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A transducer to harvest electric power from radio waves

Or how to get electric power out of radio waves.

Years ago, I invented the "snake". It's just a wire, with a LED, some elementary electronics and no battery. In most cities, when holding the snake in the direction of a 100 MHz FM broadcast emitter, it will harvest enough electric power from the radio waves to make the LED light up, often even at a few kilometers distance from the emitter. The intention was to make a simple gadget that can be folded inside a letter and that can show the presence of radio waves, with no doubt about their power because it is that power that feeds the LED.

LED of the snake lights up when
        receiving 100MHz radio waves

The schematics of the snake and building tips are available in this article:

I did try to adapt the snake to 900 MHz cell phone radio waves. I build prototypes like this one:

snake adapted for 900MHz cell phone
        radio waves

The LED did light up, up to a few meters distance from weak in-street cell phone towers. This was not of much use, since I wanted to show the presence of those waves everywhere. And their power.

The problem lies in the fact that cell phone waves are 9 times shorter than FM broadcast waves. The snake, being a half-wave antenna, has a length of 1.5 meters. The LED needs 1.5 Volts to light up. So, roughly, the snake for FM broadcast waves just needs an electromagnetic field with a force of 1 V/m. The snake for cell phone waves, being 9 times shorter, needs in the order of 9 V/m.

Instead of a LED, I tried to use a galvanometer. A 100 µA scale galvanometer needs almost no electric tension to operate. But the snake did not produce enough electric current...

Of course I tried many layouts, like Yagi-Uda reflectors and directors or several snakes put in series. It was bulky and too directional. I did get good results with 4 snake-like elements put in series, that also were each other's parasitic element. The LED would light up maybe 20 meters away from a cell phone tower. Great... but still not useful.

Till I thought of this layout. The copper wire has a diameter of 0.1 millimeters and the LED is a standard 5 millimeters one.

transducer antenna to
          harvest electric power from 900 MHz cell phone radio waves

It does harvest enough power to be connected to a 100 µA galvanometer, which allowed to assemble this box:

radio wave transducer
          in a box with a galvanometer

In most open spaces in a city, like public spaces, docks and bridges, the needle will rise at least a few lines. It should rise on most rooftops and in most rooms that have windows above the surrounding buildings.

measuring 900MHz cell
          phone electromagnetic pollution on the Place Saint-Lambert in

The needle will rise firmly if a big cell tower is in sight, even maybe 200 meters away. And of course it will go berserk when close to an in-street little cell tower. In the photo below, the antenna is the white little rod just right of the galvanometer.

galvanometer going
          full scale close to a 900MHz cell tower

Places where the needle stays on zero will for sure be considered as safer by electrosensitive persons. But I don't know if it can be used by them to check for safe places. One main drawback is that this is a resonator, dedicated to one frequency: the lower band for cell phones in Belgium, around 900 MHz. This band, and the FM broadcast band at 100 MHz, are the main overall sources of electromagnetic pollution. But in order to be safe, an electrosensitive person should rather use a broadband electronic measuring device.

This is an inside view of the device. The box is that of an Eachine H8 quadcopter turned inside out.

          view of radio wave dectector that uses no battery

The central part of the transducer is a standard diode bridge yet made of high frequency diodes. I used HSMS-285C-BLKG ones. They are very sensitive. The capacitor in the middle is not mandatory but it proved to be useful. I used a 100 pF high frequency SMD one.

electronics of radio waves transducer

Each of the 8 antenna wires has a length of ½ λ. Hence for a frequency f of 900 MHz, with a wavelength λ of c / f equal to about 33 centimeters, the length of each wire would be 16.7 centimeters, which you want to correct because the speed of a frontwave in a copper wire is slightly lower than c. I used 16 centimeters wires. This page can help do calculations: 

You must experiment and try out different lengths. In one application for cell phone waves, the optimal length of the antenna turned out to be about 2/3 of the calculated length! When using thin wire, the window will be of about 10%. In other words, if for a given frequency the optimal length of an antenna is 100 çirek then it will dimly start working at a length of 90 çirek and stop working above 110 çirek. It will work really well between 95 çirek and 105 çirek. If you need a broader window, use thick rods and some other tricks.

If you glue the wires on thick wood then they must be a little longer, because the diamagnetism of the cellulose will increase the frontwave speed of the wires.

My first trial was with the wires going straight up. This gives quite good results. The middle part of each wire must be at a distance of about λ of the center line of the device. (I used a little more; about 3.5 centimeters, for the 900 MHz transducer. Hence the distance between two diagonally opposed wires is 7 centimeters.) Each set of 4 wires will form a "cage" with a square floor section. At the top and bottom, each set of 4 wires is soldered together; they are electrically joined. Four wires, at this given distance of each other, is an optimal layout to harvest power, I believe because each wire is a parasitic element for the other ones.

radio wave transducer, first

The layout shown below is better. The end of the wires are turned inwards. It doesn't matter that they are no more aligned with the radio waves, because the ends of a dipole antenna are the least sensitive to the radio waves (yet they are necessary for the standing wave that develop inside the wire). The most important advantage is that the middle parts of the wires are now closer. This decreases the directivity and it allows the device to operate in a narrower space, possibly making a better use of a zone where echos of the radio waves add up. The fact that this also uses less space is a bonus. The height of the transducer as a whole is 14 centimeters.

transducer for radio waves, second

The main aim of this layout is to maximize the potential difference across the diodes. When the standing waves in the upper wires present their lowest potential to the diodes, the standing waves in the lower wires present their highest potential, which gives the most chance to rise above the forward voltage of the diodes.

The standing waves will increase inside the wires till the potential becomes sufficient for some current to flow through the diodes and load the capacitor.

If you want to build a parasitic element for the transducer as a whole, assemble another same transducer yet with no diode bridge in the middle (do not connect the bottom wires to the upper wires).

A more conventional looking layout would be like below. It should not be called a "full wavelength dipole antenna" because the resonances in the two wires are in phase. It rather is a "double half wavelength dipole antenna with a high impedance". I did not experiment with it.

dipole with full wavelenght

If you want a simple device that can harvest energy from strong radio waves, like close to a cell phone, use a dipole with its ends connected to the diode bridge. That would be like the antenna above but with one wire removed and replaced by the other end of the remaining wire. Hence the green wire shown in the picture below should have a length of ½ λ, probably a little less, like all the ones above. (Of course you can also use 4 wires like above, placed λ from the diode bridge in the center. But then it's no more the simplest device and it won't be handy close to a cell phone.)

folded dipole on a diode bridge to
        harvest electic power from strong radio waves

By the way, the part of the wire that receives the radio waves is the continuous line on the right. The two parts on the left, that connect to the diodes, are useless for the radio waves but they are mandatory for the wire to have the adequate length to resonate at the desired frequency. And to connect to the diodes. (A regular "folded dipole" is another type of antenna; with double the length for a given wavelength.)

An example of inappropriate use of the formulas is when I computed that a 0.5 millimeters wire should be 1.46 meters long to build such a folded dipole for 100 MHz waves (½ λ = 1.5). Later on an experiment yielded that the shortest length to get a LED to faintly light up was 1.15 meters and the longest length was 1.45 meters. The optimal length, with the LED lighting up decisively, was broadly around 1.30 meters.

MMSD701T1G diodes are a strong and safe choice for powerful applications.

Whatever you connect to the diode bridge, be it a LED or a galvanometer, if you want to get the most out of the device you will need to cope with the impedance. The impedance of those transducers is quite high, which means for example that when the diode bridge produces a tension of 20 V, it will only be able to produce a current of say 1 mA. If you connect this to one LED, the tension will fall to 1.5 V but the current will still be 1 mA. So you are wasting most of the available power. One simple solution is to use several LED in series. Three LED will need a tension of about 4.5 V and they will be traversed by still the same current of 1 mA, roughly speaking. Hence you get three times more light, still roughly speaking. A more sophisticated solution is to use a little DC/DC converter, that will adapt the impedance. For example a converter that can take any tension like 6 V or 20 V with a weak current and produce a low tension of 3 V or 2 V with a higher current.

LED in series to cope with the
        impedance of the transducer

If the LED is meant to light up dimly, then my favorite choice is a hyper red one, like the L-7113SEC-J3. If the LED is meant to light up very bright, then it will be a high yield white one.

I built a transducer for 100 MHz radio waves, hence 9 times bigger. Each of the 8 wires in the photo below is 135 centimeters long, despite ½ λ being 150 centimeters. The optimal wire length for a closed dipole with the same wire is 130 centimeters, which would mean that when connecting an end of an antenna wire to diodes it must loose 5 centimeters. The diameter of the electric conductor is about 0.5 millimeters.

transducer for 100 MHz
          radio waves with LED

It manages to harvest enough power to make a buzzer sound. After some experiments I converged to a buzzer with an impedance that matches that of the transducer (about 2 kΩ) and that can sound dimly with only 1 V. I also built something around it to separate the front wave from the back wave and focus both. That would be the transparent plastic structure on next picture.

produce sound out of
          the energy of radio waves

These two videos show the result. The first one is on a dock of the Meuse river, at about 800 meters distance from the emitter. Downtown Liège gets maybe half that amount of radio pollution. The second video is in the Parc de la Citadelle, much closer to the emitter.

Eric Brasseur  -  July 24  till  July 8 2017