This instructable demonstrates a prototype that harvests energy by collecting RF radiations from its surroundings through an antenna. The circuit when kept in the vicinity of RF emitting sources like Wi-Fi, cell phone, etc., collects RF energy from its surroundings and converts it into DC charge which is stored in super capacitor and then used for low voltage applications.

This circuit basically has three blocks:

Antenna
Impedance matching circuit
RF to DC converter circuit

The most important part in making this project is designing and fabricating an antenna.

Antenna plays an important role in the design of RF energy harvester. In this project we prefer patch antenna over other antennas due to its characteristics such as low profile, compactness, low fabrication cost and also because it supports linear and circular polarization.

Now let's jump into the making of this project.

So, the first step is designing and simulating all three blocks of the project.

Let's start with designing the antenna using a simulating software. If you start hunting for the antenna designing software you will end up finding many like HFSS, CST, ADS etc. Antenna could also be designed using MATLAB code (just make sure there is antenna toolbox in MATLAB software). Now, select and download the crack version of your choice and get started. Here, we have used HFSS software to design the antenna.

Step 1: Designing Patch Antenna Using HFSS Software

Here, we have designed an inset feed microstrip patch antenna that operates at 2.4 GHz, using HFSS (High Frequency Structure Simulator) software.

To design an antenna in a simulator it's very important to know its specifications as per the requirements. There are a few parameters to be considered while designing and fabricating an antenna.

Antenna Specifications:

Substrate: FR4 Epoxy with relative permittivity 4.4, loss tangent 0.009 and thickness 1.6 mm.

Impedance: 50 ohms

Dimensions: Lg = 38.52mm, Wg = 47.01 , Lp = 28.92 , Lg = 37.41 , a = 3mm, b = 19mm.

For more understanding, you can watch video on designing patch antenna that's embedded here.

Step 2: Simulating Impedance Matching Circuit Using Multisim Software

The next step is to simulate Impedance matching circuit.

Impedance matching is necessary for maximum power transfer between Antenna and RF-DC converter. This circuit is employed in this project to have minimum return loss and to improve the performance of the system.

We have chosen T-match network because of its high Q and low ripple factor characteristics.

T match circuit is designed and simulated using multisim software as shown in the schematic above.

The output from this circuit is noted and compared with the practical ouptut.

Step 3: Simulating RF to DC Conversion Circuit Using Multisim.

The captured low level RF energy by the antenna is fed to RF to DC Converter circuit through Impedance matching circuit. The received electromagnetic waves are converted to DC using a circuit called Voltage Doubler.

RF to DC converter is designed using multisim as shown in the schematic above.

Step 4: Gathering Components

After designing all the three blocks of the circuit i.e., Antenna, impedance matching circuit and RF to DC converter, the components as per design are gathered.

The required components as per schematic that we have designed here are:-

HSMS 2860 Schottky diode
Double layered copper clad board
PCB mount SMA connector
Coaxial cable (impedance 50 ohms)
Resistors - 47 , 1K, 10K ohms
Capacitors - 1pF, 1nF
Inductor - 10nH
Wires and jumper cables
Super capacitor
LED

Note: Sometimes you don't get the components of exact values. So, the alternative is to get the components of approximate values.

Step 5: Fabricating Antenna

When you are ready with the schematic and required components the next step is to fabricate them according to the design.

Firstly, fabricate the antenna either by chemical etching or milling process. We have fabricated antenna using chemical etching process.
Next, place and solder the SMA connector correctly onto the antenna feed.
Then, connect the coaxial cable of impedance 50 ohms to the antenna just as shown in the above image.

Step 6: Fabricating Impedance Matching and RF - DC Converter Circuits.

Rig up the circuit as per the schematic.
Then, connect all three blocks.
And connect oscilloscope probes across the output terminals.
You can also use digital multimeter to measure the output.

Step 7: Testing

Now, it's time for testing the circuit.

As observed on the oscilloscope, the final output i.e., the RMS value that we have achieved is in milli-volts which is approximately equal to the values obtained from simulation.

I would like to mention that the output that we get here is not constant as it completely depends on RF energy. You may get in millivolts or sometimes in volts depending on the intensity of RF energy waves.

The output DC voltage can be amplified by adding more number of voltage doubler circuit stages to the RF to DC converter.

Note: We obtained this output when the circuit was placed near a couple of cell phones.

Step 8: Testing With Super Capacitor As Load

A super capacitor is connected across the output terminals of this circuit and is left to charge for some time. After a few hours of charging of capacitor the charge across the capacitor is measured using a digital multimeter as shown in the image.

This prototype is feasible only for low voltage applications and further can be connected to DC voltage booster for other applications.

Acknowledgement:

This project was carried out as a part of my internship at Workbench Projects, Bangalore. I would like to thank Mr. Richard, Instructor at Workbench Projects for guiding through and also to my Co-intern Ms. Vidyashree who worked with me on this project.

We got this idea of converting RF energy to DC from a paper titled 'Development of Radio Frequency Energy Harvesting Module', the authors of which are Pankaj E Ipar, Shilpa M Lambor, Sangeeta M Joshi.