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Easy build: an FM snake

Did you see pictures of people holding neon tubes below high voltage electrical lines, with the tubes lighting up just because of the powerful AC electromagnetic field? The "snake" does the same in most big cities but using the extreme waste of energy by FM radio broadcast masts, in the 88 to 108 MHz range.

The picture below shows the LED of a prototype snake lighting up red in Liège, Belgium, about 1 kilometer away from an emitter (less than a mile):

FM snake
          lighting up

The emitter masts are the four pylons on the roof of the hospital atop the hill:

Hopital de la Citadelle

To build a snake, solder a LED at the end of a pair of wires. Note that the red wire is soldered to the longest lead of the LED:

LED at
          the end of an FM snake

About 30 centimeters below (12"), solder two radio frequency detection diodes. Note that in the picture below

          diodes of a snake

The wires below the two diodes should be about 122.5 centimeters long (48¼"). It doesn't have to be precise but try to ensure that the total length, from the LED to the end of the wire, is about 160 centimeters (63"). A snake will still operate correctly if it has 15 centimeters more length or less... (6") but the ideal length is just below 160 centimeters (63").


The second pair of wires does not need to be a pair. It can be just one wire, since the diodes are soldered together. But a pair makes the snake more visually uniform and easier to handle.

When using the snake, stretch your arm to get it far from your body.

If in doubt about the origin of radio waves that make a snake light up, try to use the radio shadows created by buildings or metallic pillars. When in the shadow, the snake will dim down, which hints that the emitter is behind the obstacle.

If you hold the snake a few centimeters in front of a metallic pillar, the brightness of the LED should be augmented, because the radio waves bounce back and hit the snake two times.

This is a schematic of the "snake":

schematic of FM snake

The prototype above uses two 1N6263 detection diodes. They are quite excellent and easy to solder. Any detection diodes that can operate at at least 100 MHz will do. If you use ones that can also operate up to cell phone frequencies (900 MHz, up to 3 GHz) that will allow to verify a snake using your cell phone placed against it (place a call with the cell phone, so it emits a strong flash of radio waves to contact a nearby tower). I usually use MMSD701T1G diodes because they are robust and quite little.

The LED I'm currently using is the KINGBRIGHT L-7113SEC-J3 or L-7113SEC-H.

Any little LED will do but the ideal LED follows some rules:
A bright white LED is great at night, close to a powerful emitter mast.

If you want to light up items in full darkness, using the snake like a torch, a high yield green LED with a wavelength around 507 nanometers should be ideal, because in the dark human eyes are most sensitive at that wavelength.

The prototype shown above will not last very long. Its wires will tear off or just make shortcuts. By cutting the component leads shorter and using heat shrink sleevings you can make a tougher one. I use UV glue to make really robust ones. UV gel is tougher once hardened but also more difficult to use. UV glue hardens in seconds and becomes quite rigid but it needs hours and even days to harden completely and become rock solid.

          up FM snake

If you want to find the FM emitters somewhere, is the reference. Click "surrounding area", click "set location", type in the name of a city then click "Location Search", choose the city in the table (usually the first one), check "radius" then type a radius of say 20 km, click "set", click "Submit Query". Possibly select "Google Map" and Satellite view.

If you'd like to build a more complex and powerful "snake":

You may wonder why the wire is knot together at the end of the prototype. That's because I verified that the ideal total length was still 160 centimeters, using those diodes. I started with a wire that was way too long, more than 2 meters. Then I patiently cut it shorter, bit by bit, till at about 2 meters lengths the LED started to light up faintly. Then I cut it to 1.4 meters and started again cutting bits away, till the LED became again very faint, at about 1.2 meters. The average of 1.2 and 2.0 being 1.6, about 160 centimeters was still the ideal length. Then I attached some length back to the wire, to get those 160 meters, hence the knot.

wires knobbed together

The working principle of the snake is that like any antenna, it's a resonator. The free electrons at the surface of the wire will oscillate back and forth along the wire, when the wire is subjected to radio waves around 100 MHz. The wavelength of 100 MHz radio waves is 3 meters (118") hence a basic oscillator is half that wavelength; 1.5 meters (59"). Why does the snake need to be a little longer? I don't know but given the presence of the diodes it is quite normal to have to adjust the length slightly.

Why are the detection diodes placed at ¼ length? Because that's where the electric power is highest. The diodes "harvest" the power of the oscillating current along the wire and create DC current for the LED. In the middle of the wire, there would be too little tension. Near the end of the wire, there would be too little current. In-between, at about ¼ length, is an optimal position.

To increase the yield, tape the snake to a long plastic rod. That way the snake can rise high above the ground, with the LED just above your hand that holds the rod.

Eric Brasseur  -  July 3 2017