Dual-Band Monopole Antenna for RFID Applications

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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].

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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 presentedsize, 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.

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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 Ω 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. Results and Discussions 3.3. Results and Discussions The motive behind the presented design is to achieve a dual-band characteristic for use in RFID TheThe motivemotive behindbehind thethe presentedpresented designdesign isis toto achieveachieve aa dual-banddual-band characteristiccharacteristic forfor useuse inin RFIDRFID applications. This has been achieved by using the presented antenna design with a modified radiation applications.applications. ThisThis hashas been achieved by using the presented antenna design with a modified modified radiation patch. Return loss characteristics of the rectangular monopole antenna (Figure 2a), the antenna with patch.patch. ReturnReturn lossloss characteristicscharacteristics ofof thethe rectangular rectangular monopole monopole antenna antenna (Figure (Figure2a), 2a), the the antenna antenna with with a a Γ-shaped radiating patch (Figure 2b), and the antenna with a modified F-shaped radiator (Figure Γa-shaped Γ-shaped radiating radiating patch patch (Figure (Figure2b), 2b), and and the the antenna antenna with with a modified a modified F-shaped F-shaped radiator radiator (Figure (Figure2c) 2c) are illustrated compared in Figure 3. are2c) illustratedare illustrated compared compared in Figure in Figure3. 3.

(a) (b) (c) (a) (b) (c) Figure 2. Different structures of the antenna, (a) rectangular-shaped, (b) Γ-shaped, and (c) F-shaped. Figure 2. Different structures of the antenna, ((aa)) rectangular-shaped,rectangular-shaped, ((bb)) ΓΓ-shaped,-shaped, andand ((cc)) F-shaped.F-shaped.

Figure 3. The return losses for the structures shown in Figure2. Figure 3. The return losses for the structures shown in Figure 2. Figure 3. The return losses for the structures shown in Figure 2.

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Future Internet 20192019,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 4 4 ofof 1010 It is observed that the lower frequency bandwidth (2.2–2.6 GHz) with a resonance at 2.6 GHz is affectedItIt isis observedobserved by Γ-shaped thatthat thethe structure, lowerlower frfr andequency the upper-frequency bandwidth (2.2–2.6 bandwidth GHz) with (5.2–6.8 a resonance GHz), at resonating 2.6 GHz is at affected5.8 GHz by, is Γ created-shaped-shaped and structure,structure, improved andand by thethe adding upper-frequencyupper-frequency a short rectangular babandwidth strip, (5.2–6.8 which GHz), modifies resonating the radiation at 5.8 GHz,patch is to created the F-shaped and improved structure. by The adding optimized a short length rectangular Lresonance strip,is setwhich to resonate modifies around the radiation 0.25λresonance patch , − totowhere thethe F-shapedF-shaped Lresonance1 structure.structure.= L3 + TheThe L2 + optimizedoptimized L + L4 + lengthlength (W LLWresonanceresonance3) +L isis1 setsetand toto L resonateresonateresonance2 aroundaround= L3 + 0.250.25 W2λresonanceresonance+ Wf/2,, wherewhere + L2 . λ λ Lresonance1resonance1resonance1 == LLand33 ++ LL22resonance2 ++ LL ++ LL44 correspond++ (W(W − WW33)) +L to+L11 the andand first LLresonance2resonance2 and second == LL33 ++ WW resonance22 ++ WWff/2/2 ++ frequencies LL22.. λresonance1resonance1 (2.4 andandGHz λresonance2resonance2 and correspond5.8 GHz), respectively. to the first and second resonance frequencies (2.4 GHz and 5.8 GHz), respectively. TheThe voltage voltage standing standing wave wave ratio ratio (VSWR) (VSWR) and and Smith-Chart Smith-Chart results results of of the the antenna antenna are are represented represented ininin FigureFigure Figure 4.4.4. ItIt is Itis seenseen is seen thatthat that thethe antennaantenna the antenna properlyproperly properly resonatesresonates resonates atat 2.42.4 andand at 2.4 5.85.8 andGHz.GHz. 5.8 AsAs GHz. shown,shown, As itit operatesoperates shown, fromfromit operates 2.22.2 toto from 2.62.6 2.2GHzGHz to 2.6(400(400 GHz MHzMHz (400 impedance-bandwidth)impedance-bandwidth) MHz impedance-bandwidth) andand fromfrom and from 5.35.3 to 5.3to 6.86.8 to 6.8 GHzGHz GHz (1500(1500 (1500 MHzMHz MHz impedance-bandwidth).impedance-bandwidth).impedance-bandwidth). InIn In addition,addition, addition, thethe the reflectedreflected reflected phasephase phase ofof of thethe the antennaantenna antenna SS S1111 andandand itsits its realreal real andand and imaginaryimaginary imaginary partsparts are are illustrated illustrated in in Figure Figure 55,, respectively.respectively. AsAs seen,seen, thethe effectseffects ofof thethe resonancesresonances atat 2.4/5.82.4/5.8 GHz are evidentevident in in the the provided provided results. results.

((a)) ((b))

FigureFigure 4. 4. ((a(a)) ) VSWRVSWR VSWR andand and ((b (b)) ) Smith-ChartSmith-Chart Smith-Chart resultresult result ofof of thethe the antenna.antenna. antenna.

((a)) ((b))

FigureFigure 5. 5. ((a(a)) ) Phase,Phase, Phase, ((b (b)) ) realreal real andand and imaginaryimaginary imaginary partsparts parts ofof of thethe the antenna.antenna. antenna.

InInIn orderorder order toto to furtherfurther further understandunderstand understand thethe the principleprinciple principle ofof of thethe dual-banddual-band characteristic,characteristic, simulatedsimulated simulated surfacesurface surface currentcurrent densitiesdensities forfor the the resonant resonant frequencies frequencies on the onradiation thethe radiationradiation patch andpatchpatch the andand ground thethe planegroundground are planeplane illustrated areare illustratedillustratedin Figure6 .inin As FigureFigure seen in 6.6. Figure AsAs seenseen6a, atinin 2.4FigureFigure GHz, 6a,6a, the atat current 2.4 GHz, ﬂow the contraries current flow on the contraries interior edge on the of theinterior ﬁrst edge of the first resonator (the main arm of the modified F-shaped radiation patch). At 5.8 GHz, the current flows are highly dominated around the second armarm ofof thethe radiationradiation patch,patch, asas illustratedillustrated inin Figure 6b. Figure 6c,d show the current distributions in the ground plane of the proposed design at

Future Internet 2019, 11, 31 5 of 10 resonator (the main arm of the modified F-shaped radiation patch). At 5.8 GHz, the current flows are highly dominated around the second arm of the radiation patch, as illustrated in Figure6b. Figure6c,d Future Internet 2019, 11, x FOR PEER REVIEW 5 of 10 show the current distributions in the ground plane of the proposed design at the resonant frequencies: the currentthe flow resonant concentrates frequencies: around the current the top flow edge concentrat of the groundes around plane. the Intop addition, edge of the it is ground observed plane. that In at 5.8 GHz,addition, the direction it is observed of current that flowsat 5.8 changesGHz, the atdirection the edges of current of the groundflows changes plane inat comparisonthe edges of tothe that at theground first resonance plane in comparison (2.4 GHz). to that at the first resonance (2.4 GHz).

Figure 6. Current distributions at (a) 2.4 GHz and (b) 5.8 GHz on the radiation patch, and (c) 2.4 GHz Figure 6. Current distributions at (a) 2.4 GHz and (b) 5.8 GHz on the radiation patch, and (c) 2.4 GHz and (d) 5.8 GHz in the ground plane. and (d) 5.8 GHz in the ground plane. Results of varying fundamental design parameters, W ,W , L, L ,L ,L ,L , and L are shown Results of varying fundamental design parameters,1 W2 1, W2,1 L, L2 1, L32, L43, L4, andgnd Lgnd are shown in Figurein7a–h. Figure Figure 7a–h.7 a,bFigure shows 7a,b the shows effects the ofeffects L and of L and2 on L the2 on impedancethe impedance matching; matching; the the operation operation frequencyfrequency bands of bands the antennaof the antenna are not are affected not affected signiﬁcantly. significantly. In In contrast, contrast, asas shown in in Figure Figure 7c,d,7c,d, the the returnreturnloss characteristic loss characteristic of the of antennathe antenna can can be tunedbe tune atd bothat both of theof the resonant resonant frequencies frequencies for for different different values ofvalues L1 and of W L11 .and Figure W1.7 Figuree,f illustrates 7e,f illustrates the impact the impact of different of different values values for L for3 and L3 and W 2Won2 on the the antenna antenna operationoperation frequency; frequency; they have they signiﬁcant have significant impacts impacts on the on second the second resonant resonant frequency frequency (5.8 (5.8 GHz GHz)) with with very little impact on the first resonant frequency (2.4 GHz). It is observed from Figure 7g,h that the

Future Internet 2019, 11, 31 6 of 10 veryFuture little Internet impact 2019 on, 11,the x FOR ﬁrst PEER resonant REVIEW frequency (2.4 GHz). It is observed from Figure7g,h 6 that of 10 the isolation and impedance-matching characteristics of the antenna at both resonant frequencies are isolation and impedance-matching characteristics of the antenna at both resonant frequencies are highly depended on values of Lgnd and L4. Figure8 illustrates the antenna fundamental radiation highly depended on values of Lgnd and L4. Figure 8 illustrates the antenna fundamental radiation characteristicscharacteristics over over the the operation operation bands. bands. AsAs shown,shown, more more than than 75% 75% radiation radiation and and total total efficiencies efﬁciencies (R.E.(R.E. and and T.E.) T.E.) have have been been achieved achieved at at the the frequency frequency bands.bands. More More than than 2 2dBi dBi and and 4 dBi 4 dBi maximum maximum gain gain levelslevels are are obtained obtained for for the the antenna antenna at at 2.4 2.4 and and 5.85.8 GHz,GHz, respectively.

(a) (b)

(e) (f)

(g) (h)

FigureFigure 7. Return 7. Return loss loss results results for for different different values values ofof ((aa)) L,L, ((bb)L) L2,,( (cc))W W11, ,((dd) )LL1, 1(,(e)e L)L3, (3f,() Wf)W2, (g2,() Lggnd)L, gnd, 4 andand (h)L (h4). L .

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FigureFigure 8. 8. TheThe antenna antenna fundamental fundamental radiation radiation characteristics. characteristics.

TopTop and bottombottom viewsviews forfor thethe prototypeprototype areare illustratedillustrated inin FigureFigure9 .9. TheThe antennaantenna hashas beenbeen 3 constructedconstructed on on a a low-cost low-cost FR4 FR4 dielectric dielectric substrate substrate with with an an overall overall dimension dimension of of40 40× 28× ×28 1.6× mm1.633 mm.. TheThe . measuredThe measured and simulated and simulated return return loss characteristics loss characteristics of the of antenna the antenna are represented are represented in Figure in Figure 10. As10 . canAs can be observed, be observed, the the antenna antenna exhibits exhibits good good return return loss loss characteristic, characteristic, covering covering 2.4/5.8 2.4/5.8 GHz GHz resonant resonant frequencies,frequencies, whichwhich comparedcompared withwith thethe simulatedsimulated result,result, thethe agreemenagreemen agreementtt isis acceptable.acceptable. However,However, However, therethere isis aa slightslight discrepancydiscrepancy betweenbetween themthem whichwhich couldcould bebe mostlymostly duedue toto possiblepossible errorserrors inin thethe prototype prototype dimensionsdimensions as as well well as the permittivi permittivitytyty of of the the high-loss high-loss FR-4 FR-4 substrate. substrate.

((a)) ((b))

FigureFigure 9. 9. FabricatedFabricated antenna, antenna, ( (aa))) toptop top andand (( bb)) bottombottom views.views.

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Figure 10. Measured and simulated return losses of the antenna. Figure 10. Measured and simulated return losses of the antenna. The simulated andFigure measured 10. Measured 2D-polar and radiation simulated patternsreturn losses of the of antennathe antenna. for H-plane and E-plane at 2.4The and simulated 5.8 GHz and are measuredillustrated 2D-polar in Figure radiation 11; the patterns antenna of provides the antenna omnidirectional for H-plane and radiation E-plane patternsat 2.4The and insimulated 5.8 the GHz H-plane, areand illustrated measuredwhile 8-shaped in 2D-polar Figure radiation 11 radiation; the patt antenna patternserns providesin E-plane of the omnidirectional antenna have been for achieved H-plane radiation andfor different E-plane patterns atresonantin 2.4 the H-plane,and frequencies. 5.8 GHz while are 8-shaped The illustrated simula radiationtion in Figureand patterns measurement 11; in the E-plane antenna haveresults provides been of achievedthe omnidirectional antenna for different gain versusradiation resonant its patternsoperationfrequencies. in frequency the The H-plane, simulation are while plotted and 8-shaped measurementin Figure radiation 12; resultsthe patt antenna oferns the in antennaprovides E-plane gain havesufficient versus been itsgainachieved operation levels for with frequencydifferent more resonantthanare plotted 2 dB frequencies.atin 2.6 Figure GHz and12 The; the3.5 simula antennadB at tion5.8 provides GHz. and measurement sufﬁcient gain results levels of with the more antenna than gain 2 dB atversus 2.6 GHz its operationand 3.5 dB frequency at 5.8 GHz. are plotted in Figure 12; the antenna provides sufficient gain levels with more than 2 dB at 2.6 GHz and 3.5 dB at 5.8 GHz.

Figure 11. Simulated and measured 2D-polar radiation patterns. Figure 11. Simulated and measured 2D-polar radiation patterns.

Figure 11. Simulated and measured 2D-polar radiation patterns.

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Figure 12. Measured and simulated gain results of the antenna. Figure 12. Measured and simulated gain results of the antenna. 4. Conclusions 4. Conclusions A design of dual-band antenna covering 2.4/5.8 GHz has been presented for RFID applications. To achieveA design the of dual-band dual-band function, antenna covering a monopole 2.4/5.8 antenna GHz has with been a radiation presented patch for RFID similar applications. to F shape Towas achieve designed the anddual-band its properties function, were a monopole investigated. antenna The with impedance-bandwidth a radiation patch similar of the to antenna F shape spans was designedfrom 2.2–2.6 and GHz its properties and 5.3–6.8 were GHz, investigated. providing The broad impedance-bandwidth bandwidth and good of radiation the antenna characteristics. spans from 2.2–2.6The antenna GHz and provides 5.3–6.8 omnidirectional GHz, providing radiation broad bandwidth patterns with and appropriate good radiation gain valuescharacteristics. at both of The the antennaoperation provides bands. Theomnidirectional antenna is simple radiation and might patterns be awith suitable appropriate candidate gain for usevalues in RFID at both systems. of the operation bands. The antenna is simple and might be a suitable candidate for use in RFID systems. Author Contributions: Conceptualization, N.O.P. and H.J.B.; methodology, N.O.P. and H.J.B.; software, N.O.P. Authorand H.J.B.; Contributions: validation, N.O.P., Conceptualization, H.J.B., and R.A.A.-A.; N.O.P. an formald H.J.B.; analysis, methodology, N.O.P., H.J.B.,N.O.P. andand R.A.A.-A.;H.J.B.; software, investigation, N.O.P. N.O.P., H.J.B., and R.A.A.-A.; resources, N.O.P., H.J.B., and R.A.A.-A.; data curation, N.O.P., H.J.B., and R.A.A.-A.; andwriting—original H.J.B.; validation, draft N.O.P., preparation, H.J.B., and N.O.P., R.A.A.-A.; H.J.B., formal and J.M.N.; analysis, writing—review N.O.P., H.J.B., and and R.A.A.-A.; editing, N.O.P.,investigation, H.J.B., N.O.P.,R.A.A.-A., H.J.B., and and J.M.N.; R.A.A.-A.; visualization, resources, N.O.P., N.O.P., H.J.B., H.J.B., R.A.A.-A., and R.A.A.-A.; and J.M.N. data curation, N.O.P., H.J.B., and R.A.A.- A.; writing—original draft preparation, N.O.P., H.J.B., and J.M.N.; writing—review and editing, N.O.P., H.J.B., Funding: This project has received funding from the European Union’s Horizon 2020 research and innovation R.A.A.-A.,program under and J.M.N.; grant agreement visualization, H2020-MSCA-ITN-2016 N.O.P., H.J.B., R.A.A.-A., SECRET-722424. and J.M.N. FundingAcknowledgments:: This projectAuthors has received wish tofunding express from their the thanks European to the Union’s support Horizon provided 2020 by research the innovation and innovation program programunder grant under agreement grant agreement H2020-MSCA-ITN-2016 H2020-MSCA-ITN-2016 SECRET-722424. SECRET-722424. Acknowledgments:Conflicts of Interest: AuthorsThe authors wishdeclare to express noconflict their thanks of interest. to the support provided by the innovation program under grant agreement H2020-MSCA-ITN-2016 SECRET-722424. References Conflicts of Interest: The authors declare no conflict of interest. 1. Finkenzeller, K. RFID Handbook: Radio-Frequency Identification Fundamentals and Applications, 2nd ed.; Wiley: ReferencesNew York, NY, USA, 2004. 2. Chawla, V.; Ha, D.S. An overview of passive RFID. IEEE Commun. Mag. 2007, 45, 11–17. [CrossRef] 1. Finkenzeller, K. RFID Handbook: Radio-Frequency Identification Fundamentals and Applications, 2nd ed.; Wiley: 3. Bell, M.S. RFID Technology and Applications; Cambridge University Press: London, UK, 2011; pp. 6–8. New York, NY, USA, 2004. 4. Siakavara, K.; Goudos, S.; Theopoulos, A.; Sahalos, J. Passive UHF RFID tags with specific printed antennas 2. Chawla, V.; Ha, D.S. An overview of passive RFID. IEEE Commun. Mag. 2007, 45, 11–17. for dielectric and metallic objects applications. Radioengineering 2017, 26, 735–745. [CrossRef] 3. Bell, M.S. RFID Technology and Applications; Cambridge University Press: London, UK, 2011; pp. 6–8. 5. Zeng, Y.; Chen, Z.N.; Qing, X.; Jin, J.-M. A directional, closely spaced zero-phase-shift-line loop array for 4. Siakavara, K.; Goudos, S.; Theopoulos, A.; Sahalos, J. Passive UHF RFID tags with specific printed antennas UHF near-field RFID reader antennas. IEEE Trans. Antennas Propag. 2018, 66, 5639–5642. [CrossRef] for dielectric and metallic objects applications. Radioengineering 2017, 26, 735–745. 6. Chang, L.; Wang, H.; Zhang, Z.; Li, Y.; Feng, Z. Compact single feed dual-mode antenna for active RFID tag 5. Zeng, Y.; Chen, Z.N.; Qing, X.; Jin, J.-M. A directional, closely spaced zero-phase-shift-line loop array for application. IEEE Trans. Antennas Propag. 2015, 63, 5190–5194. [CrossRef] UHF near-field RFID reader antennas. IEEE Trans. Antennas Propag. 2018, 66, 5639–5642. 7. Liu, Q.; Shen, J.; Yin, J.; Liu, H.; Liu, Y. Compact 0.92/2.45-GH dual-band directional circularly polarized 6. Chang, L.; Wang, H.; Zhang, Z.; Li, Y.; Feng, Z. Compact single feed dual-mode antenna for active RFID microstrip antenna for handheld RFID reader applications. IEEE Trans. Antennas Propag. 2015, 63, 3849–3856. tag application. IEEE Trans. Antennas Propag. 2015, 63, 5190–5194. [CrossRef] 7. Liu, Q.; Shen, J.; Yin, J.; Liu, H.; Liu, Y. Compact 0.92/2.45-GH dual-band directional circularly polarized 8. Wang, B.; Wang, W. A miniature tri-band RFID reader antenna with high gain for portable devices. Int. 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