Urban environments are filled with kilowatts of radio waves from commercial and government transmitters. Although you can’t make anywhere near enough electricity from this energy source to run your house, you can detect it with sensitive electronic equipment. All antennas create electric currents from radio waves; they produce small amounts of energy.
Radio Wave Energy
A radio station’s transmitter puts out anywhere from a few watts for FM to 50,000 watts for prominent AM stations. The broadcast antenna puts out energy that radiates outward in an ever-expanding bubble. The energy for a given area weakens according to a principle called the Inverse-Square Law: the amount passing through a square foot, for example, drops to one-quarter the original strength when you double the distance from the source. According to the Federal Communications Commission, the theoretical maximum electric field one kilometer away from a 50,000-watt transmitter is only 394 millivolts per meter; trees, buildings and air moisture absorb some of this energy, making the actual amount smaller.
The metal in an antenna turns radio signals into electricity; generally, the longer the antenna, the more signal you receive. Portable radios have relatively short antennas and pick up tiny amounts of radio energy. An amplifier in the radio boosts the signal, allowing you to hear broadcasts. To harvest energy, you need a run of insulated wire at least 20 feet long. Locate the wire well away from metal furniture, aluminum siding and other large metal objects that might block the signal you want to receive. Attach another insulated wire to a reliable earth ground, such as a cold water pipe. The ends of the antenna and ground wires create a voltage potential you can measure with electronic equipment.
An oscilloscope is sensitive enough to clearly indicate and measure the electrical energy you get from radio waves. To see this energy, connect the antenna to the oscilloscope’s input probe “hot” terminal and connect the ground wire to the oscilloscope probe’s ground clip. Set the vertical input range to 1 volt per division. The oscilloscope display will show a “white noise” pattern indicating radio energy from many different sources. You may have to adjust the vertical input to a more sensitive setting, such as 200 millivolts per division or less, to make the pattern clearer.
Diode and Galvanometer
The electricity you obtain from radio waves is a high-frequency alternating current; when amplified and tuned to the frequency of a specific station, the result is an audio broadcast. To use radio waves for electricity, first convert the AC signal to direct current with a diode, a device in which current flows only one way. Connect the anode of a small-signal diode to the antenna wire and then connect the cathode to one terminal of a galvanometer. Touching the ground wire to the other galvanometer terminal will cause the needle to jump, indicating DC.
Because radio waves yield very small amounts of power, using an antenna to investigate electricity is for the most part safe. Always use insulated wire, not bare metal, and avoid placing it near electrical outlets, power lines and other obvious sources of electricity. Do not conduct any experiments during thunderstorms; a nearby lightning strike can induce a large voltage in a long wire, harming yourself and any connected equipment.
The small amount of current you get during normal conditions unfortunately limits practical applications for electricity obtained from radio waves. On its own, the current is insufficient to light a light bulb or run a motor, for example. If you obtain about 1.5 volts from an antenna and diode setup, you can use it to trickle-charge a nickel-metal-hydride rechargeable battery; over a period of several hours or days, the battery may accumulate enough energy to power a radio or other small device.
by J.T. Barett