Optically transparent antennas have attracted a great deal of attention due to their advanced functional properties and aesthetic concerns. In some applications, such as cube satellites, surface area is highly limited due to solar panels. Transparent antennas are integrated onto the solar panels without any notable decrease in the conversion efficiency of sunlight to electricity, while also saving surface area [[1], [2], [3], [4], [5], [6]]. In addition, transparent antennas can be hidden inside the healthcare, automotive and industrial products, such as windshields, eye glasses, building windows, displays, and lighting surfaces, without sacrificing their functionality [2]. Optical multilayer structures also can be employed in applications, such as, in photonic crystal layered topologies (PCLT) for magnetic field sensing [7] and broadband wide-angle absorption [8].

Optically transparent antennas can be employed by coating transparent substrates by ultrathin (about 10 nm) metal (e.g., Cu, Ag, Au, etc.) layers as the radiating components. However, reducing the thickness of the metal increases the sheet resistance (Rs) due to electron scattering on the surface and grain boundaries, and high Rs causes high ohmic losses. In this respect, there is a trade-off between thickness and conductivity, in turn, optical transparency and conductivity. To overcome this limitation, metal mesh antennas have been proposed, where both the patch and ground plane are made up from metallic meshes [1]. However, mesh antennas, employing conventional metals such as silver and copper, may not be sufficiently suitable for applications where visibility is intolerable.

Another method for achieving high optical transparency as well as high conductivity is by using transparent conductive films (TCFs). Indium tin oxide (ITO) is extensively investigated for this purpose although it is costly [9]. To increase the optical visibility, thickness of the transparent conductive films is preferred to be less than the skin depth. Thinner than the skin depth, however, causes high sheet resistance, decreasing the electrical conductivity and resulting in relatively poor antenna performance [10,11]. Since the single layer transparent antenna structures have such a problem, sandwich (three-layer) structures of TCF/metal/TCF are introduced [12]. Not only optical transparency is improved by suppressing the reflection from the metal by the TCF, but also conductivity is improved due to the decrease in the equivalent sheet resistance of the multilayer structure. For example, the resistivity of ITO/Ag/ITO structure with a 10 nm Ag metal layer is lowered by a factor of 4 with respect to ITO single layer of the same total thickness and optical transmittance is not affected by the 10 nm silver layer [13]. In such three-layer sandwich structures, increasing thickness of the metal improves the antenna characteristics but definitely decreases optical transparency. The critical question is how metal content of the structure can be further increased without sacrificing optical transparency. It is not possible to add metal layers to increase optical transparency because, for example, 40 nm thick silver film has a transmittance of only 70% [10]. On the other hand, implementing multilayer structures, resonant tunneling leads to several orders of magnitude enhanced transmission [10].

Numerous research studies are being conducted in transparent antenna technology to explore innovative designs and materials for achieving specific functionalities and performance enhancements. Among these studies, a L-shaped microstrip lines are exemplified by advanced antenna systems such as wideband omnidirectional circularly polarized antennas [14] and polarization reconfigurable omnidirectional antennas [15]. Both of these studies are dedicated to developing and examining sophisticated antenna systems with distinctive features.

Transparent antennas have been studied in the literature, which are comprised of nanocarbons (graphene [16], carbon nanotube [17], etc.), transparent conductive oxides [18,19], conductive polymers [20] and metallic nanostructures [21,22]. Transparent antennas are expected to have low sheet resistance (Rs) and high optical transparency (T). The summary of various transparent antennas including various materials is presented in Table 1. However, there is no optically transparent antenna structure studied in the literature that incorporates a dielectric/metal/dielectric multilayer metallic nanostructure. We resolve the problem of sacrificing optical transparency for enhanced antenna parameters by introducing multiple sandwich structures that remind one-dimensional photonic crystals [10]. The phenomenon of multiple dielectric/metal/dielectric structures in optically transparent antennas is implemented, to the best of our knowledge, for the first time in the literature.

In this paper, we study three transparent multilayer antenna (MLA) structures. Polycarbonate (PC) is chosen as the substrate material due to its optical transparency and flexibility. Schematic representations of MLA1, MLA2 and MLA3 are given in Fig. 1. (a), (b) and (c), respectively. The ground plane structures of all the proposed three multilayer antennas are in the same form. The first transparent multilayer antenna structure (MLA1) is composed of a single layer Ag sandwiched between TiOx/SixNy/ZnAlOx (3L) layers in the rectangular patch form with a feed line on the PC substrate. The 3L′ in Fig. 1. (a) is the mirror image of the of the 3L structure be-low the Ag layer (i.e. 3L’: ZnAlOx/SixNy/TiOx). In addition, the back surface (ground) of the PC substrate is fully structured as a single layer Ag sandwiched between TiOx/SixNy/ZnAlOx layers. The full form of the first transparent MLA structure can be given as 3L’/Ag/3L/PC/3L/Ag/3L′ in compact form. The proposed second transparent multilayer antenna (MLA2) is shown in Fig. 1. (b). The antenna structure consists of double layer Ag sandwiched between 3L and 3S multilayers. Abbreviated 3S multilayer is composed of ZnAlOx/SixNy/ZnAlOx. The full form of the second transparent MLA structure can be given as 3L’/Ag/3L/PC/3L/Ag/3S/Ag/3L′ in compact form. The proposed third transparent multilayer antenna (MLA3) is shown in Fig. 1. (c). The rectangular patch and the feed line of the third transparent multilayer antenna structure (MLA3) is formed by triple layer Ag sandwiched between 3L and 3S multilayers and double 3S layer. The full form of the third transparent MLA structure can be given as 3L’/Ag/3L/PC/3L/Ag/3S/Ag/3S/Ag/3L’ in compact form. The proposed three MLA structures are fabricated and their optical, electrical and antenna characteristics are measured. The advantage of utilizing multilayer structures in terms of enhanced antenna parameters as well as sufficient optical transparency (≥68%) is demonstrated.