Energy-Based Considerations for Ungrounded Wearable UHF Antenna Design

Abstract

The robustness of wearable UHF-band ungrounded antennas with respect to body-coupling effects is addressed. Two different configurations of single-layer antennas, with different energy density distributions, are presented, and a design criterion to improve their performance with respect to the antenna-body separation is derived. Through an analysis of the antenna electric and magnetic energy density distributions, it is shown that the degradation of the antenna performance due to the proximity of the human body can be reduced if the electric energy density is confined in specific regions far from the antenna border. The proposed design criterion has been validated by numerical simulations and experimental measurements

A Design Rule to Reduce the Human Body Effect on Wearable PIFA Antennas

“…A design criterion to limit the effect of the coupling with the human body, is described in references [5][6][7][8][9][10][11], and consists of using an appropriate ground plane shape and size able to confine the electric energy density of the antenna in a region far from its border. These works, however, give no details about the design of the optimal antenna ground plane, which could allow the designer to obtain the best trade-off between compactness (i.e., wearability) and robustness with respect to the human body coupling.…”

Design Considerations on the Placement of a Wearable UHF-RFID PIFA on a Compact Ground Plane

“…In order to improve the antenna robustness with respect to the human body coupling, the ground plane size must be suitably increased to optimize the antenna performance, without enlarging too much the overall antenna size. The approach discussed in [8], [16] suggests a ground plane extension toward the region where the electric energy density exhibits a peak. Fig.…”

“…It should be noted that the slot dimension is the same for the three prototypes, and it has been optimized to maximize the input impedance matching with a Monza 3 chip [24]. As discussed in [16], [8],  and  can be considered to study the antenna robustness with respect to the distance (d) between the antenna and the human body. The power transmission coefficient is calculated as…”

“…. In order to numerically analyze the interaction between the antenna and the human body, an equivalent model at 866.5 MHz, consisting in a 5-cm-thick material with εr= 2/3, εr_muscle = 36, and σ = 2/3 σmuscle= 0.62 S/m, has been added to the simulation scenario [8]. Thus, the wearable antenna performance robustness has been estimated by means of two metrics,  and  , which are computed as a function of both d and frequency, in a range close to the ETSI UHF RFID operative band (865-868 MHz).…”

“…In particular, the antenna robustness has been assessed by means of two metrics, namely the power transmission coefficient () and the radiation efficiency (), which have been numerically evaluated versus the body-antenna distance. In [8], the analysis has been extended to ungrounded antennas, and a preliminary experimental validation has been carried out by measuring the input impedance variation as a function of the distance between the antenna and a human body phantom. Such a measurement confirmed that the power transmission coefficient () variation can be minimized by extending the antenna layout toward the directions close to an electric energy density peak.…”