Development and testing of an antenna made of an optically transparent conductive film

Optically transparent antennas are being actively developed for various industrial applications; however, their widespread deployment is often limited by low radiation efficiency. Improving efficiency while maintaining high optical transparency remains a significant challenge. This study presents the design and fabrication of a transparent monopole antenna operating at 2.4 GHz. The antenna was fabricated using a transparent conductive film (TCF) based on a metallic micromesh. The surface impedance of the TCF was experimentally characterized over the 2.0–3.3 GHz frequency range using a waveguide transmission method, yielding a value of 1.25 + j0.33 Ω/sq at 2.4 GHz. A laboratory prototype was produced by laser patterning the TCF, which was subsequently adhered to a PET substrate. Experimental measurements demonstrated a radiation efficiency of 65% at 2.4 GHz. The antenna achieved an estimated optical transmittance of 86% without the use of antireflection coatings. The proposed antenna demonstrates a high radiation efficiency for optically transparent antennas achieving this level of transparency without visible mesh structures, highlighting the effectiveness of the selected material and design approach.

Transparent conductive films (TCFs) offer the unique property of optical transparency with high radio-frequency (RF) conductivity. There is an increased effort to develop high-performance TCFs which can combine improved conductivity at high frequencies, high levels of optical transmittance, and mechanical stability at low-cost (Nie et al., 2021). TCFs are mainly based on transparent conductive and semiconductive oxides or polymers and on meshed non-transparent conductors, while other materials, such as carbon nanotubes (CNTs), graphene film, MXene and even water, have also been used for TCFs development (Sayem et al., 2022). These films find applications in a wide range of areas, from solar panels (Yu et al., 2013) to flexible electronics (Lu et al., 2021).

A novel and increasingly intriguing application of TCFs is the development of optically transparent antennas. These antennas can be seamlessly integrated into various surfaces, such as vehicle and building windows, smartphones, smart lenses, and other structures, with minimal aesthetic impact on the surrounding environment. Their seamless integration can contribute to the growing need for access points and RF components in both indoor and outdoor environments.

Several transparent antennas, from an active RFID tag (Malek et al., 2014) and rectangular patches (Kang and Jung, 2018; Hautcoeur et al., 2011a) to MIMO antennas (Li et al., 2016), have been studied in the literature. A comprehensive review of optically transparent and semi-transparent antennas, developed with various techniques and materials, is presented in (Chishti et al., 2022). The primary hurdle in developing transparent antennas lies in achieving satisfactory efficiency, which is challenged by the inherent lower conductivity of TCFs compared to opaque metals (Sayem et al., 2022). This is because optical transmittance and conductivity are inversely related in typical TCFs, and this trade-off leads to either efficient antennas of low transparency or transparent antennas of low efficiency (Chishti et al., 2022). Some efforts to develop highly efficient and highly transparent antennas have been reported for specific applications (Ding et al., 2019). It remains important and timely, however, to explore simple methods to develop such antennas either by manufacturing improved TCFs or by strategically selecting antenna designs that counterbalance the constraints of available TCFs.

To this end, this paper presents a high-efficiency transparent planar monopole based on a recently developed metal micromesh TCF. The electrical properties of the TCF were experimentally characterized over the frequency range of interest. Accordingly, for antenna design and simulation, the measured high-frequency complex sheet impedance was used instead of the corresponding DC sheet impedance, enabling more accurate modeling of the antenna at its operating frequency (Paraskevopoulos et al., 2024). The antenna geometry was selected and optimized to compensate for the low conductivity of the available materials. To the best of the authors’ knowledge, the proposed antenna exhibits one of the best combinations of high optical transmittance (86%) and enhanced radiation efficiency (measured efficiency of 65%) among metal micromesh-based transparent antennas. This level of performance is sufficient for short-range and near-field wireless applications where unobstructed visibility is critical, such as window-mounted IoT gateways, transparent access points integrated into displays or smart glass, and vehicular or architectural interfaces. These applications demand antennas with high optical transparency, even at the expense of moderate radiation efficiency.

Antennas based on metal micromesh TCFs

As mentioned in the Introduction, various innovative materials have been proposed for the conductive elements of transparent antennas (Sayem et al., 2022). However, most research focuses on two main categories: (i) conductive oxides, such as indium zinc tin oxide (IZTO), and (ii) metallic meshes, typically made from copper (Cu) or silver (Ag). Transparent conductive films (TCFs) based on oxides offer satisfactory optical transmittance and are low-cost and easy to fabricate. However, they have limited electrical conductivity, with DC sheet resistance above 10 Ω/sq, 7 Ω/sq for fluorine-doped tin oxide (FTO) (Shanmuganathan et al., 2025), or as low as 2–5 Ω/sq in IZTO multi-layered films that incorporate sandwiched metal layers (e.g., IZTO/Ag/IZTO), though at the expense of optical transmittance (Hong, Kim, et al., 2016b; Hong, Kang, et al., 2016; Farid et al., 2021; Zhang et al., 2025). Consequently, oxide-based TCF transparent antennas tend to have low efficiency, particularly in the lower microwave spectrum (Hong, Kim, et al., 2016b; Hong, Kang, et al., 2016). Similarly, hybrid graphene-based TCFs incorporating silver nanowires have been proposed to achieve high optical transparency; however, their sheet resistance typically remains above 7 Ω/sq (Hong et al., 2025).

Metal mesh TCFs achieve transparency not through inherently transparent materials but by having a sparse metal pattern that allows light to pass through open spaces. These metal meshes come in two general types, defined by their pitch, s, and metal width, w, (Figure 1A): millimeter-scale meshes (mm-mesh) and micrometer-scale meshes (micromesh) with w lower or equal to 3 μm. Mm-mesh antennas feature good efficiency and transparency, defined as the percentage of open space across the surface (Hautcoeur et al., 2011b; Abbasi et al., 2023). The mm-mesh metal lines are visible to the naked eye, however, and the antennas are typically suited for applications such as photovoltaics, where visual transparency is less critical (Liu et al., 2017; Yasin et al., 2017; Xi et al., 2020). Recently, sub-millimeter-scale mesh structures have been investigated as a means of improving visual transparency while preserving high radiation efficiency; nevertheless, the conductive radiating elements are still perceptible to the human eye (Yu et al., 2025).

In contrast, micromeshes are normally not resolved by the human eye, making them suitable for applications on screens and windows without obstructing visibility. Micromesh TCFs present enhanced optical transmittance relative to both above categories and improved conductivity relative to the conductive oxide and polymer counterparts. Thus, they are excellent candidates for visually transparent antennas. Their commercial use, however, requires further effort on improving both the optical transmittance and the antenna efficiency. As expected, the optical loss is directly linked to the amount of metal on the TCF, and thus, the TCF’s conductivity and the antenna’s radiation efficiency (Nie et al., 2021). A transparent antenna at 2.4 GHz is the edge-slot, mesh-layer design proposed for smart-glasses applications, where a metal-mesh film is integrated into the glass frame and lens stack-up, achieving a measured total efficiency of approximately 40% at 2.4 GHz on free space (Morimoto et al., 2023). Although optical transmittance is not reported, this work demonstrates the practical integration of mesh-based transparent antennas in wearable devices and highlights the associated trade-offs between transparency, material conductivity, and antenna efficiency. One of the highest documented optical transmittance for a metal micromesh TCF used in a transparent microwave antenna is 75% (Xi et al., 2020). Typically, materials with optical transmittance levels of 70%–75% correspond to microwave antennas with radiation efficiencies of approximately 40%–53% (see Table 1). An exception is the dual-polarized, stacked circular patch antenna reported in (Ding et al., 2019) with an 85% radiation efficiency; however, this design includes vias that connect two central metallic patches to ensure a galvanic connection, which partially obstructs visibility. It is important to clarify that the efficiencies of the antennas listed in Table 1 correspond to similar operating frequency ranges, specifically from 2.4 GHz to 2.9 GHz. This is significant because the skin depth—and consequently the sheet impedance of the material—varies substantially with frequency. These variations directly impact the antenna’s performance, making it essential to consider comparable frequency ranges for a fair efficiency evaluation.

In practice, visual transparency without perceivable mesh patterns requires metal line widths below approximately 3 μm, which is close to or below the resolution limit of the human eye at typical viewing distances. However, such narrow and thin conductive traces impose a fundamental limitation on microwave performance. At radio and microwave frequencies, the current is confined to a thin surface layer of the conductor due to the skin effect, with a skin depth given by where ω is the angular frequency, μ is the magnetic permeability, and σ is the electrical conductivity of the metal (Pozar, 2021). For copper at 2.4 GHz, this yields δ ≈ 1.35 μm, which is comparable to the thickness of typical micromesh metal lines. As a result, the effective current-carrying cross section is strongly limited both by the small line width and by the skin depth, leading to an increased surface resistance and therefore higher ohmic losses. This contrasts with millimeter-scale mesh transparent antennas, which are visible to the human eye, where even for similar metal-to-open-area ratios the conductor dimensions are much larger than the skin depth, resulting in a significantly lower effective resistance and reduced conductive losses.

Beyond the width and thickness of the metal lines, the mesh pitch also plays a critical role in determining the electromagnetic performance of the TCF. Increasing the pitch reduces the metal fill factor and constricts the available current paths, leading to an increase in the effective surface resistance. To first order, the surface resistance of a mesh scales inversely with the metal fill factor, such that where F is the fractional metal coverage and R s = 1 σ δ is the surface resistance of a continuous metal sheet (Whitbourn and Compton, 1985). In addition, the periodic wire geometry introduces a distributed inductive reactance due to increased magnetic flux linkage between more sparsely spaced conductors, which contributes to the imaginary part of the surface impedance. Consequently, increasing the mesh pitch raises both the resistive and inductive components of the surface impedance, resulting in higher ohmic losses and degraded antenna efficiency. This explains why meshes with larger pitch, although optically more transparent, exhibit poorer microwave performance.

These losses directly contribute to the loss resistance in the antenna, reducing the radiation efficiency according to where R rad is the antenna’s radiation resistance and R loss is the loss resistance representing the ohmic losses (Balanis, 2016). Consequently, reducing the metal coverage and line width improves optical transparency but also increases the conductive losses and limits the achievable antenna efficiency. This leads to the transparency/efficiency trade-off observed in micromesh-based transparent antennas.

It should be noted that reports of radiation efficiencies approaching or exceeding 90% generally correspond to antennas employing bulk metallic conductors, millimeter-scale meshes, or partially opaque structures that are optically transparent only in terms of open-area ratio but are not visually transparent in the sense of being imperceptible to the human eye. In such cases, the conductor dimensions are much larger than the skin depth, resulting in significantly lower surface resistance and therefore higher radiation efficiency. In contrast, the antenna considered in this work employ micrometer-scale meshes with sub-3 μm line widths specifically to ensure visual invisibility, which inherently limits the effective current-carrying cross section and increases ohmic losses.

In this paper, we design and develop a fully transparent antenna using a metal micromesh TCF with an optical transmittance of 86%, which is higher than that of materials previously used in antenna applications (Table 1). Unlike earlier studies (Hautcoeur et al., 2011a; Hautcoeur et al., 2011b), in which only the radiating element was transparent and the ground plane was realized in opaque copper, our antenna is entirely transparent, with both the radiating element and the ground plane fabricated from TCF. Our goal is to offset the limited conductivity of the TCF through optimized design, thereby achieving enhanced radiation efficiency while maintaining full optical transparency.

TCF characterization

The monopole antenna was fabricated using a TCF comprising copper (Cu)-based micromeshes (1.5 μm thick and 3 μm wide wires) embedded in an ultraviolet (UV) resin and supported on a flexible 58 μm-thick polyethylene terephthalate (PET) substrate (Figure 1B) (Panasonic, 2022). The optical microscope image in Figure 1B (right) shows the rectangular micromesh pattern, confirming that the conductor line widths are on the order of a few micrometers and therefore below the resolution limit of the human eye at typical viewing distances.

The choice of conductor material and metal thickness significantly affects the TCF conductivity, while the substrate properties influence the optical transmittance. In addition to these parameters, both electrical conductivity and optical transmittance depend on the geometrical characteristics of the metallic grid pattern, with hexagonal patterns offering the best trade-off between the two properties (Liu et al., 2014). n general, optical transmittance depends on the ratio of non-metal area to the total TCF surface area (Liu et al., 2014). In this work, a TCF with a rectangular grid pattern and a pitch of 45 μm (Figure 1B) was characterized for antenna development. The TCF was designed by Meta Materials Inc. and produced by Panasonic Industry.

Impedance and DC resistance

The surface impedance of the TCF in the 2.2–3.0 GHz frequency range was measured using the techniques described in (Costa, 2012; Wang et al., 2017). These techniques estimate the surface impedance of very thin conductive films. A pair of WR 340 rectangular waveguides were used to measure the S-parameters of the samples. The samples were cut at 110 mm × 68 mm to fully cover the gap of the waveguides and to fit between the flanges. Transmission and reflection coefficients were measured and used to extract the film’s impedance. We used permittivity value ε r = 3.2 for the substrate for applying the technique in (Costa, 2012), while the PET substrate without the conductive film was also measured to serve as reference for applying the technique in (Wang et al., 2017). The values extracted using the two approaches agree to within 1%. The measured complex surface impedance of the rectangular-patterned TCF is shown in Figure 2.

The DC sheet resistance of the film was measured independently using a four-point probe (four-terminal) method. As expected, the DC resistance is lower than the real part of the surface impedance measured at 2.4 GHz (Table 2), since DC measurements do not account for current confinement due to the skin effect or for the inductive behavior introduced by the mesh geometry at microwave frequencies.

While the DC sheet resistance characterizes the static conductivity of the material and is useful for comparing fabrication quality and material uniformity, it is not representative of the effective conductive losses experienced by the antenna under microwave excitation. At radio frequencies, current is confined to a thin surface layer of the conductor and is further constrained by the narrow and discontinuous nature of the micromesh lines, which significantly increases the effective loss resistance. Consequently, the radiation efficiency of the antenna is governed by the RF loss resistance associated with the real part of the surface impedance rather than by the DC resistance as described in Section 2.1. For this reason, the waveguide-based surface impedance measurement provides a more accurate and relevant characterization of the conductive losses affecting antenna performance.

Optical transmittance

Table 2 also shows the estimated optical loss of the mesh film (PET substrate and metal mesh), which is 6%. This value is based on an empirical model derived from extensive optical transmission measurements on films fabricated with the same mesh design but optimized for spectrophotometric characterization. Optical measurements in the visible spectrum were performed using a JASCO V-770 UV–Vis–NIR spectrophotometer (JASCO, Inc., Easton, MD, USA) prior to antenna assembly.

The spectrophotometer features a broadband Halogen white light source. The white light enters a monochromator, which includes a rotating dispersion grating, sending through a slit nearly monochromatic light, which is split into a reference beam and a measurement beam. After transmission through the sample, the measurement beam is collected from a photomultiplier tube detector, and its value divided by the value of reference beam (through air), to calculate the transmission for each wavelength. This transmission spectrum is then integrated over the visible wavelength range to product the total optical transmittance through the sample (Agilent Technologies, 2025).

The measured optical transmittance was therefore estimated at 86% (14% loss), which includes an 8% reflection loss from the two film-air interfaces.

Transparent monopole development

Antenna design and simulation

The antenna was designed to operate at 2.4 GHz. The main target was to achieve high radiation efficiency despite the TCFs’ limited conductivity, and hence confirm the potential of transparent antennas in real-life telecom applications. Our design aim was to reduce the ohmic losses on the antenna conductors by excluding narrow features and transmission lines, and minimizing the total area covered with TCF.

Based on these considerations, a monopole antenna design with short ground plane was chosen, with a 40 mm × 55 mm x 1.2 mm PET slab with ε r = 3.0 and tanδ = 0.008 used as substrate. The antenna ground plane was 40 mm × 11.5 mm, and the monopoles had length L = 38.4 mm.

The antenna was modelled and simulated with CST Studio Suite® using the experimentally acquired data for the conductor’s surface impedance (Figure 3), whereby the micromesh was represented as a homogenized conductive layer rather than by its explicit geometrical topology.The width of the monopole, W, is a key parameter for maximizing radiation efficiency, since increasing W reduces the effective current density and therefore the ohmic losses on the conductor surface. A parametric analysis was therefore performed in which W was systematically varied. In order to isolate the effect of W on antenna performance while maintaining resonance at 2.4 GHz and a 50 Ω input match, the substrate thickness t was adjusted accordingly for each value of W. Figure 4A shows the simulated reflection coefficient for different combinations of monopole width and substrate thickness, demonstrating that the resonance is maintained at 2.4 GHz for all configurations. Figure 4B presents the corresponding radiation efficiency over the 2.2–2.8 GHz frequency range, showing that increasing the monopole width leads to a systematic improvement in radiation efficiency. Since increasing substrate thickness alone is known to reduce radiation efficiency (Jackson and NG, 1986), this confirms that the observed efficiency enhancement is primarily due to the widening of the monopole strip rather than to changes in the substrate.

In addition, a separate parametric study was conducted to examine the intrinsic variation of the input impedance and resonance frequency when each design parameter is varied independently. Unlike the compensated analysis of Figure 4, in this study the substrate thickness and monopole width were varied individually without retuning the complementary parameter to enforce resonance at 2.4 GHz. The corresponding reflection coefficient results are presented in Figure 5. Figure 5a illustrates the influence of monopole width for a fixed substrate thickness (t = 1.2 mm), while Figure 5b shows the effect of substrate thickness for a fixed monopole width (W = 3.1 mm). These results reveal the natural resonance shift and impedance sensitivity associated with geometric variations, thereby clarifying the underlying design trade-offs.

We note that another approach to improve the radiation efficiency of a transparent antenna is to locally enhance conductivity with bulk metal in areas of high current density to minimize ohmic losses (Song et al., 2008). However, the proposed method of widening narrow current paths in the antenna, such as transmission lines, preserves full transparency.

Antenna fabrication and testing

For the antenna in-lab fabrication the TCF parts were cut with a CNC laser cutter and then adhered on a PET slab with the conductive face up using optically clear adhesive (OCA). The available PET substrate had 1.2 mm thickness, and thus, the monopole was cut at W = 3.1 mm wide. The developed transparent monopole antenna is shown in Figure 5. A clothing pin-like component was designed and 3D printed to attach the SMA connector to the antenna (Figure 6, inset). This technique was used to ensure the galvanic connection of the TCF radiating elements and ground with the SMA connector’s metallic parts without soldering, which would burn the PET components. The 3D-printed pin, developed from Z-HIPS with ε r = 2.2 and tanδ = 0.002 at 2.5 GHz, was specifically designed to encircle the connector. This design not only minimizes the material volume on the antenna’s surface but also ensures that mechanical stability is maintained. While this solution provided stable and repeatable connections during the measurements reported here, a systematic long-term durability study was beyond the scope of this work and is left for future investigation.

The antenna was measured using a balance-to-unbalance transformer (bazooka balun) in an anechoic chamber (Figure 7) at NCSR-“Demokritos” (NCSRD) with a VNA (Anritsu MS46122A), and the results are shown in the following section.

The simulation and measurement results for the reflection coefficient magnitude (|S 11 |) of the TCF monopole antennas are shown in Figure 8. The agreement between simulation and experimental results is good. To further elucidate the loss mechanisms and current distribution behavior, the electric field magnitude and surface current density of the selected configuration (W = 3.1 mm and t = 1.2 mm) were analyzed, as shown in Figure 9. The surface current distribution confirms that the highest current densities are concentrated along the monopole strip and near the feed region, where ohmic losses are most significant.

The simulation results for the far-field radiation pattern of the TCF antenna are shown in Figure 10. From the simulations, the gain of the rectangular patterned TCF antenna was calculated as G R_sim = 1.4 dBi.

The antenna’s measured gain was found G R = 0.6 dBi. The radiation pattern was measured at 2.4 GHz (Figure 11) in the NCSR anechoic chamber. A time-gating measurement technique was used to mitigate the significant unwanted scattering due to the antenna’s omnidirectional radiation pattern. The simulation and measurement results are in good match (Figure 11B). The measured radiation efficiency at 2.4 GHz was found 65% for the antenna with the TCF of 1.25 Ω/sq surface impedance. Although the measured efficiencies of the transparent antennas are high, a discrepancy is observed between the measured and simulated values. This discrepancy is likely due to an underestimation of the film reactance, arising from the micro-structured wire network of the TCF, during the waveguide measurement.

In this study, a metal-micromesh TCF was characterized, and the findings were applied to design, develop, and access a fully transparent monopole planar antenna. The primary objective was to realize an antenna that simultaneously achieves high optical transmittance and satisfactory radiation efficiency despite the limited conductivity of the transparent material. The rectangular-patterned TCF antenna, employing a film with an optical transmittance of 86%, achieved a measured radiation efficiency of 65%, which is the highest reported for antennas entirely constructed from transparent metal-mesh films with optical transmittance exceeding 70% (Figure 12). Figure 12 provides a graphical comparison of the proposed antenna with other micromesh-based transparent antennas reported in the literature, while Table 1 summarizes their key characteristics and enables a direct performance comparison within a similar operating frequency range.

Figure 12 and Table 1 also highlight the critical role of antenna topology in mitigating the intrinsic conductivity limitations of transparent micromesh films. For a given level of optical transmittance, radiation efficiency is influenced not only by material properties but also by the current distribution imposed by the antenna geometry. In this work, a common design, a planar monopole with a short ground plane and a relatively wide radiating element, was deliberately selected to avoid narrow, high-current-density features and lossy transmission lines. This topology reduces effective ohmic loss by distributing the RF current over a larger conductor cross-section while preserving a high open-area fraction for optical transparency. As a result, the proposed design achieves a more favorable balance between radiation efficiency and optical transmittance than many previously reported micromesh antennas, which often employ patch-based or feedline-dominated geometries with longer current paths and higher resistive losses. The added value of the proposed approach therefore lies not only in the material choice but also in the deliberate use of a geometry that counterbalances material losses and shifts the antenna toward a more optimal region of the efficiency–transparency trade-off space.

Additionally, the rectangular patterned TCF exhibits exceptionally high optical transparency. Replacing the PET substrate with materials such as PMMA or glass could further increase the overall optical transmittance of the antenna without altering its electromagnetic behavior.

More broadly, the development of cost-effective and durable TCFs with enhanced conductivity while maintaining high transparency remains a key research direction for transparent RF systems. In parallel, selecting antenna topologies that minimize resistive losses provides a practical strategy for achieving satisfactory RF performance when using low-conductivity transparent materials. Finally, application-specific constraints, such as form factor, integration environment, and operating frequency, further influence the selection of suitable antenna designs.

An alternative or complementary approach involves selectively reinforcing the conductive mesh or locally incorporating conventional metals in regions of high current density, such as the feed or matching sections. This strategy can improve performance in critical areas while preserving the overall visual transparency of the antenna.