Abstract — In this paper, a novel design of dual-band

monopole antenna for radio frequency identification (RFID) systems is proposed. The proposed antenna is designed to operate at 2.45 GHz and 5.8 GHz (ISM frequency range) and it is fabricated on an FR4 substrate with dielectric constant of 4.4. The measured impedance bandwidth for l0dB return loss is from 1.82 to 2.92 GHz (49%) and 5.53 to 6.36 GHz (13.0%), covering the 2.4/5.8 GHz RFID operating bands. Keywords — Monopole antenna, RFID, wireless communications.

I. INTRODUCTION ADIO frequency identification (RFID) is a rapidly developing technology which can be used in supply chain management, access control, inventory

management and so on [1]. It is a well-known fact that printed microstrip antenna

presents really appealing physical features, such as simple structure, small size, and low cost. Because of all these interesting characteristics, microstrip antennas are extremely attractive to be used in RFID applications and growing research activity is being focused on them. During the last years, there are various antennas with low profile, lightweight, flush mounted for RFID applications are proposed. These antennas include the CPW-Fed monopole antenna [2], mender-line antennas [3-4], H-ring antenna [5], and etc.

In this paper, a novel design of a dual-band monopole antenna is proposed, which has many advantages such as simple structure of a wide bandwidth and easy integration with active devices. In this structure, by using a minder-line strip and rotated E-shaped slit in the open-stub microstrip line structure, the antenna achieves good dual-band operation. Good return loss and radiation pattern characteristics are obtained in the frequency bands of interest. Simulated and measured results are presented to validate the usefulness of the proposed antenna structure for RFID applications.

II. ANTENNA DESIGN The presented small monopole antenna fed by a

microstrip line is shown in Fig. 1, which is printed on an FR4 substrate of thickness 1.6 mm, permittivity 4.4, and loss tangent 0.018. The basic monopole antenna structure consists of an open-stub microstrip line and a ground plane. The width of the microstrip line line is fixed at 2 mm, as shown in Fig. 1. On the other side of the substrate, a conducting ground plane is placed. The proposed antenna is

N. Ojaroudi is with the Young Researchers Club, Ardabil Branch,

Islamic Azad University, Ardabil, Iran. (E-mail: n.ojaroudi@yahoo.com).

connected to a 50- SMA connector for signal transmission. The final dimensions of the designed antenna are specified in Table 1.

Fig. 1. Geometry of the antenna (a) side view, (b) top view.

Table. 1. The final dimensions of the designed antenna Param. mm Param. mm Param. mm Wsub 10 Lsub 17 Lgnd 3.5

W 2 L 12.5 W1 1.75 L1 0.75 W2 0.5 L2 0.25 W3 0.25 L3 8.25 L4 8.25

In this study, the open stub microstrip line with minder-

line strip structure can provide a single resonant mode at the lower band and by adding an E-shaped slit good dual-band operation can be achieved. Therefore, in the

Design of Microstrip Antenna for 2.4/5.8 GHz RFID Applications

Nasser Ojaroudi

R

proposed configuration the E-shaped slit is playing an important role in the dual band characteristics of this antenna, because it can creates additional surface current paths in the antenna, so an additional resonance is excited [6-8].

III. RESULT AND DISCUSSION In The proposed monopole antenna with various design

parameters was constructed, and the numerical and experimental results of the input impedance and radiation characteristics are presented and discussed. Ansoft HFSS simulations are used to optimize the design and also in the proposed antenna, a good agreement between the simulation and measurement results is obtained [4]. The HFSS is a high performance full wave electromagnetic (EM) field simulator for arbitrary 3D volumetric passive device modelling that takes advantage of the familiar Microsoft Windows graphical user interface. It integrates simulation, visualization, solid modelling, and automation in an easy to learn environment where solutions to 3D EM problems are quickly and accurate obtained. Ansoft HFSS employs the Finite Element Method (FEM), adaptive meshing, and brilliant graphics to give unparalleled performance and insight to all of 3D EM problems. Ansoft HFSS can be used to calculate parameters such as S-Parameters, Resonant Frequency, and Fields [12].

The configuration of the presented monopole antenna was shown in Fig .1. Return loss characteristics for the open stub microstrip line structure antenna (Fig. 2(a)), with minder-line strip (Fig. 2(b)), and the proposed antenna structure (Fig. 2(c)) are compared in Fig 3.

Fig.2. (a) Open-stub microstrip line, (b) open-stub microstrip line with minder-line strip, and (c) the proposed antenna structure.

As shown in Fig.3, it is observed that the lower fre'quency bandwidth is affected by using minder-line strip and generation of dual-band performance is sensitive to the E-shaped slit. Also input impedance of the proposed antenna structure that shown in Fig. 1, on a Smith Chart is shown in Fig. 4. In order to known the phenomenon behind this dual-band performance, simulated current distributions for the proposed antenna at 2.4 GHz are presented in Fig. 5 (a). It can be observed in Fig. 8 (a) that the currents are concentrated on the interior and exterior edges of the

minder-line strip. Therefore, the antenna impedance changes at this frequency due to the resonant properties of the minder-line strip. Another important design parameter of this structure is the E-shaped slit. Fig.5 (b) presents the simulated current distributions of the proposed antenna at 5.8 GHz. As shown in Fig.5 (b), at the second resonance frequency, the current flows are more dominant around of the E-shaped slit structure. [10-12].

Fig.3. Simulated return loss characteristics for the various antenna structures shown in Fig. 2.

Fig.4. Smith chart demonstration of simulated input impedance for the proposed antenna.

The proposed antenna with final design, as shown in

Fig. 6, was built and tested. The return loss characteristic was measured using a network analyzer in an anechoic chamber. The radiation patterns have been measured inside an anechoic chamber using a double-ridged horn antenna as a reference antenna placed at a distance of 2 m. The measured and simulated return loss characteristics of the proposed antenna are shown on Fig. 7. The fabricated

antenna has a frequency band of 2.05 GHz to 2.83 GHz and 5.30 GHz to 6.32 GHz. As shown in Fig. 7, there exists a discrepancy between the measured data and the simulated results.

Fig.5. Simulated surface current distributions for the proposed antenna (a) at 2.45 GHz and (b) at 5.8 GHz.

Fig.6. Photograph of the realized printed monopole antenna.

This discrepancy is mostly due to a number of

parameters such as the fabricated antenna dimensions as well as the thickness and dielectric constant of the substrate on which the antenna is fabricated, and also the wide range of simulation frequencies. In order to confirm the accurate return loss characteristics for the designed antenna, it is recommended that the manufacturing and measurement process need to be performed carefully,

besides, SMA soldering accuracy and FR4 substrate quality needs to be taken into consideration [13-15].

Fig.7. Measured and simulated return loss characteristics for the proposed antenna.

Fig.8. Measured radiation patterns of the proposed antenna at (a) H-plane, and (b) E-plane.

Fig. 8 illustrates the measured radiation patterns, including the co-polarization, in H-plane (E in x-y) and E-plane (E in y-z) . It can be seen that quasi-omnidirectional radiation pattern can be observed on x-z plane for the three frequencies. The radiation patterns on the y-z plane displays a typical figure-of-eight, similar to that of a conventional dipole antenna. It should be noticed that the radiation patterns in E-plane become imbalanced as frequency increases because of the increasing effects of the cross polarization. The patterns indicate At higher frequencies, more ripples can be observed in both E- and H-planes owing to the generation of higher-order modes [16-18].

IV. CONCLUSION A novel dual-band monopole antenna for RFID communications is presented, in this paper. The proposed antenna can operate from 2.05 to 2.83 GHz and 5.30 to 6.32 GHz, covering the 2.4/5.8 GHz RFID operating bands. By applying a minder-line strip and a rotated E-shaped slit in the open-stub microstrip line structure, a good dual band characteristic in ISM frequency range is achieved. The designed antenna has a small size. Simulated and experimental results show that the proposed antenna could be a good candidate for RFID applications.

ACKNOWLEDGMENT The authors are thankful to Microwave Technology (MWT) company staff for their beneficial and professional help (www.microwave-technology.com).

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[13] N. Ojaroudi, Sh. Amiri, and F. Geran, “Reconfigurable Monopole Antenna with Controllable Band-Notched Performance for UWB Communications,” 20th Telecommunications Forum, TELFOR 2012, 20 – 22November, 2012, Belgrade, Serbia

[14] N. Ojaroudi, S. Amiri, and F. Geran, “A novel design of reconfigurable monopole antenna for UWB applications,” Applied Computational Electromagnetics Society (ACES) Journal, vol. 28, no. 6, pp. 633-639, July 2013.

[15] N. Ojaroudi, H. Ojaroudi, Y. Ojaroudi, “Very low profile ultra-wideband microstrip band-stop filter,” Microw. Opt. Technol. Lett., vol. 56, pp. 709-711, 2014.

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[17] N. Ojaroudi, “Compact UWB monopole antenna with enhanced bandwidth using rotated L-shaped slots and parasitic structures,”Microw. Opt. Technol. Lett., vol. 56, pp.175–178, 2014.

[18] N. Ojaroudi, “Microstrip monopole antenna with dual band-stop function for UWB applications,” Microw. Opt. Technol. Lett., vol. 56, 1045-1048.