Low-Profile Dual Hollow Octagonal Ring Shaped Optically Transparent Tri-band Antenna for WLAN and Sub-6 GHz 5G Applications

A transparent dual octagonal split ring-shaped resonator connected by a horizontal strip is proposed for tri-band applications. Stub-loaded microstrip line fed structural design of radiator consists of two slotted octagonalshaped rings connected via a strip on the top with the partial ground at the back. The low prole (40×25 mm) radiator achieves impedance bandwidth of (46.08%) 1.62-2.59, (7.78%) 3.954.27, and (12.60%) 5.13-5.82, respectively. A bi-directional (dipole shaped) radiation pattern with maximum gain and minimum e ciency of 2.5 dBi and 52%, respectively is achieved. Transparency above 80%, low pro le structure, and tri-band operation make the antenna a good contender for WLAN and Sub-6 GHz 5G applications. Good correlation is observed for the modeled and experimental parameters.


Introduction
Wireless Communication has become a crucial part of today's era of 4G and 5G technology. Transmitting information through the air medium with the help of electromagnetic waves to wireless devices can be ameliorated with the help of antennas. Wireless technology allows people to work from remote locations for several applications which rises the demand for compact, multiband, and easy to establish antennas that leads to seamless connectivity. The need has elevated due to advancements in electronics gadgets with multiple usabilities.
Many designs of antennas have been proposed for achieving multiband antennas. Among them, the applications covering WLAN (Wireless Local Area Network) and sub-6 GHz band is in very much demand. The antenna usability can be further increased if the antennas are made transparent as they can be interfaced anywhere without causing any visual clutter. The main idea behind making the antenna transparent is that it can be installed everywhere without caus- 202 c 2021 Journal of Advanced Engineering and Computation (JAEC) ing any visual clutter and it also helps in receiving the signals at every possible corner. There -two main categories include mesh [1] and conductive oxide [2] based antennas. Mesh structures can easily be detected while antennas are made up of oxides like FTO (Fluorine Tin Oxide), ITO (indium Tin Oxide), AgHT (Silver Tin Oxide), AZnO (Aluminum Doped) achieves completely transparency. Such antennas can be prepared from commercially available sheets or sputtering techniques [3], and the chemical vapor deposition process [4]. Due to the easy availability of AgHT-8 in the form of sheets along with sheet impedance of 8Ω/Sq, it is widely acceptable for fabrication of transparent antennas Multiband transparent antennas can be useful for better signal quality by attaching it to indoor ceilings and walls, window glasses, large monitors, and automobile glazing due to their unobtrusive property.
Various tri-band antennas are proposed in the literature for wireless applications [5]- [18]. A compact triple-band antenna that is made recongurable with the use of a PIN diode in the external split ring of the metamaterial structure is proposed in [5] that covers WiMAX and WLAN band. In [6] a Tri-band Y shaped slotted monopole antenna with a Split-Ring meandering slot and a symmetrical upturned L-strips pair having steady gain and the broadside radiation pattern is presented that span from (2.332.76 GHz), (3.053.88 GHz), and (5.575.88 GHz). A monopole tri-band antenna with a feeding element such as a coplanar waveguide covers the 2.4/5 GHz WLAN and 3.5/5 GHz WiMAX is proposed. It can cover more bandwidth in comparison with [5]. The radiator consists of an Sshaped strip and a rectangular ring, with a Ushaped strip in a crooked manner and a bottom layer with three strips [7]. A revised structure named compact triband printed antenna which works for 2.4,3.5,5 GHz is proposed in [8]. The size of the antenna is 17×23.5×1.6 mm 3 whereas it is capable to cover 2.4,5.2,5.8 GHz and 3.5,5.5 GHz. Bandwidth coverage ameliorated in comparison with [5]- [7]. The number of antennas used in [6] and [8] is the same but [8] has a more strip-based structure which makes its structure less complex. Amalgamating the dierent communication systems into a single antenna system is made with the implementation of the tripleband antenna with an E-plane coupled MSA antenna having two elements with a defected base having a size of 50×50×1.6 mm 3 [9]. A polarised antenna with a Y-shaped radiating element and partial ground with monopole arms is circularly polarised having a dimension of 35 × 45 mm 2 [10]. A circular slot-loaded compact cylindrical tri-band dielectric resonator antenna working in the 2.4/5.2 and 3.5 GHz regimes [11] with a similar size of the antenna as [9] is proposed. An integrated antenna system for WiGig (57 to 64 GHz) and WLAN (2.4 to 2.485 GHz and 5.15 to 5.85 GHz) and application based on magnetic electric dipole and a stacked patch antenna has been developed with a large frequency ratio however it suers from huge bandwidth dierence in certain applications [12]. A rectangular tri-band antenna is designed for an application covering WiMAX and WLAN, that received problems while fabricating small shorts without the presence of air and got 0.8 GHz deviation w.r.t simulated results [13]. Although, it exhibits multiband matching but does have a complex structure. Similarly, a compact metamaterial recongurable antenna illustrated in [14] covers 2.4, 3.5, and 5 GHz, bands. A triple-band MIMO antenna with omnidirectional radiation pattern with measured radiation gain of 0.253, 0.6, and 3.38 dBi at 0.9, 1.8, and 2.6 GHz with low correlation coecient and covers GSM900/1800 and LTE2600 bands is proposed in [15]. Nonuniform fork and meandered type grounded radiator works on a triple band which is capable of covering the WLAN band with good radiation pattern and impedance matching [16]. A slot-loaded microstrip multi-band antenna with a defected ground base has been designed for the IoT applications which resonated in 2.42, 5.22, and 5.92 GHz bands [17]. Subsequently, in this smart world GSM, Bluetooth and DCS are stated under the rudimentary categories where the antenna transmission and reception play a vital role for access in rural and urban zones. An inverted F-antenna in a folded planar is capable of covering GSM, DCS, and Bluetooth bands [18].
However, the antennas proposed above are nontransparent and both WLAN with sub-6 GHz bands are still not covered. In [25]- [27], transparent antennas with dual-band [25] and triband [26,27] is presented. The transparent antenna in [25], is a slotted split ring resonator with the slot line extended to the patch which covers bands of 2.4 and 5.26 GHz, respectively. It is capable of indoor WLAN applications whereas the opaqueness issue has been raised. In [26], a exible transparent antenna is proposed for GSM, WiMAX, WLAN, 3G, 4G, and 5G applications however exible antennas often suer from low eciency and gain due to the thin substrate and bending conditions.
In this work, a compact antenna having dual octagonal-shaped rings connected with a horizontal strip is proposed. Transparency above 80%, low prole structure, and tri-band operation make the antenna a good contender for WLAN and Sub-6 GHz 5G applications. The satisfactory agreement between the measurement and numerical computations of RF performance indicates the technical potential of an octagonal ring-shaped transparent antenna for interfacing the antenna in devices using WLAN and sub-6 GHz 5G applications.

Antenna conguration and design geometry
The transparent antenna models' top, side, and 3-D view are depicted in Fig. 1. The blending of the transparent substrate and conductive sheet helps the antenna in achieving transparency more than 85%. The lightweight transparent structure consists of a Plexiglas (thickness = 1.48 mm, dielectric constant (εr) = 2.3 and tan δ = 0.003) that serves as a substrate and AgHT-8 (silver tin oxide) as patch and ground material, respectively. AgHT-8 is a thin transparent sheet (thickness = 0.177 mm and surface resistance of 8 Ω/m). Antenna geometry consists of two octagonal splits ring-shaped structures with slots that are united using a rectangular strip is interfaced with a stub-loaded mi-

Parametric analysis
The antenna has been designed with disparate radii in the octagonally shaped ring on the top and partial ground on the bottom. The base antenna consists of a solid octagonal-shaped patch and full ground. Subsequently, the six consecutive changes are carried out for achieving the required bands resonating in WLAN and sub-6 GHz 5G. Firstly, the octagonal ring with a narrow strip and a slot has been made which is illustrated as antenna 1 and 2 in Fig. 3. Conjunction with the outer ring, a second octagonal slotted ring is added to form antenna 4. A horizontal strip is added to attach the two slotted octagonal-shaped rings. The slots of the ring are oriented in opposite directions to avoid the internal collision of radiations. Finally, in the proposed design, the ground plane is made partial which is elucidated in Fig. 3. transformed to a simple narrow octagonal ring (antenna 2). Antenna 2 has the languishing effect in comparison with the previous antenna with approximately -3 dB reection coecient. Revised Antenna 3 with a slight gap in the ring shows the improvised value of reection coecient but not up to the mark. Subsequently, Antenna 4 with the second octagonal ring has been placed with lesser radii than the outer ring. The gap in the outer ring directs in the upper direction and the gap in the second ring directs towards down which establishes the reection coefcient value below -15 dB which is considerable. Antenna 5 with horizontal strip elevates the performance of the model which results in radiation spikes below -10 dB. To further improve the radiation, the interconnection of the two octagonal rings has been made. Antenna 6 with improvised structure have dual-band performance with reection coecient spike at -34 dB which still lacks the required impedance bandwidth for WLAN operation whereas the sub-6 GHz band is still not covered. Thus, the nal antenna is proposed with the partial ground plane, which radiates at WLAN and sub-6 GHz band with a reection coecient below -10 dB. The partial ground is added at the last since the defected ground structure is known to improve the impedance bandwidth of the resonant frequency and so it was not added from the very rst stage. The antenna was rst optimized to work at dualband covering both the WLAN bands but to improve the impedance bandwidth, the partial ground was added instead of full ground. In doing so the impedance bandwidth did improve signicantly while achieving an additional band between the previously attained bands.
To understand the working of the antenna, parametric analysis is performed to analyze the eect on the reection coecient. The outer ring thickness (ORT) is varied where it can be observed that when the thickness increases, the frequency band shifts towards the higher side whereas a decrease in the thickness leads to shifting in the rst and second resonant frequency towards the lower side as shown in Fig. 5(a). The optimum value of 0.93 mm is chosen as the thickness. In Fig. 5(b), the reection coecient is observed by changing the inner ring thickness (IRT). As the value of IRT increases, the fre-quency at the third resonant shifts towards the higher side of the band while there is no signicant change in the other two bands.  (a) illustrates the eect on the reection coecient due to variation of the slot length (SL). Signicant eect on the second and third band is observed in terms of impedance bandwidth and reection coecient when the slot length is increased or decreased however the rst band is least aected due to the same. The effect on the reection coecient by rotating the connector (C) connecting the outer and inner octagonal ring is observed in Fig. 6(b). When the connector is rotated from the 5 0 position towards the 0 0 position, the impedance bandwidth at the second and third band signicantly im- a bit more towards the higher side thus deviating from the bands of interest (WLAN). While the rotation to 10 0 led to only dual-band performance. The connector position is selected as 5 0 as the same covers the required band and attains acceptable impedance bandwidth.
The eect of the stub length variation (S) is depicted in Fig. 7(a) where it is observed that optimum stub length helps in achieving the right level of reection coecient while improving the impedance bandwidth as well. The stub length of 14 mm is selected as the same achieve the required frequency bands with superior IBW compared to other results. Finally, the length of the ground plane (GL) has been varied which plays a vital role in establishing the required impedance  bandwidth of the tri-band transparent antenna. The best performance is observed for length of the ground = 5 mm as tri-band performance is observed along with the satisfactory values of the impedance bandwidth as shown in Fig. 7(b).   The radiator is fabricated and tested using a Keysight handheld N9915A vector network analyzer and anechoic chamber after carrying out numerical computations and optimization in commercial 3D electromagnetic eld solver software, Ansoft. The reection coecient results are illustrated in Fig. 8 for the proposed design. It is observed that the simulated antenna achieves tri-band resonance at (46.08%) 1.62-2.59, (7.78%) 3.95-4.27, and (12.60%) 5.13-5.82 which closely matches the measured values. The measured impedance bandwidth is slightly lower due to the tolerances in geometric fabrication and calibration errors in RF cabling-based systems.
The distribution of current in the octagonalshaped transparent antenna at 2.05, 4.08, and 5.47 GHz is presented in Fig. 9. At 2.05 GHz, the distribution of current is observed on the stub, outer ring, and the strip connecting inner and outer hollow octagonal-shaped rings. Very little current is coupled in the inner ring whereas, at 4.08 GHz, the current is also coupled with the inner ring where major distribution is observed on the sides next to the slotted regions. At 5.47 GHz, the majority of the surface current is observed near the right edges of inner and outer octagonal rings.
The 2D co/cross-polarization patterns of the transparent radiator along the E and H plane are depicted in Fig. 10 which is numerically calculated using HFSS software and experimentally measured in an anechoic chamber as illustrated in Fig. 11. At least a 10 dB dierence between co and cross-pol pattern is observed which is true for both simulated and measured results. At the E plane bidirectional (dipole shaped) pattern is achieved while at the H plane, the omnidirectional pattern is observed. Figure 12 portrays the gain and eciency of the proposed low-prole transparent antenna. The simulated gain of the antenna is 1.1, 1.9, and 2.5 dBi at 2.05, 4.08, and 5.47 GHz where eciency ranges from 58-67%, respectively. The measured gain demonstrated well correlation with the simulated results.
The performance comparison of the lowprole tri-band transparent antenna with other antennas is carried out as shown in Tab. 1. The antenna radiates at WLAN and sub-6 GHz band, with the satisfactory value of reection coecient, peak gain, and achieves more than 80% optical transparency.

Conclusions
A low-prole transparent antenna with disparate radii in the octagonally shaped ring on the top and partial ground on the bottom is proposed. The optimized antenna geometry helped in achieving tri-band performance spanning from (46.08%) 1.62-2.59, (7.78%) 3.95-4.27, and (12.60%) 5.13-5.82, gain greater than 1 dBi and bidirectional pattern at bands of interest. The triband resonance attainment and design process are backed by numerical computations, parametric studies, and experimental results. The antenna exhibits more than 80% transparency and compact size that makes it suitable for its placement in the indoor or outdoor facility without being noticed. Thus, it offers seamless connectivity to the devices working on WLAN and sub-6 GHz frequency. "This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium provided the original work is properly cited (CC BY 4.0)."