## 1 Introduction  
Due to the restrictions on cost, reliability, productivity, safety, and robustness, as well as the requirement for a good user experience in advanced communications and wireless body area networks (WBAN) for health care, health monitoring, sensing, and athlete training, etc., a tremendous effort has been devoted to minimizing traditional rigid antennas [1; 2]. However, as a compromise, the miniaturization sacrifices the antenna performance since the antenna gain is essentially related to its dimensions [3]. Alternatively, flexible wearable antennas based on textiles [4; 5; 6], polyimide [7], PDMS [8; 9; 10], etc., are supposed to be good candidates for maintaining good performance and user experience while relieving the dimension restriction. For instance, [6] reports a wearable antenna for energy harvesting applications; however, the reported antenna has a low front-to-back ratio (FBR) without ground shielding. It is well known that the unshielded design results in a significant gain decrease (e.g., it decreases by a factor of 2 or more in [6]) due to the coupling and power absorption caused by the vicinity loading of the lossy human body. In contrast, patch antennas are reported to be insensitive to the human body and can obtain high FBR due to their shielding ground, which can benefit wearable applications. Their merits of low profile and low cost are also favorable for massive usage. Additionally, one needs to address the issue of polarization mismatch, which inevitably happens due to unpredictable movements in realistic circumstances when linearly polarized (LP) antennas are used. Hence, CPantennas are a more appropriate choice [11; 12; 13; 14; 15; 16]. Ref. [11] reports an S-shape slot CP antenna with a narrow 3-dB AR bandwidth (1.23%) for Iridium and GPS communications. Ref. [12] presents an omnidirectional button antenna using conceptual magnetic dipoles for 5 GHz applications. There are some other wearable CP antennas designed using a low-profile approach since clothes mostly have a thin thickness [17; 18; 19; 20; 21; 22]. However, according to the above-mentioned literature, most of these wearable antennas have a limited CP bandwidth and a degraded performance in harsh environments. There are some antennas with bandwidth-enhancing techniques reported for traditional rigid antenna designs, for example, using aperture coupling [23], metamaterials/metasurfaces [24; 25; 26; 27; 28], parasitic elements [14; 23], thick-air substrates [14; 24], multilayer stacking [13], pin-loading [29], reflectors (e.g., AMC, PRS) [30; 31], AI-based approaches [25; 32], etc. However, most of these popular techniques are not suitable for wearable applications because of the thickness restrictions. In this paper, we present a low-profile slit-loaded CP antenna with enhanced 3-dB AR bandwidth using Characteristic Mode Analysis (CMA). Its CP bandwidth is 2.2-2.54 GHz (14.3%). This design features two essential merits over the literature: (1) a superior CP bandwidth and high gain are achieved for low-profile wearable CP antennas in the 2.4 GHz band; (2) increased bandwidth with only single layer substrate, which is easy to fabricate and low-cost. It is worth mentioning that not many designs are capable of obtaining a decent 3-dB AR bandwidth for low-profile CP antennas (LPCPA), as depicted in Table 1, whereas our work draws a clear distinction. In detail, a multilayer structure with stacked patches and parasitic elements is reported in [13], which leads to 20.7% CP bandwidth at 6 GHz. Ref. [14] reports a strip-loaded antenna with 24% CP bandwidth. However, they are only suitable for traditional rigid substrates. Ref. [17] presents a wearable antenna array fed by a sequential feeding network. The feeding is embedded in the inner layer of the antenna, which makes the fabrication complicated. A modified metasurface is also applied to the antenna design in [18], which results in a wideband property with a peak gain of 8.5 dBic.