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Dual-Band Monopole Antenna for RFID Applications - MDPI
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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; r.a.a.abd@bradford.ac.uk (R.A.A.-A.);
      j.m.noras@bradford.ac.uk (J.M.N.)
 2    Microwave Technology Company, Ardabil 56158-46984, Iran; Hale.Jahanbakhsh@gmail.com
 *    Correspondence: n.ojaroudiparchin@bradford.ac.uk; 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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Dual-Band Monopole Antenna for RFID Applications - MDPI
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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,
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and omnidirectional radiation patterns [13]. Multi-band monopole antennas can be realized by
employing      parasitic
            Printed          structures,
                        monopole             slots,are
                                       antennas     or slits
                                                        very in the antenna
                                                              attractive  and configuration       or usingor
                                                                                suitable for dual-band        various    radiating
                                                                                                                 multi-band
elements     with    different     shapes    [14].  Configuration      of  the  presented     design
      applications owing to their simple structures, compact size, good impedance matching, and        consists    of a  modified
F-shaped     radiation patch,
      omnidirectional       radiationa rectangular
                                        patterns [13].microstrip
                                                        Multi-bandfeed-line,
                                                                     monopoleand       a ground
                                                                                 antennas    can beplane.  The
                                                                                                    realized  by antenna
                                                                                                                 employing   with a
planarparasitic
         structurestructures,
                       is designedslots,on
                                         or an
                                            slitsFR-4
                                                  in the antenna configuration
                                                      substrate.   Its frequencyor      using various
                                                                                     operation   covers radiating   elementsbands
                                                                                                          the frequency
from with    different
       2.2–2.6   GHz and  shapesfrom[14].  Configuration
                                       5.3–6.8    GHz. The of antenna
                                                              the presented     design
                                                                         provides        consists of a modified
                                                                                      omnidirectional     radiationF-shaped
                                                                                                                       patterns at
      radiation    patch,   a  rectangular    microstrip
both of the desired frequency bands (2.4 and 5.8 GHz).    feed-line,  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.       The antenna
            In contrast     to the operation
                                     reported RFID bandwidth
                                                       antenna is   highly[5–12],
                                                                 designs     sensitive   to small changes
                                                                                   the presented              in the fabrication
                                                                                                    design exhibits    wider
process.
      impedance bandwidth at lower and upper operation bands with higher gain values, especiallytoatlower
           There     must    be   very   strict  guidelines   in  order  to   avoid   shifting  of the  resonance          the or
higher   frequencies
      upper    band. The   outside
                             antennathe    required
                                        operation      narrow band
                                                    bandwidth          of interest.
                                                                 is highly  sensitive This  drawback
                                                                                       to small changescould      be avoided if a
                                                                                                          in the fabrication
wideband
      process.antenna     is designed
                 There must       be veryinstead     [15]. Theinproposed
                                            strict guidelines                 RFIDshifting
                                                                  order to avoid    antennaofprovides      moretothan
                                                                                                the resonance       lower85%or total
      higher   frequencies      outside   the required   narrow   band
efficiency characteristic at the resonant frequencies (2.4/5.8 GHz).    of  interest. This drawback    could   be avoided    if
      a wideband antenna is designed instead [15]. The proposed RFID antenna provides more than 85%
     total efficiency
2. Antenna    Designcharacteristic at the resonant frequencies (2.4/5.8 GHz).
                      and Configuration
     Configuration
     2. Antenna Designof theandantenna     is illustrated in Figure 1. It is printed on a low-cost FR-4 dielectric,
                                 Configuration
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 FR4FR4  material
               dielectric    is sufficiently
                           combines              cheap and
                                       good electrical        available
                                                        features,  price, in the
                                                                           and    market and
                                                                                availability.    has been
                                                                                              Compared      widely
                                                                                                          with other used
in antenna   designing
     materials,           for frequencies
                FR4 material    is sufficientlyless than
                                                 cheap and6 available
                                                             GHz. Moreover,       FR-4and
                                                                       in the market    material   iswidely
                                                                                            has been  available
                                                                                                             usedininmore
thickness   values  than    other   ones,    which   gives   more   design    flexibility.  However,
     antenna designing for frequencies less than 6 GHz. Moreover, FR-4 material is available in more     as the  antenna
operation  frequency
     thickness  values isthan
                           increased,    the permittivity
                               other ones,     which gives of  FR-4design
                                                             more    variesflexibility.
                                                                              and loss in   the substrate
                                                                                         However,    as theincreases
                                                                                                            antenna [16].
     operation
Therefore,      frequency isfrequency
            for ultra-high     increased, designs,
                                             the permittivity  of FR-4
                                                       it is better     varies
                                                                    to use      and loss prone
                                                                             materials    in the substrate increases
                                                                                                 to loss, such  as Arlon,
     [16].
Roger,  andTherefore,
             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.
Dual-Band Monopole Antenna for RFID Applications - MDPI
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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
       andradiation  patch
            Lf length. Theof  the designed
                            width            antenna isfeed-line
                                    of the microstrip   connected     to a rectangular
                                                                  is fixed   at 3 mm. Onfeed-line  withside
                                                                                           the other     Wf of
                                                                                                            width
                                                                                                               the
 width and Lf length. The width of the microstrip feed-line is fixed at 3 mm. On the other side of the
and Lf length.
substrate,       The width of
            a conducting       the microstrip
                            ground    plane offeed-line  is fixed
                                                 Wsub width    andat 3Lgnd
                                                                        mm.   On theisother
                                                                            length          sideThe
                                                                                       placed.    of the substrate,
                                                                                                       antenna   is
 substrate, a conducting ground plane of Wsub width and Lgnd length is placed. The antenna is
a conducting
connected   to aground   planeconnector
                  50 Ω SMA      of Wsub width    and transmission.
                                          for signal Lgnd length is Theplaced.   The antenna
                                                                             parameter   valuesisfor
                                                                                                  connected    to a
                                                                                                      the antenna
 connected to a 50 Ω SMA connector for signal transmission. The parameter values for the antenna
50 Ω SMA
design       connector
        are listed      for signal
                   in Table 1.     transmission. The parameter values for the antenna design are listed
 design are listed in Table 1.
in Table 1.
                                 Table 1. Parameter values of the antenna design.
                                 Table 1.
                                 Table  1. Parameter
                                           Parameter values
                                                     values of
                                                            of the
                                                                the antenna
                                                                    antenna design.
                                                                            design.
           Parameter      Value (mm) Parameter Value (mm) Parameter Value (mm)
            Parameter
           Parameter      Value(mm)
                          Value    (mm) Parameter
                                        Parameter Value
                                                  Value   (mm) Parameter  Value  (mm)
              WSub            28           Lsub       40(mm)    Parameter
                                                                  hsub     Value
                                                                             1.6 (mm)
              WW
                WSub          28
                                28          L
                                           LL
                                              sub      40
                                                       40
                                                                  h  sub
                                                                   hsub
                                                                              1.6
                Subf            3             f
                                             sub      20          W           101.6
               W Wf            343          LLf f      20
                                                       20          WW          10
                Wf 1                       W2         4            L          810
               WW    1         44           W2
                                            W          4            LL         88
                L11             6           L22       3            L3         3
               L1L1            66           LL22       33          LL33        33
               LL4 4           33         LLgnd
                                             gnd      17
                                                       17         WW3 3       33
                  L4             3          Lgnd       17          W3          3
3. Results and Discussions
3.
 3. Results
    Results and
             and Discussions
                  Discussions
      The motive behind the presented design is to achieve a dual-band characteristic for use in RFID
       The
       The motive
            motive behind
                    behind the
                            the presented
                                 presented design
                                             design is
                                                     is to
                                                        to achieve
                                                           achieve aa dual-band
                                                                      dual-band characteristic
                                                                                 characteristic for
                                                                                                 for use
                                                                                                     use in
                                                                                                         in RFID
                                                                                                            RFID
applications. This has been achieved by using the presented antenna design with a modified radiation
applications.
 applications. This
                This has
                     has been
                         been achieved
                                achieved byby using
                                              using the
                                                     the presented
                                                         presented antenna
                                                                     antenna design
                                                                             design with
                                                                                     with aa modified
                                                                                             modified radiation
                                                                                                        radiation
patch. Return loss characteristics of the rectangular monopole antenna (Figure 2a), the antenna with
patch.
 patch. Return
         Return loss
                 losscharacteristics
                      characteristicsof
                                      ofthe
                                          therectangular
                                              rectangularmonopole
                                                             monopoleantenna
                                                                        antenna(Figure
                                                                                 (Figure2a),
                                                                                         2a),the antenna
                                                                                               the antenna with a
                                                                                                             with
a Γ-shaped radiating patch (Figure 2b), and the antenna with a modified F-shaped radiator (Figure
Γ-shaped
 a Γ-shaped radiating  patch
               radiating patch(Figure 2b),
                                 (Figure    and
                                         2b),    thethe
                                               and    antenna  with
                                                         antenna     a modified
                                                                  with          F-shaped
                                                                        a modified  F-shapedradiator  (Figure
                                                                                                radiator      2c)
                                                                                                          (Figure
2c) are illustrated compared in Figure 3.
are  illustrated compared    in Figure 3.
 2c) are illustrated compared in Figure 3.

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

                         Figure 3. The return losses for the structures shown in Figure 2.
                         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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        It is observed that the lower frequency bandwidth (2.2–2.6 GHz) with a resonance at 2.6 GHz
 is affected     by Γ-shaped
       It is observed        that thestructure,  and the upper-frequency
                                        lower frequency       bandwidth (2.2–2.6      bandwidth
                                                                                           GHz) with   (5.2–6.8    GHz), at
                                                                                                           a resonance       resonating
                                                                                                                                 2.6 GHz is    at
 5.8 GHz,by
affected      is Γ-shaped
                  created and        improved
                                 structure,  andby theadding    a short rectangular
                                                        upper-frequency           bandwidth  strip,   whichGHz),
                                                                                                  (5.2–6.8     modifies      the radiation
                                                                                                                       resonating        at 5.8
GHz, is created and improved by adding a short rectangular strip, which modifies the radiation patch ,
 patch    to the    F-shaped      structure.  The   optimized     length    L resonance   is set  to resonate    around      0.25λ   resonance
 where
to          Lresonance1
    the F-shaped             = L3 + The
                        structure.      L2 +optimized
                                              L + L4 +length (W −LW      3 ) +L
                                                                      resonance
                                                                      resonance
                                                                                     andto Lresonate
                                                                                is1 set                    = L3 +0.25λ
                                                                                             resonance2around         W2 resonance
                                                                                                                            + Wf /2
                                                                                                                            resonance
                                                                                                                                           + L2 .
                                                                                                                                     , where
Lλresonance1
   resonance1
  resonance1 = Land         + L + L44 correspond
                       λ22resonance2
                  33 + L                + (W − W33) +L to 11the
                                                             andfirst  and second resonance frequencies
                                                                  Lresonance2
                                                                    resonance2 = L33 + W22 + Wff/2 + L22. λresonance1
                                                                                                                            (2.4 GHz
                                                                                                                  resonance1 and λresonance2
                                                                                                                                            and
                                                                                                                                       resonance2
 5.8  GHz),    respectively.
correspond to the first and second resonance frequencies (2.4 GHz and 5.8 GHz), respectively.
        Thevoltage
       The    voltagestanding
                           standingwave waveratio
                                               ratio(VSWR)
                                                      (VSWR)and  andSmith-Chart
                                                                        Smith-Chartresultsresultsof  ofthe
                                                                                                         theantenna
                                                                                                             antennaare   arerepresented
                                                                                                                                represented
 in   Figure     4.    It   is  seen   that  the   antenna      properly      resonates
in Figure 4. It is seen that the antenna properly resonates at 2.4 and 5.8 GHz. As shown, it  at  2.4   and   5.8    GHz.        Asoperates
                                                                                                                                       shown,
 it operates     from    2.2   to  2.6 GHz   (400  MHz      impedance-bandwidth)
from 2.2 to 2.6 GHz (400 MHz impedance-bandwidth) and from 5.3 to 6.8 GHz (1500 MHz          and   from    5.3 to   6.8  GHz     (1500    MHz
 impedance-bandwidth). In
impedance-bandwidth).                 Inaddition,
                                         addition,thethereflected
                                                         reflected phase
                                                                       phase of of the
                                                                                    the antenna
                                                                                         antenna SS11      andits
                                                                                                       11and
                                                                                                       11       itsreal
                                                                                                                     realand
                                                                                                                           andimaginary
                                                                                                                                  imaginary
 parts   are  illustrated      in  Figure  5, respectively.     As  seen,    the   effects  of  the  resonances
parts are illustrated in Figure 5, respectively. As seen, the effects of the resonances at 2.4/5.8 GHz                at 2.4/5.8    GHzare   are
 evident     in the   provided
evident in the provided results.     results.

                                    (a)                                                                    (b)
                               Figure
                                Figure4.4.(a)
                                           (a)VSWR
                                               VSWRand
                                                    and(b)
                                                        (b)Smith-Chart
                                                            Smith-Chartresult
                                                                        resultof
                                                                               ofthe
                                                                                  theantenna.
                                                                                      antenna.

                              (a)                                                                    (b)
                                           (a)Phase,
                                Figure5.5.(a)
                               Figure          Phase,(b)
                                                      (b)real
                                                          realand
                                                               andimaginary
                                                                   imaginaryparts
                                                                             partsof
                                                                                   ofthe
                                                                                      theantenna.
                                                                                          antenna.

       Inorder
      In order to
                to further
                    further understand
                              understand the the principle
                                                 principle of
                                                            of the
                                                               the dual-band
                                                                    dual-band characteristic,
                                                                               characteristic, simulated
                                                                                                simulated surface
                                                                                                             surface
 current densities  for the  resonant  frequencies   on the radiation  patch and  the ground
current densities for the resonant frequencies on the radiation patch and the ground plane areplane  are illustrated
 in Figure 6.inAs
illustrated       seen in
                Figure   6. Figure
                            As seen6a,inatFigure
                                           2.4 GHz,
                                                  6a,the current
                                                      at 2.4 GHz, flow
                                                                     thecontraries on the
                                                                         current flow     interior edge
                                                                                       contraries  on theofinterior
                                                                                                            the 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 arm of the radiation patch, as illustrated in
Figure 6b. Figure 6c,d show the current distributions in the ground plane of the proposed design at
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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 Figure 6b. Figure 6c,d
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show the current distributions in the ground plane of the proposed design at the resonant frequencies:
the currenttheflow   concentrates
                resonant    frequencies:around   the topflow
                                            the current  edgeconcentrates
                                                               of the ground  plane.
                                                                          around  theIn
                                                                                      topaddition, it isground
                                                                                          edge of the    observed thatIn
                                                                                                               plane.
at 5.8 GHz,addition, it is observed that at 5.8 GHz, the direction of current flows changes at the edges of to
               the  direction     of  current  flows  changes  at  the edges of the ground  plane   in  comparison   the
that at theground
             first resonance       (2.4 GHz).to that at the first resonance (2.4 GHz).
                     plane in comparison

      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) 5.8and
                   GHz  in the ground plane.
                     (d) 5.8 GHz in the ground plane.

     Results ofResults
                 varying    fundamental
                         of varying          design design
                                      fundamental     parameters,    W1 , WW
                                                              parameters,    2 ,1,L,
                                                                                   WL2,1L,
                                                                                        , LL2 ,1,LL32,, LL43,, Land  L Lgndare
                                                                                                                 4, andgnd    areshown
                                                                                                                                   shown
in Figurein7a–h.   Figure   7a,b  shows    the effects  of L  and
             Figure 7a–h. Figure 7a,b shows the effects of L and   L 2 on  the   impedance           matching;
                                                                          L2 on the impedance matching; the operation  the   operation
frequency    bands ofbands
          frequency     the antenna      are not
                              of the antenna   areaffected   significantly.
                                                   not affected  significantly.InIncontrast,
                                                                                      contrast, as        shownininFigure
                                                                                                    as shown              Figure    7c,d,
                                                                                                                                 7c,d, the
the returnreturn
            loss characteristic   of the
                  loss characteristic  ofantenna    can can
                                          the antenna    be tuned   at both
                                                             be tuned        of the
                                                                        at both        resonant
                                                                                  of the  resonant     frequencies
                                                                                                           frequenciesfor   fordifferent
                                                                                                                                 different
values of values
          L1 andofWL11.and
                         Figure
                             W1. 7e,f  illustrates
                                  Figure            the impact
                                          7e,f illustrates       of different
                                                           the impact             values
                                                                        of different       for Lfor
                                                                                        values       3 andL3 and W2Won    the
                                                                                                                       2 on  theantenna
                                                                                                                                 antenna
          operation   frequency;   they  have  significant  impacts  on  the second      resonant
operation frequency; they have significant impacts on the second resonant frequency (5.8 GHz) with       frequency       (5.8 GHz)   with
            very little impact on the first resonant frequency (2.4 GHz). It is observed from Figure 7g,h that the
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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 . Figure 8 illustrates the antenna fundamental radiation
    highly depended on values of Lgnd and L4. Figure 8 illustrates the antenna fundamental radiation
characteristics      over the operation bands. As shown, more than 75% radiation and total efficiencies
    characteristics over the operation bands. As shown, more than 75% radiation and total efficiencies
(R.E.(R.E.
      andand T.E.)   have
                  T.E.) havebeenbeenachieved
                                       achievedatatthe
                                                    thefrequency
                                                         frequency bands. Morethan
                                                                   bands. More   than2 2dBi
                                                                                         dBi  and
                                                                                            and    4 dBi
                                                                                                4 dBi    maximum
                                                                                                      maximum gaingain
levels  are are
    levels   obtained
                  obtained forfor
                                thethe
                                     antenna
                                        antennaatat2.4
                                                     2.4and
                                                         and5.8
                                                             5.8 GHz, respectively.
                                                                 GHz, respectively.

                                       (a)                                                             (b)

                                       (c)                                                             (d)

                                       (e)                                                              (f)

                                       (g)                                                             (h)
          Figure
      Figure        7. Return
             7. Return     lossloss results
                                 results forfor differentvalues
                                              different   valuesof
                                                                 of(a)
                                                                    (a) L,
                                                                        L, (b)
                                                                            (b) L
                                                                                L22,, (c)
                                                                                      (c) W
                                                                                          W11, ,(d)
                                                                                                 (d)LL
                                                                                                     1, 1(e)  L3L
                                                                                                          , (e) , (f)  W2W
                                                                                                                  3 , (f) , (g)  LgndL, gnd ,
                                                                                                                             2 , (g)
      and and
          (h) L(h)
                4 . L 4.
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                           Figure 8.
                           Figure    The antenna
                                  8. The antenna fundamental
                                                  fundamental radiation
                                                              radiation characteristics.
                                                                        characteristics.

      Top and
      Top   and bottom
                 bottom views
                           views for
                                  for the
                                       the prototype
                                            prototype are
                                                        are illustrated
                                                             illustrated inin Figure
                                                                              Figure 9.   The antenna
                                                                                       9. The   antenna hashas been
                                                                                                                  been
                                                                                                  ××28               3
constructedon
constructed    onaalow-cost
                    low-costFR4FR4dielectric
                                    dielectricsubstrate
                                                substratewith
                                                           with anan  overall
                                                                    overall   dimension
                                                                            dimension    of of
                                                                                            40 40
                                                                                               × 28   1.6×mm
                                                                                                           1.6 3mm
                                                                                                               3 . The .
The measured
measured          and simulated
             and simulated         return
                              return       loss characteristics
                                      loss characteristics  of the ofantenna
                                                                      the antenna   are represented
                                                                               are represented         in Figure
                                                                                                  in Figure   10. As10.
As  can  be observed,  the  antenna  exhibits  good  return loss  characteristic, covering   2.4/5.8
can be observed, the antenna exhibits good return loss characteristic, covering 2.4/5.8 GHz resonant GHz    resonant
frequencies, which
frequencies,   which compared
                       compared with
                                   with the
                                         the simulated
                                             simulated result,
                                                         result, the
                                                                  the agreement
                                                                       agreement isis acceptable.
                                                                                      acceptable. However,
                                                                                                   However, therethere
is a slight  discrepancy   between   them   which   could  be  mostly    due to possible  errors
is a slight discrepancy between them which could be mostly due to possible errors in the prototypein the  prototype
dimensions as
dimensions     as well
                  well as
                       as the
                          the permittivity
                              permittivity ofof the
                                                the high-loss
                                                    high-loss FR-4
                                                                FR-4 substrate.
                                                                       substrate.

                            (a)                                                        (b)
                            Figure 9.
                            Figure    Fabricated antenna,
                                   9. Fabricated antenna, (a)
                                                           (a) top
                                                                top and
                                                                    and (b)
                                                                        (b) bottom
                                                                            bottom 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.
                     Figure
      The simulated and     10. Measured
                        measured         andradiation
                                   2D-polar  simulatedpatterns
                                                       return losses of antenna
                                                                 of the the antenna.
                                                                                  for H-plane and E-plane
at 2.4Theandsimulated
                5.8 GHzand  aremeasured
                                 illustrated2D-polar  radiation
                                              in Figure   11; thepatterns
                                                                   antenna of provides
                                                                              the antenna    for H-plane and
                                                                                         omnidirectional         E-plane
                                                                                                               radiation
 at 2.4 and
      The     5.8  GHz
            simulated    are
                          andillustrated
                               measured   in Figure
                                           2D-polar 11; the antenna
                                                     radiation        provides
                                                                 patterns  of theomnidirectional
                                                                                  antenna   for      radiation
                                                                                                 H-plane
patterns in the H-plane, while 8-shaped radiation patterns in E-plane have been achieved for different     and   patterns
                                                                                                                E-plane
 in 2.4
at  the H-plane,
resonant             whileare
         andfrequencies.
                5.8 GHz     8-shaped
                              The       radiation
                                 illustrated       patterns
                                              in Figure
                                     simulation   and    11;inthe
                                                               E-plane
                                                        measurement      have
                                                                   antenna     been
                                                                                  of achieved
                                                                              provides
                                                                         results                 for different
                                                                                      theomnidirectional
                                                                                           antenna              resonant
                                                                                                              radiation
                                                                                                       gain versus     its
 frequencies.
patterns   in    The
               the     simulation
                    H-plane,   while and measurement
                                       8-shaped         results
                                                 radiation      of the
                                                            patterns   antenna
                                                                      in E-plane gain
                                                                                   haveversus
                                                                                         been  its operation
                                                                                                achieved
operation frequency are plotted in Figure 12; the antenna provides sufficient gain levels with more        forfrequency
                                                                                                               different
 are plotted
resonant
than         atin2.6Figure
       2 dB frequencies.
                     GHz and12;
                              Thethe  antenna
                                      dB at 5.8provides
                                  3.5simulation           sufficient gain
                                                  and measurement
                                                GHz.                       levelsof
                                                                         results   with
                                                                                     themore     than gain
                                                                                           antenna     2 dB at  2.6 GHz
                                                                                                             versus   its
 and  3.5 dB    at 5.8 GHz.
operation frequency 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.
      A design
To achieve   theofdual-band
                   dual-bandfunction,
                              antenna covering   2.4/5.8
                                        a monopole       GHz has
                                                     antenna  withbeen  presented
                                                                    a radiation    for RFID
                                                                                patch        applications.
                                                                                       similar  to F shape
To  achieve the dual-band   function, a monopole  antenna   with a radiation patch similar
was designed and its properties were investigated. The impedance-bandwidth of the antenna  to F shapespans
                                                                                                       was
designed  andGHz
from 2.2–2.6   its properties were
                     and 5.3–6.8    investigated.
                                 GHz,   providingThe  impedance-bandwidth
                                                  broad   bandwidth and goodofradiation
                                                                                  the antenna  spans from
                                                                                           characteristics.
2.2–2.6
The antenna provides omnidirectional radiation patterns with appropriate gain values at both ofThe
        GHz   and   5.3–6.8 GHz,  providing  broad  bandwidth    and good  radiation  characteristics.  the
antenna
operationprovides
           bands. Theomnidirectional  radiation
                        antenna is simple       patterns
                                           and might   be awith appropriate
                                                           suitable candidategain   values
                                                                               for use     at both
                                                                                       in RFID      of the
                                                                                                systems.
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.
and H.J.B.;
Author      validation, N.O.P.,
        Contributions:           H.J.B., and R.A.A.-A.;
                          Conceptualization,  N.O.P. and  formal  analysis,
                                                             H.J.B.;        N.O.P., H.J.B.,
                                                                     methodology,   N.O.P. and
                                                                                             andR.A.A.-A.;   investigation,
                                                                                                 H.J.B.; software, 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.;
and H.J.B.; validation, N.O.P., H.J.B., and R.A.A.-A.;   formal  analysis, 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., investigation,
N.O.P., H.J.B.,
R.A.A.-A.,  andand   R.A.A.-A.;
                 J.M.N.;        resources,
                         visualization,     N.O.P.,
                                         N.O.P.,     H.J.B.,
                                                 H.J.B.,     and R.A.A.-A.;
                                                         R.A.A.-A.,          data curation, N.O.P., H.J.B., and R.A.A.-
                                                                     and J.M.N.
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.,  and J.M.N.;
program under            visualization,
                  grant agreement        N.O.P., H.J.B., R.A.A.-A.,
                                    H2020-MSCA-ITN-2016              and J.M.N.
                                                               SECRET-722424.
Funding: This projectAuthors
Acknowledgments:      has received funding
                             wish to       from
                                     express theirthe European
                                                    thanks to theUnion’s
                                                                  supportHorizon 2020
                                                                          provided by research and innovation
                                                                                      the innovation program
program under
under grant    grant agreement
            agreement           H2020-MSCA-ITN-2016
                       H2020-MSCA-ITN-2016               SECRET-722424.
                                               SECRET-722424.
                       The authors
Conflicts of Interest:Authors
Acknowledgments:              wishdeclare no conflict
                                   to express         of interest.
                                              their thanks  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.
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