Design and Realization of a Frequency Reconfigurable

applied sciences

Article Design and Realization of a Frequency Reconﬁgurable Multimode Antenna for ISM, 5G-Sub-6-GHz, and S-Band Applications

Adnan Ghaffar 1,* , Xue Jun Li 1 , Wahaj Abbas Awan 2 , Syeda Iffat Naqvi 3 , Niamat Hussain 4 , Boon-Chong Seet 1, Mohammad Alibakhshikenari 5,* , Francisco Falcone 6,7 and Ernesto Limiti 5

1 Department of Electrical and Electronic Engineering, Auckland University of Technology, Auckland 1010, New Zealand; [email protected] (X.J.L.); [email protected] (B.-C.S.) 2 Department of Integrated IT Engineering, Seoul National University of Science and Technology, Seoul 01811, Korea; [email protected] 3 Telecommunication Engineering Department, University of Engineering Technology, Taxila 47050, Pakistan; [email protected] 4 Department of Computer and Communication Engineering, Chungbuk National University, Chungbuk 28644, Korea; [email protected] 5 Electronic Engineering Department, University of Rome “Tor Vergata”, Via del Politecnico 1, 00133 Rome, Italy; [email protected] 6 Electric, Electronic and Communication Engineering Department, Public University of Navarre, 31006 Pamplona, Spain; [email protected] 7 Institute of Smart Cities, Public University of Navarre, 31006 Pamplona, Spain * Correspondence: [email protected] (A.G.); [email protected] (M.A.) Abstract: This paper presents the design and realization of a compact size multimode frequency Citation: Ghaffar, A.; Li, X.J.; Awan, reconfigurable antenna. The antenna consists of a triangular-shaped monopole radiator, originally W.A.; Iffat Naqvi, S.; Hussain, N.; inspired from a rectangular monopole antenna. Slots were utilized to notch the desired frequency Seet, B.-C.; Alibakhshikenari, M.; Falcone, F.; Limiti, E. Design and while the PIN diodes were utilized to achieve frequency reconfigurability. The antenna can operate Realization of a Frequency in wideband, dual-band, or tri-band mode depending upon the state of the diodes. To validate the Reconfigurable Multimode Antenna simulation results, a prototype was fabricated, and various performance parameters were measured for ISM, 5G-Sub-6-GHz, and S-Band and compared with simulated results. The strong agreement between simulated and measured Applications. Appl. Sci. 2021, 11, results along with superior performance as compared to existing works in the literature makes the 1635. https://doi.org/10.3390/ proposed antenna a strong candidate for ISM, 5G-sub-6 GHz, and S-band applications. app11041635 Keywords: multimode antenna; frequency reconfigurable; ISM band; 5G-sub-6-GHz; S-band Academic Editor: Akram Alomainy Received: 19 January 2021 Accepted: 8 February 2021 Published: 11 February 2021 1. Introduction

Publisher’s Note: MDPI stays neutral Multifunctional antennas have gained considerable attention in the past decade due with regard to jurisdictional claims in to advanced level technologies like 5G, Internet of Things (IoT), etc., along with a need published maps and institutional affil- for compact devices. The reconfigurable antennas are well-known due to the numerous iations. advantages. These antennas can replace multiple antennas by showing switching frequency, polarization, and radiation patterns using various techniques [1,2]. The RF-PIN diodes, Micro Electrical and Mechanical Switches (MEMS), Optical switch, microfluids, meta-surface, and electrically phase change materials are widely used to achieve the of reconfigurability [3,4]. Among various types of antennas proposed for sub-6-GHz appli- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. cations, most of the reported works are related to reconfigurable antennas where RF-PIN This article is an open access article diode for frequency reconfigurability was widely studied by the researchers to achieve distributed under the terms and single band, dual-band, and multiple-band reconfigurable operations [5–17]. conditions of the Creative Commons In [5], a wideband antenna was designed for Wireless Wide Area Network (WWAN) Attribution (CC BY) license (https:// and Long-Term Evolution (LTE) applications. The antenna comprises of simple structure creativecommons.org/licenses/by/ that shows good performance over a wide operational band. However, the antenna suffers 4.0/). from several drawbacks, including a bigger dimension, low gain, and it did not support

Appl. Sci. 2021, 11, 1635. https://doi.org/10.3390/app11041635 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 1635 2 of 14

reconfigurability. In [6], a compact wideband monopole antenna is converted into tri-band antenna for ISM and 5G-sub-6-GHz applications. Although the antenna offers a compact size with the advantages of flexibility, it does not support frequency reconfiguration. Similarly another antenna by using the proximity-coupling feed to cover the WLAN and 5G-sub-6-GHz was presented in [7]. However, the proposed antenna has complex antenna structure that is the combination of two substrate separated by air gap. Several single band frequency reconfigurable antennas are proposed to operate in frequency band spectrum ranges 2–4.5 GHz [8–12]. Although the antennas reported in [8–10] are offering very high gain characteristics, they have multiple layered structures, which resulted in increased thickness and structural complexity. Although the work reported antennas in [11,12] offer compact size, they cover very limited bandwidth, making them not suitable for modern-day devices operates in wideband frequency spectrums. Antennas with frequency reconfigurable characteristics for LTE and Wireless Local Area Network (WLAN) applications are presented in [13,14]. A conventional rectangular patch antenna was modified by using Defected Ground Structure (DGS) for wideband operation, and three PIN diodes were incorporate to switch the operation [13]. In [14], various slots and asymmetric feed is utilized along with two pin diodes, to achieve frequency reconfigurability. Although these antennas have compact and simple geometry, the operational bandwidth in single and dual band is narrow. A dual to tri-band and a dual-band to quad-band frequency reconfigurable antenna were presented in [15,16], respectively. Although, these antenna designs cover many bands, however, both works have impediment of narrow bandwidth, complex geometrical structures and lesser number of operational modes. In addition, the antenna presented in [15] suffers from the draw back of larger dimensions. A flexible frequency reconfigurable antenna was presented in [17] for sub-6-GHz applications. The presented work offers wideband and two dual-band operational modes with relatively simple structure. However, it has larger dimensions and limited operational bandwidth. To overcome the aforementioned challenges, a compact frequency reconfigurable antenna is proposed in this paper. The antenna radiator is composed of a triangular radiator with V-shaped slots and two PIN diodes are utilized to achieve frequency reconfigurability. The presented work shows a good combination of compact size, multimode operation, simple geometrical configuration, wide operational region, and reasonable gain. The rest of the paper is organized as follows. Section2 presents the theoretical analysis and design methodology of the proposed antenna. Section3 illustrates the discussion on various performance parameters of the presented work, while the paper is concluded in Section4.

2. Materials and Methods 2.1. Antenna Geometry The geometrical conﬁguration of the proposed antenna is illustrated in Figure1. The antenna is designed using ROGERS RT/droid 6010 having dielectric loss tangent (tan δ) of 0.0023, relative permittivity (εr) of 10.2 and 1.9-mm thickness (H) [18]. The overall dimension of the reconﬁgurable antenna is AX × AY. It can be seen from Figure1a that the proposed monopole antenna consists of a triangle-shaped radiator having a dimension of PX × PY and the feed line dimension is FX × FY. Two V-shaped slots were etched to notch the desire bands from wide bandwidth, the total length of the long slot is 2 × L1, whereas the length of the shorter slot is 2 × L2. The width of both slots had a dimension of d while the gap between two slots is g. A truncated ground plane of length GX was used to achieve wideband operational bandwidth. Table1 shows the optimized parameters of the antenna.

2.2. Simulation Setup The simulations of the proposed frequency reconfigurable antenna were performed using Higher Frequency Structural Simulator (HFSS). For frequency reconfigurability two RF PIN diodes D1 and D2 (Infineon, model # BAR50-02) were utilized. Two capacitors C1 and C2 were used to connect the pads with the radiator physically and to block current Appl. Sci. 2021, 11, 1635 3 of 16

achieve wideband operational bandwidth. Table 1 shows the optimized parameters of the antenna.

2.2. Simulation Setup The simulations of the proposed frequency reconfigurable antenna were performed Appl. Sci. 2021, 11, 1635 3 of 14 using Higher Frequency Structural Simulator (HFSS). For frequency reconfigurability two RF PIN diodes D1 and D2 (Infineon, model # BAR50-02) were utilized. Two capacitors C1 and C2 were used to connect the pads with the radiator physically and to blockﬂow fromcurrent biasing flow padsfrom tobiasing the radiator. pads to Capacitor the radiator.CDC Capacitoris utilized C toDC stop is utilized the ﬂow to of stop current the flowtoward of current the connector toward which the connector may affect which the performance may affect ofthe the performance antenna. The of pads the antenna. for diode Theare pads connected for diode using are a viasconnected to biasing using pads a vias presented to biasing at pads the backside presented of at the the antenna, backside as ofdepicted the antenna, in Figure as depicted1b. Inductor in FigureI is employed 1b. Inductor to stop I is the employed unwanted to RF-currentstop the unwanted from the RF-currentsource VDC from. The the biasing source voltage VDC. The is provided biasing voltage from the is backsideprovided of from the antennathe backside to mitigate of the antennathe effects to mitigate of DC-wires the effects on the of performance DC-wires on of the performance antenna. of the antenna.

AY H

PY d g V C DC Via pad 2 GND Via I D2 GND D1 AX

CDC G X FX X

Y FY Z (a) (b) (c)

FigureFigure 1. 1. GeometricalGeometrical configuration conﬁguration of proposed antenna:antenna: ((aa)) top-viewtop-view ( b(b)) bottom-view bottom-view (c ()c side-view.) side-view. Table 1. Dimension of the various parameter length of the proposed antenna. Table 1. Dimension of the various parameter length of the proposed antenna. Parameter Value Parameter Value Parameter Value Parameter Value Parameter Value Parameter Value AX 30 mm AY 30 mm H 1.9 mm AX 30 mm AY 30 mm H 1.9 mm GX 12 mm FX 30 mm FY 1.9 mm X X Y GPX 15.5512 mm mm PY F 30 mmmm F g1.9 1 mmmm PdX 15.55 1 mm mm L P1 Y 12.7230 mm mm L g2 9.9 1 mm mm Cd 1 1001 mm pF IL1 12.72 68 nH mm C LDC2 9.9100 mm pF C1 100 pF I 68 nH CDC 100 pF 2.3. Design of Wideband Antenna 2.3. DesignThe proposedof Wideband reconﬁgurable Antenna antenna was originally inspired by a conventional quarter-waveThe proposed rectangular reconfigurable monopole antenna antenna, was and originally the basic inspired concept ofby thea conventional proposed an- quarter-wavetenna and designed rectangular methodology monopole was antenna already, and presented the basic in [ 19concept]. The lengthof the (proposedLR) of the antennarectangular and monopoledesigned methodology antenna could was be already estimated presented by using in the[19]. following The length equation (LR) of pro-the rectangularvided in [20 ]:monopole antenna could be estimated by using the following equation Co provided in [20]: LR = √ (1) 4 fc εe f f 𝐶 where Co represents the speed of light𝐿 in= free space, fc represents the central frequency;(1) 4𝑓𝜀 for the presented case it was selected to be 4 GHz, whereas, εeff is the effective dielectric whereconstant Co represents of the substrate the speed is calculated of light in by free using space, the followingfc represents relation: the central frequency; for the presented case it was selected to be 4 GHz, whereas, εeff is the effective dielectric ( −0.5) constant of the substrate is calculatεr +ed1 byε rusing− 1 the followingA relation: ε ≈ + 1 + 12 Y (2) e f f 2 2 H

. 𝜀 +1 𝜀 −1 𝐴 𝜀 ≈ + 1+ 12 (2) 2 2 𝐻

where H is the thickness of the substrate, and AY is the width of the substrate. The resultant antenna resonates at 3.8 GHz with an impedance bandwidth of 850-MHz ranging 3.55–4.4 GHz, as depicted in Figure 2. Although the antenna offers a wide impedance bandwidth, still it does not cover the targeted 5G sub-6-GHz band Appl. Sci. 2021, 11, 1635 (3.1–4.5 GHz) allocated globally for future wireless communications. Thus, perturbation4 of 14 techniques were utilized to widen the impedance bandwidth of the antenna. Different from common techniques including DGS and slots to achieve wideband behavior, this design employed truncation of radiator corners to improve the bandwidth. Both lower cornerstechniques were including truncated DGS using and an slots equilateral to achieve triangle wideband which behavior, results this in design increasing employed the effectivetruncation length of radiator of the radiator, corners tothus improve shifting the of bandwidth.resonance toward Both lower the lower corners frequencies, were trun- ascated depicted using in an Figure equilateral 2. Moreover, triangle truncation which results of the in increasingcorner causes the more effective current length to offlow the fromradiator, the feedline thus shifting to the of resonanceradiator. Due toward to the the triangular lower frequencies, shape, the as depictedcurrent uniformly in Figure2 . distributeMoreover, itself truncation on the of surface the corner of causesthe radiator. more current The increase to ﬂow fromin flow the feedlineand uniform to the distributionradiator. Due of tocurrent the triangular resulted shape, in improvement the current uniformlyin impedance distribute matching itself onat thelower sur- frequenciesface of the radiator.to enhances The the increase impedance in ﬂow bandwi and uniformdth of the distribution antenna. The of currentresultant resulted antenna in exhibitsimprovement a wide in impedance matchingbandwidth at lowerof 2.2 frequenciesGHz ranging to enhances2.35–4.55 the GHz impedance which correspondsbandwidth ofto the62.8% antenna. of central The frequency, resultant antenna as depicted exhibits in Figure a wide 2. impedance It is worthy bandwidth to note hereof 2.2 that GHz besides ranging 5G sub-6-GHz 2.35–4.55 GHz band which spectr correspondsum the antenna to 62.8% also covers of central 2.4 GHz frequency, WLAN as band,depicted 2.45 inGHz Figure ISM2. Itband, is worthy 2.5GHz to noteWi-Max here ba thatnd, besides and more 5G sub-6-GHzthan 90% of band S-band spectrum (2–4 GHz),the antenna thus making also covers it suitable 2.4 GHz for WLANmultiple band, applications. 2.45 GHz ISM band, 2.5GHz Wi-Max band, and more than 90% of S-band (2–4 GHz), thus making it suitable for multiple applications.

-5

-10

-15

| (dB)

11 -20 |S -25

-30

-35 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Frequency (GHz)

FigureFigure 2. 2. S11S11 forfor rectangular rectangular shape shape monopole monopole wi withth and and without without truncated truncated corners. corners.

2.4.2.4. Design Design of of Notch Notch Band Band Antenna Antenna ToTo overcome overcome the problemproblem ofof band band congestion congestion due due to theto the closely closely allocated allocated band band spec- spectrumstrums for variousfor various applications, applications, the widebandthe wideband antenna antenna designed designed in the in previous the previous section sectionis transformed is transformed into a notch into banda notch antenna. band Usageantenna. of additional Usage of filtersadditional is the commonfilters is waythe commonto notch theway desire to notch bands the [21 ].desire However, bands it may[21]. resultHowever, in various it may drawbacks, result in includingvarious drawbacks,the requirement including of additional the requirement matching circuitsof additional to match matching the impedance circuits of to the match filter withthe impedancewideband antennaof the filter [22]. with Furthermore, wideband it alsoante requiresnna [22]. a biggerFurthermore, area to fitit thealso whole requires circuit a biggerallowing area very to fit little the spacewhole for circuit other allowing components very thuslittle limitsspace itsfor usage other incomponents compact size thus de- limitsvices [its23 ].usage Therefore, in compact the researcher size devices adopted [23] various. Therefore, techniques the researcher including adopted etching slotsvarious [24], loading metamaterials [25], insertion of Split Ring Resonator (SSR) [26], and stub loading techniques [27] to notch the desire bands. Furthermore, to avoid complex geometrical configuration, a simple yet effective technique to achieve a notched band is adopted, i.e., etching the slot in the radiator. One of the key parameters is to select the shape and position to place the slot, therefore, the shape and position of the slot must be chosen carefully to achieve a high value of VSWR to minimize the interference of the notch bands [28]. For the said purpose two V-shaped slots were used to achieve dual notch bands, the effective length (LS) of the slot for desire frequency (fd) can be calculated by [29]:

Co LS(n) = √ (3) 2 fd(n) εe f f Appl. Sci. 2021, 11, 1635 5 of 16

techniques including etching slots [24], loading metamaterials [25], insertion of Split Ring Resonator (SSR) [26], and stub loading techniques [27] to notch the desire bands. Furthermore, to avoid complex geometrical configuration, a simple yet effective technique to achieve a notched band is adopted, i.e., etching the slot in the radiator. One of the key parameters is to select the shape and position to place the slot, therefore, the shape and position of the slot must be chosen carefully to achieve a high value of VSWR to minimize the interference of the notch bands [28]. For the said purpose two V-shaped slots were used to achieve dual notch bands, the effective length (LS) of the slot for desire frequency (fd) can be calculated by [29]: Appl. Sci. 2021, 11, 1635 5 of 14 𝐶 𝐿() = (3) 2𝑓()𝜀

() where (n) = 1,2 which shows the number of slot while 𝜀 ≈ (ε +1). Moreover, for the r where (n) = 1, 2 which shows the number of slot while εe f f ≈ 2 . Moreover, for the proposed work LS1 = 2 × L1 while LS2 = 2 × L2. Figure 3 illustrates the comparison between proposed work LS1 = 2 × L1 while LS2 = 2 × L2. Figure3 illustrates the comparison between simulated values of S11 for wideband antenna and antenna with a single and dual notch. simulated values of S11 for wideband antenna and antenna with a single and dual notch. It Itcould could be observedbe observed that that with with a single a single V-shaped V-shaped slot the slot antenna the antenna mitigates mitigates the band spectrumthe band 11 spectrumof 2.47–3.11 of GHz 2.47–3.11 having GHz a maximum having valuea maximum of S11 = value−0.78 dBof S at 2.82= −0.78 GHz. dB Similarly, at 2.82 whenGHz. Similarly,two slots werewhen etched, two slots the were antenna etched, mitigates the antenna the two mitigates band spectrums the two band of 2.48–2.96 spectrums GHz of 2.48–2.96and 3.47–3.76 GHz GHz.and 3.47–3.76 To have GHz. a better To have understanding a better understanding of the slot to of mitigate the slot theto mitigate desired thefrequency, desired current frequency, distribution current distribution plots were presented plots were in presented Figure4a,b. in ItFigure could 4a,b. be observed It could befrom observed Figure 4froma that Figure for the 4a ﬁrst that notchfor the band first thenotch maximum band the current maximum is around current the is biggeraround V-shapedthe bigger slots,V-shaped which slots, causes which the mitigationcauses the ofmitigation the lower of band. the lower Similarly, band. the Similarly, maximum the maximumcurrent was current observed was along observed with aalong smaller with V-shaped a smaller slot V-shaped for the second slot for notch the second band, whichnotch band,causes which its mitigation causes its from mitigation the resonating from the band resonating spectrum. band spectrum.

-10

-20 | (dB) | 11

|S -30

-40

-50 2.02.53.03.54.04.55.0 Frequency (GHz) Appl. Sci. 2021, 11, 1635 6 of 16 Figure 3. S11 for wideband antenna and wideband antenna with single and dual-notch bands. Figure 3. S11 for wideband antenna and wideband antenna with single and dual-notch bands.

(a) (b) FigureFigure 4. 4. CurrentCurrent distribution distribution of of proposed proposed antenna antenna at at notch-band notch-band of: of: ( (aa)) 2.82 2.82 GHz GHz ( (bb)) 3.64 3.64 GHz. GHz.

2.5.2.5. Numerical Numerical Analysis Analysis FigureFigure 55 depictsdepicts thethe equivalentequivalent circuitcircuit ofof thethe UWBUWB antennaantenna alongalong withwith thethe UWBUWB antennaantenna with with a a single single and and dual-notch dual-notch band. band. The The equivalent equivalent model model of of the the UWB UWB antenna waswas designed designed using using a a series-connected co combinationmbination of of RLC RLC components, where where each each groupgroup of RLCRLC components components show show the the independent independent single single resonance. resonance. The operationalThe operational region regionof each of resonance each resonance overlaps overlaps due to closelydue to spacedclosely thusspaced results thus inresults a Wide in Banda Wide Antenna Band Antenna (WBA) [30,31]. Similarly, the notch bands occurred due to the presence of V-shaped slots were also model using an RLC group, where the LC equivalent notch frequency fno [32].

1 1 𝑓 = (4) 2𝜋 𝐿𝐶

where Ceq is the total equivalent capacitance and can be obtained by summing up the capacitances generated by individual leg of the V-shaped slot. Leq is the equivalent inductance generated by the etched slot and can be calculated by using the following relation: 4𝐸 𝐿 = 0.0002E 2.303 log ( )−𝜃 𝜇𝐻 (5) 𝐷 where E was assumed to be the finite length of a wire of rectangular cross-section equals to the notch area, D is the length approximately equals to the cross-section of the slot, and θ is the angle between the legs of the V-shaped slot. Appl. Sci. 2021, 11, 1635 6 of 14

(WBA) [30,31]. Similarly, the notch bands occurred due to the presence of V-shaped slots were also model using an RLC group, where the LC equivalent notch frequency fno [32]. s 1 1 fno = (4) 2π LeqCeq

where Ceq is the total equivalent capacitance and can be obtained by summing up the capacitances generated by individual leg of the V-shaped slot. Leq is the equivalent inductance generated by the etched slot and can be calculated by using the following relation:

4E L = 0.0002E 2.303 log ( ) − θ µH (5) eq 10 D

where E was assumed to be the ﬁnite length of a wire of rectangular cross-section equals to Appl. Sci. 2021, 11, 1635 the notch area, D is the length approximately equals to the cross-section of the slot, and7 ofθ is16

the angle between the legs of the V-shaped slot.

R1 RN-1 RN

L1 LN-1 LN Zin C1 CN-1 CN

Equivalent RLC Model of Wide Band Antenna (WBA) V-shaped Slot WBA

RS1

LS1 Z Zin CS1

Equivalent RLC Model of WBA with Single Notch Band Outer Inner Slot Slot WBA

RS1 RS2

LS1 LS2 Z Zin CS1 CS2

Equivalent RLC Model of WBA with Dual Notch Band

FigureFigure 5.5. RLCRLC equivalentequivalent modelmodel ofof widewide bandband antennaantenna withwith andand withoutwithout notch-bands.notch-bands.

AsAs thethe currentcurrent distributiondistribution isis perturbedperturbed duedue toto slotsslots onon thethe radiatingradiating patch,patch, thethe pathpath ofof thethe currentcurrent isis elongated.elongated. TheThe lengthlength ofof thethe currentcurrent pathpath exhibitsexhibits thethe seriesseries inductance,inductance, whereaswhereas thethe etchedetched gapsgaps ofof thethe slotsslots accumulateaccumulate thethe charge,charge, hencehence determinesdetermines seriesseries capacitance. Collectively, this behavior can be represented by the equivalent lumped capacitance. Collectively, this behavior can be represented by the equivalent lumped element circuit with LC conﬁguration. The equivalent lumped element parallel RLC element circuit with LC configuration. The equivalent lumped element parallel RLC circuit for the proposed antenna is illustrated in Figure5. LS1, CSi, and RSi represent the circuit for the proposed antenna is illustrated in Figure 5. LS1, CSi, and RSi represent the inductance, capacitance, and radiation resistance for ith radiating mode at each resonant frequency band, respectively. The lumped effect of the slots causing the resonant frequency is represented as a series combination of all lumped elements with an inductive patch antenna and ground plane as a capacitive load. Both inductive and capacitive effects as well as feeding effects are considered for higher-order modes generation and significantly contribute to the input impedance of the antenna.

inductance, capacitance, and radiation resistance for ith radiating mode at each resonant frequency band, respectively. The lumped effect of the slots causing the resonant frequency is represented as a series combination of all lumped elements with an inductive patch antenna and ground plane as a capacitive load. Both inductive and capacitive effects as well as feeding effects are considered for higher-order modes generation and signiﬁcantly contribute to the input impedance of the antenna.

2.6. Parametric Analysis Parametric analysis of the key parameters of the antenna is performed to investigate their effects on the performance of the antenna, as depicted in Figure6. As shown in Figure6a , when the length of the truncated corner was changed from 15.55 mm to 12.73 mm, Appl. Sci. 2021, 11, 1635 all the resonances were shifted toward the higher side. An increment in bandwidth8 wasof 16 observed for the third while the bandwidth of the ﬁrst resonance decreases considerably, as depicted in Figure6a. On the other hand, when the value of PX was increased from 15.55 mm to 18.35 mm, all resonances shift toward the lower side along with a decrement the bandwidth are due to the number of current passes from feedline to radiator while in the bandwidth of the third resonance. The increase and decrease in the bandwidth are duethe shift to the in number resonance of currenttoward passeslower and from higher feedline frequency to radiator is due while to change the shift in in the resonance effective towardlength of lower the andradiator, higher as frequency discussed is earlier due to changein Section in the2.2. effective It is worthy length to of note the radiator,that the asnotched discussed frequencies earlier in remain Section unchanged 2.2. It is worthy for both to notehigher that and the lower notched variations, frequencies as depicted remain unchangedin Figure 6a. for both higher and lower variations, as depicted in Figure6a.

0 0

-5 -5 -10 -10 -15 | (dB)

| (dB) -15 11 -20 11 |S |S -20 -25 PX = 12.73mm L = 14.15mm -25 1 PX = 15.55mm -30 L1 = 12.72mm PX = 18.35mm L = 11.32mm -35 -30 1 2.0 2.5 3.0 3.5 4.0 4.5 5.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Frequency (GHz) Frequency (GHz) (a) (b)

0 0

-5 -5 -10 -10 -15

-15 | (dB) | (dB) 11

11 -20 |S |S -20 -25 L = 11.3mm g = 0.5 mm -25 2 L2 = 9.9mm -30 g = 1.0 mm L = 8.5mm 2 g = 1.5 mm -30 -35 2.0 2.5 3.0 3.5 4.0 4.5 5.0 22.533.544.55 Frequency (GHz) Frequency (GHz) (c) (d)

Figure 6. Parametric analysis of the various parameters of the antenna: (a) PX (b) L1 (c) L2 (d) g. Figure 6. Parametric analysis of the various parameters of the antenna: (a)PX (b)L1 (c)L2 (d) g.

FigureFigure6 b6b illustrates illustrates the the effects effects of variation of variation in the dimensionin the dimension of L1. It of could L1. beIt observedcould be thatobserved when that the valuewhen oftheL 1valuehas increased of L1 has from increased 12.72 mmfrom to 12.72 14.15 mm mm to the 14.15 first mm notch the band first shiftsnotch fromband 2.8 shifts GHz from to 2.55 2.8 GHz, GHz also to 2.55 the bandwidthGHz, also the of the bandwidth first resonance of the decreases first resonance while thedecreases bandwidth while of the the bandwidth second resonance of the second increases resonance as depicted increases in Figure as depicted6b. On thein Figure other hand,6b. On when the other the value hand, of whenL1 was the decreased value of toL1 11.32was decreased mm, the notch to 11.32 band mm, shifted the notch to 3.1 GHzband alongshifted with to increased3.1 GHz along bandwidth with atincreased first resonance bandwidth and decreased at first resonance bandwidth and at thedecreased second resonance.bandwidth A at similar the second phenomenon resonance. was A also simi observedlar phenomenon for the second was notchalso observed band by varying for the second notch band by varying the dimension of L2, as depicted in Figure 6c. It is worthy to note that the second notch band and third resonance remain conserved for all variation in L1 while the first notch and first resonance remain conserved for all variation in L2. Moreover, from Figure 6d, it could also be observed that the gap between the slots (g) also has less impact of the performance of the antenna. Thus, the proposed antenna becomes a promising candidate for the application where independently controllable notch band antennas are required.

2.7. Design Methodology Figure 7 depicts the flow chart of the design methodology of the proposed antenna. Initially, a quarter-wave rectangular monopole antenna was designed. Various parameters of monopole, including length and width of the radiator, feedline, and truncated ground plane, were optimized to get maximum bandwidth. Afterwards, lower Appl. Sci. 2021, 11, 1635 8 of 14

the dimension of L2, as depicted in Figure6c. It is worthy to note that the second notch band and third resonance remain conserved for all variation in L1 while the ﬁrst notch and ﬁrst resonance remain conserved for all variation in L2. Moreover, from Figure6d, it could also be observed that the gap between the slots (g) also has less impact of the performance of the antenna. Thus, the proposed antenna becomes a promising candidate for the application where independently controllable notch band antennas are required.

2.7. Design Methodology Figure7 depicts the ﬂow chart of the design methodology of the proposed antenna. Appl. Sci. 2021, 11, 1635 9 of 16 Initially, a quarter-wave rectangular monopole antenna was designed. Various parameters of monopole, including length and width of the radiator, feedline, and truncated ground plane, were optimized to get maximum bandwidth. Afterwards, lower corners of the cornersmonopole of radiatorthe monopole were truncated radiator bywere using truncated an equilateral by using triangle an equilateral such that the triangle hypotenuse such thatof the the triangle hypotenuse is PX. of the triangle is PX.

Rectangular Monopole Antenna Tune Parameters L1, LY, g, d

No Tune Parameters GX, FX, FY, PY

Desire Notch- No bands Achieved

Maximum Bandwidth Yes Achieved

Insertion of diode and Parameters Tunning

Yes

No Etching Triangular Slots from corners

Expected Results Achieved

Tune Parameters PX, PY

Yes

No Design Criteria Fullfilled

Maximum Bandwidth Achieved

Proposed Reconfigurable Yes Antenna is ready

Insertion of Triangular slots on Patch

Figure 7. Flow chart of the design methodology of the proposed antenna.

The hypotenusehypotenuse of of the the triangular triangular slot andslot widthand width of the radiatorof the wasradiator further was optimized further optimizedto get the ultra-wide to get the bandwidth.ultra-wide bandwidth. The resultant The antenna resultant shows antenna an impedance shows an bandwidth impedance of bandwidth62.8%. In the of next62.8%. step, In twothe next triangular step, two slots triangular were etched slots from were the etched radiator from to achievethe radiator two tonotch achieve bands two from notch an already bands designed from an ultra-wideband already designed antenna. ultra-wideband The length andantenna. width The slot lengthalong withand separationwidth slot betweenalong with them separation were well-tuned between tothem achieve were the well-tuned desire notch to achieve bands. Thethe desire resultant notch antenna bands. exhibits The resultant a tri-band antenna operational exhibits modea tri-band along operational with two notchmode bandsalong − wherewith two the notch value bands of S11 iswhere greater the than value 2of dB. S11 Finally, is greater two than RF-PIN −2 dB. diodes Finally, were two inserted RF-PIN at diodesthe center were of inserted the slots, at such the center that when of the the slots, diode such is that in ON-state when the the diode slot becomesis in ON-state inactive the slot becomes inactive while in OFF-state the slot becomes active and a notched band is achieved. Biasing circuits were designed at the backside of the antenna to minimize the effects of basing components and wires. Final tuning of various parameters was performed due to the addition of diodes, capacitors, and inductors to achieve the desired frequency reconfigurable antenna having dual-band, tri-band, and wideband operational mode.

3. Results and Discussion 3.1. Measurement Setup To measure the S-parameters of the proposed antenna, the Anritsu S820E Antenna Analyzer is used. Figure 8 shows the fabricated antenna prototype. The far-field Appl. Sci. 2021, 11, 1635 9 of 14

while in OFF-state the slot becomes active and a notched band is achieved. Biasing circuits were designed at the backside of the antenna to minimize the effects of basing components and wires. Final tuning of various parameters was performed due to the addition of diodes, capacitors, and inductors to achieve the desired frequency reconﬁgurable antenna having dual-band, tri-band, and wideband operational mode.

3. Results and Discussion Appl. Sci. 2021, 11, 1635 10 of 16 3.1. Measurement Setup To measure the S-parameters of the proposed antenna, the Anritsu S820E Antenna Analyzer is used. Figure8 shows the fabricated antenna prototype. The far-field character- characteristicsistics of the antenna of the areantenna measured are measured through a through commercial a commercial far-field measurement far-field measurement system in a systemshielded in RF a anechoicshielded chamber.RF anechoic The chamber. horn antenna The (SGH-series)horn antenna having (SGH-series) a standard having gain ofa standard24 dBi is utilizedgain of as24 a dBi transmitter, is utilized while as thea transmitter, proposed antenna while the was proposed measured antenna as a receiving was measuredantenna. Poweras a receiving amplifiers antenna. were used Power to provideamplifiers stable were power used reception.to provide The stable antenna power is ° reception.rotated in The 360◦ antennato measure is rotated the far-field in 360 results. to measure the far-field results.

(a) (b)

FigureFigure 8. 8. FabricatedFabricated prototype prototype of of the the proposed proposed antenna: antenna: ( (aa)) top-view top-view ( (bb)) bottom-view bottom-view.

3.2.3.2. S-Parameter S-Parameter

FigureFigure 9 shows the simulated and measuredmeasured SS1111. .It It can can be be observed observed that that the the antenna antenna eithereither operates inin wideband wideband mode, mode, two two dual-band dual-band modes, modes, and tri-band and tri-band mode, asmode, depicted as depictedin Figure in9a–d. Figure The 9a–d. biasing The voltage biasing ofvoltage the PIN of diodethe PIN is diode 3V. When is 3V. both When diodes both Ddiodes1 and D12 andwere D in2 were ON in state, ON which state, referswhich torefers case-11, to case-11, the antenna the antenna act as a act wideband as a wideband antenna antenna having havingS11 < − S1011 dB< − impedance10 dB impedance predicted predicted bandwidth bandwidt of 2.29–4.47h of 2.29–4.47 GHz whereas, GHz thewhereas, measured the measuredbandwidth bandwidth was observed was toobserved be 2.25–4.53 to be GHz,2.25–4.53 as depicted GHz, as indepicted Figure9 ina. Figure When either9a. When the eitherdiode theD1 ordiodeD2 wasD1 or kept D2 inwas On-state kept in while On-state keeping while the keep othering diode the other in Off-state diode in the Off-state antenna theexhibits antenna dual-band exhibits mode, dual-band as depicted mode, in as Figure depicted9b,c. Thein Figure predicted 9b,c. bandwidth The predicted of the bandwidthproposed antenna of the proposed for case—01 antenna was observed for case to−01 be was 2.3–2.65 observed GHz andto be 3–4.5 2.3–2.65 GHz, GHz while and for 3–4.5case-10 GHz, it was while observed for case-10 to be 2.1–2.4it was GHz observed and 2.92–4.37 to be 2.1–2.4 GHz, respectively.GHz and 2.92–4.37 GHz, − respectively.On the other hand, the measured S11 < 10 dB bandwidth for case-01 was observed to be 2.1–2.78 GHz and 3.09–4.57 GHz, respectively, as depicted in Figure9b, while for On the other hand, the measured S11 < -10 dB bandwidth for case-01 was observed to becase-10 2.1–2.78 it was GHz observed and 3.09–4.57 to be 1.86–2.5 GHz, respectively, GHz and 3.03–4.47 as depicted GHz, asin depictedFigure 9b, in while Figure for9c. At last, when both diodes were kept in Off-state, referring to case-00, the antenna exhibits case-10 it was observed to be 1.86–2.5 GHz and 3.03–4.47 GHz, as depicted in Figure 9c. triband operational mode having predicted passbands of 2.17–2.35 GHz, 2.64–2.85 GHz, At last, when both diodes were kept in Off-state, referring to case-00, the antenna exhibits and 3.09–4.34 GHz. The measured impedance bandwidths for case-00 were reported to triband operational mode having predicted passbands of 2.17–2.35 GHz, 2.64–2.85 GHz, be 1.93–2.4 GHz, 2.62–2.97 GHz, and 3.2–4.65 GHz, as depicted in Figure9d. In general, a and 3.09–4.34 GHz. The measured impedance bandwidths for case-00 were reported to strong agreement was observed between simulated and measured results, while the small be 1.93–2.4 GHz, 2.62–2.97 GHz, and 3.2–4.65 GHz, as depicted in Figure 9d. In general, a discrepancy between the results was owing to fabrication and measurement circuitry. strong agreement was observed between simulated and measured results, while the small discrepancy between the results was owing to fabrication and measurement circuitry. Appl.Appl. Sci. Sci.2021 2021, 11, ,11 1635, 1635 11 of10 16 of 14

0 0

-5 -5

-10 -10

-15 -15 | (dB) | (dB) 11 11 |S -20 |S -20

-25 -25 Simulated Simulated Measured Measured -30 -30 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Frequency(GHz) Frequency(GHz) (a) (b) 0 0

-5 -5

-10 -10

-15 -15 | (dB) | (dB) 11 11 |S |S -20 -20

-25 -25 Simulated Simulated Measured Measured -30 -30 1.52.02.53.03.54.04.55.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Frequency(GHz) Frequency(GHz) (c) (d)

Figure 9. Simulated and measured results of S11: (a) case-11 (b) case-01 (c) case-10 (d) case-00. Figure 9. Simulated and measured results of S11:(a) case-11 (b) case-01 (c) case-10 (d) case-00.

3.3. Radiation Patterns 3.3. Radiation Patterns Figure 10 illustrates the simulated and measured radiation patterns of the proposed Figure 10 illustrates the simulated and measured radiation patterns of the proposed an- antenna at resonating frequencies for various switching states. For all resonating tenna at resonating frequencies for various switching states. For all resonating frequencies, frequencies, the antenna exhibits an omnidirectional radiation pattern in principle the antenna exhibits an omnidirectional radiation pattern in principle H-plane (θ = 90◦). H-plane (θ = 90°). On the other hand, for all resonating frequencies, the antenna exhibits a On the other hand, for all resonating frequencies, the antenna exhibits a monopole like monopole like bi-directional radiation pattern in principle E-Plane◦ (θ = 0°). Moreover, it is Appl. Sci. 2021, 11, 1635 bi-directional radiation pattern in principle E-Plane (θ = 0 ). Moreover, it is observed12 of 16 for observed for all switching states that the proposed antenna shows good agreement allbetween switching simulated states and that measured the proposed results. antenna shows good agreement between simulated and measured results.

H-Plane Sim. H-Plane Mea. E-Plane Sim. E-Plane Mea. 0 0 0 5 0 -5 -10 300 60 300 60 300 60 -15 -20 -25 -30 -25 -20 -15 -10 240 120 240 120 240 120 -5 0 5 180 180 180 (a) (b) (c)

FigureFigure 10. 10.Simulated Simulated and and measured measured radiationradiation patterns for for case-00: case-00: (a ()a )2.24 2.24 GHz GHz (b ()b 2.74) 2.74 GHz GHz (c) (3.71c) 3.71 GHz. GHz.

3.4. Gain and Radiation Efficiency Figure 11 shows the predicted and measured gain of the proposed reconfigurable antenna. It can be seen from Figure 11a, when both diodes are in ON-state the antenna exhibits a gain of <2.5 dBi in the operational band along with radiation efficiency of <90%. However, the antenna in multiband mode shows the gain of <2.4 dBi in passband regions along with radiation efficiency of <89% (Figure 11b–d), while for the band stop region the gain and radiation efficiency of the proposed antenna decreases up to −8 dBi and 22%, respectively. A strong agreement between simulated and measured results state the performance stability of the antenna. Table 2 shows the comparison between measured and simulated results for different states of PIN diodes.

Appl. Sci. 2021, 11, 1635 11 of 14

3.4. Gain and Radiation Efficiency Figure 11 shows the predicted and measured gain of the proposed reconfigurable antenna. It can be seen from Figure 11a, when both diodes are in ON-state the antenna exhibits a gain of <2.5 dBi in the operational band along with radiation efficiency of <90%. However, the antenna in multiband mode shows the gain of <2.4 dBi in passband regions Appl. Sci. 2021, 11, 1635 13 of 16 along with radiation efficiency of <89% (Figure 11b–d), while for the band stop region the gain and radiation efficiency of the proposed antenna decreases up to −8 dBi and 22%, respectively. A strong agreement between simulated and measured results state the performance stability of the antenna. Table2 shows the comparison between measured and simulated results for different states of PIN diodes.

4 100 4 100

2 2 80 80 0 0

-2 60 -2 60 Gain (dB) Gain (dB) -4 -4 Sim. Gain 40 Sim. Gain 40 -6 Mea. Gain -6 Mea. Gain RadiationEfficiency (%) Radiation Eff. RadiationEfficiency (%) Radiation Eff. -8 20 -8 20 2 2.5 3 3.5 4 4.5 5 2 2.5 3 3.5 4 4.5 5 Frequency (GHz) Frequency (GHz)

(a) (b) 4 100 4 100

2 2 80 80 0 0

-2 60 -2 60 Gain (dB) Gain (dB) -4 -4 Sim. Gain 40 Sim. Gain 40 -6 Mea. Gain -6 Mea. Gain RadiationEfficiency (%)

Radiation Eff. RadiationEfficiency (%) Radiation Eff. -8 20 -8 20 2 2.5 3 3.5 4 4.5 5 2 2.5 3 3.5 4 4.5 5 Frequency (GHz) Frequency (GHz)

(c) (d) Figure 11.FigureSimulated 11. Simulated and measured and measured gain along gain with alon radiationg with efﬁciency:radiation efficiency: (a) case-11 (a (b) )case-11 case-01 (b (c)) case-01 case-10 ( (cd) ) case-00. case-10 (d) case-00. Table 2. Comparison among simulated and measured values for various switching states. Table 2. Comparison among simulated and measured values for various switching states. Simulated Measured Simulated Measured Switching SimulatedDiode Measured Simulated Measured Switching Diode Bandwidth Bandwidth Peak Gain Peak Gain State BandwidthState Bandwidth Peak Gain Peak Gain State State (GHz) (GHz) (dBi) (dBi) (GHz) (GHz) (dBi) (dBi) D1-On and Case-11 2.29–4.47 2.25–4.53 3.86 3.8 Case-11 D1-On and D2-On 2.29–4.47D2-On 2.25–4.53 3.86 3.8 2.3–2.65 2.1–2.78 2.44 2.39 D1-On and D1-On and 2.3–2.65 2.1–2.78 2.44 2.39 Case-01 Case-01 D2-Off D2-Off3–4.5 3.09–4.573–4.5 3.09–4.57 3.91 3.91 3.82 3.82 D1-Off and D1-Off2.1–2.4 and 1.86–2.52.1–2.4 1.86–2.5 2.21 2.21 1.89 1.89 Case-10 Case-10 D2-On 2.92–4.37D2-On 2.92–4.373.03–4.47 3.03–4.47 3.89 3.89 3.82 3.82 2.17–2.35 1.93–2.4 2.26 2.17 D1-Off2.17–2.35 and 1.93–2.4 2.26 2.17 D1-OffCase-00 and 2.64–2.85 2.62–2.93 2.01 1.96 Case-00 2.64–2.85D2-Off 2.62–2.93 2.01 1.96 D2-Off 3.19–4.34 3.19–4.65 3.9 3.79 3.19–4.34 3.19–4.65 3.9 3.79

3.5. Comparison With State-Of-The-Art Works Table 3 presents the comparison of the proposed antenna with state-of-the-art works for similar applications. It could be observed clearly that the proposed antenna overperforms the existing antennas by providing the best combination of compact size, multi-mode operation, wide operational region, relatively high gain, and simple geometrical structure.

Table 3. Comparison of the proposed antenna with state-of-the-art works for similar applications.

Ref. Dimension Operational Mode Reconfigurable Frequency Peak Appl. Sci. 2021, 11, 1635 12 of 14

3.5. Comparison with State-of-the-Art Works Table3 presents the comparison of the proposed antenna with state-of-the-art works for similar applications. It could be observed clearly that the proposed antenna overperforms the existing antennas by providing the best combination of compact size, multi-mode operation, wide operational region, relatively high gain, and simple geometrical structure.

Table 3. Comparison of the proposed antenna with state-of-the-art works for similar applications.

Frequency Peak Dimension Reconﬁgurable Ref. Operational Mode Range Gain (mm2) Technique (GHz) (dBi) [5] 65 × 10 Wideband N. A 1.6–2.2 2.45 [6] 25 × 30 Tri-band N. A 2.35–5.5 3.75 [8] 100 × 100 Single band PIN-diode (2) 2.2–3.8 6.5 [9] 70 × 70 Single band Varactor (1) 2.6–3.8 5.92 [10] 50 × 50 Single band Mechanical 2–4.5 5.6 [11] 27 × 27 Single band Varactor (1) 1.4–2. 2.48 [12] 14 × 36 Single band PIN-diode (2) 2–2.6 N. R [13] 25 × 27 Single and Dual Band PIN-diode (2) 2–6 2.8 [14] 20 × 16 Single and Dual Band PIN-diode (2) 2–6 3.13 [15] 50 × 45 Dual and Tri Band PIN-diode (2) 2.5–5 5.8 [16] 30 × 28 Dual and Quad band PIN-diode (1) 1.5–11 3.1 [17] 50 × 33 Wide and Dual Band PIN-diode (2) 2–3.75 3.2 This Work 30 × 30 Wide, Dual and Tri band PIN-diode (2) 1.8–4.5 3.72

4. Conclusions A compact frequency reconﬁgurable antenna is proposed in this paper. Initially, to cover 5G sub—6 GHz band, a wideband triangle-shaped monopole antenna was designed, which was originally inspired from a rectangular monopole antenna. Afterwards, two V-shaped slots were utilized to achieve two notch bands, their length could be adjusted independently to achieve the desire notch bands. Finally, two RF-PIN diodes were inserted at the middle of the slots such that when diodes are in Off-state they active the slot and thus result in notching the band. The overall size of the antenna is 0.19λo × 0.19λo × 0.0081λo, where λo correspond to the free space wavelength at the lowest resonating frequency. The proposed antenna can operate in wideband mode, two different dual-band modes, or tri-band, depending upon the state of the diodes. Strong agreement between predicted and measured results, along with reasonable gain in passbands and very low gain in the band-stop region, makes the proposed antenna a suitable candidate for 2.1 GHz 4G-LTE band, 2.45 GHz ISM band, 5G-sub-6 GHz band, and S-band applications.

Author Contributions: Conceptualization: A.G., W.A.A., and M.A.; methodology, A.G., N.H. and S.I.N.; software, A.G., W.A.A., N.H., and X.J.L.; validation, A.G., S.I.N., and M.A.; investigation, A.G., W.A.A., S.I.N., and M.A.; resources, M.A., F.F., and E.L.; writing—original draft preparation, A.G., W.A.A., and N.H.; writing—review and editing, X.J.L., B.-C.S., S.I.N., M.A., F.F., and E.L.; supervision, X.J.L., and B.-C.S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article. Acknowledgments: This work is partially supported by RTI2018-095499-B-C31, Funded by Ministe- rio de Ciencia, Innovación y Universidades, Gobierno de España (MCIU/AEI/FEDER,UE). Conﬂicts of Interest: The authors declare no conﬂict of interest. Appl. Sci. 2021, 11, 1635 13 of 14

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