# Effects of the FR 4 Substrate Realized in a Circularly Polarized UHF-RFID Reader Antenna with Fractal Geometry for Enhancing Parameters

Authors: Chitra Varadhan; S. Arulselvi; Fekadu Ashine Chamatu
Journal: Advances in Materials Science and Engineering
 (2021)

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## Abstract

The proposed RFID reader antenna progressed with perfect electric conductor (PEC) as the radiating element and FR 4 as the substrate to achieve circular polarization, enhancement in bandwidth, and read range. The design presents a CPW feed RFID antenna for near-field reading applications, between the range of 903 MHz to 929 MHz. The operating frequency of the proposed design is 900 MHz, axial ratio of the model is less than 3 dB, impedance bandwidth is 256 MHz, and axial bandwidth is 36 MHz, proving to be adequate for near-field RFID reader applications such as item-level tagging and smart shelf. The proposed antenna model is realized with fractal structure to achieve miniaturization. The developed antenna is optimized using EM software for numerical analysis. The designed antenna is fabricated, and the prototype is characterized in terms of dielectric constant and loss tangent. The obtained results indicate high correlation with simulation results.


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## Body

## 1. Introduction

Monopole RFID antennas are widely applicable in the field of mobile applications and military, biomedical, and aerospace industries. The important property of monopole antenna selection is broad-band achievement. The performance of the antenna greatly depends upon the substrate parameters such as uniformity, dielectric constant, and loss tangent [1]. Extensive literature surveys were performed regarding the parameters stating that certain substrates produce an improvement in bandwidth performance [2–8]. Another survey reported that the material contains less values of loss tangent and dielectric constants and produces better result compared to higher values [9].In modern wireless communication industry, the RFID system plays major role in accessing data. RFID uses two components to exchange information, namely, a reader and tag. The product information is transmitted by the tag, and the stored data can be retrieved by the reader. RFIDs are operated within various microwave frequency bands; among these, UHF-RFIDs provide specific significance in wireless technology. In the RFID domain, reader construction in the UHF band is widely used due to the long read range (1 m–10 m), and this is mostly applicable in container tracking, pallet tracking, parking lot access, and electronic toll fee collection [10–16]. The construction and design of RFID antennas comprise challenges such as size, read range, and bandwidth coverage. In general, high frequencies and microwave frequency scale are reserved for UHF-RFID system applications. Every nation adopts its own ranges similar to 840.5–844.5 MHz in China, 865–867 MHz in India, and 902–928 MHz in USA and Argentina. Considering all these bands, the experiment selected an RFID range from 840 to 928 MHz.FR 4 glass fiber-reinforced and epoxy-based substrate fabricated by high-pressure thermoplastic lamination is very popular and resourceful as it exhibits good strength-to-weight ratios. The FR 4 substrate is generally used as an electrical insulator in applications such as semiconductive dielectric, with significant mechanical properties, solvent-resistant properties, and also water absorption rate taken as approximately zero (0.125 in <0.10%). The importance of the FR 4 substrate is it can maintain the most important property of electrical insulation and mechanical specification in both wet or dry conditions. In the proposed RFID reader antenna, FR 4 is used as a substrate as it exhibits better performance [17–23].Perfect electric conductor (PEC) is used as the radiating element as well as ground plane in the design of an antenna. The patch and ground planes are superimposed on the same plane. PEC demonstrates one of the most unique characteristics, that is, zero resistivity. By freezing the magnetic plasma to zero or if the flux value of initial condition is zero, it maintains zero value constantly. Zero flux is allowed, but not required to perform. With the help of this unique property, the antenna is coated with PEC to provide the maximum power transfer throughout the radiating plane at that instant, explained in detail in the result section. From the literature, the coplanar waveguide (CPW) circularly polarized (CP) RFID reader antenna can access the tag data without any degradation from its polarization and is not sensitive to orientation of the receiver and transmitter. But, the CP-RFID antenna producing smaller amount reading capability is the major challenge in this domain [24, 25]. Basically, the CP can be achieved either by using diagonal feed, dual feed, or by allocating asymmetry in antenna structures. To achieve circular polarization in the antenna, various methods such as dual feed exciting by the orthogonal mode, truncated edge, and slotted types are prominently used [26–31].In the proposed design, highlight of the antenna achieving, a modified monopole CPW fractal structure is incorporated to provide higher bandwidth at 900 MHz. The bandwidth is increased by 4-5% compared with the normal patch antenna employed, due to the usage of the FR 4 substrate and 20% improvement with the CPW monopole structure [32–34]. The fractal concept is included in the antenna design to reduce the size along with circular polarization. The communication is detailed as follows: Section 2 gives the design concepts, the result and discussions are detailed in Section 3, and calculations and tabulated results are provided in Section 4.


## 2. Design of the RFID Reader Antenna

The proposed CPW-UHF-RFID reader antenna implemented with FR 4 as the substrate with an optimum thickness of 1.6 mm and medium dielectric constant of 4.3 to achieve circular polarization with good bandwidth performance at 900 MHz resonance frequency. PEC is considered in the antenna design as the radiating element and ground plane in same surface on the rectangular FR 4 glassy substrate, thus reducing the cost of fabrication. The thickness selection of perfect electric conductor is 0.036 mm. A common microstrip coplanar waveguide monopole antenna structure is displayed in Figure1(a). The abovementioned parameters incorporated flawlessly result in an omnidirectional radiation pattern but reduction in antenna directivity. The directivity of the antenna is enhanced by implanting additional planes in the design. The bandwidth of the antenna is also improved by means of introducing fractal design in the radiating pole. Implementing fractal structure accomplishes greater bandwidth as well as circular polarization. The fractal geometry creates several operating frequencies, but the fundamental mode is obtained by increasing the total electrical length. Increment in length results in issues in compactness. The basic structures can be a line or surface or even a volume, which are used in the design. Selecting a suitable fractal geometry design to acquire high directivity, smaller size, and multiband behaviour are the electrical challenges. The design of proposed fractal antenna is illustrated in Figure 1(b).Figure 1
(a) Monopole CPW antenna without the fractal structure with the FR 4 substrate. (b) Monopole CPW proposed fractal antenna with the FR 4 substrate.
(a)(b)In the proposed design, the contribution of substrate FR 4 plays an important role in increasing the bandwidth of the antenna. The structure and shape of the substrate in the design of the antenna is the main parameter such as the dielectric constant and loss tangent. Initially, the glass epoxy matrix is reinforced with woven glass cloth and cured in 130°C to remove the surplus components. The configuration of FR 4 varies with thickness results in shift of the operating frequency. It is also direction dependent; that is, the composite material varies with length and width and the bandwidth changes accordingly. Thus, the selection of dimension of the FR 4 substrate raises a crucial flag in the design of the RFID reader antenna. Even the thickness of PEC selected between 0.033 mm to 0.0356 mm results in not obtrusive degradation at lower frequencies but reflects substantial deprivation and nonlinearities at high frequencies. Thus selection of material, fractal design structure, and dimension of antenna is a challenge in the design of RFIDs.Numerous ways are there in practice to bring out the circular polarization in antenna design. In this manuscript, the 1st-order simple fractal design is applied to create circular polarization. The overall dimension of the antenna is 90 mm × 105 mm, and FR 4 material with εo = 4.3 and loss tangent = 0.025 is considered as a substrate of the antenna with the thickness t = 1.6 mm. The design also involves λ/4 CPW feed and ground plane. In the zeroth iteration, a single monopole having a length of λ/8 and by projecting slant 45° plane initiates axial ratio in the design as illustrated in Figure 2(a). As described in Figure 2(b), the first iteration introduces an inverted L shape with perimeter 3λ/16. The successive complete corresponding iterations are shown in Figures 2(c) and 2(d). The perimeter of the conductive material PEC is increased by implementing fractal dimensions such that the designed antenna resonates at 900 MHz. The inverted L shape maintains a diagonally opposite relation at each and every iteration, and this introduces circular polarization in the proposed RFID reader antenna. The impedance 50 Ohm is realized, and axial ratio is improved by selecting the stub length of the ground plane as L = 30 mm, width 3 mm is overhanging from ground plane. The proposed RFID reader fractal antenna structure using PEC creates infinity conductivity and generates right-hand polarization while the flipped arms generate left-hand polarization accordingly. Also, it exhibits the fractal structure supporting the generation of RHCP surface current distribution all over the RFID reader antenna.Figure 2
(a) L-shaped monopole; (b), (c), (d) iterations of the inverted L-shaped fractal structure,λ = 160 mm.
(a)(b)(c)(d)Table1 lists the substrate parameters and PEC thickness of the design specifications of the proposed RFID reader antenna. Table 2 enumerates various materials with corresponding dielectric constants and loss tangents. Lower value of dielectric constant and less loss tangent offer wider bandwidth and directivity. Thereby, FR 4 is selected for the proposed antenna. The corresponding relative permittivity and loss tangents are summarized in Table 2.Table 1
Substrate and PEC design specification of the proposed RFID reader antenna.
FR 4 substrate and PEC design specificationsDielectric constant (εr)4.3Thickness of the substrate1.6 mmLoss tangent (δ)0.019∼0.02Conductor thickness (perfect electric conductor)0.035 mmTable 2
Various dielectric materials with dielectric constants and loss tangent comparison.
Dielectric materialDielectric constant (εr)Loss tangent (δ)Alumina (95%)9.750.0003Beryllia6.50.0004CEM4.50.025Gallium arsenide130.006Roger 58702.330.0012Roger 58802.20.0004Silicon11.90.004Teflon2.080.0004Vaseline2.160.001FR 4 (proposed antenna)4.30.02


## 3. Result and Measurement

The proposed antenna is simulated using EM software; the obtained S11 results are verified with a typical monopole structure and are illustrated in Figure 3. The result indicates that the bandwidth is expanded significantly due to deployment of the composite materials along with the fractal structure in the design antenna. The calculated parameters incorporated in the design and simulation result in increased bandwidth compared to the monopole antenna around 325 MHz (35.7%) with a frequency of operation of 910 MHz and impedance bandwidth of 28.5% for 900 MHz. Thus, the simulated results indicate that the proposed antenna is suitable for item-level tagging and smart shelf applications in manufacturing industries.Figure 3
Simulation and measured characteristics of the reflection coefficient vs. frequency of the proposed antenna.Figure4 is composed of the axial ratio that compares the multiple iterations exclusively to describe its gradual progress and the axial ratio that declines to approximately 0 dB along with respect to the main lobe direction. The simulated result shows various proposals taken and an optimum length of PEC is considered in the design and had attained perfect resonating frequency with an appreciable axial ratio. This fidelity can be achieved by the uniform surface current distribution due to zero resistivity of the PEC.Figure 4
Simulated result: AR setup along the direction of the main lobe.The calculation of axial ratio is verified by both frequency-domain and time-domain solvers. The observed results of AR are approximately in 3 dB bandwidth of 36 MHz (14%) of impedance bandwidth, width = 3 mm and a stub length = 30 mm is protruded outside from the CPW plane, thus diminishing the axial ratio down from 3 dB to 0 dB. Similarly, various lengths of stub are taken and simulated. Variation of length of PEC offers various axial ratios with respect to frequency. Optimum length of PEC is considered in the design of RFID reader application and equipped in the fabrication; the effects and optimized result of the proposed antenna are spotlighted in Figure5.Figure 5
Microstrip stub length variation results in AR.


## 4. Prototype Fabrication and Measurement of Results

The read range parameter is used for measurement of the efficiency of the designed antenna. The efficiency can be deviated by the operating frequency in terms of the wavelengthλ if the thickness selection of substrate varies. The assumed transmitted power (Pt) is 4 Watt; similarly, the approximated received power (Pr) is 1 mW. The minimum detectable power Pd of 1 mW and the reader gain (1.35 dBi) are maintained. The read range can be calculated by means of the Friis transmission equation.Power density can be calculated from(1)p=PT4πR2.Considering the transmit antenna gainGt, the power density becomes(2)p=PT4πR2Gt.Assuming the effective aperture of the antenna, the received power becomes(3)PR=PTGtGrλ24πR2.Thereby, the read range is calculated by the abovementioned Friis transmission equation. The optimum read range is taken, and the length and width of the antenna are theoretically calculated before the design of the antenna. The theoretical and simulated results are verified, and the results are optimized. Then, the design of the RFID reader antenna is fabricated as presented in Figure6.Figure 6
Fabricated prototype of the RFID reader antenna.The fabricated antenna is compared with the existing products in terms of operating frequency, gain of the antenna, read range, and the size of the antenna, as shown in Table3. The first two antennas attain circular polarization with larger gain, but the size of the antenna is very large, as these kinds of antennas use stacked layers of composite substrates, which are commercially available. The third antenna generates circular polarization with four different slots with a square patch with FR 4 suggested as the substrate of the RFID antenna (45° plane). Though the antenna uses the FR 4 substrate, it produces very low gain due to the deformities present on the patch surface; also, the bandwidth of the antenna is very less.Table 3
Proposed antenna vs. existing product parameters.
Product/IDFrequency range (MHz)Max gain (dBi)Read range (m)Antenna dimension (L × W × H mm3)Laird/S9025PR902–9285.53.85133 × 133 × 18Motorola/AN720902–9286.54.19133 × 133 × 18ACSSMP antenna/FR 4918–9290.51.1690 × 90 × 4.8Proposed antenna902–9281.351.9105 × 90 × 1.6From Table4, it can be seen that the proposed RFID reader antenna bandwidth is possibly upgraded from 3% to 20% compared to various substrate materials. It is proven that the fabricated RFID reader antenna uses FR 4 as the substrate to achieve cost reduction and also produces appreciable return loss. Many different materials and their corresponding return loss and bandwidth are compared with the proposed antenna. It is observed that the designed antenna gives extraordinary return loss and bandwidth (%). The reason behind the uncertainty is due to frequency dependence of relative permittivity and loss tangent. Normally, the relative permittivity is not constant for a material; rather, it falls down with increasing operating frequency. The proposed fractal structure operates well between 902–928 MHz bandwidth, accomplishing circular polarization, the CPW feed provides wider bandwidth with a considerable gain of 1.35 dBi, thus owing to be highly suitable for near field applications, and the fractal structure is used in size reduction of the antenna when compared to other commercial types.Table 4
Parameter comparison with the proposed antenna.
SubstrateThickness (inches)Return loss (dB)Bandwidth (%)Duroid 58800.050−20.42376.25Duroid 58700.062−17.108012.76RO 30030.06−10.39124.34RO 30060.050−6.67313.26TMM–10i0.050−18.513020.83Duroid 60100.025−26.02057.60GIL MC 50.030−27.604310.86FR 4 (proposed antenna)0.0014−51.3714


## 5. Conclusions

The proposed UHF-RFID reader antenna is realized using FR 4 as the substrate with optimum thickness, and perfect electric conductor (PEC) is equipped as the radiating stub and accomplishes circular polarization with an operating frequency of 900 MHz. The selected material FR 4 is covalently tethered with PEC, thus forming a strong network structure. It is very much compact due to implementation of the fractal structure, and an acceptable gain of 1.35°dBi and 1.9 m read range are achieved. It attains circular polarization (<3 dB) from 903 to 929 MHz of 4% with the central frequency, the impedance bandwidth of 256 MHz, and axial bandwidth of 36 MHz. The iteration sequence results in inverted fractal L shapes giving rise to attain circular polarization, which is very much suitable for the RFID near-field reader antenna applications such as item-level tagging and smart shelf.



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*Source: 8475621-2021-07-28.xml*