# Nearfield Validation of a Flexible Solid Phantom for mmW Antennas

Rossella Rizzo, Giuseppe Ruello, Maxim Zhadobov, Giulia Sacco

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Rossella Rizzo, Giuseppe Ruello, Maxim Zhadobov, Giulia Sacco. Nearfield Validation of a Flexible Solid Phantom for mmW Antennas. BioEM 2025, Jun 2025, Rennes, France. hal-05236168

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# Nearfield Validation of a Flexible Solid Phantom for mmW Antennas

Rossella Rizzo¹,², Giuseppe Ruello², Maxim Zhadobov¹, Giulia Sacco¹

¹Univ Rennes, CNRS, Centrale Supelec, Nantes Université, IETR UMR 6164, F 35000 Rennes

²Department of Electrical Engineering and Information Technology (DIETI), University of Naples Federico II, Italy
[rossella.rizzo@univ-rennes.fr](mailto:rossella.rizzo@univ-rennes.fr), [ruello@unina.it](mailto:ruello@unina.it), [maxim.zhadobov@univ-rennes.fr](mailto:maxim.zhadobov@univ-rennes.fr), [giulia.sacco@cnrs.fr](mailto:giulia.sacco@cnrs.fr)

## INTRODUCTION

Millimeter-wave (mmW) frequencies are gaining prominence in wireless communications, biomedical sensing, and radar applications [1]. More in particular, the 60-GHz unlicensed band has attracted attention for high-data-rate applications such as 5G, and body-centric networks [2], [3]. To avoid testing such devices directly on human beings for ethical reasons and to facilitate experimental studies, phantoms—artificial structures replicating the electromagnetic (EM) human tissue properties—can be employed in antenna characterization and EM exposure assessments. Liquid phantoms are commonly used for in-body applications, such as implantable antenna studies and wireless power transfer systems, where conductivity and permittivity closely replicate biological tissues [4]. Semi-solid phantoms offer tunable electrical and mechanical properties, making them useful for temporary test setups and controlled experiments [5], [6]. Solid phantoms are preferred for near-body and on-body applications due to their structural stability, repeatability, and ability to recreate complex anatomical shapes. These phantoms are particularly relevant for nearfield testing, where the interaction between antennas and the human body must be characterized in realistic conditions [7].

In [8] we proposed the first solid phantom based on electrotextile and silicon charged with carbon powder. The phantom has been optimized in farfield in the 60 GHz band and it has been designed to reproduce the skin reflection coefficient instead of its permittivity. Thanks to its thin and flexible structure it may be used to model curved body parts and to replicate even micro-movements of the body surface. The main scope of this abstract is to determine in which conditions the phantom can be used to reproduce the skin behavior in nearfield. The results will provide insights into the phantom's potential applications in wearable antenna testing and nearfield EM interaction studies.

## NEARFIELD ANALYSIS

Unlike farfield testing, which focuses on radiation characteristics at distances beyond several wavelengths, nearfield testing is crucial for wearable and implantable devices. In these applications, the antenna operates in close proximity to human tissues, significantly affecting impedance matching, radiation efficiency, and propagation behavior. The nearfield region can be further divided into two zones:

* reactive nearfield, the closest region to the antenna, up to $0.62\sqrt{D^3/\lambda}$ for a radiating structure, where D is the antenna's largest dimension. In this zone the electric and magnetic fields are strongly coupled, the energy storage effects dominate, and the antenna's radiation characteristics are highly sensitive to nearby objects.

* radiating nearfield (Fresnel region) up to approximately $2D^2/\lambda$. In this zone the wavefronts begin to form but are not yet fully developed. This region is particularly relevant for wearable antennas, as it influences both radiation efficiency and directional properties.

For near-body applications, understanding antenna-body interactions under realistic conditions is essential to discern the impact of human tissues on the functionality of these devices.

As an example, in this work, a patch antenna operating at 60 GHz is used to study nearfield interactions with human skin and the phantom model of [8]. The objective is to compare the EM behavior of the phantom and a realistic skin model, determining how accurately the phantom replicates skin characteristics in near-body conditions.
**Simulation setup:**

The setup consists of a patch antenna, interacting with a homogeneous skin model, and the multi-layered phantom we designed in [8], in the 50–70 GHz frequency range. The antenna used in this study is a rectangular microstrip patch antenna [Figure 1(a)], designed to operate at 60 GHz. The substrate is Rogers RO4350B, modeled as loss-free to simplify the analysis. The antenna is fed through a microstrip line, and excited using a waveguide port, the substrate is backed with a ground plane. The skin model is represented as a planar, homogeneous layer, characterized by the relative permittivity $\epsilon_r = 7.98(1 - j \cdot 1.4)$ [9], and a thickness of 5 mm. The phantom model is a flat, multilayered structure designed to replicate the reflectivity of human skin [8]. The main layer is a silicone-carbon mixture with a relative permittivity of $\epsilon_p = 13.7(1 - j \cdot 0.36)$ and a thickness of 2.1 mm, backed with an electrotextile that acts as a ground plane [Figure 1(b)]. The composition of the phantom (40% of carbon in weight) and its thickness were optimized to replicate the reflectivity of the air/skin interface. The surface dimensions are 15 x 15 mm for both the skin and the phantom models, which are positioned parallel to the patch antenna at varying distances ($d$) in the 0.1 – 5 mm range [Figure 1(a)]. These distances are carefully chosen to fall within the reactive, radiating, and farfield regions of the patch antenna, as shown in Figure 1(c).

<figure><img src="image_2.png" /><figcaption>Figure 1: (a) Setup of the simulation, (b) realized flexible phantom and (c) definition of nearfield and farfield for the patch antenna.</figcaption></figure>
# RESULTS

Using numerical simulations, the phantom's performance is compared against the skin reference, focusing on key parameters such as S-parameters, radiation efficiency, total efficiency, and radiation patterns.

The $S_{11}$ results indicate that the phantom closely follows the behavior of the skin model, with an increasing agreement as the distance increases [Figure 2(a) and (b)]. As expected, in the reactive nearfield ($d < 0.6$ mm), the absolute error in $S_{11}$ is higher due to the strong nearfield coupling effects and energy storage mechanisms that are difficult to replicate. The highest error is 4.8 dB at 0.5 mm, but, across all the distances, the mean absolute difference remains lower than 0.7 dB in the 50–70 GHz range. When transitioning into the radiating nearfield, the agreement improves, with maximum errors below 2 dB. It is worth noticing that the highest differences appear around the resonant frequency of the patch antenna.

A similar pattern is observed for radiation efficiency [Figure 2(c) and (d)] and total efficiency [Figure 2(e) and (f)]. The phantom exhibits higher discrepancies at small distances, where localized reactive fields dominate, reaching a maximum absolute error of 2.7 dB for radiation efficiency and 2.1 dB for total efficiency at 0.1 mm. However, as the distance increases, the phantom successfully replicates the attenuation and propagation characteristics of the skin model, with errors below 0.2 dB for $d \ge 1$ mm.

## Radiation pattern:

The radiation pattern is crucial for evaluating the antenna's performance in near-body environments. In nearfield conditions, the radiation pattern can be affected by multiple factors, including impedance mismatching, field perturbations, and diffraction effects caused by the presence of an object. Particularly, in the reactive zone, strong field interactions can distort the expected radiation pattern.

To assess the accuracy of the phantom in reproducing the radiation patterns, the mean error in the elevation plane ($\phi = 90°$) was analyzed across different distances and frequencies, as summarized in Table 1. The results indicate that the mean absolute errors are generally higher in the reactive nearfield ($d \le 0.6$ mm), but decrease significantly beyond this range, following the same trend observed in the $S_{11}$ and efficiency analysis. For example, at 50 GHz, the highest error is 1.2 dB and is observed at 0.1 mm, which is expected due to the strong reactive interactions. However, beyond 0.5 mm, the error stabilizes around 0.5–0.6 dB, suggesting that at moderate distances, the phantom effectively reproduces the radiation behavior of human skin. At higher frequencies (60–70 GHz), the mean error remains within the 0.4–0.8 dB range across all distances, indicating better overall agreement between the phantom and skin model. The lowest observed error occurs between 60 GHz and 65 GHz, suggesting that the phantom performs particularly well near the designed resonance frequency of the patch antenna.

These results show that while nearfield interactions are more complex to model at extremely short distances, the phantom still provides a reasonably accurate representation of near-body effects.
<figure><img src="image_3.png" /><figcaption>Radiaition efficiency - Reactive nearfield</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Total efficiency - Reactive nearfield</figcaption></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>Radiaition efficiency - Radiating nearfield</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Total efficiency - Radiating nearfield</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Figure 2: Comparison between the skin and the phantom models of S11, a) and b), radiation efficiency, c) and d), total efficiency, e) and f).</figcaption></figure>
<table><caption>Table 1: Mean absolute error of the phantom compared to the skin in the radiation pattern as a function of the frequency and the distance.</caption><thead><tr><th></th><th>0.1 mm</th><th>0.5 mm</th><th>1 mm</th><th>2 mm</th><th>5 mm</th></tr></thead><tbody><tr><th>50 GHz</th><td>1.2 dB</td><td>0.5 dB</td><td>0.5 dB</td><td>0.5 dB</td><td>0.6 dB</td></tr><tr><th>55 GHz</th><td>0.7 dB</td><td>0.6 dB</td><td>0.5 dB</td><td>0.6 dB</td><td>0.5 dB</td></tr><tr><th>60 GHz</th><td>0.5 dB</td><td>0.4 dB</td><td>0.5 dB</td><td>0.5 dB</td><td>0.5 dB</td></tr><tr><th>65 GHz</th><td>0.4 dB</td><td>0.5 dB</td><td>0.4 dB</td><td>0.7 dB</td><td>0.6 dB</td></tr><tr><th>70 GHz</th><td>0.5 dB</td><td>0.5 dB</td><td>0.5 dB</td><td>0.8 dB</td><td>0.6 dB</td></tr></tbody></table>

## CONCLUSIONS

The results of this study demonstrate that the silicone-carbon-based solid phantom that we proposed in [8] for farfield applications, can be a reasonable substitute for the skin for nearfield characterization at mmW frequencies. Despite higher errors in the reactive nearfield, due to strong field coupling and energy storage effects, the phantom maintains a good level of accuracy. Considering all evaluated parameters—S₁₁, radiation efficiency, total efficiency, and radiation pattern—errors were highest in the reactive nearfield, but the mean absolute error remained below 1.6 dB across all the distances. In the radiating nearfield and beyond, deviations decreased, particularly in the 60–70 GHz range, demonstrating that the phantom effectively replicates the EM behavior of human skin in near-body conditions.

Overall, these findings validate the use of this solid phantom for nearfield antenna testing, particularly in wearable device applications. Its flexibility enables it to accurately replicate curved body parts and complex anatomical structures, allowing for testing in realistic scenarios that account for surface curvature and micro-movements of the body.

## ACKNOWLEDGMENT

This project has received funding from the European Union's Horizon Europe research and innovation program under the grant agreement N°101063966 (Marie Skłodowska-Curie IN-SIGHT project), from France2030 EUR DIGISPORT program under the grant agreement ANR-18-EURE-0022 (WEARSPORT project), from the European Union through EU Regional Development Fund (ERDF), Ministry of Higher Education and Research, CNRS, Brittany Region, Conseils Départementaux d'Illeet-Vilaine and Côtes d'Armor, Rennes Métropole, and Lannion Trégor Communauté, through the CPER Project CyMoCod, from the French National Research Program for Environmental and Occupational Health of Anses (ANSES-23-RF-07), and from the University of Rennes through the Defis Scientifiques (E-VISION project).

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