A Compact Microstrip Slotted Antenna for Dual-band RFID Applications

This paper presents a new dual-band microstrip antenna with embedded slots on a rectangular radiator. It is capable of operating in the two radio frequency identification (RFID) frequency bands simultaneously (UHF and microwave) and has a miniaturized size designed to be easily integrated into portable RFID readers. The simulation results, carried out using the two commercial software packages CST Microwave Studio electromagnetic and ADS, show stable radiation pattern performances and good matching input impedance at the desired operating frequencies. The proposed antenna has overall dimensions of 47×46 mm² and is mounted on an FR4 substrate with dielectric permittivity constant 4.4, thickness 1.6 mm and loss tangent 0.025. The creation of a prototype of the proposed optimal antenna allowed validation of the dual operating bands with -10 dB return-loss bandwidths of approx. 28 MHz centred at 0.868 GHz and of approx. 90 MHz centred at 2.45 GHz, which shows that the proposed antenna is a good candidate for implementation in handheld RFID readers.

Antenna; Dual-frequency operation; L-shaped slot; Microstrip fed; RFID (Radio Frequency Identification); Slot antenna

During the last few years, radio frequency identification (RFID) applications have been growing significantly in many fields, such as medicine, supply chain, intelligent trace, shop security and particularly in the area of electronic article surveillance, in which it has become one of the most important techniques (Brown, 2007; Finkenzeller, 2010; Poespawati & Nugroho, 2016). The RFID system generally consists of two elements: the tag and the reader. The RFID reader acts as an interrogator equipped with an antenna that sends a radio frequency interrogation signal and receives the backscattered signal containing the data stored in the tag, which is composed of an integrated circuit chip connected to an antenna. The basic block diagram of an RFID system is illustrated in Figure 1.

The RFID system can be described as a fusion of different modern technologies from several areas: monolithic integrated circuits; identification technology; coding and modulation (Mujahid & Najam-ul-islam, 2017); cryptography, providing the secure mutual authentication between reader and tag (Paret, 2008; El Hamraoui et al., 2016); and wireless transmission RFID technology operates in the LF band (125–134 kHz), the HF band (13.56 MHz), the Ultra-high frequency (UHF) band (860–960 MHz) and the microwave band (2.45 GHz and 5.8 GHz), the last of which uses the antenna’s far field electromagnetic properties and enables interrogations over a long reading range (Foster & Bueberry, 1999). When operating at microwave frequencies, the RFID reader design becomes crucial, especially the RF front end circuit including the antenna element (Kumagai et al., 2011; Hattan et al., 2012).

Figure 1 Block diagram of RFID system

In the literature, wireless communication devices mostly employ integrated compact microstrip patch antennas due to their light weight, low profile, low cost and easy fabrication, which allows better efficiency and wide bandwidth (Chen, 2002; Chen & Chen, 2004; Abbas et al., 2012; Roy et al., 2014; Zulkifli & Saputro, 2016). Hence they are good candidates for RFID applications. Several printed antennas have been designed for dual band RFID applications in the UHF band (860–960 MHz) and the microwave band (2.45–5.8 GHz); such configurations enable the reader to cover the different allocated frequency bands for RFID technology. A miniaturised multifrequency antenna with circular ring and Y-shaped strip was designed for WiMAX and WLAN operations (Anabi et al., 2011). The Minkowski fractal microstrip antenna presented in (Ghiotto et al., 2006) operates at 868 MHz and 2.45 GHz with radiating patch dimensions of 66×66 mm². In (Sabran et al., 2011), a dual frequency band antenna with an overall size of 190×190×7.635 mm³ was achieved by controlling the shape and size of the diamond shaped patch; the obtained bandwidths were 18 MHz (902–920 MHz) and 80 MHz (2.42–2.5 GHz) in the UHF band ISM bands, respectively. This antenna includes an aluminium plate with a fixed 5 mm air gap, which makes it voluminous and complex to achieve. (Mabaso & Kumar, 2018) presents a dual band antenna consisting of a rectangular patch which is fed by the coaxial probe feeding technique; the ground plane is loaded with two rectangular strip slots and one elliptical slot with dimensions of 50×50 mm².

A new dual-band and miniature microstrip antenna was numerically and experimentally studied. By optimising the lengths and the locations of the inserted slots, it is possible to obtain the two resonating frequencies for the desired bands for RFID applications. The simulated and experimental results validate the proposed antenna with two bandwidths: 90 MHz (2.41–2.5 GHz) and 28 MHz (0.850–0.878 GHz). According to these results, the dual-band antenna can cover the RFID UHF in the European region and microwave bands (2.45 GHz bands). Hence, the proposed antenna is an excellent candidate for handheld RFID readers.

The authors would like to thank Mr Mohamed Latrach, Professor in ESEO, Engineering Institute in Angers, France for providing support and assistance to perform the simulations using software and to obtain the measurements using the VNA and the anechoic chamber available in his laboratory.

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