Optimization of Log-Periodic TV Reception Antenna with UHF Mobile Communications Band Rejection

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Article Optimization of Log-Periodic TV Reception Antenna with UHF Mobile Communications Band Rejection

Keyur K. Mistry 1, Pavlos I. Lazaridis 2,* , Zaharias D. Zaharis 3 , Ioannis P. Chochliouros 4 , Tian Hong Loh 5 , Ioannis P. Gravas 3 and David Cheadle 5

1 Oxford Space Systems, Harwell OX11 0RL, UK; [email protected] 2 Department of Engineering and Technology, University of Huddersﬁeld, Huddersﬁeld HD1 3DH, UK 3 Department of Electrical and Computer Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece; [email protected] (Z.D.Z.); [email protected] (I.P.G.) 4 Hellenic Telecommunications Organization S.A. Member of the Deutsche Telekom Group of Companies, 15122 Athens, Greece; [email protected] 5 5G and Future Communications Technology Group, National Physical Laboratory, Teddington TW11 0LW, UK; [email protected] (T.H.L.); [email protected] (D.C.) * Correspondence: [email protected]

Received: 19 October 2020; Accepted: 31 October 2020; Published: 3 November 2020

Abstract: The coexistence of TV broadcasting and mobile services causes interference that leads to poor quality-of-service for TV consumers. Solutions usually found in the market involve external band-stop ﬁlters along with TV reception log-periodic and Yagi-Uda antennas. This paper presents a log-periodic antenna design without additional ﬁltering that serves as a lower cost alternative to avoid interference from mobile services into the UHF TV. The proposed antenna operates in the UHF TV band (470–790 MHz-passband) and rejects the 800 MHz and 900 MHz bands (stopband) of 4G/LTE-800 and GSM900 services, respectively. Matching to 50 Ohms is very satisfactory in the passband with values of S11 below 12 dB. Furthermore, the antenna is highly directive with a − realized gain of approximately 8 dBi and a front-to-back ratio greater than 20 dB.

Keywords: CST simulations; GSM900 band rejection; log-periodic dipole antennas; 4G rejection

1. Introduction The adoption of digital modulation and advanced video compression techniques for terrestrial TV broadcasting, replacing analog broadcasting, resulted in considerable improvements in the efficiency of frequency spectrum utilization, making it possible to broadcast several TV programs in multiplex in a single 8 MHz TV channel. This transition set a part of the TV spectrum free for utilization for other applications, and thus, the wireless communications industry saw this transition as an opportunity to use the released frequency spectrum for mobile communication services. In turn, that led to European Member States being urged by the European Parliament to make sure that the frequency spectrum is efficiently utilized [1]. The European Commission (EC) investigated the allocation of the 790–862 MHz frequency band (the so-called 800 MHz band and part of the former UHF TV band) to Long Term Evolution (LTE-800) wireless services as a “digital dividend”, thus redistributing the benefits of digital TV migration [2]. The EC decision described in [3] suggests a harmonized plan that was adopted by European states (ITU Region 1) for LTE network deployment in the 800 MHz band. This band is particularly important for the mobile communications industry because of its relatively low building material penetration radio losses and the fact that most of the time mobile phones are used indoors. According to this plan, the spectrum of 790–862 MHz involves the use of chunks of 30 MHz uplink and 30 MHz downlink transmissions in the FDD (Frequency Division Duplex). Moreover, the uplink

Electronics 2020, 9, 1830; doi:10.3390/electronics9111830 www.mdpi.com/journal/electronics Electronics 2020, 9, x FOR PEER REVIEW 2 of 12 Electronics 2020, 9, 1830 2 of 12 11 MHz-wide frequency gap. On the other hand, DTT (Digital Terrestrial Television) is allocated the 470 MHz to 790 MHz band and that just leaves a 1 MHz-wide frequency gap between DTT spectrum is separated from the downlink spectrum using an 11 MHz-wide frequency gap. On the broadcasting and LTE-800 mobile communications. A useful study performed in [4] investigates the other hand, DTT (Digital Terrestrial Television) is allocated the 470 MHz to 790 MHz band and that just effects of interference on the degradation of the QoS (quality-of-service) in the TV broadcasting band leaves a 1 MHz-wide frequency gap between DTT broadcasting and LTE-800 mobile communications. due to its vicinity with the LTE-800 band. Following the clearance of the 800 MHz band, the A useful study performed in [4] investigates the effects of interference on the degradation of the QoS requirement of allocating the 700 MHz band (694–790 MHz) was stated in a worldwide resolution (quality-of-service) in the TV broadcasting band due to its vicinity with the LTE-800 band. Following the [5], which was later approved in the 2012 World Radiocommunication Conference (WRC-12) of clearance of the 800 MHz band, the requirement of allocating the 700 MHz band (694–790 MHz) was ITU-R [6]. Thereafter, the 2015 World Radiocommunication Conference (WRC-15) [7] of ITU-R states stated in a worldwide resolution [5], which was later approved in the 2012 World Radiocommunication the provisions used to deploy the 700 MHz band in ITU Region 1. Another significant study is Conference (WRC-12) of ITU-R [6]. Thereafter, the 2015 World Radiocommunication Conference presented in [8] and investigates the interference effects due to coexistence of the DTT broadcasting (WRC-15) [7] of ITU-R states the provisions used to deploy the 700 MHz band in ITU Region 1. service and LTE service in both the 700 MHz and 800 MHz bands. Another significant study is presented in [8] and investigates the interference effects due to coexistence This paper is an extended version of a paper submitted to the IEEE NEMO-2019 conference [9]. of the DTT broadcasting service and LTE service in both the 700 MHz and 800 MHz bands. Moreover, the proposed antenna in this paper has been further optimized to achieve higher and This paper is an extended version of a paper submitted to the IEEE NEMO-2019 conference [9]. flatter realized gain (RG) along with a higher FBR (front-to-back ratio) throughout the DTT reception Moreover,passband. the proposed antenna in this paper has been further optimized to achieve higher and flatter realized gain (RG) along with a higher FBR (front-to-back ratio) throughout the DTT reception passband. 2. Conventional LPDA Geometry 2. Conventional LPDA Geometry LPDAs (Log Periodic Dipole Arrays) or simply log-periodic antennas are frequently used for LPDAs (Log Periodic Dipole Arrays) or simply log-periodic antennas are frequently used for TV reception since the 1960s because of their excellent wideband properties and high directivity TV reception since the 1960s because of their excellent wideband properties and high directivity despite the fact that their peak gain is generally lower than that of similar size Yagi-Uda antennas despite the fact that their peak gain is generally lower than that of similar size Yagi-Uda antennas [10]. [10]. Furthermore, LPDAs provide flat gain throughout their operating band and are thus Furthermore, LPDAs provide flat gain throughout their operating band and are thus considered as considered as the best solution for applications where broadband behavior and gain flatness are the best solution for applications where broadband behavior and gain flatness are important [11,12]. important [11,12]. Compared to Yagi-Uda antennas, which are generally narrowband, LPDAs Compared to Yagi-Uda antennas, which are generally narrowband, LPDAs provide better gain flatness provide better gain flatness but a relatively lower RG. The performance of these antennas differs also but a relatively lower RG. The performance of these antennas differs also because of their different because of their different feeding pattern [13]. Conventional LPDAs are usually designed using feeding pattern [13]. Conventional LPDAs are usually designed using calculation guidelines proposed calculation guidelines proposed by Carrel in [14]. The basic LPDA geometry is shown in Figure 1. by Carrel in [14]. The basic LPDA geometry is shown in Figure1.

Figure 1. Geometry of a conventional LPDA (Log Periodic Dipole Array) proposed by Carrel. Figure 1. Geometry of a conventional LPDA (Log Periodic Dipole Array) proposed by Carrel. A conventional LPDA is constructed using two booms arranged parallel to each other and separatedA conventional by air using LPDA insulating is constructed or metallic using fasteners two booms at the arranged rear end, parallel as well to as each the frontother end,and andseparated sometimes by air in using the middle insulating of the or antennametallic (seefasten Figureers at 3).the Therear two end, booms as well are as forming the front a end, two-line and air-dielectricsometimes in transmission the middle of line, the which antenna feeds (see the Figu antennare 3). dipoles.The two The booms rear are end forming fastener a is two-line usually air-dielectric transmission line, which feeds the antenna dipoles. The rear end fastener is usually metallic and acts also as a short-circuit stub. The spacing between the two conducting booms along with metallic and acts also as a short-circuit stub. The spacing between the two conducting booms along their cross-section size and shape controls the transmission line characteristic impedance, and therefore, with their cross-section size and shape controls the transmission line characteristic impedance, and it is important for matching of the antenna to a feeder cable [15]. In most cases, the antenna is matched therefore, it is important for matching of the antenna to a feeder cable [15]. In most cases, the antenna to a feeding cable impedance of 50 Ohms in general or 75 Ohms for TV reception. The dipoles are ﬁxed is matched to a feeding cable impedance of 50 Ohms in general or 75 Ohms for TV reception. The to the booms in such a way that the length of the dipoles decreases as we move from the rear end of the

Electronics 2020, 9, 1830 3 of 12 antenna toward the front end. In addition, half-dipoles are alternatively connected to the upper and lower booms, and two half-dipoles together form a complete dipole. The phase inversion between the feeds of the two consecutive dipoles is achieved because of the dipoles being arranged in a criss-cross fashion. This phase inversion ensures that the antenna always radiates as an end-fire dipole antenna array [16]. Contrary to Yagi-Uda antennas, where there is only one dipole that is active and all the other dipoles are passive, all the dipoles in an LPDA are fed by the conducting booms transmission line, and therefore all the dipoles are active. Depending on dipole length and thickness, each dipole of the LPDA resonates around a specific frequency. Antenna gain and/or bandwidth significantly increase with the total number of dipoles used [17]. The feeding is provided at the front end of the booms (left end of the booms in Figure 3) using a coaxial cable that is usually passing though one of the booms that are hollow. Carrel [14] introduced design equations for the calculation of the dimensions in the case of the conventional LPDA geometry. The apex angle is the half angle in which all the dipoles are confined, and it is mathematically expressed as 1 1 τ α = tan− − . (1) 4σ

In the above expression, the parameter τ is called the “scaling factor” and is the ratio of the lengths or diameters of two consecutive dipoles as shown by the following expression:

Ln+ dn+ τ = 1 = 1 (2) Ln dn where Ln and dn are, respectively, the length and the diameter of the nth dipole. In addition, the parameter σ shown in (1) is called the “spacing factor” and is defined as: s σ = n (3) 2Ln where sn is the spacing between the nth dipole and its consecutive (n + 1)th dipole. The overall physical dimensions of the antenna significantly depend on the above two factors (τ and σ). Sometimes, a fixed diameter is used for the dipoles in order to simplify the antenna design and reduce costs.

3. Proposed LPDA Geometry The proposed LPDA geometry can be used to mitigate the interference caused by the LTE-800 and GSM-900 mobile communications band transmissions to the TV broadcasting service band. This solution is actually a UHF TV reception log-periodic antenna composed, in this example, of 10 dipoles that can act as a filter and reject frequencies in the 800 MHz and 900 MHz bands. One of the already existing solutions that is most widely used to mitigate interference is the use of a conventional TV reception antenna along with an external bandstop filter that rejects the undesired 800 MHz and 900 MHz bands. However, not enough research work has been performed on TV reception antennas without using external bandstop filters. In [18], an LPDA is proposed that is optimized by a variant of the PSO (Particle Swarm Optimization) method, i.e., by PSOvm (PSO with velocity mutation). This antenna design is capable of rejecting signals, which reside in the 800 MHz band and are directed toward the antenna axis; however, relatively low VSWR values in the 800 MHz band still raise a concern of receiving interference signals from other directions. In the above-mentioned LPDA design, the first dipole is longer in length compared to its adjacent dipoles (i.e., 2nd and 3rd dipole) and therefore acts as reflector for signals that reside in the 800 MHz band. An early effort was made in [19] to design an LPDA that rejects the 800 MHz band without using any external filters and avoids LTE-800 signals from every direction of arrival. As a starting point, Carrel’s design procedure [14] was used to calculate the antenna dimensions. Then, the shorter dipole lengths and the distances between these dipoles were optimized using the TRF optimization algorithm Electronics 2020, 9, 1830 4 of 12 in CST (Trusted Region Framework) in order to obtain rejection in the 800 MHz band. This type of optimization achieves LTE-800 rejection by finding appropriate values for the lengths of some (two or three) of the shorter dipoles and for the distances between them. Extending the LPDA design presented in [19], an improved version of this LPDA [9,20] is proposed in this paper. Instead of optimizing the whole antenna geometry, which would lead to an increased computational burden and doubtful convergence, this design is the result of optimizing the lengths of the front three dipoles only as well as the distances between these dipoles. The rest of the antenna is identical to the conventional design. In comparison to the conventional LPDA design (Carrel’s model) and the LPDA design given in [19], the antenna presented in this paper achieves lower values of S11 (i.e., better matching) as well as higher and flatter RG and FBR throughout the passband, and it concurrently provides better rejection of the LTE-800 and GSM-900 services. The TRF algorithm, which is available in the CST environment, is employed to optimize the antenna according to the goals listed in Table1. The antenna optimization could also have been carried out using several other algorithms, such as IWO (Invasive Weed Optimization) in [21–24], PSO in [25,26], and PSOvm in [27].

Table 1. Optimization speciﬁcations and goals.

Parameter Goal Frequency (MHz) Weight S11 < 14 dB 470–790 (Passband) 10.0 − Realized gain >10 dBi 470–790 (Passband) 1.0 Front-to-back ratio >14 dB 470–790 (Passband) 0.2 S11 > 1 dB 810–960 (Stopband) 5.0 − Realized gain > 10 dBi 810–960 (Stopband) 1.0 −

The reason that only the front three dipoles take part in the optimization is due to the fact that the front dipoles determine the behavior of an LPDA at higher frequencies. So, the lengths of the ﬁrst three dipoles, i.e., L1,L2 and L3, and their distances, i.e., s0, s1, s2, and s3, are the only parameters considered for optimization. The optimization settings for the TRF algorithm are taking into consideration that the optimal values of these parameters cannot deviate more than 30% from the respective values of a conventional LPDA design. Therefore, by multiplying the parameter values of this conventional design by 0.7 (30% less) or by 1.3 (30% more), we ﬁnd the parameter boundaries, which are listed in Table2. These boundaries are used to restrict the search area of every parameter and thus help the optimization algorithm converge faster.

Table 2. Lower and upper boundaries of the parameters to be optimized.

Parameter Initial Values from [18] Lower Boundary Upper Boundary

L1 138 96.6 mm 179.4 mm L2 111.8 78.26 mm 145.34 mm L3 126.2 88.34 mm 164.06 mm s0 28 19.6 mm 36.4 mm s1 15.3 10.7 mm 19.9 mm s2 18.6 13.0 mm 24.2 mm s3 22 15.4 mm 28.6 mm

Table1 shows the optimization goals that were used to optimize the LPDA parameters that are specified in Table2. This LPDA optimization resulted in a design, where the first four dipoles are arranged in an unusal pattern compared to the conventional LPDA design. The arrangement of first four dipoles were such that the 1st (front) dipole is longer than the 2nd dipole. Furthermore, the length of 2nd dipole is longer than the 3rd dipole, and the length of 4th dipole is longer than the 3rd dipole. The optimized antenna design follows the conventional LPDA design rule after the 4th dipole, whereafter, the lengths of dipole keep increasing until the last (rear) dipole. Electronics 2020, 9, x FOR PEER REVIEW 6 of 12 Electronics 2020, 9, x FOR PEER REVIEW 6 of 12 Electronics 2020, 9, 1830 gap 10 11.7 5 5 of 12 Stub gaplength 7910 11.7 45 45 5 Stub length 79 45 45 The physicalStub dimensions thickness of the optimized 35 LPDA design 35 are 356 mm 35 302.6 mm 35 mm Stub thickness 35 35 35× × (length width thickness). In order to mantain the seperation gap between the two booms, 4. LPDA× Simulations× and Measurements a cuboidal fastener is embodied at the rear end of the antenna, which also provides an advantage of 4. LPDA Simulations and Measurements actingMaxwell’s as a short-circuit equations stub. are Additionally,often solved thisusing fastener FDTD is (Finite-difference used to mechanically time-domain) attach the variants antenna in to variousany externalMaxwell’s electromagnetic support. equations The simulation physicalare often dimensions solvedsoftware using pack of this FDTDages. cuboidal (Finite-differenceCST fasteneris one of are the 45time-domain) many mm 15electromagnetic mm variants35 mm in × × simulation(lengthvarious electromagneticwidth softwarethickness). packages simulation that utilizes software FDTD pack for ages.simulations. CST is This one softwareof the many offers electromagnetica user-friendly × × interfacesimulationFigure that 2software presents allows thepackages the user top viewto that model ofutilizes the fully conventional FDTD parametric for simulations. LPDA CAD designmodels. This as Itsoftware well has asa wide the offers optimized range a user-friendly of solvers LPDA thatdesign.interface can Thebe that used side allows viewfor simulationsthe of theuser LPDA to model such design asfully time is shownparametric domain in Figure solvers,CAD3 .models. The frequency dimensions It has domain a wide of the solvers,range conventional of and solvers an integralLPDAthat can design, solver. be used the However, antennafor simulations design any electromagnetic in such [19], as and time the proposed dosimainmulation solvers, antenna requires frequency design an areaccurate domain shown inmeshing solvers, Table3. andof It alsothe an model.showsintegral theAnother solver. list of advantage parametersHowever, of any used CST electromagnetic foris that designing it provides the si CAD mulationa fast, model automatic requires of the meshing antenna,an accurate with where mesh meshing Ln refinementis length of the of capabilitiesthemodel.nth dipole,Another thatsn adaptadvantageis distance the quality betweenof CST ofis the thatmeshingn thit provides and dep (n +ending 1)tha fast, dipole, on automatic the s0 model is the meshing distanceand uses with between Perfect mesh therefinementBoundary start of Approximationsthecapabilities boom and that the (PBA)adapt first dipole, the[28]. quality For L-boom this of paper, ismeshing length the ofdep Finite theending boom, Integration on W-boom the modelTechnique is width and of(FIT)uses the Perfect boom,included H-boomBoundary in CST is washeightApproximations used of the to boom,simulate (PBA) and the gap[28]. proposed is For the this distance antennapaper, between the des Finiteign. the The twoIntegration model parallel wasTechnique booms. hexahedrally The (FIT) cuboidal included meshed fastener inusing CST for 4,668,482thewas antenna used mesh to was simulate cells, designed and the usingthe proposed simulations the dimensions antenna were specifieddes performedign. The for Stub_length, modelwith − 50was dB Stub_width,hexahedrally accuracy. Open and meshed Stub_height boundary using conditionsin4,668,482 Table3. An meshwith LPDA acells, reflection can and be the matched level simulations of toapproximately the desired were performed impedance 10−4 were with byused varying − to50 modeldB the accuracy. seperationthis antenna Open gap inboundary between CST. A discretetheconditions two conducting50 Ωwith port a thatreflection booms. connects Thelevel initialthe of centerapproximately model points that wasof 10 both− designed4 were the usedbooms using to atmodel Carrel’s front thisend design antenna of the equation antenna in CST. had isA usedadiscrete matching to provide 50 Ω impedance port the thatexcitation. ofconnects 75 ohms, The the antenna wherecenter thewaspoints booms simulated of both were inthe 10 the booms mm frequency apart. at front However,range end from of the in 450 case antenna MHz of the to is 1000proposedused MHz to provide optimized with 10the MHz LPDA,excitation. resolution, the separationThe antenna because gap was the between sifrequencymulated the booms inof theinterest wasfrequency reducedinvolves range from UHF from 10 TV, mm 450 LTE-800, to MHz 5 mm to andin1000 order GSM-900 MHz to matchwith bands. 10 the MHz The antenna anechoic resolution, to 50 chamber ohms because impedance. at thethe Nafrequencytional Thereafter, Physical of theinterest proposedLaboratory involves antenna (NPL), UHF was UKTV, fabricatedwas LTE-800, used tousingand test GSM-900 alluminiumand measure bands. components the The fabricated anechoic at theLPDA chamber University design. at the of Na Huddersfieldtional Physical Manufacturing Laboratory (NPL), laboratory. UK was Figure used4 showsto test theand fabricated measure the model fabricated that follows LPDA the design. exact dimensions of the CST-optimized LPDA design.

Figure 2. CAD (Computer-Aided Design) model of Carrel’s conventional model (left) and the Figure 2. CAD (Computer-Aided Design) model of Carrel’s conventional model (left) and the proposed proposedFigure 2. LPDA CAD (right(Computer-Aided) shown as a top Design) view. model of Carrel’s conventional model (left) and the LPDAproposed (right LPDA) shown (right as) a shown top view. as a top view.

Figure 3. Side-viewSide-view of of the the CAD CAD model model of of the the proposed proposed LPDA design. Figure 3. Side-view of the CAD model of the proposed LPDA design.

Electronics 2020, 9, 1830 6 of 12

Table 3. Conventional and proposed LPDA dimensions.

Parameter Carrel’s Design Proposed Design in [19] Proposed Design Variable name Value (mm) Value (mm) Value (mm) L1 98 138 145.4 L2 110 111.8 128.4 L3 124 126.2 112 L4 140 121.4 121.4 L5 160 157.2 157.2 L6 180 180.2 180.2 L7 206 203.6 203.6 L8 232 235.4 235.4 L9 264 267 267 L10 298 302.6 302.6 L-boom 356 356 356 H-boom 15 15 15 W-boom 15 15 15 Dipole diameter 4 4 4 Stub width 15 15 15 s0 30 28 24.1 s1 16 15.3 11.4 s2 18 18.6 14.9 s3 20 22 17.9 s4 22 25.3 25.3 s5 26 27.3 27.3 s6 28 27 27 s7 34 36.3 36.3 s8 37 40.4 40.4 s9 42 40.6 40.6 gap 10 11.7 5 Stub length 79 45 45 Stub thickness 35 35 35 Electronics 2020, 9, x FOR PEER REVIEW 7 of 12

Figure 4. Fabricated LPDA built using the optimized LPDA dimensions.

4. LPDAThe Simulationssurface current and density Measurements of the optimized antenna was simulated in order to obtain the currentMaxwell’s distribution equations at four are specific often solvedfrequency using points: FDTD (a) (Finite-di 470 MHz,fference (b) 630 time-domain) MHz, (c) 790 variantsMHz, and in various(d) 960 MHz electromagnetic as shown in simulation Figure 5. The software four freque packages.ncy points CST iswere one specifically of the many selected electromagnetic such that threesimulation of the software frequency packages points thatlie in utilizes the passband FDTD for (470–790 simulations. MHz), This and software a frequency offers point a user-friendly is in the stopband/rejectioninterface that allows band the user(810–960 to model MHz) fully of parametricthe antenna. CAD The models.surface Itcurrent has a widedensity range presented of solvers in Figurethat can 5 validates be used forthe simulationsfact that each such dipole as time resonate domains at a solvers, specific frequencyfrequency. domain Figure 5a solvers, suggests and that an theintegral longest solver. dipole However, of the antenna any electromagnetic has the highest simulation surface requirescurrent dens an accurateity, as the meshing longest of dipole the model. will Anotherresonate advantageat the lowest of CST frequency is that it providesof bandwidth. a fast, automatic However, meshing Figure with5b shows mesh refinement a shift in capabilitiesmaximum surfacethat adapt current the quality density of to meshing the 7th dependingdipole at 630 on MHz, the model which and is usesapproximately Perfect Boundary the center Approximations frequency of the bandwidth. Figure 5c demonstrates that the maximum current density is at the 2nd and 6th dipoles of the antenna, thereby stating that both the dipoles resonate at 790 MHz. The reason for this is the intentionally created anomaly in the antenna design, which results in the energy getting trapped between these two dipoles, which ultimately leads to high rejection beyond this frequency. As seen in Figure 5d, at 960 MHz, very minimal current density is seen throughout the antenna model, which suggests that none of the dipoles of the antenna resonates at this frequency.

(a) (b) (c) (d)

Figure 5. Surface current density of the optimized antenna at (a) 470 MHz, (b) 630 MHz, (c) 790 MHz, and (d) 960 MHz from simulation.

The S11 values of the simulated and the measured LPDA design using Carrel’s design guidelines are shown in Figure 6. In addition, the S11 values of the optimized antenna are also shown in Figure 6. The graphs suggest that the S11 values of the optimized antenna are relatively lower compared to the Carrel’s model in the passband, hence validating the improved S11 of the

Electronics 2020, 9, x FOR PEER REVIEW 7 of 12

Electronics 2020, 9, 1830 7 of 12

(PBA) [28]. For this paper, the Finite Integration Technique (FIT) included in CST was used to simulate the proposed antenna design. The model was hexahedrally meshed using 4,668,482 mesh cells, and the simulations were performed with 50 dB accuracy. Open boundary conditions with a reflection level 4 − of approximately 10− were used to model this antenna in CST. A discrete 50 Ω port that connects the center points of both the booms at front end of the antenna is used to provide the excitation. The antenna was simulated in the frequency range from 450 MHz to 1000 MHz with 10 MHz resolution, because the frequency of interest involves UHF TV, LTE-800, and GSM-900 bands. The anechoic chamber at the National Physical Laboratory (NPL), UK was used to test and measure the fabricated LPDA design. Figure 4. Fabricated LPDA built using the optimized LPDA dimensions. The surface current density of the optimized antenna was simulated in order to obtain the current distributionThe surface at four specificcurrent frequencydensity of points:the optimized (a) 470 MHz,antenna (b) was 630 MHz,simulated (c) 790 in MHz,order andto obtain (d) 960 the MHz ascurrent shown distribution in Figure5. at The four four specific frequency frequency points points: were (a) specifically 470 MHz, (b) selected 630 MHz, such (c) that790 MHz, three and of the frequency(d) 960 MHz points as lie shown in the in passband Figure 5. (470–790 The four MHz), freque andncy apoints frequency were pointspecifically is in the selected stopband such/rejection that bandthree (810–960 of the frequency MHz) of points the antenna. lie in the The passband surface (470–790 current MHz), density and presented a frequency in Figure point 5is validates in the thestopband/rejection fact that each dipole band resonates (810–960 atMHz) a specific of the frequency. antenna. The Figure surface5a suggests current thatdensity the presented longest dipole in ofFigure the antenna 5 validates has the fact highest that each surface dipole current resonate density,s at a specific as the frequency. longest dipole Figure will 5a suggests resonate that at the the longest dipole of the antenna has the highest surface current density, as the longest dipole will lowest frequency of bandwidth. However, Figure5b shows a shift in maximum surface current resonate at the lowest frequency of bandwidth. However, Figure 5b shows a shift in maximum density to the 7th dipole at 630 MHz, which is approximately the center frequency of the bandwidth. surface current density to the 7th dipole at 630 MHz, which is approximately the center frequency of Figure5c demonstrates that the maximum current density is at the 2nd and 6th dipoles of the antenna, the bandwidth. Figure 5c demonstrates that the maximum current density is at the 2nd and 6th thereby stating that both the dipoles resonate at 790 MHz. The reason for this is the intentionally dipoles of the antenna, thereby stating that both the dipoles resonate at 790 MHz. The reason for this createdis the anomalyintentionally in the created antenna anomaly design, in whichthe ante resultsnna design, in the energywhich results getting in trapped the energy between getting these twotrapped dipoles, between which these ultimately two dipoles, leads which to high ultimately rejection leads beyond to high this rejection frequency. beyond As seenthis frequency. in Figure5 d, at 960As seen MHz, in very Figure minimal 5d, at current960 MHz, density very isminima seen throughoutl current density the antenna is seen model,throughout which the suggests antenna that nonemodel, of the which dipoles suggests of the that antenna none of resonates the dipoles at thisof the frequency. antenna resonates at this frequency.

(a) (b) (c) (d)

FigureFigure 5. 5.Surface Surface current current density density ofof thethe optimized antenna antenna at at (a ()a 470) 470 MHz, MHz, (b) ( b630) 630 MHz, MHz, (c) 790 (c) 790MHz, MHz, andand (d )(d 960) 960 MHz MHz from from simulation. simulation. The S11 values of the simulated and the measured LPDA design using Carrel’s design guidelines The S11 values of the simulated and the measured LPDA design using Carrel’s design areguidelines shown in Figureare shown6. In in addition, Figure the6. In S11 addition, values ofthe the S11 optimized values of antenna the optimized are also antenna shown inare Figure also 6. Theshown graphs in suggestFigure 6. thatThe thegraphs S11 suggest values ofthat the the optimized S11 values antenna of the optimized are relatively antenna lower are comparedrelatively to thelower Carrel’s compared model to in the the Carrel’s passband, model hence in the validating passband, the hence improved validating S11 the of theimproved optimized S11 of antenna. the This also demonstrates excellent antenna matching for the reception in the passband. Furthermore, in the LTE-800 and GSM-900 mobile service bands (stopband), the S11 values of the optimized antenna are signiﬁcantly higher compared to the Carrel’s model. This demonstrates the ability of the optimized antenna to reject the interference from the stopband from all the directions. This ﬁgure also indicates a Electronics 2020, 9, x FOR PEER REVIEW 8 of 12 optimizedElectronics 2020 antenna., 9, x FOR This PEER also REVIEW demonstrates excellent antenna matching for the reception in 8the of 12 passband. Furthermore, in the LTE-800 and GSM-900 mobile service bands (stopband), the S11 valuesoptimized of the antenna. optimized This antenna also demonstrates are significantl excelly higherent antenna compared matching to the for Carrel’s the reception model. inThis the demonstratespassband. Furthermore, the ability of inthe the optimized LTE-800 antenna and GSM-900 to reject mobile the interference service bands from the(stopband), stopband the from S11 allvaluesElectronics the directions. of2020 the, 9 ,optimized 1830 This figure antenna also areindicates significantl a goody higheragreement compared between to thethe Carrel’ssimulated model. and 8theThis of 12 measureddemonstrates S11 values the ability of the of proposed the optimized antenna antenna design. to The reject measured the interference S11 values from have the been stopband corrected from inall order the todirections. take into accountThis figure some also extra indicates losses in a the good fabricated agreement antenna. between the simulated and the measuredgood agreement S11 values between of the the proposed simulated antenna and the desi measuredgn. The S11measured values S11 of the values proposed have antennabeen corrected design. inThe order measured to take S11into values account have some been extra corrected losses in in the order fabricated to take intoantenna. account some extra losses in the fabricated antenna.

Figure 6. S11 plot of Carrel’s conventional LPDA design and the optimized LPDA design of this paper. Figure 6. S11 plot of Carrel’s conventional LPDA design and the optimized LPDA design of this paper. Figure 6. S11 plot of Carrel’s conventional LPDA design and the optimized LPDA design of this Figurepaper.Figure 77 compares compares the the simulated simulated and and measured measured RG RG of of Carrel’s Carrel’s conventional conventional LPDA LPDA with with the the simulatedsimulated RG RG of ofthe the optimized optimized LPDA LPDA and and the the measur measureded results. results. The The measured measured RG RG of ofthe the proposed proposed Figure 7 compares the simulated and measured RG of Carrel’s conventional LPDA with the antennaantenna has has been been corrected corrected in inorder order to totake take into into account account some some extra extra losses losses in inthe the fabricated fabricated antenna. antenna. simulated RG of the optimized LPDA and the measured results. The measured RG of the proposed ThisThis result result shows shows that that the the optimized optimized antenna antenna achi achieveseves a ahigher higher RG RG compared compared to tothat that derived derived by by antenna has been corrected in order to take into account some extra losses in the fabricated antenna. Carrel’sCarrel’s model. model. Furthermore, Furthermore, the the optimized optimized antenna antenna demonstrates demonstrates a steep a steep rejection rejection in inthe the stopband, stopband, This result shows that the optimized antenna achieves a higher RG compared to that derived by wherewhere the the RG RG drops drops below below 0 0dBi. dBi. This This ensures ensures that that th thatat antenna antenna rejects rejects the signals from allall thethe anglesangles of Carrel’s model. Furthermore, the optimized antenna demonstrates a steep rejection in the stopband, ofarrival arrival of of radiation. radiation. The The measured measured realized realized gain gain of the of proposed the proposed antenna antenna follows follows closely theclosely simulated the where the RG drops below 0 dBi. This ensures that that antenna rejects the signals from all the angles simulatedRG inside RG the inside passband. the passband. However, However, some diﬀerence some candifference be seen can between be seen the between measured the and measured simulated of arrival of radiation. The measured realized gain of the proposed antenna follows closely the andRG simulated of the proposed RG of the antenna proposed from antenna 700 MHz from to 1000 700 MHz.MHz to The 1000 RG MHz. drops The faster RG in drops the stopband faster in inthe the simulated RG inside the passband. However, some difference can be seen between the measured stopbandmeasurement in the thanmeasurement in the simulation. than in the simulation. and simulated RG of the proposed antenna from 700 MHz to 1000 MHz. The RG drops faster in the stopband in the measurement than in the simulation.

FigureFigure 7. Realized 7. Realized gain gain plot plot of ofCarrel’s Carrel’s conventional conventional LPDA LPDA and and the the proposed proposed LPDA LPDA of ofthis this paper. paper.

The simulated FBR of the optimized LPDA is compared with Carrel’s conventional LPDA in Figure 7. Realized gain plot of Carrel’s conventional LPDA and the proposed LPDA of this paper. Figure8. This plot shows that the proposed antenna exhibits highly directive characteristics in the passband. In contrary to this, Carrel’s model demonstrates a highly directive nature in the passband

as well as stopband, therefore being vulnerable to the interference. Furthermore, in comparison to Carrel’s model, the optimized antenna also achieves a somewhat improved FBR in the passband. Electronics 2020, 9, x FOR PEER REVIEW 9 of 12

The simulated FBR of the optimized LPDA is compared with Carrel’s conventional LPDA in Figure 8. This plot shows that the proposed antenna exhibits highly directive characteristics in the passband. In contrary to this, Carrel’s model demonstrates a highly directive nature in the passband as well as stopband, therefore being vulnerable to the interference. Furthermore, in comparison to Carrel’s model, the optimized antenna also achieves a somewhat improved FBR in the passband. ElectronicsFigure 2020 9 ,shows 9, x FOR the PEER test REVIEW setup in an NPL anechoic chamber for the measurement of realized gain9 of 12 and radiation patterns of the optimized LPDA. The Schwarzbeck USLP-9143B LPDA was used as a referenceThe antennasimulated in FBRthese of measurements. the optimized TheLPDA AUT is co(Antennampared withUnder Carrel’s Test) wasconventional placed 2 mLPDA apart in fromFigure the 8. reference This plot antenna, shows thatand theboth proposed the antennas antenna were exhibits placed athighly 1.2 m directive above the characteristics absorbing floor. in theA schematicpassband. diagram In contrary of the to this,test setupCarrel’s for model gain measur demonstementsrates ais highly shown directive in Figure nature 9. The in S12 the frompassband the testas wellantenna as stopband, to the reference therefore antenna being was vulnerable measured to usingthe interference. a Vector Network Furthermore, Analyzer in (VNA).comparison Then, to theseCarrel’s data model, were used the optimized along with antenna the already also knownachieves gain a somewhat values of improvedthe reference FBR antenna in the passband. to calculate the RGFigure of the 9 testshows antenna. the test As setup shown in an in NPL Figure anechoic 10, the chamber reference for antenna the measurement and the test of antennarealized gainare orientedElectronicsand radiation 2020parallel, 9, 1830patterns to the ground of the optimized in order to LPDA. perform The E-plane Schwarzbeck measurements. USLP-9143B LPDA was used9 of as 12 a reference antenna in these measurements. The AUT (Antenna Under Test) was placed 2 m apart from the reference antenna, and both the antennas were placed at 1.2 m above the absorbing floor. A schematic diagram of the test setup for gain measurements is shown in Figure 9. The S12 from the test antenna to the reference antenna was measured using a Vector Network Analyzer (VNA). Then, these data were used along with the already known gain values of the reference antenna to calculate the RG of the test antenna. As shown in Figure 10, the reference antenna and the test antenna are oriented parallel to the ground in order to perform E-plane measurements.

Figure 8.8. FBRFBR (front-to-back(front-to-back ratio) ratio) plot plot of Carrel’sof Carrel’s conventional conventional design design and the and optimized the optimized LPDA design.LPDA design. Figure9 shows the test setup in an NPL anechoic chamber for the measurement of realized gain and radiation patterns of the optimized LPDA. The Schwarzbeck USLP-9143B LPDA was used as a reference antenna in these measurements. The AUT (Antenna Under Test) was placed 2 m apart from the reference antenna, and both the antennas were placed at 1.2 m above the absorbing ﬂoor. A schematic diagram of the test setup for gain measurements is shown in Figure9 . The S12 from the test antennaFigure 8. to FBR the reference(front-to-back antenna ratio) was plot measured of Carrel’s usingconventional a Vector design Network and Analyzerthe optimized (VNA). LPDA Then, these datadesign. were used along with the already known gain values of the reference antenna to calculate the RG of the test antenna. As shown in Figure 10, the reference antenna and the test antenna are oriented parallel to the ground in order to perform E-plane measurements.

Figure 9. A schematic diagram of the test setup for gain measurements in an anechoic chamber.

ElectronicsFigure 2020 9., 9,A x FOR schematic PEER REVIEW diagram of the test setup for gain measurements in an anechoic chamber. 10 of 12 Figure 9. A schematic diagram of the test setup for gain measurements in an anechoic chamber.

(a) (b)

FigureFigure 10. 10.The The test test setup setup to measure to measure the radiation the radiation patterns: patterns: (a) E-plane (a) andE-plane (b) H-plane and (b of) H-plane the fabricated of the optimizedfabricated LPDA optimized in an NPLLPDA anechoic in an NPL chamber. anechoic chamber.

Figure 11 presents the simulated and the measured E-plane normalized radiation patterns of the optimized antenna at (a) 470 MHz, (b) 630 MHz, (c) 790 MHz, and (d) 960 MHz. These plots suggest that the simulated radiation patterns and the measured radiation pattern in the E-plane are in good agreement. It also validates the highly directional behavior of the antenna in the passband as shown in Figure 11a–c. The directional behavior of the antenna degrades in the stopband as shown in Figure 11d at 960 MHz.

(a) (b) (c) (d)

Figure 11. E-plane normalized radiation patterns at (a) 470 MHz, (b) 630 MHz, (c) 790 MHz, and (d) 960 MHz.

After the radiation pattern measurements on the E-plane, the radiation patterns of the optimized LPDA were measured at the same frequency points on the H-plane. The test setup for H-plane radiation pattern measurements used exactly the same test setup as the E-plane radiation pattern measurements, where only the orientation of the test antenna and the reference antenna were changed to be perpendicularly aligned with respect to the absorbing floor, as shown in Figure 10. The simulated and the measured radiation patters at four frequency points are compared in Figure 12. The results show satisfactory agreement between measurements and simulations.

(a) (b) (c) (d)

Electronics 2020,, 9,, xx FORFOR PEERPEER REVIEWREVIEW 10 of 12

(a) (b)

ElectronicsFigure2020 ,10.9, 1830The test setup to measure the radiation patterns: (a) E-plane and (b) H-plane of the10 of 12 fabricated optimized LPDA in anan NPLNPL anechoicanechoic chamber.chamber.

FigureFigure 1111 presents presents the the simulated simulated and and the the measured measured E-plane E-plane normalized normalized radiation radiation patterns patterns of the of optimizedthe optimized antenna antenna at (a) at 470 (a) MHz, 470 MHz, (b) 630 (b) MHz, 630 (c)MHz, 790 (c) MHz, 790and MHz, (d) and 960 MHz.(d) 960 These MHz. plots These suggest plots thatsuggest the simulatedthat the simulated radiation radiation patterns patterns and the measuredand the measured radiation radiation pattern in pattern the E-plane in the areE-plane in good are agreement.in good agreement. It also validates It also validates the highly the directional highly di behaviorrectional ofbehavior the antenna of the in antenna the passband in the passband as shown inas Figureshown 11 ina–c. Figure The 11a–c. directional The directional behavior of behavior the antenna of the degrades antenna in thedegrades stopband in the asshown stopband in Figure as shown 11d atin 960Figure MHz. 11d at 960 MHz.

(a) (b) (c) (d)

FigureFigure 11.11. E-planeE-plane normalizednormalized radiationradiation patternspatterns atat ((aa)) 470470 MHz,MHz, ((bb)) 630630 MHz,MHz, ((cc)) 790790 MHz,MHz, andand ((dd)) 960960 MHz.MHz.

AfterAfter thethe radiation radiation pattern pattern measurements measurements on the on E-plane, the E-plane, the radiation the radiation patterns ofpatterns the optimized of the LPDAoptimized were LPDA measured were atmeasured the same at frequency the same pointsfrequency on thepoints H-plane. on the TheH-plane. test setupThe test for setup H-plane for radiationH-plane radiation pattern measurements pattern measurements used exactly used the exactl samey the test same setup test as setup the E-plane as the radiationE-plane radiation pattern measurements,pattern measurements, where only where the orientation only the orientation of the test antenna of the test and antenna the reference and antennathe reference were changedantenna towere be perpendicularly changed to be perpendicularly aligned with respect aligned to the with absorbing respect ﬂoor,to the as absorbing shown in floor, Figure as 10 shown. The simulated in Figure and10. The the measuredsimulated radiationand the pattersmeasured at fourradiation frequency patters points at four are comparedfrequency inpoints Figure are 12 compared. The results in showFigure satisfactory 12. The results agreement show satisfactory between measurements agreement between and simulations. measurements and simulations.

(a) (b) (c) (d)

Figure 12. Normalized radiation patterns on the H-plane at (a) 470 MHz, (b) 630 MHz, (c) 790 MHz, and (d) 960 MHz.

5. Conclusions An optimized interference-rejecting 10-dipole LPDA that mitigates interference problems because of UHF DTT broadcasting co-existence with LTE-800 mobile communications band has been presented. Compared to conventional LPDAs, the proposed optimized LPDA demonstrates improved performance in the UHF DTT reception band (passband), which improves the quality-of-service (QoS) of the TV reception. Another advantage is that the proposed antenna offers high-rejection capabilities in the LTE-800 band without the use of external bandstop filters, which reduces the final cost of the antenna. Furthermore, this paper highlights a novel design procedure for LPDAs in order to achieve desired passband and stopband characteristics by simply optimizing antenna geometry [19]. The proposed antenna exhibits excellent matching in the passband with low S11 values below 12 dB. Moreover, − Electronics 2020, 9, 1830 11 of 12 the antenna achieves a relatively flat gain of approximately 8 dBi in the passband and also demonstrates a highly directive behavior with a front-to-back ratio of 20 dB. Finally, this paper also presents the S11, RG, and radiation pattern measurements of the fabricated proposed antenna in an anechoic chamber. The measured results show good agreement with simulated results.

Author Contributions: Conceptualization, K.K.M. and P.I.L.; methodology, K.K.M., P.I.L., Z.D.Z.; software, K.K.M.; validation, P.I.L., Z.D.Z. and I.P.G.; formal analysis, K.K.M. and P.I.L.; investigation, K.K.M. and P.I.L.; resources, P.I.L., T.H.L. and D.C.; data curation, K.K.M., T.H.L. and D.C.; writing—original draft preparation, K.K.M. and P.I.L.; writing—review and editing, Z.D.Z., I.P.G. and I.P.C.; visualization, K.K.M.; supervision, P.I.L., Z.D.Z. and I.P.C.; project administration, P.I.L.; funding acquisition, P.I.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the “European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie”, grant agreement number 861219, and the “European Union’s Horizon 2020 Research and Innovation Staff Exchange programme”, grant agreement number 872857. Acknowledgments: The work of T. H. Loh and D. Cheadle were supported by The 2017-2020 National Measurement System Programme of the UK government’s Department for Business, Energy and Industrial Strategy (BEIS), under Science Theme Reference EMT20 of that Programme. Conflicts of Interest: The authors declare no conflict of interest.

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