## 1 Introduction
The downsides of the IPT are short operation distances and sensitivity to misalignments, which are the main obstacles to achieving sufficiently high PTE in the suit charging application. Namely, the precise coil placement required to avoid misalignments and large separation is challenging because the Rx coil is attached to a soft material of the suit. The antenna-based power transfer systems [11; 12] show great flexibility owing to high spatial freedom and larger operating distances, making them advantageous in applications where power receivers are mounted high above the ground, e.g., tree health monitoring, high-rise building structural integrity monitoring, etc. [13]. However, high path loss at high frequencies and large antennas at low ones make them unsuitable for smart suit charging. Though adaptive beamforming can mitigate the high path-loss issue, it requires more complex and sophisticated tracking mechanisms and bulky antenna arrays [14; 15]. The conducting walls make cavity-based WPT systems immune to the interference caused by external RF signal sources while causing little interference to electronic devices in the cavity exterior. For this reason, cavity-based WPT systems might be particularly important in applications that benefit from the use of operating frequencies within licensed communication bands. Hence, this WPT technology is expected to be widely applicable in a range of practical scenarios, such as charging closets, toy storage, wireless charging rooms, etc. However, the need for a conducting enclosure confines the use of cavity-based WPT systems to indoor applications. Recently, it was demonstrated that by exciting a cavity with copper walls by electrically small loops ensuring weak coupling with the magnetic field, a PTE of up to 72% can be achieved [16]. Short capacitive dipoles (capacitive probes) also ensure efficient power transfer inside metallic enclosures, as reported in [17]. The coupled mode theory [18] has been shown to provide reasonable PTE estimation accuracy for both the loop- and dipole-based configurations, enabling circuit analysis-based treatment to accelerate the WPT system's optimization [19]. Operating a cavity in a multi-mode regime yields a higher PTE over larger volumes than in the single-mode regime, as reported in [20], making the PTE less sensitive to receiver position and orientations. In [21], the implantable receiver orientation insensitivity was achieved using a biaxial receive resonator system designed for animal studies involving rodents. A retro-directive beamforming-inspired method was proposed in [22] to improve the PTE achieved by an over-moded cavity. Cavities operated in a single-mode [23; 24] or multi-mode [25] sub-wavelength regime (quasi-magneto-static regime) can also ensure sufficiently high PTE and power levels while meeting safety requirements. This enables the deployment of room-scale enclosed WPT systems for charging various electronic devices. In [26], it is shown that a PTE higher than 37.1% can be achieved throughout a 3 m \(\times\) 3 m \(\times\) 2 m enclosure. Operating a resonant cavity at frequencies far above the dominant one, resulting in over-moding, has also been shown to extend the coverage. This can be achieved by affecting the field distribution within an over-moded cavity by varying the operating frequency (frequency stirring) [27] or altering the cavity interior (mechanical stirring) [28]. The effect of several relatively large scattering objects inside a heavily over-moded cavity was studied in [29], showing that it is possible to ensure a PTE of 30% even when there is no Line-of-Sight between the receiver and transmitter. The inability to establish reliable communication links between the cavity interior and the outside world was addressed [30; 31; 32] by using meshed walls. In the present study, the power transfer inside the carbon steel closet is accomplished using a pair of antennas. Figure 1 shows the schematic diagram of the cavity-based (closet-based) WPT system under study.