What you will learn:
- What is a metasurface and how can it be used to capture ambient RF energy?
- How an energy harvesting metamaterial was constructed using this technique.
- The test setup used and the results of the RF collection scheme on various parameters.
Energy harvesting, with its many manifestations, is often an attractive and viable solution to providing long-term low levels of DC power. Harvesting RF energy is particularly appealing because this energy is ubiquitous, while its capture does not cause any discernible negative impact on intended users – it is truly ‘wasted’. Additionally, unlike many (but not all) fixed solar, vibration, or thermal installations, most RF harvesting devices can be mobile to accompany the system they are powering.
The key to using ambient RF energy as an exploitable source is the energy sink. It acts as a transducer to pick up electromagnetic energy and transform it into useful electrical energy and power in the form of voltage and current. Although an ordinary antenna can be used for this function – after all, that’s what an antenna does – the capture efficiency is quite low and usually insufficient unless a fairly large, resonant antenna is used.
Antenna based on the metasurface
To address this challenge, a team at the University of South Florida developed a metasurface-based antenna that achieves a self-reported utility “threshold” of delivering 100 µW of power from its 16×16 cm surface. . With its delivered energy level of 0.4 µW per square centimeter, or about the intensity of radio waves 100 meters from a cell phone tower, they were able to capture and use this power in real time to power a small LEDs.
“With the huge explosion of radio wave-based technologies, there will be a lot of electromagnetic waste emissions that could be collected,” said Jiangfeng Zhou, associate professor of physics and research team leader. “This, combined with advances in metamaterials, has created a ripe environment for new devices and applications that could benefit from collecting this wasted energy and using it.”
The team used a harvesting device based on a metamaterial perfect absorber (MPA) with built-in Schottky diodes as a rectenna (rectifier antenna) to convert the captured RF waves to direct current. The Fabry-Perot (FP) cavity resonance of the MPA greatly improves the amount of energy captured. Additionally, FP resonance exhibits a high Q factor and significantly increases the voltage across the Schottky diodes, which improves rectification efficiency, especially at low currents.
“We also placed a cellphone very close to the antenna during a phone call, and it picked up enough energy to power an LED during the call,” Zhou said. “Although it would be more practical to harvest power from cellphone towers, it demonstrated the antenna’s power-harvesting capabilities.”
Electromagnetic (EM) metamaterials are metallic resonant synthetic structures, usually periodically arranged, that behave as homogeneous media with effective electrical permittivity (ε) and effective magnetic permeability (μ). Both of these values can be engineered to provide unique EM properties that do not exist in nature, such as negative refractive index, diffraction-unlimited optical imaging, EM invisibility masking, and perfect absorption.
Metamaterials also provide the flexibility to design electrically small and low profile antennas. Additionally, the ability to manipulate their electrical permittivity and magnetic permeability further provides the ability to match the input/output impedance of the antennas to that of the surrounding environment for optimal energy transfer.
Take advantage of the Perfect Absorber metamaterial
In this project, the designed MPA helps convert RF waves to DC more efficiently by perfectly capturing and storing the RF wave energy inside the Fabry-Perot metacavity. The MPA-based rectenna consists of a 4×4 array of double-spaced split-ring resonators (SRR) and a copper ground plane (not shown) of the same size, separated by a distance(s) (Fig.1).
The sample was fabricated on a copper-clad FR4 printed circuit board (dielectric constant ε = 4.34) using lithography followed by chemical etching. A Schottky diode (Skyworks SMS-7630-079LF) was soldered across a gap in each SRR to create a DC voltage by rectifying the resonant current excited by the incident RF wave. The rows of SRRs are connected via copper strips along the x-direction, thus forming a series connection of four efficient “batteries”.
Four rows of SRRs are connected by two thicker strips at the left and right ends, forming a parallel connection along the y direction. The diode polarities alternate in adjacent rows and columns to create the correct “battery” polarities effective in series and parallel connections. The alternating arrangement of the diodes also helps harvest both the forward and reverse currents induced by the positive and negative half cycles of the incident RF wave, respectively.
The team performed full-wave 3D simulations to solve Maxwell’s equations and obtain numerical solutions through Computer Simulation Technology (CST) Microwave Studio using finite integration technology.
Tests and results
Testing was performed in an anechoic chamber located at The MITER Corp. (Bedford, Mass.), sample rectenna being connected to a 1 kΩ load as an indicator of a functional load. (Fig.2).
A calibrated and fully controllable signal was transmitted by a normal incidence horn antenna to the specimen rectenna, which was placed approximately 380 cm from the transmitting antenna. A metallic ground plane was placed behind the sample in an attempt to create a Fabry-Perot cavity to increase the amount of RF radiation captured. All instruments were controlled by a LabVIEW program and the measurement was automated by scanning both the power and the frequency of an incident RF wave.
The team performed performance and efficiency tests on a range of parameter values. They first measured the sample for relatively high intensities of incident RF waves (Fig.3).
The intensity range of 2.6 μm/cm2 at 65 μm/cm2 is well above what is typically expected from ambient RF signals. It would likely only be in close proximity (
With or without a ground plane, the maximum energy harvesting efficiency occurs at 0.90 GHz, which is comparable to the frequency predicted by the absorption cross section. Without the ground plane, the SRR network achieves the highest efficiency of ∼60% at 0.90 GHz when the incident RF wave intensity reaches 65 μm/cm2.
However, when the ground plane was placed at the optimum distance (s=4cm), the energy harvesting efficiency improved significantly and reached the highest efficiency of about 140%. (Yes, it is possible: an efficiency greater than 100% indicates that the ground plane has made the rectenna’s effective area significantly larger than its physical area.)
They also evaluated the effectiveness at intensities comparable to what can be found from ambient sources (up to 1.0 μW/cm2) (Fig.4). These intensity levels were chosen to match those encountered on the streets of urban environments, such as a GSM cell phone (2 at 900 MHz and 2 at 1800 MHz) and 3G (2 at 2110 MHz). The efficiency here is significantly lower than it was for high power measurements. However, the presence of the ground plane increases the efficiency by a significant factor (up to 16) in this case.
The team concluded that designing RF harvesting rectennas based on perfect metamaterial absorbers is a promising solution for harvesting ambient RF energy in low power density environments, as they are tunable, highly efficient and electrically small. .
The perfect absorption design of the metamaterial rectenna greatly improves the efficiency of RF-dc conversion (especially at lower available power densities) by increasing the effective area of the antenna. This eliminates reflection due to impedance mismatch and helps overcome high resistance below the turn-on threshold.
The current version of the antenna is much larger than most devices it could potentially power, so researchers are working to make it smaller. They would also like to create a version that could simultaneously collect energy from multiple types of radio waves so that more energy can be collected.
Their research, which was funded by the Asian Office of Aerospace Research and Development and the Alfred P. Sloan Foundation, is detailed in a crisp, highly readable nine-page article titled “High-Efficiency Ambient RF Energy Harvesting by a perfect absorber of metamaterial” published in Express Optical Materials. Surprisingly enough, there is no additional material or video accompanying the document.