Communication systems using millimeter waves have received much attention in wireless body area networks (WBANs) because of their ultra-reliability, high speed, and low latency characteristics [1–3]. To establish good on-body communication linking body-mounted devices with a millimeter wave, an antenna with a directional radiation pattern toward the body surface is commonly used to compensate for the high path loss [4]. The H-plane substrate integrated waveguide (SIW) horn antenna is a promising candidate for such applications because of its directional radiation pattern and low-profile characteristics. Many different technologies for such SIW horn antennas have been proposed to improve their performance. A printed transition, a pair of slots, and mushroom-type meta-materials have been placed in front of the SIW horn aperture to improve the radiation and matching performances [5–7]. However, such antennas have substantial radiation toward the human body. In the millimeter-wave frequency band, the Federal Communications Commission (FCC) and International Commission on Non-Ionizing Radiation Protection (ICNIRP) regulate electromagnetic exposure through power density [8, 9]. The power density limit is 10 W/m², and meeting this regulation at millimeter-wave frequencies is challenging. As a result, the input power of the antenna is limited, thus addressing the overall system power budget problem [10, 11]. Aiming to suppress radiation toward the body surface, a SIW H-plane horn antenna with an extended ground with a meta-surface and a large conducting plane was proposed [12, 13]. However, the extended ground and large conducting plane make the radiation beam pattern of the antenna tilted toward the broadside.

In this paper, an all-textile H-plane SIW horn antenna with corrugated ground is proposed for on-body WBAN applications. The proposed textile antenna operates at 28 GHz, which is a potential frequency band for 5G millimeter-wave communications. An extended corrugated ground is placed in front of a traditional SIW H-plane horn antenna (TSHA). The extended ground is used to suppress radiation toward the body surface. Corrugation is implemented on the extended ground to solve the beam tilting issue by utilizing its wave guiding characteristics [14, 15]. The extended ground corrugation acts as an inductively reactive surface, creating a directional radiation pattern toward the near surface direction (θ = 90°) and reducing the radiation toward a human body. All antenna elements are fabricated using textile materials, such as conductive fabric, conductive thread, and textile substrates, because they are flexible and easily embedded into clothing.

Fig. 1 shows the geometry of the proposed antenna. The overall dimensions are 10 mm × 24.5 mm × 2.5 mm. A felt fabric (ɛ_r = 1.2) with a thickness of 2.5 mm is used as the substrate. The proposed antenna is composed of a coaxially fed TSHA for generating a surface wave and an extended corrugated ground for guiding the wave towards the near surface direction (θ = 90°). The corrugation is realized by six walls with a height of 1.3 mm separated by a distance of 1.4 mm. The top plane and ground plane of the SIW H-plane horn and the extended ground are made of conductive fabric (σ = 500,000 S/m). Conductive thread (σ = 20,000 S/m) with a radius of 0.2 mm is used for the via.

To analyze the effect of the corrugations on the radiation properties, the peak gain and gain toward the body surface direction (θ = 90°) are simulated for the different numbers of corrugations (N) and height of corrugations (H). The real and imaginary parts of the wave impedance (Re[Z] and Im[Z], respectively) are averaged in the line above the corrugation walls (A–B in Fig. 1). Fig. 2(a) and (b) show the trend of the gain and average impedance for the different N and H values. As the height of the corrugations increases, the corrugated surface becomes an inductively reactive surface, leading the surface wave to be bounded to a corrugated surface [16]. Although the average value of Im[Z] increases up to H = 2 mm, the gain toward the body surface direction (θ = 90°) has a peak value of H = 1.3 mm, and it decreases beyond H = 1.3 mm because of the impedance mismatching between the corrugation wall and the aperture of TSHA. The average value of Im[Z] increases with more corrugations being implemented on the extended ground and is not significantly changed after N = 4. The gain toward the body surface direction (θ = 90°) has a similar trend on the average value of Im[Z]. The peak gain is nearly constant for different N values, with a maximum of 8.25 dBi and a minimum of 7.53 dBi. Fig. 2(c) and (d) show the return loss characteristics and the E-plane radiation patterns of the TSHA, TSHA with extended ground (N = 0), and TSHA with corrugated ground (N = 6). Fig. 2(c) shows that the return loss characteristics improve when the ground of TSHA is extended and become even better with corrugation. The simulated 10 dB return loss bandwidth of the proposed antenna is 2,400 MHz (26.7–29.1 GHz). In Fig. 2(d), the proposed antenna has an improved gain of 6.17 dBi toward the body surface direction (θ = 90°) in comparison with the other cases (TSHA = 4.33 dBi; TSHA with extended ground = 3.9 dBi). The peak gains of TSHA, TSHA with extended ground, and TSHA with corrugated ground are 5.06 dBi at θ = 109°, 7.93 dBi at θ = 46°, and 7.9 dBi at θ = 61°, respectively. For a practical on-body situation, the effect of bending the antenna should be considered. To investigate the effect of bending on the antenna performance, the antenna is bent along a cylinder with various radii (r = 15, 25, 100, and 1,000 mm). Fig. 2(e) and (f) show the simulated return loss and radiation characteristics of the proposed antenna for various bending setups. Although the antenna is bent, the 10 dB return loss bandwidth remains stable and is sufficient to cover the 28 GHz band. The bent antenna has a radiation pattern tilted in the direction along the end edge of the antenna. As the antenna consists of waveguide structures, the performance of the antenna is not severely affected by bending. Fig. 3(a)–(f) illustrate the simulated electric field distributions in the yz-plane. A substantial amount of the E-field propagates toward a body surface in Fig. 3(a). In Fig. 3(b), the E-field distribution of TSHA with the extended ground shows that the extended ground attached in front of the aperture obstructs the propagation of the E-field toward a body surface and reflects the field, thus making the main radiation direction tilted toward the broadside.

However, the simulated E-field distribution of TSHA with the corrugated ground in Fig. 3(c) shows that the fields are guided along the body surface direction (θ = 90°) by confining the field in the corrugated ground, which is inductively reactive at 28 GHz. Fig. 3(d) and (e) show that the corrugated ground maintains a guiding characteristic even if the antenna is bent.

Fig. 4 shows the peak power density on an arbitrary 40 mm × 80 mm rectangular surface located below the proposed antenna when the input power is 1 W. The peak power density of TSHA with corrugated ground becomes less than that of TSHA in a separation distance range of 0–30 mm at 28 GHz because of the suppression of the waves radiating normally to the body surface by the corrugated ground. When the distance is 5 mm, which is a typical separation distance between an antenna and a body, the peak power density of TSHA is 1,808.25 W/m² and that of TSHA with corrugated ground is 401.56 W/m², which is 22% of the TSHA peak power density. The maximum allowable input power to satisfy the INCIRP guideline is 13.86 dBm for TSHA with corrugated ground and 6.53 dBm for TSHA.

Fig. 5(a) and (b) show the fabricated textile antenna. The top and bottom ground planes are made of a Shieldex conductive metalized nylon fabric (Zell) with a thickness of 0.1 mm (surface resistance of 0.02 Ω/sq), and the vias for the corrugation and wall of TSHA are made of a Shieldex conductive thread (resistivity of 0.0025 Ω/cm²) with a radius of 0.1 mm. Circuitworks conductive epoxy CW2400 was used to attach a 2.92 mm (K) connector instead of soldering. The simulated and measured return loss characteristics are shown in Fig. 6(a). The measured results agree well with the simulation. The measured 10 dB return loss bandwidth is 2,800 MHz (26.66–29.54 GHz). Fig. 6(b) and (c) show the simulated and measured far-field radiation patterns in the yz- and xy-planes. The measured results are slightly deteriorated because of fabrication errors. The measured peak gain and gain toward the body surface direction (θ = 90°) are 6.54 dBi and 4.14 dBi, respectively, at 28 GHz.

In this paper, an all-textile H-plane SIW horn antenna with corrugated ground for millimeter-wave WBAN applications is proposed. The extended corrugated ground is implemented to guide waves toward the near body surface direction and to suppress radiation toward the human body. The maximum output power to satisfy the ICNIRP guideline is 13.86 dBm when the separation distance between the antenna and the human body is 5 mm. The fact that the proposed antenna has good radiation performance toward the body surface direction and more allowable power than TSHA shows that it has advantages in the millimeter-wave communication environment with higher path loss. The proposed antenna can be easily implemented into wearable devices because it is made completely of textile materials.