ES2342816B2 - SELECTIVE SURFACE IN FREQUENCY AND FLAT ARTIFICIAL MAGNETIC DRIVER AT FREQUENCIES LOWER TO 1GHZ, AND ITS USES. - Google Patents

Abstract

Selective surface in frequency and plane artificial magnetic conductor at frequencies below 1 GHz, and its uses

Frequency selective surface (FSS) comprises a periodic structure of metallizations (D) constituted by the repetition of the metallization of a unit cell on a dielectric substrate (Y). The artificial magnetic conductive plane (AMC) comprises at least one FSS, located on a mass plane driver (F). The invention is applicable in systems of communications, especially in those that require sizes reduced, such as laptops, for example for implementation of filters or as ground plane for antennas. Of special interest is its application in passive labeling in identification systems by radiofrequency in UHF bands on metals or in environments with the presence of metals, as in hospital logistics, airports, stations, in manufacturing, transportation, automotive, food and metallurgical industry, among others.

Description

Selective surface in frequency and plane artificial magnetic conductor at frequencies below 1 GHz, and its uses

The present invention relates to a selective surface in frequency and to a magnetic conducting plane artificial at frequencies below 1 GHz. The invention is also refers to the uses of the frequency selective surface and the artificial magnetic conductor plane synthesized with it, in communications systems, for the implementation of filters, such as ground plane for antennas and for labeling, and in systems of radio frequency identification on metals, in the band of UHF

The invention is applicable in the sectors in those that are designed, produced, manufactured and / or used materials and devices for radiofrequency identification preferably on metals or in environments with the presence of metals, such as in hospital logistics, airports, stations, in manufacturing, transportation, automotive, food industry, metallurgical and manufacturing industrial products, among others.

State of the art

Identification systems by radio frequency (RFID) are experiencing a boom due to its applications in identification, tracking, location or detection of a wide variety of objects and also of living beings (L. Boglione, IEEE Microwave Magazine, Vol. 8, No. 6, 2007, pp. 30-32; R. Das, P. Harrop. IDTECHEX 2008. RFID Forecasts, Players & Opportunities 2008-2018; S. Miles, S. Sarma, J. Williams Co-Editors, Cambridge University Press, RFID Technology and Applications. 2008; S. Miles, Auto-ID Research: Auto-ID-Development and Adoption-A Worldwide Perspective. 2008. Laboratory for Manufacturing Productivity. Massachusetts Institute of Technology (MIT); M. Stewart, USA DOD Logistic AIT Office. DLA Expo Workshop, 2005).

An RFID system is normally composed of three elements: the tag ( tag or transponder ), the reader ( reader ) and the data processing subsystem ( middleware ). In turn, a label that works by electromagnetic coupling is formed by an antenna and an integrated circuit or chip (K. Finkenzeller. Wiley, Chichester, 2004). The labels are classified as active, passive and semi-passive depending on whether or not they incorporate a battery.

Recently, RFID technology in the band of UHF has gained popularity in many applications, given that Provides a wide range of label reading, capacity larger information storage and faster speed of label reading that lower frequency bands (LF and HF). (S. R. Aroor, D. Deavours, IEEE Systems Journal, Vol. 1, No. 2, December 2007; EPCglobal Inc .: EPC radio frequency identification protocols class-1 generation-2 UHF RFID protocols for communication at 860-960 MHz, V 1.2.0. Technical report, October, 2008; ISO / IEC Standard: ISO 18000-6C). Also, in this UHF band it is possible Simultaneous reading of a considerable number of tags. Without However, one of the major drawbacks for the development and implementation of RFID systems in the UHF band in, by example, production and supply chains, is nature metal of the objects to be identified or the presence of metals in the surroundings (L. Mo, H. Zhang, IEEE International Symposium on Microwave, Antenna, Propagation, and EMC Technologies For Wireless Communications 2007, pp 803-806; D. M. Dobkin, S. M. Weigand, IEEE APS 2005, pp135-138; J. T. Prothro, G. D. Durgin, J. D. Griffin IEEE APS 2006, pp 3241-3244). This is extended to the identification of living beings in these environments.

In RFID systems the material to which adheres the label should have minimal effect on it, to It works properly. However, the antennae of the labels do not operate independently of the objects they have close, but these can degrade their radiation properties in different degree In fact, the reading range of the labels and their stability change depending on the characteristics of the surface materials on which they are placed. So by example, if the surfaces are made with materials dielectric, the reading range may decrease due to resonance frequency shift. Can also be seen Reduced radiation efficiency based on electrical property of surface materials. Moreover, if the objects have high conductivities, as in the case of metallic objects, then the performance degradation in the readings is done significant, since the tangential component of the electric field on the metal surface it decreases a lot (in the case of a Perfect electrical conductor (PEC) is canceled). This disadvantage is more accused by increasing the frequency, so the labels of the UHF bands are more susceptible than those of lower bands than inconveniences arising from the presence of metals in the system and its environment.

Another problem caused by the placement of a antenna near a metal surface is the decrease in its impedance. For a passive RFID system to work, it is necessary get a level of sufficient power, coming from the wave that affects the label. The presence of metals and degradation of the impedance of input causes very little power to be attached to the chip, and very little of The modulated signal can be returned from the chip. This inconvenience is partly solved with the use of active tags, although at the cost of an increase in the price of the label, for the Need to include a battery in it.

In addition, the design of antennas for RFID tags is subject to a series of restrictions, if it is intended that the labels replace a massive labeling system like the barcode at the object level. These restrictions are of cost, since the antenna must be cheap; in size, so that the antenna footprint is small and thus be able to identify objects of small dimensions; and of small thickness so that, for example, Be flexible

Passive tags are known based on microstrip patch antennas, flat antennas shaped inverted F (PIFA) and tuned dipoles on foam (foam). However, microstrip patch antennas have a low bandwidth, need a non-negligible thickness and are usually fragile (brittle), eliminating the possibility of using them in flexible sticker type labels. The inverted F-shaped flat antennas (PIFA) are smaller and have greater bandwidth, but still small for RFID radiofrequency identification. Attempts to increase bandwidth have led to complex structures that imply an increase in manufacturing difficulty and cost (M. Hirvonen, P. Pursula, K. Jaakkola, K. Laukkanen, Electron. Lett., Vol.40 , pp 848-850, July 2004). In addition, the thickness is still not negligible. Likewise, the PIFA antennas need at least a small metallization ( shorting plate or shorting pin ) lateral of connection of the radiating element with the plane of metallic mass, which makes them not suitable for flexible labels of type sticker because when folding the label, such metallization could break easily. The dipoles tuned to work at a height of a quarter wavelength at the frequency of interest on the ground plane, require separate the dipole of the ground plane by a foam layer (foam) the thickness mentioned, ie of a quarter wavelength at the frequency of interest, which is not negligible at the frequencies of the UHF band. An improved version called FAT constitute Tags (Foam Attached Tags), which also make use of a foam layer (foam) between the label and the object to be identified. Some designs are PIFA antennas that also incorporate foam (W. Choi et al , ETRI Journal, Vol 28, No 2, April 2008). Although the thickness is smaller than a quarter of a wavelength, it is still considerable (since layers of dielectric substrate and foam are usually sandwiched) for labeling small objects. In addition, the greater the thickness of the foam layer, the less suitable the design for a flexible or sticker type label is.

\hbox{Octubre 2003, pp
2678-2689) lo cual les hace inutilizables en
etiquetas RFID flexibles.}

There are no known designs of artificial magnetic conductors (AMCs) operating at frequencies below 900 MHz and having a unit cell size small enough to be usable as a mass plane for REID tags in the European UHF band and, in general, in portable communications systems in the UHF band. In addition, few designs of artificial magnetic conductors are known for frequencies in the range of 900 MHz to 1 GHz, such as that described in WO / 2003/107484, not serving for the UHF band of European radiofrequency identification frequencies; The scarce designs of these structures for frequencies of UHF bands other than the European ones are complex and difficult to manufacture, with nothing negligible thicknesses and large base cell size, which makes their use in the labeling of small objects impossible. Another added disadvantage is that they have very small artificial magnetic conductor behavior bandwidths. Most of them are based on several layers of frequency selective surfaces (FSS) of considerable thickness, which further increases the thickness of the artificial magnetic conductor. In addition, some designs use metallic holes that pass through the substrate (S. Clavijo, RE Díaz, AE McKinzie, IEEE Transaction on Antennas and Propagation, Vol 51, No. 10,

 \ hbox {October 2003, pp
2678-2689) which makes them unusable in
flexible RFID tags.} 

Description of the invention

For the purposes of the present invention and its description, FSS refers to frequency selective surface, understanding as such that surface constituted by the periodic repetition of a unit cell and designed to reflect or transmit electromagnetic waves with frequency discrimination. AMC refers to artificial magnetic conductor. Until the present no perfect magnetic conductors have been discovered (PMCs) in Nature and therefore, have to be synthesized. A possible way to synthesize an AMC is to place one or more FSS on A driving plane. AMCs behave like PMCs on a certain frequency band, which is considered the band of AMC operation. This band is centered on the frequency of AMC resonance, at which the phase of the reflection coefficient at the level of the FSS it is null and has as lower limits and higher frequencies for which said phase of the coefficient Reflection takes the values + 90º and -90º respectively. In what this invention refers to, the AMC bandwidth in% is the difference between the upper and lower frequency limits of the cited AMC operating band divided by frequency central or resonance and multiplied by 100. Lambda (λ) refers to the wavelength in free space at the frequency of resonance of the structure.

In addition, for the purpose of the present invention and its description, metallic holes, or also known as tracks or metallic holes connecting between layers of a circuit, it refers to those who, crossing the substrate, serve to connect the periodic metallization of the FSS with the ground plane lower metallic added to constitute the AMC.

The present invention relates to a frequency selective surface (FSS) comprising a periodic structure of metallizations constituted by repetition of the metallization of a unit cell on a substrate dielectric. Said unit cell is not larger than λ / 25, and the relative dielectric constant of the substrate dielectric takes values between 18 and 140. In addition, it lacks holes metallized that cross the substrate. In one embodiment preferred, the unit cell is as illustrated in Fig. 1 of the drawings, where W = λ / 32.55, P = λ / 262.60 and C = λ / 651.04.

The geometry of the unit cell is divided into a centered internal grid of 8 x 8 cells each side P = λ / 262.60 that are numbered starting at the top end left successively from 1 to 64 from left to right and from from top to bottom. In that grid they are completely metallized cells 1, 5, 6, 7, 8, 10, 12, 13, 16, 17, 18, 19, 24, 25, 27, 28, 29, 31, 32, 34, 37, 39, 42, 45, 46, 49, 50, 51, 52, 54, 55, 57, 58, 61, 62 and 64. The pairs of cells 1-10, 25-34, 52-61 and 55-64 are connected by a metallic chamfer of width C = λ / 651.04 forming an angle of 135 ° with the line horizontal that follows the sequence of cells 1 to 8 or their parallels. The pairs of cells 12-19, 27-34, 39-46 and 45-52 are connected by a metallic chamfer of width C = λ / 651.04 that forms a 135º angle with the vertical line that follows the sequence of cells 1 to 57 or its parallels.

The invention also relates to the use of said FSS in communications systems.

Another aspect of the invention is a plane artificial magnetic conductor (AMC) comprising at least one frequency selective surface (FSS), located on a plane of conductive mass The mentioned frequency selective surface (FSS) in turn comprises a periodic structure of metallizations constituted by the repetition of the metallization of a unit cell on a dielectric substrate. Said unit cell is one size not exceeding λ / 25, and with constant values relative dielectric of the dielectric substrate between 18 and 140 and It lacks metallic holes that pass through the substrate.

In a preferred embodiment, the AMC comprises only one of the mentioned frequency selective surfaces on a plane of conductive mass. In a more preferred embodiment, the resonant frequency is less than or equal to 1 GHz. In a Even more specific embodiment, the thickness of the AMC is not more than λ / 100. In another even more specific embodiment, the Metallizations are copper. In another even more specific embodiment, The metallizations are silver.

In another preferred embodiment, the AMC comprises at least one of the aforementioned FSS, with a unit cell such as the illustrated in Fig. 1 of the drawings, where W = λ55, P = λ / 262.60 and C = λ / 651.04. In one more embodiment preferred, the AMC comprises a single selective surface in frequency on a plane of conductive mass. In one embodiment yet more preferred, the resonance frequency is less than or equal to 1 GHz. In an even more specific embodiment, the thickness of the AMC is not greater than λ / 100. In another even more preferred embodiment, The metallizations are copper. In yet another embodiment Specific, the metallizations are silver.

Another aspect of the invention is the use of a artificial magnetic conductor plane (AMC) comprising only one of the aforementioned FSS located on a plane of conductive mass, in communications systems

Another object of the invention is the use of a artificial magnetic conductor plane (AMC) comprising only one of the aforementioned FSS with a unit cell as illustrated in the Fig. 1 of the drawings, where W = λ / 32.55, P = λ / 262.60 and C = λ / 651.04, in communications systems.

In a preferred embodiment the use in systems of communications, is like ground plane for an antenna.

In another preferred embodiment the use in systems of communications, it is for the implementation of filters in said systems.

Another aspect of the invention is the use of an AMC comprising only one of said selective surfaces in frequency on a plane of conductive mass and where the frequency of resonance is less than or equal to 1 GHz, and the thickness of the AMC is not greater than λ / 100, in communications systems.

Another object of the invention is the use of a artificial magnetic conductor plane (AMC) comprising only one of the aforementioned FSS with a unit cell as illustrated in the Figure 1 of the drawings, where W = λ / 32.55, P = λ / 262.60 and C = λ / 651.04, and where the frequency of resonance is less than or equal to 1 GHz, and the thickness of the AMC is not greater than λ / 100, in communications systems.

In a preferred embodiment the use in systems of communications, is like ground plane for an antenna. In a most preferred embodiment, the antenna is of an object tag Metallic in radiofrequency identification systems. In other most preferred embodiment, the antenna is of a passive tag on metal objects in radiofrequency identification systems in the UHF band.

In another preferred embodiment the use in systems of communications, it is for the implementation of filters in said systems.

One of the advantages over the state of the current technique is to have achieved a selective surface in frequency with a small basic cell, not exceeding λ / 25, and thickness not exceeding λ / 100, at frequencies less than 1 GHz. This allows a considerable reduction of size and thickness of the artificial magnetic conductor plane and, in consequence of the label that contains it, allowing the identification of small objects in UHF bands at the level international. In addition, the design of this basic cell gives the artificial magnetic conductor plane high bandwidth (between 4 and 6%), which is an important achievement in this band of frequencies, given the small size and thickness of the cell.

The use of the magnetic conductor plane artificial as mass plane on which the antenna of the label, allows to isolate it from the metal thus achieving a excellent performance on metal objects, giving with it solution to one of the big problems that RFID is finding in the UHF bands for its massive development and implementation. In addition, it represents an increase in the gain and bandwidth of the antenna, with respect to other conventional mass planes, as well as a considerable reduction or lobe suppression rear radiation and the antenna size decrease and with Tag it. Therefore, objects can be labeled smaller than what the state of the art allows.

Since the design is planar, with a single layer dielectric substrate and also does not incorporate holes metallized, can be mass produced by common techniques of printed circuits, which significantly reduces the cost of manufacture of both the frequency selective surface of the artificial magnetic conductor plane and label on your set. These characteristics enable its implementation on flexible substrates, which does not happen with alternatives current that incorporate metallic holes.

One of the advantages of the preferred embodiment of the design of the unit cell shown in Fig. 1 of the FSS, is that its geometry has symmetry, which imposes less restrictions to the polarization of the antenna that is placed on the AMC with it synthesized, thus facilitating the reading of the label and thereby increasing the chances of identification of objects.

Another advantage of the design of the conductive plane artificial magnetic object of this invention compared to other alternatives used in antenna designs for it application, is its scalability. For the same geometric design of the metallization of the FSS unit cell, the resonance frequency for values less than or equal to 1 GHz, by corresponding scaling of the unit cell, of size always not exceeding λ / 25, and / or by varying the relative dielectric constant ε of the substrate in the range between 18 and 140. The scalability gives the design here Presented great versatility. This advantage extends to all uses that are mentioned for both the FSS and the AMC in systems of communications, such as the implementation of filters

The lower the frequency, get a design of a small FSS and AMC constitutes one more achievement important. Due to the scalability property, the same design Geometric achieved for a UHF RFID frequency band casualties, like the European band, can be used in the bands of higher UHF RFID frequencies, such as from the USA and countries Asians, reducing the size of the unit cell using the same dielectric substrate, or maintaining the size and reducing the relative dielectric constant ε of the substrate. This avoids the costs of carrying out a design completely New for each case.

The invention is applicable in those sectors in which surfaces are designed, produced or used frequency selective, such as in systems of communications, for the implementation of filters, as a map of mass for antennas and for labeling, in identification systems by radiofrequency on metals, in the UHF band or lower or equal to 1 GHz and especially in those systems that require Small sizes, such as laptops.

Particularly, one of the applications of the AMC of the present invention is its use as a mass plane for antennas at frequencies less than or equal to 1 GHz due to the improved antenna performance such as improved directivity and radiation efficiency, as well as the decrease of the rear lobes of radiation. Of special interest to the industry is its use as mass plane for antennas in labels passive in the UHF band for identification of metallic objects or in environments where there are metals. Especially interesting is its use in the UHF band of European RFID, since the frequencies assigned (866 to 869 MHz) are of lower value than the UHF bands of RFID of countries like USA (902 to 928 MHz) and countries of Asia / Pacific (950 to 956 MHz). Its use extends to all applications of identification of metal parts and / or final products, as per example: parts in the automotive and aeronautical industry as well as electronic equipment, metal containers or cans in food industries, metallurgical and chemical industries, recycling, identification of patients in hospitals and location of luggage at airports, among others.

Brief description of the drawings

For the best understanding of how much is left written here, are accompanied by drawings in which a practical case of realization of the conductive plane is represented artificial magnetic

Fig. 1 corresponds to a top view of the geometry of the unit cell (Z) or basic cell designed with its constituent elements. In this figure, (X) indicates metallization of the unit cell (Z), e (Y) indicates the dielectric substrate of said unit cell (Z); W refers to the size of the cell side unit (Z) (which is square), where W = \ lambda / 32.55. P refers to the base size of the regular grid used in metallization (X) of the unit cell (Z), where P = λ / 262.60; (T1) and (T2) they refer to the two types of chamfers (T1) and (T2) in the metallization (X) of the unit cell (Z). In Fig. 1 also indicates the symmetry of the unit cell (Z) according to the diagonal plane a-a '.

Fig. 2 represents a detail of the two types of chamfers (T1) and (T2) in the metallization (X) of the unit cell (Z) bounded by P and referenced to the grid regular used in the metallization (X) of the unit cell (Z), where C = \ lambda / 651.04.

Fig. 3 represents a perspective view of an artificial magnetic conductive plane from the repetition of 3 x 9 unit cells (Z) whose geometry corresponds to that of the Fig. 1, where (D) refers to the periodic structure of metallizations of the FSS, (Y) refers to the dielectric substrate and (F) refers to the plane of conductive mass.

Fig. 4 represents the variation of the phase of the reflection coefficient in degrees, <\ Gamma> (º) measured on the periodic structure of metallizations of the FSS (D), in frequency function, f.

Explanation of a preferred embodiment

For a better understanding of this invention, the following preferred embodiment example is set forth, described in detail, which should be understood without limitation of the scope of the invention.

A magnetic conductor plane was implemented artificial (AMC) usable as a ground plane for an antenna a passive RFID tag in the European UHF band. To do this used a single FSS on a plane of conducting mass (F). According to this, the AMC in question consisted of a conductive mass plane (F), a dielectric substrate (Y) on the conductive mass plane (F) and a periodic structure of metallizations (D) constituted by the repetition of the metallization (X) of a unit or basic cell (Z), on the dielectric substrate (Y). Specifically, it was used silver in the periodic structure of metallizations (D).

For the construction of the AMC they were used conventional printed circuit manufacturing techniques, and specifically, the structured laser, although you can also manufacture by mechanical structuring or high milling precision.

For the European UHF frequency band, it used as a unit cell (Z) a design with the dimensions W = λ / 32.55 sideways and λ / 152 thick, for a substrate (Y) with a relative dielectric constant \ varepsilon_ {r} of value 25.

Through a grouping of 3 x 9 unit cells (Z) with the constitutive characteristics just mentioned, obtained a preferred but not the only realization of the plan artificial magnetic conductor in the frequency band of interest. Specifically, and as follows from Fig. 4, obtained a bandwidth of 4.33%.

Claims (14)

1. Frequency selective surface comprising a periodic structure of metallizations (D) constituted by the repetition of the metallization (X) of a unit cell (Z) on a dielectric substrate (Y), characterized in that the unit cell (Z) is square of a size W not exceeding λ / 25, because the geometry of the unit cell (Z) is divided into a centered internal grid of 8 x 8 cells each with a side P = λ / 262.60 that are numbered starting with the upper left end successively from 1 to 64 from left to right and from top to bottom, the cells 1, 5, 6, 7, 8,10,12, 13,16,17,18, 19 being completely metallized in said grid. , 24.25, 27.28.29, 31, 32, 34, 37, 39.42, 45.46, 49, 50, 51, 52, 54, 55, 57, 58, 61, 62 and 64, being the pairs of cells 1-10, 25-34, 52-61 and 55-64 connected by a metallic chamfer of width C = λ / 651.04 that forms an angle of 135 ° with the horizontal line that follows the succession of celd as 1 to 8 or its parallels, and the pairs of cells 12-19, 27-34, 39-46 and 45-52 being connected by a metallic chamfer of width C = λ / 651.04 that forms an angle of 135 ° with the vertical line that follows the sequence of cells 1 to 57 or their parallels, because the relative dielectric constant of the dielectric substrate (Y) takes values between 18 and 140, because it lacks metallic holes that cross the substrate (Y), and because the frequency The resonance is less than or equal to 1 GHz, with λ being the wavelength in free space at the resonant frequency of the structure. 2. Artificial magnetic conductor plane comprising one or more frequency selective surfaces according to claim 1 located on a conductor mass plane (F), characterized in that the resonance frequency is less than or equal to 1 GHz. 3. Artificial magnetic conductor plane according to claim 2 characterized in that its thickness is not greater than λ / 100. 4. Artificial magnetic conductor plane according to claim 2 characterized in that the metallizations (D) are copper. 5. Artificial magnetic conductor plane according to claim 2 characterized in that the metallizations (D) are silver. 6. Use of frequency selective surface of claim 1 in communication systems. 7. Use of the artificial magnetic conductor plane of claim 2 in communication systems. 8. Use according to claim 7 as the plan of mass for an antenna. 9. Use according to claim 7 for the Implementation of filters in communications systems. 10. Use of the artificial magnetic conductor plane of claim 3 in communication systems. 11. Use according to claim 10 as the plan of mass for an antenna. 12. Use according to claim 10 for Implementation of filters in communications systems. 13. Use according to claim 11 as the plan of mass for an antenna of a label for metallic objects in radio frequency identification systems. 14. Use according to claim 11 as a plan of mass for an antenna of a passive tag on objects metallic in radiofrequency identification systems in the UHF band.