UHF Power Harvesting


[Alanson Sample] and [Joshua R. Smith] have been experimenting with wireless power transfer for their sensing platform. Their microcontroller of choice is the MSP430, which we used on our e-paper clock. They chose it specifically for its ability to work with low voltages and they discus its specific behavior at different voltages. The first portion of their paper uses a UHF RFID reader to transmit to the sensor’s four stage charge pump. They added a supercap to provide enough power for 24 hours of logging while the node isn’t near a reader. For the second half of the paper, they use a UHF antenna designed for digital TV with the same circuit and pointed it at a television tower ~4.1km away. It had an open circuit voltage of 5.0V and 0.7V across an 8KOhm load, which works out to be 60uW of power. They connected this to the AAA battery terminals of the thermometer/hygrometer pictured above. It worked without issue. The thermometer’s draw on a lab power supply was 25uA at 1.5V.

It’s an interesting approach to powering devices. Do you have an application that needs something like this? For more on wireless power, checkout this earlier post on scratch building RFID tags.

[via DVICE]

15 thoughts on “UHF Power Harvesting

  1. i live next door to a high-power am radio station. everything in my house that has a speaker (and an amplifier) picks up this station, and i get a measurable voltage between myself (or any large metal object) and ground. i don’t know how safe this is, but it would be interesting to see if i could power something using myself as an antenna. :)

  2. Ryan: you could try putting ferrite beads on all signal and power cables to stuff that is picking up the signal.

    A large-ish antenna tuned to the frequency of the radio station could probably pull quite a bit of power from the air.

  3. When I worked in Chicago there was a 50 KW am radio station across the road from our office. The previous tenants had made a 10 foot on each side square coil antenna. Which fed a 8 inch speaker at decent volume from a germanium diode detector. 50 KW several hundred feet away you know.

  4. I’ve actually been wondering for a couple of weeks if it was possible to pull adequate power from the cell networks to run MEMS devices. It seems like it might be!
    Very timely hack.

  5. Re: MEMS from Cell networks. Duty cycle of the waveform is your primary issue in accumulating DC power. Smoothed waveforms like AM broadcast simplify the design for efficient conversion. Digital modes sadly tend to have “ringy” harmonics in conversion applications. Which can quickly become parasitics and worse still rebroadcast on unwanted frequencies. With very potentially bad results. But to keep on the original post’s track requires less heavy math and just a few simple mnemonic style rules. Up Frequencies down lengths. The old term “Wavelength” literally embedded the concept. MF to HF waves are in Hundreds to Tens of Meter resonant lengths-absent of course coil loadings or fractional wavelength antennas. Bear with me for a moment as it’s really to a point worth exploring. VHF to UHF Bands “6 meters to the centimeter bands” are then resonant at those lengths. Making elements shorter as is seemingly obvious. The trade offs? At the 1 MHZ or so of that Chicago radio station coupling in a magnetic field mode over short distances is the dominant mode. At the UHF or centimeter wave ranges you shift modes into the distant E field mode/s for simplification. Which produces a slightly “looser” coupling. Hey- I’m trying to dredge up memories of RF lectures heard at age 15 or so:>

    How the hell does this link to a hack article? Well- understanding the concepts makes building this stuff easier for me at least. The wrapup of all my seeming ramble is? Think of the RF sources you are “Harvesting” energy from as the “Primary” of a switch mode power supply! Albeit one with a non-trivial air gap between the primary element and your secondary coil on… So the induced RF voltages get rectified and either directly used or could feed other up converters as in the original post. Where this is leading? Depending on the field energies ion any given situation multiple energy harvesters on multiple frequencies could harvest energy and sum their outputs just as the cells of a solar panel do…

  6. Isn’t this cold war spy technology? I recall reading about audio bugs that could be planted anywhere (foreign embassies) then you just beamed a radio frequency at the bug every day to keep it charged up.

  7. The original device which broke open awareness of remote powered eavesdropping tech was a cavity resonator. Hidden Trojan Horse style in a wood carving given to America’s ambassador to Russia in the early 50’s Discovered in 1953, it’s stark simplicity was a watershed moment in several areas of overall tech advance. That cavity resonator has descendants all around us in the taken for granted fabric of our lives. And someday any one of our “hacks” might sow a similar legacy of descendants.

  8. Looks cool, but on a visit to a local short wave transmission station I was told such acts (here in the UK at least) are illegal. Apparently some guy in the local village had tried to power a number of devices in his home, as the station had power in the range of hundreds of kilowatts it actually worked! Unfortunately he didn’t realise this would reduce the transmission range significantly, and they quickly caught him!

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