96.9MHz Dipole Harvester

May 2018   Love

Project Documents

This energy harvester was created with a team of 4 other members as part of the MIT class 6.013: Electromagnetics and Applications. This project was very open ended, and had to incorporate a topic covered in the class at some point in the semester. This project focuses on dipole antenna design and includes some basic circuit design. As one of the tallest in Boston, the Prudential center is optimal for transmitting radio stations from its antenna/mast at the top of the building. This 96.6MHz dipole harvester harvests energy transmitted by the Prudential building and uses the harvested energy to flash an LED.

Overview

The final report for this project can be found in the "Project Documents" section above.  I encourage you to read the report in addition to this project page.  See the bottom of this page to view a video of the harvester in action!

In a world full of wireless communication, the amount of free energy surround us at any given time is astonishing.  To bring to light (quite literally) this fact, a team of 4 of us set out to create an energy harvester which harvests energy from a radio station transmitted by the Prudential center in downtown Boston.  Anyone familiar with Boston will most likely recognize the iconic skyline punctuated by the tall Prudential center (shown in the image below). 

The Boston skyline as seen from MIT's campus.  The Prudential center is the tallest building directly to the left of the flag pole.

The Prudential center broadcasts numerous radio stations from its tall mast atop its observation deck.

Source: http://www.necrat.us/prudential.html

The harvester for this project was tuned to harvest energy from the 96.9MHz radio station transmitted from the main antenna mast.  While the Prudential center transmits 105.7MHz at higher power, 96.9Mhz proved to work well.  Energy harvesting at 105.7MHz was not attempted.

According to Google Maps, the Prudential center is approximately 1 mile from the electrical engineering labs on MIT's campus, which, for the frequencies of interest, is far enough away to consider far-field radiation patterns, i.e. energy transfer can occur (good).

The distance between the MIT EE labs and the Prudential center.

A simple test was performed in lab - close to where the antenna would be tested.  A long wire was attached to an oscilloscope probe and and FFT was performed on the signal.  The FFT indicates clear voltage peak for the radio frequencies transmitted by the Prudential center - this is promising for the project!

An FFT of the stray RF waves in the displayed on an oscilloscope in the lab.  Note the large peak at 105MHz as we'd expect being so close to the Prudential!  The scope probe is attached to a long (~8in) wire.

The design of the energy harvester comprises of 2 main elements: the antenna and the harvesting circuitry.  This page will briefly discuss both and design iterations for each element.

Antenna

Since this project was part of an electromagnetics class, the focus of this project was on the antenna design; the circuit design came second.  Multiple antennas were made: first a loop antenna was tested.  A variable capacitor was added in parallel with the loop antenna in hopes of tuning the antenna to resonate.  Unfortunately, this proved fruitless, and it is hypothesized that the load the Cockcroft-Walton Voltage Multiplier (see below) presented to the antenna interfered with the tuning of the antenna.

Initial loop antenna testing setup with variable capacitor on breadboard.  More loops were added later with not much improvement.

After many attempts to get the small loop antenna working, a half-wave dipole antenna was created using some copper tubing acquired from an MIT machine shop.  The copper tubing was trimmed until a reasonable output voltage was obtained from the voltage multiplier circuit.  In fear of trimming too much, once the antenna performed sufficiently well (high output voltage), trimming stopped - which is why the antenna is tuned to 96.9MHz, as opposed to 105.7MHz.

The half-wave dipole antenna with a Delrin block in the middle, separating the two, quarter-wavelength copper rods.

The half-wave dipole antenna performed much better than the loop antenna and the output of the voltage multiplier could charge to a voltage sufficient to forward bias an LED.

The completed half-wave dipole antenna with an acrylic stand.

Circuit Design

The circuit for this harvester is a simple Cockcroft-Walton Voltage Multiplier.  This simple circuit acts as a RF to DC converter that ultimately charges a load capacitor that stores the energy harvested from the RF waves in the air.

The Cockcroft-Walton Voltage Multiplier circuit shown connected to the antenna and a load.  Six stages were used (each stage comprises of 2 diodes and 2 capacitors).

In reality, the load resistor (R_L) was exchanged for an LED in series with a simple SPST button that, when pressed, would discharge C_Y through the LED, illuminating the LED for a split second.  If the received power was "too good", a ~100Ω resistor was used to limit the current to prevent the LED from burning out.  1N5711 Schottky diodes were used minimize power loss attributed to the forward voltage drop across the diodes and to ensure fast enough recovery time.  The following video provides an overview of the harvester operating with the push button:

Modifications to the circuit

After the class ended, a modification was made to the circuit to allow the LED to blink without the need for the pushbutton.  This was not incorporated into the initial design as time did not permit.  From some reading online, it turns out that 2N2222 transistors exhibit negative resistance when the the collector-emitter is reversed biased above a certain voltage.  From my experiments, this voltage seems to be around ~7.5V.  I utilized this negative resistance to automatically discharge the large load capacitor into the LED after it was sufficiently charged.

The IV characteristic of a 2N2222 shown on a curve tracer.  One can clearly see the negative resistance of the transistor as indicated by the negative slope of the line.  Note: the 2N2222 is plugged into the curve-tracer backwards, that is the emitter is placed in the collector plug, and the collector is placed in the emitter plug.  The base was left floating.

PCB

After the circuit was created and tested on a breadboard, I decided to make a PCB.  The schematic of the PCB is included under "Projects Documents", where you can also find the gerber files for the PCB (RF_HARVESTER.zip).  Altium was used for the schematic and layout artwork.  OSH Park was used to fabricate the PCB.  Feel free to use these materials to make your own!

The PCB and the components.

The front (left) and back (right) of the final PCB

A coax SMA cable was used to connect the dipole antenna to the PCB.

The final populated PCB.

The populated PCB with the coax cable.

The following video shows the harvester harvesting the RF waves on the 8th floor of building 36 on MIT's campus, which has a direct line of sight to the Prudential center.  The multimeter is measuring the voltage across the large load capacitor; every time the voltage drops, the LED flashes.  The resistor on the PCB (in series with the LED) was shorted out, which made the flashing more reliable.

While this wraps up this project, I would like to share a schematic of another harvester, created by Joe Sousa, from Analog Devices Inc., who built a harvester which also harvests RF waves.  This harvester has been living outside the 6.101 lab at MIT for as long as I can remember, and is partly what inspired this project.  His schematic (drawn by him) is shown below:

Joe Sousa's RF Harvester Schematic.