Compact Wideband Loop Antenna for Earbuds

This paper presents the compact wideband loop antenna with a small size of 7.05mm’s radius covering ultra-wideband above 6 GHz, Wi-Fi, and Bluetooth bands for earbuds. The wide bandwidth of the proposed compact loop antenna is obtained with the antenna structure consisting of the loop antenna and the capacitive loading pin inserted in the middle of the loop antenna. The loop antenna with the capacitive loading pin supports multi-resonant modes. A wider bandwidth can be obtained by adjusting the position of the capacitive loading pin. The half-wavelength resonant frequency of the basic loop can easily be tuned through the physical capacitance between the capacitive loading pin and loop antenna. Additionally, the impedance loop is considered for the impedance matching of the compact wideband loop antenna. The simulated and measured results are included to evaluate the performance of the proposed compact wideband loop antenna. Consequently, we obtained that the simulation and measurements results are consistent.


I. INTRODUCTION
After the Federal Communications Commission approved the operation of ultra-wideband (UWB) in February 2002, related technologies have been continuously developed [1]. UWB technology has recently been commercialized in small electronic devices such as mobile phones and accessories. The most significant advantage of UWB is that it can significantly reduce the position error indoors using a modulation-based pulse without a carrier. Through this, users can receive various advertisements and service benefits while moving. Earbuds like True Wireless Stereo (TWS) earphones are products that can maximize UWB service. It is not simply a wireless earphone connected to a mobile phone to stream music based on Bluetooth (BT), but it can be an independent accessory that gets voice services in various places. Because the Wi-Fi repeater has location information, Wi-Fi technology is essential for this indoor positioning-based UWB service. Therefore, research on UWB antennas, including a Wi-Fi band for earbuds, is necessary.
However, research on antennas for earbuds has been conducted only to cover the BT band and reduce the size [2]- [6]. It is essential to develop an advanced antenna to operate UWB and Wi-Fi band for local-based service (LBS) in earbuds. Small-sized wideband antennas have been studied to review the antennas applicable to the earbuds [7]- [18]. There are methods to Planar Inverted-F Antenna (PIFA) with slot [7], [8], slot antenna with Inverted-F Antenna (IFA), dipole, and monopole [9]- [11], appropriate design cases of feeder and slot antenna [12]- [15], and how to make additional resonant modes between the resonant modes of the loop antenna by adding a capacitance factor in the middle of the loop antenna in series [16]- [18]. Although the previously reported methods have differences in the limited ground size of the earbuds and antenna positions, it is shown that a wideband design is possible by designing a basic antenna and creating additional operation modes. Antennas that require a lot of antenna areas, such as PIFA and slots, cannot be designed in earbuds to secure the touch sensor area. Loop antennas require a wider bandwidth than previous studies; thus, a different approach is required. This paper proposes a method to operate in UWB above 6 GHz, Wi-Fi, and BT bands by reflecting additional elements to the loop antenna in earbuds. The capacitive loading pin and ground are used to design the loop antenna for wideband performance and miniaturization. Two sub-loops are created by adding a capacitive loading pin and ground, enabling wideband design. The loop antenna with ground generates 0.5λ resonant mode to enable a miniatured loop antenna. Thus, 0.5λ, 1.0λ, 1.5λ, and 2.0λ resonant modes are implemented in the basic loop antenna; 0.5λ, and 1λ resonant modes are implemented in the sub-loop antenna to operate eight resonant modes. These resonant modes are later verified through current distribution and impedance analysis. Broadband performance can be realized through the location of the capacitive loading pin. Additionally, the 0.5λ resonant frequency of the basic loop can easily be shifted through the physical capacitance between the capacitive loading pin and radiating loop. The impedance loop is added to match the impedance of a wideband. The proposed antenna operates UWB above 6 GHz (6-10.6 GHz), Wi-Fi (5.15-5.825 GHz), and BT (2.4-2.4835 GHz) bands. In the UWB band above 6 GHz, it operates S 11 -10 dB, which is the UWB standard for LBS use, and in the BT and Wi-Fi bands, it is designed to be S 11 ≤−6 dB based on a small antenna for earbuds [2]- [5]. The proposed antenna is designed within a radius of 7.05 mm, considering the size of a commercial coin battery of the ultra-    small earbuds. Therefore, it is suitable for earbuds and wearable devices. The performance of the proposed loop antenna is confirmed using CST [19] and measurement. The equivalent structure is illustrated in Fig. 2 to demonstrate the operation principle. The basic loop consists of a feeding pin, radiating loop, shorting pin, and ground. The sub-loops are formed by dividing the main loop through the capacitive loading pin for broadband performance. There is a physical capacitor between the capacitive loading pin and radiation loop to tune the 0.5λ resonant mode of the basic loop. The impedance matching pin is added to match the impedance of 50 ohms. The size of the circular ground has a radius of 7.05 mm, considering the size of the coin-type battery used in the earbuds. The circular ground is Taconic RF-35 substrate ( 3.5, loss tangent = 0.0018, and thickness = 0.5 mm). The other parts are Taconic RF-35 substrate ( 3.5, loss tangent = 0.0018, and thickness = 0.25 mm). Table I presents the parameter values of the proposed antenna. Fig. 3 shows the current distributions of the proposed loop antenna, where the cross " " represents the current null point. In the basic loop, 0.5 , 1 , 1.5 , and 2 resonant modes operate, and in each sub-loop, 0.5 and 1 resonant modes work. Fig. 4 shows the input impedance of the proposed loop antenna. The feeding and shorting pins are perpendicular to the ground, and the radiating loop is parallel to the ground. The direction of the current changes in each mode, and the width of each pin affects the impedance. Therefore, the width of each pin is optimized to match the impedance of 50 ohms. Table II shows the wavelength for the operating frequency according to each operation mode. In the case of sub modes, looking at the current distribution in Fig. 3, other sub-loop and basic loop exist as parasitic components. As a result, the wavelength seems to be increased.  9.03 1 λ 0 * λ 0 is the wavelength of resonant frequency in free space It is necessary to use a 0.5λ resonant mode for a miniaturized loop antenna. One way to create a 0.5λ resonant mode is using a ground [20]. The loop antenna with the ground is an unbalanced feeding structure. One end of the loop antenna is connected with the shorting pin to the ground plane, and the other end serves as the feeding point. Fig. 5 shows the ground effect of the loop antenna when the width of the loop antenna is 1 mm. Half-wavelength resonance of the loop antenna is possible due to the ground effect in the earbud model.

B. EFFECT OF THE CAPACITIVE LOADING PIN
The capacitive loading pin is used for wideband performance and moving 0.5 resonant frequency in basic loop. Openended antennas such as PIFA [21], IFA [22], and dipole [23] use a capacitive loading to reduce antenna size. For loop antenna, capacitive loading allows additional wideband designs. Fig. 6 shows that new resonant modes are generated when a capacitive pin is inserted. Additionally, as CTL increases, the UWB, and Wi-Fi bands become broader. CTL is the parameter for the position of the capacitive loading pin. CTL is the difference in length between the two sub-loops.
is the length of the radiating loop from the feeding pin to shorting pin 2 /2 . is the length of the radiating loop from the feeding pin to the capacitive loading pin /2 /2 , and is the length of the radiating loop from the capacitive loading pin to the shorting pin /2 /2 . Fig. 7 shows the simulated input impedance to verify the new resonant modes. Fig. 7(a) shows the added resonant modes of two sub-loops created by the two subloops because of the capacitive loading pin. Fig. 7(b) shows that as the CTL increases, the resonant frequencies of the sub-loop1 move to a high-frequency band, whereas the resonant frequencies of the sub-loop2 move to a lowfrequency band. Additionally, as the CTL value increases, the electrical length of sub-loop1 decreases, whereas the electrical length of sub-loop2 increases. Therefore, as the CTL value increases, a wider bandwidth is satisfied.
The loop antenna has the same current distribution as the quarter-wavelength monopole in half-wavelength mode. Therefore, when the capacitive loading pin is inserted at the midpoint of the radiating loop where the current becomes null in the half-wavelength mode of the loop antenna, the operating frequency can be moved to the lower frequency band. Thus, it is possible to satisfy the BT band without increasing the antenna's physical length. The shift of the resonant frequency is explained using Equation (1). The resonant frequency shifts to a lower frequency as the inductance and capacitance values increase.
The capacitance of physical capacitor can be expressed as follows: (2) where C is the capacitance, is the permittivity, A is the area of the capacitor, and d is the space of both plates. The spacing d of both plates is fixed 0.25 mm, which is the thickness of the PCB's substrate. The relative permittivity of the designed PCB is 3.5. Because the physical capacitor plate area is 0.2 mm mm, the half-wavelength resonant frequency of the basic loop can be adjusted through . Fig. 8 shows that the resonant frequency of the half-wavelength resonant mode of the basic loop can be shifted to the BT band according to

C. EFFECT OF THE IMPEDANCE MATCHING PIN
IFA can easily match 50 ohms by adding one loop to the Inverted-L antenna and increasing the low input resistance by adjusting the length of the matching loop. As the length of the impedance matching decreases, the current value to the impedance matching increases, reducing the current value input to the radiation antenna. Therefore, the input resistance value entering the radiating antenna increases [24]. The technique of matching the impedance through the additional current path can also be applied to the loop antenna. Fig. 9 shows the simulated reflection coefficient according to the length of the impedance matching pin. Fig.  10 shows the simulated input impedance to analyze the change in input impedance. The input resistance value increases as the length of the input impedance matching pin reduces.

III. RESULTS AND DISCUSSION
The proposed loop antenna with optimized parameters is conducted as shown in Fig. 11. The fabricated antenna is fed with a coaxial cable, as shown in Fig. 11(e). Fig. 12 shows the simulated and measured reflection coefficients in free space (FS) and in-the-ear (ITE). Head model provided by CST is used for simulation, and EAR-A2-P10 model is used for measurement. Bandwidth is defined as S 11 −6 dB for BT and Wi-Fi, and S 11 -10 dB for UWB. The Comparing the results of ITE and FS conditions, the resonant frequencies move to a slightly lower frequency band due to the body effect [5]. And the measured results show the reduced bandwidth compared with the simulated results because the measurement environments such as the position of the antenna and the ear shape are not exactly matched with the simulation and there exists some minor manufacturing error due to the coaxial feeding cable. Fig. 14 shows the difference in total efficiency and peak gain measured in FS and ITE. The measured total efficiency is 90.57% and 36.13% in FS and ITE at 7.5GHz, respectively, reducing the total efficiency by about 40%. Total efficiency degradation through the human body effect is like the previous study [5]. Similarly, the measured peak gain is 6.89 dBi and 1.69 dBi in FS and ITE at 5.75GHz, respectively, reducing the peak gain by 5.2 dB.
For LBS using UWB, an omnidirectional radiation pattern is required. Fig. 15 shows the simulated and measured radiation patterns in FS. It has an omnidirectional characteristic. This is because the loop configuration is designed to make the current distribution in the x, y, and z directions possible. Additionally, the current distribution is diversified as the resonance mode of the loop antenna changes according to the frequency. The abnormal peaks appear in the -z direction of the measured patterns due to the elongated coaxial cable which is fabricated for the ITE mode measurement.
The performance of the proposed loop antenna is compared to the wideband antennas in Table Ⅲ. Compared with reported other wideband antenna designs, the proposed

IV. CONCLUSION
This paper proposes the compact loop antenna for earbuds that covers the application of UWB, Wi-Fi, and BT. It is designed to cover the application of UWB and Wi-Fi to provide an LBS in a streaming service based on simple BT communication.
The proposed antenna designs a basic loop antenna on the ground and puts the capacitive loading pin in the middle to create two sub-loops. Multiple resonances can be created to ensure wideband performance by adjusting the capacitive loading pin's position to tune the sub-loop antenna's length. The capacitor for capacitive loading is physically designed. The capacitive effect can be controlled by adjusting the area of the physical capacitor. Consequently, the frequency of the 0.5λ resonant mode of the basic loop can work at BT band. The impedance matching loop can be added to match low impedance to high impedance. The impedance matching loop is in the minimum area of PCB used as ground. The proposed loop antenna is designed in a very small size. Therefore, it is suitable for earbuds and other wearable devices.