FM Snake Feeds Off Radio Waves

[Eric Brasseur] built a radio-detecting snake that consists of a LED that lights up when around reasonably strong radio waves. Near an FM radio mast you’ll find a huge amount of waste energy being dumped out in the 88 to 108 MHz range.

[Eric]’s rig consists of a pair of 1N6263 Schottky diodes, flip-flopped with one set of ends soldered to the antenna and the other ends soldered to the leads of the LED with about a foot of wire in between. The antenna can be a single wire as the diodes are soldered together. This one is around 4 feet in length for a total length of around 160 cm or a little over 5 feet. He went with a red LED just to give it a greater chance of being seen when illuminated by a distant or weak source of radio waves.

Hackaday loves its radio hacks; check out our posts on improving WiFi throughput with FM radio and building a modern DIY FM radio.

[Thanks, Alain!]

54 thoughts on “FM Snake Feeds Off Radio Waves

    1. Inverse square law suggests we would “Waste” less energy if we put the transmitters right next to the receivers… of course at that point using a radio is silly and we should just use a digital signal over copper… like a cat5 or cat6 cable…

      I’m just saying running all radio through ethernet would do away with all this wasted power.
      Would it be awkward to hook cars up to ethernet while they are driving? Sure, but think of the range increase for electric cars if we can get PoE to them!

      Just wireless is a waste of energy, cords for all the things!

        1. Marvell claims that for 30W going into a 100m line, 25.5W comes out making for 85% efficiency. Wireless charging’s roughly 50% efficient so that’s that debate done. Next up charging time, that’s about 163 days at 25.5W. 17kWh wasted in the cable but across 163 days the heating will be minimal, as will the cost compared to the amount of petrol station sandwiches for the driver to survive that long while charging.

          1. That’s funny. Of course, the standby loss in the car just to run the keyfob receiver, cellphone data, and alarm system probably exceeds that, so it actually never will charge…

            It’s amazing how fast a car “recharges” at a normal filling station: Last week I clocked mine at an average charge rate of 10 megawatts for the 90 seconds it took to refill its 250 kWh tank with energy-dense liquid fuel. Elon Musk is a huge promoter of “dense propellants”, and uses them to great success in his SpaceX rockets. I think he should promote their use for terrestrial vehicles too. Seriously. He’s proposing sucking the CO2 out of the Mars atmosphere to make rocket fuel there with solar power. Why not do it here first? Make dense, environmentally-friendly motor fuel literally out of thin air and sunshine.

          2. Methane and methanol have a problem on earth, they can use many supercheap feedstocks that nobody makes much of a profit off, therefore they don’t have a lobby… Also any serious effort to make bio/renewable-methane or methanol can be undercut indefinitely by the oil companies dumping natural gas on the market. Most of the cost of natural gas is transportation and some places storage. It’s a waste product as far as many oil companies are concerned, unless they are sufficiently near a domestic heating/power market, I mean we had to write laws some places to stop them just burning it off.

          3. @Paul – Sucking CO2 out of the Mars atmosphere to make rocket fuel? Wouldn’t people living on Mars want to add CO2 to the atmosphere, not take it away? This makes as much sense to me as using lunar water to create rocket fuel as opposed to using it to drink and grow plants and then recycling it.

          4. @Paul there’s something to be said for a car that you can ‘refill’ at home though. Stops you having to go visit a filling station and means you can turn on the air conditioning before you reach the car. Now sure, you could do that with petrol cans but petrol isn’t fun to keep at home next to your toddler, dog, or houseplants.

          5. @Max Siegieda (@CampGareth): ” there’s something to be said for a car that you can ‘refill’ at home though.”

            Nope. That’s the funny thing about my car: I’m never home with it when I want to fill it up. I’m always somewhere else. That’s kinda what a car is for. Last week I did a 600 km trip in one day on one fillup. Impossible to do with an electric car. The month before, 1300 km in two days. Also impossible, despite a Supercharger along the route. Commuting and errands around town? Heck, my bicycle is better for that. I have no use for a vehicle that ‘refills’ at home or trickle-charges at specific limited places (wait 30 mins of charging for 200 km of driving? gimme a break), when I can get a 850-km liquid refill in a couple of minutes virtually anywhere.

          6. @Paul I just a few days ago saw the CEO of nissan/renault talk about an 600km range electric car.
            But you just have fun with your sluggish coal powered relic that you toddle around in.
            And incidentally, if you go hundreds and hundreds of km perhaps you should just take a plane eh.

          7. @Whatnot, despite your troll, you remind me my car, and motorcycle, and 3/4-tonne work truck all once *were* powered by coal, because I filled with petrol manufactured from coal, by Sasol’s Fischer-Tropsch process:

            Coincidentally, it’s a process not conceptually different from the Sabatier process Musk is promoting. (Sasol uses raw coal as both a feedstock and energy source to make liquid fuel, instead of carbon dioxide and fission or sunshine, but the objective and methods are similar.)

      1. Inverse-square does suggest that, but physically it is the EM ‘near-field’. You need an antenna as an impedance match between the signal source and space itself to reach the far-field proper, and even then the energy goes through the near-field.

        Somehow two antenna-like devices are able to detect the presence of each other in the near-field and couple to provide much higher efficiency. We just know how this detection is physically possible. It is a gaping hole in the sky which everybody seems content to ignore.

        1. “We just [don’t] know how this detection is physically possible. It is a gaping hole in the sky which everybody seems content to ignore.”

          What the heck are you talking about?

          Two antennas close to each other couple better because the higher-order portions of the field are more significant there, as opposed to the far field where the (1/r) terms in the field dominate. The complete electric field and magnetic field for simple antennas (e.g. a Hertzian dipole) is completely solvable, and you can compute the mutual inductance between nearby antennas directly.

          1. Interesting. I wonder what “pilot-wave theory” could contribute to this already thoroughly-understood concept. Is there any way to explain it simply and without woo-woo?

    2. Oh yes it can be waste energy, but most of all harmful !!!
      In his page
      Eric explains that a great part of the broadcast from the nearby Citadelle hill is uselessly and harmfully sent down to the Liège city below instead of horizontally were it is intended to be received. And that happens in other places too.
      Some people badly suffer from it. Eric knows what he’s talking about.

    1. It would be great to build this and direct it at the loud stereos of the cars stopped at our street corner. If only it could be done without also directing it at those cars’ drivers.

  1. Wow. According to the fmscan link, that hospital has a total of EIGHT broadcast-band FM transmitters on top of it, transmitting a total of 15 kW from those relatively small masts (and hospitals are worries about a few milliwatts from cell phones?).

    Europeans are so sensible and frugal. Here in the Great White North, the transmitter nearest my house uses a 280m tower, and it broadcasts 300 kW (effective radiated power). Of course, you can hear it 300 km away too – equivalent to this guy’s site covering the region including Paris, Frankfurt and Amsterdam..

    1. Europe used FM for local stations and AM for regional stations (gross oversimplification but true for a lot of stations). Nowadays countrywide coverage is more often by chains of medium power FM stations dotted across the county and the AM band is almost abandoned, there were plenty of >100kW AM stations and a few >1000 kW AM stations in the day though.

  2. Did he mention that he was sitting in a microwave oven when he got this to work? You would need to be in a very strong RF field to get that to work. And as far as how long it would take to charge a large battery, probably forever. My guess is that you would not be putting in much above the self discharge rate of the battery.

    1. I suppose that depends on your notion of what a “very strong RF field” is. TFA says he’s sited about 1 km from those masts, which (from the fmscan site) are putting out 15 kW total. At that range, that’s in the ballpark of 4 mW/m^2 and 1 V/m field strength. It would take a resonator with just a modest Q to make a LED visible from that.

        1. (in which I notice I forgot a pi in my power density calculation…)
          If you know the power density you can figure electric field strength from basic principles, once you know the impedance of free space is (approximately) 120*pi ~ 377 ohms. Or you can use any of a bunch of web calculators. These two actually show the equations:


  3. IIRC, Radio Electronics published in the early 1980’s a simple microwave leakage detector.
    It was an LED soldered in reverse polarity to a Schottky Barrier Diode. The leads of the SBD acted as antennas.
    When placed near a leaky microwave oven the LED would light up as the SBD rectified the microwaves its leads received.
    I made about 10 of those attached them to Popsicle sticks (perpendicular and about 2 inches from one end).
    I tested them against our professional microwave leakage detector. They all managed the light up or flicker near the professional detectors safety threshold. I gave most away to friends, keeping one for myself.

  4. Pffft… 1N6263 Schottky. Try this instead (diode P/N & Spice model below). It will work work much better, even through low-mid VHF. Also, choose low Vf & If “High-Brightness” LED’s and cut/lengthen the wire (antenna) for best results for the frequency of interest.

    * 1N34A 60V 85mA Ge Signal Diode DO-41 1=A, 2=K
    .MODEL 1N34A D ( bv=75 cjo=0.5e-12 eg=0.67 ibv=18e-3
    + is=2e-7 rs=7 n=1.3 vj=0.1 m=0.27 )

  5. it’s called a rectenna and I’ve been trying to make one off and on for at least a year.I’m trying to get it to work at ~900mhz. I’ve got some hsms-286 diodes for high frequency rectifiers but the capacitance and antenna are also critical. It’s some very tough math and then your physical prototype never matches the calculations. I guess that’s why they call rf “black magic”.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.