Design of a 2.45 GHz Rectenna for Electromagnetic (EM) Energy Scavenging Gianfranco Andia Vera1,2, Apostolos Georgiadis1, Ana Collado1, and Selva Via1 1
Centre Tecnologic de Telecomunicacions de Catalunya (CTTC), Castelldefels, 08860, Barcelona, Spain, Tel: +34 93 645 2900, Email:
[email protected] 2 Communications Engineering Dept., Pontifica Universidad Catolica del Peru, San Miguel, Peru.
Abstract — A compact dual polarized rectenna operating at 2.45 GHz is presented. It consists of a square aperture coupled patch antenna with a cross shaped slot etched on its surface that permits a patch side reduction of 32.5%. The patch size is 3.4 cm by 3.4 cm. The antenna is dual linearly polarized with each orthogonal polarization received by an appropriately placed coupling slot. The received signal from each slot output is rectified by a voltage doubling circuit and the doubler DC output signals are combined allowing the rectenna receive signals of arbitrary polarization. The circuit is optimized for low input power densities using harmonic balance. Simulated rectifier maximum RF-to-DC conversion efficiency values of 15.7% and 42.1% were obtained for input available power levels of -20 dBm and -10dBm respectively at 2.45 GHz. The measured results are in agreement with the simulation. Index Terms — Rectenna, electromagnetic energy harvesting, harmonic balance, patch antenna, voltage doubler.
I. INTRODUCTION Wireless power transmission has received significant attention in the past, in relation to high power applications [1]. Considerable interest has been directed in the recent years towards low profile, low power, energy efficient and self-sustainable sensor networks [2-4]. The possibility of recycling the ambient electromagnetic energy especially in densely populated urban zones is actively being explored [5]. In this work a dual polarized aperture coupled patch rectenna is proposed. The rectenna has a compact size due to the use of a cross-shaped slot at the patch surface [6]. Dual linear polarization is employed in order to be able to receive arbitrarily polarized input RF signals. The RF-toDC conversion efficiency of the rectenna is optimized for low input power density signals such as the ambient electromagnetic fields using harmonic balance (HB) simulation. The aperture coupled feeding structure makes the design suitable for implementation on flexible or textile based substrates [7]. The design of the antenna is first presented followed by the rectifier circuit optimization and measurements. Finally, the complete rectenna performance is evaluated.
II. ANTENNA DESIGN The antenna is first designed separately from the rectifier. The radiating structure is based on an aperture coupled dual linearly polarized patch antenna. The patch and antenna feed lines are placed on two Arlon A25N substrates with thickness ta = 20mil, separated by a Rohacell 51 foam layer. The foam layer has a thickness tr = 6mm. A 32.5% size reduction in the length of the radiating patch side is obtained by etching a cross-shaped slot in its surface. The fabricated circuit is shown in Fig. 1. This topology is inspired from the antenna design presented in [6], where a rectangular patch was proposed for a single feed dual band operation with a different (orthogonal) polarization for each band. In contrast to [6] a square patch is used in this work, leading to single band operation, targeting the 2.4 GHz ISM band. A different feed for each orthogonal polarization is used by etching two perpendicular coupling slots (Fig. 1b). The slot edges are bended so that they can be accommodated within the available size. The antenna was simulated using Agilent’s Momentum. The measured s-parameters of one of the antenna ports is shown in Fig. 2 with very good agreement with the simulation. The simulated antenna directivity was 8.8 dB at 2.45 GHz, however the measured value was 7.5 dB. The difference is attributed to the finite ground plane of the fabricated circuit not considered in simulation, as well as fabrication errors. In addition, spray adhesive (3M Super 77) was used to glue together the various layers, and it was not considered in simulation. III. RECTIFIER DESIGN Rectification of the received RF signals at each polarization is achieved using two voltage doubler circuits. Each doubler consists of two stages. The first stage is formed by a series capacitor (C1 = 1.2 pF) and a shunt Skyworks SMS7630 Schottkty diode. The second stage uses a series SMS7630 diode and a shunt capacitor (C2 = 2.7 pF). The rectifier input was matched to 50 Ohms using
an L-network consisting of a shunt L1 = 1nH and a series inductor L2 = 3.9nH. The circuit schematic is shown in Fig. 1c, where it is seen that the output capacitor C2 and load RL are shared between the two doublers.
within the operating frequency band for RL = 8.2 KOhm is shown in Fig. 3.
Fig. 2. Antenna input s-parameter simulation and measurement.
Fig. 3. Rectifier simulated efficiency versus frequency (RL = 8.2 KOhm). Fig. 1. Fabricated rectenna: a) top view of radiating patch, b) bottom view of antenna feed networks and rectifiers, c) rectifier circuit schematic. tr = 6mm, ta = 0.5mm, Lp = 3.5mm, Wc = 1.27mm, Lc = 31.95mm, Lp = 33.95mm, Ls1 = 8.5mm, Ls2 = 19.2mm, Ls = 6.28mm, W1 = 1.2mm. L1 = 1nH, L2 = 3.9nH, C1 = 1.2pF, C2 = 2.7pF, RL = 8.2KOhm.
The rectifier was simulated using Agilent’s harmonic balance simulator (HB) and the matching network was optimized in order to maximize the RF-to-DC conversion efficiency for a low input power level of -20 dBm. The RF-to-DC efficiency n was defined as the ratio of the DC power that is delivered to the load RL (Pdc = Vout,dc2/RL), over the available input power at the rectifier terminal (Pav = -20 dBm). The simulated efficiency of the rectifier
In addition the efficiency for different load values is indicated in Fig. 4, indicating a maximum value at approximately 5KOhm to 6 KOhm. A prototype rectifier was built and its performance was evaluated using single tone as well as modulated signal measurements. The input signal was applied using an Agilent ESG signal generator. A 10MBPS QPSK signal was used for the measurements. The measured efficiency of the rectenna for RL = 8.2 KOhm is shown in Fig. 5. Good agreement with the simulated values (Fig. 3) was obtained. One may observe that the QPSK modulated signals resulted in higher efficiency values than the tone inputs. This is potentially attributed to the fact that QPSK signals
have a certain peak-to-average value that due to the nonlinearity of the rectifier results in higher efficiency.
effective area of 67.1 cm2 and a maximum available input power of -20.4 dBm. The measured efficiency of the rectenna within the operating frequency band is shown in Fig. 6 for S = 0.135 uWcm-2. The values are in agreement with the simulated and measured values of the rectifier circuit only. Fig.7 shows the measured variation of the efficiency versus the input power density. As expected, there is a strong dependence on the input power density.
Fig. 4. Rectifier simulated efficiency versus the load RL.
-2
Fig. 6. Measured rectenna efficiency (S = 0.135 uWcm , RL=8.2 KOhm).
Finally, the angle between the input wave polarization and the rectenna was varied. The DC output that was measured for RL=8.2 KOhm is shown in Fig. 8 for three different frequencies. The result shows that the rectenna can efficiently receive input signals with arbitrary polarization.
Fig. 5. Rectifier prototype measured efficiency for tone and QPSK modulated input signals (RL = 8.2 KOhm).
III. RECTENNA PERFORMANCE A prototype rectenna was built (Fig. 1) and evaluated. The measurements were performed in an anechoic chamber using a reference antenna with gain 6.8 dB placed at a distance of 3.75m from the rectenna. A load of 8.2 KOhms was used in the measurements. Due to equipment limitations a maximum input power of 17 dBm was applied at the reference antenna. This corresponds to a maximum input power density of S = 0.135 uWcm-2 at the antenna center. If one considers the measured rectenna directivity of 7.5 dB at 2.45 GHz, it corresponds to an
Fig. 7. Measured rectenna efficiency (RL=8.2 KOhm) versus the input power density at the antenna center.
ACKNOWLEDGEMENT This work has been supported by the Spanish Ministry of Science and Innovation project TEC2008-02685/TEC, the PTQ-06-02-0555, PTQ-08-01-06432 grants and the COST Action IC0803 "RF/Microwave Communication Subsystems for Emerging Wireless Technologies (RFCSET)". REFERENCES
Fig. 8. DC output voltage variation for varying orientation angles of the input wave polarization relative to the rectenna (S = 0.135 uWcm-2, RL=8.2 KOhm).
VII. CONCLUSION A compact dual polarized aperture coupled patch rectenna is proposed. The patch has dimensions 3.4 cm x 3.4 cm with a substrate thickness of 7mm. Harmonic balance was used to optimize the RF-to-DC conversion efficiency of the rectifier circuit for low input signal power levels. A simulated maximum conversion efficiency of 42.1% at a 5.0 KOhm load for an input signal power of -10dBm was obtained for the rectifier. The rectenna is able to receive arbitrarily polarized signals by combining the DC output from two voltage doublers corresponding to two orthogonal linear polarizations.
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