Design of an High Frequency RFID Multi-Loop Antenna for Applications in Metallic Environments.
RFID (Radio Frequency IDentification) is an active research field nowadays, which has led to the emergence of many applications for commercial or consumer markets. Although these systems are working properly in the proximity of human bodies or liquid environments , they have many problems when are in the proximity or surrounded by metallic environments [2-5]. Inductive coupling is used in order to establish a communication between the transmitter and the receiver for this kind of systems. Through this coupling, the magnetic field generated by the reader's antenna (transmitter) feeds the tag (receiver) attached to a product to be identified . Once this tag receives enough energy required to enable the chip, it will transmit in many cases the EPC (Electronic Product Code) information to an RFID reader. Thus, this magnetic field generated by the reader's antenna is the key for proper functionality of the whole RFID system .
According to Lenz's law, if a metallic plane is placed into a magnetic field, on its surface will appear Eddy currents that will generate an equal and opposite magnetic field with the magnetic field generator. Thus, magnetic field lines moving orthogonally on the surface of the metal plane will be diverted into field lines that move tangentially to the metal surface. The region where the maximum field line deviation occurs is located near the metal surface [8-9]. The occurrence of this phenomenon in RFID systems can be noticed when the tag's antenna is placed at 0 degree orientation to a metal surface. Basically, the tag didn't get enough magnetic field lines to pass through its antenna and to provide enough energy for the chip to be activated and to transmit the information requested by the reader.
With the emergence of the magnetic field generated by the Eddy currents, the magnetic flux generated by the antenna will decrease, thus reducing its inductance, according to equation (1).
L = [[PSI]/I], (1)
where [PSI], L and I are the magnetic flux, inductance and the current that flows in the antenna, respectively. As a result, the inductance decrease will produce a mismatch in the resonant frequency of the antenna, frequency expressed through the equation (2).
[mathematical expression not reproducible] (2)
where [f.sub.0], L, and C are the resonant frequency, inductance, respectively capacitance. This mismatch in the resonant frequency produces a reading distance attenuation that can be totally if the metallic environment is at a very small distance (in order of mm) from the antenna. Most antenna models used for RFID readers in metallic environments assume that the tags will have always a 0 degree orientation with the reader's antenna. In case when they are placed at 90 degrees orientation each other (worst case scenario), the identification distance decreases a lot, up to 95% or more when are placed in the center of the reader's antenna.
In this paper is related a novel design of an HF (High Frequency) RFID reader antenna that provide good results when it's used in metallic environments and can achieve better reading distances of tags placed at 90 degrees orientation in the center of the antenna. The efficiency of the proposed model increases by 9 times comparing with a standard HF RFID reader antenna, due to its uniformed magnetic field created over its entire surface. The paper structure is as follows: Section I presents a short introduction, Section II summarizes the state of the art in the field, Section III is dedicated to the proposed antenna design, followed by Section IV where performances are evaluated. In Section V the model is validated, followed by the conclusions.
II. STATE OF THE ART
The influence of metallic environments on an HF RFID antenna can be found in [10-13] where are reported the main aspects for resonant frequency shifting and the reading distance attenuation.
In , the authors present a study which involves the influence of metallic environments on an RFID reader's antenna. Starting from the ideas mentioned in  and approaching the antenna style from , the authors analyzed different types of metallic planes of different sizes and thicknesses, and tried to highlight the main effects when the reader's antenna is attached to a metallic plane. The same analysis found in  is also taken up in , to show an increase in resonant frequency of the RFID reader antenna as the distance between the antenna and the metallic plane decreases. A solution to attenuate these effects is to make a "static" antenna model that has a metallic plane attached that provides a relative immunity of the antenna to metallic environments from its proximity. This antenna model is calibrated only once when is installed at the final position. This makes any other metallic environment that comes in contact with the new antenna model have a very low influence on the RFID system. At the same time, this new model also has drawbacks. With the metallic plane near the reader's antenna, the reading distance decreases as the magnitude of the magnetic field generated by it decreases, even if the transmitting power is increased. This antenna can only be used for a certain application type because it will be able to communicate with the tags in only one direction, opposite to the metallic plane. Also in 2009, an RFID antenna model for HF frequency is proposed in  to reduce metal influences on systems using RFID technology. This new model is designed for libraries and especially for metallic shelving applications. As a dielectric material, between the antenna and the metal plane, polystyrene foam is used, due to its low dielectric constant of only 1.07. According to the authors, the new antenna model was implemented to manage books from a library, providing 95% accuracy in the identification process. A major disadvantage of this antenna structure is the angle at which tags can be targeted to be identified. The magnetic field in the center of the antenna will be attenuated due to the appearance of Eddy currents in the metallic plane of the physical structure of this antenna model. Thus, the identification percentage will be much smaller in the center than at the edges, and if the tag is placed 90 degrees to the antenna, this identifier percentage will be almost null in the center of the reader's antenna. Another disadvantage is given by the way this antenna is calibrated. If the structure of the metallic plane of the antenna changes, or other metallic environments occurs in the proximity, the antenna will be mismatched from resonant frequency and the percentage of tag identification will decrease. An eventual antenna calibration involves the use of special equipment, such as the VNA (vector network analyzer).
III. ANTENNA DESIGN
The performance of RFID systems depends exclusively on the energy transfer between the reader's antenna and the tag antenna. A solution to improve the performance of RFID systems that works in the proximity of metallic environments can be implementing an antenna model for the RFID reader that provides lowest possible losses and can read RFID tags regardless of their orientation in space.
An HF RFID antenna model proposed to work in the proximity of metallic environments is shown in Fig. 1 . The physical structure of this model involves using 2 turns of conductive wire in a multi-loop configuration. This antenna model is chosen because the magnetic field generated due to its physical structure is almost uniform throughout its surface, regardless of the action of a metallic environment in the vicinity. This is explained in more detail in the following sections.
The physical implementation of this antenna model involves the use of insulated copper wire with a diameter of 1 mm. The segments of the antenna are numbered from 1 to 10, so that the inductance can be depicted more easily for calibrating it at the resonant frequency of the RFID reader. The physical dimensions of the conceived model are presented in Table I. The antenna length and width (L1 and W1) as well as its shape are chosen in such a way that the performance of this antenna can then be compared to the performance of other commercially available RFID antenna models and are available in our RFID research laboratory. We marked with x1 and respectively x2 the distances between the antenna turns, in length and width respectively.
In order to depict the antenna inductance of the proposed antenna model, the mathematical formulas expressed in  were taken into account. There are included also the mutual inductance formed between antenna segments of the multiloop structure. Thus, the total value of the inductance can be expressed by equation (3), value depicted in [mu]H.
[[L.sub.ant] = [L.sub.segments] + [M.sub.+] + [M.sub.-]] (3)
[mathematical expression not reproducible] (4)
According to the same reference, the value of one antenna segment can be calculated with equation (5), the value of the inductance being in [mu]H
[mathematical expression not reproducible] (5)
where [l.sub.x] is the length w is the width and t is the thickness in cm for the wire used in the antenna design. The mutual inductance can be calculated with equation (6), the value obtained being this time in nH.
[M.sub.x] =2 x [l.sub.x] x Q (6)
where Q represents the mutual inductance parameter and can be expressed with equation (7).
[mathematical expression not reproducible] (7)
where [l.sub.x] is the length of the segment for which Q is calculated and GMD is the geometric distance between two adjacent conductors and can be calculated with the equation (8).
[mathematical expression not reproducible] (8)
where d is the physical distance between segments.
Using the values from Table I and the mathematical approach, the value of the antenna inductance between points A and B is 1.8 [micro]H. This value is experimentally verified by measuring the antenna inductance using the Agilent FieldFox N9912A VNA.
This inductance value is also verified by measurement, using a precision L meter, built using a development platform with a microcontroller. By performing measurements with this precision meter, a value of 1.808 [micro]H is obtained for the antenna inductance (see Fig. 3), validating the measurements made with the VNA and the mathematical approach presented.
To prove the efficiency of the antenna model, we need to analyze the magnetic field created by it. Based on this field, we can estimate the maximum reading distance for RFID tags, and with this we can demonstrate the antenna efficiency. The value of 94 dBuA/m or 0.05 A/m for the magnetic field is required to activate a standard HF RFID tag and is also the reference value to determine the antenna reading distance in this paper. This value is the lower limit imposed on various RFID tag models operating in the HF frequency range .
IV. PERFORMANCE EVALUATION OF THE PROPOSED DESIGN
The efficiency of an antenna is given by the magnetic field it generates over its entire surface. Usually, the field is evaluated at the center of the antenna, because there is strongly attenuated when influences from various environments are present [20-21]. A theoretical approach for determining the magnitude of the magnetic field that the proposed antenna model generates, involves dividing the structure of the antenna into a number of finite length segments. The next step is to calculate the magnetic field values generated by each segment. The overlap of the magnetic field of all the segments gives us the value of the total magnetic field generated by that antenna. Another approach for obtaining the magnetic field value is the use of the indications presented in  where the total value of the magnetic field can be expressed by the equation (9) when the antenna structure has a rectangular shape with a x b dimensions.
[mathematical expression not reproducible] (9)
where N represents the number of turns, I the current, a and b represents the width and the length respectively, and x represents the distance in the Z plane for which the magnetic field value of the antenna model used is determined.
This case, the thickness of the conductor, as well as the distance between two parallel segments of the respective antenna, is neglected. Using the proposed model, the magnetic field value is shown in Fig. 4. One can see the total magnetic field value required to activate an HF tag. The maximum reading distance can be determined of about 50 cm for a tag when there is no disruptive environment in the proximity of the reader's antenna. It should be noted that this distance involves 0 degree orientation of the tag to the antenna, and is calculated from the center of the antenna.
When the antenna of the RFID reader is in the vicinity of a metal plane, the magnetic field lines produce Eddy currents, which, according to Lenz's law, have an opposite orientation relative to the magnetic field lines that generated them. In order to make a mathematical model that will allow us to approximate the Eddy currents, we can assume that the metallic plane can be replaced by an antenna, having the same characteristics as the antenna influenced by the metallic plane, through which an equal current with opposite value will flow. Equation (10) describes this:
[mathematical expression not reproducible] (10)
The antenna that will replace the metallic plane will have a symmetrical orientation as per the RFID antenna, as shown in Fig. 5.
where d represents the distance between the reader antenna and the antenna replacing the metallic plane.
An important role in this mathematical approximation is the distance between the two symmetrically oriented antennas. The lower the distance, the more magnetic field of the RFID reader antenna will be canceled due to the field generated by the antenna replacing the metallic plane. If the two antennas are much closed, then the magnitude of the magnetic field will be zero.
The magnitude of the determined magnetic field, with a distance between the antenna and the metal plane, which varies between 1 mm and 50 mm, is shown in Fig. 6. In this case, the reading distance of the RFID tags is approximately 27 cm if the metallic plane is at a distance of 50 mm.
The magnetic field generated by the proposed antenna model can also be expressed through EM simulation software. In this case, the Ansoft HFSS suite was used to determine the value of the maximum field generated by the antenna model. At the same time, using this simulation environment we can validate the proposed mathematical model related to equations (9) and (10).
In Fig. 7 we can observe the simulation of the magnetic field of the proposed antenna model when there is no disturbing environment in its proximity. From the obtained results, in the middle of the antenna there is a uniform magnetic field leading to a high percentage of identification of the tags being interrogated. The magnetic field has a value of 0.05 A/m at about 50 cm from the reader's antenna. The results obtained in this case are similar to the results obtained using the mathematical approach.
If the proposed antenna model is located at 1 cm from a metallic plane, the obtained values can be seen in Fig. 8. In this case, a magnetic field value of 0.05 A/m is obtained at a distance of only 10 cm. One may note that this value of the magnetic field is constant in the center of the antenna. As in the previous case, the mathematical model is comparable to the simulated, the results being approximately identical.
V. ANTENNA MODEL VALIDATION
In order to validate the performance of the proposed antenna model different test measurements must be performed. The main objective is to determine and evaluate the reading distances in close proximity of a metallic plane. The obtained distances are compared to read distances for other commercially available HF RFID antennas, working at 13.56 MHz, considered as reference antennas. Two types of RFID antennas were selected (Fig. 9) with external dimensions close to the dimensions of the proposed antenna model.
The two reference antennas are firstly evaluated by simulation. These simulations are made to observe the magnetic field distribution in the presence and in the absence of a metal plane. Both, the Single turn and the PCB antenna, provides low performance in the proximity of a metallic plane. The magnetic field simulated for the two antennas gives us information about the coverage area on the antenna surface. In both cases, (Fig. 10), the magnetic field is strongly attenuated in the center. If a 90 degree oriented tag reading is necessary, the identification percentage will be very low, almost equal to 0.
The physical characteristics of these antennas as well as the maximum distance at which an RFID tag in the center of the HF field can be identified on the Z axis can be seen in Table II.
Data was obtained by practical measurements of the reading distance, using the same tag pattern oriented at 90 degrees and 0 degree position with the antenna. It should be noted that all the antennas used in this paper, including the proposed model, are calibrated on the resonant frequency only once, in the absence of a metal plane in the proximity. According to the presented results, the maximum reading distance of the proposed antenna model should also be determined in the absence of metallic plane, oriented both in 0 and 90 degrees orientation to the reader's antenna. Using the same type of RFID reader as with measurements made with standard antennas, if the tag is 0 degree oriented with the antenna, a reading distance of 50 cm is obtained and if the tag is 90 degrees oriented with the antenna is obtained 27 cm. The results obtained from the measurements are compared for each antenna model used.
In order to highlight the influence of metallic environments on the reader's antenna, a metal plane with 80 x 60 cm made from galvanized steel and having a thickness of 0.5 mm is used. The setup for measurements is shown in Fig. 11. The metal plane provides uniform coverage over the entire surface of the antenna. The same metallic plane is used for each type of antenna, thus determining the maximum reading distance.
The distance between the metal plane and the antenna of the RFID reader varies between 1 mm and 50 mm. Reading of distance values have been taken both if the tag is placed 0 and 90 degrees orientation to the reader's antenna.
The analysis of the values can also be seen in Fig. 12. The proposed antenna model performs similar to the standard single turn antenna model used for comparison.
The proposed antenna model is also tested if the tags are oriented 90 degrees to the reader's antenna. The values obtained in this case can also be seen in Fig. 13.
In Fig. 13 the proposed antenna model for the case where the tag is 90 degrees oriented with the antenna has superior performance compared to the results of other standard RFID antenna models. The magnetic field generated by the proposed antenna model ensure uniformity over its entire surface, even if there is a metallic plane in close proximity. At a distance of just 8 mm between the metallic plane and the antenna, a reading distance of approximately 10 cm is obtained. It is noteworthy that the values achieved using the mathematical and the simulated models are almost identical to the measured values for the practical implementation of the antenna. If the tag is 0 degree oriented to the antenna, the proposed antenna model has approximately the same reading distances as the models taken for comparison, but when the tag is 90 degrees oriented with the antenna, the reading distance for the proposed antenna increase by 3 and 9 times respectively, comparing with the standard HF RFID antennas, for a distance between the metallic plane and the antenna of 50 mm.
This paper presents a new approach of a multi-loop RFID reader antenna that can be successfully used in metallic environment applications. The new design ensures that RFID tags can be correctly identified and read even if they are placed 90 degrees in orientation to the reader's antenna. The mathematical approach and computer simulations are proven by measurements performed using specialized measurements equipment's like a VNA and a high quality L meter. This way, the antenna matching circuit can be easily obtained by configuring only the capacitance of the resonant circuit. This kind of antenna can be implemented with success for smart shelf applications, where each item can be identified without any issues, without taking into account its orientation to the reader's antenna.
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Adrian Ioan PETRARIU (1,2), Alexandru LAVRIC (1), Eugen COCA (1)
(1) Stefan cel Mare University of Suceava, 720229, Romania
(2) Integrated Center for Research, Development and Innovation in Advanced Materials, Nanotechnologies, and Distributed Systems for Fabrication and Control (MANSiD), Stefan cel Mare University of Suceava, Romania
This paper was partially supported by the project "Integrated Center for research, development and innovation in Advanced Materials, Nanotechnologies, and Distributed Systems for fabrication and control", Contract No. 671/09.04.2015, Sectoral Operational Program for Increase of the Economic Competitiveness co-funded from the European Regional Development Fund.
Digital Object Identifier 10.4316/AECE.2018.02005
TABLE I. ANTENNA PARAMETERS Parameter Value [cm] L1 32 W1 22 x1 7 x2 6 TABLE II. MEASUREMENT RESULTS FOR THE REFERENCE ANTENNAS Tag reading distance [cm] Antenna Dimensions 0 degrees 90 degrees type [cm] oriented tag oriented tag Single Turn 33 x 31 43 10 Antenna PCB 33 x 23 46 15 Antenna
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|Author:||Petrariu, Adrian Ioan; Lavric, Alexandru; Coca, Eugen|
|Publication:||Advances in Electrical and Computer Engineering|
|Date:||May 1, 2018|
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