Experimental Results with two Wireless Power Transfer Systems

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I. INTRODUCTION
This paper describes two systems that make use of RFbased
wireless power transfer. The first, WISP (Wireless
Identification and Sensing Platform)[1], is powered
deliberately by a commercial off the shelf UHF RFID
reader that transmits 4W EIRP. The second system
harvests ambient VHF or UHF energy from TV towers.
We describe an experiment in which we powered a
commercially available thermometer/hygrometer with
LCD display using only RF power harvested from a TV
transmission tower.

II. WISP:WIRELESS IDENTIFICATION AND SENSING
PLATFORM
A block diagram of the WISP is shown in Figure 1.
The system consists of an antenna and impedance
matching components, RF power harvester, demodulator
to extract reader-to-WISP data, backscatter modulator for
WISP-to-reader data, voltage regulator, programmable
microcontroller, and optional external sensors.


III. RF HARVESTING FROM AMBIENT SOURCES
From a balcony at the Intel Research Seattle lab (47° 39'
41”N, 122° 18' 60” W), we harvested RF power from the
KING-TV tower at (47° 37' 55" N, 122° 20' 59" W) which
broadcasts 960kW ERP on channel 48, at 674 - 680 MHz.
This is a distance of 4.1km. We used a broadband log
periodic antenna (5 dBi) designed for TV applications and
a 4 stage power harvesting circuit of the same design as
WISP, with a front end tuned to the desired channel. The
bandwidth of the tuned front end was approximately
30MHz. With the antenna manually oriented toward the
transmit tower, the measured open circuit voltage was
5.0V (i.e. the only load on the power harvester was the
voltmeter). Across an 8K Ohm load, we measured 0.7V,
which corresponds to 60uW of power harvested. This is
equivalent to the net power budget many of the WISP
sensing applications.

VII. CONCLUSION
The possibilities of combining the two power
harvesting techniques described here are exciting. One
can imagine RF powered sensor devices that log their data
until they are interrogated by and RFID reader. For
applications in which a suitably large, outdoor, and fixed
orientation antenna can be accommodated, this should
enable a perpetual sensing platform with no batteries. The
only required maintenance would be periodic data
download.