Characteristic Mode-Based Dual-Mode Dual-Band of Single-Feed Antenna for On-/Off-Body Communication

Abstract

A dual-band, dual-mode button antenna is proposed for emerging fifth-generation (5G) networks and Industrial, Scientific, and Medical (ISM) communication systems, as it operates at 3.5 GHz and 5.8 GHz, respectively. At the lower band, a monopole-like omnidirectional radiation pattern is achieved by loading shorting pins on curved strips for on-body communication. At the higher band, broadside circularly polarized radiation is achieved by loading an asymmetric U-shaped slot in the central chamferd patch for off-body communication. By using Characteristic Modal Analysis (CMA), a clear physical insight into the formation of dual polarization is provided. The −10 dB impedance bandwidth ranges from 3.48 to 3.60 GHz and 5.65 to 6.03 GHz, respectively. The 3 dB axial ratio (AR) bandwidth ranges from 5.71 to 5.85 GHz in the high band. Additionally, the antenna achieves a peak gain of 1.2 dBi in on-body mode and 6.9 dBi in off-body mode. The maximum specific absorption rate (SAR) calculated on the body tissues is below the US/EU standard thresholds of 1.6 W/kg and 2 W/kg. The measured results indicate that the antenna experiences only slight impact from human body loading and structural deformations. Given its notable features, the proposed design is well suited for Wireless Body Area Network (WBAN) applications.

1. Introduction

With the increasing demand for high-speed communications, research in the field of 5G technology has become intensive. The 3.5 GHz band, as a mainstream frequency for 5G communications, is expected to continue playing a critical role in the future of wireless communications technology due to its high data rates and low latency [1]. The advancement of 5G technology also continues to broaden the application scenarios for wearable devices [2,3,4]. For example, in the medical field, smart wearable devices can monitor patients’ vital signs in real time and provide instant feedback to healthcare professionals, enabling remote health monitoring. In the field of sports and fitness, these devices improve the collection and transmission of exercise data, thus providing a better experience for users. WBAN is a network technology designed for efficient and secure proximity communications in the vicinity of the human body, with requirements for antennas in compact devices to be flexible, lightweight, and conformal, as well as having superior performance [5,6,7]. In WBAN communication systems, two commonly used communication modes are on- and off-body. The optimal radiation patterns of the antenna required for these two modes are different. In on-body communication, sensors located on different parts of the body transmit collected data to a central hub. For this mode, the maximum radiation pattern of the antenna should be tangent to the body surface to enable effective communication between wearable devices on various parts of the body. This requires omnidirectional radiation along the body surface. In contrast, for off-body communication, the central hub communicates with external data collection devices (e.g., cell phones, computers). In this mode, the direction of the maximum radiation pattern should be perpendicular to the body surface and directed outward to ensure robust external communication while minimizing the impact of the antenna’s backward radiation on the body. Consequently, multifunctional antennas that can adapt to the varying radiation characteristics required by different application scenarios are increasingly sought after in wearable technologies.

The effectiveness of various antennas in WBAN has been extensively examined [8,9,10,11,12]. A button-like wideband wearable chassis antenna was introduced in [8], utilizing a ferrite ring around the feed cable to mitigate common mode currents, albeit at the expense of increased fabrication cost and complexity. In [10], a dual-band dual-polarized antenna with an omnidirectional radiation pattern is achieved by exciting the ground layer slit in odd order mode; the use of the ground layer as a radiator leads to larger specific absorption rate. Circular patch antennas operating in higher order modes [11,12] have comparable performance to monopole antennas for on-body communication. Despite these advancements, the majority of the discussed antennas are limited to working in either on-body mode or off-body mode, which limits their multifunctionality.

The antennas in [13,14] both achieve dual-band, dual-mode characteristics based on a planar inverted-F structure for on-/off-body communication. An approach utilizing a common feed mechanism to activate distinct patches for dual-mode functionality was introduced in [15]. The antennas in [16,17] both propose the use of circular patch higher-order modes ${\mathrm{TM}}_{11}$ and ${\mathrm{TM}}_{02}$ to achieve dual radiation characteristics. While the designs presented in references [15,16,17,18] successfully achieve dual-band and dual-mode functionality, they exhibit certain limitations. Specifically, at lower frequencies, the radiation patterns tend to be broadside, whereas at higher frequencies, they become omnidirectional. These designs may experience relatively high link loss, which is a factor to consider for optimal performance. When it comes to on-body information transmission, wearable antennas that exhibit omnidirectional radiation patterns at lower frequencies are preferred. This is due to their longer wavelengths and enhanced diffraction properties, which allow for more reliable coverage in human communication scenarios.

Moreover, unidirectional radiation with circular polarization (CP) [19,20,21,22,23,24] for off-body communication is preferred as it can mitigate polarization mismatches due to movement and postural changes in the human body. The wearable CP antennas studied in [20,21] were implemented through the incident of an x-polarized wave, emanating from a monopole antenna, onto a polarization conversion metasurface (PCMS). Due to the complexity of the technology and the high cost of fabrication, very few studies have been conducted to combine the dual-mode operation of LP and CP for both modes of communication.

Among the diverse wearable antenna designs, button antennas [25,26,27,28,29] exhibit advantages due to their rigid form factor, high design flexibility, compact size, and resistance to crumpling and bending. These characteristics typically result in more stable radiating performance compared to wearable antennas [30,31,32,33,34] that use textiles as substrates. This feature is quite important for data transmission or acquisition. Additionally, when button antennas are kept at a distance from human tissue, the SAR is reduced. Due to their excellent RF performance and detachability, button antennas are considered a practical and economical RF connectivity solution for wearable devices. In [35], a wearable directional button antenna consisting of artificial magnetic conductor (AMC) units is proposed for future wireless power transmission applications on the human body. In [36], a dual-band, dual-mode button antenna sensor is proposed. It achieves omnidirectional radiation at 3.5 GHz, ideal for on-body information transmission, and circularly polarized broadside radiation in the 5 GHz WLAN band, suitable for off-body wireless power harvesting. Nevertheless, the design uses an essential AMC structure, resulting in a larger size.

In this study, a dual-band, dual-mode button antenna is proposed, featuring an omni-directional radiation pattern at 3.5 GHz under the 5G communication standard to support on-body communication and a circularly polarized broadside radiation pattern at 5.8 GHz in the ISM band for off-body communication. The asymmetric U-slot loading technique is used to form a pair of orthogonal modes with the ${\mathrm{TM}}_{01}$ mode of the patch, achieving stable circularly polarized radiation characteristics and introducing an additional resonance frequency at 5.8 GHz, which helps to expand the operating bandwidth. In addition to the ground plate, the reflector plate on the upper layer further improves the gain and significantly diminishes the radiation of the antenna toward the human body. The short pins of the four surrounding monopoles ensure the mechanical stability of the antenna. Furthermore, the principle of dual polarization generation is clearly explained using CMA. The proposed design integrates two operating modes, which provides a valuable candidate to meet the functional requirements of different WBAN communication application scenarios.

2. Antenna Design

Figure 1 shows the design details and the dimensions used in the antenna structure. The specific parameters of the antenna are shown in Table 1. The button section is a circular disc made of FR-4 with a dielectric constant of 4.4 and a loss tangent of 0.02. The textile layer is felt with a dielectric constant of 1.2 and a loss tangent of 0.02. Furthermore, conductive textiles were placed at both the top and bottom of the felt. The top layer of conductive textile acts as a reflective layer, forming a circular gap with a radius of 1 mm at its center for the feed probe to pass through, while the bottom conductive textile was employed as a ground. This approach produces broadside radiation in the high-frequency range, resulting in higher gain. The button disc is separated from the textile layer by a 3 mm high air layer, and the overall height of the antenna is 6.2 mm.

The main radiator is located on the top of the button disc. The top center is the square patch with chamferd and loaded with an asymmetric U-slot. The feed pin starts at the SMA connector and runs from bottom to top, connecting directly to the center of the modified patch, which is excited directly by the probe. The four equal-length radiation strips on the outside of the patch are shorted to the ground by the action of four shorting pins. Thus, the outer ring can be considered as four monopole antennas coupled and excited by the center patch.

The resonance frequency of the quarter-wavelength strip was designed at 3.5 GHz, while the resonance frequency of the patch was designed at 5.8 GHz, approximated by the following equation [37]:

$$f=\frac{c}{2l_{\mathit{eff}}\sqrt{{\epsilon }_{\mathit{eff}}}}$$

(1)

$${\epsilon }_{\mathit{eff}}=\frac{{\epsilon }_{\mathit{r}}+1}{2}+\frac{{\epsilon }_{\mathit{r}}-1}{2}{(1+\frac{10\mathit{h}}{\mathit{W}})}^{-1/2}$$

(2)

where c is the speed of light in free space, $l_{\mathit{eff}}$ is the effective length of the patch, ${\epsilon }_{\mathit{eff}}$ is the effective permittivity of the substrate, ${\epsilon }_{\mathit{r}}$ is the dielectric constant of substrate, and W is the length of the patch.

As illustrated in Figure 2, we constructed an equivalent circuit model based on the principles of transmission line and microstrip patch, providing a clear physical perspective of the proposed antenna structure [38]. In this model, the patch loaded with a U-slot is emulated as a parallel resonant circuit represented by $R_P$, $L_P$, and $C_P$, while the four monopole antennas of the outer loop are simulated as another parallel resonant circuit, characterized by $R_m$, $L_m$, and $C_p$. These two parallel resonant circuits are connected in series, and electromagnetic coupling is achieved through the capacitor $C_g$.

Traditionally, to achieve broadside radiation for the patch, the patch should be fed with an offset feed. However, this distorts the omnidirectional radiation pattern at a low frequency. Therefore, an asymmetric U-shaped slot is introduced to compensate for the feed offset, thereby relocating the probe feed position to the center. The dimensions of the U-slot are designed slightly below 5.8 GHz. By adjusting the position of the slot and the dimensions of its two arms appropriately, additional resonance frequencies can be introduced.

Figure 3 shows the simulated current distribution at the two resonances obtained from the HFSS software. Figure 3a illustrates that the currents are primarily concentrated in the strips located at the outer ring of the patch. Four in-phase currents form a ring that generates monopole-like omnidirectional radiation patterns. In Figure 3a,b, it is seen that at phase = 0°, the main part of the current flows in the −y direction. At phase = 90°, the maximum current flows in the +x direction. This indicates that the current rotates in the anti-clockwise direction, confirming that the right hand circular polarization (RHCP) is effectively excited at 5.8 GHz.

3. Characteristic Mode Analysis

CMA was performed to authenticate the dual polarization generation mechanism. In CMA theory, the modal significance value (MS) quantitatively measures the degree of contribution or importance of a specific characteristic mode in the electromagnetic field at a given frequency, typically manifested through its normalized amplitude:

$$\mathrm{MS}=\left|\frac{1}{1+j{\lambda }_n}\right|$$

(3)

where ${\lambda }_n$ is the characteristic eigenvalue. A value of MS = 1 refers to the most dominant resonating mode. The characteristic angle (CA) is an equally significant metric, representing the phase difference between the modal current and its corresponding characteristic field, defined as follows:

$$\mathrm{CA}={180}^{\circ }-{tan}^{-1}\left({\lambda }_n\right)$$

(4)

Specifically, when the CA is equal to 180°, the mode achieves resonance, indicating its effectiveness as a radiator.

The CMA results for the first six modes of the button structure are shown in Figure 4, with the substrate and ground plane extending indefinitely in the x-y plane, and without external excitation. To obtain a thorough understanding of the antenna’s radiation performance, Figure 5 illustrates the modal current distribution and far-field patterns for these initial six modes. It is observed that mode 1 and mode 2 are a pair of orthogonal modes of the conducting textile. At 3.33 GHz, mode 3 is dominant, and the current is mainly distributed on the four curved strips of the outer ring and the copper post. At this time, the maximum radiation direction of mode 3 is perpendicular to the z-axis, which is similar to the omnidirectional mode of a monopole, and it can be identified as a quarter-wavelength monopole mode. Mode 4 has a resonance frequency of about 5.26 GHz, the strong current density is mainly concentrated near the U-shaped slot, and the maximum radiation appears on the broadside, which can be called the slot mode. Mode 5 is not considered because the radiation pattern in the Z direction appears as a radiation null due to the phase cancellation of the currents. Mode 6 has a resonant frequency of 5.8 GHz, and the current is mainly distributed on the patch, similar to the typical ${\mathrm{TM}}_{01}$ patch mode, which can be named the ${\mathrm{TM}}_{01}$-like mode. As shown in Figure 5d,f, the slot mode and the ${\mathrm{TM}}_{01}$-like mode have the same far-field radiation pattern with orthogonal polarization states. This allows for the simultaneous excitation of mode 4 and mode 6 as candidates for generating circularly polarized radiation.

Introducing the feed probe structure, the CMA results of the antenna structure are shown in Figure 6. The slot mode moves from 5.26 GHz to about 5.76 GHz. This is because the feed probe generates a strong inductive component in the impedance, neutralizing the capacitive component generated by the U-slot. In addition, in the absence of a feed probe, the current distribution in a U-shaped slot is purely magnetic. With the introduction of the probe, the current pattern changes to a combined pattern, consisting of an electrical component of characteristic current on the probe and a magnetic component of characteristic current in the slot. The combined mode exhibits the same magnitude as the MS curve of the ${\mathrm{TM}}_{01}$-like patch mode at 5.89 GHz. The CA plot shows that the phase of the combined mode lags behind that of Mode 6, with a phase difference of about 90°. Therefore, it can be deduced that the combined mode of the probe and the U-slot is the key reason for the CP.

Based on the modal current distribution, it can be seen that the resonant frequency equation for the ${\mathrm{TM}}_{01}$-like patch mode needs to be modified since its current is interrupted by the U-slot:

$$l_{eff}^{\mathrm{Slot}}=l_1+l_2+l_3-w_1-w_3-2w_2$$

(5)

$$l_{eff}^{{\mathrm{TM}}_{01}-\mathrm{like}}=W-w_3+d+2\Delta l\left(W\right)$$

(6)

$$\Delta l\left(W\right)=0.412h\left(\frac{{\epsilon }_{eff}+0.3}{{\epsilon }_{eff}-0.258}\right)\left(\frac{W/h+0.264}{W/h+0.8}\right)$$

(7)

where $l_1$, $l_2$, $l_3$, $w_1$, $w_2$, $w_3$, and W are structural parameters listed in Table 1. The notation $\Delta l\left(W\right)$ represents the extension of the effective length of the patch due to edge effects.

Substituting the above equation into the empirical formula in Equation (2), we are able to derive the effective resonant frequency of the ${\mathrm{TM}}_{01}$-like mode and the slot mode. The key parameters mentioned in the equation facilitate the subsequent rapid estimation and optimization of the antenna size. The results exhibited by the CMA demonstrate a high degree of agreement with the results obtained through the empirical formula.

4. Simulated and Measured Results

4.1. Antenna Performance Analysis

The antenna performance in the on-body case was investigated using a three-layer model with dimensions of 150 × 150 × 30.5 ${\mathrm{mm}}^3$, as shown in Figure 7. The antenna was placed 10 mm above the model to simulate the thickness of a real-life multilayered garment. Table 2 includes the dielectric properties of whole tissues at 3.5 GHz and 5.8 GHz, respectively.

To validate the efficacy of the aforementioned design approach, a button antenna prototype was fabricated, as shown in Figure 8a. The simulated reflection coefficients cover a −10 dB working band from 3.48 to 3.6 GHz and 5.65 to 6.03 GHz in free space. It is clear that the combination of the resonant frequency of the slot and that of the patch expands the high-frequency impedance matching bandwidth. The measurements taken on different parts of the body show slight shifts, which are tolerable and primarily attributed to limitations in precision when manually cutting conductive fabrics and errors in the welding process. Nevertheless, the measured impedance bandwidths in both frequency bands are adequate for effective WBAN applications, thus verifying the robustness of the antenna design.

The radiation performance was evaluated in a SATIMO microwave anechoic chamber, and the results were compared to simulations. Figure 9 shows the comparison between the measured and simulated 2D far-field normalized radiation patterns. A monopole-like omnidirectional radiation pattern can be observed at 3.5 GHz, while a broadside radiation pattern with a right-handed circular polarization can be seen at 5.8 GHz. Both patterns align well with the intended operational modes.

In addition, Figure 10 presents a comparison between the simulated and measured peak gains. At 3.5 GHz, the experimental peak gain stands at 1.1 dBi, slightly lower than the simulated value of 1.2 dBi. Similarly, for the frequency of 5.8 GHz, the measured maximum gain is 6.8 dBi, which closely matches the simulated result of 6.9 dBi. In Figure 11, the measured 3 dB AR bandwidth is 130 MHz (5.73–5.86 GHz). Compared with the simulation results of 140 MHz (5.71–5.85 GHz), it has a certain degree of offset towards higher frequencies. The antenna’s radiation efficiency is more than 80% across the entire bandwidth for both on-body and off-body conditions, as shown in Figure 12.

Since the antenna might tilt due to unavoidable minor movements, Figure 13 illustrates an example of the antenna’s radiating part tilting along the y-axis. The simulated results in Figure 14 and Figure 15 indicate that different tilt angles have minimal impact on the antenna’s reflection coefficient and axial ratio and they still cover the target bandwidth. This demonstrates the high robustness of the button antenna in actual use, maintaining stable performance even in dynamic environments.

4.2. SAR Performance Analysis

When designing wearable antennas that operate in close proximity to the lossy human body, SAR evaluation is paramount as it provides a critical metric to quantify the RF energy absorbed by human tissues. Accurately assessing and controlling SAR levels within safe thresholds can effectively protect human health. As shown in Figure 16, the calculated SAR distributions are achieved by using the software CST Microwave Studio. The IEEE C95.3 standard was used to calculate the simulated SAR values, with the reference power received by the antenna set at 500 mW and a distance of 10 mm between the antenna and human tissues. It can be seen that for both on-body and off-body modes, the 1g-averaged SAR peaks are 0.660 W/kg and 0.101 W/kg, respectively, which are much lower than the Federal Communication Commission (FCC) standard of 1.6 W/kg. The 10g-averaged SAR peaks are 0.331 W/kg and 0.057 W/kg, well below the Council of Europe (CE) standard of 2 W/kg. Therefore, it can be concluded that SAR will not be an issue for future applications.

4.3. Comparison of Antenna Performance

In Table 3, a comparative analysis of previous designs and the proposed antenna is listed. Notably, our antenna achieves dual-mode operation with a streamlined design utilizing a single port and cost-effective textile substrates, significantly cutting down on production expenses. Unlike conventional wearable antennas intended for on-body applications, which typically exhibit LP in single- or dual-band operations, our antenna uniquely combines LP at lower frequencies with CP and high gain at higher frequencies. This dual-mode capability offers significant advantages for wearable applications, enhancing link reliability and making it a prime candidate for WBAN.

5. Conclusions

A dual-band, dual-mode button antenna is introduced for emerging 5G networks and ISM applications. Specifically, for the on-body mode at 3.5 GHz, it exhibits a vertically polarized monopole-like radiation pattern; for the off-body mode at 5.8 GHz, the antenna generates a CP broadside radiation pattern. An analysis of the antenna’s dual-mode operational principles was conducted employing the CMA technique, while also exploring the implications of human body loading. To substantiate the proposed design, a prototype was crafted and evaluated in diverse settings. Notably, all experimental outcomes closely align with the anticipated simulations. Furthermore, the SAR values indicate that the antenna fulfills health and safety standards. Overall, the button antenna possesses the merits of dual-band, dual-mode, robust, and flexible performance, positioning it as a promising candidate for future multifunctional wearable devices in emerging 5G networks.