(Tco F) What Are the Constraints on the Read Distance of a Tag?

Open admission peer-reviewed chapter

Troubleshooting RFID Tags Problems with Metal Objects Using Metamaterials

Submitted: October 15th, 2010 Reviewed: April quaternary, 2011 Published: July 20th, 2011

DOI: 10.5772/16888

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• Ma Elena de Cos*

  • Universidad de Oviedo, España

• Fernando Las-Heras

  • Universidad de Oviedo, España

*Address all correspondence to:

1. Introduction

Radiofrequency Identification (RFID) is a engineering science that is being rapidly developed and that uses radiofrequency (RF) signals for the automatic identification of objects or persons. Although the first commodity regarding modulated electromagnetic backscattering (basic principle of passive RFID) was published in 1948 (Stockman, 1948) information technology has been a long style to progress for reaching today levels (Rao, 1999; Finkenzeller, 2004; Pozar, 2004). Nowadays RFID finds many applications in logistics, supply chain management, access control, electronic toll systems, targets identification, vehicle security, animals tracking and patients' identification in hospitals.

An RFID system is composed of a reader, a reader antenna (usually circularly polarized patch antenna), RFID 'tags' or transponders and a middleware or subsystem of information processing. A passive RFID tag consists of an antenna and an application specific integrated circuit (ASIC) scrap. IC chips have complex input impedances, and their impedances vary with frequency. A fundamental point for tag antenna design is that information technology must be conjugately matched with the desired IC chip for the maximum power transfer (Geyi, 2004; Rao et al, 2005).

The different types of RFID systems are distinguished past ii major characteristics: the ability source of the tag and the frequency of functioning. With regards to the power source of the tag, they tin either be active (powered by battery), passive (powered by the reader field) or semi-passive (battery assisted backscatter). According to the frequency of operation the RFID systems are generally distinguished into iv frequency ranges; i.e., low frequency (LF) (125-134.ii kHz), loftier frequency (HF) (13.56 MHz), ultra loftier frequency (UHF) (433, 860-960 MHz) and microwave frequency (2.45, 5.viii GHz). In improver, the standards of the UHF RFID are different for each state: 866-869 MHz in Europe, 902-928 MHz in America and 950-956 MHz in Asia. The communication frequencies used depends to a large extent on the application. Regulations are imposed by nearly countries (grouped into iii Regions: US, Europe and Asia) to command emissions and prevent interference with other Industrial, Scientific and Medical equipment (ISM).

The higher the frequency ring the faster the speed of tag reading and also the larger the data storage capacity. This is the reason why UHF RFID has gained popularity in many applications and information technology can exist expected that the same will happen in the near future with microwave RFID.

In a typical awarding tags are attached to objects (or persons). Each tag has a sure corporeality of internal memory (EEPROM) in the flake in which it stores data well-nigh the object (or person), such as its EPC (electronic product lawmaking) or unique identification (ID) series number and some other data depending on the application, i.e. industry date and production composition, (or personal information for access control or health care matters).

A passive dorsum-scattered RFID arrangement operates as follows: a modulated signal with periods of unmodulated carrier is transmitted by a reader and is received past the tag antenna. Then the RF voltage adult on antenna terminals during unmodulated period is converted to dc. The flake is powered up with this dc voltage and sends back the information past varying its forepart end circuitous RF input impedance. The modulation of the dorsum-scattered betoken is carried out by toggling the impedance between two different states, i.e., conjugate match and some other impedance (Rao et al, 2005)

The tag antenna, together with the chip sensitivity, plays a key role in the RFID system operation, such as the reading range (VanBladel, 2002) and compatibility with the tagged object. In sum, the requirements for RFID tag antennas are the following (Foster & Burberry, 1999):

• Good impedance matching for receiving maximum signals from the reader to power up the chip;

• Insensitive to the attached object to keep performance consistent;

• Required radiation patterns (omnidirectional, directional or hemispherical);

• Pocket-size enough and depression profile to be attached to or embedded into the specified object (Rao et al, 2005);

• Robust in mechanical structure (since they could exist bent in some applications);

• Low toll in both materials and fabrication.

Antennas do not operate independently of nearby objects. On the opposite, these objects tin ruin the radiation properties of the antenna to different extent. In RFID systems, the material of the objects the tags are attached to should have minimum result on tag antenna behaviour, so that the reading performances of tags, such equally readable range and reading stability, do non modify. However, the performance of a tag antenna varies when it is mounted on different objects (Dobkin & Weigand, 2005; Clarke et al, 2006). On the i hand if the object surface is made of a dielectric material, and then the readable range is decreased due to frequency shift of the resonance frequency. On the other mitt, metallic objects which are commonly tagged in RFID applications seriously degrade the terminal impedance matching, bandwidth, radiations efficiency and readable range of the tag antenna. This is such a critical problem that global deployment of passive UHF RFID systems is being hindered by the performance deposition of tag antennas placed nearby metallic objects. As it has already said, in the vicinity of conductors, the antenna radiation parameters are modified; for case radiation efficiency is decreased. In improver, a metallic surface typically decreases the input impedance of the antenna (which makes that lower or non enough power can exist supplied to the IC flake, so the reading range is reduced or fifty-fifty the tag is not read at all) and varies its resonance frequency. The electromagnetic moving ridge is profoundly reflected by the conductor surface yielding a significant reduction of the RFID tag operating distance or its total malfunctioning (Dobkin& Weigand, 2005; Clarke et al, 2006; Rao et al, 2005). These negative effects are increased at higher frequencies and then, RFID operation in the SHF ring with tags attached to metallic objects presents an fifty-fifty more critical problem to be solved.

To overcome these bug and to obtain RFID tags usable with metallic objects, researchers have proposed different approaches:

1. To design novel antennas rather than dipole based antennas (with the inconvenient of large thickness or with shorting planes). As for example patch antennas (Ukkonen et al, 2006) that already take a metallic ground aeroplane but they show some shortcomings as narrow bandwidth and not negligible thickness. Another possibility that has been already explored are tag antennas using a planar inverted-F structure (Hirkonen et al, 2004; Kwon & Lee, 2005) that can operate well on metallic objects, since they already have large ground planes, but they accept several important drawbacks such every bit high cost and difficulty in manufacturing, because they require multiple shorting pins and a big ground plane, as well as thick dielectric substrates.

2. To use dipoles separated λ/iv from the metallic object (for example using foam, which leads to thick antenna designs and more circuitous manufacturing procedure)

3. The adoption of ferroelectric textile to insulate the tag from metallic (which is rather expensive).

4. To use Perfect Magnetic Conductors (PMCs) since they accept a +1 reflection coefficient with magnitude of 1 (in the platonic lossless example) and a phase of 0º. Then, they show in-stage reflection, which seems to exist a proper solution to the destructive interference problem when the antenna is placed very close to the metallic plate. Thus, the PMC can be used every bit a barrier between the antenna and the metallic plate in gild to electromagnetically insulate the antenna from the disturbing metal plate effects. For this reason, this approach is going to exist analyzed in this chapter. In addition, other advantages such as enhanced efficiency can be obtained equally a reward for the use of PMCs. PMCs do not be in nature and so they accept to be synthesised. For this reason they are known as Artificial Magnetic Conductors (AMCs) and behave equally PMCs over a sure frequency band.

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2. Design of AMC structures for different RFID frequency bands

An Artificial Magnetic Conductor (AMC) is dual to a Perfect Electric Usher (PEC) from an electromagnetic point of view. For design and analysis purposes, AMC condition is indicated by a reflection coefficient with magnitude of 1 (in the ideal lossless example) and a phase of 0º. The reflection phase on the AMC plane varies continuously from -180º to 180º related to the frequency and is naught at the resonance frequency. The useful bandwidth of AMC functioning is defined in the range from +90º to -90º, since in this range, the phase values would not cause subversive interference between direct and reflected waves (Sievenpiper, 1999; Sievenpiper et al, 1999). The surface impedance of an AMC is very high in its bandwidth of AMC performance, and then they are also known as High Impedance Surfaces (HIS).

A commonly used technique for AMCs implementation consists in using two-dimensional periodic metallic lattices patterned on a usher-backed dielectric surface, known as PEC-baked metallo-dielectric Frequency Selective Surfaces (FSSs) and too chosen Electromagnetic ring-gap (EBG) surfaces, as they take 1 or multiple frequency band-gaps in which no substrate mode can exist. However, in the absenteeism of via holes, the AMC and EBG frequency bands exercise not always coincide (Goussetis et al 2006). Their unique backdrop take been applied to pattern antennas with a improve proceeds and efficiency, lower sidelobes and backlobe level (Mosallaei & Sarabandi, 2004; Feresidis et al, 2005; Mantash et al, 2010a, 2010b). Several narrow band antennas, such every bit Microstrip patches and dipoles accept been mounted on these periodic structures in previous works (McVay et al, 2004; Liang & Yang, 2007; Zhu & Langley, 2009). With the aim of obtaining AMC designs that tin exist easily integrated in depression profile antennas and microwave and millimeterwave circuits, recent research efforts focus on the development of planar unilayer EBGs (in contrast to the use of multilayered FSS (Monorchio et al, 2002)) that do not need vias (Yang et al, 1999; Zhang et al, 2002; McVay et al, 2004; Kern et al, 2005). The primary drawback of using unilayer FSSs over a metallic ground aeroplane is the very narrow AMC operation bandwidth, due to EBGs' inherent resonant nature. In add-on, designing compact AMCs for frequencies below 1GHz equally those required in UHF RFID applications is by itself quite challenging and peculiarly when intended to be used for RFID tags due to their size and thickness restrictions.

Each AMC unit-cell can be seen as implementing a distributed parallel LC network having one or more resonant frequencies. The resonance frequency is where the loftier impedance and AMC conditions occur and for a parallel LC circuit is equal to 1 / (2π LC ) , while in-phase reflection bandwidth is proportional to Fifty / C .The resonance frequency and the bandwidth of an AMC depend on the unit-cell geometry together with substrate's relative dielectric permittivity and thickness. And so, information technology is necessary to increase L and reduce C in order to obtain a wider AMC functioning bandwidth. Lower frequency applications crave college L and/or C values. L tin exist increased using a thicker dielectric substrate and also including in the geometry narrow and long strips (lines). C can be reduced by reducing substrate'south relative dielectric permittivity ε_r and increasing the gap betwixt the metallization edge and the unit-cell edge (and so the gap between adjacent unit-cells). In club to obtain both meaty size and broad AMC functioning bandwidth a trade-off solution regarding ε_r and substrate thickness has to exist adopted.

With the aim of searching the frequency band in which the periodic structure behaves as an AMC, its reflection coefficient for a uniform incident plane wave is simulated, using Finite Element Method (FEM) together with the Bloch-Floquet theory, modelling a single cell of the structure with periodic boundary atmospheric condition (PBC) on its sides, resembling the modelling of an infinite structure (Sievenpiper et al, 1999; Yang & Rahmat-Samii, 2003). The periodic surface is chosen equally the phase reference plane. Normal plane waves are launched to illuminate the periodic surface using a waveport positioned a half-wavelength higher up it. The phase of the reflection coefficient of the AMC plane is compared to that of a PEC plane taken as reference, in the same manner as in (Sievenpiper et al, 1999).

The aim of this section is to testify an AMC construction design proper to be used for European UHF RFID frequency band tags and for 2.4GHz and five.8GHz SHF RFID frequency ring tags, using the aforementioned geometry for the AMC unit of measurement-cells and just changing the dielectric substrate and/or the unit-cell size. AMC structures for other UHF RFID bands can be hands obtained just by scaling the unit-jail cell metallization from the European UHF unit-cell design, and/or slightly scaling the whole unit of measurement-prison cell.

Unit of measurement cell size W (mm)	Thickness h (mm)	εr	BW (%)	Reso. freq (GHz)
16.93 ( λ/xx)	2.54 ( λ/136)	25.0	4.63	0.864
16.93 ( λ/7)	one.27 ( λ/98)	10.two	v.24	2.480
xi.52( λ/five)	0.81 ( λ/64)	3.38	seven.20	5.820

Table 1.

AMC Unit-cell design parameters and resulting resonance frequencies and bandwidths of AMC functioning.

Tabular array one shows the unit-cell dimensions and the dielectric substrate parameters to attain the indicated resonance frequencies and bandwidths of AMC performance. The 3 AMC designs apply commercial dielectric substrates: Transtech MCT-25 with relative dielectric permittivity ε_r=25 and loss tangent less than 0.001, Rogers RO3010 with ε_r=10.two and loss tangent 0.0035 and RO4003C with ε_r=3.38 and loss tangent less than 0.0027.

Effigy 1.

Simulation model and Reflection phase of the simulated AMC prototypes

The faux reflection phase of normally incident aeroplane moving ridge (field strength 1 Five/m) on the AMC surface versus frequency for the designed unit of measurement cell geometry with the three different dielectric substrates is shown in Fig. 1. The bandwidth of AMC operation increases with the thickness of the dielectric substrate merely decreases as the relative dielectric permittivity gets higher values. The iii presented designs (see Fig.1 and Table 1) show broad bandwidth using neither via-holes nor multilayered structures, which simplifies manufacturing process and reduces the costs.

Information technology is remarkable that the broad AMC functioning bandwidth of this specific unit of measurement prison cell geometry makes possible its combination with an antenna without significantly reducing the antenna bandwidth, which is the common drawback pointed out when dealing with AMC structures due to their inherent narrow bandwidth.

Another major concern on AMCs operation is related to their angular stability (Simovski et al, 2005). This can exist analyzed from two different points of view: the first assay is performed with regards to AMC operation under normal incidence condition when the polarization of the incident field is varied. The second assay is focused on the AMC performance under oblique incidence. Both of them are very important considering when combining the AMC with the antenna, the angular stability of the AMC will influence the antenna radiation performance and this will have direct touch on on the angular reading range of the final RFID tag depending on the position of the reader with respect to the tagged object. Following this, an AMC pattern with as higher angular stability every bit possible is desirable.

As pointed out in section 1, the negative effects of metallic objects in RFID tags are increased at higher frequencies and so the following discussions are going to be focused on an AMC to be used on v.8GHz SHF RFID frequency ring tags.

The reflection phase of the designed AMC surface has been faux for dissimilar incident field (E_inc) polarization angles (φ). The unit of measurement jail cell pattern symmetry makes possible the AMC to operate identically for any polarization of the incident field (assuming normal incidence), as shown in Fig. two. This also ways that reflection phase of both TE and TM polarizations of the incident wave volition exist identical for normal incidence.

Regarding AMC operation under oblique incidence, it can be extracted from Fig. 3 that resonance atmospheric condition are met within an athwart margin of θ_inc= ±58º (due to the unit cell design symmetry) for TE polarization. In this range the deviation of the resonance frequency is less than 1%. For higher incident angles the resonance frequency shifts to another band. Information technology is likewise remarkable that the AMC operation bandwidth decreases from 7.20% to iii.39% as the incident angle θ_inc is increased from 0º to +58º. However, the 5.8GHz frequency of involvement is inside the AMC operation bandwidth for all the incident angles in the θ_inc= ±58º angular margin. This means that there is almost a 120º angular margin in which the structure performs as an AMC at 5.8GHz. For TM polarization, the angular margin reduces to θ_inc= ±40º, the divergence of the resonance frequency is 6.83% and the AMC operation bandwidth is preserved. So in that location is a 80º angular margin in which the structure performs equally an AMC at five.8GHz for both Te and TM polarizations of the incident wave, which tin be considered as a very stable AMC structure.

Figure 2.

Simulated Reflection phase of the AMC surface for different incident field (E_inc) polarization angles φ=0º, 15º, 30º, 45º, 60º and 90º.

It is important to point out that angular stability under oblique incidence depends not only on the unit cell design geometry but also on the thickness of the dielectric substrate and on the unit of measurement prison cell size (periodicity) compared to the dielectric substrate thickness (Hosseini et al, 2006; Simovski et al, 2005).

Figure 3.

False Reflection phase of the AMC surface for TE (upwardly) and TM (downward) polarizations for different incident angles θ_inc=0º, º, 30º, 45º, 55º and 58º.

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3. Antenna on AMC to be used in 5.8GHz SHF RFID tags over metallic objects

Firstly, a miniature printed CPW-fed slot antenna (Lin et al, 2005) for operating in the 5.8GHz frequency band has been designed (meet Fig.4) using RO4003C, with ε_r=three.38, loss tangent less than 0.0027 and 0.813mm thickness, as dielectric substrate. A slot antenna has been chosen considering it volition provide wider bandwidth making easier the combination with the narrower bandwidth of AMC functioning. At that place is no metallic layer under the antenna dielectric substrate. This antenna has a simple structure with simply one layer of dielectric substrate and metallization.

The antenna dimensions together with fake return losses for the antenna are shown in Fig. 4. The faux operating bandwidth of the antenna (range of frequencies with S11-10dB) is ane.48GHz (22.0%).

Figure 4.

Return loss of the antenna; Geometry and dimensions; Manufactured prototype.

The faux antenna gain at five.8GHz is 5.0 dB with very small variation along the antenna bandwidth (meet Fig 5). The simulated E and H-plane radiation pattern in polar grade for the antenna at 5.8GHz are shown in Fig.five. Both the CP and the XP components are represented. The E-airplane radiations pattern is broadside and bidirectional. The H-plane radiation pattern is well-nigh omnidirectional.

Regarding the AMC arrangement with respect to the antenna, several ideas have been considered. The get-go one is that the AMC used as antenna ground plane would electromagnetically insulate the antenna from the metallic object, without disturbing the antenna performance. The 2d is to minimize the size of the last prototype and to facilitate manufacturing process.

Two AMC arrangements having respectively 5x5 and 5x4 AMC unit cells have been combined with the CPW-fed slot antenna and the resulting prototypes (encounter Fig. 6) take been tested in terms of return loss. In both cases the antenna is fixed to the AMC structure by a 0.1mm double sided non-conducting adhesive tape.

Effigy 5.

CPW-fed slot antenna fake radiation pattern (normalized, in dB) CP (dark-green) and XP (carmine) components for E plane (up, left) and H plane (upwards, correct). Three-dimensional imitation radiation design (downwards, left). Simulated antenna gain (down, right)

Prototypes of the antenna and the antenna on AMC have been manufactured using laser micromachining. The render losses of each manufactured prototype have been measured. As it tin be observed in Fig.4, the measured operating bandwidth of the slot antenna is ane.5GHz (24.0%), which is wider than the 1.48GHz (22.0%) obtained by simulation. The departure in bandwidth and the frequency shift could be due to manufacturing tolerances.

From the measurements results shown in Fig. half-dozen information technology can be concluded that although the antenna on 5x5 AMC shows better return loss results than the antenna on 5x4 AMC at some frequencies, both prototypes have the same operating bandwidth and the render loss of the antenna on 5x5 AMC is also proper. Then the increase of the prototype size due to the use of 5x5 unit of measurement cells is not profitable from the operation point of view. Taking this into account, the 5x4 AMC has been selected to be combined with the CPW-fed slot antenna.

The selected AMC arrangement in terms of a trade-off between performance and size is the one shown in Fig.7. The dimensions of the concluding construction, antenna on AMC (Fig. seven)), are Lp=57.60mm and Wp=46.08mm. The thickness is one.626mm in the function corresponding to the antenna on the AMC and 0.813mm in the role respective only to AMC unit-cells.

Effigy 6.

Manufactured prototypes of the antenna on 5x5 cells AMC, the antenna on 5x4 cells and the antenna (up). Return loss of the Antenna, the antenna on 5x5 cells AMC and the antenna on 5x4 cells (down).

Equally it could be expected, when placed on a metallic plate the antenna resonance frequency has been shifted out of the SHF RFID band leading to its total malfunctioning (meet Fig.4.). However, from Fig.7, it can be extracted that the antenna on AMC combination keeps the antenna operating properly in the whole antenna bandwidth, fifty-fifty when placed on a metal plate, as the AMC electromagnetically insulates the antenna from the metallic plate. The measured input return loss for the antenna on AMC prototype shows two resonances: the get-go one is due to the joint performance of the antenna and the AMC, since the AMC operation bandwidth starts at 5.625GHz (Come across Fig.1). Whereas the second resonance is due to an antenna resonance out of the AMC functioning bandwidth, since at that place is an additional RO4003C metallic-backed layer below the original antenna.

Co-ordinate to the measurements, metallic plates do non bear on the resonance frequency of the antenna on AMC. In add-on, the metallic plates practise not degrade the bandwidth of the antenna on AMC.

Figure vii.

Measured input return loss for the antenna and the antenna on AMC on a metallic plate; manufactured epitome.

The radiation blueprint of the manufactured prototypes has been measured in anechoic chamber (see Fig. 8). The prototypes are placed in the XY plane. The measured antenna radiation design is in very adept agreement with the simulated one, as can be concluded by comparison Fig 5 and Fig 10.

Every bit tin can exist observed in Fig 9, when the antenna is placed on the AMC, the maximum of the radiation pattern is displaced (the direction of maximum radiations changes). Nonetheless when the antenna on AMC epitome is stock-still over a metal plate, this maximum is preserved with respect to the antenna on AMC epitome. As could be expected, the dorsum radiation of the antenna on AMC is reduced with respect to the antenna prototype due to the in stage reflection properties of the AMC. And so despite the pocket-sized AMC construction, the antenna on AMC has a relatively low back radiation. Radiations blueprint properties of the Antenna on AMC for RFID application are still preserved even when placed on a metallic plate.

Prototype	Gain (dB)	Pattern directivity (dB)	Efficiency (%)
Antenna	4.2	6.3	59.8
Antenna on AMC	2.two	7.0	32.0
Antenna on AMC over metallic plate	3.8	10.0	22.7

Table two.

Measured gain, directivity and radiations efficiency of the manufactured prototypes.

Figure viii.

Measurement set-up in anechoic bedchamber. Antenna measurement (left) and antenna on AMC over metallic plate measurement (correct).

Figure 9.

Iii-dimensional representation of the normalized measured radiations pattern for the three manufactured prototypes: antenna (left), antenna on AMC (heart) and antenna on AMC over metallic plate (right)

In improver, the gain of the antenna on AMC fixed over a metallic plate virtually preserves with respect to the gain of the antenna alone equally it is shown in table ii, which represents a significant achievement.

In general, when placing an antenna on an AMC radiation properties such equally gain and radiations efficiency are enhanced with respect to the antenna lonely. This is due to the fact of using the AMC equally a footing plane for the antenna substituting a conventional metallic ground aeroplane i.east. antenna topologies that already have a metal ground plane under the antenna metallization, such as microstrip patch antennas. As pointed out in department 1, these antennas can perform well with metallic objects but have narrow bandwidth and not negligible thickness. Other approaches combining antennas without metal layer under the dielectric substrate (such equally CPW-fed antennas) with AMCs for gain enhancement purposes, separate the antenna from the AMC by using an additional layer of foam. This likewise increases the antenna thickness which is not convenient in RFID applications. However, the slot

Figure ten.

Measured radiation pattern (normalized, in dB) of antenna, antenna on AMC and antenna on AMC over metallic plate. Planes φ = 90º (YZ-plane) and φ = 0º (XZ-plane).

antenna presented here has no metallic layer nether the dielectric substrate and so when placing the AMC directly under the antenna to electromagnetically insulate the antenna from the metallic object, the antenna operation is slightly disturbed to some extent in terms of gain and radiations efficiency (see table Two), whereas the obtained prototype exhibit proper operation over metallic objects and the gain is almost preserved compared to the antenna operating alone. All these backdrop are suitable for RFID application.

Another possibility tested to try to obtain enhanced efficiency (or at least preserved it) is to remove the AMC'southward unit of measurement cells beneath the antenna but this significantly reduces the antenna bandwidth every bit tin can be seen in Fig. 11 and the resonance at 5.8GHz disappears. For this reason the use of this organization has been declined. The only resonance that appears is at 6.45GHz and it is due to antenna having and additional dielectric substrate layer and a metallic ground plane bellow this dielectric substrate layer.

Also the antenna could exist centred on the AMC organization which might preserve and/or raise the radiation properties of the antenna, but this would crave changing the antenna feeding increasing the complexity of the prototype and also its price. The aim of this affiliate it is to show that it is possible to obtain a compact, low profile and low cost antenna on AMC combination proper to be used over metallic objects.

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4. Conclusion

A novel CPW-fed-slot antenna on AMC combination prototype suitable to be used in five.8 GHz RFID tags on metal objects has been presented. It has been shown that metallic plates

Figure 11.

Measured input return loss for the antenna and the antenna on AMC when the unit cells under the antenna are removed.

do not affect the resonance frequency of the antenna on AMC. In addition, the metallic plates exercise not degrade the bandwidth of the antenna on AMC.

As a reward for the AMC addition, the manufactured prototype, using a sparse and low dielectric permittivity commercial substrate, exhibits proper operation both alone and when placed on a metallic plate.

The presented CPW-fed-slot antenna on AMC combination meets near of the RFID tag antennas requirements pointed out in department one. Further inquiry is existence carried out to obtain a paradigm in a bendable dielectric substrate.

By using the other presented AMC designs for UHF and two.4GHz SHF with antennas operating at those frequency bands, problems related to RFID tags performance with metallic objects tin be overcome.

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Acknowledgments

Authors thanks Ramona C. Hadarig and Dr Yuri Álvarez for their comments and useful discussions. This work has been supported past the "Ministerio de Ciencia e Innovación" of Spain /FEDER" under projects TEC2008-01638/TEC (INVEMTA) and CONSOLIDER CSD2008-00068 (TERASENSE), by PCTI Asturias under project, PEST08-02 (MATID) and past the Principado de Asturias/FEDER Project IB09-081 (CAMSILOC).

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Written By

María Elena De Cos and Fernando Las-Heras

Submitted: Oct 15th, 2010 Reviewed: April fourth, 2011 Published: July 20th, 2011

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