Imagine a world where antennas vanish, seamlessly blending into their surroundings. Transparent antennas are turning this concept into a reality. This article examines the technical complexities and approaches involved in designing and manufacturing transparent antennas for high volume production, highlighting the interplay between RF design principles, material science, physics, mechanical engineering and advanced manufacturing processes. There are numerous applications for transparent antennas, from automotive sunroofs to electric vehicle (EV) charging display screens and more to be discovered. By unraveling the art, science and magic behind this invisible technology, this article aims to inspire further advancements, paving the way for a future of covert connectivity.
WHAT IS A TRANSPARENT ANTENNA?
Transparent antennas are made from transparent conductive films and are designed to be virtually invisible to the human eye. These ultra-low-profile, flexible antennas are placed on transparent, nonmetal surfaces, such as glass, plastic or other clear materials, allowing light to pass through without obstructing visibility. Their transparency enables concealed antenna placement and allows them to be placed on various surfaces, which would previously have been undesirable or unusable for other antenna types. Typical applications include glass surfaces such as windows, screens and sunroofs of automotive and commercial transportation, EV charging and parking bays, digital signage and display screens and point-of-sale kiosks.
Figure 1 TFX62.A, a transparent 5G/ 4G cellular antenna.
Taoglas now offers six different Taoglas Invisible Antenna™ products in its portfolio. The TFX series can be used standalone or in a custom combination to enhance cellular, Wi-Fi and GNSS antenna installations. Each antenna comes with a pre-adhered adhesive for ease of installation and has an enclosed carrier terminated with a FAKRA or an SMA connector. Figure 1 shows the TFX62.A, a 5G/4G cellular antenna with coverage from 600 MHz to 6 GHz.
Taoglas has also worked on multiple custom antenna designs based on this patented transparent technology. One custom solution created an 8-in-1 combination antenna, integrating cellular, Wi-Fi, GNSS and other antenna technologies into a single transparent film. In this particular use case, the edges of the transparent film were rounded to fit the unique requirements of the application and to offer a covert appearance.
DESIGN & DEVELOPMENT CONSIDERATIONS
Taoglas first began development of transparent antennas in 2020 and commercially introduced the first three products in the Taoglas Invisible Antenna series in February 2023 with support for cellular (TFX62.A), Wi-Fi (TFX257.A) and GNSS (TFX125.A). The motivation was to design a solution that was as transparent as possible, building upon the company’s technical knowledge in different antenna types, including flexible printed circuit board (PCB) antennas. Flexible antennas are attached via a “peel and stick” process and can be bent, folded or stretched to conform to various shapes and surfaces. This flexibility enables the integration of antennas into unconventional locations, such as the curved edges of a smartphone or the interior of a vehicle. However, these flexible antennas are typically black and would be highly visible on a transparent surface, such as glass.
The core challenge in designing transparent antennas lies in reconciling two seemingly contradictory properties: conductivity and transparency. Conductive materials, essential for efficient antenna performance, typically absorb or reflect light, making them opaque. The more transparent the material, the less conductive it is, which can degrade antenna performance. Transparency is measured in visible light transmission (VLT), which is the percentage of visible light that passes through the material as opposed to being reflected or absorbed. Finding the right balance between performance and transparency is crucial.
Figure 2 Simulated and measured results for copper and transparent material.
Figure 3 The unique PCB adapter board with FAKRA connector solution.
Figure 4 TFX62 antenna efficiency versus frequency.
Taoglas has observed several responses to translating an antenna design from copper to a transparent material. Whether or not a resonance shift occurs is dependent on the design of the antenna. More often than not, a resonance shift is not seen and the impedance response is fairly similar. However, a drop in performance for both antenna efficiency and peak gain can be expected. The designs are also reflected fairly accurately in simulation models, as shown in Figure 2.
Taoglas regularly provides custom antenna solutions to customers. These antennas are designed to optimize RF performance for a specific environment. These projects often involve combining and integrating several different technologies. Taoglas is thus ideally suited to identify technical challenges, propose and evaluate potential solutions and provide expert opinions to ensure technical challenges are overcome.
Several types of materials can be used for transparent antennas. Each presents a different compromise between RF performance and transparency. One of the materials is a metal mesh conductive film that exhibits properties that make it an excellent choice for antenna applications when considering sheet resistance, VLT, power, color and haze.
Transparent films are not solid metals, making it difficult to solder cables directly. For high volume production, a reliable, repeatable and easy-to-manufacture connection method is needed. The design has to ensure that the antenna remained invisible, even with the necessary cables and connections. An electromechanical connection method is most often required to ensure an optimal RF connection is in place. This method is also the friendliest from production and assembly points of view.
Another challenge with transparent films is the difficulty of creating a multi-layer stacked film with a physical electrical connection between these layers. This structure is similar to using via holes in PCBs. This poses a considerable challenge for RF engineers and the ability to create controlled impedance transmission lines such as on-ground coplanar waveguides (CPWG).
Finally, there are mounting and adhesion considerations. The double-sided adhesive must be clear enough to maintain invisibility and provide strong adhesion for the expected product lifetime. For instance, considering the potential placement of the transparent antenna on a car window, the adhesive cannot yellow from the sun’s UV rays or lose strength from excessive heat.