Miniaturized Antipodal Vivaldi Antenna with Improved Bandwidth Using Exponential Strip Arms

electronics

Article Miniaturized Antipodal Vivaldi Antenna with Improved Bandwidth Using Exponential Strip Arms

Mohammad Mahdi Honari * , Mohammad Saeid Ghaffarian and Rashid Mirzavand

Intelligent Wireless Technology Laboratory, Electrical and Mechanical Engineering Department, University of Alberta, 9211 116 Street NW, Edmonton, AB T6G 1H9, Canada; [email protected] (M.S.G.); [email protected] (R.M.) * Correspondence: [email protected]

Abstract: In this paper, a miniaturized ultra-wideband antipodal tapered slot antenna with exponential strip arms is presented. Two exponential arms with designed equations are optimized to reduce the lower edge cut-off frequency of the impedance bandwidth from 1480 MHz to 720 MHz, resulting in antenna miniaturization by 51%. This approach also improves antenna bandwidth without compromising the radiation characteristics. The dimension of the proposed antenna structure including the feeding line and transition is 158 × 125 × 1 mm3. The results show that a peak gain more than 1 dBi is achieved all over the impedance bandwidth (0.72–17 GHz), which is an improvement to what have been reported for antipodal tapered slot and Vivaldi antennas with similar size.

Keywords: antipodal Vivaldi antenna (AVA); tapered slot antenna (TSA); ultra-wideband (UWB)

1. Introduction Due to the rapid development of wireless communication, ultra-wideband antennas/systems are becoming highly attractive in many wideband applications, such as broad Citation: Honari, M.M.; Ghaffarian, band wireless communications, ultra-wideband interference, and imaging systems [1,2]. M.S.; Mirzavand, R. Miniaturized They can be used in electromagnetic compatibility (EMC) test and measurement of emerg- Antipodal Vivaldi Antenna with ing wireless technology devices and UWB security radar detection [3]. Different antennas Improved Bandwidth Using such as double ridge horn antennas, fractal antenna or planar Vivaldi antennas could be Exponential Strip Arms. Electronics 2021, 10, 83. https://doi.org/ utilized for mentioned applications. Due to the relatively large and expensive fabrication 10.3390/electronics10010083 process of horn antennas and poor radiation of fractal antennas, Vivaldi antennas seem to be a good choice because of their ultra-wideband performance with acceptable radiation Received: 15 November 2020 characteristics. Accepted: 31 December 2020 Vivaldi antennas which are also called exponential tapered slot antennas (ETSA) were Published: 4 January 2021 introduced by Gibson and Gazit as a new class of UWB antennas [4,5]. The dual exponential tapered slot antennas (DETSA) are a type of conventional Vivaldi antennas [5–7]. Recently, Publisher’s Note: MDPI stays neu- many types of TSA antennas such as linear TSA, conformal TSA, parabolic TSA, and tral with regard to jurisdictional clai- logarithmic TSA were investigated extensively in [4–11]. ms in published maps and institutio- Several modiﬁcations have been carried out on the basic co-planar TSA structure, nal afﬁliations. resulting in antipodal Vivaldi antennas (AVAs) which consist of two radiating arms on either side of the substrate with 180◦ differential phase excitation to obtain the best radiation characteristics. The AVAs possess some advantages such as ultra-wideband performance compared to other Vivaldi antennas. Various kinds of AVA structures have been reported Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. in [11–18] and some techniques have been proposed to reduce the size of AVAs and improve This article is an open access article their performance [15–19]. In these antennas, although the high edge of frequency band of distributed under the terms and con- interest could be increased to ultra-high frequencies, the lower edge cut-off is still limited ditions of the Creative Commons At- by the antenna effective aperture size. tribution (CC BY) license (https:// In this paper, a miniaturized AVA with improved impedance bandwidth is proposed. creativecommons.org/licenses/by/ Two exponential arms with designed equations are added to the conventional structure 4.0/). of antipodal Vivaldi antenna in order to miniaturize the whole size of the antenna. By

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adding two of these additional arms, it is demonstrated that the lower edge of impedance bandwidth starts from 0.72 GHz instead of its original value of 1.48 GHz, resulting in antenna miniaturization by 51% size reduction. The measured results show that the proposed antenna perfectly works over 0.72 GHz to 17 GHz. The paper is organized as follows: First, the antenna conﬁguration and design procedure are presented. By using antenna current distribution, the antenna radiation mech- anism is demonstrated to bring a physical insight into the improvement of the antenna performance. Then antenna design simulation with parametric study and measured results are investigated. Finally, a conclusion is given in the last section.

2. Antenna Conﬁguration and Design Figure1a shows conventional antipodal antennas with different widths of tapered Electronics 2021, 10, x FOR PEER REVIEW m 3 of 12 strips labeled as 1. To investigate the effect of tapered strips’ width of the antenna on cut-off frequency and antenna size miniaturization, four cases with different widths of tapered strips of 10 mm, 22 mm, 50 mm, and 80 mm are considered in Figure1. As shown in FigureIn order1b, by to changing further understand the width of the tapered behavior strips, of the the antenna AVA structure, cut-off frequency especially changes in the betweenlower frequencies, 1.55 GHz to the 1.4 current GHz. Although distribution m1 can of bethe used AVA for with antenna arms matching, (proposed this structure) variable cannotand without be obviously them (conventional utilized for antenna structure) size is miniaturization. shown in Figure In addition,3. As shown the antennain Figure gain 3a, forat 1.6 all GHz, four casesboth instructures Figure1b with/without shows that the arms, width approximately of tapered strips have in the antipodal same current antennas dis- affectstribution. mainly However, the antenna at 0.8 GHz, gain. shown It should in Figure be noted 3b, that the increasingarms change the the length current of the distribu- strips shiftstion significantly. the cut-off frequency At this frequency, of the antipodal the strip antennas arms in to the lower proposed frequency structure with thecreate cost an- of havingother resonance a larger antenna.since they In behave this manuscript, like a dipole we antenna investigate with theelectrical possibility length of of integrating a quarter- anotherwavelength. resonator In the into structure the antipodal without Vivaldiarms, no antenna, resonance which happens could at help 0.8 toGHz. reduce As a the result, size of antipodal Vivaldi antennas, without compromising the radiation characteristics. at low frequencies, the additional arms contribute to the antenna miniaturization.

(a) (b)

FigureFigure 1. AntipodalAntipodal Vivaldi Vivaldi antenna antenna configuration configuration with with (a ()a different) different widths widths of oftapered tapered strips, strips, and and (b) (theirb) their reflection reflection co- coefficientefficient and and gain. gain.

Figure2 demonstrates the proposed antenna. The proposed antenna includes two main tapered strips (conventional structure) and two additional strip arms. We will investigate how these additional strip arms contribute to antenna size reduction. The 2 dimensions of the antenna are set to be 158 × 125 mm , which is approximately 0.38λ0 × 0.3λ0, where λ0 is the free space wavelength of 0.72 GHz. The proposed microstrip-fed antenna is designed on Taconic TLT substrate with dielectric constant and thickness of

Figure 2. The proposed Vivaldi antenna configuration.

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In order to further understand the behavior of the AVA structure, especially in the lower frequencies, the current distribution of the AVA with arms (proposed structure) and without them (conventional structure) is shown in Figure 3. As shown in Figure 3a, at 1.6 GHz, both structures with/without arms, approximately have the same current distribution. However, at 0.8 GHz, shown in Figure 3b, the arms change the current distribution significantly. At this frequency, the strip arms in the proposed structure create another resonance since they behave like a dipole antenna with electrical length of a quarter- wavelength. In the structure without arms, no resonance happens at 0.8 GHz. As a result, at low frequencies, the additional arms contribute to the antenna miniaturization.

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2.55 and 1 mm, respectively. In order to design a compact ultra-wideband antenna, four exponential curves are employed. The equations of these curves are given as:

 a x y(x) = a1 e 2 + a3 upper curve  b x , for main tapered strips  y(x) = b1 e 2 + b3 lower curve c x (1)  y(x) = c1 e 2 + c3 upper curve  d x , for additional strip arms y(x) = d1 e 2 + d3 lower curve

where a2, b2, c2, and d2 demonstrate the degree of the exponential curves and a1, b1, c1, (a) (b) d1, a3, b3, c3, and d3 are related to the length and width of the arms and depend on the Figure 1. Antipodal Vivaldiposition antenna ofconfiguration the origin ofwith the (a Cartesian) different coordinatewidths of tapered system strips, in the and antenna (b) their structure. reflection To co- have efficient and gain. the optimum reﬂection coefﬁcient, the values of a2, b2, c2, and d2 must be optimized.

FigureFigure 2.2. TheThe proposedproposed VivaldiVivaldi antennaantenna conﬁguration.configuration.

As shown in Figure2, the tapered strips and arms in the proposed antipodal tapered Vivaldi antenna are terminated with elliptically-shaped features. The added exponential strip arms can create another dipole-shaped resonator for surface current, in comparison with the conventional structures (without the arms), which potentially may result in having lower edge of impedance bandwidth reduced (antenna miniaturization) and having more bandwidth. However, it may increase the complexity of the structure for the design optimization. The proposed antenna optimum parameter values are given in Table1. The proposed structure is designed, simulated and optimized using ANSYS High Frequency Structure Simulator (HFSS).

Table 1. Final optimum parameter values of the proposed antenna.

Parameter Value Parameter Value

Ws 158 mm m1 22 mm Ls 125 mm m2 26 mm Wstrip 66 mm n1 7 mm Wt 2 mm n2 11 mm Wg 32 mm a2 −80 mm Lg 10 mm b2 −80 mm s 59 mm c2 −100 mm d 16 mm d2 −100 mm P 7 mm

at 1.6 GHz, both structures with/without arms, approximately have the same current distribution. However, at 0.8 GHz, shown in Figure3b, the arms change the current distribution signiﬁcantly. At this frequency, the strip arms in the proposed structure create another resonance since they behave like a dipole antenna with electrical length of a Electronics 2021, 10, x FOR PEER REVIEW 4 of 12 quarter-wavelength. In the structure without arms, no resonance happens at 0.8 GHz. As a result, at low frequencies, the additional arms contribute to the antenna miniaturization.

(a)

(b)

FigureFigure 3. 3. (a()a )Surface Surface current current distribution distribution on on the the prop proposedosed antenna structure with/withoutwith/without additionaladdi- tionalstrip strip arms arms at the at frequency the frequency of (a) of 1.6 (a GHz,) 1.6 GHz, and ( band) 0.8 (b GHz.) 0.8 GHz.

Table 1.Figure Final 4optimum shows theparameter reflection values coefficient of the proposed of the antenna. AVA structure with/without arms. As indicated, it is easy to find that by integrating the new arms into the conventional Parameter Value Parameter Value structure, another resonance at 800 MHz is created which shifts the lower edge of cut-off Ws 158 mm m1 22 mm from 1480 MHz to 720 MHz resulting in antennas’ size reduction by 51%. As shown in

FigureL4,s the new arms do125 not mm change or shift them other2 resonances of the26 mm conventional VivaldiWstrip antenna. This is66 observable mm by looking atn1 the surface current density7 mm along the entire structureWt at 1.6 GHz2 inmm Figure 3a, where it is seenn2 that the integration11 of mm the arms does not changeWg the current distribution32 mm on the main tapereda2 strips. In addition−80 tomm the antenna miniaturization,Lg the additional10 mm strip arms can helpb2 in improving the antenna−80 mm matching at low frequencies.s In fact,59 using mm the extra variablesc of2 the dipole antenna−100 structure mm (i.e., n2, m2, p, c1d, c3, d1, and d3), the16 antennamm matching can bed2 better optimized. More−100 specifically,mm as shownP in Figure4, the additional7 mm arms improve the reflection coefficient over 2–5 GHz.

Figure 4 shows the reflection coefficient of the AVA structure with/without arms. As indicated, it is easy to find that by integrating the new arms into the conventional structure, another resonance at 800 MHz is created which shifts the lower edge of cut-off from 1480 MHz to 720 MHz resulting in antennas’ size reduction by 51%. As shown in Figure 4, the new arms do not change or shift the other resonances of the conventional Vivaldi antenna. This is observable by looking at the surface current density along the entire structure at 1.6 GHz in Figure 3a, where it is seen that the integration of the arms does not change the current distribution on the main tapered strips. In addition to the antenna miniaturization, the additional strip arms can help in improving the antenna matching at low frequencies. In fact, using the extra variables of the dipole antenna structure (i.e., n2, m2, p, c1, c3, d1, and d3), the antenna matching can be better optimized. More specifically, as shown in Figure 4, the additional arms improve the reflection coefficient over 2–5 GHz. To better show the effect of additional arms, the reflection coefficient at low frequencies for conventional antenna (Vivaldi antenna without strip arms), proposed structure,

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and antenna with only strip arms is plotted in Figure 5. As shown, the proposed antenna has both resonances of conventional antenna and structure with only strip arms at around 1600 MHz and 800 MHz, respectively. The electric field distribution of the proposed antenna above its top layer is demonstrated at different frequencies in Figure 6. As shown, at 800 MHz, the strip arms antenna resonates and radiates and the contribution of the main tapered strips in radiation is negligible. On the contrary, as shown in Figure 6b, the main tapered strips radiate at 1.6 GHz, and strip arms with dipole-shaped structure do not have any effect on the resonant frequency and radiation. As it is known, the dipole antenna resonates at its odd harmonics. In Figure 6c, the electric field distribution of the antenna is illustrated at 4 GHz (the 5th harmonic of the dipole structure). It can be seen that both the dipole-shaped strip arms and tapered strips resonate and radiates. Since these radiations at low frequencies are not in-phase, a reduction in the gain of the proposed antenna compared to conventional antenna is expected. To demonstrate this fact, the gain of the proposed antenna and conventional Vivaldi antenna is plotted in Figure 7. As shown, the integration of the strip arms into the antipodal Vivaldi antenna deteriorates the antenna gain (or antenna radiation gain) in S frequency band between 3rd and 5th harmonics of the dipole structure. This is, however, what we expected due to the radiation of the dipole antenna. Furthermore, because of the in-phase radiation of the dipole antenna (at its harmonics) and tapered strips, there are frequency ranges, within which the gain of the proposed antenna is higher than the conventional one. The minimum gain of proposed antenna over the impedance bandwidth is better than 1 dBi that offers an appreciable gain in lower edge of the impedance bandwidth. As shown in Figure 8, a parametric study on the size and degree of the exponential curves of the additional arms shows that reflection coefficient and antenna gain can be optimized by the features of the arms. This is due to the fact that the dimensions of the additional arms affect the amplitude and phase of current distribution. The size of the additional arms, as expected, has more effect at the lower edge frequency band. As shown in Figure 8a, to have a good reflection coefficient and antenna gain, the width of the additional arms need to be m2 = 26 mm. For lower values such as m2 = 10 mm, the antenna gain drops at 4.3 GHz, and the reflection gets higher values. For higher values such as m2 = 34 mm, the reflection coefficient gets worse around 7.5 GHz. Furthermore, the degree of exponential curve of the arms affects the antenna reflection at low frequencies and antenna Electronics 2021, 10, 83 gain at higher frequency. According to Figure 8b, this parameter needs to be set at5 ofC 122 = −100 for the best performance.

FigureFigure 4.4.Reﬂection Reflection coefﬁcient coefficient of of the the AVA AVA (Antipodal (Antipodal Vivaldi Vivaldi antenna) antenna) structure structure with with arms arms (proposed (pro- antenna)posed antenna) and without and without arms (conventional arms (conventional antenna). antenna).

To better show the effect of additional arms, the reflection coefficient at low frequencies for conventional antenna (Vivaldi antenna without strip arms), proposed structure, and antenna with only strip arms is plotted in Figure5. As shown, the proposed antenna has both resonances of conventional antenna and structure with only strip arms at around 1600 MHz and 800 MHz, respectively. The electric field distribution of the proposed antenna above its top layer is demonstrated at different frequencies in Figure6. As shown, at 800 MHz, the strip arms antenna resonates and radiates and the contribution of the main tapered strips in radiation is negligible. On the contrary, as shown in Figure6b, the main tapered strips radiate at 1.6 GHz, and strip arms with dipole-shaped structure do not have any effect on the resonant frequency and radiation. As it is known, the dipole antenna resonates at its odd harmonics. In Figure6c, the electric field distribution of the antenna is illustrated at 4 GHz (the 5th harmonic of the dipole structure). It can be seen that both the dipole-shaped strip arms and tapered strips resonate and radiates. Since these radiations at low frequencies are not in-phase, a reduction in the gain of the proposed antenna compared to conventional antenna is expected. To demonstrate this fact, the gain of the proposed antenna and conventional Vivaldi antenna is plotted in Figure7. As shown, the integration of the strip arms into the antipodal Vivaldi antenna deteriorates the antenna gain (or antenna radiation gain) in S frequency band between 3rd and 5th harmonics of the dipole structure. This is, however, what we expected due to the radiation of the dipole antenna. Furthermore, because of the in-phase radiation of the dipole antenna (at its harmonics) and tapered strips, there are frequency ranges, within which the gain of the proposed antenna is higher than the conventional one. The minimum gain of proposed antenna over the impedance bandwidth is better than 1 dBi that offers an appreciable gain in lower edge of the impedance bandwidth. ElectronicsElectronics 2021 2021,, 10, 10,, 83x, xFOR FOR PEER PEER REVIEW REVIEW 6 6of of 12 12

Figure 5. Reﬂection coefﬁcient of the AVA structure with arms (proposed antenna) and without arms FigureFigure 5. 5. Reflection Reflection coefficient coefficient of of the the AVA AVA structur structure ewith with arms arms (proposed (proposed antenna) antenna) and and without without (conventional antenna), and only strip arms. armsarms (conventional (conventional antenna), antenna), and and only only strip strip arms. arms.

E-fieldE-field E-fieldE-field MaxMax MaxMax

ResonanceResonance ResonanceResonance MinMin MinMin

(a(a) ) (b(b) )

E-field E-field Max Max

TaperedTapered strip strip StripStrip atms atms radiationradiation radiationradiation

Min Min

(c(c) )

FigureFigureFigure 6. 6.6. Electric ElectricElectric field field ﬁeld distribution distribution distribution above above above the the top the top la toplayeryer layerof of the the of proposed proposed the proposed antenna antenna antenna structure structure structure at at ( a(a) )0.8 0.8 at GHz, (b) 1.6 GHz, and (c) 4 GHz. GHz,(a) 0.8 (b GHz,) 1.6 (GHz,b) 1.6 and GHz, (c) and 4 GHz. (c) 4 GHz.

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FigureFigure 7.7. GainGain ofof thethe AVAAVA structure structure with with arms arms (proposed (proposed antenna) antenna) and and without without arms arms (conventional (conven- antenna).tional antenna).

As shown in Figure8, a parametric study on the size and degree of the exponential curves of the additional arms shows that reflection coefficient and antenna gain can be optimized by the features of the arms. This is due to the fact that the dimensions of the additional arms affect the amplitude and phase of current distribution. The size of the additional arms, as expected, has more effect at the lower edge frequency band. As shown in Figure8a, to have a good reflection coefficient and antenna gain, the width of the additional arms need to be m2 = 26 mm. For lower values such as m2 = 10 mm, the antenna gain drops at 4.3 GHz, and the reflection gets higher values. For higher values such as m2 = 34 mm, the reflection coefficient gets worse around 7.5 GHz. Furthermore, the degree of exponential curve of the arms affects the antenna reflection at low frequencies Figureand antenna 7. Gain of gain the atAVA higher structure frequency. with arms According (proposed to antenna) Figure8 b,and this without parameter arms (conven- needs to be tionalset at antenna).C2 = −100 for the best performance.

(a) (b)

Figure 8. Reflection coefficient and antenna gain for (other parameters are constant) (a) different width and (b) the degree of exponential curve of the additional arms.

3. Results and Discussion Figure 9 shows a prototype of the fabricated AVA antenna. The proposed antenna has very simple configuration which leads to the fabrication without any complexities. Figure 10 shows the reflection coefficient of the proposed AVA antenna. Due to antenna’s simple configuration, an acceptable agreement between the simulated and measured reflection coefficient can be observed. The maximum measured reflection happens at 2.3 GHz, where a reflection coefficient of −9.4 dB is measured. The measured impedance bandwidth of 185% has been achieved over 0.72–17 GHz with acceptable radiation characteristics such as antenna(a) gain. The gain of the proposed antenna(b) is drawn in Figure 11.

FigureOver the 8. Reflection measured coefficient impedance and bandwidth antenna gain (0.7 for2–17 (other GHz), parameters the measured are constant) antenna (a) gain different var- Figure 8. Reflection coefficient and antenna gain for (other parameters are constant) (a) different widthies between and (b) 1 the dBi degree and 12.5 of exponential dBi with the curve maximum of the additional gain around arms. 10 GHz. In Figure 11, there widthare some and (b discrepancies) the degree of exponentialbetween simulation curve of the and additional measurement arms. results at higher frequen- 3.cies, Results which and may Discussion be due to the measurement errors/tolerance of the measurement devices. 3. Results and Discussion The Figuremeasured9 shows antenna a prototype gain confirms of the fabricatedthe great reliability AVA antenna. and performance The proposed of antenna the pro- hasposedFigure very antenna simple 9 shows especially configuration a prototype at the which lowerof the leadsedgefabricated of to the the AVAbandwidth. fabrication antenna. without The proposed any complexities. antenna hasFigure veryThe 10 simple simulatedshows configuration the and reflection measured coefficientwhich radiation leads of to the pattern the proposed fabrications of the AVA proposed without antenna. antennaany Due complexities. to in antenna’s both or- Figuresimplethogonal 10 configuration, shows planes the ZoY reflection an(H-plane) acceptable coefficient and agreement XoY of (E-plane) the betweenproposed at 0.7, the AVA simulated2.4, antenna.5.8 and and 10 Due measuredGHz to antenna’sare shown reflec- simpletionin Figure coefficient configuration, 12. As can shown, be an observed. acceptable there is Thea goodagreement maximum agreement between measured between the reflection simulated simulated happens and and measured measured at 2.3 GHz, re- re- flectionwheresults in acoefficient all reflection four frequencies. coefficientcan be observed. At of −lower9.4 dBThe frequencies, is ma measured.ximum the measured The antenna measured has reflection a impedance bidirectional happens bandwidth behavior at 2.3 GHz,ofwith 185% wheresmooth has a been variations.reflection achieved coefficient As overthe operating 0.72–17 of −9.4 GHz freqdB is withuency measured. acceptable increases, The radiationdue measured to the characteristics higherimpedance order bandwidth of 185% has been achieved over 0.72–17 GHz with acceptable radiation characteristics such as antenna gain. The gain of the proposed antenna is drawn in Figure 11. Over the measured impedance bandwidth (0.72–17 GHz), the measured antenna gain varies between 1 dBi and 12.5 dBi with the maximum gain around 10 GHz. In Figure 11, there are some discrepancies between simulation and measurement results at higher frequencies, which may be due to the measurement errors/tolerance of the measurement devices. The measured antenna gain confirms the great reliability and performance of the proposed antenna especially at the lower edge of the bandwidth. The simulated and measured radiation patterns of the proposed antenna in both orthogonal planes ZoY (H-plane) and XoY (E-plane) at 0.7, 2.4, 5.8 and 10 GHz are shown in Figure 12. As shown, there is a good agreement between simulated and measured results in all four frequencies. At lower frequencies, the antenna has a bidirectional behavior with smooth variations. As the operating frequency increases, due to the higher order

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mode excitations, the unwanted ripples appear in the peripheries of the radiation patterns. Electronics 2021, 10, 83 mode excitations, the unwanted ripples appear in the peripheries of the radiation patterns.8 of 12 TheThe proposedproposed antennaantenna hashas thethe maximummaximum gaingain atat thethe mainmain lobelobe inin thethe axialaxial directiondirection (+Y-(+Y- direction)direction) ofof thethe taperedtapered exponentialexponential arms.arms. Moreover,Moreover, thethe half-powerhalf-power beamwidthbeamwidth (HPBW)(HPBW) of the proposed antenna decreases as the frequency increases which results in a higher suchof the as proposed antenna antenna gain. The decreases gain of as thethe proposedfrequency antennaincreases is which drawn results in Figure in a higher 11. Over antenna gain. As seen in Figure 12, the measured cross-polarization levels of the H- and theantenna measured gain. As impedance seen in Figure bandwidth 12, the measured (0.72–17 GHz),cross-polarization the measured levels antenna of the gainH- and varies E-planesE-planes withinwithin thethe HPBWHPBW areare atat leastleast 2020 dBdB belowbelow thethe correspondingcorresponding co-polarizationsco-polarizations betweenwhich indicates 1 dBi and the 12.5 extreme dBi with linearity the maximum of polarization gain aroundof the fabricated 10 GHz. In AVA. Figure This 11 ,light- there are somewhich discrepanciesindicates the extreme between linearity simulation of polarization and measurement of the fabricated results at AVA. higher This frequencies, light- weightweight andand compactcompact antennaantenna designdesign providesprovides excellentexcellent matchingmatching overover aa broadbroad frequencyfrequency whichrange maywhich be makes due to the the antenna measurement most attractive errors/tolerance for ultra-wideband of the measurement applications. devices. The The measuredrange which antenna makes gainthe antenna conﬁrms most the greatattractive reliability for ultra-wideband and performance applications. of the proposed The smallsmall sizesize ofof thethe proposedproposed antennaantenna makesmakes itit eveneven moremore appealingappealing forfor lowlow frequencyfrequency ap-ap- antenna especially at the lower edge of the bandwidth. plicationsplications suchsuch asas sub-sixsub-six measurementmeasurement setups.setups.

FigureFigure 9. 9. PhotographPhotograph ofof of fabricatedfabricated fabricated antenna:antenna: antenna: ((aa)) ( atoptop) top layerlayer layer andand and ((bb)) (bottombottomb) bottom layer.layer. layer.

Figure 10. Reflection coefficient of the proposed antenna. Figure 10. ReflectionReﬂection coefficient coefﬁcient of of the the proposed proposed antenna. antenna.

The simulated and measured radiation patterns of the proposed antenna in both orthogonal planes ZoY (H-plane) and XoY (E-plane) at 0.7, 2.4, 5.8 and 10 GHz are shown in Figure 12. As shown, there is a good agreement between simulated and measured results in all four frequencies. At lower frequencies, the antenna has a bidirectional behavior with smooth variations. As the operating frequency increases, due to the higher order mode excitations, the unwanted ripples appear in the peripheries of the radiation patterns. The proposed antenna has the maximum gain at the main lobe in the axial direction (+Y- direction) of the tapered exponential arms. Moreover, the half-power beamwidth (HPBW) of the proposed antenna decreases as the frequency increases which results in a higher antenna gain. As seen in Figure 12, the measured cross-polarization levels of the H- and E-planes within the HPBW are at least 20 dB below the corresponding co-polarizations which indicates the extreme linearity of polarization of the fabricated AVA. This lightweight and compact antenna design provides excellent matching over a broad frequency range which makes the antenna most attractive for ultra-wideband applications. The small size Electronics 2021, 10, 83 9 of 12

Electronics 2021, 10, x FOR PEER REVIEW 9 of 12 of the proposed antenna makes it even more appealing for low frequency applications such Electronics 2021, 10, x FOR PEER REVIEWas sub-six measurement setups. 9 of 12

Figure 11. Gain of the proposed antenna. Figure 11. Gain of the proposed antenna. Figure 11. Gain of the proposed antenna.

Figure 12. Cont.

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FigureFigure 12. 12. 12. Simulated SimulatedSimulated and and and measured measured radiation radiation patterns patterns of the ofdesigned the designeddesigned antenna antennaantenna in ZY (H-plane) inin ZYZY (H-plane)(H-plane) and andXY (E-plane)XYXY (E-plane) (E-plane) at at (a at)(a 0.7 )( 0.7a) GHz, 0.7 GHz, GHz, ( b(b)) 2.4 2.4(b) GHz,GHz, 2.4 GHz, ( (cc)) 5.8 5.8 ( cGHz) GHz 5.8 andGHz and (d (and) d10) 10GHz. (d GHz.) 10 GHz.

TheTheThe group group delay delay is anotheris another important important characteristic characteristic for an for ultra-wideband an ultra-wideband antenna antenna applicationapplication which which which shows shows shows how how how the thesignal the signal signal in time in in timedomain time domain domain is distorted is is distorted distorted by the proposed by by the the proposed proposed AVA AVA AVA antenna. In this paper, two identical AVAs were placed face to face over a distance of 50 antenna.antenna. In this paper, twotwo identicalidentical AVAs AVAs were were placed placed face face to to face face over over a distance a distance of 50of cm50 cm and the measured result in Figure 13 shows that the group delay of the proposed de- and the measured result in Figure 13 shows that the group delay of the proposed design is signcm and is around the measured 2 ns in the result whole infrequency Figure 13band shows of interest that thewith group the variation delay ofless the than proposed ±1 de- around 2 ns in the whole frequency band of interest with the variation less than ±1 ns in nssign in theis around most frequencies. 2 ns in the In whole other words,frequency the result band demonstrates of interest with that thea transmitted variation sig- less than ±1 nalnsthe inwill most the not most frequencies. be harmfully frequencies. In affected other In other words,by the words, proposed the result the antenna. result demonstrates demonstrates that a transmittedthat a transmitted signal willsig- nalnot will be harmfully not be harmfully affected affected by the proposed by the proposed antenna. antenna.

Figure 13. Measured group delay of the proposed antenna.

Table 2 compares the proposed AVA antenna with some recent published works in theFigureFigure literature. 13. 13. MeasuredMeasured As indicated, group group thedelay performance of the proposed of the proposedantenna. antenna in this paper is com- parable with other works. The proposed antenna offers an acceptable gain all over the operatingTableTable frequency 22 comparescompares band. thethe Besides, proposedproposed the impe AVAAVAdance antennaantenna bandwidth with with some some(0.72–17 recentrecent GHz) publishedpublished of the pro- worksworks inin posedthethe literature. literature. antenna isAs Aswider indicated, indicated, than other the the performance referred performance structures. of the of theTheproposed proposedmaximum antenna gain antenna (12.5in this indBi) paper this of paper is com- is theparablecomparable proposed with antenna withother other works.at X-band works. The is higher Theproposed proposed than antenna its counterparts. antenna offers offers an Except acceptable an acceptablefor the designedgain gainall over all over the antennaoperatingthe operating in [19],frequency the frequency proposed band. band. antenna Besides, Besides, is smallerthe impe the than impedancedance any of bandwidth other bandwidth structures (0.72 listed–17 (0.72–17 GHz) in Table GHz) of the of pro- the 2 which are fabricated on substrates with almost higher permittivity values. It should be posedproposed antenna antenna is wider is wider than than other other referred referred structures. structures. The The maximum maximum gain gain (12.5 (12.5 dBi) dBi) of noted that the antenna reported in [16] has a very complicated configuration and design thethe proposed proposed antenna antenna at at X-band is higher thanthan its counterparts. Except for the designed procedure compared to the proposed antenna structure which makes it undesirable. In addition,antennaantenna inthe in [19], [fractal19], the the structures proposed proposed used antenna antenna in [16] is deteriorate smaller is smaller than the than anylinearity anyof other of the other structures antenna structures due listed to listedin Table in increasing2Table which2 which are the fabricated cross-polarization. are fabricated on substrates on substrates with withalmost almost higher higher permittivity permittivity values. values. It should It should be notedbe noted that that the the antenna antenna reported reported in in[16] [16 has] has a avery very complicated complicated configuration conﬁguration and and design design procedure compared to the proposed antenna structure which makes it undesirable.undesirable. In addition,addition, the the fractal fractal structures structures used in [16] [16] deteriorate the linearity of the antenna due to increasingincreasing the the cross-polarization. cross-polarization.

Electronics 2021, 10, 83 11 of 12

Table 2. Comparison between some AVAs and TSAs (tapered slot antennas) in literature and the proposed antipodal Vivaldi antenna.

Operating Impedance Min and Ref No. ε Size (λ2) r 0 Frequency (GHz) BW (%) MaxGain (dBi) [15] 4.5 0.67 × 0.89 4–30 152 5&7 [16] 2.6 0.39 × 0.4 1.3–17 171 3.5&9.3 [17] 4.4 0.39 × 0.48 2.4–14 141 3.7&10 [18] 3.5 0.37 × 0.43 0.8–9.8 170 0&10.5 [19] 4.4 0.22 × 0.27 1.3–20 176 −2&10 This work 2.5 0.3 × 0.38 0.72–17 184 1&12.5

λ0: Free space wavelength at the lowest operating frequency.

There are several techniques to increase the Vivaldi antenna gain. Several researchers have introduced different methods to increase the antenna directivity [20–23]. Various lens structures such as metamaterial-loaded [21], fractal inspired [22], and phase adjusted [23] structures were proposed to be placed in front of the radiating area to improve the antenna directivity and radiation characteristics of Vivaldi antennas. Using one of these techniques could increase the directivity of the proposed antenna if higher gain is required.

4. Conclusions The presented AVA antenna employs extra exponential strip arms with elliptical- shape termination, to reduce the lower edge of the operating frequency bandwidth. The decrease in the low-end operating band demonstrates that the proposed structure can effectively miniaturize the size of the Vivaldi antennas. It provides an impedance frequency bandwidth of around 185%. Moreover, the modiﬁed AVA presents an appreciable gain all over the band of interest. A good agreement between simulated and measured results illustrates that the proposed antenna is a good candidate for ultra-wideband measuring systems or other communication applications.

Author Contributions: M.M.H. has designed and fabricated the antenna structure. M.M.H. and M.S.G. have measured the antenna and wrote the manuscript. R.M. supervised the project, con- tributed to discussion, and reviewed the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Testforce Systems Inc., Alberta Innovative Technology Futures (AITF) and the Canadian National Science and Engineering Research Council (NSERC) under the NSERC-AITF Industrial Chair Program. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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