Tikalon Blog by Dev Gualtieri

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Harvesting Radio Frequency Energy

November 22, 2011

The light switch for my cellar is the type with a neon bulb built into the switch handle. When the cellar lights are off, the switch handle is illuminated. Well, that's how it worked for the first few decades. Then, either some of the neon leaked out of the bulb, or the electrodes became contaminated, and the switch handle wasn't illuminated all the time. However, when it was off, touching the switch with a finger would cause the neon bulb to light.

The magic involved in lighting the neon bulb had a simple technical explanation.[1] My body was acting as an antenna for radio frequency signals , and my finger would capacitively couple this extra energy into the neon gas to allow it to produce a plasma . There's a lot of radio frequency radiation everywhere, from broadcasting stations to cellphones . Much of it is harvested by the power lines that enter our homes.

Transformers will block radio frequency signals on the power lines, but there are still long runs of highly conductive wire from the nearest step-down transformer to your household wiring. That's one reason why there's too much interference from various harmonic generators to get good AM radio reception in your home.

Now that many electronic devices require so little power to operate, research in environmental energy harvesting has become popular. Radio frequency energy harvesting is a subset of this research area. The table below is a summary of environmental energy available from various sources.[2]

Source Power Density
μW /cm² Power Density
μW/cm²

Vibration/Motion 4
(Human) 100
(Industrial)

Temperature Difference 25
(Human) 1,000-10,000
(Industrial)

Light 10
(Indoors) 10,000
(Outdoors)

Radio Frequency 0.1
(GSM) 1
(Wi-Fi)

As can be seen from the table, ambient radio frequencies have lower energy than other environmental energy sources. However, if we accept the 1 μW/cm² factor, this means that a 10 cm x 10 cm antenna would harvest 100 microwatts . This is enough power for many sensors that just need to wirelessly update their state a few times each hour.

Of course, ambient radio frequencies cover a wide bandwidth . There's AM radio at about a megahertz , cellphones at about a gigahertz and beyond, and Wi-Fi data networks at 2.4 and 5 GHz. Low frequency signals are usually ignored, since their long wavelengths require very large antennas for efficient energy collection. A Georgia Institute of Technology team, led by Manos Tentzeris , a professor of electrical and computer engineering , has been conducting research on small, wideband antennas for energy harvesters that collect energy from radio frequencies in a band from 100 MHz to potentially 60 GHz.[3]

Spiral antenna

A spiral antenna for radio frequency energy harvesting.

Spiral antennas are broadband antennas that are especially suited to this application.

(Illustration by the author)

The Georgia Tech research is facilitated by the process they use to fabricate the antennas. Tentzeris and his team have been printing their antennas since 2004 using inkjet techniques. Some of their research is in the conductive inks for this process, and they can print on a variety of substrate materials, although polyimide offers the best high frequency performance. Their RF energy scavenging systems have produced hundreds of microwatts of power.[3]

Nokia , at its Cambridge, UK , research laboratory, has also been investigating RF energy scavenging systems that operate at high frequencies, from 500 MHz to 10 GHz. Although details of the device and test environment have not been disclosed, Nokia claimed 3-5 milliwatt power in 2009, with a short-term goal of 20 milliwatt, and a stretch goal of 50 milliwatt. Their logic is that, since one transmitter will give a few tens of microwatts, reception of a thousand transmitters will give tens of milliwatts.[4-5] I'm somewhat skeptical, but such power levels are possible with a large enough antenna.

Robert Wilson and Arno Penzias at Crawford Hill, New Jersey

Now, that's an antenna!

Robert Wilson (left) and Arno Penzias in front of the Bell Labs Crawford Hill, New Jersey, horn antenna they used in their discovery of the cosmic microwave background radiation .

(Via Wikimedia Commons).

In 2009, Intel powered a small weather station using RF energy scavenged from the television transmitter of KING-TV , located about two and a half miles away. This transmitter broadcasts on frequencies between 674 and 680 MHz at almost a megawatt of effective radiated power. The combination of a nearby, high power transmitter, and a harvesting antenna with a 5 dB gain , resulted in 60 microwatts of harvested power.[6]

Radio frequencies are an alternating current power source, so rectifier circuitry is needed to convert the power to direct current . Since the typical antenna source impedance is fifty ohms , a milliwatt of power corresponds to about 0.2 volts . Silicon rectifiers don't work well below 0.7 volts, and low voltage electronic circuitry operates at about 1.8 volts, so some step-up method is needed.

As a first step, the voltage can be increased by a suitable step-up transformer . After that, the most useful rectification method is the use of a switched capacitance filter , in which the diodes of a voltage multiplier circuit are replaced by field effect transistors .[7] The Intel demonstration used a four stage charge pump , which is a type of voltage multiplier .

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