future internet Article Dual-Band Monopole Antenna for RFID Applications Naser Ojaroudi Parchin 1,* , Haleh Jahanbakhsh Basherlou 2 , Raed A. Abd-Alhameed 1 and James M. Noras 1 1 Faculty of Engineering and Informatics, School of Electrical Engineering and Computer Science, University of Bradford, Bradford BD7 1DP, UK; [email protected] (R.A.A.-A.); [email protected] (J.M.N.) 2 Microwave Technology Company, Ardabil 56158-46984, Iran; [email protected] * Correspondence: [email protected]; Tel.: +44-7341436156 Received: 30 December 2018; Accepted: 26 January 2019; Published: 30 January 2019 Abstract: Over the past decade, radio-frequency identification (RFID) technology has attracted significant attention and become very popular in different applications, such as identification, management, and monitoring. In this study, a dual-band microstrip-fed monopole antenna has been introduced for RFID applications. The antenna is designed to work at the frequency ranges of 2.2–2.6 GHz and 5.3–6.8 GHz, covering 2.4/5.8 GHz RFID operation bands. The antenna structure is like a modified F-shaped radiator. It is printed on an FR-4 dielectric with an overall size of 38 × 45 × 1.6 mm3. Fundamental characteristics of the antenna in terms of return loss, Smith Chart, phase, radiation pattern, and antenna gain are investigated and good results are obtained. Simulations have been carried out using computer simulation technology (CST) software. A prototype of the antenna was fabricated and its characteristics were measured. The measured results show good agreement with simulations. The structure of the antenna is planar, simple to design and fabricate, easy to integrate with RF circuit, and suitable for use in RFID systems. Keywords: dual-band antenna; microstrip-fed printed antenna; monopole antenna; RFID; wireless communication 1. Introduction RFID is a recent outstanding technology which uses radio frequency (RF) signals for the identification of objects and has been employed in many applications, such as service industries and moving vehicle identification [1]. In general, RFID is a paste and tag antenna for the purpose of identification and tracking by radio. An RFID system contains three major components: a transponder, a reader, and a computer for data processing. The reader (Interrogator) includes an antenna, which communicates with the TAG [2]. The antenna should be inexpensive, light, simple, and easy to fabricate [3]. Different frequency bands of the electromagnetic spectrum, such as 130 kHz (low-frequency), 13.5 MHz (high-frequency), 900 MHz (ultra-high-frequency), 2.4 GHz, 5.8 GHz, and 24 GHz (microwave frequency) are allocated for RFID applications [4]. Recently, several antenna designs with single, dual, and multi-band characteristics were reported in the literature for RFID applications [5–8]. A closely-spaced loop antenna array with directional radiation patterns is proposed in [5] to operate at 900 MHz. In [6], an active RFID tag antenna operating at 2.4 GHz has been introduced for RFID applications. In [7], a dual-band circularly-polarized antenna is designed for covering both 900 MHz and 2.4 GHz frequency bands. Another design with multi-band function is proposed in [8], which can cover 0.9/2.4/5.8 GHz RFID frequency bands. In this study, 2.4 GHz and 5.8 GHz are selected as the desired dual operation bands for RFID applications [9–12]. Future Internet 2019, 11, 31; doi:10.3390/fi11020031 www.mdpi.com/journal/futureinternet Future Internet 2019, 11, 31 2 of 10 Printed monopole antennas are very attractive and suitable for dual-band or multi-band applications owing to their simple structures, compact size, good impedance matching, Future Internet 2019, 11, x FOR PEER REVIEW 2 of 10 and omnidirectional radiation patterns [13]. Multi-band monopole antennas can be realized by employingPrinted parasitic monopole structures, antennas slots, are or slitsvery inattractive the antenna and configurationsuitable for dual-band or using or various multi-band radiating elementsapplications with different owing to shapes their [simple14]. Configuration structures, compact of the size, presented good designimpedance consists matching, of a modifiedand F-shapedomnidirectional radiation patch,radiation a rectangular patterns [13]. microstrip Multi-band feed-line, monopole andantennas a ground can be plane. realized The by antennaemploying with a planarparasitic structure structures, is designed slots, onor anslits FR-4 in the substrate. antenna configuration Its frequency or operation using various covers radiating the frequency elements bands fromwith 2.2–2.6 different GHz andshapes from [14]. 5.3–6.8 Configuration GHz. The of antennathe presented provides design omnidirectional consists of a modified radiation F-shaped patterns at bothradiation of the desired patch, frequencya rectangular bands microstrip (2.4 and feed-line, 5.8 GHz). and a ground plane. The antenna with a planar structure is designed on an FR-4 substrate. Its frequency operation covers the frequency bands from In contrast to the reported RFID antenna designs [5–12], the presented design exhibits wider 2.2–2.6 GHz and from 5.3–6.8 GHz. The antenna provides omnidirectional radiation patterns at both impedance bandwidth at lower and upper operation bands with higher gain values, especially at the of the desired frequency bands (2.4 and 5.8 GHz). upper band.In contrast The antenna to the operationreported RFID bandwidth antenna isdesi highlygns [5–12], sensitive the presented to small changesdesign exhibits in the fabricationwider process.impedance There mustbandwidth be very at lower strict and guidelines upper operation in order ba tonds avoid with higher shifting gain of values, the resonance especially to at lower the or higherupper frequencies band. The outside antenna the operation required bandwidth narrow bandis highly of interest.sensitive Thisto small drawback changes couldin the fabrication be avoided if a widebandprocess. antenna There must is designed be very insteadstrict guidelines [15]. The in proposed order to avoid RFID shifting antenna of providesthe resonance more to than lower 85% or total efficiencyhigher characteristic frequencies outside at the the resonant required frequencies narrow band (2.4/5.8 of interest. GHz). This drawback could be avoided if a wideband antenna is designed instead [15]. The proposed RFID antenna provides more than 85% 2. Antennatotal efficiency Design characteristic and Configuration at the resonant frequencies (2.4/5.8 GHz). Configuration2. Antenna Design of the and antenna Configuration is illustrated in Figure1. It is printed on a low-cost FR-4 dielectric, whose relative permittivity, loss tangent, and thickness are 4.4, 0.025, and hsub = 1.6 mm, respectively. Configuration of the antenna is illustrated in Figure 1. It is printed on a low-cost FR-4 dielectric, The FR4 dielectric combines good electrical features, price, and availability. Compared with other whose relative permittivity, loss tangent, and thickness are 4.4, 0.025, and hsub = 1.6 mm, respectively. materials,The FR4 FR4 dielectric material combines is sufficiently good electrical cheap andfeatures, available price, inand the availability. market and Compared has been with widely other used in antennamaterials, designing FR4 material for frequencies is sufficiently less cheap than and 6 av GHz.ailable Moreover, in the market FR-4 and material has been is widely available used in in more thicknessantenna values designing than for other frequencies ones, which less than gives 6 GHz. more Moreover, design flexibility. FR-4 material However, is available as thein more antenna operationthickness frequency values isthan increased, other ones, the permittivitywhich gives ofmore FR-4 design varies flexibility. and loss inHowever, the substrate as theincreases antenna [16]. Therefore,operation for ultra-highfrequency is frequency increased, designs,the permittivity it is better of FR-4 to usevaries materials and loss pronein the substrate to loss, such increases as Arlon, Roger,[16]. and Therefore, others. for ultra-high frequency designs, it is better to use materials prone to loss, such as Arlon, Roger, and others. Figure 1. (a) Side, (b) top, and (c) bottom views of the antenna. Figure 1. (a) Side, (b) top, and (c) bottom views of the antenna. FutureFuture Internet 2019,, 11,, x 31 FOR PEER REVIEW 33 of of 10 10 Future Internet 2019, 11, x FOR PEER REVIEW 3 of 10 The radiation patch of the designed antenna is connected to a rectangular feed-line with Wf The radiation patch of the designed antenna is connected to a rectangular feed-line with Wf widthThe and radiation Lf length. patch Theof width the designed of the microstrip antenna is feed-line connected is fixed to a rectangular at 3 mm. On feed-line the other with side Wf ofwidth the width and Lf length. The width of the microstrip feed-line is fixed at 3 mm. On the other side of the substrate,and Lf length. a conducting The width ofground the microstrip plane of feed-line Wsub width is fixed and at 3Lgnd mm. length On the is otherplaced. side The of the antenna substrate, is substrate, a conducting ground plane of Wsub width and Lgnd length is placed. The antenna is connecteda conducting to a ground 50 Ω SMA plane connector of Wsub widthfor signal and transmission Lgnd length is. The placed. parameter The antenna values is for connected the antenna to a connected to a 50 Ω SMA connector for signal transmission. The parameter values for the antenna design50 W SMA are listed connector in Table for signal1. transmission. The parameter values for the antenna design are listed design are listed in Table 1. in Table1. Table 1. Parameter values of the antenna design. Table 1.1. ParameterParameter valuesvalues ofof thethe antennaantenna design.design. Parameter Value (mm) Parameter Value (mm) Parameter Value (mm) ParameterParameter Value (mm) (mm) Parameter Value Value (mm) Parameter Parameter Value Value (mm) (mm) WSub 28 Lsub 40 hsub 1.6 WSub 28 Lsub 40 hsub 1.6 WWSubf 283 LLsubf 2040 W hsub 101.6 WWf 33 LLf 20 W W 10 10 Wf 1 4 Wf2 4 L 8 WW1 1 44 WW22 4 L L 8 8 L1 6 L2 3 L3 3 L1L1 66 LL22 3 L L33 33 L4 3 Lgnd 17 W3 3 L4 3 Lgnd 17 W3 3 L4 3 Lgnd 17 W3 3 3.