FULLY TRANSPARENT METAMATERIAL AMC BACKED CPW FED MONOPOLE ANTENNA FOR IOT APPLICATIONS

In this paper, a fully transparent antenna comprising of an Artificial Magnetic Conductor (AMC) backed Co-planar Waveguide (CPW) fed dual-ring monopole is presented. The monopole antenna and AMC structure achieve transparency due to the use of AgHT-8 conductive oxide and Plexiglas substrate. Measured antenna performance shows an impedance bandwidth of 5.3 – 6 GHz (12.4 %) in the U-NII-1 to U-NII-4 frequency band with a peak gain of 5.7 dBi which is approximately an increase of 4.5 % and 3.9 dBi, respectively, as compared to the stand-alone antenna. The simulation and the measurement results agree well with each other which proves the validity of the proposed design. To the best of our knowledge, the proposed antenna is the first fully transparent antenna design combining a transparent radiator and a transparent AMC structure.


INTRODUCTION
Monopole antenna is one of the most widely used antenna types in modern technology as it is simple, low cost, and suitable for multiple commercial applications [1,2]. Despite having a compact size and low -profile, the planar monopole antenna suffers from low radiation efficiency because the main electric component has its radiation canceled by staying on the same plane as the ground plane [3]. Metamaterials are an interesting research topic because antenna performance (e.g. bandwidth, gain, size) can be improved with appropriate implementation [4 -6]. AMC, a type of metamaterial, is a well-known technique to improve gain performance by placing on the backside of the antenna. The AMC structure consists of multiple unit cells that are duplicated along with its fixed periodicity and have its operating bandwidth defined where the reflection phase of the unit cell model is ranged from 90˚ to -90˚ [7]. It makes the antenna design more complex as the topology of the unit cell, which is the core of the AMC structure, strongly depends on its shape and size but the improvement in bandwidth and gain compared to the stand-alone antenna is also adequate. There are many published works regarding AMC-backed antenna design. In [8], a bowtie antenna that was backed by 9 × 6 unit cells of the circular patch was proposed. The antenna has a relatively small size of 75 × 110 mm 2 over the bandwidth from 1.64 -1.94 GHz (16.7 %). The main disadvantage of the design was the low gain (6.5 dBi) as compared to the gain reported in [9] (10.1 dBi), where Pooja Prakash et al. introduced a ring slot AMC unit cell in a 4×4 AMC array to improve gain for a rectangular monopole antenna. Though the antenna in [9] exhibited a wider bandwidth of 4.25 -6.9 GHz (47.53 %), its overall dimension in terms of wavelength was larger than that of the antenna proposed in [8].
Recent advances in technology have greatly reduced the size of IoT devices. The main disadvantage of using an AMC structure at the back of the antenna results in the consumption of useful board space due to the distance between the antenna and the AMC structure. If the antenna is made transparent, it can be installed anywhere within the device without occupying the functional space of other electronic components. Transparent antennas can be installed anywhere in the room, most ideally on transparent surfaces such as windows, glasses, overhead lights, etc. There are 2 main ways to achieve a transparent antenna, that is using metal mesh or transparent material. In the metal mesh approach, transparency is achieved through gaps in the copper mesh [10,11]. Although the antenna performance is good, the transparency is much lower than when using transparent material. Transparent materials such as Plexiglas substrates or AgHT conductive sheets with greater than 75 % transparency are widely used for transparent antennas despite their low conductivity. This problem leads to low efficiency and gain, which can be overcome by using the AMC structure. However, to our knowledge, even though there is a transparent monopole antenna by using metal mesh [12] or optically transparent material [13], the use of AMC structure for this type of antenna is still limited.
Furthermore, the extreme rise in data ingestion requires Wi-Fi to operate at high speeds, driven by new applications, devices, and use cases. The Unlicensed International Infrastructure (U-NII) radio band, which is a part of IEEE 802.11a, offers a new spectrum. This spectrum especially operates over 4 bands: from U-NII-1 to U-NII-4 which ranges from 5.15 GHz to 5.925 GHz [14], making it suitable for many indoor and outdoor uses as well as Dedicated Short Range Communication applications [15]. Therefore, this paper proposes a transparent dual ring monopole antenna with AMC support, operating in the spectrum range from U-NII-1 to U-NII-4 (5.15 -5.925 GHz). The AMC unit cell design and its effect on antenna bandwidth and gain are discussed in Section II. Section III shows the simulation and measurement results of the proposed design using CST software. Finally, conclusions on the antenna is given in Section IV.

ANTENNA GEOMETRY AND PARAMETRIC STUDY
The dual ring CPW based transparent antenna geometry is illustrated in Fig. 1 (a,b) [16]. The antenna structure achieves transparency by using conductive oxide AgHT-8 and Plexiglas where AgHT-8 has a thickness and plate impedance of 0.177 mm and 8Ω/sq, respectively.
The designed transparent antenna is interfaced with an AMC structure that has 4×4-unit cells and is held below the antenna at an optimized distance (d) as shown in Fig. 1 (c, d). The artificial magnetic conductor (AMC) consists of three layers including the top layer of a 4×4unit cell array formed using AgHT-8, a middle layer made up of Plexiglas substrate, and a bottom layer as the ground plane with a total thickness of T = 1.83 mm. The optimized antenna dimensions of the dual ring antenna are shown in Table 1.   Figure 2 (a) depicts the internal radius variation of the hollow circular ring. As the radius decreases the frequency band shifts towards the lower side and the reflection coefficient decreases, while the opposite happens as the radius increases. For best results, the size of the radius (R2) was chosen to be 8.3 mm.

Antenna Parameters Dimensions (in mm) Antenna Parameters Dimensions (in mm)
The variation of the gap between the CPW feed and the microstrip line (G1) affects the impedance bandwidth of the antenna. As the gap decreases, the impedance bandwidth increases as can be seen in Fig. 2 (b). The optimum gap between the CPW feed and the microstrip line is carefully chosen to be 0.5 mm for more precise fabrication.  The AMC simulation model consists of an AMC unit cell enclosed inside a unit cell boundary and provided the feed from the top side. The reflection phase illustrates that the inphase region of the frequency range 5.3 -6.2 GHz is spanned between ± 90° as shown in Fig. 3.
The effect on |S11| and gain is analyzed by varying the AMC distance from the transparent radiator as shown in Fig. 4 (a, b). As the air gap between the antenna and the AMC structure increases, the impedance bandwidth decreases while the reflection coefficient improves greatly. A value of air gap distance (d) of 5 mm shows the maximum impedance bandwidth as compared to the radiator with AMC distance of 0 mm and 10 mm, respectively. The gain plot as shown in Fig. 4(b) depicts that the AMC at a distance of 5 mm from the transparent radiator shows an average gain of more than 5 dBi for the proposed bands. The value of gain is negative for frequencies lower than 5.56 GHz when the distance (d) is 0 mm and positive gain is obtained when the value of d is 10 mm. However, the gain value is still smaller than that achieved for d = 5 mm.
The mean E-field distribution at 5.67 GHz is plotted as shown in Fig. 5 by varying the distance between the AMC and the transparent radiator. It can be seen that when d = 5 mm (Fig.  5b), the E-field distribution on the AMC is not only spread out evenly on all sides but also concentrated on the dual ring, which is the main radiating element. When d = 0 mm, the electric field magnitude on the lower side of the AMC is very weak, opposite to the case of d = 10 mm.    Furthermore, regarding the magnitude of the electric field, the distribution on the dual ring is not high in the case of d = 0 mm and relatively low, especially in the central area, in the case of d = 10 mm when compared with the case of d = 5 mm. The air gap distance d = 5 mm shows not only the strong distribution on both the dual ring monopole and the AMC structure, but also the uniformity on the surface of the AMC structure. Therefore, the value of the air gap distance is chosen to be 5 mm.
It is observed from Fig. 6 that a transparent antenna achieves higher efficiency in the operating bandwidth with AMC than an antenna without AMC. The fabricated transparent radiator front and top views are depicted in Fig. 7 (a, b). A transparent antenna with AMC structure at a distance of 5 mm is fabricated by using Styrofoam columns (εr = 1) in the middle to create the required air gap. The side and top views of the antenna with AMC are visible in Fig. 7 (c, d).

RESULTS AND DISCUSSION
The fabricated antennas, both with and without AMC structure, have been tested for reflection coefficient. The measured frequency band of the single element fabricated antenna spans from (7.55 %) 5.48 to 5.91 GHz, which is in good correlation with the simulated value ranging from (8.8 %) 5.46 to 5.96 GHz as observed from Fig. 8 (a). The antenna with AMC resonates in the frequency range of (12.01 %) 5.32 -6.0 GHz, close to the simulated value spanning from (13.06 %) 5.292 to 6.032 GHz as observed from Fig. 8 (b). The simulation results and the measured results are in good agreement with each other due to the precise fabrication technique used to achieve the desired shape of the monopole which is performed using the simulation. The antenna geometry is patterned using a laser cutter to achieve the utmost precision. An extra thin double-sided adhesive sheet is used to stick the conductive sheet with the substrate. The main purpose of using this method is to avoid air gaps that can occur if conventional adhesives are used. The low-loss SMA connector is interfaced with the antenna using a conductive adhesive (Silver/Graphene paste) instead of hot solder. Finally, the VNA and the anechoic chamber used for measurement are calibrated and used under conditions of very low noise levels at IF (intermediate frequency) bandwidth.  The performance of the antenna with AMC in terms of gain is depicted in Fig. 9, where it can be observed that a stand-alone transparent antenna shows a lower value of gain as compared to the antenna with a 4×4 AMC array at the back. This is significant since the AMC structure makes the radiation pattern more directive, thus helping to achieve more gain. The antenna shows an average gain in the range of 5.47 dBi for the proposed band.
The radiation patterns for the E and Hplanes depicted in Fig. 10 (a, b) at 5.5 GHz and 5.85 GHz, respectively, were measured in an anechoic chamber. The back lobes are greatly reduced in the antenna with the AMC structure, which helps to make the radiation patterns more directional. Therefore, it enhances the gain of the antenna. The setup for measuring the radiation pattern and gain in anechoic chamber is shown in Fig. 11.  Table 2 shows a comparison of the proposed design with the previously published monopole antenna designs. In general, published designs are divided into antennas having either transparency [13, 19 -20], or AMC structure [9, 17 -18]. AMC-backed monopole antenna has the advantage of high gain despite its much larger average size than transparent antennas. On the other hand, transparent antennas have a wider fractional impedance bandwidth by using a radiator structure, but low gain performance is inevitable due to the inherently low conductivity of the material. To the best of our knowledge, the proposed antenna is the first to combine a transparent material with an AMC structure. The design has high gain and comparable size and fractional bandwidth making it suitable for many IoT applications.  Note: is the wavelength at the lowest frequency

CONCLUSIONS
A 4×4 metamaterial array-backed transparent antenna with a dual-ring structure is proposed. Complete transparency is achieved by the use of AgHT-8 and Plexiglas as transparent conductive oxides and substrate, and AMC, respectively. Analysis of distance between AMC cell arrays and the transparent radiator is carried out to improve the impedance bandwidth and gain of the antenna. The AMC structure with a 4×4 array size at a distance of 5 mm from the main radiator shows the maximum value of gain and bandwidth, causing an increase of 3.9 dBi and 4.5 %, respectively as compared to the stand-alone antenna. The transparency coupled with the enhanced performance in gain, bandwidth and directional radiation pattern make the proposed AMC-backed antenna suitable for use in the spectral range from U-NII-1 to U-NII-4.
Declaration of competing interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.