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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014
A Small Dual-Band Asymmetric Dipole Antenna for 13.56 MHz Power and 2.45 GHz Data Transmission Yuji Tanabe, Member, IEEE, Tammy Chang, Student Member, IEEE, Alexander J. Yeh, and Ada S. Y. Poon, Senior Member, IEEE
Abstract—Despite the disparity between typical small antenna designs for wireless powering and far-field radiation, this letter proposes a single asymmetric dipole antenna for both applications on a printed circuit board (PCB) of dimension 10 24 mm . In this design, current cancellation is applied as a technique to enhance far-field radiation efficiency while maintaining inductive wireless power transfer performance. The measured radiation efficiency at 2.45 GHz is 57%, and the power transfer efficiency at 13.56 MHz over a distance of 20 mm is 2.15 dB. The results demonstrate the feasibility of a single dual-band antenna operating at two distinct frequencies while attaining reasonable efficiencies. Index Terms—Bluetooth, electrically small antenna, near-field communication (NFC), wireless power transfer.
I. INTRODUCTION
W
ITH the advance of wireless powering technologies, there is a growing demand for devices that incorporate both data transmission and wireless powering capabilities. Available technologies for such applications include Bluetooth for communication and near-field communication (NFC) for wireless powering. However, a challenge for antenna design is the integration of both Bluetooth and NFC technologies within an increasingly confined space while still providing good radiation and power transmission performance. As wireless devices continue to shrink in size, electromagnetic coupling between different antennas becomes more difficult to mitigate, and the need to use a single antenna for multiple applications becomes necessary. As a result of the severe space constraints for modern wireless devices, this letter proposes a dual-band antenna that has the ability to operate in both near-field and far-field regimes, with separate operating bands at 2.45 GHz and 13.56 MHz for Bluetooth and NFC, respectively. For our application, the antenna was designed under the constraints for a printed circuit board (PCB) (Fig. 1) to be completely sealed within an athletic mouthguard for in vivo head impact research at Stanford University, Stanford, CA, USA [1], [2]. This application required Bluetooth capability for short-range, real-time data transmission [3]. Due to the nature of the design, near-field power transfer was required Manuscript received March 07, 2014; revised May 05, 2014; accepted May 29, 2014. Date of publication June 12, 2014; date of current version June 20, 2014. The authors are with the Department of Electrical Engineering, Stanford University, Stanford, CA 94035 USA (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2014.2330496
Fig. 1. Proposed antenna design. (a) Perspective view. (b) Top view. (c) Bottom view. (d) Side view.
to wirelessly power the system. NFC was chosen for power transfer at 13.56 MHz as it requires a lesser number of turns
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TABLE I DETAILED DIMENSIONS IN MILLIMETERS
than the Qi standard to couple with the transmitter, whose frequency range is in the low kilohertz [4]–[7]. Dual-band and multiband antenna designs are popular in the far-field regime for traditional frequency band separation within approximately 1–3 GHz (e.g., 800–1900 MHz for cellular low and high bands, 2.4–5 GHz for WLAN) [8]–[12], where different current paths are utilized for different bands. However, antennas are not typically designed for dual-band operation in both near- and far-field regimes. In this letter, we propose a dual-band asymmetric dipole antenna for two disparate communication channels (far-field radiation and near-field inductive coupling), which is, to the best of our knowledge, the first of its kind. II. ANTENNA DESIGN Fig. 1 shows the schematic for the design of the proposed antenna including pads on the FR-4 board for sensor integration, where the board is 10 24 mm in dimension. It is used to hold the motion sensors, random access memory, analog-to-digital convertor, and other electronics. Because of the PCB property reserved by the electronics, the antenna placed on this board is forced to follow the meander line technique for small antennas [13]–[15] around the components. In this design, the primary focus is to minimize coupling effects that degrade radiation efficiency for Bluetooth operation at 2.45 GHz; lower frequency inductive coupling is less affected by smaller pieces of conductor since these are much smaller than the wavelength at 13.56 MHz. Dimensions for the design are provided in Table I and are optimized for operation at 2.45 GHz (Port 1) for Bluetooth communication and 13.56 MHz (Port 2) for wireless charging using the full-wave electromagnetic field simulation software CST Microwave Studio, which adopts a finite integration technique [16], [17]. In the simulation, the antenna is matched to 50 using a matching circuit composed of a 1-nF capacitor in series, and 2-nF capacitor in parallel. The two arms of the antenna reside on the front side of the sensor board, as shown in Fig. 1(b). The antenna is based on an asymmetric dipole, in which the upper arm of the dipole is connected to a 3-turn loop on the back side. The current flow switches at the end of the dipole arms that are in length and allow for phase shift of 180 and is transmitted to the backside loop [Fig. 1(c)]. However, the loop element extends in the opposite direction against the upper arm, thus both currents will
Fig. 2. Simulated current distribution and simplified demonstration of the enhancement and cancellation of current flow on the proposed antenna.
Fig. 3. Simulated radiation pattern of the proposed antenna at 2.45 GHz. (a) -plane. (b) -plane.
be in phase and hence produce more efficient radiation. The other (bottom) arm of the dipole and the loop elements are located close to one another. Under this condition, the currents in the top pair of loop elements will mutually cancel, allowing the coupling between the Bluetooth and NFC loop antenna to be mitigated, while the current flow on the bottom arm will be enhanced when the current direction of the loop is in phase. The technique described can be seen in Fig. 2, which shows the simulation results for the current flow at 2.45 GHz. It is clear that the strongest current path is along the asymmetric dipole at the front side, and in the loop at the back side. The dielectric-loaded half-quarter-wavelength at this frequency is around 15 mm, thus the current on the backside loop is canceled because the currents run in opposite directions at 2.45 GHz, and the canceled current leads to thermal loss. While the current level of the upper arm is slightly higher than the bottom arm due to reflection from the ends of the dipole arms, it is confirmed that the current is strong in the vicinity of the feeding and weak at the end of the dipole, confirming that the Bluetooth antenna is still acting as a dipole at 2.45 GHz despite the connected loop configuration. The simulated radiation pattern of the proposed antenna at 2.45 GHz is shown in Fig. 3 and approximates a dipole-like pattern. This suggests that the dominant current on the antenna is directed along the longer dimension of the PCB. Therefore, the current along the short side is minimized at 2.45 GHz and does not contribute to radiation. As shown in Fig. 2, the current
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014
Fig. 5. Measured radiation efficiency of the antenna. (a) Measurement setup. (b) Simulated and measured results (2.45 GHz is marked by the gray line).
Fig. 4. Simulated and measured reflection coefficients of the proposed antenna. (NFC at Port 2). (c) (Port 1 to Port 2). (a) (Bluetooth at Port 1). (b)
in the 3-turn loop is strong, but because it is mostly out of phase, it does not contribute to radiation. III. MEASUREMENTS Fig. 4 shows the reflection coefficients of the fabricated antenna as measured in free space, where Port 1 is used to drive the antenna for Bluetooth communication and Port 2 is used to harvest inductive wireless power. It is worth noting that the antenna itself will only be driven at Port 1 for real applications, but Port 2 is included here to measure the amount of power coupled
into the matching network as a result of radiation at 2.45 GHz. The discrepancy between the measured and simulation results seen in Fig. 4(a) can be attributed to the inaccuracy of the simulation sweep over a wide range of frequencies. The measured results show that the antenna operates as intended, centered at 13.56 MHz and 2.45 GHz. From Fig. 4(c), it is evident that the coupling between the two ports is not strong, thus allowing for high radiation efficiency at 2.45 GHz. This letter applies a simple technique for measuring the radiation efficiency of small antennas by using a tunable spherical Wheeler cap [18]–[20] made of brass with an inner diameter of 70 mm. The Wheeler cap method utilizes a multimode tuner designed as an adjustable brass stub with a ring to compensate the undesirable reductions of the measured efficiency caused by both TM and TE mode resonances of the Wheeler cap. Fig. 5 shows the results of radiation efficiency obtained from the tunable Wheeler cap, where it is clear that the efficiency of the antenna yields a smoothed curve when the resonant modes are appropriately tuned. The experimental results, measured in the frequency range of 2000–3000 MHz, show that the efficiency drop can be minimized by enveloping over all maxima of efficiency curves measured for five different tuner positions, giving a resulting radiation efficiency of 57% for the proposed antenna at 2.45 GHz. This value is comparable to or higher than the reported radiation efficiencies for dual-band antennas in literature [11], [12].
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antenna design in small devices for wireless charging and communication devices. Future work includes embedding the sensor board and antenna in the device as well as evaluating the performance of the system as a whole. ACKNOWLEDGMENT The authors would like to acknowledge CST, Inc., and AET, Inc., for their assistance and support in the antenna simulations and measurements. REFERENCES
Fig. 6. Measured power transfer efficiency and experimental setup (20 mm is marked by the gray line).
We use a light-emitting diode (LED) to measure the received power of the 13.56-MHz NFC antenna. The minimum power required to turn on the LED is 800 W. At this power level for the LED load, the impedance of the rectifier is approximately 2000 . However, it is nearly impossible to design an NFC antenna with a matched input impedance without including lump components. Furthermore, near-field antennas typically consist of loops that are inductive. Thus, an impedance transformation is needed to interface with the rectifier. For the measurements, a 100-nF series capacitor and 720-pF parallel capacitor are used for matching. Fig. 6 shows the experimental setup and the power transfer efficiency (PTE) measurement results for variable distance between the aligned transmitting and receiving antennas. The PTE is defined as (1) where is the input power to the transmitter and is the LED turn-on power level (800 W). As seen in Fig. 6, the proposed antenna exhibits a power transfer efficiency above 5 dB up to a distance of 50 mm, reaching a maximum of 2.15 dB at a distance of 20 mm for the designed matching circuit. This PTE is below the theoretical limit [21] due to power losses in the LED/rectifier circuitry and the limits imposed by small antennas [7], but is sufficient for applications such as the mouthguard scenario [1], [2], where wireless charging can be achieved within a close distance of the mouthguard under test. IV. CONCLUSION In this letter, a single small dual-band antenna with 2.45-GHz Bluetooth communication and 13.56-MHz NFC wireless powering capability has been proposed. The antenna is based on the idea of the asymmetric dipole with a 3-turn loop connected to one side of the dipole arm, which can cancel the effects from the 3-turn loop at 2.45 GHz in order to enhance radiation efficiency. Experimental results show that the designed antenna has good radiation efficiency of 57% at 2.45-GHz Bluetooth, and high power transfer efficiency of 2.15 dB at 13.56-MHz NFC at a distance of 20 mm between the transmitter and receiver. This letter indicates the feasibility of integrating near- and far-field radiation in small devices, offering a new design approach for
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