Nicolas P Rougier
email: nicolas.rougier@inria.fr

Konrad Hinsen
email: konrad.hinsen@cnrs.fr

ReScience C: a Journal for Reproducible Replications in Computational Science

Keywords: Open science, Computational science, Reproducibility

Independent replication is one of the most powerful methods to verify published scientific studies. In computational science, it requires the reimplementation of the methods described in the original article by a different team of researchers. Replication is often performed by scientists who wish to gain a better understanding of a published method, but its results are rarely made public. ReScience C is a peer-reviewed journal dedicated to the publication of high-quality computational replications that provide added value to the scientific community. To this end, ReScience C requires replications to be reproducible and implemented using Open Source languages and libraries. In this article, we provide an overview of ReScience C's goals and quality standards, outline the submission and reviewing processes, and summarize the experience of its first three years of operation, concluding with an outlook towards evolutions envisaged for the near future.

Introduction

The question of how to attain reliable outcomes from unreliable components pervades many aspects of life. Scientific research is no exception. Individual research contributions are prone to mistakes, and sometimes fraud, and therefore error detection and correction mechanisms are required to reach a higher level of reliability at the collective level. The two main methods for error detection are critical inspection, starting with peer review of article submissions but continuing well after publication, and independent replication of published work. But replication is more than a verification technique. For the researchers performing the replication, it yields a level of understanding and insight that is impossible to achieve by other means. This is in fact the main motivation for much replication work, verification being merely a side effect.

The power but also the limitations of replication as an approach to verification are best illustrated by the recent discussion of replication crises in various scientific domains [START_REF] Ioannidis | Why Most Published Research Findings Are False[END_REF][START_REF] Baker | 1,500 scientists lift the lid on reproducibility[END_REF][START_REF] Munafò | A manifesto for reproducible science[END_REF][START_REF] Iqbal | Reproducible Research Practices and Transparency across the Biomedical Literature[END_REF], which are all based on the observation of frequent failures to replicate published scientific findings. However, a replication failure does not necessarily mean that the original study is flawed. First of all, it could well be the replication work that is at fault. But it is also possible that both the original and the replication work are of excellent quality and yet yield different conclusions, if some important factor has escaped everyone's attention and accidentally differs between the two studies (see [START_REF] Smart | The war over supercooled water[END_REF][START_REF] Palmer | Comment on "The putative liquid-liquid transition is a liquid-solid transition in atomistic models of water" [I and II[END_REF] for a recent example that led to a seven-year search for the cause of the disagreement). In this situation, independent replication can become the starting point of completely new lines of research.

Replication is thus an important contribution to science, and its findings should be shared with the scientific community. Unfortunately, most journals do not accept replication studies for publication because originality is one of their selection criteria. For this reason, we launched ReScience in 2015 (now called ReScience C for reasons explained later) as a journal dedicated to replications of computational research. In this article, we outline its mode of operation and summarize our experience from the first few years. A more complete account, also containing more background references, has been published recently [START_REF] Rougier | Sustainable computational science: The ReScience initiative[END_REF].

Terminology: reproducible replications

The replication crisis has given rise to an active debate in various domains of science, in which some terms, in particular "reproducible" and "replicable", are used with very different meanings. We therefore explain the definitions that we are using in this article and more generally in ReScience C. Our definitions are formulated in the specific context of computational science, and are not easily transferable to experimental science [START_REF] Hinsen | Scientific software is different from lab equipment[END_REF].

A computation is reproducible if the code and input data is available together with sufficient instructions for someone else to re-do or reproduce the computation. The only point in reproducing the computation is to verify its reproducibility, which in turn is evidence that the archived code and data is (1) complete and (2) indeed the code and data that was used in the original published study. A failed reproduction means that the description of the original code and data is incomplete or inaccurate. A frequent form of incompleteness is the lack of a detailed description of the computational environment, i.e. the infrastructure software (operating system, compiler, ...) or code dependencies (libraries, ...) that were used in the original work. Reproducible computations are the most detailed and accurate possible description of a computational method within the current state of the art of computational science.

A replication of computational work involves writing and then running new software, using only the description of a method published in a journal article, i.e. without using or consulting the software used by the original authors, which may or may not be available. Successful replication confirms that the method description is complete and accurate, and significantly reduces the probability of an error in either implementation. A replication failure can be caused by such errors or by an inexact or incomplete method description. It requires further investigation which, as explained above, can even lead to new directions of scientific inquiry.

A reproducible replication is a replication whose code and data has been archived and documented for reproducibility. It is especially useful in the still dominant situation that the target of the replication was not published reproducibly. In that case, the replication provides not only verification, but also the missing code and data.

ReScience C

The definition of a replication given above should be sufficient to show that performing replications is a useful activity for a researcher. Moreover, whether successful or not, a replication yields additional insight into the problem that are worth sharing with the scientific community. For example, minor omissions or inaccuracies are inevitable in the narratives that make up for most of a journal article, meaning that replication authors have to do some detective work whose results are of use to others.

Unfortunately, the vast majority of scientific journals would not consider such work for publication, with the possible exception of a failed replication of particularly important findings, because novelty is for them an important selection criterion. Moreover, the reviewing process of traditional scientific journals, designed in the 20th century for experimental and theoretical but not for computational work, cannot handle the technical challenges posed by a verification of reproducibility and successful replication. For these reasons we created the ReScience C journal (at the time called simply ReScience) in September 2015 as a state-of-the-art venue for the publication of reproducible replication studies in computational science.

The criteria that a submission must fulfill for acceptance by ReScience C are the following:

-It must aim at reproducing all or a significant part of the figures and tables in an already published scientific study.

-The text of the article must discuss which results were successfully replicated and which, if any, could not be replicated. It should also provide a description of problems that were encountered, e.g. additional assumptions that had to be made. -The complete source code of the software used for the replication must be provided, and should have only Open Source software as dependencies in order to allow full inspection of the complete software stack. -In order to ensure the independence of the replication, its authors should not include any authors of the original study, nor their close collaborators.

A newly submitted replication is assigned to a member of the editorial board, which at this time is composed of 12 scientists from different research domains. The handling editor recruits two reviewers from a pool of currently nearly 100 volunteers. The reviewing process consists of a dialog between the reviewers, the authors, and the handling editor whose goal is to improve the submission to the point that it can be accepted. In particular, the reviewers verify that they can reproduce the results from the supplied code and data, and judge if the replication claims made by the authors are valid subject to the criteria of their scientific domain. The entire reviewing process is openly conducted on the GitHub platform, meaning that contributions are open to read for anyone, and anyone with a GitHub account can participate by leaving a comment. Once the submission is deemed acceptable, it is added to the table of contents and to the ReScience archive, with links to the submission repository, the review, and a PDF version which permits the article to handled like a standard scientific paper in personal and institutional databases and bibliography management software. An additional copy is deposited on Zenodo [1], which, being an archiving platform, makes stronger promises about long-term preservation than GitHub, whose primary goal is to support dynamic development processes. An additional advantage is that Zenodo issues a DOI that serves as a persistent reference.

The outstanding feature of this reviewing process, even compared to other journals practicing open peer review, is the rapid interaction between reviewers and authors that does not require the constant intervention of the handling editor. This rapid exchange has turned out to be essential in the quick resolution of the technical issues that inevitably arise when dealing with software and data.

Another outstanding feature of ReScience C is its reliance on no other infrastructure than two digital platforms, GitHub and Zenodo, which are both free to use. Considering that editors and reviewers as well as authors are unpaid volunteers, this means that ReScience C has so far been able to operate without any budget at all, and thereby avoid being subjected to any political pressure. We note however that this may not always be true for the individual volunteers contributing to ReScience C because the open reviewing process provides no anonymity. It is therefore imaginable that authors or reviewers of a ReScience article pointing out a mistake in prior work by an influential scientist could be exposed to sanctions by that scientist in grant or tenure reviews.

4 Learning from the past to prepare the future After three years of operation, our original ideas for ReScience C have turned into concrete practical experience which has mostly confirmed our expectations. It has also shown a few weaknesses, most of which concern technical details, which we are currently addressing in an overhaul of the ReScience C publishing workflow. In the following we summarize this experience and the conclusions we have drawn from it, referring to the full account [START_REF] Rougier | Sustainable computational science: The ReScience initiative[END_REF] for the details.

ReScience C has so far published 27 articles. Most submissions are from computational neuroscience, the other represented domains are neuroimaging, computational ecology, and computer graphics. No submission was ever rejected.

All submitted replications were successful, but this is probably due to a selection bias: publishing a failed replication is equivalent to publicly accusing the authors of the target work of having made a mistake, which is a potential source of conflict. One idea we have put forward to alleviate this obstacle is pre-publication replication. In that scenario, researchers submit their original work to a new type of journal, for which we use the name CoScience to indicate that we imagine it as the successor of ReScience. The journal then invites other scientists to do a replication, and publishes the original work and the replication together as a single joint work by the original authors and the replication team.

Achieving reproducibility has been much more challenging than expected. It is the reviewers' task to verify reproducibility, but our experience has shown that this is not sufficient to ensure that someone else can reproduce the work as well. Reviewers typically work in the same field as the authors and are likely to have similar software installation on their computers, meaning that unlisted dependencies can easily go unnoticed. There are a few approaches that would improve reproducibility, but each has its downsides as well. IPOL [?] provides online execution via its Web site, which is extremely convenient for both reviewers and readers. However, it is feasible only because IPOL's narrow domain scope (signal processing) makes the restriction to a small number of computational environments (C/C++, Python, Matlab) acceptable. We could also impose higher technical reproducibility guarantees on authors, e.g. the submission of an archived environment in the form of a virtual machine or a container image, which would also open the door to online execution via services such as Binder [?]. Such a requirement might, however, also become an additional barrier discouraging researchers from publishing their replication work.

The open reviewing process has overall worked very well. The exchanges between reviewers and authors have been constructive and courteous without exception. The handling editor intervenes mainly at the beginning, by inviting reviewers, and at the end, by judging if the reviewers' feedback is satisfactory for publication. Occasionally, reviewers or authors ask the handling editor for help with specific, mostly technical, issues. Another common task for the handling editor is to gently nudge authors or reviewers towards completing their tasks within reasonable delays. It is rare for third parties to intervene, but in one case a reviewer suggested asking the author of the target study for the permission to re-use some data, which he did by commenting directly on the GitHub platform.

An unexpected and so far unresolved consequence of the open reviewing process is the impossibility to handle replications that process confidential data. In some fields of science, confidentiality is inevitable, be it for ethical reasons (e.g. in medical research) or for commercial ones (e.g. data on stock market transactions not freely available). This is an issue of wider concern for the Open Science community, and we hope that satisfactory solutions will emerge in the near future.

The use of the GitHub platform has turned out to be a good choice overall. Since a ReScience C submission combines a narrative and source code, with the code taking center stage during the reviewing process, a platform designed for collaborative software development and code reviewing is a better match than the traditional manuscript management platforms used by scientific journals, which have no provision at all for reviewing code. We are, however, currently revising several technical details. Submissions currently take the form of a pull request to the ReScience repository, which is counterintuitive for an article submission. More importantly, the final steps of publishing in our current workflow are laborious and not automated, causing too much hassle mainly for the handling editor. In the future workflow, articles are submitted as individual repositories of which ReScience retains a fork upon acceptance.

Finally, an evolution that has motivated the name change from ReScience to ReScience C is the imminent launch of ReScience X, a new journal dedicated to replications of experimental research, under the auspices of Etienne Roesch and Nicolas Rougier. We hope that it will be able to profit from the experience gained with ReScience C, although the challenges it will face are of a quite different nature. ReScience C will continue to focus on improving computational research, joining forces with the wider Reproducible Research community wherever possible. For example, we envisage proposing the publication of dedicated issues to reproducibility-related workshops such as the Reproducible Research on Pattern Recognition workshop [?] (part of the International Conference on Pattern Recognition) or the Enabling Reproducibility in Machine Learning workshop (part of the Internatiponal Conference on Machine Learning).


Marjorie Grzeskowiak
email: marjorie.grzeskowiak-lucas@u-pem.fr

Antoine Diet
email: antoine.diet@geeps.centralesupelec.fr

Megdouda Benamara

Stéphane Protat

Christophe Conessa

Marc Biancheri-Astier
email: marc.biancheri-astier@u-psud.fr

Franciso De Oliveira Alves

Yann Le Bihan

Gaelle Lissorgues

Coaxially Distributed Diameter sub-Coil Twisted Loop Antenna in HF RFID

Keywords: HF RFID, coil, near field, magnetic coupling. I

This paper proposes an HF (High Frequency) transmitting coil less sensitive to the angular and position misalignments of the small receiving coil. The DDC (Distributed Diameter Coil) shape and TLA (Twisted Loop Antenna) allow respectively minimizing the disturbance of the magnetic link due to the lateral misalignment and the relative tilting direction of the transmitting coil to the receiving coil. The magnetic coupling link obtained from DDC TLA coils is illustrated by comparison with conventional TLA in the case of HF RFID.

INTRODUCTION

In RFID (IDentification by RF communication) and HF WPT (Wireless Power Transfer), higher magnetic flux, proportional to induced current on small receiving coil, provides wider useful operating plane to activate a receiving system [START_REF] Finkenseller | RFID Handbook: Radio-Frequency, Identification Fundamentals and Application[END_REF]. In RFID applications, the transponders responses are detected by the interrogator as load modulations of the induced field, while for the WPT applications [START_REF] Heebl | Comprehensive Analysis and Measurement of Frequency-Tuned and Impedance-Tuned Wireless Non-Radiative Power-Transfer Systems[END_REF], the budget link is determined versus the energy efficiency [START_REF] Rajagopalan | Improving Power Transfer Efficiency of a Short-Range Telemetry System Using Compact Metamaterials[END_REF]. In the both cases, stronger H-field enhances the power for the receiving hardware at the required read-out distance. With the miniaturization of receiving coils [START_REF] Zierhofer | Geometric approach for coupling enhancement of magnetically coupled coils[END_REF], it is a challenge to power a receiving system whose effective area is relatively smaller in comparison with the surface of the transmitting coil. These RFID and WPT applications would like to activate area of receiving systems whatever their angular and position misalignments versus the transmitting coil [START_REF] Fotopoulou | Wireless Power Transfer in Loosely Coupled Links: Coil Misalignment Model[END_REF], [START_REF] Kim | Free-positionning Wireless Power Transfer using multiple coupling coils in a transmitter[END_REF]. The objectives in this paper are two-fold: sizes of the receiving coil in comparison with the transmitting coil, and angular /position misalignments. Design of a planar transmitting coil allowing the modification of the magnetic field (intensity and orientation) could be suitable for a HF RFID reader antenna. This later is installed under a movable tray, on which are placed tagged arbitrary oriented objects.

To enhance the magnetic coupling with a small and arbitrary oriented receiving coil, DDC (Distributed Diameter Coil) shape [START_REF] Diet | Improvement of RFID HF tags detection with a distributed diameter coil, Antennas and Wireless Propagation Letters[END_REF] that favors the magnetic link with a small misaligned coil, is inserted in the TLA (Twisted Loop Antenna) [START_REF] Benamara | Twisted Loop Antenna for HF RFID detection surface[END_REF]. The goal is to improve the magnetic coupling when the coil is coplanar to the axis of the TLA plane.

First, the authors describe the studied transmitting coils. In a second part, the performance of the system in magnetic coupling with a small receiving coil is presented: the measured values of mutual inductance between the transmitting and receiving coils are compared with the simulated one as function of both position misalignment and distance for parallel and normal positions of the receiving coils over transmitting area. In the last part, RFID set-ups illustrate the activate area of the receiving coils with the detection distances for two sizes of tags and for the both orientations.

II. THE DDC TLA STRUCTURE

The TLA [START_REF] Benamara | Twisted Loop Antenna for HF RFID detection surface[END_REF] and DDC [START_REF] Diet | Improvement of RFID HF tags detection with a distributed diameter coil, Antennas and Wireless Propagation Letters[END_REF] are mixed to enhance respectively the magnetic field coupling whatever the orientation and position of the receiving coil versus the transmitting coil. In the TLA structure, the two coils are complementary and for each sub-coil, the direction of the current is assumed to be equal in all the turns.

In the conventional coil, the sub-coil diameter of the transmitter antenna is 4.5 wider than the receiving coil, while in the second and third structures, in keeping constant the self-inductance of the DDC TLA coil, a second and third sub-coil are respectively added. The third sub-coil presents a diameter comparable to the receiving coil, while the second one is intermediate between the upper and downer diameters. It should be noted that the coupling with the receiving coil, was calculated without matching the coils to source and load impedances. The S-parameter won't be done in order to appreciate the effect of variations in magnetic flux on the RFID and WPT systems.

The transmitting coils are designed with the same inductance value 𝐿 ! and the quality factor Q is in the same order to generate an equivalent magnetic energy. Three combinations of turns with diameters in centimeter 𝑑 ! , 𝑑 ! and 𝑑 ! , are evaluated empirically in neglecting the mutual coupling between each of sub-coils and in summing the inductance 𝐿 ! ! ! , expressed in nH, corresponding to each subcoil of diameter 𝑑 ! of N i turns and used in Ib Technology UK [9].

𝐿 ! ! ! = 2𝜋𝑑 ! 𝑙𝑛 !! ! ! ! 𝑁 !.! ; 𝐿 ! = 𝐿 ! ! ! ! + 𝐿 ! ! ! ! + 𝐿 ! ! ! ! (1)
If we consider the mutual inductance between pair of coaxial sub-coils, we can calculate the mutual inductance 𝑀 !,! for N ! and N ! turn numbers of the considered subcoils.

In the case of perfect alignment of coplanar loops, the mutual inductance 𝑀 !,! [START_REF] Zierhofer | Geometric approach for coupling enhancement of magnetically coupled coils[END_REF] is

𝑀 !,! = 𝑁 ! 𝑁 ! µμ ! 𝑎𝑏 ! К -К 𝐾 К - ! К 𝐸 К (2)
Where The external diameters d 1 , d 2 and d 3 are respectively set to 4.5 cm, 2.5 cm and 0.45 cm: the A (3d 1 -0d 2 -0d 3 ) coil, with each number corresponding to the number of turns of the associated diameter, correspond to the conventional coil in [START_REF] Heebl | Comprehensive Analysis and Measurement of Frequency-Tuned and Impedance-Tuned Wireless Non-Radiative Power-Transfer Systems[END_REF] and B (2d 1 -3d 2 -0d 3 ), and E (1d 1 -3d 2 -7d 3 ) coils to evaluate the impact on the DDC profile. The width and the depth of the slot are respectively equal to 2mm and 5 mm. The empirical value of the DDC TLA inductance is reported and compared with the simulated and the measured ones in the table 1. We consider the internal diameters of the coils to calculate the self-inductances. Electrical characteristics (empirical, simulated and measured) of the DDC TLA coils. In table I, empirical, simulated and measured inductances for each coil are compared, and show an important difference with the measured one, that can be explained by length of wires to join the sub-coils and to feed the coils by SMA connector in realized structure. For the practical one, enamel thickness, due to uncertainty, doesn't correspond to the width of the strip. We can precise the neglecting of the conductor losses because of the null thickness of the strip. The electromagnetic simulation has been implemented with PEC strip, the aim is not accuracy but to understand the trend of variation of magnetic coupling with changing physical coil parameter. The magnetic field is displayed (Fig. 2) in the 𝑦0𝑧 plane on the left side for the H-field vector and in the 𝑥0𝑦 plane for the H-field magnitude for each DDC TLA structure. While the orientation of the H-field vector seems similar for each structure, the distribution of the magnitude is different and the contribution of the different diameter can be observed. The displaying is useful to illustrate the magnetic field but to see clearly the magnetic coupling with a small receiving coil, and compare the structure between themselves, it is necessary to use a quantified parameter, as the mutual inductance.

К = !!" !!! ! ! ! (3)

III. MUTUAL INDUCTANCE

The above studies have been tested using FEM (Finite Element Method) with the commercial software Ansys HFSS. The comparison of the different DDC TLA structures versus the inductive coupling between the coils, is possible on the simulated software, and compared with the measured ones, thanks to the experimental setup defined in figure 3. The simulation was performed with the assumption that the coils aren't matched to the 50Ω ports. The simulated and measured values of the DDC coil are reported on the table 1, the experimental spiral loop presents an impedance equal to 10+j311 (L C =3.6 µH), while in simulation it is calculated to 0.31+j2.67 (L C =0.0313 µH). The structure of receiving coil is it is a spiral coil on both sides of the FR4 substrate in the experimental case while it corresponds to a single coil in the simulated study. In this setup, DX relative location of the DDC TLA transmitting coil to the receiving coil and DZ height are varied in VM (and HM) modes, when the TAGs surface is perpendicular (and parallel) to this one of the DDC TLA surface. The center of the coil tested is the origin. DY will not be used and only positive values for DX are considered for reasons of symmetry. We report, at the height DZ=10mm, the mutual inductance M versus DY variation in simulation (Fig. 4) and in measure (Fig. 5) and the Figures of Merit (Fig. 6 and Fig. 7), to evaluate the improvement of the Coupling Capability (2) and the peak of mutual inductance. The CC FoM, by means of the integral of the mutual inductance M(y) absolute value along the dY axis (Fig. 1), allows to estimate the detection of tag whatever its orientation, while the peak of mutual inductance characterizes a maximum of detection range at one point. The improvement of these both parameters can permit to check the best structure to feed a tag by magnetic coupling. The aim is to increase the distance and the area ranges. HM mode in Fig. 2 VM mode in Fig. HM mode in Fig. 2 VM mode in Fig. The comparison of the different DDC TLA structures as efficient WPT transmitting coils, is possible with a parametric study on the simulated software versus the height d=DZ for each HM and VM mode (Fig. 6). We have reported the mutual inductance M, the coupling is less sensitive to vertical displacement for B coil that keeps a greater value M than the other structures in HM mode. Each On-center structure is loaded with a capacitor 𝐶 // to resonate around 13.56MHz in parallel with a resistance 𝑅 // to equalize the quality factor 𝑄 !"#! of the DDC On-center structure around 25. In order to analyze the frequency impact, dB 20 (magZ 12) is reported versus the misalignment and the Z bandwidth is calculated in 3 dB frequency-band. Weaker coupling between the transmitting coil and small receiving coil could result in a frequency shift (less than 1%) of the trans-impedance. The bandwidth of the transimpedance of the links at the resonant frequency is slightly affected by the lateral misalignment. However, table II shows that the Z bandwidth seems constant with the different DDC TLA. Furthermore, larger the value of M, higher will be the efficiency, with a kept constant bandwidth frequency and negligible frequency shift. The measured quality factor (𝑄 !"#! ) is reported in table II. The values of 𝑄 !"# are impacted by the inter-turns capacitive effects in HF and by the wire losses whose length that are respectively 40.52 (A), 48.69 (B), and 40.69 (E) cm: a parallel resistance is added on the DDC TLA to equalize the quality factor 𝑄 !"#! after tuning.

𝐶𝐶 = 𝑀(𝑦) 𝑑𝑦 ! ! ! ! (2)
To validate the improvement of the mutual inductance with a very weak frequency shift in keeping constant the frequency bandwidth, we realize RFID detection as used case with the DDC On center and two types of tag.

IV. RFID VOLUME DETECTION

RFID read-out distance illustrates the impact of the different DDC TLA on the power transfer efficiency and characterizes the magnetic coupling. The comparison concerns structures with almost the same values for 𝐿 ! and Q (tables I and II). The RFID tags have different diameters (2cm for Mifare chip (Fig. 8)[11] and 0.7cm for NXP chip (Fig. 9) [12]) and are in the same order than respectively the intermediate (d 2 =2.5 cm in Fig. 1) and small sub-coils diameter (d 3 =0.45 cm in Fig. 1).

The tag is placed in an area where the read-out distance is optimal, i.e. above the center of the sub-coils in HM mode and in the center of the DDC TLA antenna in VM mode. In VM and HM modes, we can notice an enhancement of performances of the E structure for the smaller tag in comparison with the wider one while the inverse phenomenon is observed for the A structure. This improvement is probably due to the similar geometry between the sub-coil and the tag diameters. Parametric studies on the tag diameter should be done to confirm the previous assumption and to explain the improvement of the B structure with wider (and supposed less efficient) tag in VM mode. V. CONCLUSION Symmetrical sub-coil to WPT and RFID systems allow changing of the distribution and orientation of magnetic field. The studied parameters to optimize the read-out distance whatever the orientation of the receiving coil over a large area are respectively the mutual inductance M and the coupling capability CC, with a receiving coil surface corresponding to one-fortieth of the transmitting coil. This design approach results in an enhancement of the mutual inductance and of the coupling capability, while achieving weak resonant frequency shift (less than 1% at 10mm above the transmitting coil) and keeping constant the trans-impedance bandwidth frequency versus the relative position of the receiving coil. The effectiveness of the optimized design is proved by the results of RFID detection range in the close regions of antenna. This study could be applied to free-positioning small RFID tags placed on arbitrary oriented objects for instance on a movable tray. A parametric study on the diameter of the receiving coil could be realized to compare the impact on the mutual inductance and coupling capability, and RFID detection would be measured with the same RFID chip to validate the enhancement of the selected coupling parameters M and CC. Future work will consider studies on asymmetrical DDC sub-coil shapes for finding critical parameters impacting the efficiency of the magnetic coupling.

a

  and b are the radii of the coaxial sub-coils, 𝐾 К and 𝐸 К respectively the complete elliptic integrals of the first and second kind. N ! and N ! are the turn numbers of the considered sub-coils diameter. The value of the self-inductance of the empirical inductance 𝐿 !! is calculated in considering (1) and (2) with enameled copper wire having an external diameter of 0.022 cm, whose enamel thickness is known to be around 5 to 50 um. The value of the copper wire diameter is comprised between 0.012 cm and 0.021 cm, and the 𝐿 !! respectively between 𝐿 !!!"# and 𝐿 !! !"# . The simulated inductance 𝐿 !! is carried out using HFSS [HFSS] (High Frequency Structure Simulator [10]). The metalized wire of the coil is replaced by copper boundaries on strip of 1µm width without thickness, to reduce the time process. The measured inductance 𝐿 !! is measured on a Rhode Schwarz ZNB8 VNA. The diameters are set in the fabricated 3-D printing base, represented in figure1.

Fig. 1 .

 1 Fig. 1. Layout for 3D fabricated plastic base (d1=4.5cm, d2=2.5cm, d3=0.45cm)

Fig. 2 .

 2 Displaying of the magnetic field

Fig. 3 .

 3 Fig. 3. Experimental setup with DX and DZ variation in millimeter Fig.4 and Fig.5 depict respectively the mutual inductance M between the DDC TLA and the small coils as a function of the DY misalignment for the simulated and measured results.

2

 2

Fig. 4 .

 4 Fig. 4. Simulated mutual inductance M versus lateral misalignement DY at height DZ=10 mm The simulated and measured values are different because the structure of receiving coil is different, but the behaviour of mutual inductance is the same one and confirms the enhancement of the B structure in HM mode, and the

2

 2

Fig. 5 .

 5 Fig. 5. Measured mutual inductance M versus lateral misalignement DY at height DZ=10 mm

Fig. 6 .

 6 Fig. 6. M versus d (from 10mm to 50 mm by 10mm step) in HM and VM modes

Fig. 7 .

 7 Fig. 7. RFID experimental setup

Fig. 8 .

 8 Fig. 8. Dectection Range in HM and VM modes for 2cm diameter tag [11]

TABLE I

 I

	.	ELECTRICAL CHARACTERISTICS (EMPIRICAL, SIMULATED
	AND MEASURED) OF THE DDC TLA COILS
	DDC TLA	A	B	E
	L1E(µH)_min	2.068	2.126	1.6658
	L1E(µH)_max	2.19	2.248	1.781
	L1S(µH)	1.94	1.98	1.77
	R1S(Ω)	2.2	2.2	3.4
	L1M(µH)	2.38	2.79	2.26
	R1M(Ω)	3.1	4.4	3.4

TABLE II .

 II ELECTRICAL CHARACTERISTICS OF THE DDC ON-CENTER (𝐶 // , 𝑅 // , Q) AND MINIMUM AND MAXIMUM VALUES OF FREQUENCY BANDWIDTH OF THE TRANS-IMPEDANCE VERSUS THE DY MISALIGNMENT

	DDC TLA	A	B	E
	𝐶 // (pF)	70	64	78
	𝑄 !"#! =	𝛥𝐹 𝑓 !	79.8	79.5	43.3
	𝑅 // (kΩ)	6500	6500	9000
	𝑄 !"#! =	𝛥𝐹 𝑓 !	25.6	25.03	25.52
	Z BP_min (MHz)	490	500	520
	Z BP_max (MHz)	530	520	530
	𝑄 !"#! =	𝛥𝐹 𝑓 !	32	25	35
	𝑄 !"#!	25	25	25

ACKNOWLEDGMENT

This work was supported by the Université Paris-Est thought a visiting period of 6 months accorded to M.Grzeskowiak in the laboratory GEEPS (CNRS, CentraleSupelec, Paris-Sud, UPMC).


M Grzeskowiak

M Benamara

P Poulichet

P Protat

G Lissorgues

Esycom Ea

A Diet

M Biancheri-Astier

Y Le Bihan

C Conessa

F Alves

Sub-coil in reader antenna for HF RFID volume detection improvment

Keywords: HF RFID, Distributed Diameter Coil, Twisted Loop Antenna, off-center subcoils, volume detection, read range improvement I

This paper proposes a reader antenna, improving the inductive coupling with a small coil whatever its position or orientation against the reader antenna. Sub-coils are inserted and the turn number is optimized in order to improve efficiency of the inductive coupling in the case of HF RFID volume detection in keeping constant the magnetic energy generated by the reader.

INTRODUCTION

In 13.56 MHz RFID (Radio Frequency IDentification) or in inductive WPT (Wireless Power Transfer), the received power is affected by the distance and by the angular misalignment between the coils. Futhermore, greater the size ratio between the coils is, weaker is the received power. In the case of RFID, the reader input impedance depends on the magnetic coupling and on the induced field in the transponder [START_REF] Finkenseller | RFID Handbook: Radio-frequency Identification fundamentals and application[END_REF]. Although, stronger magnetic coupling could result in frequency shift: the transponder could be not detected. The objectives will be to improve the magnitude of the reader input impedance in minimizing its phase shift.

In this study, sub-coils [START_REF] Diet | Improvement of RFID HF tags detection with a distributed diameter coil[END_REF] in complementary coils [START_REF] Benamara | Twisted Loop Antenna for HF RFID detection surface[END_REF] are combined to improve the RFID detection for a small tag (in comparison with the size of the reader coil). The complementary coils allow detection of a tag placed perpendicularly to the reader plane (defined as VM mode), not only on the edges of the loop but also in the center of the structure. The sub-coils increase the magnetic coupling when the tag surface and this of the reader surface are parallel (HM mode) and perpendicular. Preliminary studies on coaxially sub-coil in complementary coils were done in [START_REF] Grzeskowiak | Coaxially Distributed Diameter sub-Coil Twisted Loop Antenna in HF RFID[END_REF]: RFID read-out distance at one point verify the improvement of the sub-coil structure for both orientations of the tag, but the magnetic coupling is weaker in the case of VM mode. In order to optimize the detection volume whatever tag's orientation, weaker magnetic coupling in VM mode has to been improved.

In the following section, the components of the magnetic field generated by the reader are carried out using magnetostatics module Radia [5]. The coupling coeffcient is calculated for both orientations of tag versus the misalignment on the (y) axis firstly for each radius of coils. Then the current of sub-coils is modified and its impact on the maximum mutual inductance is reported versus the reader inductance with electromagnetic software [6]. The better structures, in terms of mutual inductance and defined Figure Of Merit, are selected and compared with a quantified parameter, as input reader impedance: the reader inductances are around the same order to generate comparable magnetic energy. RFID volume detection is illustrated via detection cartographies for selected structure in both orientations of the tag.

II. OFF CENTER TWISTED LOOP ANTENNA

A. Contexte

In the case of RFID, the volume to prospect could be wide in comparison with the object to detect. This latter one could be oriented arbitrary and can be displaced; for instance, tagged pebbles [START_REF] Diet | LF RFID Chequered Loop Antenna for pebble detection and area localization[END_REF] with a movable reader coil on sea-beach, clinical patient in a hospital [START_REF] Mirbozorgi | A Smart Cage With Uniform Wireless Power Distribution in 3D for Enabling Long-Term Experiments With Freely Moving Animals[END_REF] or surgeon devices sliding in an inclined tube [START_REF] Diet | Detection tube for small HF RFID tags, based on mutual coupling with a coil resonator[END_REF]. A solution could be to design a planar transmitting antenna that generates magnetic field with the same intensity over the entire surface of the reader antenna but several orientations versus the zone of the reader antenna. The mutual inductance with the received antenna will be increased on a large area in both orientations, when the transponders are perpendicularly and parallel to the plane of the transmitting antenna, that can be useful to receive power when the transponder is oriented to 45°degrees from the planar transmitting antenna. First, the objective is to increase the magnetic coupling in the worst case, i.e. when the transponder is perpendicular to the reader surface and then to optimize the geometrical coil in the best case, i.e. when the transponder surface is placed parallel to this of the reader one.

Complementary coils [START_REF] Benamara | Twisted Loop Antenna for HF RFID detection surface[END_REF] increase magnetic coupling in the intersection of the coils when the transponder is perpendicular to the reader surface (VM mode in Fig. 1.b) and Distributed Diameter Coil [START_REF] Grzeskowiak | Coaxially Distributed Diameter sub-Coil Twisted Loop Antenna in HF RFID[END_REF] allows especially the detection in HM mode, i.e. when the transponder is placed parallel (Fig. 1.a). The figures 1.a and 1.b represent respectively the coils system in HM and VM modes with the (0xyz) coordinates system. In the following part, the magnetic induction in calculated for each component, and its integration on the receiving coil allows evaluating the inductive coupling.

B. Optimization of coil radius

The magnetic induction generated by the transmitting coil (Fig. 2 and Fig. 3), and the mutual inductance with a receiving coil (Fig. 4) are carried out using magneto-statics module Radia [5]. The receiving coil is one-turn coil of 1cm diameter. The transmitting coil is simulated in considering firstly the connection between the complementary coils.

In the perpendicular direction away from the coil (integration of B-field around the whole circular receiving coil for HM mode), the maximum magnetic induction appears in the center of the sub-coils ( = 0; = 0; = max ) : at 10mm away from the transmitting coil, the maximum of B-field is obtained in the center of the complementary coils and is higher with the smallest value of radius.

In the parallel direction away from the coil ((x) and (y) axis), the maxima appear for the center of the structure ( = ; = ; = 0 ). Although, the (xOz) plane is a symmetry plane for the current distribution, so the magnetic induction ⃗ would be perpendicular to this plane and ⃗ belongs to the anti-symmetry (y0z) plane: would be null on the (y) axis, and especially in the center of the structure, while it is not the case on the Fig. 2. To explain the origin of B , the connection between the complementary sub-coils is removed in keeping the current flow direction as previously (Fig. 1.b). In this case, for R=22.5mm, x-component of B-field is 10 times smaller than this y-component and can be considered negligible. The observation of the B-field generating by the transmitting coil with and without the receiving coil (Fig. 3) allows seeing its disturbance on B-field. The greater modification appears for the y-component: the connection decreases the level of the Bfield in the center of the structure with and without the receiving coil (comparison for DY=0 mm in Fig. 2). Furthermore, in presence of the receiving coil, this component is completely modified: the value is superior above the exterior edge of the structure. To observe the impact of the B-field on the received power, the coupling coefficient between the transmitting and receiving coils is calculated in Fig. 4 with the connection and the presence of the receiving coil.

For the HM mode, the inductive coupling is better for the smallest coil while the available area is greater for the widest coil, as seen for the B-field (Fig. 2).

In the case of VM mode, corresponding to the integration of the B-field y component , the received power can be decreased due to the integration of the B-field x-component and the B-field disturbance caused by the receiving coil: the maximum doesn't appear in the center of the structure but above the edge of the coils. The impact of two radii for the complementary coils on the previous parameter confirms the interest to use these two radii in terms of level of coupling and available area in HM mode. The current can be modified to modify the distribution and orientation of B-field in order to maximize the received power. In VM mode, the disturbance of the B-field in the center of the structure (due to connection and presence of the receiving coil) could cause probably less enhancement as expected.

C. Optimisation of coil current

The following simulations were made with the help of full wave electromagnetic field simulator HFSS (High Frequency Structure Simulator) [6] to take into account the time variation, and its impact on the current distribution and on the B-field generation. The current is changed by modifying the turn number of each sub-coil. For the transmitting coil, the external diameters d1 and d2 of each sub-coil are respectively set to 4.5 cm and 2.5 cm, for instance, conventional 400 corresponding to (4d1-0d2), which 4 is the number of turn of the associated diameter d1. A 1-turn loop receiving coil is used in simulation to reduce the computing time. The metalized part of the coils was substituted with infinite perfect electric conducting plane (PEC). The coils were supplied with the power of 1W.

The simulated maximum mutual inductance is obtained at 10 mm in HM mode (receiving coil parallel to the plane of the transmitting coil) when the coils are coaxial and in VM mode (receiving coil perpendicular to the plane of the transmitting coil) when the receiving coil is oriented perpendicularly in the center of the TLA structure. The disturbance of B-field caused by the connection and the presence of the receiving coil isn't observed in this case (Fig. 2 and3). Fig. 5 shows the simulated maximum mutual inductance versus the inductance of the transmitting coil for both VM and HM modes.

With 2μH self-inductance of the transmitting coil, the mutual inductance is higher in HM mode than in VM mode, while around 3μH, the VM mode is better than the HM mode. The off-center 240 structure that favors the using of 12,5mm radius seems more efficient in HM mode for self-inductance around 3.3μH, as suggested in Fig. 2, in comparison with the conventional 400 and off-center 320. The same enhancement for HM mode appears for the off-center 220 against the conventional 300 coil.

To quantify the ability of coupling with a small coil, several parameters are calculated, like previously M (Mutual inductance), and CC (Coupling Capability) between transmitting and receiving coils. M and CC (1) can be used to optimize the wireless power transfer for best performances and have been identified for enhancement of reading distance for the inductive system and extension of the range of active area of the system (on DY axis in Fig. 5), respectively. In order to normalize FoM(2), the maximum value M of M and inductance L of the DDC TLA are mixed with CC.

CC = ∫ |M(y)|dy

FoM = × (1)
FoM can be varied by fluctuating the following parameters: sub-coil numbers of turns (Ni) and diameters of sub-coils (di). FoM is calculated for the structures in Fig. 6. FoM is reported in HM and VM modes versus the simulated self-inductance (Fig. 6) when moving according the receiving coil to dY (from 0 to 100mm with 2 mm step) at a distance DZ of 1 cm.

FoM is better for VM mode except for the conventional 400, while the mutual inductance is maximum for HM mode for reader inductance superior to 2μH. As expected in Fig. 9, the surface where the read-out distances is more important with the off-center 240. Although, the volume of detection is better for the conventional 400. The field generated by the enameled wire is too weak to be detected when the turn number is equal and inferior to 2: these structures would be realized with copper strip to reduce losses. The detection in VM mode appears just in the center of the structure away 0.2mm from the structure: the hypothesis is that the connection between the complementary sub-coils degrades the magnetic induction, as seen in Fig. 2 and that the presence of the receiving coil disrupt the B-field lines, as observed in Fig. 3. orientation of the magnetic induction. The objective with this design is to enhance the maximum coupling factor with a small receiving coil with an average coupling capability over a larger area. Magneto-static and magneto-dynamic softwares were used respectively to observe the magnetic induction and to calculate the input impedance of the reader in magnitude and phase. Numerical and experimental results show that the RFID distance read-out is improved, while the RFID detection area isn't as expected, due to wire losses and connection between sub-coils. Future work will focus on the realization of the subcoil structures by micro-etching insertion to reduce wire losses: the connection that disrupts the magnetic induction and the detection in perpendicular orientation of the transponder has to be optimized.

V. CONCLUSION

Sub-coil in reader coil allows changing distribution and

Fig. 1 .

 1 Fig.1. Simulated coils systems in HM and VM modes

Fig. 2

 2 Fig.2 Simulated magnetic induction versus (y) misalignment at 1 cm for radius R of 22.5mm and 12.5mm

Fig. 3

 3 Fig.3 Simulated magnetic induction versus (y) misalignment at 1 cm for radius R of 22.5mm with and without the receiving coil

Fig. 4

 4 Fig.4 Simulated coupling coefficient versus (y) misalignment at 10cm for radius R of 22.5mm and 12.5mm in HM and VM modes

Fig. 5

 5 Fig.5 Simulated mutual inductance M(nH) versus the reader inductance LDDC(μH) in HM and VM mode at 1 cm

Fig. 6

 6 Fig. 6 Simulated figure of merit versus the inductance of the transmitting coil in VM and HM modesIn the following part, the off-center 240 is compared with the conventional 400: the mutual inductance and FoM are better for the off-center 240 in HM mode.

Fig. 10 .

 10 Fig.10. Photography of the fabricated 3D printing base, with Ib technology [10] and RFID home-made transponder. The experimental test of increasing detection volume and surface is depicted in Fig.11 with sub-coils structure associated with Ib Technology UK [10].The RFID transponder is a spiral coil made by micro-etching (strip of 110 μm, inter-spire of 110 μm and 20 turns of 1cm diameter) on both side of the FR4 substrate.The figure11is respectively the 2D plots of tag detection in HM mode with 400 conventional (a) and with off-center 240 (b). The (DY, DX) axes correspond respectively to a lateral misalignment according to the width and the length of the surface comprising the structures, the color scale defined the

  Fig.11. Surface and distance of detection in HM with conventional 400 (a), and with off-center 240 (b)

ACKNOWLEDGMENT

This research funded by Paris-Est University was conducted by M. Grzeskowiak over a six-month visiting period in Paris Electrical and Electronic Engineering Laboratory (GeePs) at Paris-Sud University.

A. Electrical model

In order to observe the impact on the RFID detection, an electrical model is associated to the coils system and the reader impedance input allows quantifying the RFID detection; higher is the magnitude of the impedance input and weaker the phase shift, better will be the detection of the transponder. These simulations are carried out using HFSS: the impedance of the reader ( ) is reported versus frequency firstly to fix the lumped electrical element and so the quality factor of the reader and then to observe the phase shift of the input impedance of the reader ( ). As seen in Fig. 7, the input voltage and the output voltage are given from currents , , , and from the transimpedance , the impedance of port 1 , this one of port 2 and the load impedance .

The input impedance of the reader can be deducted as:

is the load impedance and corresponds to the chip impedance in the case of RFID The impedance matrix is extracted directly from electromagnetic simulation with HFSS (High Frequency Structure Simulator) without determination of and .

To better sensitivity, the variation of input impedance from the impedance of port 1 ( ) (5) is calculated and reported versus at 1cm distance.

B. Simulated electrical model of the reader coil

On the tag side, resonance results from the parallel combination of capacitance of the chip and the tag coil inductor . In simulation, the diameter of 1-turn tag coil is 1cm and the electrical model corresponds to a resistance of 0.31 Ω and of 0.031μH; a parallel capacitance of 3.7nF is added and a quality factor of 10 is obtained (6).

= =

. × ( . . )

= 10 (6)

The quality factor is no greater than 30 for the reader coil in order to have enough bandwidth for communication. is reported in table 1 for the 400 conventional coil and off-center 240 structures, after parallel resistance and capacitance added to the reader coils. In the following part, in the (y0z) plane (Fig. 1), H-field is displayed for each selected structure with the same color scale.

C. Display of H-field

In the near field zone (Fig. 8), the impact of the sub-coil can be seen clearly for the distribution and orientation of the magnetic field: the zone where the field is high is wider for the off-center structures and the orientation is more diverse. The input impedance of the reader versus the transponder's displacement is reported for each structure.

D. Simulated imput impedance versus misalignment

The magnitude and phase of delta input impedance is reported versus the DY misalignment of the transponder in HM mode (Fig. 9). In HM mode, the phase of ( ) is around 165 degrees with variation of ±10 degrees. The variation of its magnitude is greater with the off-center 240 structure but the available area is smaller in this case.


M Benamara
email: benamara@univ-miv.fr

M Grzeskowiak

G Lissorgues

Calculation of the Equivalente Mutual Impedance in Complex HF RFID Systems

Keywords: rjid, mutual impedance, HF

In this work the improvement of HF RFID detection is obtained by the addition of small resonator in the reader coil. In conventional RFID system, the link between the reader and the tag coils is the mutual inductance, the added resonator is magnetically coupled with the reader coil, and then the link is expressed by the mutual impedance. Theoretical calculation of the equivalent mutual impedance of such system is reported and validated by comparison with HFSS simulation and measures.

I. INTRODUCTION

An inductively coupled RFID HF system is based on the magnetic coupling between the reader and the tag coils. In the case of weak coupling, the detection performance of the tag is fruitfully evaluated by the mutual inductance as a figure of merit [START_REF] Eason | Validity check of mutual inductance formulas for circular filaments with lateral and angular misalignments[END_REF][2][3] [START_REF] Hackl | A Novel Method for Determining the Mutual Inductance for 13.56MHz RFID Systems[END_REF]. A current challenge in HF RFID is to increase the surface and, if possible, the volume of detection by customizing the shape of the reader antenna [START_REF] Diet | Improving LF Reader Antenna volume of detection for RFlD token tag thanks to Identical Coaxial Loops (ICLs) and infout-of phase multiple-loops structures[END_REF][6] [START_REF] Grzeskowiak | Pebbles Tracking Thanks to RFID LF Multi-loops Inductively Coupled Reader[END_REF]. More precisely, a way of improvement is studied and obtained by the addition of a small resonator coil. The addition of a resonator concentrates and modifies the vectorial distribution of the magnetic field generated by the reader, as it can be shown in Fig. l. Accordingly, optimal detection areas can appear. In the following section of this paper, we investigate the formula defining equivalent mutual impedance between the dual-coils structure and the tag coil, and validate these relations, in section III, by simulations under HFSS (High Frequency Structure Simulator) and measurements using a Vectorial Network Analyzer. In simulation, each coil is fed by a lumped port, so we have matrix results with [3x3] data dimensions, but in measurements we used only the two ports of the vector network analyzer (VNA): one port for feeding the reader dual coils structure (coil 1 with the influence of coil 2 loaded by Cz), and the second port used for feeding the tag coil (coil 3). Consequently, we have experimental ZIO matrix results with [2x2] data dimensions. Using analytical formulas, we can develop the equivalent matrix of the system (see Fig. 2).

We consider the expressions of V I and V 3 mentioned in (I) with:

V -i12 (2)

2-C2ffi

The expression of current 12 in the resonator is given by (3):

jOJM1 2 jOJM23 12= 11+ 13 a a
Where:

a = -j --( j{)) L 2 + n) {)) C 2
The equation's system becomes: From these equations, the Z' matrix elements are equal to:

{)) 2 M2 1 2 . Z1 1 =r]- +j{)) LI a (3) (4) I. ()) 2 M 12 M 23 Z13 = j{)) M 13 - ( 5
) a I . {)) 2 M3 2 M1 2 Z31 = j{)) M31---�--=-= a ()) 2 M 23 2 . Z33 = r3 - + j{)) L3 a
Tn the following step of the calculus, the capacitors C1 and C3 are inserted in order to find the final Z,o-matrix of the entire system: a conversion from impedance matrix to chain matrix is used. The final ZIO matrix, considering the capacitors C1 and C3, has the following terms:

{)) 2 M 21 2 .
Z ii = r I - + j ()

) L I + Z C I a . {)) 2 M 12 M 23 Zi o = j{)) M13 - (8) a . ()) 2 M 32 M 12 Z o i = j{)) M 31 ---�---"-"' a ()) 2 M 23 2 . Z 00 = r 3 - + j ()) L 3 + Z C 3 a
When we develop, the parameters of the Z,o matrix can be expressed as: It can be seen in equations (9) that the resonator influences all terms of the Z-matrix. Tn this study, we are interested in the equivalent mutual impedance parameters (Zio or Zoi): the improvement of the RFID detection is done by the maximization of the module of these parameters. We use in the following sections the terms of equivalent mutual inductance and equivalent mutual resistance for, respectively, the imaginary part divided by 0) and the real part of the equivalent impedance parameter. Their calculated expressions are given from equation 3: As the addition of a coil resonator on the surface of the reader is studied by calculation and simulation, we developed a specific measurement set-up to evaluate the equivalent mutual impedance improvement, in function of the tag orientation and a lateral misalignment. This measurement setup (Fig. 3) is ensuring lateral variations between the coils, using graduate millimeter scale to read accurately the position of the tag relative to the reader one. For tag misalignment (-SO mm<y<SOmm), the equivalent mutual impedance is evaluated according to the calculated expression (equation 10), and compared to the simulated and measured results in both parallel and perpendicular configuration (FigA). The addition of the resonator can increase the equivalent mutual inductance and mutual resistance respectively by 10 nH (from IlnH t02lnH) and 1.1 Q(from OQ to 1.1 Q) above the resonator in parallel configuration (the comparison with and without resonator corresponds respectively to -SO mm<y<Omm and 0 mm<y<SOmm where y=-30mm is the center of the resonator and y=±SOmm are the edges of the reader coil) (Fig. S). However, in perpendicular configuration, the equivalent mutual inductance and the equivalent mutual resistance are improved respectively by l.S nH (from 3.S nH to 4.S nH) and 0.11 Q (from 0 Q to 1.1 Q). The positions of the maxima of the equivalent mutual inductance and mutual resistance are always on above the resonator (y=-30 mm).

Z ii = j OJ [ L 1 -_ 1 _ + OJ M 21 2 r] + ai M 21 2 t5 + r] .
The calculated mutual impedance (real and imaginary parts) is confirmed by comparison to simulation and measures of the structure in both parallel and perpendicular configuration. Because of the few values of the mutual resistance in the case of perpendicular configuration, only calculation results are presented.

TV. CONCLUSION

Analytical calculation of the equivalent impedance matrix of complex HF RFTD system is reported in this article. The principle of adding resonators in the reader coil modifies the distribution of the magnetic field, and can then increase detection volume and surface. The performances of RFID systems are evaluated by the mutual inductance, the added resonator changed this operation. New formulas of equivalent mutual impedance must be defined with the presence of resonator. These formulas are presented in this article and validated by simulations and measures of a dual-coil reader antenna. In future works, the equivalent self impedance parameters will be studied for evaluating the degradation of RFID performances by the added resonators. The degradation concerns the shift frequency in the reader and the tag coils.

Fig. I .

 I Fig. I. Proposed structure and its generate magnetic field

Fig. 2 .

 2 Fig.2 is the electrical model of the system composed by the dual-coils reader and the tag coil, where. L1=0.3 uH, Lz= 0.058 uH and L3=0.058 uH are respectively the self-inductances of coils I, 2 and 3. r1=0. H1, r2=0.06[2 and r3=0.06 [2 are their internal resistances. C1=0.45 nF, C2=2.32 nF and C3=2.32 nF are the series tuning capacitors at 13.56 MHz.

  VI = (n + j{J)Ll )11 + jOJM1313+ jOJM1 2 ( j ()) M 12 j ()) M 23 J -"--------'-=-11 + 13 . a a V3 = j{)) M3111 + (r3 + j{)) L3) J3 + j{)) M32 ( j ()) M 12 j ()) M 23 J ----'-=-11 + 13 . a a

( 6 )

 6 Where (j3( 1)) and (j3( 3)) are the chain matrix of both capacitors C1 and C3•[START_REF] Grzeskowiak | Pebbles Tracking Thanks to RFID LF Multi-loops Inductively Coupled Reader[END_REF]

CIOJ 2 Z 1 _

 21 io = j OJ (M 31 + OJ M 12 M 23 r)+ OJ 2 M 12 M 23 t5 Z oi = j OJ (M 13 + OJ M 32 M 21 r)+ OJ 2 M 32 M 21 t5 Z oo = j OJ [ L 3 -__

Fig. 3 .

 3 Fig. 3. Misalignment measurement-setup system fabricated

Fig. 4 .

 4 Fig. 4. Parallel (left side) and perpendicular (right side) configurations

4 _MeasureFig. 5 .

 45 Fig.5. Equivalent mutual inductance (b, c) and equivalent mutual resistance in parallel and perpendicular configuration


M Benamara
email: benamara@univ-mlv.fr

M Grzeskowiak

G Lissorgues

Esycom Ea

A Diet

Y Le Bihan

C Conessa

Reader Antenna Including Resonators for HF RFID Detection

Keywords: RFID, magnetic coupling, detection, HF I

In this article, HF RFID reader antenna including small resonant coil, operating with the magnetic coupling with the reader coil, is reported. The proposed system is used to improve surface and volume of small tag detection. The performances of such system are validated by maximization of input impedance (load modulation principle) and equivalent mutual inductance between the reader dual-coils and the tag coil compared to a conventional RFID reader antenna. Analytical formulas of theses parameters are developed. The proposed system is validated by detection measurements in parallel and perpendicular configurations between the reader and the tag coils.

INTRODUCTION FID (Radio Frequency Identification) LF (Low

Frequency, 125 kHz) and HF (High Frequency, 13.56MHz) is a non-contact technology used to identify objects. The communication is done by load modulation principle between the reader and the tag. Principally, the performances of such system are evaluated according to the range of the tags. In the context of such near field communications, where the magnetic coupling phenomenon is the medium, the range tags depends on the respective sizes between the reader and the tag coils and their spatial orientations [START_REF] Finkenzeller | RFID Handbook[END_REF][2] [START_REF] Zierhofer | Geometric approach for coupling enhancement of magnetically coupled coils[END_REF].

In the context of small tags identification, both surface and volume of tag detection are limited. Optimizing RFID detection requires the modification of the reader antenna structure on a multi-loop antenna. This solution can be applied in LF, but presents limits in HF application because of the line length of the reader antenna [START_REF] Grzeskowiak | Pebbles Tracking Thanks to RFID LF Multi-loops Inductively Coupled Reader[END_REF][5] [START_REF] Diet | Improving LF Reader Antenna volume of detection, for RFID token tag, with a combination of ICLs and in/out-of phase multiple-loops structures[END_REF]. Herein, we proposed in this article, a reader antenna including small resonant coil operating with the magnetic coupling with the reader coil. We focus on a "bi-coils prototype" whose performance illustrates the principle. Consequently, the optimization and maximization of communication parameters (mutual inductance and input impedance) is our figure of merit.

The proposed structure seen in Fig. 1 consists in inserting a resonator (small co-planar coil), included in the surface of a reader coil. The centers of the two coils are the furthest possible without coil intersection. The structure in Fig. 1 represents the interaction between the two reader coils (1 and resonator) and the coil of the tag (2). Coil 2 has a radius of R2 = 15 mm. Coils and the resonator coil present respectively a radius of 50mm and 15mm. In this study, the volume of detection is approximated by the different parallel surfaces of detection at different height values. Each one of these surfaces, parallel to the plane including the dual-coils structure, is defined as areas where the effective surface of the TAG is perpendicular to the magnetic field generated by the reader [START_REF] Hackl | A Novel Method for Determining the Mutual Inductance for 13.56MHz RFID Systems[END_REF]. The addition of a resonator concentrates and modifies the vectorial distribution of the R Coil 1

Resonator

Coil 2 magnetic field generated by the reader, as it can be shown in Fig. 3. This increases and decreases locally the mutual impedance in function of the tag coil position. Accordingly, optimal detection areas can appear because a minimum value of mutual induction is needed in order to power the chip of RFID tags.

In the following section of this paper, formulas defining the input impedance and the equivalent mutual inductance are investigated. The formulas are used in HFSS (High Frequency Structure Simulator) simulations results for parallel and perpendicular configurations. The last part exhibits a first optimized HF RFID detection, checking the operation principle, with a matched fabricated prototype constituted of added resonators in a coplanar coil, connected to a commercial RFID module. In conventional system, the link between the tag and the reader is the mutual inductance, while in the proposed system, the reader is two magnetically coupled coils, the link between the reader and the tag depends on the mutual impedance. In This part, we are interesting on the influence of the added resonator on the input impedance and the equivalent mutual impedance; these two parameters are used, later in this paper, to analyze RFID performances.

In conventional structure, the input impedance of the reader coil and the mutual inductance are expressed as:

chip Z L j r L j r Z in M      2 2 2 2 1 1    (1)  ) Im( 12 Z M  (2)
In the proposed design, all impedance matrix parameters are changed compared to the conventional system:

    r M res M res C L j Z M res M M res M res M j eq Z M r M r M res M res M j eq Z r M res M res C L j Z eq res eq 2 2 1 ) ( 2 2 2 ) ( 2 2 2 2 1 2 22 1 2 1 2 21 2 1 2 2 1 12 . ) ( 1 2 2 ) ( 1 2 2 1 1 1 11                                                       (3) With     r res L res C res r res r res C res L res L res C res              1 2 , 1 2 1  
The resonator influence is depicted in both reader and tag impedances by the M 1res and M 2res parameters but also by impedance coupling (in this case, the mutual impedance has a real and an imaginary part).The equivalent input impedance and equivalent mutual inductance are then expressed by:

Z Z chip eq eq eq eq Z Z Z in    22 12 11 (4)    M M M f Z M eq eq 23 12 13 2 ) Im( 12    (5)
In the next part, the calculated formulas are used for evaluated the performances of the proposed system.

III. EFFECT OF THE ADDED RESONATOR ON (EQUIVALENT) MUTUAL INDUCTANCE

In this part, the interest of adding resonators on the surface of the reader is discussed according to the different radius tag and distance between the reader and the tag coils.

Equivalent Mutual Inductance for Different Radius Tag Values

The evolution of the equivalent mutual inductance is parametrically studied according to the radius of the tag for different cases: two coils, with or without resonator. Fig. 5a shows two circular and parallel coils (R2≤R1). They are located at a distance of 10mm. The mutual inductance is evaluated for lateral displacement of the coil 2 along Oy. In this case, the mutual inductance can be shown to be maximal in the center of coil1for R2~R1. For a radius ratio R1/R2>5/4, the maximum of mutual inductance is shown in the edge of coil 1. The addition of resonator (Fig. 5b) has therefore no influence on the position of the maximum of the equivalent mutual inductance (when R1/R2~1), but rather on the level of the equivalent mutual inductance. This influence is clearly seen for a radius ratio of 5 (R1/R2=5) between dual-coils 1resonator and coil 2. The maximum of the magnetic coupling in this case is obtained in the center of the resonator.

The added resonator in the co-planar reader surface shift the null of mutual inductance versus lateral misalignment and increases the mutual resistance above the resonator: the mutual impedance is affected by the resonator but the levels stay weak in comparison with those obtained in the parallel configuration.

A1.

Equivalent Mutual Inductance for Distance Variation To assess the validity of this work on increasing RFID detection volume, we are also interested in the detection distance. For lateral misalignments (Y=0mm and Y=-30mm are respectively de center of the coil 1 and resonator, see Fig. 1) and distance (d) changes between the tag and the reader, the equivalent mutual inductance of the system (M eq ) is evaluated according to the simulation results presented in Fig. 6a.

At a distance of 10 mm, the addition of the resonator can increase the equivalent mutual inductance by 10nH (from 11nH to 21nH) above the resonator. However, the equivalent mutual inductance presents two peaks: the first one, at a position y =-30mm, corresponds to the alignment with the center of the resonator and the second one, y= 30mm, is due to the mutual inductance between coil 1 and coil 2. The mutual inductance is proportional to the sensed flow (magnetic field integration) of the reader and decreases with increasing distance d between the reader (coil 1 and resonator) and the tag (coil 2) (coherent with Biot and Savart law). Similarly, in perpendicular configuration, in the case of prototype with a single coil (coil 1) wider than the tag (coil 3), the mutual inductance has two maxima at the edges of the coil 1 and a zero at its center; it is therefore symmetric, the addition of the resonator modifies this operation.

The positions of the maxima of the equivalent mutual inductance are always on the edges of the coil 1 (increased above resonator), but at the center of coil 1, the zero of M eq is shifted from y = 0 mm to y = -10 mm. This is due to the mutual inductance between the resonator and the coil 1, which creates some additional maxima at the edges positions of the resonator.

IV. EFFECT OF THE ADDED RESONATOR ON SHOWN

IMPEDANCE

In Fig. 7, the input impedance in both parallel and perpendicular configuration is studied by simulation using the calculation in equations 1 and 4. The distance here is fixed at 10mm. In conventional system (Fig. 7a) the magnitude of the input impedance has maximum above the edges of the reader antenna: 5 Ω in parallel configuration and 2.1 Ω in perpendicular configuration, and minima at the center of the reader antenna: 3 Ω in parallel configuration and 0.5 Ω in perpendicular configuration. In this case the variation of the input impedance depends only on the mutual inductance between the reader and the tag coils. The magnitudes of the input impedance are changed with the added resonators (Fig. 7b). This is due to the transformed impedances. In both parallel and perpendicular configurations, the variation of the impedances due to the mutual inductance is negligible compared to the variation due to the added resonator: the improvement of input impedance corresponds to 5 Ω in parallel configuration and 2 Ω in perpendicular configuration.

V. EXPERIMENTAL VALIDATION

An experimental test of increasing detection volume and surface with adding resonator was made using a RFID reader from Ib Technologies, which requires tuned coil antenna (1uH reference value with quality factor Q around 30). The previous simulated structure, constituted of a 1-turn coil of 370 nH inductance value, and 30cm diameter, could not be matched and associated with the Ib Technologies reader. A new rectangular coil of 50 cm width and 35 cm height was realized with copper ribbon of 2 cm width (Fig. 8) and used in measurements. The global system consists in a rectangular coil antenna reader of 1750 cm 2 surface, a circular coil antenna tag of 4 mm diameter and two hexagonal resonators antennas of 42 cm 2 surfaces. The principle of adding resonator on the co-planar surface of the reader coil was applied. The detection of parallel and perpendicular tag was evaluated for lateral misalignment and different distances between the tag and the reader antenna. When the tag is detected, a detection message appears on the computer screen (Reader software interface).

Fig. 9 are the 3D plots of tag detection in parallel and perpendicular configurations without resonator (a, b), with one (c, d) and two resonators (e, f). The (X, Y) axes correspond respectively to a lateral misalignment according to the width and the length of the reader antenna (the rectangular one in Fig. 8), the color scale defined the distance of detection For resonator with size corresponding to 2.4% of the reader surface, both the volume and the surface of detection for parallel and perpendicular configuration were increased (table I). The surface of the reader was increased by (17%, 33%), and (6%, 15%) respectively for parallel and perpendicular configuration using one and two resonators, since a new detection zone appears above the resonator. The RFID detection results (TABLE I) exhibit clearly that new detection zones appear with the addition of the resonator in the parallel and perpendicular orientations and favor the RFID detection.

VI. CONCLUSION

In this paper, the improvement of HF RFID system performances, such as surface or/and volume of tag detection, was obtained by reader coil including resonators. The improvement of equivalent mutual inductance and input impedance was achieved thanks to the addition of a co-planar resonator, made of coil with a size in the range of those of the tag coil (with geometrical similarity).

The resonator, based on a tuned coil, was fruitful for detection in both parallel and perpendicular orientation of the tag. A dual-coils HF antenna prototype was studied. The design includes was including a resonator coil (15 mm diameter) whose surface occupies 30% of the reader surface, corresponding to a comparable tag surface (15 mm diameter).

Analytical formulas for equivalent mutual inductance and equivalent mutual resistance of the system were developed using electrical model of the structure, constituted of dualcoils reader and tag. The formulas are used in HFSS simulation to validate the operation principle.

In perspectives, the same principle will be also applied in wireless power transmission (WPT) to maximize the power transfer and the energy efficiency at the receiver and we will realize an optimized prototype with multiple resonating coils.

Fig. 1 .

 1 Fig.1. Fabricated tag (coil 2) (left) and dual-coils 1-resonator (right)Two configurations for the reader and the tag coils were investigated: parallel (0°) and perpendicular (90°) orientations, as defined in Fig.2. In the context of volume detection for any tag orientation, the mutual inductance has to be optimized in both of these cases. Several other geometrical factors are involved in this study such as radii of coils, lateral misalignment and distance between the tag and the reader dual-coils.

Fig. 3 .

 3 Fig. 3. Magnetic field with (left ) and without (right ) additional coil resonator

Fig. 4 .

 4 Fig.4. Electrical model for conventional (a) and proposed (b) RFID system.

5 .

 5 Mutual inductance versus the lateral misalignment in parallel configuration, for different radii a. without resonator b. with resonator

6 .

 6 Simulated equivalent mutual inductance for different values of tag distance d in parallel (a) and perpendicular (b) configuration

Fig. 7

 7 Fig.7 Input impedance (a) and equivalent input impedance (b) in parallel and perpendicular configurations

Fig. 8 .

 8 Fig.8. Experimental setup of detection

Fig. 9

 9 Fig.9 Surface and distance of detection in: Parallel configuration without resonator (a), with one resonator(c), with two resonators (e) and perpendicular configuration without resonator (b), with one resonator ,(d) with two resonators (f)


M Benamara
email: benamara@univ-mlv.fr

M Grzeskowiak

M Salhi

G Lissorgues

Esycom Ea

A Diet

Y Le Bihan

3D HF RFID Reader Antenna for Tag Detection in different angular orientations

Keywords: HF RFHD, magnetic coupling, reader antenna, detection, 3D antenna

Herein, a 3D 13, 56 MHz (HF) RFID reader antenna is proposed in order to optimize detection performance whatever the tag angular positioning. The design is made of a multi-loop structure, based on serial complementary antennas, as said "twisted" antennas. The RFID tag detection is optimized by two factors which rely on the modifications of the magnetic field (i) vectorial distribution and (ii) magnitude density. The reader antenna design is analyzed with electromagnetic simulation under HFSS (High Frequency Electromagnetic Field Simulation), and validated by detection measurements, in coplanar mode. A multi-loop structure, composed by 4 sub-loops, is then conformed onto a tube surface to provide the 3D structure. The goal of this improvement is to provide tag detection for any angular positions. At the center of the tube (3D reader structure), the detection of the tag is performed whatever its angular orientation, that is to say for any radial orientation.

INTRODUCTION

HF RFID (High Frequency Radio Frequency IDentification) systems are used in several applications for tracking and logistics. An inductively coupled RFID HF system is based on the magnetic coupling between the reader and the tag loops [1]. The main drawback of such RFID technology is the difficulty in providing a sufficient magnetic coupling for different scenarii of tag orientation and positions. This is all the more difficult than the tag size is small compared to the reader loop antenna. Herein, we focus on the case of small tags falling into a tube of detection, as can be used for any small tagged object in logistic applications. The challenge of the proposed system is to improve range and tag detection in function of its position and especially orientation. To optimize the generated magnetic field distribution, and consequently the ability of magnetic coupling with small tags for any orientation, the reader antenna must have large size and a complex structure made of complementary sub-loops [START_REF] Benamara | A twisted loop antenna to enhance HF RFID detection areas for different tag positioning[END_REF]. A current challenge in HF RFID is to increase the surface and the volume of detection by changing the shape of the reader antenna [3][4][5] [START_REF] Liu | 3D RFID Simulation and Design -Factory Automation[END_REF]. In the state-of-the-art bibliography, several reader antennas are proposed in coplanar or 3D model and the range and the surface of detection are often maximized in parallel configuration [5][7][8]. However, all other angular orientations of the tags, especially for a perpendicular configuration, reach their limit of detection close to the antenna loop.

In this study, an improvement of tag detection ability is obtained by using complementary loops conformed onto the tube external surface. The sub-loops of the proposed design allow to reduce the difference in size between the tag and the reader loops, and consequently to increase the magnitude of the mutual coupling values. Additionally, these complementary sub-loops modify the distribution of the magnetic field above their common edges. The design is validated by HFSS electromagnetic simulations and measurements of detection in parallel and perpendicular configuration. The same design is used to realize the 3D structure. The use of complementary loops consequently creates a peak of equivalent mutual inductance, which is fruitful in the 3D structure for a randomly orientated tag. The proposed structure in Fig. 1 has four sub-loops of (10x17cm 2 ). The tag coil corresponds to 7% of the surface of the conventional antenna (10*68cm 2 ), while in the proposed design the tag coil corresponds to 26.6% of each sub-loop size.

The use of sub-loops fed with opposite phase current creates the reorientation of the magnetic field lines at the common zones between each two juxtaposed sub-loops (Fig. 2a,2b). Also the magnitude field is also improved with the multi-loop antenna compared to the conventional antenna (Fig. 2c,2d). In conventional configuration, the mutual inductance is calculated by the Neumann formula as:

          = R Γ T Γ d T dl . R dl 4π 0 μ M (1)
Where, R and T are the two circulations fed respectively by the currents I R and I T . d is the distance between each elementary displacement In the case of a multi-loops antenna design, the mutual inductance is the sum of the mutual inductances between the tag and each one of the sub-loops R 1 , R 2 , R 3 : and R 4

T R M T R M T R M T R M R Γ T Γ R Γ T Γ d T dl . R dl d T dl . R dl R Γ T Γ d T dl . R dl R Γ T Γ d T dl . R dl 4π 0 μ M 4 3 2 1 3 4 4 3 2 2 1 1 + + + =                     + +   +   = (2)

B. Simulations and measurements

The coplanar design is validated by (i) HFSS simulations of the mutual inductance, and (ii) RFID detection tests using commercial reader module, in parallel and perpendicular configuration. The two reader structures are realized with 6 mm width copper ribbon. The inductances and quality factors are respectively 0.93 uH and 28 for the conventional loop, 1.44 uH and 31 for the multi-loop antenna. The tag is a commercial card corresponding to a simulated rectangular coil of 8x5cm with an inductance of 21.5 nH.

The mutual inductance and detection range are evaluated in function of the tag positioning. In conventional antenna the maximum values of mutual inductance is seen at its edges. While at the surface of the conventional antenna, the mutual inductance has fixed values: 55 nH in parallel configuration and zero in perpendicular configuration. The tag detection is then performed at a distance of 4 cm above the surface of the reader antenna in parallel configuration, while in perpendicular configuration the tag is detected only above the edges of the antenna from a distance of 2.5 cm.

The mutual inductance is improved by the proposed design in both parallel and perpendicular configuration. The maximum values are seen above the common zones between the juxtaposed sub-loops, herein, the range detection is maximized up to 11.5 cm in parallel configuration and 6 cm in perpendicular configuration.

The perpendicular configuration is not optimized using the coplanar structure because the tag is not detected above the surface of the sub-coils. To overcome this drawback, we proposed then, to use the 3D model antenna in step 2.

III. STEP 2:3D STRUCTURE

A. Magnetic field

Previously, the concept of multi-loops antenna has been validated. In this part, the multi-loop antenna is conformed onto a tube external surface to provide a 3D structure. Each juxtaposed sub-loop is fed with opposite phase current. At the common zones, the magnetic field lines have a distribution in which the field line follows a circle, as seen in Fig. 4.a. The generated magnetic field distribution by the subloop "in face to face visibility" is directed outside the tube (i. e. loop 1 and 3), and towards the center of the tube (loops 2 and 4 in Fig4c). The structure permits the maximization of the magnitude of the magnetic field inside the tube but its center the magnetic field has minimum values (Fig 4 .b). The tag is fixed at the center of the tube. In Fig. 4c, different angular tag orientations are reported (0°<θ<360°).

-For θ=0°: the tag is parallel to the sub loops 1 and 3 and perpendicular to the sub-loops 2 and 4.

-For θ=90°: the tag is parallel to the sub loops 2 and 4 and perpendicular to the sub-loops 1 and 3.

-For θ=45°: the tag is parallel to the common zones between sub-loops 1 and 4 and sub-loops 2 and 3, also the tag is perpendicular to the other common zones between subloops 1 and 2 and 3 and 4.

The mutual inductance between each sub-loop and the tag is reported in Fig. 5. According to the angular misalignment, the mutual inductance has maximum values when the tag is parallel to each one of the sub-coils and zero values the tag is perpendicular to the surface of the sub-loop.

-Sub-loops 1 and 3 are optimized for θ=0°, 180° and 360° (maximum values of mutual inductance: 10nH), however when θ=90°, 270° the mutual inductance has zero values.

-Sub-loops 2 and 4 are optimized for θ=90° and θ= 270° (maximum values of mutual inductance: 10nH) , however when θ=0°, 180° and 360°, the mutual inductance has zero values. to two sub loops, the two other sub loops are parallel to the tag surface, then, the 3D structure eliminate theoretically all possible zero mutual inductance.

-For θ=45 °, 135°, 225° and 315°: the mutual inductance has maximum values of 30.5 nH. It corresponds to the sum of the mutual inductances between visible common zones Fig. 6. Mutual inductance between the tag and the 3D antenna according to the angular tag orientation

The mutual inductance is then optimized with the 3D structure for all angular misalignment of the tag.

IV. EXPERIMENTAL VALIDATION

The 3D design is realized with a tube of 20 cm diameter. The tag is positioned at the center of the tube at a distance of 10 cm from the structure. The tests of detection are done inside and outside of the tube.

-Compared to the coplanar design, where only parallel configuration ensures tag detection above the reader antenna and where all the other angular configurations present null zones detection, this proposed design permits tag detection, inside the tube, in all possible angular orientations of the tag (0°<θ<360°) (Fig. 7a). It can be fruitfully used in practical for tracking applications like as animals or objects.

Outside the tube, the maximal distance of detection is respectively 25 cm when the tag is parallel to the tube (Fig. 7b) and 19.5 cm when the tag is perpendicular to the edges of the tube (Fig. 7c). The distance of detection is then improved up to 14 cm (from 11 cm to 25 cm) in parallel configuration and 13.5 cm in perpendicular configuration (from 6 cm to 19.5 cm). Inside the tube, the detection is also performed in different configuration, more precisely in parallel configuration the tag is detected at the concave surface of the tube, the design can then be used for maximizing range detection.

V. CONCLUSION

In this paper a new 3D HF RFID reader antenna is reported. The design is based on multi-loops antenna loaded by opposite phase currents (complementary principle). The coplanar design improved tag detection but especially for parallel configuration.. The proposed 3D design antenna which is a coplanar design conformed onto a tube surface, allow tag detection in any angular orientation of the tag inside the tube.

Fig. 1

 1 Fig.1 Proposed design antenna in coplanar (step 1) and 3D model (step 2)

Fig. 2 .

 2 Fig. 2.Lines and magnitude of the magnetic field for conventional reader antenna (a, c) and the proposed multi-loop antenna (b, d)A. Theoritecal studyHF RFID detection is based on load modulation. The performances are depending on the distance and positioning (lateral and angular) between the tag and the reader. In this study the evaluation of the performance is based on mutual inductance value in function of the position.

Fig. 3 .

 3 Fig. 3.Mutual inductance (a, c) and distance od detection (b, d) for conventional and multi-loop antenna in parallel and perpendicular configuration

Fig. 4 .

 4 Fig.4. Lines (a) and magnitude (b) of the magnetic field and angular orientation of the tag inside the tube (c)

Fig. 5

 5 Fig.5 Mutual inductance between the tag and the different sub-loops according to the angular tag orientation Inside the tube (Fig.6), the mutual inductance corresponds to the sum of absolute values of the mutual inductances between each sub loop and the tag coil. The mutual inductance has minimum values of 20 nH and maximum values of 30.5 nH:-For θ=0 °, 90°, 180°, 270° and 360°: the mutual inductance corresponds to the sum of the mutual inductances between the visible sub-loops. When the tag is perpendicular

Fig. 7 .

 7 Fig.7. Realized 3D design with the tag inside the tube (a), parallel to the tube (b) and perpendicular to the edges of the tube (c)