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,two- two-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,

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