Coplanar Stripline-Fed Wideband Yagi Dipole Antenna with Filtering-Radiating Performance

electronics

Article Coplanar Stripline-Fed Wideband Yagi Dipole Antenna with Filtering-Radiating Performance

Yong Chen 1, Gege Lu 2, Shiyan Wang 2 and Jianpeng Wang 2,*

1 School of Physics and Electronic Electrical Engineering, Huaiyin Normal University, Huaian 223300, China; [email protected] 2 Ministerial Key Laboratory of JGMT, Nanjing University of Science and Technology, Nanjing 210094, China; [email protected] (G.L.); [email protected] (S.W.) * Correspondence: [email protected]

Received: 6 July 2020; Accepted: 4 August 2020; Published: 6 August 2020

Abstract: In this article, a wideband filtering-radiating Yagi dipole antenna with the coplanar stripline (CPS) excitation form is investigated, designed, and fabricated. By introducing an open-circuited half-wavelength resonator between the CPS structure and dipole, the gain selectivity has been improved and the operating bandwidth is simultaneously enhanced. Then, the intrinsic filtering-radiating performance of Yagi antenna is studied. By implementing a reflector on initial structure, it is observed that two radiation nulls appear at both lower and upper gain passband edges, respectively. Moreover, in order to improve the selectivity in the upper stopband, a pair of U-shaped resonators are employed and coupled to CPS directly. As such, the antenna design is finally completed with expected characteristics. To verify the feasibility of the proposed scheme, a filtering Yagi antenna prototype with a wide bandwidth covering from 3.64 GHz to 4.38 GHz is designed, fabricated, and measured. Both simulated and measured results are found to be in good agreement, thus demonstrating that the presented antenna has the performances of high frequency selectivity and stable in-band gain.

Keywords: CPS (coplanar stripline); Yagi antenna; filtering-radiating performance; frequency selectivity

1. Introduction There is an increasing demand for RF front end to possess much more potential characteristics for application in modern wireless communication systems, such as compact structure, low cost, high efficiency, multiple functions, and so on. It is well known that both antennas and filters are two key components in the RF front end as they play important roles in whole communication systems [1–7]. If the antenna and filter can be integrated into one module, which possesses not only the radiation characteristics but also the filtering function, the extra matching network between these two components can be removed and the footprint of whole system will be reduced efficiently. In this context, antennas with filtering performance have been attracting more and more attention [8–10]. Antennas with unidirectional radiation are much more practical in some modern wireless communication systems [8–14], such as missiles, aircrafts, and vehicles. To accommodate to this tendency, Yagi antennas have been widely used as a kind of classical structure since its original design and operating principles were first described by Uda and Yagi [15,16]. Quite recently, filtering Yagi antennas have been proposed and investigated [17–19]. In [17], a filtering quasi-Yagi antenna was designed by using cascade strategy. Multimode balun bandpass filter was directly integrated into the antenna so as to achieve filtering performance. In [19], the principle from filter to antenna was adopted. Yagi structure here acted as the last-stage resonator of a filter. However, antennas designed by these two kinds of methods are bulky. Actually, the Yagi structure can exhibit the filtering performance

Electronics 2020, 9, 1258; doi:10.3390/electronics9081258 www.mdpi.com/journal/electronics ElectronicsElectronics 20202020,,9 9,, 1258x FOR PEER REVIEW 22 of of 10 9

designed by these two kinds of methods are bulky. Actually, the Yagi structure can exhibit the itself.filtering As demonstratedperformance itself. in [20 ],As the demonstrated out-of-band gain in [20], suppression the out-of-band of Yagi antenna gain suppression can be improved of Yagi by optimizingantenna can the be length improved and spacing by optimizing of directors the and length reflectors, and spacing while narrowing of directors the operatingand reflectors, bandwidth while comparednarrowing withthe operating the conventional bandwidth counterpart. compared Meanwhile, with the conventional it is well known counterpart. that the CPSMeanwhile, structure it isis muchwell known appreciated that the by engineeringCPS structure according is much to apprecia its advantagested by engineering in greatly simplifying according the to diitsff erential-fedadvantages networkin greatly for simplifying unidirectional the radiation differential-fed antenna andnetwork convenient for unidirectional integration with radiation the active antenna circuits andand monolithicconvenient microwaveintegration integratedwith the active circuits circuits [21–25 an].d monolithic microwave integrated circuits [21–25]. TheThe main motivation motivation of of this this article article is isto toprop proposeose a CPS-fed a CPS-fed wideband wideband Yagi Yagi dipole dipole antenna antenna with withfiltering-radiating filtering-radiating performance. performance. The intrinsic The intrinsic filtering filtering performance performance of the of Yagi the structure Yagi structure has been has beenutilized utilized here to here produce to produce two radiation two radiation nulls emer nullsging emerging at both at lower both lowerand upper and upperpassband passband edges, edges,respectively. respectively. To overcome To the overcome narrow theoperation narrow band operationwidth caused bandwidth by this filtering caused scheme, by this an filtering open- scheme,circuited anhalf-wavelength open-circuited resonator half-wavelength is introduced resonator between is CPS introduced and driven between dipole. CPSAs such, and both driven the dipole.gain selectivity As such, and boththe operating the gain bandwidth selectivity have and been the operatingenhanced simultaneously. bandwidth have It is been demonstrated enhanced simultaneously.that the introduced It is demonstratedresonator herein that serves the introduced as a first-order resonator resonator. herein servesMoreover, as a first-orderin order to resonator. improve Moreover,the selectivity in order of the to upperimprove passband the selectivity edge, a of pair the of upper U-shaped passband resonators edge, a pairare employed of U-shaped and resonators coupled areto CPS employed directly. and Finally, coupled an antenna to CPS directly.prototype Finally, with operation an antenna frequency prototype band with covering operation from frequency 3.64 GHz bandto 4.38 covering GHz is fabricated from 3.64 and GHz measured. to 4.38 GHz All isresults fabricated are observed and measured. as being in All good results agreement, are observed thereby as beingverifying in good the validity agreement, of this thereby design. verifying the validity of this design.

2.2. Design of the Proposed Antenna FigureFigure1 1 illustrates illustrates thethe configuration configuration ofof thethe proposed proposed filteringfiltering YagiYagi dipole dipole antenna, antenna, which which is is fabricatedfabricated onon aa polytetrafluoroethylenepolytetrafluoroethylene (PTFE)(PTFE) substratesubstrate withwith aa relativerelative permittivitypermittivity ofof 2.2,2.2, thicknessthickness 2 of 1 mm, and dimensions of 30 42 mm 2. The filtering antenna is composed of four parts: CPS for of 1 mm, and dimensions of 30× × 42 mm . The filtering antenna is composed of four parts: CPS for didifferentialfferential signal excitation, excitation, a apair pair of ofU-shaped U-shaped resonators resonators symmetrically symmetrically coupled coupled to the to CPS, the CPS,twotwo-elementelement radiators radiators consisting consisting of a of driven a driven element element and and a reflector, a reflector, as as well wel as as a a folded open-circuited half-wavelengthhalf-wavelength resonator inserted inserted between between the the feed feedlineline and and driver. driver. All All the the four four parts parts are are printed printed on onthe thetop top side side of ofthe the substrate substrate while while no no metal metal parts parts exist exist on on its its bottom bottom to ensure thethe operatingoperating environmentenvironment ofof dipoledipole structure.structure. The antenna structurestructure is symmetricalsymmetrical with respectrespect toto thethe referencereference lineline alongalong thethe middlemiddle axisaxis ofof CPSCPS alongalong thethe xx-axis.-axis.

Figure 1. Geometry of the proposed ﬁlteringfiltering Yagi dipoledipole antennaantenna inin 3D3D view.view. 2.1. Modiﬁed Dipole with Bandwidth Improved (Type A) 2.1. Modified Dipole with Bandwidth Improved (Type A) Firstly, the frequency selectivity of dipole antenna is investigated. For clear illustrating, both the Firstly, the frequency selectivity of dipole antenna is investigated. For clear illustrating, both the conventional dipole antenna and the new dipole structure named type A are presented and depicted conventional dipole antenna and the new dipole structure named type A are presented and depicted in Figure2a,b, respectively. It should be mentioned that these two dipole antennas are designed on the in Figure 2a,b, respectively. It should be mentioned that these two dipole antennas are designed on same substrate and operate at the same frequency. As indicated in Figure2b, for the new dipole structure, the same substrate and operate at the same frequency. As indicated in Figure 2b, for the new dipole

ElectronicsElectronics 2020 2020, ,9 9, , 1258x x FOR FOR PEER PEER REVIEW REVIEW 333 of of of 10 9 9 structure,structure, anan open-circuitedopen-circuited half-wavelengthhalf-wavelength resonaresonatortor hashas beenbeen introducedintroduced asas oneone filteringfiltering elementelement betweenanbetween open-circuited thethe feedlinefeedline half-wavelength andand dipole.dipole. resonatorByBy virtuevirtue has ofof this beenthis scheme,scheme, introduced bothboth as thethe one selectivityselectivity filtering element andand bandwidthbandwidth between the ofof dipolefeedlinedipole antennaantenna and dipole. areare expected Byexpected virtue to ofto thisbebe improved.improved. scheme, both CompComp thearative selectivityarative resultsresults and bandwidthincludingincluding reflectionreflection of dipole antennacoefficientscoefficients are andexpectedand normalized normalized to be improved. realized realized gains Comparativegains of of antennas antennas results are are including shown shown in in reflectionFigure Figure 3. 3. coeIt It can canfficients be be observed observed and normalized that that by by inserting inserting realized thegainsthe half-wavelengthhalf-wavelength of antennas are resonator,resonator, shown in anan Figure additionaladditional3. It can resonanceresonance be observed pointpoint that appearsappears by inserting atat aboutabout the 3.653.65 half-wavelength GHz,GHz, naturallynaturally resultingresonator,resulting inin an enhancedenhanced additional bandwidth.bandwidth. resonance Besides, pointBesides, appears forfor thethe at typetype about AA 3.65 antenna,antenna, GHz, itsits naturally in-bandin-band resulting gaingain becomesbecomes in enhanced flatterflatter andbandwidth.and thethe out-of-bandout-of-band Besides, forsuppressionsuppression the type A isis antenna, betterbetter thanthan its in-band thethe conventionalconventional gain becomes counterpart,counterpart, flatter and whichwhich the out-of-band meansmeans anan improvedsuppressionimproved selectivity.selectivity. is better than the conventional counterpart, which means an improved selectivity.

((aa)) ((bb))

FigureFigure 2.2. Dipole Dipole antennas. antennas. ( (aa))) Conventional. Conventional.Conventional. ( ((bb))) Proposed ProposedProposed type typetype A. A.A.

FigureFigure 3. 3. Comparison Comparison about about reflection reflectionreflection coefficients coecoefficientsfficients and and re realizedrealizedalized gainsgains between betweenbetween thethe conventional conventionalconventional dipole dipole antennaantenna and and proposed proposed TypeType A.A. A. 2.2. Study of the Filtering Performance of Yagi Antenna 2.2.2.2. StudyStudy ofof thethe FilteringFiltering PePerformancerformance ofof YagiYagi AntennaAntenna Herein, the filtering performance of Yagi structure itself is discussed. A typical three-element Herein,Herein, thethe filteringfiltering performanceperformance ofof YagiYagi structurstructuree itselfitself isis discussed.discussed. AA typicaltypical three-elementthree-element Yagi structure, including both reflector and director, is utilized and shown in Figure4. By optimizing YagiYagi structure, structure, including including both both reflector reflector and and director, director, is is utilized utilized and and shown shown in in Figure Figure 4. 4. By By optimizing optimizing the parameters, a filtering-radiating Yagi antenna can be obtained. Table1 tabulates the detailed thethe parameters,parameters, aa filtering-radiatingfiltering-radiating YagiYagi antennaantenna cancan bebe obtained.obtained. TableTable 11 tabulatestabulates thethe detaileddetailed dimensions of traditional and optimized Yagi antenna. A comparison about normalized realized gain dimensionsdimensions of of traditional traditional and and optimized optimized Yagi Yagi ante antenna.nna. A A comparison comparison about about no normalizedrmalized realized realized gain gain and reflection coefficient curves versus frequency between them is provided in Figure5. It can be easily andand reflectionreflection coefficientcoefficient curvescurves versusversus frequencyfrequency bebetweentween themthem isis providedprovided inin FigureFigure 5.5. ItIt cancan bebe observed that after the optimization process, the roll-off performance for both the realized gain and |S11| easilyeasily observed observed that that after after the the optimization optimization process, process, the the roll-off roll-off performance performance for for both both the the realized realized gain gain curves have been improved remarkably, revealing the desired filtering performance. In fact, the filtering andand |S11||S11| curvescurves havehave beenbeen improvedimproved remarkably,remarkably, revealingrevealing thethe desireddesired filteringfiltering performance.performance. InIn property, especially out-of-band gain suppression is here caused by the enhanced loaded Q-factor, fact,fact, thethe filteringfiltering property,property, especiespeciallyally out-of-bandout-of-band gaingain suppressionsuppression isis herehere causedcaused byby thethe enhancedenhanced which is influenced by the distance between the driver and parasitic elements and will be higher under loadedloaded Q-factor,Q-factor, whichwhich isis influencedinfluenced byby thethe distandistancece betweenbetween thethe driverdriver andand parasiticparasitic elementselements andand small element spaces [10]. The described optimization exactly reduces the distances among elements, willwill bebe higherhigher underunder smallsmall elementelement spacesspaces [10][10].. TheThe describeddescribed optimizationoptimization exactlyexactly reducesreduces thethe thus increasing the loaded Q-factor. Besides, it is seen that not only the gain of the optimized one distancesdistances amongamong elements,elements, ththusus increasingincreasing thethe loadedloaded Q-factor.Q-factor. Besides,Besides, itit isis seenseen thatthat notnot onlyonly thethe becomes flatter, but also the upper and lower radiation nulls are generated. One thing should be gaingain of of the the optimized optimized one one becomes becomes flatter, flatter, but but also also the the upper upper and and lower lower radiation radiation nulls nulls are are generated. generated. mentioned that radiation nulls are produced by the intrinsic characteristic of the Yagi antenna. It is OneOne thing thing should should be be mentioned mentioned that that radiation radiation nulls nulls are are produced produced by by the the intrinsic intrinsic characteristic characteristic of of the the known that the parasitic element working as director or reflector is determined by the phase condition YagiYagi antenna. antenna. It It is is known known that that the the parasitic parasitic element element workingworking as as director director or or reflector reflector is is determined determined by by

Electronics 2020, 9, x FOR PEER REVIEW 4 of 9 Electronics 2020, 9, 1258 4 of 10 Electronics 2020, 9, x FOR PEER REVIEW 4 of 9 Electronicsthe phase 2020 condition, 9, x FOR PEERbetween REVIEW the currents on the driver and the element. If the phase of current4 ofon 9 parasitic element is prior to that of the driver, the parasitic one will act as a director. Similarly, if the betweenthe phase the condition currents between on the driver the currents and the element.on the driv If theer phaseand the of element. current on If parasiticthe phase element of current is prior on phase of the current on the parasitic element is delayed to that of driver, a reflector can be achieved. toparasitic that of element the driver, is prior the parasitic to that of one the will driver, act as the a director. parasitic Similarly, one will act if the as phasea director. of the Similarly, current on if the Besides, there is no doubt that phase condition is related to the operating frequency. As such, the parasiticphase of elementthe current is delayed on the parasitic to that of element driver, a is reflector delayed can to bethat achieved. of driver, Besides, a reflector there can is no be doubtachieved. that radiation pattern and beam direction will turn 180 degrees at some certain frequencies where the phaseBesides, condition there is is no related doubt to that the operatingphase condition frequency. is related As such, to the the radiation operating pattern frequency. and beam As such, direction the reflector or director change their role. In this context, when the front-to-back ratio reaches its willradiation turn 180pattern degrees and atbeam some direction certain frequencieswill turn 180 where degrees the at reflector some certain or director frequencies change where theirrole. the minimum, radiation nulls appear. The radiation nulls at the upper and lower band edges are caused Inreflector this context, or director when thechange front-to-back their role. ratio In reaches this context, its minimum, when radiationthe front-to-back nulls appear. ratio The reaches radiation its by the director and reflector, respectively. It should also be noted that the resonant frequency will nullsminimum, at the radiation upper and nulls lower appear. band edges The radiation are caused nulls by theat the director upper and and reflector, lower band respectively. edges are It caused should also be changed slightly with different distance values as the distances will affect the coupling alsoby the be director noted that and the reflector, resonant respec frequencytively. will It should also be also changed be noted slightly that with the diresonantfferent distancefrequency values will condition for the Yagi dipole antenna [26–32]. To verify the aforementioned statement, the radiation asalso the be distances changed will slightly affect with the coupling different condition distance forvalues the Yagias the dipole distances antenna will [26 affect–32]. the To verifycoupling the patterns at these two radiation nulls are displayed in Figure 6. It can be clearly found that beam aforementionedcondition for the statement, Yagi dipole the antenna radiation [26–32]. patterns To at verify these the two aforementioned radiation nulls arestatement, displayed the in radiation Figure6. direction has been reversed. The optimization process changes the distances among elements, which Itpatterns can be clearlyat these found two radiation that beam nulls direction are displayed has been reversed.in Figure The6. It optimization can be clearly process found changes that beam the determines coupling and phase condition. Therefore, the location of radiation nulls can be controlled. distancesdirection has among been elements, reversed. which The optimization determines coupling process andchanges phase the condition. distances Therefore, among elements, the location which of radiationdetermines nulls coupling can be and controlled. phase condition. Therefore, the location of radiation nulls can be controlled.

Figure 4. Configuration of a three-element Yagi antenna. Figure 4. Configuration of a three-element Yagi antenna. Figure 4. 0 ConfigurationConﬁguration of a three-element Yagi antenna.0 0 0 -50 0-5 -5 -5 -10-5 -5-10

(dB) -10-15 -10-15

⏐ -10 -10 11 S (dB) -15 -15

(dB) ⏐ ⏐ -15-20 S _Traditional -15-20

⏐ 11 11 11 S S _Optimized S ⏐ -20 S11_Traditional -20 ⏐ -20-25 S11_Traditional -20-25 Realized Gain_Traditional11 S11_Optimized -25 Realized Gain_Optimized S11_Optimized -25 -25-30 Realized Gain_Traditional -25-30 Normalized Realized Gain (dBi) 2.5 3.0 3.5 Realized 4.0 Gain_Traditional 4.5 5.0 5.5 Realized Gain_Optimized -30 Frequency Realized (GHz) Gain_Optimized -30 Normalized Realized Gain (dBi) -302.5 3.0 3.5 4.0 4.5 5.0 5.5-30 Normalized Realized Gain (dBi) 2.5 3.0 3.5 4.0 4.5 5.0 5.5 Frequency (GHz) Figure 5. Comparison about normalized Frequencyrealized (GHz)gain and reflection coefficient curves versus Figurefrequency 5.5.Comparison betweenComparison the about traditionalabout normalized normalized and optimized realized realized gain Yagi andgain antenna. reﬂection and reflection coeﬃcient coefficient curves versus curves frequency versus Figure 5. Comparison about normalized realized gain and reflection coefficient curves versus betweenfrequency the between traditional the traditional and optimized and optimized Yagi antenna. Yagi antenna. frequency between the traditional and optimized Yagi antenna. 0 0 330 30 330 30 0-5 0-5 330 0 30 330 0 30 300 60 300 60 330 -10 30 330 -10 30 -5 -5 -5 -5 300 -15 60 300 -15 60 300 -10 60 300 -10 60 270 -10-20 90 270 -10-20 90 -15 -15 -15 -15 270 -20 90 270 -20 90 270 -20 90 270 -20 90 240 120 240 120

240 120 240 120 240 210 150 120 240 210 150 120 180 180 210 150 210 150 210 180 150 210 180 150 (180a) (180b) FigureFigure 6. 6.E E planes planes of of thethe optimizedoptimized(a) YagiYagi operatingoperating atat twotwo radiationradiation(b) nulls. ( (aa)) 3.32 3.32 GHz. GHz. (b (b) )4.66 4.66 GHz. GHz. Figure 6. E planes of the optimized Yagi operating at two radiation nulls. (a) 3.32 GHz. (b) 4.66 GHz. Figure 6. E planes of the optimized Yagi operating at two radiation nulls. (a) 3.32 GHz. (b) 4.66 GHz. In addition, as indicated in Table 1, the most difference between traditional and optimized antennas lies in the length of parasitic element and distance from it to the driven one. In this context, In addition, as indicated in Table 1, the most difference between traditional and optimized antennas lies in the length of parasitic element and distance from it to the driven one. In this context,

Electronics 2020, 9, 1258 5 of 10

Table 1. Dimensions of Yagi antenna in Figure4

Type Parameters WLy1 Ly2 Ly3 D1 D2 Traditional Values(mm) 2 30 31.22 26.33 18.98 15.3 Optimized Values(mm) 2 30 27.55 33.67 2.02 3.06

ElectronicsIn addition,2020, 9, x FOR as PEER indicated REVIEW in Table1, the most di fference between traditional and optimized5 of 9 antennas lies in the length of parasitic element and distance from it to the driven one. In this context, thethe relationshiprelationship betweenbetween the the occurring occurring frequency frequency of lower of lower radiation radiation null and null the and spacing the spacing from reflector from reflectorto driven to element driven elementD2, as wel D2, as as the well length as the of length reflector of reflectorLy3 are selectedLy3 are selected for investigation for investigation and shown and shownin Figure in 7Figure. It can 7. beIt can found be found that when that whenD2 remains D2 remains unchanged, unchanged, increasing increasingLy3 will Ly3 will move move down down the thefrequency frequency location location of radiationof radiation null. null. Moreover, Moreover, with with a a fixed fixedL yL3y3,, increasing increasingD D22 cancan alsoalso result in a a variation ofof thethe frequency frequency of of radiation radiation null. null. Herein, Herein, according according to the to abovethe above discussions, discussions, both theboth length the lengthof the parasiticof the parasitic element element and the distanceand the fromdistance the drivenfrom the element driven to element parasitic to one parasitic in Yagi structureone in Yagi has structurebeen properly has been selected properly to obtain selected good to gain obtain selectivity good gain for selectivity better filtering for better performance. filtering performance.

3.7 Ly3=32mm 3.6 L =33mm y3

3.5 Ly3=34mm L =35mm 3.4 y3 L =36mm 3.3 y3 3.2 3.1 Frequency (GHz) Frequency 3.0 2.9 02468101214 D (mm) 2 FigureFigure 7. RelationshipRelationship between between the the occurring occurring frequency frequency of of lower lower radiation radiation null null and and the the spacing spacing from from

reflectorreflector to driven element D2,, as as well wel asas thethe lengthlength ofof reflectorreflector LLyy33. 2.3. Realization of the Proposed Filtering Yagi Dipole Antenna Table 1. Dimensions of Yagi antenna in Figure 4 Based on the aforementioned dipole antenna with improved selectivity and enhanced bandwidth, Type Parameters W Ly1 Ly2 Ly3 D1 D2 a Yagi dipole antenna named type B is presented by adding a reflector in the dipole structure (Type A), Traditional Values(mm) 2 30 31.22 26.33 18.98 15.3 as shown in Figure8a. As we have proved that the reflector can reflect electromagnetic waves and Optimized Values(mm) 2 30 27.55 33.67 2.02 3.06 enhance the directivity of antenna. Also, it actually can improve the out of band suppression. Figure9 sketches the realized gain of proposed Type B. From the gain results, it is found that two radiation nulls 2.3. Realization of the Proposed Filtering Yagi Dipole Antenna appear at the lower and upper passband edges; nevertheless, the suppression of the upper stopband is not veryBased good. on Thisthe aforementioned phenomenon is causeddipole byantenna the fact with that theimproved introduced selectivity folded resonatorand enhanced hardly bandwidth,influences the a Yagi radiation dipole of antenna driver andnamed cannot type work B is aspresented a director by due adding to its a limitedreflector length. in the Thisdipole is structureexactly the (Type reason A), why as theshown filtering in Figure performance 8a. As ofwe Type have B atproved the upper that bandthe reflector edge is worsecan reflect than electromagneticthe optimized Yagi waves antenna. and enhance Limited the by directivity the CPS feedline, of antenna. the Also, director it actually has not can been improve utilized the in thisout ofproposed band suppression. structure. Moreover, Figure 9 sketch insidees the the operation realized band,gain of there proposed also exist Type two B. resonanceFrom the gain points, results, so as itto is maintain found that a wide two bandwidth.radiation nulls Compared appear withat the type lower A antenna,and upper the passband filtering characteristicedges; nevertheless, of type Bthe is suppressionenhanced. Undoubtedly, of the upper st theopband increment is not on very filtering good. performance This phenomenon is achieved is caused by intrinsic by the fact property that the of introducedYagi antenna. folded resonator hardly influences the radiation of driver and cannot work as a director due toTo its further limited improve length. theThis suppression is exactly the in thereas upperon why stopband, the filtering a pair performance of U-shaped of resonatorsType B at the are upperdeployed band and edge directly is worse coupled than the to optimized the CPS feedline. Yagi antenna. Until Limited now, the by implementation the CPS feedline, of the wideband director hasYagi not dipole been filteringutilized antenna—i.e.,in this proposed antenna structure. type Moreover, C—is finally inside accomplished, the operation as band, shown there in Figurealso exist8b. twoFigure resonance9 depicts points, its realized so as gain. to maintain It is found a thatwide one bandwidth. additional radiationCompared null with has type been A achieved antenna, at the filteringnear edge characteristic of operating of frequency type B is enhanced. band, thus Undo manifestingubtedly, an the improved increment gain on filtering selectivity performance than the one is achievedcaused by by the intrinsic reflector. property Herein, of the Yagi introduced antenna. U-shaped folded line functions as an open-circuited half-wavelengthTo further improve resonator the corresponding suppression toin athe specific upper frequency; stopband, thus, a pair signals of U-shaped at this frequency resonators cannot are deployed and directly coupled to the CPS feedline. Until now, the implementation of wideband Yagi dipole filtering antenna—i.e., antenna type C—is finally accomplished, as shown in Figure 8b. Figure 9 depicts its realized gain. It is found that one additional radiation null has been achieved at the near edge of operating frequency band, thus manifesting an improved gain selectivity than the one caused by the reflector. Herein, the introduced U-shaped folded line functions as an open-circuited half-wavelength resonator corresponding to a specific frequency; thus, signals at this frequency cannot pass through the feedline but couple to the U-shaped resonator. One thing should be mentioned that the U-shaped folded line is not same as those reported structures with notch since ground is nonexistent. The introduced U- shaped line radiates here and works just like a folded dipole antenna, which possesses an

Electronics 2020, 9, 1258 6 of 10 pass through the feedline but couple to the U-shaped resonator. One thing should be mentioned that the U-shaped folded line is not same as those reported structures with notch since ground is Electronics 2020,, 9,, xx FORFOR PEERPEER REVIEWREVIEW 6 of 9 nonexistent. The introduced U-shaped line radiates here and works just like a folded dipole antenna, whichomnidirectional possesses radiation an omnidirectional pattern. As radiation such, the pattern. omnidirectional As such, the radiation omnidirectional caused by radiation U-shaped caused line worsensby U-shaped the front-to-back line worsens ratio, the front-to-back leading to an ratio, additional leading radiation to an additional null. radiation null.

((a)) ((b))

FigureFigure 8. 8. ConfigurationConﬁguration of of proposed proposed Yagi Yagi dipole antennas. ( (aa)) TypeType B. B. ( (b)) Type Type C.C.C.

55 00 -5-5 -10-10 -15-15 -20-20 -25-25 RealizedRealized Gain_TypeGain_Type AA Realised Gain (dBi) Gain Realised Realised Gain (dBi) Gain Realised Realized Gain_Type B Realised Gain (dBi) Gain Realised Realized Gain_Type B -30-30 RealizedRealized Gain_TypeGain_Type CC -35-35 2.52.5 3.0 3.0 3.5 3.5 4.0 4.0 4.5 4.5 5.0 5.0 5.5 5.5 Frequency (GHz) Frequency (GHz) Figure 9.9. ComparisonComparison about about reflection reﬂection coefficients coeﬃcients and and normalized normalized realized realized gains between the conventional dipole antenna andand proposedproposed TypeType A.A.

3. Fabrication and Experimental Results 3. Fabrication and Experimental Results Figure 10 shows the photograph of fabricated antenna prototype. The dimensions of the antenna Figure 10 shows the photograph of fabricated antennatenna prototype.prototype. TheThe dimensionsdimensions ofof thethe antennaantenna prototype are: W1 = 3.8 mm, W2 = 1 mm, W3 = 0.8 mm, Wdri = 1 mm, Wref = 1 mm, L1 = 13 mm, prototype are: W1 = 3.8 mm, W2 = 1 mm, W3 = 0.8 mm, Wdri = 1 mm, Wref= 1 mm, L1 = 13 mm, L2 = L2 = 28.4 mm, L3 = 27.4 mm, L4 = 13 mm, Ldri = 30 mm, Lref = 36 mm, g1 = 0.2 mm, g2 = 0.3 mm, 28.4 mm, L3 = 27.4 mm, L4 = 13 mm, Ldri = 30 mm, Lref = 36 mm, g1 = 0.2 mm, g2 = 0.3 mm, g3 = 5 g3 = 5 mm, g4 = 1 mm, g5 = 5 mm, and g6 = 0.3 mm. The CPS feedline is achieved by a pair of mm, g4 = 1 mm, g5 = 5 mm, and g6 = 0.3 mm. The CPS feedline is achieved by a pair of parallel parallel coupled lines which are respectively connected with the inner and outer conductor of SMA, coupled lines which are respectively connected with thethe innerinner andand outerouter conductorconductor ofof SMA,SMA, thusthus thus forming a balun structure. Besides, the ﬂange of SMA is selected with a compact size for reducing formingforming aa balunbalun structure.structure. Besides,Besides, thethe flangeflange ofof SMASMA isis selectedselected withwith aa compactcompact sizesize forfor reducingreducing itsits its inﬂuence on radiation. The proposed antenna is easily fabricated and assembled. influenceinfluence onon radiation.radiation. TheThe proposedproposed anteantenna is easily fabricated and assembled.

FigureFigure 10. 10. PhotographPhotograph of of the the fabricated fabricated antenna antenna prototype. prototype.

Measured and simulated reflection coefficients and gains of the fabricated antenna prototype are provided in Figure 11. It can be found that the measured operating frequency band covering from 3.64 GHz to 4.38 GHz (18.5%) is little higher than the simulated one. Moreover, these two gain curves have the same tendency with the variation of frequency. The emerged radiation nulls at specific frequencyfrequency locationslocations agreeagree wellwell withwith ourour expectatioexpectations. Compared with simulated result, the measured

Electronics 2020, 9, 1258 7 of 10

Measured and simulated reflection coefficients and gains of the fabricated antenna prototype are provided in Figure 11. It can be found that the measured operating frequency band covering fromElectronics 3.64 2020 GHz, 9, tox FOR 4.38 PEER GHz REVIEW (18.5%) is little higher than the simulated one. Moreover, these two gain7 of 9 curvesElectronics have 2020, the9, x FOR same PEER tendency REVIEW with the variation of frequency. The emerged radiation nulls7 of at 9 specificone also frequency slightly moves locations toward agree a higher well with frequency, our expectations. which corresponds Compared to the with results simulated of reflection result, theonecoefficients. measuredalso slightly Figure one moves also 12 slightlydepicts toward movesthe a measuredhigher toward frequency, and a higher simulated which frequency, radiationcorresponds which patterns corresponds to the of results fabricated to theof reflection results antenna of reflectioncoefficients.prototypes coe operatingFigurefficients. 12 Figuredepictsat 3.66 12 GHzthe depicts measured and the4.13 measured andGHz, si whichmulated and means simulated radiation the radiationresonant patterns frequencies patternsof fabricated of fabricatedinside antenna the antennaprototypesworking prototypes band. operating Measured operating at 3.66 patterns GHz at 3.66 and match GHz 4.13 and well GHz, 4.13 with GHz,which the which simulatedmeans means the ones, resonant the resonantexcept frequencies the frequencies measured inside inside cross- the theworkingpolarization working band. band.is aMeasured little Measured high patternser than patterns the match simulated match well with well one. the with simulated the simulated ones, ones,except except the measured the measured cross- cross-polarizationpolarization is a little is a high littleer higher than the than simulated the simulated one. one.

Figure 11. Measured and simulated reflection coefficients and realized gains of the fabricated antenna Figureprototype. 11. 11. Measured Measured and and simulated simulated reflection reﬂection coefficients coeﬃcients and realized and realized gains of gains the fabricated of the fabricated antenna antennaprototype. prototype.

0 90 330 30 120 60 0 90 330 -10 30 120 -10 60 300 60 150 30 -10-20 -10-20 300 60 150 30 -20-30 -20-30

270 -30-40 90 180 -30-40 0

270 -40 90 180 -40 0

240 120 210 330

210 150 240 300 180 270 210 150 240 300 180 270 (a) (a) 0 330 30 0 330 -10 30

300 -10-20 60 300 60 -20-30

270 -30-40 90 270 -40 90

240 120 240 120 210 150 180 210 150 180 (b) (b) FigureFigure 12. 12.Measured Measured andand simulated simulated radiationradiation patternspatterns ofof fabricated fabricatedantenna antennaprototype prototypeoperating operatingat at (Figurea(a)) 3.66 3.66 12. GHz. GHz. Measured ( b(b)) 4.13 4.13 GHz.and GHz. simulated radiation patterns of fabricated antenna prototype operating at (a) 3.66 GHz. (b) 4.13 GHz. To highlight the advantages of this work, the performances in comparison with other reported counterpartsTo highlight are summarizedthe advantages in Tableof this 2. work, It can thebe concludedperformances from in this comparison table that with our presentedother reported work counterpartsexhibits good are properties summarized of compac in Tablet size, 2. It widercan be bandwidth, concluded fromsimple this st ructuretable that of our one-layer presented circuit, work as exhibitswell as angood improved properties gain of selectivity. compact size, wider bandwidth, simple structure of one-layer circuit, as well as an improved gain selectivity.

Electronics 2020, 9, 1258 8 of 10

To highlight the advantages of this work, the performances in comparison with other reported counterparts are summarized in Table2. It can be concluded from this table that our presented work exhibits good properties of compact size, wider bandwidth, simple structure of one-layer circuit, as wel as an improved gain selectivity.

Table 2. Comparison among ﬁltering Yagi antennas

Bandwidth Central Realized Size Number of Number Ref. Design Principle (%) Frequency Gain (dBi) (λ0) Radiation Null of Layer [17] 16.7 1.72 GHz Cascade 5.9 0.6 0.6 0.007 0 3 × × [18] 5.5 1.82 GHz Cascade 5.1 0.75 0.6 0.007 0 3 × × [19] 25 5.2 GHz From filter to antenna 7.0 0.65 0.62 0.009 1 1 × × [20] 8.7 0.95 GHz From antenna to filter 10.6 0.55 0.15 N.A 2 1 × × Our work 18.5 4.01 GHz From antenna to filter 5.8 0.56 0.4 0.013 3 1 × ×

4. Conclusions In this article, a wideband filtering-radiating Yagi dipole antenna is presented and investigated. With resorting to the CPS differential feed network, the design complexity has been effectively reduced. Then, by introducing an open-circuited half-wavelength resonator between CPS and dipole, a new dipole antenna with improved selectivity and extended bandwidth is proposed. Afterwards, the intrinsic filtering performance of Yagi structure has been studied and implemented on the dipole antenna. Meanwhile, a pair of U-shaped resonators are employed and coupled to CPS directly so as to accomplish the filtering radiation purpose. An antenna prototype has been fabricated and tested. Measured results demonstrate a wide bandwidth covering from 3.64 GHz to 4.38 GHz (18.5%) and three desired radiation nulls. Since no filtering and matching networks are involved, the structure of the proposed antenna is very simple. In addition, this antenna has the features of low profile, compact size, and easy fabrication, which will make it a good candidate for modern 5G wireless communication systems.

Author Contributions: Methodology, investigation, writing—original draft, and data curation, Y.C.; Conceptualization and project administration, G.L.; Resources, S.W.; Supervision, writing—review and editing, J.W. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (grant no. 61771247). Acknowledgments: The authors wish to express their thanks for the support provided by the National Natural Science Foundation of China (grant no. 61771247). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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