Design of Compact Tri-Band Fractal Antenna for RFID Readers

In this paper, a multiband and miniature rectangular microstrip antenna is designed and analyzed for Radio Frequency Identification (RFID) reader applications. The miniaturization is achieved using fractal technique and the physical parameters of the structure as well as its ground plane are optimized using CST Microwave Studio. The total area of the final structure is 71.6 x 94 mm 2 . The results show that the proposed antenna has good matching input impedance with a stable radiation pattern at 915 MHz, 2.45 GHz, and 5.8 GHz.


INTRODUCTION
Radio Frequency Identification (RFID) is the wireless use of electromagnetic field to identify tagged objects and is used in a variety of fields such as access control, transport, banks, health, and logistic. An RFID system is generally composed of a reader and tags. The communication between the reader and the tags is achieved by modulated backscattering of the reader's carrier wave signal. Most RFID systems operate in either the low frequency band (30-300 kHz), the high frequency band , the ultra-high-frequency band (300 MHz-3 GHz), or the microwave band (3GHz-40 GHz) [1][2][3][4][5].
In this paper, a miniature low cost microstrip multiband antenna, based on the Koch fractal structure, is proposed. Using CST-MW Studio, the antenna is designed and optimized to operate at 915 MHz, 2.45 GHz, and 5.8 GHz frequencies.

RESEARCH METHOD
The proposed antenna is a rectangular radiating patch fed by a 50 ohms microstrip line and uses an FR4 substrate with dielectric constant ε r = 4.4, loss tangent tan = 0.025, thickness H = 1.60 mm, and metal thickness t = 0.035 mm.

Conventional Patch Antenna
Rectangular patch antenna has two dimensions, the length L patch and width W patch , which are related to the resonant frequency, to the permittivity and to the thickness of substrate by the following conventional equations discussed in [24]: where c is the speed of light, f is the resonant frequency and is the substrate's dielectric constant L eff is the effective length given by: and ΔL is the length extension, given by: where, h is the height of substrate and is the effective dielectric constant which can be determined by: In this case, the dimensions of the conventional patch antenna at frequency 915MHz are: L patch = 76 mm, W patch = 99 mm.

Fractal and Slots Techniques
Fractals antennas use the space-filling properties to miniaturize the classic antenna elements. The line that is used to represent the fractal can meander to fill the available space. This line is electrically long but compacted into a small physical space [24].
The size reduction of the proposed microstrip antenna is achieved by etching the patch edges according to Koch curve. Multiple iterations of the Koch fractal are shown in Figure 1. To form the first iteration, the original line segment is partitioned into n=9 equal line segments of 1/5 the original length. This corresponds to a reduction of 9/5 for the first iteration, (9/5) 2 for the second iteration, and so on.
Three slots are inserted into the patch antenna in order to adjust the other resonances generated by the fractal technique. By this insertion the current is forced to flow through a long path around the slots and change the additional resonance around 2.45 GHz and 5.8 GHz. Figure 2 shows the proposed patch antenna with fractal side of length L and width W, a microstrip feed line of length L f and width W f , and three inserted slots. The conducting ground plane with length L gnd , which is placed on the other side of the substrate, is optimized to have a good gain. The width W f of the microstrip feed line is fixed at 2.7 mm. The dimensions of the proposed antenna are presented in Table 1.

RESULTS AND ANALYSIS 3.1. Simulation Results
Using the fractal structure and the slots technique to minimize the patch size, we have proposed an antenna structure with fractal side and three slots. The influence of different parameters of the proposed antenna has been studied by using CST simulation software which is based on the Finite Integration Technique. Figure 3, shows the influence of different slots lengths values (L s1 , L s2 , L s3 ) to adjust the resonance antenna at 915 MHz, 2.45 GHz and 5.8 GHz. Figure 4 shows the return loss S 11 of the proposed antenna after many optimizations, which has good matching input impedance at the three resonant frequencies of 915 MHz, 2.45 GHz, and 5.8 GHz. It is also noted that a return loss of less than -10 dB was achieved for all three frequencies with a bandwidth varying from 68 MHz to 168 MHz.   Table 2, are very suitable for RFID applications.  Table 3 compare the size of the proposed patch structure (69.6 x 72 mm 2 ) with the size of a traditional rectangular patch antenna (99 x 77.6 mm 2 ) at the same operating frequency of 915 MHz. This corresponds to 34.77% reduction in size. This antenna is also smaller than the fractal antenna proposed in

Experimental Results
A prototype of the antenna has been realized as shown in Figure 9.  Table 4, show good agreement with a return loss of approximately -10 dB at the tri-band frequencies of 915 MH, 2.45 GHz, and 5.8 GHz.

CONCLUSION
This paper proposed a new tri-band patch antenna for RFID readers. The design was based on fractal structures and slot techniques to achieve a size reduction of 35% when compared with a conventional rectangular patch. The antenna was designed using a standard FR4 substrate and realized with conventional Printed Circuit Board (PCB) techniques. A return loss of less than -10 dB was achieved with a bandwidth varying from52 MHz to 370 MHz and a gain between 2.62 dBi and 3.31 dBi.