Microstrip patch antennas—widely used due to their low profile, compact size, and conformal design—are highly suitable for wearable applications, such as patient monitoring, sports activities, security, and firefighting. In patient monitoring applications, transmission and reception involving high data rates are necessary for various on-body/off-body devices. However, since microstrip antennas usually have a narrow bandwidth, they are insufficient for use in such wearable applications. Nonetheless, various techniques for bandwidth enhancement have been proposed in the literature. For instance, the use of a double U-shaped defective ground structure [1], folded defects [2], and L-shaped defects [3] have all been proposed for bandwidth improvement. Furthermore, stripline and L-shaped probes have been used to feed multilayer structures [4]. Some studies have also employed sequential phase feeding methods for bandwidth enhancement [5, 6]. In addition, using capacitive coupling with a small disc on top offered improvements in bandwidth in [7], while introducing multiple slots on a patch close to the resonant frequency also had the same effect [8, 9]. Similarly, implementing two parasitic patches in the annular area helped improve bandwidth [10], and so did a coplanar waveguide-fed antenna using modified ground [11]. Other proposed methods for bandwidth improvement include using a vertical coupling-based method with modified ground [12], and employing the dual resonance method using ethylene-vinyl acetate (EVA) foam as substrate [13]. Furthermore, the literature has attempted to improve antenna performance by experimenting with different substrate materials created using EVA foam and plastic polymers [14]. In addition, different techniques for realizing multiband operations have been explored, such as employing two stacked patches operating at two different frequencies [15], using three independent radiators that are switched using PIN diodes to operate at 1.575 GHz for global positioning systems (GPS) and at the 2.45 GHz and 5.2 GHz industrial, scientific, and medical (ISM) bands [16], and implementing a loop inspired dual/triple band operation that uses switching by PIN diodes for the 4G, 5G, and X bands [17]. In modern wireless applications, the use of reconfigurable antennas is preferred due to their ability to radiate more than one pattern at different frequencies and polarizations. For instance, [18] reported frequency and polarization reconfiguration using PIN switching diodes for L1, worldwide interoperability for microwave access (WiMAX), wireless local area network, and mobile bands. Frequency reconfiguration for multi-bands using microelectromechanical system (MEMS) switches has also been demonstrated in [19, 20]. A liquid dielectric-controlled polarization reconfigurable antenna for radio frequency identification has been proposed in [21], while stub switching by employing PIN diodes has been recommended for frequency reconfigurability in [22]. In addition, a frequency reconfigurable planar inverted-F antenna for GSM 850/900 and UMTS 2100 was presented in [23].

In this work, stacked layers and multiple slots near the resonant frequency are combined to improve the bandwidth of a wearable antenna. The upper patch bearing resonant slots provides the desired bandwidth enhancement. Moreover, low-cost flexible EVA foam material is used as the substrate and copper tape is used as the patch, while frequency reconfigurability is incorporated using a PIN diode. Finally, the antenna structure was tested and validated for wearable applications using an equivalent human body model.

The dimensions of the proposed antenna were calculated using standard Eqs. (1)–(3) for microstrip design [24], as noted below:

where W is the width of the antenna, L is its length, ɛ_r indicates the relative permittivity of the substrate, ɛ_eff refers to the effective permittivity, h is the thickness of the substrate, f_o denotes the resonant frequency, and c is the velocity of light.

Notably, the antenna was simulated using the CST Microwave Studio. Its ground plane, created using copper material, had dimensions of 62 mm × 58 mm × 0.05 mm, while the substrate material was EVA foam having a relative permittivity of 1.2, loss tangent of 0.02, and size of 62 mm × 58 mm × 1.65 mm. The patch consisted of a copper tape of size 24.64 mm × 21.33 mm × 0.05 mm. A slot of size 8 mm × 11.66 mm was cut into the patch. Furthermore, a feed line of size 15.3 mm × 3 mm was used to excite the patch. The spacing between the feed and the patch was optimized to 2.5 mm on the left and right sides and 1.7 mm on the top of the feed. The basic antenna design is presented in Fig. 1.

The EVA foam substrate (thickness = 1.65 mm) was chosen for the proposed antenna because it is flexible and suitable for wearable applications, owing to its electrical and mechanical properties. First, the basic antenna with an EVA foam substrate of height 1.75 mm was designed for the 5.8 GHz frequency band. Energy was fed into the patch as coupling by the feed line. However, the bandwidth obtained at this frequency was 122 MHz, which is narrow and insufficient for high data rate transmission/reception. Therefore, for bandwidth enhancement, another patch was placed onto the first patch. As shown in Fig. 2(a), an upper patch of size 23 mm × 20 mm × 0.05 mm is placed at a height of 8.25 mm from the first patch, with air being the dielectric in the gap. Notably, since the thickness of the upper patch was 0.05 mm, it was difficult to suspend it in air and keep it parallel to the lower patch. Therefore, expanded polyurethane foam with a relative permittivity of ɛ_r = 1 was used in the air gap to support the upper patch. Consequently, the bandwidth increased to 203 MHz. Furthermore, two slots of equal size (4 mm × 4 mm) were created at the center of the upper patch, as shown in Fig. 2(b), with the spacing between them being 9 mm. The size of the slots and the upper patch were optimized to attain an enhanced bandwidth. Due to the creation of slots on the upper patch, the overall bandwidth increased to 223 MHz at a resonant frequency of 5.8 GHz. The flowchart of the entire fabrication process is shown in Fig. 2(c), and the antenna dimensions are noted in Table 1.

A wearable antenna is usually used along with on-body devices, which are responsible for establishing communication between the on-body and off-body devices for patient data transmission. For this kind of communication, the radiation pattern must be directional and propagate away from the body. Moreover, radiation directed toward the body is hazardous and, therefore, must be minimized. The simulated 3-dimensional radiation depicted in Fig. 12(a) shows that the radiated field is in the broadside direction, while minimal radiation is present in the back direction toward the body. Furthermore, to calculate the specific absorption rate (SAR), an equivalent human body model of size 94 mm × 90 mm × 70 mm was considered, as shown in Fig. 12(b). The thickness of the skin, fat, and muscle layers was 3 mm, 7 mm, and 60 mm, respectively. The dimensions and electrical parameters of the cubic human body model are noted in Table 5 [25]. The antenna was kept 2 mm away from the body model to provide space for wearable clothes. The calculated results presented in Fig. 12(c) show that the SAR value is 0.145 W/kg for 10 gram of tissue, which is well within the limits of the SAR value (2 W/kg for 10 gram tissue) prescribed by the Federal Communications Commission and the International Commission on Non-Ionizing Radiation Protection [26, 27].

Furthermore, the back radiation was calculated in accordance with Eq. (4) as the ratio of the radiated power in the back hemisphere from the antenna to the total radiated power, which can be expressed as follows:

Here, E is the electric field, θ is a variation of the angle of the electric field in the vertical plane, and φ is a variation of the angle of the electric field in the horizontal plane.

An equivalent cylindrical structure that accounts for the air gap and the skin layer of the body was created, as shown in Fig. 13(a). The outer cylinder with 2 mm thickness represents the air gap, while the inner cylinder with 3 mm thickness represents the skin layer. The bending effects of the antenna were studied by placing it on the surface of the cylinders with different radii. The results of S₁₁ based on the different radii are shown in Fig. 13(b). It was observed that the resonant frequency of the antenna drifted toward the higher side, while the return loss became poorer, as the radius (R) of the cylinder increased. However, no significant effect on the radiation pattern and efficiency of the antenna was observed due to bending.

This work proposes a simple design for a low-cost frequency reconfigurable dual-band antenna that can be used to establish wireless connectivity between on-body and off-body devices, enabling data transmission from the patient’s body to other remote monitoring places. Substantial gain, bandwidth, and directional radiation patterns are the primary requirements of high-rate data transmissions in biomedical telemetry. Exhibiting enhanced bandwidth and directional radiation, the proposed antenna met all these requirements. Furthermore, the antenna achieved low back radiation toward the body, a high front-to-back ratio, and a very low SAR value, all of which are highly desired parameters for wearable applications. A parametric study was conducted to understand the effects of variation in dimension on the parameters of an antenna. The physical properties of the EVA foam substrate and the stability of the antenna parameters, such as resonant frequency, bandwidth, directivity, and front-to-back ratio, under body loading conditions make it suitable for telemetry applications.