How can I create an energy harvesting device covering cell phone frequencies?

[novicefedora]

I live in an area with a lot of cell phone towers and WiFi networks, is there any single or multiple antenna energy harvesting device I can build which can absorb these frequencies?

[cliffyk]

If it were possible to "harvest" energy from those signals, cell phones would not need batteries...

[0culus]

You can build all the antennas you want (you know, like cell phones have!) but the amount of energy at the receiver will always be tiny. Not gonna work, unless you go stand next to the transmitter! 

[Whales]

Yes you can, lookup "crystal receiver" and "crystal radio". 

Recalculate the antenna coils + cap to be approximately tuned for the target frequency range you are after (eg 900MHz).  Instead of headphones use a capacitor to build up a stored amount of charge (make sure it's low leakage, ie use a small capacitor with a high voltage rating). 

As mentioned by others: you will only get tiny amounts of power.  Micro-watts, or smaller?

[TheUnnamedNewbie]

There is some work on energy harvesting solutions for sensor nodes. The power you can get out is miniscule, but for ultra-low-power electronics, it can be enough (say, temperature logger that uses back-scatter to transmit the information back).

The main problem is usually rectification. The way I've seen it done is using very high-impedance nodes (vitually only possible in a single-chip solution because anything on a PCB or such will have way to high parasitic capacitance), so even with these tiny powers you can get enough voltage to actually activate rectifiers and so on.

Lets just do some math to get an idea of how much power we can expect in the best case:

Look at a WiFi node (for now lets assume we use the 2.4 GHz band) - these usually have in the neighborhood of 15-20 dBm EIRP. Over 10 meters, the FSPL at 2.4 GHz is about 60 dB, which means we have -40 dBm at our receiver. That is 100 nW. So lets say we somehow manage to get 100% efficiency in the rectification, I don't think there are any commercial microcontrollers that have that low power, but there are some ASIC solutions that can pull it off, especially if you do something where you spend most of your time in sleep, and only wake up a handfull of milliseconds every few minutes.

*Also keep in mind that this assumes a always-on CW transmitter, which is not the case, so in practice you will have less power still.

[VooDust]

There used to be a fashion that people hanging those LED ornaments on their phones that light up when someone calls you. Those operate by harvesting GSM radio energy to light up some LEDs.

Well, that's interesting. Never seen one of those.

[Ian.M]

Maybe *some* of them worked by energy harvesting, but I doubt it   I remember that fad and the LED ornaments had a couple of button cell batteries in them to power their LED + a circuit to switch it on when a strong RF E field was detected, similar to a 'voltstick' non-contact live wire detector but tuned for GSM frequencies not 50Hz/60Hz.

If you live next to a cell tower, on a high enough floor to be in the main lobe of one of its beams, with direct line of sight, you *might* get as much as 12mW/m², incident on a  surface normal to the line of sight.  However popular paranoia and the resulting risk of vandalism makes cellular network operators very unlikely to aim a beam at or even close to a neighbouring residential building.    Most locations, even in dense urban areas will have power density levels orders of magnitude smaller.   A well oriented, carefully optimised 10m² rectenna array near a cell tower could harvest enough power to run a LED nightlight, at a cost of at least an order of magnitude above solar panels harvesting ambient daylight.

[radiolistener]

You can use Yagi–Uda antenna tuned at specific frequency and directed into base station.

Also you can make antenna array created from these Yagi–Uda antennas with proper phase adjust between each antennas in the array. In such way you can increase received power.

The problem is that such antenna array will cost you money and will take up significant space, but obtained energy will be small. It will depends on your antenna array size, the distance from the base station and base station power.

What is the distance to the nearest base station directed into your sector?

[m3vuv]

sounds like this guy has been watching too many youbend nut job vids!!

[cdev]

No, collecting RF for low power electronics is something a lot of people have been working on for a long time. And the amount of power which can be collected is still quite low but stands to increase significantly with the rollout of 5G. Will it still remain fairly low? I actually don't think so, but here on the surface of the earth the earth's curvature serves as a natural limit to RF based on height. The higher up you are the more RF exposure you may get, within limits, and the more of that energy you might be able to "harvest"  . My guess is for most real world applications the power will still be too small, except for the least energy hungry devices. It may be enough for very low power, wake up to ping the mother ship GPS tracking of objects though. Imagine something like a super RFID tag that works anywhere, forever, tied to your ipv6 address.  If people are interested in this kind of atuff there is a lab at Georgia Tech that has been working with very low power electronics and broadband antennas for a long time, and they publish their research.

sounds like this guy has been watching too many youbend nut job vids!!

[wraper]

Maybe *some* of them worked by energy harvesting, but I doubt it   I remember that fad and the LED ornaments had a couple of button cell batteries in them to power their LED + a circuit to switch it on when a strong RF E field was detected, similar to a 'voltstick' non-contact live wire detector but tuned for GSM frequencies not 50Hz/60Hz.

Never seen one containing a battery. They were too thin to contain any power source. Also there is nothing surprising, GSM phone can cause audible noise in audio equipment a few meters away. Peak power is about 2 watts. And sticker was placed on top of antenna location.

[Ian.M]

Different LED ornament? The ones I'm thinking of were about the size of the tip of your little finger and weren't stickers.  I think the idea was ladies could attach one to the exterior of their handbag to see if a phone on silent in it was ringing.

[wraper]

Different LED ornament? The ones I'm thinking of were about the size of the tip of your little finger and weren't stickers.  I think the idea was ladies could attach one to the exterior of their handbag to see if a phone on silent in it was ringing.

[Ian.M]

Those are a similar level of naffness.  The ones I'm thinking of were a long time ago - IIRC mid '90s when cellphones still had external antennae.   

[coppice]

Yes you can, just don't expect any real amount of power.

There used to be a fashion that people hanging those LED ornaments on their phones that light up when someone calls you. Those operate by harvesting GSM radio energy to light up some LEDs.

Those things work by harvesting energy from the transmitter they are wrapped around, not the cell tower far away. They don't start flashing when the call request comes in. They flash when the acknowledgement goes back. Harvesting over a centimetre or two is easy.

[cdev]

Build a broadband antenna that covers the ranges most strogn transmitters are in your area, and then figure out a way to turn it into electricity. the simplest approach is to use something like a bridge rectifier made with the approriate RF friendly diodes and capacitors, but I am sure that one really needs to do impedance matching. A good antenna might be something like a log spiral or Archimedes spiral that captures radiation at a wide range of frequencies and polarizations, however if the frequency is very high you might lose a lot in the feedline and you dont want that, so perhaps to rectify right on the antenna. (this kind of setup goes by the catchy name "rectenna". Seriously.

This is only likely to be a usable strategy for applications that take very very little power.  Applications that sleep a long time, wake up, sense something, and send it off then go back to sleep, seem like good candidates.

[novicefedora]

sounds like this guy has been watching too many youbend nut job vids!!

Is your comment only because of this thread or some other thread?

[cliffyk]

Those are a similar level of naffness.  The ones I'm thinking of were a long time ago - IIRC mid '90s when cellphones still had external antennae.

I had a 5W "bagphone", you could feel your brain sizzling when you made a call...

[cdev]

For a couple of years when I was much younger I used to sell consumer electronics in a store not far away from a huge radio and TV transmission tower. I also lived in the shadow of this tower. In addition to causing a lot or various radio artifacts, which I also would deal with at home, (it made choosing hifi equipment difficult because it would bleed into the electronics) we used to constantly have customers who would tell us stories of the strange things all this RF would do in the neighborhoods right under this tower, especially when the fog was rolling in. One thing that I remember quite well because I used to walk my dog in those neighborhoods a lot, it would make florescent light bulbs flicker, which you could even see from outside on the street.  You can also see that under high tension lines. (I've done it near here) Teke an old school flourescent bulb, bury one end in the ground the other end pointing straight up, underneath a power line. You'll see the bulb flicker.

Maybe *some* of them worked by energy harvesting, but I doubt it   I remember that fad and the LED ornaments had a couple of button cell batteries in them to power their LED + a circuit to switch it on when a strong RF E field was detected, similar to a 'voltstick' non-contact live wire detector but tuned for GSM frequencies not 50Hz/60Hz.

If you live next to a cell tower, on a high enough floor to be in the main lobe of one of its beams, with direct line of sight, you *might* get as much as 12mW/m², incident on a  surface normal to the line of sight.  However popular paranoia and the resulting risk of vandalism makes cellular network operators very unlikely to aim a beam at or even close to a neighbouring residential building.    Most locations, even in dense urban areas will have power density levels orders of magnitude smaller.   A well oriented, carefully optimised 10m² rectenna array near a cell tower could harvest enough power to run a LED nightlight, at a cost of at least an order of magnitude above solar panels harvesting ambient daylight.

I have read that the amount of power from 5G is quite a lot higher, but the difference is that its pulsed and also sread over a wide numbers of bands. Basically the industry wants to establish rights to as much spectrum as they can. By claiming it they create a legal entitlement. Thing "gold rush" that is what is really going on.   Similarly with the ITU wanting to take parts of several ham bands now, something I just heard about.  they claim there is no significant opposition!

The way the poilicy is set up something that is free and doesnt involve economic activity you can put a number on is always going to lose out to the use that claims it will be generating huge profits from something, even if those applications are total hype. 

The world is just in a sort of land grab, gold rush phase right now.

Watch out!

EDIT: Correction! ITU found out there was significant opposition and called the theft off. For now.

[vk3yedotcom]

For those curious about this sort of stuff a VHF/UHF transmitter of a few watts can just get a little buzzer to sound.

Very small amounts of power though.