Easy build: an FM snake
Did you see pictures of people
holding neon tubes below high voltage electrical lines
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
The emitter masts are the four pylons on the roof of the hospital
atop the hill:
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:
About 30 centimeters below (12"), solder two radio frequency
detection diodes. Note that in the picture below
- the diodes are head to tail,
- the black ring of the one on the left is towards the red wire
- At the bottom of the picture the leads of the diodes are
soldered together, altogether with the wires,
- the distance between the body of the LED and the body of the
diodes should be about 37.5 centimeters (14¾"). It doesn't need
to be precise.
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":
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
Any little LED will do but the ideal LED follows some rules:
- It lights up with a low tension, like 1.5 Volts. Hence it must
be a red LED. The closer to blue the LED is, the higher the
tension it needs to light up.
- It produces a hue of red that the human eye can see easily. A
deep red LED (650 nm) is not easy to see when it lights up
dimly. Better use a slightly more orange LED, like 630 nm.
- A very weak current is enough to make it light up dimly, even
just a few micro-amperes.
- It focuses its light towards your eyes.
A bright white LED is great at night, close to a powerful emitter
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
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.
If you want to find the FM emitters somewhere, fmscan.org
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
If you'd like to build a more complex and powerful "snake": http://ericbrasseur.org/radio_wave_transducer.html
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.
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