The potential utilization of RF signals for DC power is experimentally investigated. The aim of the work is to investigate the levels of power that can be harvested from the air and processed to achieve levels of energy that are sufficient to charge up low-power electronic circuits. The work presented shows field measurements from two selected regions: an urbanized hence signal congested area and a less populated one. An RF harvesting system has been specifically designed, built, and shown to successfully pick up enough energy to power up circuits. The work concludes that while RF harvesting was successful under certain conditions, however, it required the support of other energy harvesting techniques to replace a battery. Efficiency considerations have, hence, placed emphasis on comparing the developed harvester to other systems.
RF harvesting is presenting itself as a viable source of energy. This is based on the notion that the level of transmitted and received signals and powers over the RF spectrum of frequencies has dramatically increased. The power that is required to charge contemporarily is mostly within microwatts or milliwatts [
There are many signals of different frequencies in the surrounding that can be reclaimed. Even though these signals carry a small amount of energy, the possibility of using their rms power to charge or run a microchip can be studied. In this paper, utilization of air signals for energizing a low voltage circuit is investigated.
As the demand for remote and even disposable sensors increases, there is an increasing interest in battery-less systems that use energy harvesters [
The spectra of different areas have different characteristics that depend on many environmental variables. For instance humidity of the location and the distance from the signal transmitters will affect the results of spectrum measurements. Most users will generally fall into two main regions: an urbanized location such as a city or a less populated area such as countryside or desert.
To inspect the RF signal level of a low density spectrum, measurements were conducted using a spectrum analyzer (Rohde & Schwarz FSP) and a dipole antenna. Figure
Snapshot of a low-density area Spectrum.
The spectrum was measured for both indoor and outdoor locations (in random pattern). The overall results show that the highest power levels are within the GSM bandwidth (920 MHz to 950 MHz). The maximum power of signals for outdoor measurements was found to be approximately −1 dBm. However, maximum signal level was about −5 dBm and even −10 dBm in some indoor locations. These results are illustrated, for the outdoor case, in Table
Spectrum variations at different time instances.
Frequencies (MHz) | ||||||||
931.2 | 932.6 | 934 | 935.4 | 936.8 | 938.2 | 939.6 | 941 | |
Power variations (dBm) | −22.2 | −14.0 | −6.7 | −4.1 | −3.4 | −3.4 | −4.6 | −9.7 |
−25.1 | −16.1 | −10.3 | −7.6 | −6.8 | −6.9 | −9.5 | −14.5 | |
−20.6 | −11.9 | −4.4 | −2.2 | −1.5 | −1.6 | −3.3 | −9.0 | |
−29.5 | −20.9 | −13.7 | −10.7 | −9.6 | −9.7 | −11.1 | −15.1 | |
−23.7 | −14.6 | −8.0 | −4.8 | −4.0 | −4.1 | −6.1 | −11.2 | |
−35.8 | −28.2 | −21.3 | −19.0 | −17.9 | −18.5 | −21.0 | −24.3 | |
−30.6 | −21.8 | −14.7 | −12.1 | −10.9 | −10.7 | −12.5 | −14.2 | |
−39.2 | −30.1 | −24.7 | −23.0 | −25.9 | −31.2 | −36.0 | −43.4 | |
−36.7 | −27.8 | −21.6 | −20.7 | −20.3 | −27.5 | −32.1 | −41.2 |
As can be seen from the measurements in Table
Another spectrum analysis was run in a busy city location. Figures
RF air signal Spectrum at a city location (100 KHz to 3 GHz).
RF signal spectrum at a city location (948–950 MHz).
As can be observed the signals with maximum power level exist in the frequency range of 940 MHz to 960 MHz.
In Table
Frequency variations at different time instances.
Frequencies (MHz) | ||||||||
959.08 | 959.26 | 959.2 | 959.24 | 959.28 | 959.32 | 959.36 | ||
Power variations (dBm) | −21.75 | −18.06 | −17.84 | −18.26 | −17.81 | −20.31 | −26.02 | |
−17.91 | −14.29 | −14.07 | −14.38 | −13.97 | −16.10 | −22.31 | ||
−17.73 | −14.34 | −14.18 | −14.55 | −14.07 | −16.66 | −22.06 | ||
−18.34 | −15.15 | −14.92 | −15.18 | −14.80 | −16.94 | −22.44 | ||
−18.38 | −14.53 | −14.31 | −14.56 | −14.84 | −16.46 | −21.92 | ||
−18.17 | −14.78 | −14.59 | −14.88 | −14.38 | −16.87 | −25.76 |
From the spectrum of Figure
The main target was the use of passive components in order to improve system efficiency. The size of the energy scavenger (harvester) in terms of number of targeted frequency ranges was another important point.
The input voltage to the harvester should be high enough to make the Schottky diodes forward biased. The input voltage to the harvester can be calculated by
The block diagram of the designed energy scavenger is shown in Figure
System diagram of RF harvester.
Since several signals of different frequencies are to be detected by this energy scavenger, there was a need for a double sided Microstrip GSM embedded antenna shown in Figure
The air signals carry very low powers. In order to get more power from these signals, there was a need for a bigger antenna. The reason was that the directivity of the antenna increases with its size. The gain of the antenna is directly proportional to its directivity [
Also, the increase in size of the antenna increases the antenna’s bandwidth, the frequency range within which the antenna performance is efficient and it is usually centered at the resonant frequency [
Because of this, the power that can be gathered using this technique is going to be small. This fact limits the choice of the air signals to only one or two high power signals that carry the maximum possible detected energy. The signals to be detected can be Bluetooth signals (operating at 2.4 GHz) or GSM signals (operating at 960 MHz) since they appear to carry higher power than the other air signals and also they exist almost wherever there are cell phones. There will still be some other signals that fall within the bandwidth of the antenna. Since the signals detected by the antenna carry low power, a booster circuit was added to the system. A Villard voltage multiplier was used for this purpose.
GSM embedded antenna.
Voltage multipliers are used to generate bias voltages of a few volts or tens of volts for purposes such as lightning safety testing. The most common type of voltage multiplier is the half-wave series multiplier, also called the Villard cascade (Figure
The output voltage that can be produced by an n-stage Villard cascade is 2
However, if the output current increases, there will be a voltage drop across the capacitors due to alternating current which results in a lower input voltage for the succeeding stages. The formula for this voltage drop is given by [
As can be seen from (
The number of stages cannot be increased to any number. There are two main constraints. The first one is the decrease in the output current due to increase of the output voltage. The second issue is the restriction on the output voltage ripple. To find the most appropriate number of stages several simulations were run and it was found that for a six stage voltage multiplier the output ripples and the current are tolerable.
To find the appropriate value for the capacitors, several simulations were also run. Given the randomness and constant change of the input energy to the harvester, no discrete formula was found in the literature. In order to add signals of different frequencies, the output voltage of the Villard voltage multipliers should be converted into DC voltage using passive peak detectors.
Since a Villard multiplier only boosts alternating signals, it was decided to be placed prior to the DC converter.
Villard voltage multiplier.
To convert the detected signals into DC, a passive peak detector was implemented. The simplest circuit consists of a Schottky diode with a low built in potential of approximately 0.2 Volts [
The overall circuit of the proposed harvester up to this stage is shown in Figure
Higher output power can be gained by cascading several harvesters and adding their outputs together. To add these DC voltages, a passive adder is required.
Passive peak detector.
The RF harvester schematic.
The capacitors of the harvester’s multiplier are to be found. Since no empirical formula was found in literature, simulations were used. AWR Analog Office was used and the circuit in Figure
Different capacitances for Villard multiplier.
As a result, 1 pF capacitance was found to be the most appropriate. After the power of RF off-air signals was found for different frequencies and locations, the harvester’s circuit was analyzed for different input powers. For this purpose, simulation signals of different powers were applied to the RF signal harvester at frequency of 1 GHz.
Figure
Harvester’s voltage response to different input powers.
As can be seen in Figures
Harvester’s Response to a −15, −20, −25 dBm Input.
The outputs shown in the graphs are for a single stage of this harvester. The voltage level is considerable for the high input powers, but it is not high enough. However, to further increase the obtained output voltage from the harvester without any further decrease in current, several stages of such circuit can be added together. To inspect the effect of addition, three of such harvesters were combined and a higher output DC voltage with higher current than that of a single stage with the same output was obtained. The result for this three stage harvester with three different input signals of −20, −17 and −18 dBm is shown in the graph of the Figure
Three-stage harvester’s output voltage.
To check the performance of the designed harvester a prototype was designed and implemented. This is shown in Figure
RF harvester’s prototype.
The fabricated harvester was taken into different environments and its response to different input powers was analyzed. First, the harvester was left in an environment which was more than 500 meters away from the closest cellular phone base station. Also, there were not many mobile phones transmitting in the area were the tests took place. Figure
Voltage harvested by the RF signal harvester in an indoor area (more than 500 meters away from the base station).
The harvester was successfully used to empower an LED. This is shown in Figure
A LED, empowered by the proposed harvester.
Furthermore, by using a nearby mobile phone as a transmitter, a calculator was successfully powered. A snapshot of this effect is shown in Figure
Back and front sides of the calculator running with the proposed RF signal harvester.
As can be seen from the snapshot of Figure
Comparison between commercialized piezoelectric harvester and the proposed RF signal harvester.
Harvester | Size (cm) | Output Voltage (mV) | Cost ($) | Special considerations |
---|---|---|---|---|
Commercial Piezoelectric | 0.25– 0.3 | 100 | Voltage based on tested measurements | |
Prototype of proposed RF Signal Harvester | 0.15–0.5 | 11.89 | Cost based on cost of prototype assembly components |
A wireless mobile charger prototype was built and is currently being evaluated. The wireless charger comprised three different harvesters: a solar harvester, a piezoelectric harvester, and the proposed RF harvester.
Based on measurements and simulations, it can be concluded that it is possible to use radiated, off-air RF signals as a source for energy harvesting. Even though the output powers of such harvester are expectedly relatively low, it was sufficient for running low consumption sensors and switches.
Improvements on efficiency of the RF signal harvesting is important. This will enable more current to be re-cycled and operate low-power circuits. The possibility of using this harvester in energizing sensor networks appears to be the most practical use at the moment.
The authors would like to acknowledge and extend their gratitude to K.T.C. Company for their sponsorship.