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.