A Dual-Frequency Ultralow-Power Efficient 0.5-g Rectenna. Robert Scheeler, Sean Korhummel, and Zoya Popović

Transcription

1 IMS2013 STUDENT DESIGN COMPETITION WINNER Wireless Energy Harvesting A Dual-Frequency Ultralow-Power Efficient 0.5-g Rectenna Robert Scheeler, Sean Korhummel, and Zoya Popović The second annual Student Wireless Energy Harvesting (WEH) Design Competition was held during the 2013 IEEE Microwave Theory and Techniques Society (MTT S) International Microwave Symposium (IMS2013) in Seattle, Washington, United States. This year, the competition parameters were modified from those of last year [1], and a new figure of merit (FoM) was established. The overall goal of the competition was to demonstrate low-mass hardware that can efficiently receive and rectify extremely low-incident power densities at two frequencies, with a fixed dc load. As the radio-frequency (RF) environment gets more saturated with spurious power, designs from this competition will become a feasible way to energize ultralow-powered or low-duty-cycle hard-to-reach sensors. Concepts such as Internet-of-Things, in which small ubiquitous devices and sensors will log data and send it to the cloud, could benefit from wireless energy harvesters. These sensors will not have convenient ways to stay powered unless power harvesting circuits are used for the sensor hardware. The requirements for the energy harvester presented in this article are as follows: incident power density: 1 μw/cm 2 at 2.45 GHz and 915 MHz power transmission: separately at each of the two frequencies with linear vertical polarization and known position of source total mass of prototype: less than 15 g, including connection for dc output, but not including any type of stand minimum harvested power at each frequency: 1-μW dc power delivered to a 2.2-kX load. Robert Scheeler (robert.scheeler@colorado.edu), Sean Korhummel (sean.regaliakorhummel@colorado.edu), and Zoya Popović (zoya.popovic@colorado.edu) are with the University of Colorado, Boulder. Digital Object Identifier /MMM Date of publication: 21 January 2014 January/February /14/$ IEEE 109

2 If the entire power reception device is codesigned such that the antenna impedance is matched directly to the nonlinear rectifying circuit, losses from the matching network can be reduced significantly, increasing overall efficiency. The FoM is defined as the total dc output power PDC normalized to 10 μw, divided by the square of the largest dimension D of the WEH device normalized to 100 cm 2 : R PDC ^nwh V S W FoM log 10 nw = 10 S W S D cm, 2 2 ^ h W S 2 W 100 cm T X where the total output dc power is weighted at each frequency as follows: PDC = 017. PDC at 915MHz PDC at GHz. It can be seen by inspection of the FoM expression that the higher frequency is weighted more heavily. In addition, when considering the size of the structure, the design should be optimized for 2.45 GHz operation as the antenna will be smaller. But the minimal received dc power at 915 MHz still needs to be ensured, although the electrical size of the antenna is reduced. The design of a wireless energy harvester requires designing an antenna integrated with a nonlinear rectifying circuit. Dual-frequency rectenna designs have been presented in the literature for ISM and GSM frequencies [2], [3]. Many examples of rectenna designflows follow the same path, e.g., [4] [6]: 1) design a rectification circuit, 2) design an antenna, 3) implement a matching network to couple as much of the received power as possible to the rectifier, and 4) design a dc collection circuit (RF block) that delivers the rectified power to the dc load. If the entire power reception device is codesigned such that the antenna impedance is matched directly to the nonlinear rectifying circuit, losses from the matching network can be reduced significantly, increasing overall efficiency. When designing low-power harvesting circuits, such as in the case of this competition, minimizing the number of components in the design becomes a worthwhile endeavor. With the above constraints in mind, the rectenna prototype shown in Figure 1 is designed to have the following features: Direct matching of antenna impedance to the rectifier impedance eliminates the loss incurred by using a separate matching network. This technique has been shown to achieve high RF to dc conversion efficiencies at low incident power densities in previous work [7]. Only two lumped components are used: the diode and a capacitor, resulting in minimal loss. The rectenna is fabricated on a flexible Rogers Ultralam substrate [8] to eliminate one linear dimension and obtain low mass and conformal shape. The design methodology will be presented by first discussing the rectifying element, in this case, a Schottky diode, to determine the optimal impedance for the antenna. Due to the nonlinear diode impedance, some assumptions about the input power are required at this stage of the design. The optimal impedance that needs to be presented to the rectification element is determined at a given power level and frequency, and then used as a design requirement for the antenna impedance and gain. Diode Impedance Analysis and Antenna Considerations The choice of the rectifying element is paramount when designing a WEH circuit. The rectifying element parameters determine the frequency range at which rectification is efficient. The main parameters are the diode nonlinear capacitance Cd^Vdh, the reverse breakdown voltage Vb, and the forward turn-on voltage V f, which in turn determines the dynamic range of the RF power the device is capable of rectifying. The resistance Rs of the diode not only limits the efficiency of the rectifier directly due to power dissipation but it also influences the range of impedances the antenna/matching network will need to present to best match the diode. The exact impedance will vary due to the nonlinear resistance, Rd^Vdh, which is dependent on the incident power level and the frequency of operation. The diode selected for this prototype is a Skyworks GaAs Schottky SMS with RS= 20 Ω, Cd= 014. pf, and V f = 0. 16V. To analyze the rectifier circuit, the power delivered to the diode must be determined, and therefore the gain of the antenna must be known. The source of the harvested energy in this case is given as a linearly polarized wave incident from a single direction. Therefore, a Yagi-Uda antenna was implemented with the rectifier integrated at the feed-point of the driven element. The Yagi-Uda antenna provides relatively high gain for a given area and can be printed on a thin flexible substrate, thus positively affecting two of the quantities that maximize the FoM given above. A three-element (single director and reflector) antenna is chosen as the directivity is approximately D0 = 8 dbi [9] and -3 dbi at 2.45 GHz and 915 MHz, respectively. The gain at 915 MHz is determined by simulation in Ansys HFSS after the Yagi-Uda antenna design was optimized to match the diode at 2.45 GHz. The power received by 110 January/February 2014

3 At the 2013 Student WEH Design Competition, the dc power harvested at 915 MHz and 2.45 GHz were measured to be 11.6 μw and 65.6 μw, resulting in FoM = db. Figure 1. The prototype dual-frequency Yagi rectenna implemented on a flexible substrate. the antenna was estimated by first determining the maximum effective area from the directivity A eff 2 m = D0. 4r Then, the power density is multiplied by the maximum effective area, resulting in an estimate of the input power of -11 dbm and dbm for 2.45 GHz and 915 MHz, respectively. Using these power levels, source-pull simulations are performed using AWR Microwave Office s harmonic balance nonlinear simulator to determine the optimum impedance that needs to be presented to the diode to provide the highest dc voltage across the given 2.2-kX load. A nonlinear spice model for the Schottky diode, provided by Skyworks Inc. [10], is used in the simulations and the results of the source-pull simulations are shown in Table 1. Both the 915 MHz and 2.45 GHz frequencies are considered, and the RF power is determined by an estimation of the effective area times the power density of 1 μw/cm 2. For both frequencies an RF-dc conversion efficiency of greater than 40% is possible with an output power of greater than 18 μw. Dual Frequency Impedance Match of Antenna Once the impedances that should be presented to the diode rectifier at the two frequencies for optimal rectification are known, a Yagi-Udi antenna is designed to be matched to the values of impedances corresponding the optimal reflection coefficients from Table 1. Generally, Table 1. Optimum reflection coefficient determined from source-pull simulations using AWR Microwave Office. Frequency Source-Pull C Input Power dc Output Power a Yagi-Uda antenna array is designed to be resonant at the design frequency and matched to 50 X. Given the source-pull data, it is clear that an inductive reactance is needed to match the diode at 2.45 GHz. The inductive reactance is achieved by using a well-known inductive feed method from the design of RF identification (RFID) tags [11]. The three-element Yagi antenna with an inductive feed consists of a reflector, a driver, and a director element with lengths of 0.5, 0.43, and 0.4 m, respectively, and widths of 4 mm. The distance of the reflector from the driven element is 0.25 m and the distance of the director element from the driven element is 0.12 m. These dimensions were varied to achieve a match directly at the diode rectifier terminals where the source-pull reference plane is defined. A dc collection network is designed to isolate the dc load from the antenna, i.e., in the same manner as a bias circuit. The dc collection line uses two series quarter-wave shorted coplanar stripline (CPS) lines, as seen in Figure 2(a). A capacitor that is an RF short ensures RF-dc isolation, and the dc load does not affect the RF match. The desired performance of the shorted quarter-wave CPS lines is to look like open circuits at the diode feed point as seen in Figure 2(b) but, however, still provide a path to deliver power to the dc load ^RDCh. The three-element Yagi-Uda antenna achieves greater than 6-dBi gain when matched at 2.45 GHz, and the gain is reduced to -3 dbi at 915 MHz. The reduced gain at 915 MHz is to be expected as the physical size is the same but the electrical size is much smaller; as a result it is not as directive and efficient. Though low, this is still sufficient to provide dbm of power to the antenna feed point at 915 MHz, which will turn on the diode, provided the antenna is matched. The next step is to match the antenna at 915 MHz to the diode. Upon further inspection of the bias line, it can be seen that at 915 MHz, the shorted CPS line is now electrically short (<m/4) and inductive, with a reactance given by Zin = jz0 tan bl. Figure 2(c) shows the RF-dc Efficiency 915 MHz dbm dbm 40.7% 2.45 GHz dbm dbm 56.2% equivalent rectenna circuit at 915 MHz. The value of the bypass capacitor CS is not critical at 2.45 GHz since it only needs to provide an RF short, assuming the package inductance of the diode is not appreciable. At 915 MHz, however, the capacitance can be used to tune the inductive reactance of the shorted stubs, which is in parallel with the diode. Varying the capacitance January/February

4 To analyze the rectifier circuit, the power delivered to the diode must be determined, and therefore the gain of the antenna must be known. Yagi Yagi D 1 D 1 Short (a) C S C S m/4 at 2.45 GHz Schematic View at 2.45 GHz R dc R dc + V dc - + V dc - will enable an impedance match at 915 MHz without affecting the 2.45 GHz match as the quarter-wave CPS lines will still look like open circuits. The antenna complex input impedance is simulated using the three-dimensional planar method-ofmoments solver Axiem, available in AWR Microwave Office. The simulated full-wave performance is then combined with measured capacitor data and an ideal 2.2 kx dc load to determine the input impedance seen at the feed point of the antenna. The simulated results in Figure 3 show the input impedance at 915 MHz and 2.45 GHz as the capacitor value is varied 1 27 pf. The source pull contours of constant dc power (Pdc) for the diode are shown at both 915 MHz and 2.45 GHz to demonstrate how much power is delivered to a 2.2-kX load for various source impedances where the colors represent dc power levels as shown in the legend. The capacitor is modeled using measured data provided by American Technical Ceramics (ATC) for 600L 0402 package capacitors. Note that the 2.45 GHz input impedance does not change for capacitor values between 1 and 27 pf. On the other hand, the capacitor value can be used to design the input impedance for an improved match to the diode at 915 MHz. Without the capacitor CS, the input impedance at 915 MHz is capacitive as shown in Figure 4 by the diamond symbol on the dashed line. This is due to the small electrical size of the antenna. If a relatively small value of capacitance is used for CS, the antenna capacitance is still dominant in the parallel impedance. As the value of the CS increases, it becomes closer to an RF short, thus allowing the inductance of the shorted stubs to dominate the (b) j1.0 Yagi D 1 Schematic View at 915 MHz C S R dc + V dc 2.45-GHz Contours 915 MHz 2.45 GHz j2.0 j P dc (dbm) MHz Contours -20 (c) Figure 2. (a) A schematic of the rectenna, including the dc collection network, (b) an equivalent circuit at 2.45 GHz shows that the dc load is isolated from the antenna, and (c) an equivalent circuit at 915 MHz shows the inductive shorted CPS lines can be combined with an appropriate value of the capacitor Cs to match the diode to the antenna at 915 MHz. Figure 3. The simulated antenna input impedance versus shunt capacitor value CS, which varies from 1 to 27 pf is shown in the black solid line for the 915-MHz frequency and can be used to match the diode impedance at that frequency, as shown by the source-pull contours for optimal dc power delivered to a 2.2-kX dc load. The 2.45-GHz antenna impedance, shown with the cross symbol, is not sensitive to the shunt capacitor value and provides a good match at 2.45 GHz as seen by the source-pull contours. 112 January/February 2014

5 parallel impedance of the antenna with the stubs. The circular symbols on the solid impedance curve show that the antenna impedance at 915 MHz and 2.45 GHz is well matched to the diode impedance. The impedance at 915 MHz moves to a considerably better match when the appropriate capacitor value is chosen. To analyze the rectifier circuit, the power delivered to the diode must be determined, and therefore the gain of the antenna must be known. Measured Rectenna Results The antenna gain pattern is simulated using the full-wave finite element method (FEM) solver Ansys HFSS. The copolarized gain pattern at 915 MHz is shown in Figure 5(a) for both the elevation and azimuth planes. At 915 MHz, the Yagi-Uda antenna array has a dipole-like pattern with a maximum gain of dbi. At 2.45 GHz, the antenna is more directive, achieving a maximum gain of 7.59 dbi as seen in Figure 5(b). Using the definition of effective area and the gains calculated from Ansys HFSS, the power received at 915 MHz and 2.45 GHz is expected to be and dbm, respectively. The final rectenna design is manufactured on Rogers Ultralam 3850 (1 mil thick, f r = 29. ) for low mass. The component used for CS was an ATC 600-L 18pF capacitor. The final weight after metal etching is approximately 0.5 g, which was well below the specified limit of 15 g. Figure 6 shows the final dual-frequency rectenna, including the dc collection and matching networks and all relevant dimensions. Using the simulated gains and reflection coefficients of the antenna, the dc power can be estimated by using the nonlinear model for the Skyworks SMS diode. In a harmonic-balance simulation, an ideal source tuner is used to present the simulated impedance of the antenna, and the power incident on the antenna and supplied by the transmitter is determined from the gain. The expected dc power delivered to the 2.2-kX load at 915 MHz and 2.45 GHz is 15 μw and 35.5 μw, respectively. Therefore, the estimated FoM for this rectenna with the largest dimension being 7.5 cm is calculated to be FoM = 7.6 db i, z 0 i = 90 z = 0 i = (a) i, z db -10 db z = db Z in w C S Z in 2.45-GHz Contours 2.45-GHz Match db MHz Contours 915-MHz Match P dc (dbm) (b) Figure 4. The simulated antenna input impedance versus frequency from 500 MHz to 2.6 GHz for the antenna with (solid blue line) and without (dashed red line) the shunt capacitor CS. The impedance is compared to the diode source pull contours for optimal dc power delivered to a 2.2-kX dc load at both frequencies. Figure 5. The simulated copolarized gain patterns in the elevation (blue, z=0 ) and azimuth (red, i=90 ) planes at (a) 915 MHz and (b) 2.45 GHz. The gain patterns are normalized to the maximum gain which at 915 MHz was dbi and at 2.45 GHz was 7.59 dbi. The simulations are performed using HFSS. January/February

6 At the 2013 student WEH design competition, the dc power harvested at 915 MHz and 2.45 GHz were measured to be 11.6 μw and 65.6 μw, resulting in FoM = db. Table 2. Measured and simulated performance of the rectenna. Frequency L a S a Expected dc Power S 2 S 1 L 3 L 1 L b L c W 2 Figure 6. The manufactured prototype rectenna. The relevant dimensions are given in mm as La = 61., 5 Lb = , Lc = 49., 3 Sa = , Sb = 14., 3 W1 = 4, W2 = 1, L1 = , L2 = 4, L3 = 3, S1 = 05., and S2 = 04.. Expected FoM Measured dc Power 915 MHz 15 μw 11.6 μw 7.6 db 2.45 GHz 35.5 μw 65.6 μw Measured FoM db At the 2013 student WEH design competition, the dc power harvested at 915 MHz and 2.45 GHz were measured to be 11.6 μw and 65.6 μw, resulting in FoM = db. The increase from the estimated FoM is most likely due to more power being incident on the antenna L 2 W 1 S b than was radiated by the dedicated transmitters and was likely provided by outside sources such as Wi-Fi hot spots and any other wireless devices. Although we specifically designed the antenna to operate at 915 MHz and 2.45 GHz, it is very likely that a reasonable match to the diode impedance exists at other frequencies where there are some ambient sources, and these would contribute to the total rectified power, resulting in an increase relative to the nominal available power level. The final measured results for the prototype are summarized in Table 2. Power circuits utilized in these competitions have the capability to extend operation of low-powered devices, perhaps indefinitely. References [1] S. Ladan, N. Ghassemi, A. Ghiotto, and K. Wu, Highly efficient compact rectenna for wireless energy harvesting application, IEEE Microwave Mag., vol. 14, no. 1, pp , Jan [2] Y-H. Suh and K. Chang, A high-efficiency dual-frequency rectenna for and 5.8-GHz wireless power transmission, IEEE Trans. Microwave Theory Techn., vol. 50, no. 7, pp , July [3] H. Sun, Y-X. Guo, M. He, and Z. Zhong, A dual-band rectenna using broadband yagi antenna array for ambient RF power harvesting, IEEE Antennas Wireless Propagation Letters, vol. 12, pp , [4] E. Falkenstein, M. Roberg, and Z. Popovic, Low-power wireless power delivery, IEEE Trans. Microwave Theory Tech., vol. 60, no. 7, pp , July [5] R. J. Vyas, B. B. Cook, Y. Kawahara, and M. M. Tentzeris, E-WEHP: A batteryless embedded sensor-platform wirelessly powered from ambient digital-tv signals, IEEE Trans. Microwave Theory Tech., vol. 61, no. 6, pp , June [6] Y.-J. Ren and K. Chang, 5.8-GHz circularly polarized dual-diode rectenna and rectenna array for microwave power transmission, IEEE Trans. Microwave Theory Tech., vol. 54, no. 4, pp , June [7] S. Korhummel, D. G. Kuester, and Z. Popovic, A harmonicallyterminated two-gram low-power rectenna on a flexible substrate, in Proc. IEEE Wireless Power Transfer, May 2013, pp [8] Rogers Corporation. ULTRALAM 3000 Liquid Crystalline Polymer Circuit Material. [Online]. Available: com/documents/730/acm/ultralam-3000-lcp-laminate-data-sheet-ultralam-3850.pdf [9] C. A. Balanis, Traveling wave broadband antennas, in Antenna Theory Analysis and Design, 2nd ed. New York: John Wiley and Sons, 1997, ch. 10, sec , p. 530, Table [10] Skyworks Solutions, Inc. Surface Mount Mixer and Detector Schottky Diodes. [Online]. Available: Diodes_200041W.pdf. [11] G. Marrocco, The art of UHF RFID antenna design: Impedancematching and size-reduction techniques, IEEE Antennas Propagat. Mag., vol. 50, no. 1, pp , Feb January/February 2014