sensors

Communication Horizontal Polarized DC Grounded Omnidirectional for UAV Control Station

Muhammad Shahzad Sadiq 1, Cunjun Ruan 1,2,* , Hamza Nawaz 3 , Shahid Ullah 1 and Wenlong He 4

1 School of Electronic and Information Engineering, Beihang University, Beijing 100191, China; [email protected] (M.S.S.); [email protected] (S.U.) 2 Beijing Key Laboratory for Sensing and Security Applications, Beihang University, Beijing 100191, China 3 School of Electrical, Information and Electronics Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; [email protected] 4 College of Electronics and Information Engineering, Shenzhen University, Shenzhen 518060, China; [email protected] * Correspondence: [email protected]; Tel.: +86-135-0120-5336

Abstract: A new slot-based antenna design capable of producing horizontal for un- manned aerial vehicle (UAV) ground control station (GCS) applications is outlined in this paper. The proposed antenna consists of oversize coaxial cylinders, slots, and slot-feed assembly. Each of the four vertical slots, arranged periodically around the antenna’s outer cylinder, emits a horizontally polarized broad beam of radiation, in phase, to produce an omnidirectional pattern. The antenna possesses a low-ripple ±0.5 dB in azimuth gain (yaw) due to its symmetric axis shape and an enclosed  feed within itself, which does not radiate and interfere with the main azimuth pattern. This is crucial  for a UAV GCS to symmetrically extend its coverage range in all directions against yaw planes. Simu- Citation: Sadiq, M.S.; Ruan, C.; lation and measurement results reveal that the antenna maintains stable gain in the omnidirectional Nawaz, H.; Ullah, S.; He, W. pattern (+0.5 dB) over the entire operational frequency band (2.55 GHz to 2.80 GHz), where S11 is Horizontal Polarized DC Grounded lower than −10 dB. A further advantage of this configuration is its enhanced polarization purity for UAV of −40 dB over the full frequency band. The direct-current (DC) grounding approach used in this Sensors 2021 Ground Control Station. , antenna is beneficial due to its electrostatic discharge (ESD) and lightning protection. Furthermore, 21, 2763. https://doi.org/10.3390/ its aerodynamic, self-supporting, and surface-mount structural shape makes this antenna a good and s21082763 worthy choice for a UAV GCS.

Academic Editors: Shuai Zhang and Razvan D. Tamas Keywords: horizontal polarization; UAV ground station; Omni-directional

Received: 16 January 2021 Accepted: 5 April 2021 Published: 14 April 2021 1. Introduction Since the discovery of the interdependency between electrical parameters and electro- Publisher’s Note: MDPI stays neutral magnetic radiation, antennas have been developed that actively exploit this phenomenon. with regard to jurisdictional claims in Antennas convert electrical parameters (current and voltages) into electromagnetic param- published maps and institutional affil- eters (electric and magnetic fields) and vice versa. Hence, an antenna can be regarded as iations. a transducer or a sensor as it converts electrical energy to electromagnetic energy, or the opposite [1]. Antennas are always considered essential parts of communication systems, and their radiation and polarization characteristics play a vital role in defining such systems’ performance and efficiency [2,3]. Copyright: © 2021 by the authors. The use of unmanned aerial vehicles (UAVs) is rapidly expanding to commercial, Licensee MDPI, Basel, Switzerland. scientific, agricultural, and military applications [4]. To overcome the difficulty of finding This article is an open access article the exact location of mobile UAVs from ground control stations (GCS), omnidirectional distributed under the terms and antennas are utilized to resolve acquisition and pointing complications [5–10]. It has conditions of the Creative Commons been proven that using horizontally polarized antennas can achieve a 10 dB improvement Attribution (CC BY) license (https:// in terms of system gain as compared to vertically polarized antennas [11]. For GCS creativecommons.org/licenses/by/ deployment, where the antenna is intended to cover a wide range of angles at variant 4.0/).

Sensors 2021, 21, 2763. https://doi.org/10.3390/s21082763 https://www.mdpi.com/journal/sensors Sensors 2021, 21, 2763 2 of 11

distances, it is essential to utilize a low-gain ripple to ensure continuous coverage in the yaw plane (azimuth plane) [12–14] as gain ripple fluctuates and reduces the coverage range at variant horizontal plane angles [15]. The challenging aspect of designing horizontally polarized omnidirectional antennas is producing a uniform and in-phase current in the antenna’s azimuth plane. That necessary condition can be fulfilled by utilizing a single loop or multi-element arrangements [16]. Three primary topological schemes are used with omnidirectional horizontally polarized antennas. In the first topology, a single radiating structure, such as a loop, is utilized to achieve horizontally polarized radiations [17–20], but it is inherently band-limited due to an open feed. The second group imitates the loop arrangement of first with dipole elements arranged in a ring or circular array format [21–26] at the expense of a complex open feeding arrangement. The third topology utilizes slots to complete the horizontally polarized antenna. There are a few slot-based omnidirectional antennas described in the literature. In [27], an omnidirectional antenna operating at X band used an array of slot doublets etched in the broadside wall of the rectangular waveguide. However, there was no mention of the azimuth gain, gain fluctuations, and operating band. A slot- based antenna capable of producing horizontal polarization was constructed by arranging alternate slots with opposite tilt angles along the axis with intervals of λg/2. To improve the antenna’s performance, alternate axial slot arrays were shifted by λg/4 along the axis. Even then, it was not improved by more than −7 dB [28]. In [29], an omnidirectional antenna was proposed, but it was circularly polarized. Moreover, it was not direct-current (DC) grounded and had a built-in main beam frequency scanning problem. In [30], a slant polarized omnidirectional antenna was presented. All slot-based horizontal polarized topologies were arranged in a series of fed axial arrays to achieve the required polarization. The other two methodologies had open feeding networks that interfered with the radiating apertures and perturbed antenna radial symmetry causing an uneven azimuth gain pattern, which further reduced antenna coverage range. This paper proposes an omnidirectional antenna capable of achieving low azimuthal gain variations of ±0.5 dB. This work is the first single-element design based on slots capable of horizontal polarization and stable gain without making a complex axial array to achieve the required polarization. The flaunted antenna comprises four slot apertures evenly spaced around the antenna’s outer circumference. It also encloses the feeding topology, so antenna symmetry is not disturbed. The device’s compactness, ruggedness, and direct-current grounding are further important features of this antenna design. The pro- posed technique has improved polarization stability since the cross-pols are very weak relative to the co-pols. The antenna structure is exhibited and explained in Section2. Section3 elaborates on the slot-feed mechanism. In Section4, simulation verification is performed. Section6 describes the manufacturing and measurements of the antenna prototype. Section6 presents a comparison of the proposed work with those published. Finally, Section7 details our conclusions.

2. Antenna The structure of the suggested horizontally polarized omnidirectional antenna is shown in Figure1. It must be shaped like a pole due to vehicle-mounting requirements. It is based on the coaxial line and is composed of inner and outer conductors. There are etched slots around the coaxial cylinder, and the internal and external coaxial cylinders are separated by air. The primary radiation is emitted via the slots (each slot is matched to a dipole with the magnetic current source), which are periodically positioned along the antenna’s outer cylinder as depicted in Figure1a. According to Babinet’s principle, the slots are complementary to the . The far field of the linear dipole [31], is found using:  −jkr Eθ = j60Imcos cos (klcosθ) e /rsinθ Sensors 2021, 21, x FOR PEER REVIEW 3 of 12

퐸 = (푗60퐼 푐표푠 푐표푠 (푘푙푐표푠휃) 푒−푗푘푟)/푟푠푖푛휃 휃 푚 In the equation, θ is the angle between the line direction and the dipole. This means the pattern function of the dipole is the same as the . 퐹 = (푐표푠 푐표푠 (푘푙푐표푠휃) − 푐표푠푘푙)/푠푖푛휃 휃 Sensors 2021, 21, 2763 3 of 11 For idea half wavelength slot, 2l = λ/2, and 퐹 = 퐶표푠(2휋푐표푠휃)/푠푖푛휃 휃

(a) (b)

FigureFigure 1. 1. TheThe geometry geometry of the of thehorizontally horizontally polarized polarized omnidirectional omnidirectional antenna: antenna: (a) 3D view, (a) 3D (b) view, (b) cross- cross-sectional view. sectional view. The pattern of the slot antenna is the same as the dipole with the same length, but In the equation, θ is the angle between the line direction and the dipole. This means their elevation plane (E-plane) and omnidirectional plane (H-plane) are exchanged accordingthe pattern to function the duality of principle. the dipole Each is the slot same aperture as the produces slot antenna. horizontally polarized radiation. Four apertures around the circumference complete the antenna, as illustrated = ( ( ) − ) in Figure 1a,b, and radiate in Fanθ omnidirectionalcos cos klcos pattern.θ coskl The SMA/sin θconnector smoothly converts the TEM modes from SMA to a large antenna assembly with a matching structure λ that isFor an optimized idea half wavelengthinner pin height, slot, as 2l given = /2, in Figure and 1b. The diameter of the outer cylinder, the diameter of the feed pin that connects the F = Cos( cos ) sin inner cylinder to the outer part of theθ antenna,2π andθ the/ lengthθ of the slots are what primarily impact the performance of the antenna. The optimal specifications are listed in Table The1. pattern of the slot antenna is the same as the dipole with the same length, but their elevation plane (E-plane) and omnidirectional plane (H-plane) are exchanged according Tableto the 1. Optimal duality parametric principle. values Each of the slot antenna. aperture produces horizontally polarized radiation. Four apertures around the circumference complete the antenna, as illustrated in Figure1a,b , Parameter Value (mm) and radiate in an omnidirectional pattern. The SMA connector smoothly converts the Outer Cylinder Diameter A 60 TEM modes from SMA to a large antenna assembly with a matching structure that is an Inner Cylinder Diameter B 25 optimized inner pin height, as given in Figure1b. Slot feed Pin Diameter P 10 The diameter of the outer cylinder, the diameter of the feed pin that connects the inner Antenna Height H 75 cylinder to the outer part of the antenna, and the length of the slots are what primarily Slot Length L 65 impact the performance of the antenna. The optimal specifications are listed in Table1. Slot Width W 5 SMA Pin Height T 7.25 Table 1. Optimal parametric values of the antenna.

At UAV GCSs, thereParameter are relaxed limitations with regard to size and Value weight (mm) compared to aerial platforms [32]. For military operations, the UAV operator at the GCS is located Outer Cylinder Diameter A 60 Inner Cylinder Diameter B 25 Slot feed Pin Diameter P 10 Antenna Height H 75 Slot Length L 65 Slot Width W 5 SMA Pin Height T 7.25

At UAV GCSs, there are relaxed limitations with regard to size and weight compared to aerial platforms [32]. For military operations, the UAV operator at the GCS is located in a harmless, secured place while the desired information or strategic data from the battlefield is gathered remotely. For such applications, antennas must be capable of withstanding all terrain operational area requirements and should be able to function correctly under extreme weather scenarios. So the required antenna should be mechanically robust and sturdy without external supports as these supports would increase the antenna’s size [4] and result in more drag, which might weaken the antenna’s structure due to rigorous Sensors 2021, 21, 2763 4 of 11

terrain and weather conditions [4,33]. Thus, it is crucial to use a compact, aerodynamic design. There is often a chance that an instance of peak instantaneous power (PIP) happens inside the printed circuit board- (PCB) based feed network. Such an event would easily damage the PCB [34], so the feed must be capable of bearing sudden PIP. The antenna would also be the primary source to channel electrostatic discharge (ESD) and lightning into the electronic systems. An ESD incident would place the functionality and safety of these systems at risk, while a lightning bolt would annihilate them. Keeping the antenna DC grounded is the most feasible and efficient strategy used in combat [35]. This antenna design would circumvent all the problems described above. The axis-symmetrical, all- metal rugged antenna is primarily constructed of brass and is DC-grounded. The solid metal feed network is enclosed inside the antenna’s conformal and compact shape.

3. Feed Mechanism Horizontal slots induce vertical polarization as they can quickly interrupt the longitu- dinal surface current on the antenna’s outer surface [36], as seen in Figure2. Conversely, the longitudinal slots in the antenna’s outer surface cannot be stimulated due to their orien- tations that are in line with the surface current, and even a short circuit would not modify the flow of the surface current [28]. So it is not easy to produce horizontal polarization using a slot configuration on a coaxial cylinder. In our design, feed pins are inserted to excite the vertical slots, which connect the outer conductor of the oversized coaxial cable with its inner conductor, as shown in Figure1b. Thus, these slot apertures are energized sideways while the opposing sides are kept floating. The slot is regarded as a dipole having a magnetic current source [29], so the slot is λg/2 long. Normally, the external feed has a built-in problem where it radiates along with the main radiating elements and causes a significant gain ripple in the omnidirectional pattern. Here, we have designed an internal feed that runs inside the radiating part and does not interfere or radiate. As for the actual feeding of the antenna, a standard SMA connector is used for feeding. The SMA connector is a coaxial structure and the antenna designed in this section is also based on coaxial structure, so the matching structure is designed and inserted between the radiation part and the feed part according to impedance transformation of coaxial transmission line [37],

Zoversize = Zmatch(Zsma + jZmatchtanβT)/(Zmatch + jZsmatanβT)

where the T is the length of the matching pin and Zsma, Zmatch, and Zoversize are the charac- teristic impedances of the SMA connector, matching pin, and oversize antenna assembly, respectively. The SMA connector’s inner pin’s optimized height ensures a seamless transi- tion from normal Coaxial TEM mode to oversized TEM cable mode. The slot excitation of the proposed antenna is simple and easy without involving baluns or impedance trans- Sensors 2021, 21, x FOR PEER REVIEW 5 of 12 formers. Four pins join the inner cylinder to the vertical slots in the antenna’s outer cylinder. The feed and antenna can be conveniently integrated by arranging the manufactured parts together around the central axis.

(a) (b)

Figure 2. FigureSlot configurations: 2. Slot configurations: (a) vertical (a and) vertical horizontal and horizontal slot, (b) vertical slot, (b slot) vertical feed. slot feed.

4. Simulation Verification CST Microwave Studio was been used to simulate and optimize the antenna design. Figure 3 demonstrates the mutual connection between the antenna azimuth gain and the total number of slots along the antenna’s circumference. Each slot radiates a directed pattern. With each increment in the number of slots along the antenna’s circumferential axis, these directional radiations widened, as shown in Figure 3. Four slots made the radiation patterns combine and generate a low-ripple horizontal polarized omnidirectional radiation pattern.

Figure 3. Azimuthal gain vs. the number of slots along the antenna’s circumferential axis.

A. Determination of Pin Diameter and Slot Size Effect Figure 4 helps us to see the impact of the slot-feed pin diameter on the antenna input reflection. The change of diameter changed the antenna’s matching, as shown in Figure 4a. Figure 4b depicts how the slot-length variations shifted the antenna resonance region.

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Sensors 2021, 21, 2763 (a) (b) 5 of 11 Figure 2. Slot configurations: (a) vertical and horizontal slot, (b) vertical slot feed.

4. Simulation4. Simulation Verification Verification CSTCST Microwave Microwave Studio Studio was was been been used used to simulate to simulate and and optimize optimize the theantenna antenna design. design. FigureFigure 3 demonstrates3 demonstrates the the mutual mutual connection connection between between the the antenna antenna azimuth azimuth gain gain and and the the totaltotal number number of ofslots slots along along the the antenna’s antenna’s circumference. circumference. Each Each slot slot radiates radiates a directed a directed pattern.pattern. With With each each increment increment in the in the number number of slots of slots along along the the antenna’s antenna’s circumferential circumferential axis,axis, these these directional directional radiations radiations widened, widened, as asshown shown in inFigure Figure 3. 3 Four. Four slots slots made made the the radiationradiation patterns patterns combine combine and and generate generate a low-ripple a low horizontal-ripple polarized horizontal omnidirectional polarized omnidirectionalradiation pattern. radiation pattern.

FigureFigure 3. Azimuthal 3. Azimuthal gain gain vs. vs.the thenumber number of slots of slots along along the theantenna’s antenna’s circumferential circumferential axis. axis. Sensors 2021, 21, x FOR PEER REVIEW 6 of 12 A. A. Determination Determination of of Pin Pin Diameter Diameter and and Slot Slot Size Size Effect Effect FigureFigure 4 helps4 helps us usto tosee see the the impact impact of ofthe the slot slot-feed-feed pin pin diameter diameter on on the the antenna antenna input input reflection.reflection. The The change change of ofdiameter diameter changed changed the the antenna’s antenna’s matching, matching, as as shown shown in Figure4 a.

4a. FigureFigure4 4bb depicts depicts how how the the slot-length slot-length variations variations shifted shifted the the antenna antenna resonance resonance region. region.

(a) (b)

Figure 4. Effect on S11(a) by changing the feed pin diameter and (b) by changing the slot length. Figure 4. Effect on S11 (a) by changing the feed pin diameter and (b) by changing the slot length.

B.B. Field Field Verification Verification TheThe field field simulations simulations were were performed performed with with the the help help of CST of CST Microwave Microwave Studio Studio soft- ware.software. The electric The electric and magnetic and magnetic fields’ cross-sectional fields’ cross-sectional views through views the through SMA connector the SMA areconnector shown Figureare shown5a,c. TheFigure cross-sectional 5a,c. The cross views-sectional of the electric views andof the magnetic electric fields and magnetic through fields through the pins that connect the inner coaxial cylinder to the slot apertures in the outer coaxial cylinder are shown in Figure 5b,d. At the input SMA connector of the antenna, the electric field is spread radially outward (TEM mode). At the edge of the oversized assembly near the SMA connector, the electric field is again radially outward as that of the TEM mode. This demonstrates that the SMA connector’s inner pin’s adjusted length effectively converted the connector TEM mode into the oversized coaxial assembly TEM mode, as shown in Figure 5a. As this mode travels toward the pin connected to the slot, the field circulates the slot area. All four slots have the same circulation pattern, which indicates that they are all in phase, as seen in Figure 5b. The electric field steadily travels from the slot middle toward the slot end and thereby emits a horizontally polarized field, as seen in Figure 5a,b. The directional radiation slot patterns are large enough to converge to generate an omnidirectional, horizontally polarized outward wave. Correspondingly, the magnetic fields form closed loops (TEM mode) at the SMA feed and eventually transform to perpendicular loops corresponding to the E field outside the antenna, as clearly visible in the simulated field trajectories in Figure 5b,d.

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the pins that connect the inner coaxial cylinder to the slot apertures in the outer coaxial cylinder are shown in Figure5b,d. At the input SMA connector of the antenna, the electric field is spread radially outward (TEM mode). At the edge of the oversized coaxial antenna assembly near the SMA connector, the electric field is again radially outward as that of the TEM mode. This demonstrates that the SMA connector’s inner pin’s adjusted length effectively converted the connector TEM mode into the oversized coaxial assembly TEM mode, as shown in Figure5a. As this mode travels toward the pin connected to the slot, the field circulates the slot area. All four slots have the same circulation pattern, which indicates that they are all in phase, as seen in Figure5b. The electric field steadily travels from the slot middle toward the slot end and thereby emits a horizontally polarized field, as seen in Figure5a,b. The directional radiation slot patterns are large enough to converge to generate an omnidirectional, horizontally polarized outward wave. Correspondingly, Sensors 2021, 21, x FOR PEER REVIEWthe magnetic fields form closed loops (TEM mode) at the SMA feed and eventually trans-7 of 12

form to perpendicular loops corresponding to the E field outside the antenna, as clearly visible in the simulated field trajectories in Figure5b,d.

(a) (b)

(c) (d)

FigureFigure 5. 5.TheThe cross cross-sectional-sectional views views of of the the fields fields at at the the SMA SMA connector: ( a) the electricelectric field,field, ((bb)) thethe magnetic magnetic field field cross- cross- sectionalsectional views views of of the the fields fields at at the the slot slot-feeding-feeding pins, pins, ( c) the electric fieldfield ((dd)) the the magnetic magnetic field. field.

5. Antenna Fabrication and Measurement Result An antenna was manufactured using brass for the design validation. This antenna can be assembled using CNC-machined parts or expensive 3D printing. This antenna was built utilizing the first approach. The antenna had a reduced footprint and conformal shape to maintain low air resistance. The simulated and measured antenna test results are discussed in this section. Figure 6a displays an image of the prototype antenna. The input scattering parameter S11 of the manufactured antenna was measured with Agilent N5242A VNA’s help. In Figure 6b, the simulated and measured S11 are plotted. Measurements were less than −10 dB from 2.5 GHz to 2.8 GHz, which were in good harmony with simulations. The antenna was a reasonably broadband structure (achieved bandwidth of 11.3%).

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5. Antenna Fabrication and Measurement Result An antenna was manufactured using brass for the design validation. This antenna can be assembled using CNC-machined parts or expensive 3D printing. This antenna was built utilizing the first approach. The antenna had a reduced footprint and conformal shape to maintain low air resistance. The simulated and measured antenna test results are discussed in this section. Figure6a displays an image of the prototype antenna. The input scattering Sensors 2021, 21, x FOR PEER REVIEW 8 of 12 parameter S11 of the manufactured antenna was measured with Agilent N5242A VNA’s help. In Figure6b, the simulated and measured S 11 are plotted. Measurements were less than −10 dB from 2.5 GHz to 2.8 GHz, which were in good harmony with simulations. The antenna was a reasonably broadband structure (achieved bandwidth of 11.3%).

(a) (b)

Figure 6. (a) The fabricated prototype antenna. (b) Simulated and measured S11 of the antenna. Figure 6. (a) The fabricated prototype antenna. (b) Simulated and measured S11 of the antenna.

MeasuredMeasured and and simulated simulated vertical vertical elevation elevation planes planes and and horizontal horizontal azimuth azimuth planes planes of of the antennathe antenna at 2.6 at GHz2.6 GHz and and 2.7 2.7 GHz GHz are are plotted plotted in Figurein Figure7. Measurements7. Measurements were were done done in in thethe the the compact compact antenna antenna testtest rangerange (ATR)(ATR) ofof March Microwave Microwave Systems Systems B.V., B.V., which which uses usesa a source source antenna that that radiates radiates a a spherical spherical wavefront wavefront and and two two secondary secondary reflectors reflectors to to collimatecollimate the radiatedthe radiated spherical spherical wavefront wavefront into into a planar a planar wavefront wavefront within within the desiredthe desired test test zonezone where where the test the antennatest antenna is placed is placed and precalibrated and precalibrated standard standard gain antennas gain antennas are used are to used determineto determine the absolute the absolute gain of the gain AUT(antenna of the AUT under(antenn test).a underThe simulated test). The and simulated measured and co-polarizationmeasured co (normalized)-polarization and (normalized) cross-polarization and cross (normalized)-polarization radiation (normalized) patterns radiation in ◦ the omnidirectionalpatterns in the omnidirectional plane (H-plane) plane are (H shown-plane) in are Figure shown7a. in The Figure 360 7a.radiation The 360° at radiation the horizontalat the horizontal plane helps plane to maintain helps to completemaintain yawcomple planete yaw operation. plane operation. The measured The measured cross- polarizationcross-polarization levels in the levels azimuth in the plane azimuth are moreplane thanare more−40 dBthan down, −40 dB which down, agrees which with agrees the simulation.with the simulation. Figure7b depicts Figure the 7b simulated depicts and the measured simulated co-polarization and measured (normalized) co-polarization radiation(normalized) patterns radiation in vertical patterns elevation in (E-plane). vertical Figure elevation8a,b (E shows-plane). a measured Figure 8a,b azimuth shows a ◦ gainmeasured ripple of ± azimuth0.5 dB, whereasgain ripple the azimuthof ±0.5 dB, pattern whereas phase the ripple azimuth is only pattern 10 peak-to-peak. phase ripple is Theseonly were 10° measuredpeak-to-peak. at 2.6 These GHz were and measured 2.7 GHz, respectively.at 2.6 GHz and Both 2.7 affirmGHz, respectively. the excellent Both stabilityaffirm of the the excellent antenna pattern.stability of the antenna pattern. In Figure9a, the measured and simulated azimuth gain ripple are plotted for the entire0 frequency range for the clear visibility of the gain fluctuations. The maximum peak- Simulated Co-Pol 330 30 Simulated Co-Pol 0 to-peak value is 1 dB in the azimuth plane, confirming a good0 omnidirectionality. Measured Co-Pol Figure9b -10 illustrates the Mesured simulated Co-Pol and measured gain0 of our-30 antenna. This DC-grounded30 antenna -20 Simulated X-Pol -10 -30 300 demonstrated Measured reliable60 X-Pol gain within the entire band. These results indicate promising and -40 effective radiation characteristics in the-20 yaw-60 plane, making this antenna60 an appealing -50 option for GCS applications. -30 -60 -40 -70 270 90 -60 -50 -90 90 -50 -40 Normalized Gain (dB) -40 -30

-30 240 120 Normalized Gain (dB) -20 -20 -120 120 -10 -10 0 210 150 0 -150 150 180 180 Azimuth Angle (degree) Elevation Angle (degree)

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(a) (b)

Figure 6. (a) The fabricated prototype antenna. (b) Simulated and measured S11 of the antenna.

Measured and simulated vertical elevation planes and horizontal azimuth planes of the antenna at 2.6 GHz and 2.7 GHz are plotted in Figure 7. Measurements were done in the the compact antenna test range (ATR) of March Microwave Systems B.V., which uses a source antenna that radiates a spherical wavefront and two secondary reflectors to collimate the radiated spherical wavefront into a planar wavefront within the desired test zone where the test antenna is placed and precalibrated standard gain antennas are used to determine the absolute gain of the AUT(antenna under test). The simulated and measured co-polarization (normalized) and cross-polarization (normalized) radiation patterns in the omnidirectional plane (H-plane) are shown in Figure 7a. The 360° radiation at the horizontal plane helps to maintain complete yaw plane operation. The measured cross-polarization levels in the azimuth plane are more than −40 dB down, which agrees with the simulation. Figure 7b depicts the simulated and measured co-polarization (normalized) radiation patterns in vertical elevation (E-plane). Figure 8a,b shows a Sensors 2021, 21, 2763 measured azimuth gain ripple of ±0.5 dB, whereas the azimuth pattern phase8 of ripple 11 is only 10° peak-to-peak. These were measured at 2.6 GHz and 2.7 GHz, respectively. Both affirm the excellent stability of the antenna pattern.

0 Simulated Co-Pol 330 30 Simulated Co-Pol 0 0 Measured Co-Pol -10 Mesured Co-Pol 0 -30 30 -20 Simulated X-Pol -10 -30 300 Measured60 X-Pol -40 -20 -60 60 -50 -30 -60 -40 -70 270 90 -60 -50 -90 90 -50 -40 Normalized Gain (dB) -40 -30

-30 240 120 Normalized Gain (dB) -20 -20 -120 120 -10 -10 0 Sensors 2021, 21, x FOR210 PEER REVIEW 150 -150 150 9 of 12 0 180 180 Azimuth Angle (degree) Elevation Angle (degree)

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0 0 Measured Co-Pol 0 -30 30 0 330 30 Simulated Co-Pol -10 -10 Mesured Co-Pol -20 Simulated X-Pol -20 Simulated Co-Pol -60 60 -30 300 0 Measured60 X-Pol -30 0 Measured Co-Pol 0 -30 30 0 330 30 Simulated Co-Pol -40 -10 -40 -50 -10 Mesured Co-Pol -20 Simulated X-Pol -20 -50 -60 -60 60 -30 300 Measured60 X-Pol -30 -70 270 90 -60 -90 90 -40 -40 -60 -50 -50 -50 -50 -60

Normalized Gain (dB) -40 -40 -70 270 90 -60 -90 90 -30 -60 -50 Normalized Gain (dB) -30 240-50 120 -120 120 Normalized Gain (dB) -40 -20 -20 -40 -10 Normalized Gain (dB) -30 -30 240 120 -10-120 120 0 -20 -20 210 150 0 -150 150 -10 180 -10 180 0 Azimuth210 Angle (degree)150 0 -150 Elevation Angle150 (degree) 180 180 Azimuth Angle (degree) Elevation Angle (degree)

(a) (b) (a) (b) FigureFigure 7. (a)Figure Measured7. (a) Measured7. (a) Measured and simulatedand and simulated simulated normalized normalized normalized co-polarization co co-polarization-polarization and and crossand cross-polarization -crosspolarization-polarization in the omnidirectional in the omnidirectional omnidirectional plane, or plane, plane, or H-plane;H- plane;top 2.6 top GHz; 2.6 GHz; bottom bottom 2.7 2.7 GHz. GHz. ( (bb) Measured and and simulated simulated normalized normalized co-polarization co-polarization in the elevation in the plane, elevation or plane, or or H-plane; top 2.6 GHz; bottom 2.7 GHz. (b) Measured and simulated normalized co-polarization in the elevation plane, E-plane;E- plane;top 2.6GHz; top 2.6GHz; bottom bottom 2.7 2.7 GHz. GHz. or E-plane; top 2.6 GHz; bottom 2.7 GHz.

(a) (b)

Figure 8. (a) MeasuredFigure 8. (a) gainMeasured ripple gain vs. ripple azimuth vs. azimuth angle, angle, (b )(b measured) measured phase phase angle angle ripple ripple vs. azimuth vs. azimuth angle, atangle, 2.6 GHz at and 2.6 GHz and 2.7 GHz, respectively. 2.7 GHz, respectively. (a) (b) In Figure 9a, the measured and simulated azimuth gain ripple are plotted for the Figure 8. (a) Measured gain rippleentire vs. frequency azimuth range angle, for ( theb) measuredclear visibility phase of the angle gain ripple fluctuations. vs. azimuth The maximum angle, at peak2.6 GHz- and 2.7 GHz, respectively. to-peak value is 1 dB in the azimuth plane, confirming a good omnidirectionality. Figure 9b illustrates the simulated and measured gain of our antenna. This DC-grounded antennaIn Figure demonstrated 9a, the measured reliable gain and within simulated the entire azimuth band. Thesegain ripple results are indicate plotted for the entirepromising frequency and effective range forradiation the clear characteristics visibility in of the the yaw gain plane, fluctuations. making this The antenna maximum peak- an appealing option for GCS applications. to-peak value is 1 dB in the azimuth plane, confirming a good omnidirectionality. Figure 9b illustrates the simulated and measured gain of our antenna. This DC-grounded antenna demonstrated reliable gain within the entire band. These results indicate promising and effective radiation characteristics in the yaw plane, making this antenna an appealing option for GCS applications.

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(a) (b)

FigureFigure 9. 9.( a()a Measured) Measured and and simulated simulated azimuth azimuth gain gain ripple ripple vs. vs. frequency. frequency. (b) Measured(b) Measured and simulatedand simulated antenna antenna gain vs.gain frequency. vs. frequency. 6. Comparison 6. Comparison Table2 compares this study to the prior works published in the literature that also featuredTable horizontal 2 compares polarization. this study to All the works prior works tabulated published were designedin the literature with external that also open featuredfeed and horizontal lack lighting polarization. protection All capability. works tabulated The polarization were designed purity with was external also not open so high. feedThis and work lack proposes lighting forprotection the first capability. time an antenna The polarization that could purity produce was also horizontal not so high. polariza- This work proposes for the first time an antenna that could produce horizontal tion utilizing slot radiators and form a stable gain at the azimuth plane like traditional polarization utilizing slot radiators and form a stable gain at the azimuth plane like omnidirectional design topologies such as loop or printed dipole antenna arranged in the traditional omnidirectional design topologies such as loop or printed dipole antenna form of a circle. Due to vertical slot radiations, its cross-polarization levels are extremely arranged in the form of a circle. Due to vertical slot radiations, its cross-polarization levels low, as shown in the Table. The proposed antenna is novel because it has an internal are extremely low, as shown in the Table. The proposed antenna is novel because it has axis-symmetric feeding system. Due to its enclosed nature, the feed does not radiate an internal axis-symmetric feeding system. Due to its enclosed nature, the feed does not and interfere with the main radiating slots. So low-gain ripples in the azimuth plane are radiate and interfere with the main radiating slots. So low-gain ripples in the azimuth planeachieved are achieved as compared as compared to the listed to the works. listed Moreover,works. Moreover, it is DC-grounded, it is DC-grounded, which iswhich essential isfor essential any practical for any deployment. practical deployment.

Table 2. PerformanceTable 2. Performance comparison comparison of the proposed of the proposed antenna antenna with the with existing the existing literature. literature.

Reference PolarizationReference Purity Polarization Gain Purity Ripple (dB)Gain Ripple (dB) DC GroundDC Ground Feed Feed Type Type [17] 11[17] 11 ±5.5±5.5 NoNo Exposed Exposed [18] 20[18] 20 NANA NoNo Exposed Exposed [19] 20[19] 20 1.31.3 NoNo Exposed Exposed [20] 25[20] 25 1.01.0 NoNo Exposed Exposed [21] 18 1.5 No Exposed [21] 18 1.5 No Exposed [22] 20 1.5 No Exposed [23] 15[22] 20 1.51.5 NoNo Exposed Exposed [24] 15[23] 15 NA1.5 NoNo Exposed Exposed [25] 20[24] 15 2.0NA NoNo Exposed Exposed [26] 20[25] 20 2.12.0 NoNo Exposed Exposed ± Proposed work 40[26] 20 0.52.1 YesNo Exposed Enclosed Proposed 40 ±0.5 Yes Enclosed 7. Conclusionswork A novel horizontally polarized omnidirectional antenna that is built on the slot struc- tures is introduced in this work. By positioning four vertical slots along the antenna circumference and energizing them with a robust central feeding mechanism, steady gain with improved polarization stability is realized in the antenna’s azimuth axis. The internal axis-symmetric feed network itself radiates no power, hence it does not interfere with the

radiating structure. Low ripples appear among the antenna azimuth gain. Steady yaw Sensors 2021, 21, 2763 10 of 11

plane gain with low gain fluctuations enhances coverage area or increases the link efficiency. So it is desirable in numerous ground station-based applications, such as UAV communica- tion and direction finding, to have a low azimuth gain ripple antenna. This antenna also possesses requisite mechanical features, which are crucial for its smooth operation. It has a sturdy, DC-grounded construction that does not need any exterior framework or support. Its conformal and aerodynamic shape minimizes air drag and any corresponding degrada- tion attributed to military operations’ environmental and terrain conditions. Altogether, this may make this antenna a favorable candidate for ground stations.

Author Contributions: M.S.S., in collaboration with H.N., proposed the antenna’s configuration and simulated it in CST. M.S.S. executed the fabrication and tuning of the antenna prototype. H.N. did the measurements. M.S.S. and S.U. did the plotting and manuscript preparation. C.R. and W.H. provided step-by-step supervision, reviewed the work, and did manuscript refinement. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by the National Natural Science Foundation of China (Grant No. 61831001) and the High-Level Talent Introduction Project of Beihang and the Youth-Top-Talent Uni- versity (Grant No. ZG216S1878) and Support Project of Beihang University (Grant No. KG12060401).). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: Authors declare no conflict of interest.

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