Wearable antennas are becoming more and more popular because of their tiny size, conformal nature, lightweight, and compactness. These antennas are in high demand in the electronics and telecom industries for usage in a variety of applications including the military, RADAR, healthcare, public safety, and many more. The Federal Communication Commission (FCC) in 2002 defined an ultra-wideband frequency with a bandwidth of 7.5 GHz, extending from 3.1 to 10.6 GHz, to be an unlicensed spectrum [
1]. Similarly, for 5G communication, it is preferable to go with the existing UWB or millimeter wave (mmWave) band to enhance the requirement of bandwidth [
2‐
4]. Although it is preferable to use the entire 7.5 GHz spectrum from UWB, the maximum power a transmitter can use is just about 0.5 mW. This is a small portion of the IEEE 802.11 a/b/g standards 2.45 GHz ISM (Industrial, Scientific, and Medical) band's capabilities. One bit of information must always be carried by combining many low-energy UWB pulses due to the extremely low transmit power that is available. In theory, increasing the number of pulses needed to carry one bit can be utilized to trade data rate for link distance. The data rate is lower and the transmission distance is higher when there are more pulses per bit. With high data rates, this meritoriously elevates UWB to indoor, short-range communications. Wide bandwidth has led to the development of wireless UWB and personal area network applications that are designed to transport data at speeds of up to several Gbps over distances of one to ten meters [
5].
In perspective to these, the features of UWB push the need for wearable antennas specifically for healthcare applications. Earlier antennas were fabricated using substrate materials like FR4, RT-duroid, etc., but it was challenging to achieve the antenna's flexibility and other desired properties. At the moment, flexible wearable antennas come in a wide range of shapes and substrates, including denim, felt, Velcro, PDMS [
6], and foam [
7]. The future flexible wearable antennas are likely to be based on textile material, polymers, or flexible ceramics [
8‐
10]. The new kind of material, such as foam, which is sufficiently flexible, light, and durable and can be readily incorporated into the wearer's clothing, is the main emphasis of the antenna. The features of the following foam material types are illustrated here.
1.3 EVA foam
EVA stands for ethylvinyl acetate. EVA foam refers to a closed-cell ethylvinyl acetate copolymer that is often given in the form of plates and is offered in a wide range of densities and colors as presented in Fig.
1c. Due to its ability to be thermoformed into both simple and complex shapes, it enables us to manufacture a variety of products that are suitable for a wide range of uses [
9,
10]. Copolymers of ethylene and vinyl acetate are used to create EVA (ethylene–vinyl acetate) foam. The weight percentage of vinyl acetate typically ranges from 10 to 40% in one sheet of EVA foam.
An additional crucial component for the creation of EVA foam is polyethylene material. EVA foam's density, hardness, color, durability, and other properties can vary depending on how much foaming catalysts and additives are used during the molding process. Materials made of EVA foam have a closed-cell foam structure. They offer many great qualities, such as robust heat insulation, long-term durability, superior cushioning and shock absorption, and outstanding water and moisture resistance [
13].
Among all these types of foam as shown in Fig.
1, EVA is the best suitable candidate from flexibility, bending, and water resistance points of view. It also makes antenna fabrication easier and more conformal. Wearable antennas are those which can be easily worn over the body parts or the parts of clothes. Ultra-wideband systems attract research interest in implementing wireless applications. As UWB offers very low power operation [
5], it has reduced the system complexity. The conventional substrate materials are not suitable for stretching, folding, or bending as it results in cracks or damage of the material directly. Also, it has increased the weight of the antenna [
14].
The author [
15] has implemented the wearable UWB antenna over a felt textile substrate (
εr = 1.2) with a thickness of 0.7 mm resulting in an operating frequency of 3.1 GHz to 11.3 GHz. However, the overall realized gain is 4 dB. The antennas made up of textile material are bendable and can readily adjust to the curves of the human body to enhance user comfort, but they must maintain their full performance while being bent. As a result, it is important to examine the performance parameters of flexible wearable antennas under various bending conditions [
16]. In [
17], the author has designed and tested a wideband antenna for wearable applications using flexible jeans substrate (
εr = 1.8). The antenna is operative over the frequency of 0.9 GHz to 6 GHz. A novel technique of five parallel metal plates is used for bandwidth enhancement. The author [
18] designed and fabricated the antenna of size
\(33 \times 35\,{\text{mm}}^{2}\) using three different textile materials such as jeans, flannel, and cotton for wearable applications in the UWB frequency range. The tested antenna resulted in a maximum bandwidth of 5 GHz for the dielectric constant of 1.7 and a thickness of 1 mm. In [
19], a UWB circular patch antenna for wearable application is proposed with the jeans fabric. A partial ground technique with the two-layer substrate is incorporated into the fabrication of the antenna. The feed line is hidden with the second layer of the substrate which made the antenna function in the range of 2 GHz to 14 GHz.
The authors [
20] have proposed the textile UWB antenna for biomedical communication with a dielectric as jeans (denim) material ((
εr = 1.67). The thickness of the dielectric substrate is taken as 2 mm, and adhesive copper tape of 0.75 mm thickness is used as conducting material. The simulated design performance analysis of the UWB antenna with FR4 and jeans substrates is carried out in [
21]. However, the results are compared in terms of
S11, gain, and radiation pattern of two different substrates. In [
22], the UWB antenna has been fabricated for breast cancer diagnosis using FR4 substrate of thickness 1. 6 mm and operates between 2 and 12 GHz. The antenna is very compact in size
\(28 \times 14\,{\text{mm}}^{2}\), and the SAR investigated is about 0.98 W/kg with a maximum gain of 4.2 dB. In recent literature [
23], a graphene-assembled film (GAF) and a flexible ceramic substrate are used in designing of UWB antenna for wearable applications along with the co-planar waveguide feed structure of two H-shaped slots. The antenna has a size of
\(32 \times 52 \times 0.28 \,{\text{mm}}^{3}\) with a maximum gain of 4.1 dB over the frequency range of 4.1 to 8 GHz. Also, the geometrical design of an antenna can be accomplished with pre-fractals called fractal antennas. The pre-fractal modeling has recently provided enormous versatility in engineering and applied science. These antennas are multiband with efficient miniaturization and have the properties of self-similarity and space-filling, which provide numerous benefits [
24]. A fractal has a structure that is too irregular to be described by a traditional mathematical theory (Sierpinski gasket antenna). A fractal approach to modeling its geometrical configuration can be considered in order to minimize the antenna size while maintaining a high radiation efficiency along with the entropy computation based on the fractal dimension and complexity of shape [
25]. The fractal geometries such as Sierpinski, Cantor, Hilbert [
26], Minkowski, Koch, and the fractal tree are being used for wearable multiband and wideband applications. A hexagonal-square-shaped fractal-based approach is used to achieve the ultra-wideband in the range of frequency from 2.1 to 13.5 GHz [
24,
25],
27.
Based on the literature, this research incorporates a foam-based flexible ultra-wideband antenna for healthcare applications using a partial ground approach. The substrate material is made of foam. Its small size and ability to operate in a broad frequency range of 2.6 GHz to 11.3 GHz make it the ideal material for ultra-wideband wearable applications. This paper is organized as follows. Section
1 provides an overview of foam material and related literature, Sect.
2 provides the antenna design method, Sect.
3 shows experimental work carried out, Sect.
4 includes simulated and measured findings, followed by discussion of the results, and Sects.
5,
6, and
7 provide a bending, on-body and SAR analysis followed by a conclusion in Sect.
8.