# Eco-Friendly Metadome-Antenna Innovations for Wearable Millimeter Wave Radar Sensing
## 1 Introduction  
Wearable devices have revolutionized the biomedical field. Force and pressure sensors, ultrasonic sensors and inertia measurement units (gyroscopes, magnetometers and accelerometers) are widely used and thoroughly studied for biomedical applications. However, the use of wearable radar sensors is still largely uncharted despite their potential having been noted in recent years. Fall prevention, especially for successful ageing and long-lasting quality of life, is an example of a research topic that can profit from wearable radar sensors [1]. Assistance for blind or visually impaired people (to overcome daily tasks, e.g., moving and navigating around unfamiliar environments) is another application in vogue, since there is a large number of people affected and the forecasts are for growth due to the aging of the global population. Furthermore, the number of people with vision impairment and blindness is expected to more than double over the next 30 years [2]. Electronic Travel Aid (ETA) systems to assist blind people aim at providing full autonomy (not ensured by a white cane and guide dog), and protect the upper body from collisions. They must be light, comfortable, compact and cost-reduced. Although ultrasonic radar systems have the advantage of reduced cost [3] and, in fact, are much cheaper than the other alternatives to be mentioned, given their wide radiation pattern, they have serious limitations in detecting narrow apertures and smooth surfaces [4, 5]. Systems based on infrared sensors require direct visibility between the sensors [6], which is a serious disadvantage. Optical technologies (e.g., cameras) are extremely sensitive to natural light, which disables them in conditions of reduced visibility and limits theiroverall resolution [7]. Near-field communication (NFC)-based systems are only accurate over very short distances [6]. Combinations of the aforementioned technologies have also been proposed [5; 8], with the drawback of providing more complex and/or expensive systems. However, mm-wave radar-based ETA systems [9] are advantageous in terms of scope, accuracy, and operation capacity in all environmental conditions. The 24 GHz ISM frequency band (comprising the frequencies from 24.05 GHz to 24.25 GHz) is suitable for high-resolution radar applications in foggy, smoky and dusty environments. Antennas are critical in radar performance. For the desired application, aiming to warn the visually impaired person of obstacles located in the short to medium range (up to 2 m), both according to the literature [10] and to recent results [11], antennas with radiation pattern beamwidths of around 30-40 degrees are desirable, given that, as in any radar system, a trade-off between range and coverage area has to be adopted. Grid array antennas (GAAs), advantageous for achieving a high radiating element density while minimizing the number of feeding lines external to the antenna structure, were introduced by [PERSON] [12]. Their microstrip implementation [13] outperforms conventional patch-type radiators in terms of gain, cross polarization control and bandwidth. Other authors [14; 15; 16; 17] contributed to the development of GAAs. Still, their use in mm-wave radar applications is not as well explored as patch-type arrays, and their fabrication on unconventional flexible materials is challenging. Most applications one can think of make it desirable for the antennas to be protected, both from inclement weather and from possible impacts or damage of any kind, whether accidental or intentional. Even more so, wearable antennas require protection. Radomes are protective covers used to shield antennas and other electronic equipment from weather conditions and impacts without interfering with the signals they send or receive. A radome should protect the antenna with minimal impact on its performance. Ideally, it should be electrically invisible (fully transparent and lossless). In practice, it must ensure high transmission and low reflection and absorption within the antenna operation frequency band.