Eco-Friendly Metadome-Antenna Innovations for Wearable Millimeter Wave Radar Sensing

Featured Application

mm-wave wearable radar Electronic Travel Assistance (ETA) systems to aid visually impaired individuals and provide full autonomy.

Abstract

A compact and low-cost meta-radomized wearable grid array antenna (MTR-GAA) for radar sensing application at 24 GHz is presented. It is based on eco-friendly aluminum-cladded Polypropylene (PP) substrate. The overall MTR-GAA size is 40 × 40 × 1.74 mm³. Prototypes are fabricated and tested, achieving consistent agreement between simulation and measurements and meeting typical requirements for the envisioned Electronic Travel Aid (ETA) radar sensing applications to aid visually impaired people. A comparison with state-of-the-art 24 GHz wearable radar antennas is also provided to endorse the advantages of the proposed metadome-antenna ensemble for the target application.

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 their overall 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 Kraus [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. Metasurfaces (2-dimensional counterparts of metamaterials with sub-wavelength thickness) are useful to control the electromagnetic response for a wide scope of applications in RF and microwave sensing [18,19,20]. In recent years, metasurfaces have been proposed to be combined with radomes, to reduce their negative effect on antenna performance, or to be used as radomes themselves [21], named metaradomes or metadomes [22].

The increasing amount of waste electrical and electronic equipment (WEEE or E-waste) has become an obstacle in reducing the ecological footprint. Eco-friendly materials [23,24,25,26,27,28,29,30] and manufacturing technologies are solutions paving the way towards circular economies and green technologies. A few mm-wave antennas on eco-friendly substrates [31,32] were recently proposed, but the metallic parts were left out of such consideration. Moreover, sensitivity to moisture and low radiation efficiency prevent suitability for wearable applications [31]. Recent research [33,34] has shown that polypropylene (PP) outperforms widely used commercial antenna materials, such as RO3003, for medium- to long-range anti-collision radar applications. The present contribution aims to show that, in addition, PP can be used to design a metaradome, achieving an advantageous meta-radomized antenna, in this case for short-medium range applications. Accordingly, a wearable meta-radomized grid array antenna (MTR-GAA) for ETA application at 24 GHz, based on eco-friendly materials (Polypropylene (PP) substrate and aluminum conductor), is presented.

The development of a 24 GHz grid array antenna protected by a metaradome, both fabricated from eco-friendly materials, represents a significant advancement in radar technology for Electronic Travel Aid (ETA) systems designed to assist visually impaired individuals. This innovative approach leverages polypropylene (PP) as the dielectric substrate and aluminum for metallization, ensuring both environmental sustainability and high performance. The use of eco-friendly materials addresses the growing demand for green technologies, reducing the ecological footprint of electronic devices. Additionally, the integration of a metaradome enhances the durability and reliability of the antenna system, providing robust protection against environmental factors while maintaining optimal signal integrity (it is worth noting that wearable antenna designs in the literature do not include a radome, which makes them very prone to damage or even unusable in realistic applications). This combination of sustainable materials and advanced design not only meets the technical requirements for high-frequency radar applications but also sets a new standard for environmentally conscious engineering in assistive technologies. The proposed solution is particularly novel in its application to ETA systems, offering a practical and sustainable means to improve the mobility and safety of visually impaired individuals.

The manuscript is organized as follows: the eco-friendly materials (both dielectric substrate and metallic conductor) selected for the devices are presented first. The design and performance of a metaradome operating at 24 GHz is explained after. Then the design of the antenna is presented. The combination of the antenna with a conventional radome and with the metaradome is analyzed next. Afterwards, the fabrication of prototypes is shown and measurement results are provided, as well as an analysis of their matching with the simulation ones. Then, a comparison with the state of the art on 24 GHz radar antennas is provided to back up the benefits of the featured MTR-GAA ensemble. Finally, some conclusions are drawn.

2. Materials and Methods

This section details the materials utilized in the design and subsequent fabrication of both the antenna and the metasurface. The designs of the antenna and metasurface, conducted through 3D electromagnetic simulation using commercial software, are subsequently presented. The most significant operational parameters of the antenna and metasurface individually, as well as in combination, are shown and confronted with the operational requirements for the intended application.

2.1. Eco-Friendly Materials Used for the Antenna and the Metasurface

Both dielectric and conductor materials used for the sensor (antenna with metaradome) are eco-friendly. It should be noted that, in general, the focus so far has been mainly on reducing the environmental impact of dielectrics, while for metals, the focus has been more conformist, with the sole focus on compliance with RoHS (Reduction of Hazardous Substances) and not so much on whether they are also environmentally friendly or, moreover, whether they truly contribute to the circular economy.

2.1.1. Eco-Friendly Dielectric Substrate: Polypropylene (PP)

Polypropylene (PP) is a flexible, light, resistant to fatigue, inherently hydrophobic (due to its nonpolar chemical structure) and eco-friendly thermoplastic polymer. PP does not release as many toxins and CO₂ as other plastics (PVC, PET and PS) and can be recycled multiple times [24,25,26]. It is much cheaper than the dielectrics commonly used for antennas. As a reference, an A4-like-sized panel of RT-Duroid 5850 costs more than 400 euros whereas an A4-sized PP sheet cost only cents of a euro. Furthermore, neither PTFE (Teflon^®), ABS, PVC, PLA, and ASA plastics nor KYDEX, FRP, synthetic foams, prepregs, or ceramic materials used for radomes are eco-friendly.

For conventional radome applications the material properties to consider include: low relative dielectric permittivity, dissipation factor (loss tangent) and water absorption, high impact resistance, strength and modulus, weatherability, rain erosion and UV resistances, surface hydrophobia, as well as machining and fabrication characteristics. PP properties make it an excellent option.

PP was introduced as a substrate for antennas in [27], achieving a device operating in the low range of microwave frequencies (more specifically at 2.45 GHz) and other authors successfully used it thereafter [28,29,30]. Recently, its use at mm-wave frequencies has been explored [32,33,34]. However, its suitability for metaradomes has not yet been investigated and there are also pending challenges for mm-wave antenna applications. In fact, it must be taken into account that, at high frequencies, any small variation in the dimensions of the device (antenna, metaradome) during manufacturing can result in a significant shift in the operating frequency band, resulting in a useless device, hence the relevance of being able to fabricate with suitable techniques, in addition to starting from a correct electromagnetic characterization of the novel material at the design stage.

The electromagnetic properties of PP in the 24 GHz ISM band have recently been determined [33] using a combination of resonant cavity, microstrip line and T-resonator methods. The resulting values, relative dielectric permittivity ε_r = 2.2 and loss tangent tanδ = 0.002 are used in this contribution.

2.1.2. Eco-Friendly Conductor: Aluminum (Al)

Aluminum (Al) is widely used in many day-to-day objects since it is highly abundant, relatively cheap, has adequate mechanical strength and low density, and resists corrosion. It is soft, ductile and malleable.

For electronics, it is highly conductive, lighter and more flexible than copper, recyclable and eco-friendly. In fact, it is one of the most environmentally friendly metals on earth and can be recycled infinitely to create the exact same product, requiring less energy and time than other metals (recycling aluminum only requires 5% of the energy it takes to produce it the first time around and its CO₂ footprint is close to zero) [35], which paves the way for a truly circular economy. Hence, it is a good alternative for conductive parts of wearable antennas and metaradomes, despite the fact that it cannot be soldered with ordinary soldering irons.

2.2. Design of the Metasurface-Based Radome

Starting, first of all, with what refers to an ordinary radome, there are five basic styles defined according to dielectric wall construction: (1) Monolithic half-wave wall solid, (2) thin-wall monolithic (with thickness ≤ 0.1 λg at the highest operating frequency); (3) A-Sandwich multi-layered wall (a core layer between two skin layers with relative dielectric permittivity higher than the one they overlay, at 0.25 λg thickness); (4) Multi-layered wall comprising five or more layers (odd number of high-permittivity layers and an even number of low-permittivity core layers. The broadband performance improves as the number of layers is increased); and (5) other wall constructions not fitting into the aforementioned ones, including the B-Sandwich that comprises a high-permittivity core between two low-permittivity skins.

The monolithic half-wave wall radome at 24 GHz requires a thickness of 4.1 mm for PP and being arranged at least 6.21 mm from the antenna, which is not convenient at all for a wearable application because it increases the thickness of the device too much, making it bulkier, heavier, less discreet and less comfortable to wear. It goes without saying that all other styles of multilayer radomes (styles 3 to 5) are not suitable for the intended purpose either, both because of the thickness required and consequent weight and lack of comfort, and because of the complexity and higher cost of their implementation. On the other hand, a thin-wall monolithic radome, requiring a thickness ≤ 0.83 mm if PP-based, will be considered for comparison with the metasurface based radome approach, which is proposed as a much more convenient solution to protect the antenna without performance degradation for the target ETA sensing application.

To achieve an operational metasurface design suitable for use as a radome in the 24 GHz frequency band, a systematic procedure has to be followed. First, the specific performance requirements for the intended application must be defined: transmission should be maximized (S₂₁ (dB) as close to 0 dB as possible) and reflection minimized (S₁₁ (dB) bellow −10 dB) across the target bandwidth (24.05–24.25 GHz), while aiming to maximize angular stability (S₂₁ and S₁₁ meeting the aforementioned requirements for the highest possible incident angles, for both TE and TM polarized incident plane-waves. Next, the geometry of the unit cell has to be chosen (square unit-cell in this case) and the specific geometry of the unit-cell metallization defined. To achieve a metasurface that resonates at the high frequency of 24.15 GHz and is suitable for fabrication with conventional microwave circuit techniques, thereby keeping costs affordable, the metallization geometry cannot be very complex. Moreover, the designer must conceptualize an equivalent LC resonant circuit for the dielectric-backed metallization geometry (which will be further explained, along with the metasurface operation). In this case, an aluminum metallic cross on a PP dielectric slab has been chosen. Once the unit cell geometry and the materials are chosen (implying a specific thickness, relative dielectric permittivity and loss tangent for the dielectric, and conductivity for the metallic parts), the dimensions are optimized using electromagnetic software to meet the desired specifications. Iterative simulations and adjustments are crucial to refine the design. Throughout the iterative simulation process, potential size constraints of the geometric elements must be considered to ensure compatibility with the chosen fabrication technique. This aspect of the design process relies not only on specifications provided in datasheets of the machine (milling machine in this work) and/or the materials but also on the designer’s experience and understanding of the mechanical properties of the selected materials

The metasurface unit-cell geometry (Al cross on PP slab) and dimensions, as well as the simulation set-up using Floquet ports and master–slave boundary conditions, are shown in Figure 1. It is remarkable that, according to the retrieved transmission and reflection coefficients for both TE- and TM-polarized incident plane waves (see Figure 2), very high transmission and very low reflection is achieved in the target band (24.05 GHz–24.25 GHz), as intended.

The operation of the metasurface can be explained on the basis of a simple LC parallel circuit. The metallic strips have an inductive behavior (either seen from the point of view of a TE-polarized or TM-polarized incident plane wave), while the dielectric between the strips gives rise to a capacitive effect, which can be relatively weak if the distance between the strips is large or, more relevant, if it is reduced. A parametric analysis, to clarify the metasurface behavior and to identify the basis on which the optimal design values have been obtained, reveals the following: the lengthening of ls (and, therefore, of p), causes a downward shift in the operating frequency at which reflection (S₁₁) is minimum and transmission (S₂₁) is maximum, due to its direct effect on an increase of the equivalent inductance. On the other hand, increasing Ws results in a reduction of the equivalent inductance, which not only shifts the operating frequency upwards, but also slightly worsens the transmission (reducing S₂₁) and reflection (increasing S₁₁), because it also slightly modifies the capacitance by increasing it. Finally, as might be expected, reducing the thickness (h) of the dielectric, in addition to causing the metasurface to act as a radome (metaradome) at a higher frequency, improves its operation by reducing reflectivity (lower S₁₁) and, consequently, increasing transmission (higher S₂₁), making it more transparent. Taking all these considerations into account, in addition to the available PP thicknesses, the optimized design of Figure 1 has been achieved.

The angular stability is a key property for a metasurface usable as a radome. According to Figure 3, the metaradome (MTR) is fully stable under oblique incident TE-polarized plane waves (up to θ = 48°) and highly stable for TM-polarized plane waves (up to θ = 26°), considering both transmission and reflection coefficients in the target band.

2.3. Design of the Grid Array Antenna for Radar Sensing

In a GAA, the directivity increases as the number of loops rises, but this reduces the bandwidth. Therefore, it is necessary to adopt trade-offs between directivity and bandwidth, which has implications for the number of loops. High directivity is required for long distance detection, whereas wider beamwidth is preferable for detection at shorter distance when illuminating the whole scene at a time. As mentioned in the introduction, for the target application aiming to detect obstacles up to 2 m, pattern beamwidths of around 30–40 degrees are pursued. The GAA comprises a metallic radiating grid on a dielectric slab backed by a metallic ground plane. The radiating grid of the GAA (see Figure 4) comprises four rectangular loops of conductors on the top of a dielectric substrate backed by a metallic ground plane. The short side of the loops with length W = λg/2 (being λg the guided wavelength at the center frequency of operation [13]) acts as a radiator. This yields nine radiating elements in total. The other requirement for the GAA resonance is that the length of the long sides should be approximately l = λg. The losses in the long side of the loops (acting as transmission line) and the radiation level of cross-polar components are controlled by Wl. The radiation coefficients of the array and hence the width of the aperture and the side lobe level (SLL) depend on Ws. To fix and flatten the antenna for measurements, there are four holes of 2 mm diameter at the four corners of the substrate. The radiating grid is fed near its center by a coaxial probe with 0.38 mm, 0.5 mm and 2 mm diameters, respectively, for the probe, inner and outer conductor, and with Teflon filling.

PP dielectric is considered as the substrate and aluminum (Al) as the conductor for the radiating grid and the ground plane. For the antenna optimization, starting from the calculation of the guided wavelength λg at the central frequency of the band (24.15 GHz) on the PP material, which results in 9.85 mm, the initial values of W and l are established for the antenna resonance at that frequency, while Ws and Wl take initial values that consider manufacturing constraints. Through iterative parametric sweeps with FEM-based electromagnetic simulation software, W and l are adjusted to set the band, and once this is achieved, Ws and Wl are modified to optimize the radiation pattern. The resulting optimized dimensions are indicated in Figure 4.

2.3.1. Impedance Matching of the GAA

The S₁₁(dB) obtained in simulation for the optimized dimensions is shown in Figure 5. Very good impedance matching is achieved for the designed GAA in the target 24.05–24.25 GHz band, with close to 4% bandwidth (see

Table 1. Frequency band and bandwidth of the array antenna on PP.

Frequency Band
Freq (GHz)		BW
fLow	fUp	Total (MHz)	%
23.60	24.51	910	3.8

).

2.3.2. Radiation Characteristics of the GAA

The simulation results regarding peak realized gain (G), peak directivity (D), radiation efficiency (η) and the front-to-back ratio (FTBR) of the GAA, for the center and end frequencies of the target band, are indicated in

Table 2. Radiation property results obtained in simulation.

Freq (GHz)	G (dBi)	D (dB)	η (%)	FTBR (dB)	SLL (dB) φ = 0°	HPBW (°) φ = 0°	SLL (dB) φ = 90°	HPBW (°) φ = 90°
24.05	14.2	14.4	94	22.9	−30	30	−15	40
24.15	14	14.3	93	22.9
24.25	13.7	14.2	89	22.9

. High levels of G (≥13.7 dBi) and D (≥14 dB) are achieved, endorsing the suitability of the GAA as short- to medium-range radar sensor. Furthermore, η and FTBR are key for wearable applications and the obtained levels, η ≥ 89% and FTBR ≥ 22.9 dB, are optimal.

Broadside radiation, with low side lobe level (SLL) and a half-power beam width (HPBW) ≥ 30° for Phi = 0° and Phi = 90° pattern cuts (see Figure 6) is achieved at 24.15 GHz (band center frequency), as indicated in Table 2, which is suitable for the aimed ETA application. Furthermore, broad 3 dB gain-drop bandwidth (11%) is obtained.

The above results account for both the finite conductivity and thickness (33 μm) of the Al, which reduces 0.4 dB the G and 5% the η with respect to ideal simulation using a perfect electric conductor (PEC).

2.4. Metaradome-GAA Combination

As already mentioned in Section 2.2, a conventional radome optimized at 24.15 GHz should be 4.1 mm thick if PP-based and be arranged at 6.21 mm from the GAA. This is rather bulky and uncomfortable for the target wearable application and that is why its use is ruled out. In any case, according to simulations, it would provide G = 12.9 dBi, η = 88% and FTBR = 25.7 dB. A thin-wall monolithic radome (hereinafter referred to as radome) and a metaradome (MTR) at much closer distances are proposed instead. Both are h = 0.52 mm thick and are arranged at hr distance above the GAA (see Figure 7).

The radome has to be at hr = 1.4 mm to provide 88% radiation efficiency, whereas the MTR can be much closer, hr = 0.7 mm, for identical η and similar G and FTBR (see

Table 3. Radiation properties vs. height from GAA.

	GAA+Radome			MTR-GAA
hr (mm)	G (dBi)	η (%)	FTBR (dB)	G (dBi)	η (%)	FTBR (dB)
0.3	12.0	69	23.2	12.3	82	21
0.5	12.8	77	24.4	13.1	87	21.5
0.7	12.3	81	23.5	13.4	88	22.7
0.9	13.4	83	23.8	13.6	88	24.7
1.2	13.7	86	23.9	13.8	88	25.1
1.4	13.8	88	24.5	13.6	87	25

). However, the MTR further increases SLL, but keeps it ≤−15 dB and preserves the HPBW. At low hr the radome mismatches the GAA more than the MTR.

3. Results

This section describes the fabrication of the GAA and MTR prototypes as well as the measurement process and the achieved results.

3.1. Fabrication

Adhesive backed Al-tape with 33 μm thickness was used for the metallic parts of the GAA and for the cross metalization geometry of the metaradome. Both the GAA and the metaradome prototypes on aluminum-cladded PP were fabricated using conventional milling machining with LPKF ProtoMat H100 machine (LPKF, Garbsen, Germany) (see Figure 8) instead of laser micromachining (proven very suitable for simple geometry patch type array [32]) to avoid adhesive residue getting stuck on the many slots when the laser heats the aluminum before removal.

The fabrication of each prototype using the LPKF ProtoMat H100 involves several key steps. The designs created using FEM simulation software are exported in Gerber format along with the drilling files in Excellon format. These files are then imported into CircuitCAM software from LPKF, which converts the design data into milling paths. Any necessary adjustments to the design, such as modifying line widths or adding aluminum areas, are made at this stage. Subsequently, the BoardMaster software from LPKF is used to control the ProtoMat H100, where the processed files are loaded, and the milling tools and parameters are configured according to the design specifications. The PCB material (Al-Cladded-PP) is then secured in the ProtoMat H100, and the milling process is initiated via BoardMaster, allowing the machine to execute the milling paths and drill the holes as per the design. Upon completion of the milling process, the PCB is inspected to ensure there are no errors.

Silver loaded epoxy adhesive and hardener are used to fix by hand an SMA connector operating up to 26 GHz, to feed the GAA. The fabricated prototypes are shown in Figure 8.

3.2. Measurement

3.2.1. Impedance Matching of the GAA Prototypes

The GAA prototype alone and when covered by the radome and the MTR using a 0.9 mm thick foam slab as separator (in blue in Figure 9) has been measured and proper impedance matching in the target 24.05–24.25 GHz band is achieved. Simulations do not account for connectorization effects and manufacturing tolerances, which cause a slight shift downwards in frequency. From Figure 9, the impedance matching bandwidths obtained from measurements for the prototypes can be derived. The results are 961 MHz (4%) and 950 MHz (4%) for the GAA and MTR-GAA, respectively, while the result is 716 MHz (3%) for the GAA+radome. The radome degrades the GAA matching regarding S₁₁ level and bandwidth reduction.

3.2.2. Radiation Characteristics of the GAA Prototypes

The measurement set-up in anechoic chamber (see Figure 10) includes a 3D-printed, polylactic acid (PLA)-based piece for fixing the prototypes. Both Phi = 0° and Phi = 90° pattern cuts (see Figure 11) exhibit HPBW and SLL almost identical to simulations [36], with slightly rising XP levels, but still good for the envisioned application. Intercomparison [37] with a Flann Microwave Standard Horn 20240-25 was used to measure the gain, and the results are indicated in

Table 4. Radiation properties results in measurements.

Freq (GHz)	G (dBi)
	GAA	GAA+Radome	GAA+Metaradome
24.05	12.5	11.7	12.7
24.15	12.5	11.5	12.5
24.25	12.1	11.0	12.0

.

Regarding radiation efficiency, the following results have been obtained: 82% for the GAA and MTR-GAA, and 64% for the GAA+radome. This indicates that the radome significantly impairs the antenna’s performance in terms of radiation efficiency.

The S₁₁ of the GAA is shifted down in frequency, and so is the band for agreement in terms of gain (G). The discrepancies between simulation and measurement results in terms of │S₁₁│, gain, pattern deformation and XP level rise are mainly attributable to manufacturing tolerances and connectorization (slight manual shifting of optimal position and the crucial curing of the conductive paste). The effect of a drop of silver-loaded epoxy adhesive and hardener for connector fixing reduces by 0.4 dB the gain [34] compared to an ideal simulation. Also, the cable, reflections in the set-up and misalignment with the horn can disturb the G results [37].

4. Discussion

Most 24 GHz radars aim at avoiding collision at >2 m distance and so high gain (G > 15 dBi) and pencil-beam pattern are pursued for the antennas [16,32,33,34,35,36,37,38,39,40,41,42], in contrast to the current target application. Moreover, recent antennas in literature operating at 24 GHz are mostly based on substrates with relative dielectric permittivity higher than 2.9 [43,44,45,46,47,48]. In general, using substrates with higher permittivities helps to miniaturize the devices, but it also reduces the bandwidth in microstrip technology and favors surface waves (more so, the higher the thickness). In addition, at millimeter wave frequencies the devices are already physically quite small, so this is not a plus, and in fact may present more manufacturing challenges. Furthermore, when a fair comparison is to be made, it seems more logical to conduct it considering antennas on substrates with dielectric permittivities as close as possible to the one proposed. Confronted to antennas on dielectrics with ε_r values close to the PP one (see

Table 5. Size, relative dielectric permittivity, bandwidth and impedance matching.

Ref.	Size (mm3)	εr	BW (GHz)	BW (%)	S11 (dB)
[16]	60 × 60 × 0.787	2.2	1.99	8.2	−34
[31]	20 × 20 × 0.68	2.9	2.0	8.3	−35
[32]	98.7 × 14.4 × 0.52	2.2	0.28	1.2	−25
[34]	60 × 60 × 0.52	2.2	0.96	4.0	−26
[37]	74.9 × 74.9 × 3.3	-	0.2	1.0	−8
[39]	160 × 43.2 × 0.5	2.3	0.32	1.3	−20
[40]	230 × 31 × 0.254	2.17	0.39	1.6	−20
[41]	73.5 × 55 × 0.254	2.2	0.83	3.4	−42
[42]	143 × 18.5 × 3.625	2.54	2.25	9.7	−17
[49]	32.4 × 15 × 1.016	2.2	2	8.0	−28
[50]	24 × 21 × 43.3	2.2	2.75	11.7	−38
[51]	40 × 6 × 0.8	2.2	4.44	18	−20
[52]	86 × 86 × 0.254	2.2	0.77	3.2	−25
This work	40.8 × 40.8 × 0.52	2.2	0.94	3.9	−24
This + MTR	40.8 × 40.8 × 1.74	2.2	0.70	2.9	−35

and

Table 6. Radiation properties.

Ref.	G (dBi)	η (%)	SLL (dB) φ = 0°	HPBW (°) φ = 0°	SLL (dB) φ = 90°	HPBW (°) φ = 90°
[16]	20.6	-	−15	16	−25	16
[31]	7.4	35	-	54	−25	48
[32]	16.8	91	−18	9.2	−25	64
[34]	20.8	98	−17	15	−20	16
[37]	17	-	−20	12	−20	50
[39]	17	-	−18	8.2	−18	80
[40]	21.7	60	−21	3.6	−21	46
[41]	22.7	96	−21	13	−23	10
[42]	18.7	-	−19	8	-	-
[49]	8	-	−10	60	−20	30
[50]	20.6	88	−15	15	−15	15
[51]	9.8	87	-	-	-	-
[52]	25.9	-	−20	15	−24.5	13.3
This work	14	93	−30	30	−15	40
This + MTR	13.4	88	−20	30	−15	40

), the GAA is thinner than [16,31,38,42,49,50,51] and more compact than [16,32,33,34,35,36,37,38,39,40,41,42,52]. It exhibits wider bandwidth than [32,39] and equal to [34] for identical thickness being more compact. It almost doubles the gain of [31,49], being thinner. Furthermore, it provides very high radiation efficiency (even using Al with thickness instead of PEC, as usual and in [34]) which is crucial. The MTR-GAA preserves the HPBW of the GAA with high gain, radiation efficiency (it overcomes [31,40], and equals [50,51] neither of which has the protection of a radome) and proper bandwidth.

5. Conclusions

A compact, light, flexible and low-cost meta-radomized microstrip grid array antenna (MTR-GAA) was designed and fabricated on eco-friendly Al-cladded PP, providing suitable matching and radiation properties for wearable ETA radar application at 24 GHz.

The use of the metadome to protect the antenna allows for obtaining a device (MTR-GAA ensemble) that is even more compact than the monolithic thin radome and with better performance, preserving the good conditions of impedance matching bandwidth and radiation properties of the starting GAA.

The MTR-GAA based on Al-cladded PP can be fabricated with common printed circuit board techniques (milling machining); therefore, the cost is not increased compared to using conventional materials that are neither environmentally friendly nor flexible.

Moreover, PP is hundreds of times cheaper than commercial dielectrics with relative dielectric permittivity values in the same range.

In addition, the assembly of the MTR-GAA ensemble by means of screws facilitates both the reuse and the disassembly and subsequent recycling of the devices.

Furthermore, the recyclability of both PP and Al materials contributes to the circular economy and reduces the environmental footprint.