rectangular microstrip trace 112, and a substrate 140 having a first surface 140a and an opposite second surface 140 b. The distance between the first and

second surfaces

140a and 140b defines a thickness "H" of the substrate 140.

The microstrip assembly 110 further includes a ground plane 150 and is configured such that the microstrip line 112 is located on the first surface 140a of the substrate 140 and the ground plane 150 is located 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 dielectric spacer 164 may be an air gap (suitable structural support not shown). First end 114a of cable 114 is electrically coupled to microstrip antenna 112 and second end 114b is electrically coupled to ground plane 150.

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

Substrate 140 is a dielectric material, which may typically comprise a ceramic or FR-4 dielectric material, having a thickness "H" and an overall width "W_s", and ground plane 150 is below. At the terminating end 118 of the linear microstrip 112, a terminating resistor R1 electrically couples the end 118 of the linear microstrip 112 to the ground plane 150.

The input impedance "Z" of linear microstrip antenna 112 at feed point 116 is designed to be approximately equal to the characteristic impedance of cable 114 supplying the feed signal in order to maximize the power coupled from the reader (the reader is part of feed system 124 and is an electronic system separate from cable 114 or the transmission network. antenna assembly 110 is coupled to the reader system by cable 114). The ratio W/H is generally greater than or equal to 1 and can be, in particular, from about 1 to about 5.

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

wherein,

ε_ris the relative dielectric constant of the substrate 140. Therefore, the impedance "Z" is mainly determined by the microstrip width W and the substrate height H.

In one embodiment, the substrate relative permittivity“ε_r"from about 2 to about 12. In another embodiment, the length "L" of the linear microstrip near field antenna assembly 110 corresponds to the equivalent or effective length of a half-wave to full-wave device, the equivalent physical length being about

Where c is the speed of light (about 3X 10)⁸m/s), f is the operating frequency in Hz, and ∈_r"is the substrate relative permittivity, and n is from about 0.5 for an equivalent half-wave dipole antenna to about 1.0 for an equivalent full-wave dipole antenna.

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

In another embodiment, the linear microstrip antenna 112 has first and second

longitudinal edges

112a and 112b, and the microstrip antenna 112 is located substantially in the center of the substrate 140 and the ground plane 150 such that the

longitudinal sides

142a and 142b of the substrate 140 and the

longitudinal sides

152a and 152b of the ground plane 150 each extend from the first and second

longitudinal edges

112a and 112b a distance at least twice the width "W" ("2W"). As a result, the substrate 140 and the ground plane 150 each have a total width "Ws" that is at least five times the width "W" ("5W"). The substrate 140 further includes a lateral side 142c at the feed point 116 and a lateral side 142d at the terminating resistor R1. Similarly, the ground plane 150 further includes a lateral side 152c at which the feed point 116 is located and a lateral side 152d at which the terminating resistor R1 is located.

The near field antenna assembly 110 intentionally attenuates the far field and enhances the near field region. More specifically, near field RFID antenna assembly 110 includes a unit antenna 112, unit antenna 112 configured such that the localized E electric field emitted by antenna 112 is substantially within the area defined by the near field, and the radiated field emitted by antenna 112 is substantially within the area defined by the far field with respect to antenna 112. Thus, the near field antenna assembly 110 has many advantages for ease of adjustment. The real impedance of such an antenna assembly is very low without a 50 ohm terminating impedance. Therefore, the radiation resistance is low. A termination impedance R1 of 50 ohms is typically added so that the input impedance is approximately 50 ohms in order to match the feed system 124 supplied by the cable 114. This configuration and method of operation also results in a very low antenna "Q" factor, making the antenna wideband.

Theoretically, as shown in FIG. 5, the microstrip antenna 112 is a half-wave "λ/2" antenna with the current distribution along the length of the trace microstrip antenna 112.

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

As shown in fig. 5, this current-distributing linear microstrip antenna assembly 110 operating in a half-wave dipole configuration generates a positive E-field at the feed 116 and a negative E-field at the termination 118.

Fig. 6 illustrates near-field E-field coupling over the near-field microstrip antenna 112. More specifically, fig. 6 is a graphical representation of the normalized time-varying E-field over the microstrip antenna 112 at a certain time instant for the half-wavelength case. At the feed point 116, the E-field reaches a maximum. At the midpoint of the microstrip antenna 112, the E-field decreases to zero. At the terminating end 118, the E-field is reduced to a negative peak or minimum. Since RFID tag 120 is located directly above such an antenna (see fig. 2), the differential E-field from microstrip antenna 112 drives or directs current along the length of RFID tag antenna 120, thereby activating RFID tag 120 so that it can then be read or written by an RFID reader, i.e., near field antenna assembly 112.

As a result, RFID tag 120, which is positioned above microstrip antenna 112 and oriented along the length "L" of microstrip antenna assembly 110, then transmits information to microstrip antenna 112. It should be noted that for a half-wave dipole antenna configuration, the substrate 140 effectively forms a slow-wave structure, depending on the material of the substrate 140, resulting in a total antenna length "L" of

Where c is the vacuum speed of light, f is the operating frequency, and ∈_r"is the relative permittivity or relative dielectric constant of the substrate material. Thus, the relative permittivity or relative dielectric constant "ε of the substrate 140_r"increase, total antenna assembly length" L "is shortened, so that such antenna assemblies may be used for smaller RFID tags. For example, if a ceramic substrate with a dielectric constant of 12.5 is used, a total microstrip length of 4.7cm is achieved in the experiment, with a theoretical length of 4.6 cm. A smaller antenna assembly is used to read or detect RFID tags at a smaller item level.

In one embodiment, the length of the linear microstrip antenna assembly 110 is extended to a length corresponding to a full wave. Fig. 7 and 8 show the time-varying E-field at a time over a full-wave microstrip antenna assembly, such as linear microstrip antenna assembly 110, at 0 ° and 90 ° phase, respectively.

Since the feed signal supplied at the feed point 116 through the cable 114 experiences the entire 360 ° phase, two specific snapshots of the differential E-field at a certain time can be observed. At zero phase, there are two pairs of differential E-fields, while at 90 ° phase there is only one pair. The actual differential E-field coupled with the upper RFID tag 120 sweeps along the length "L" of the linear microstrip antenna 112. This facilitates alignment between the linear microstrip antenna 112 and the RFID tag 120. Increasing the dielectric strength (or relative permittivity ∈ of the material of the substrate 140_r") at least partially compensates for the need to increase the overall antenna length" L ".

Referring to fig. 9, a series of RFID tags 120a through 120e are separated by a gap distance "d", one of which, 120c, is located above a single linear microstrip antenna assembly 110. RFID tags 120a through 120e are oriented such that the antenna dipoles of RFID tags 120a through 120e are oriented longitudinally along the length "L" of linear microstrip antenna assembly 110.

To prevent fromThe anti-near-field linear microstrip antenna assembly 110 reads and writes a

tag

120b or 120d close to an addressed tag 120c, and the microstrip width "W", the length "L" and the total substrate width "W" can be adjusted correspondingly_s". As the gap "d" between the RFID tags 120 a-120 e shrinks, the microstrip width "W" and the total substrate width "W" of about "5W" must be reduced_s". The size of the gap "d" properly positions

adjacent tags

120a, 120b, 120c, 120d outside the

sides

142a, 142b of the substrate 140 of the linear microstrip antenna 112 so that the microstrip antenna assembly 110 does not detect the presence of

adjacent RFID tags

120a, 120b, 120c, 120 d. Trace width W, length L, and substrate parameters W/H and ε_rIs adjusted so that the current distribution is achieved to correspond effectively to a half-wave to full-wave configuration.

In one embodiment as shown in fig. 10 and 11, the linear microstrip antenna assembly 110' includes an extended or wrapped around ground plane. More specifically, the linear microstrip antenna assembly 110 'is identical to the linear microstrip 110 except that instead of the ground plane 150, the microstrip line 112 is on the first surface 140a of the substrate 140, and the ground plane 150' is on at least a portion of the first surface 140a of the substrate 140 and is not in contact with the microstrip line 112. The ground plane 150' is also on the first and

second edges

142a and 142b of the substrate 140 and the second surface 140b of the substrate 140, respectively. 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 overlie the first surface 140a and extend inward toward the

edges

112a and 112b, respectively, a distance "WG" but do not contact the trace microstrip 112.

As shown in fig. 11, RFID tags 120a through 120e may be located very closely above antenna assembly 110 'such that while one tag 120c is above the trace linear microstrip 112,

adjacent tags

120b and 120c are generally above flaps or ends 180a and 180b, respectively, of ground plane 150'. As shown in fig. 12, antenna assembly 110 'controls the location of radio frequency energy by propagating near field energy and by a ground plane 150' surrounded by flaps or ends 180a and 180b that extend inward toward

edges

112a and 112b, respectively, by a distance WG but do not contact trace microstrip 112. Thus, the E-field extends substantially only from trace microstrip 112 to flaps or ends 180a and 180b, effectively terminating the E-field and preventing antenna assembly 110' from coupling with

adjacent tags

120b and 120 d.

Fig. 13 illustrates a temporal diagram of the coupling of the time-varying near-electric field E over the near-field microstrip antenna 112 of the antenna assembly 110 ' as viewed from one of the sides, such as side 152b, of the ground plane 150 ' of the antenna assembly 110 '. More specifically, FIG. 13 is a graphical representation of the normalized E-field for the half-wavelength case. In a similar manner as shown in fig. 6, the E-field is maximum at the feed point 116. At the midpoint of the microstrip antenna 112 along the length "L", the E-field decreases to zero. At the termination point 118, the E-field is reduced to a negative peak or maximum.

Since RFID tag 120 is located directly above antenna assembly 110', as shown in FIG. 12, the differential E-field from microstrip antenna 112 drives or directs a current along the length of RFID tag 120 and thus activates RFID tag 120 so that the RFID reader, i.e., near-field antenna assembly 112, can then read or write. As a result, RFID tag 120c, which is located above microstrip antenna 112 and oriented along length L of microstrip antenna assembly 110', is also properly coupled to microstrip antenna 112. And, trace width W, length L and substrate parameters W/H and ε_rIs adjusted such that an effective current distribution is achieved that effectively corresponds to a half-wave to full-wave configuration.

Referring to fig. 14 and 15, in one embodiment, the linear microstrip antenna assembly 110 (or 110') may be mounted within or on the conductive housing 160. The conductive housing 160 includes a base plate 162, two generally

longitudinal side walls

162a and 162b, and two generally orthogonal connecting lateral side walls 162c and 162 d. The bottom surface of the ground plane 150 is positioned on the bottom plate 162 to electrically couple the conductive housing 160 with the ground plane 150. Thus, the conductive housing 160 is grounded through the ground plane 150.

Sidewalls 162a through 162d may be spaced from edges 142a through 142d of base 140. The edges 142 a-142 d may contact the conductive housing 160, but may require a space margin to enclose the antenna assembly 110 (or 110') within the housing 160. The sidewalls 162 a-162 d may also be separated from the linear microstrip antenna 112 by a dielectric isolation material 170 such 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 isolation material 170 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 contemplated that adding a conductive housing 160 having extended side surfaces realized by sidewalls 162a through 162d adjacent sides 142a through 142d of base 140 of microstrip antenna assembly 110 may further reduce undesirable coupling of adjacent RFID tags 120 to linear microstrip antenna assembly 110.

In one embodiment of the present disclosure as shown in fig. 16-18, the meander-line element microstrip antenna assembly 210 is used to make the apparent antenna length "L" longer for a given overall antenna size, e.g., used for reading small RFID tags. The meander line antenna assembly 210 is similar in many respects to the linear microstrip antenna assembly 110 and, therefore, is described herein only to the extent necessary to identify differences in 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 trace 212 spans the width "W" of the substrate 140 as it proceeds along the length "L" from the feed point 116 to the terminating resistance R1 at the terminating end 118_s"" zigzag ". The meander-line microstrip antenna trace 212 has a thickness "t" and is electrically coupled to the cable 114 at the feed point end 116 and terminated at the termination end 118 by a terminating resistor R1, typically 50 ohms.

The meander-line microstrip antenna 212 differs from the linear microstrip antenna 112 in that the meander-line microstrip antenna 212 guides current two-dimensionally. More specifically, in one embodiment, the meander-line microstrip assembly 210 comprises a plurality of alternating orthogonally-contacted

conductive segments

214 and 216, the

conductive segments

214 and 216 being respectively arranged in a square wave pattern forming a meander-line microstrip trace antenna 212. Conductive segment 214 and length "L_M"are aligned and substantially parallel to at least one of the

longitudinal side edges

142a and 142b of the substrate 140. Conductive segment 216Laterally aligned with and contacting the flat row of conductive segments 216 to form a square wave pattern. Each of conductive segments 216 is oriented with respect to a centerline axis C-C that extends along a length Ls of the conductive segment and bisects the width. The respective

conductive segments

214 and 216 of the contacts may be integrally formed as a single microstrip trace. The meander line antenna 212 may be formed with other patterns that do not follow a square wave pattern, wherein the alternating contact

conductive segments

214 and 216 are non-orthogonal. The embodiments are not limited in this regard. The configuration of the

segments

214 and 216 enables the localized E-field to drive or steer current in two dimensions.

At least one

edge

142a, 142b of the substrate 140 has a length "L_M", and the orthogonal contact

conductive segments

214, 216 are arranged in alternating transverse and longitudinal orientations with respect to at least one

edge

142a, 142 b.

As shown in fig. 17, the conductive segments 214 are arranged in a longitudinal orientation and together define an overall length "L" of the meander-line shaped microstrip trace 212 extending from the feed point 116 to the terminating end 118 terminating resistor R1 and including the terminating resistor R1 of the terminating end 118_M". Width "W" of meander-line-like trace 212_M"is defined as the width of one of the longitudinally oriented conductive segments 214.

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

In one embodiment, the meander-line microstrip antenna assembly 210 has a ratio "W" that may be greater than or equal to 1, and particularly from about 1 to about 5_MAnd H ". The substrate 140 may have a relative dielectric constant from about 2 to about 12. At least one

edge

142a, 142b of substrate 140 may be configured to extend transversely from electrically conductive segment 214 arranged in a longitudinal orientation substantially greater than or equal to width "W" of meander-line-like microstrip trace 212_M"twice (2W)_M") of a distance. In another embodiment, at least one of the ground planes

150Each edge

152a, 152b may extend transversely from the longitudinally oriented conductive segment 214 substantially greater than or equal to the width "W" of the meander-line-like microstrip trace 212_M"is measured. It is also contemplated that the meander-line antenna assembly 210 may include a capacitive load 122 electrically coupled to the meander-line microstrip trace 212 generally near the terminating resistor R1.

As shown in fig. 17-19, and described in a manner similar to linear antenna assembly 110 shown in fig. 9, a series of RFID tags 120a through 120e are separated by a gap distance "d", one of the RFID tags 120c being located above a single meander-line shaped microstrip antenna assembly 210. The meander-line microstrip antenna assembly 210 is configured such that the localized E electric field of the meander-line antenna 212 couples with one RFID tag or label 120 oriented longitudinally along the length of the meander-line microstrip antenna assembly 210. The localized E-field drives or directs current along the antenna 212 in two dimensions.

To prevent the near field meander-line microstrip antenna assembly 210 from reading and writing a

tag

120b or 120d that is close to the addressed tag 120c, the microstrip width "W" may be adjusted accordingly_M", length" L_M"and Total substrate Width" W_s". As the gap "d" between the RFID tags 120a through 120e shrinks, the microstrip width "W_M"and Total substrate Width" W_s"also shrinks. The size of the gap "d" properly positions

adjacent tags

120a, 120b, 120c, and 120d outside the

sides

142a, 142b of the substrate 140 of the meanderline-like microstrip antenna 212 so that the microstrip antenna assembly 210 does not detect the presence of

adjacent RFID tags

120a, 120b, 120c, 120 d. In the case of a meander-line microstrip antenna, the trace width W_MTotal effective length L_MAnd the substrate parameters are adjusted so that an effective current distribution corresponding to a half-wave to full-wave structure is achieved. This may be done by increasing each given fixed length L_MMeander line trace period L'_MNumber of cells.

In one embodiment, such as the embodiment shown in fig. 20 and 21, the meander-line microstrip antenna assembly 210' includes an extended or surrounding ground plane. More specifically, the meander-line microstrip antenna assembly 210 'is identical to the meander-line microstrip 210, except that instead of the ground plane 150, a microstrip line 212 is on the first surface 140a of the substrate 140, and the ground plane 150' is on at least a portion of the first surface 140a of the substrate 140 and is not in contact with the microstrip line 212. In a similar manner to the linear microstrip 110 ', the ground plane 150' is also located on the first and

second edges

142a and 142b of the substrate 140 and the second surface 140b of the substrate 140, respectively. The ground plane 150' may also be separated from the substrate by one or more dielectric spacers 164.

Ground plane 150' may also include a distance "W" overlying first surface 140a and extending inward toward

edges

112a and 112b, respectively_G"but not in contact with trace microstrip 212.

As shown in fig. 21, RFID tags 120a through 120e may be located very closely above antenna assembly 210 'such that while one tag 120c is above trace meander-like microstrip 212,

adjacent tags

120b and 120c are generally above flaps or ends 180a and 180b, respectively, of ground plane 150'.

Still further, 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 assembly 210 (or 210') may be electrically coupled to the conductive enclosure 160. Sidewalls 162a through 162d may be spaced from edges 142a through 142d of base 140. The edges 142 a-142 d may contact the conductive housing 160, but may require a space margin in order to enclose the antenna assembly 110 (or 110') within the housing 160. The sidewalls 162 a-162 d may also be separated from the meanderline microstrip antenna 212 by a dielectric isolation material 170, such 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 previously, the trace width W_MTotal effective length L_MAnd the substrate parameters are adjusted such that an effective current distribution corresponding to a half-wave to full-wave structure is achieved. This can be done by increasing each given fixed lengthDegree L_MMeander line trace period L'_MNumber of cells.

The foregoing embodiments of the near field antenna assemblies 110, 110 ', 210' are disclosed as being powered in a unit configuration by the cable 114 and terminating resistor R1. Those of ordinary skill in the art will recognize that the near field antenna assemblies 110, 110 ', 210' may also be powered by a dipole configuration that includes a transformer. The embodiments are not limited in this regard.

In view of the foregoing, embodiments of the present disclosure are directed to reading a near field antenna assembly 110, 110 ', 210' of an RFID tag, wherein the near field antenna assembly 110, 110 ', 210' is configured such that the localized E electric field emitted by the antenna assembly 110, 110 ', 210' at the operating wavelength "λ" is substantially within a region defined by the near field and the radiation field emitted by the antenna assembly 110, 110 ', 210' at the operating wavelength "λ" is substantially within a region defined by the far field relative to the antenna assembly 110, 110 ', 210'.

Various embodiments of the present disclosure are designed to increase the magnitude of the localized E-field relative to the magnitude of the radiated field, and to read the RFID tag or label 120c by the antenna or antenna assembly 110, 110 ', 210' only when the tag or label 120c is located in the near-field region (not when the tag or label 120c is located in the far-field region). Further, the amplitude of the radiated field may be reduced relative to the amplitude of the localized E-field such that the RFID tag or label 120c is read by the antenna or antenna assembly 110, 110 ', 210' only when the tag or label 120c is located in the near-field region (and not when the tag or label 120c is located in the far-field region). The antenna components 110, 110 ', 210 ' have a relative dielectric constant ' epsilon_r”。

The antenna or antenna assembly 110, 110 ', 210' is configured to define a near field region at a distance from the antenna assembly 110, 110 ', 210' equal to "λ/2 π", where "λ" is the operating wavelength of the antenna or antenna assembly 110, 110 ', 210'. In one embodiment, the antenna or antenna assembly 110, 110 ', 210' operates at a frequency of about 915MHz such that the near field region distance is about 5 cm.

A method of reading and writing an RFID tag or label 120c is also disclosed, the method comprising the steps of: providing a near-field antenna assembly 110, 110 ', 210', the near-field antenna assembly 110, 110 ', 210' being configured such that the localized E electric field emitted by the antenna or antenna assembly 110, 110 ', 210' at the operating wavelength "λ" is substantially within a region defined by the near-field and the radiation field emitted by the antenna or antenna assembly 110, 110 ', 210' at the operating wavelength "λ" is substantially within a region defined by the far-field relative to the antenna assembly 110, 110 ', 210'; and coupling the localized E-electric field of the near-field antenna assembly 110, 110 ', 210' with the RFID tag or label 120c located within the near-field region.

Effective length L or L of

antenna assembly

110, 110', 210_MIt may be that the current distribution directed through the antenna causes a waveform with a wavelength proportional to nv/f, where v is the propagation wave speed equal to the speed of light divided by the square root of the relative permittivity of the antenna components 110, 110 ', 210', f is the frequency in Hz, and n is from about 0.5 for a half wave to about 1.0 for a full wave.

The method may further comprise the steps of: the magnitude of the localized E-field is increased relative to the magnitude of the radiated field such that the RFID tag or label 120c is read by the antenna assembly 110, 110 ', 210' only when the tag or label 120c is located in the near-field region, but the antenna assembly 110, 110 ', 210' does not read the RFID tag or label 120c when the tag or label 120c is located in the far-field region.

The method may further comprise the steps of: the amplitude of the radiated field is reduced relative to the amplitude of the localized E electric field so that the RFID tag or label 120c is read by the antenna assembly 110, 110 ', 210' only when the tag or label 120c is located in the near-field region, but the antenna assembly 11 when the tag or label 120c is located in the far-field region0. 110 ', 210' do not read the RFID tag or label 120 c. The method may comprise the steps of: the antenna assemblies 110, 110 ', 210' are configured to define a near field region at a distance from the antenna assemblies 110, 110 ', 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 is operated at a frequency of about 915MHz such that the near field region distance is about 5 cm. Effective length L or L of

antenna assembly

It is contemplated that advantageous characteristics of the disclosed near field antenna assemblies include:

(1) the range of reading and writing the RFID tags 120a through 120e is limited to the near field distance d < λ/2 π;

(2) the main field energy of the

near field antenna

112 or 212 is dissipated in the terminating load resistor R1;

(3) the near-field antenna assembly exhibits a low Q factor compared to the radiating far-field antenna assembly;

(4) the wide operating bandwidth due to the low Q factor can be used for global UHF broadband applications;

(5) the wide operating bandwidth and low Q factor allow for simplified RFID reader electronics, without the need for frequency hopping to prevent reader interference with each other;

(6) the near-field antenna assembly exhibits low radiation resistance and radiation efficiency compared to the radiating antenna assembly. Thus, far field radiation is significantly reduced;

(7) near-field antenna assemblies equipped with microstrip-type antennas with trace dimensions, substrate characteristics, and ground planes are designed to operate from half-wave antennas to full-wave antennas;

(8) the unit 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 on the bottom surface of the substrate, provides a simpler, more cost effective feed configuration than an alternative differential feed configuration that may require a transformer;

(9) the conductive housing of the near field antenna assembly at its open top side is connected to the ground plane of the antenna assembly. The conductive housing helps to minimize stray electric fields that tend to couple with adjacent RFID tags that are adjacent to RFID tags located directly above the microstrip antenna; and

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

As a result of the foregoing, embodiments of the present disclosure allow for programming RFID tags in close proximity to one another. For example, RFID tags on a reel have the feature that the separation distance between each tag is small. Embodiments of the present disclosure do not require that the tags be widely separated and prevent multiple tags from being read and programmed together. In addition, embodiments of the present disclosure facilitate identifying defective tags that are located in proximity to tags having proper functionality.

While the above description contains many specifics, these specifics should not be construed as limitations on the scope of the disclosure, but merely as exemplifications of preferred embodiments thereof. Those of ordinary skill in the art may devise many other possible variations that are within the scope and spirit of the present disclosure.

Claims

1. A near field RFID antenna assembly comprising a substantially linear element microstrip antenna, comprising:

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

a feed point at an end region of the basic linear element microstrip antenna;

a terminating resistor at an end region of the basic linear element microstrip antenna opposite the feed point;

a linear microstrip line coupled to a cable at the feedpoint and terminated to the terminating resistor, the linear microstrip line having a width W;

the substantially linear element microstrip antenna further comprises:

a ground plane is provided on the substrate,

wherein the linear microstrip line is located on a first surface of the substrate and the ground plane is located on a second surface of the substrate,

the antenna is configured such that:

the localized E electric field emitted by the antenna is substantially within the region defined by the near field;

the localized E-field directs a current distribution along an effective length of the antenna corresponding to a half-wave to full-wave structure;

an input impedance Z in ohms at the feed point substantially equal to an impedance of the termination resistor; and

the W/H ratio ranges from about 1 to 5,

wherein the substrate and ground plane each have a width at least five times the width W, and

wherein the linear microstrip line has first and second longitudinal edges and is located substantially at the center of the substrate such that the edges of the substrate and the edges of the ground plane each extend from the first and second longitudinal edges a distance of at least twice the width W.

2. The antenna assembly of claim 1, the terminating resistor being electrically coupled to a ground plane.

3. The antenna assembly according to claim 2, wherein the linear microstrip line has a width W and the substrate has a thickness H such that an input impedance Z in ohms of the antenna assembly is substantially equal to the following equation:

wherein,

and epsilon_rIs the relative dielectric constant of the substrate.

4. The method of claim 3An antenna assembly, wherein the relative dielectric constant ε of the substrate_rFrom 2 to 12.

5. The antenna assembly of claim 2, wherein the linear microstrip line has a length L extending from a feed point to a termination resistance and including the termination resistance, the length L given by the equation:

where c is the speed of light in m/s, the speed of light being 3X 10⁸m/s, f is the operating frequency in Hz, ε_reIs that

And n ranges from 0.5 for an equivalent half-wave dipole antenna to 1.0 for an equivalent full-wave dipole antenna.

6. The antenna assembly of claim 2, wherein an antenna input impedance at the feed point is approximately equal to a characteristic impedance of a cable supplying the feed signal at the feed point.

7. The antenna assembly of claim 1, wherein the linear microstrip line has a thickness from 10 to 30 microns.

8. The antenna assembly of claim 1,

the substrate having first and second edges along a length of the substrate; and

the ground plane is also located on at least a portion of the first surface of the substrate but not in contact with the linear microstrip line, and on the first and second edges of the substrate.

9. The antenna assembly according to claim 8, wherein the linear microstrip line has a width W and the substrate has a thickness H such that an input impedance Z in ohms of the antenna assembly is substantially equal to the following equation:

wherein,

and epsilon_rIs the relative dielectric constant of the substrate.

10. The antenna assembly of claim 1, wherein the ground plane of the antenna assembly is electrically coupled with the conductive housing.

11. The antenna assembly of claim 10, wherein the conductive housing is separated from the antenna by at least one dielectric spacer.

12. The antenna assembly of claim 11, wherein the dielectric spacer comprises an air gap.

13. The antenna assembly of claim 1, wherein the antenna assembly is configured such that the localized E-electric field of the antenna assembly couples with an RFID tag oriented longitudinally along a length of the antenna assembly.

14. The antenna assembly of claim 1, further comprising a capacitive load electrically coupled with the linear microstrip line.