“This video of so called “plasma radio” is popular lately on Russian blogs. The shown effect is very cool. Here is a short story to understand it:
Location: Brovary town, Russia [Actually, Ukraine].
Transmitter output: 150 kW
Also used: old Soviet antennas, 90 meteres (280 ft) long cable.
That’s all what you need for UNSAFE experiments to receive radio waves. In the area of the effect of radio waves very big potential difference emerges. Because of the modulation of the radio signal the electical spark arc starts vibrating with the human voice and begins to “speak”. The intensity of current in such “radio” can reach hundreds of ampers and it easily can molt metals. Such electricity doesn’t have any barriers, it can come through the thick tree. Don’t try this at home!” [English Russia]
Is this real? An elaborate hoax? Another urban legend? Here in Forgetomori, the Answer reads YOU!! Keep reading.
Among the simplest radios that can be built are the crystal radios. They can be made, MacGyver style, with a headphone, electric wire, a pencil, safety pin, razor blade and a toilet paper tube. In the video below, Bre Pettis shows how to do it:
This is no trick! These improvised crystal sets, as they needed no batteries and could be made with household items were very useful to GIs in the Second World War. Building them is a common science fair experiment to this day as well as a nice introduction for ham radio enthusiasts.
No batteries required? It sounds like a hoax, but in fact, no power source in the receiver is required, because there’s a power source, and that is the radio transmitter. It can be miles away, but the closer, the better.
The radio antenna, when properly tuned to the frequency of the radio transmitter, will induce small electric currents. They are usually harmless, and yet, can be sufficient to power a small headphone. And in the case of AM radio, the induced electric currents will even be directly related in their amplitude to the sound being transmitted, which means no complicated circuitry is needed to hear them.
One very important electronic element is necessary, however, and that its a rectifier, acting as a diode. In the video above it was made from the safety pin to hold a pencil over a rusty razor blade. The important thing there is the point of contact between the pencil’s lead and the rusty razor blade, a junction which is capable of rectifying the electric current. Without it, no sound will come out of the headphone.
Put everything together, and you have a crystal radio set. You can read more about if on HowStuffWorks.
Now that we know more about how one of the simplest radio receivers works, we may speculate that in the Russian (or Ukrainian) video, somehow the steel cables, either slightly rusty or with some kind of coating could, when in contact, form a rectifying junction. That would be fabulous… except that this first hypothesis has its problems.
Watching the video, especially the ending part, it’s clear that although the sound starts when the cables are in contact, they can be separated, enlarging the electric arc… and the “plasma radio” still works! Does plasma also work as a rectifier? Probably not.
“I think things are much more simple than that. Cutting the cable at the right length and you ‘tune’ the radio. The radio waves induce a current in the antenna, and as it’s AM, the intensity of the current is modulated by the voice in the radio. The electric arc already makes a buzz; modulating the intensity of the current you modulate the buzzing”, fellow Murilo Queiroz told me.
Indeed, it can be as simple as that, without rectifiers and their solid state physics, for the small detail that as it seems, the sound produced by the electric arc, contrary to the sound produced by a headphone, doesn’t need a rectified current!
Which reminds me that I need to mention where does the sound in the “plasma radio” comes from. Well, it comes from the plasma, not much different from the thunder. The electric current going through the air creates a plasma which, by varying in intensity, can expand and contract, producing sound waves. The principle is explored in ionic speakers, but it’s much more interesting with these fantastic videos of Tesla coils buzzing to the tune of Super Mario Brothers:
If the plasma radio doesn’t have a rectifier like the crystal radios, why did we spend the first half of this post talking about them? Because knowing crystal radios made with pencil and razor blades, utterly cool by themselves, just makes it even clearer how amazingly nice are the plasma radios.
That is: a steel cable, with the right length to tune into the frequency of a 150.000W AM radio transmitter antenna, induces electric currents sufficient to melt steel. By putting another grounded steel cable in contact with it, the current flows and then fly through the air, creating a plasma which changes along with the radio signal, translating into audible sound.
The plasma radio is therefore simpler than a crystal radio set, and mind-bogglingly fantastic, from the absence of a rectifier and filter, to the plain danger and coolness of playing with dancing electric arcs.
Or at least, that was how we could understand the effect. We needed some confirmation, and an ordinary investigation soon paid off as we found this other video of the same phenomenon:
In this case, the effect is seen (or heard) directly in the transmitter antenna. As one operator points out:
“All you need is a powerful enough radio (I think you can get this starting at the 50 to 100W mark as corona, flames take at least a couple hundred watts), and an antenna that is not rated for the power level or a really high SWR.”
Here the overloaded antenna discharges in the air, and even without rectification, the arcs produce audible sound. It looks like magic, even something supernatural, but its science, or at least, the very natural explanation we can find to the phenomenon.
Some further ordinary investigation reveals the effect is not that rare. There’s the lovely tale of the “haunted crane”, as told by electrical engineer Antonio Panicalli, who had already solved the case of an electronic stethoscope that was receiving country music and was thus called to investigate the talking crane.
Besides scaring the construction workers, it shocked and formed electric arcs in the air. As you can guess, exactly like in the Russian (or Ukrainian) video, the crane’s steel cables were near a powerful radio transmitter in São Paulo, Brazil.
There’s also the story by Mike Wooton, that “during the Second World War the same phenomenon gave rise to many erroneous reports of the "foreign invaders" in areas of Britain close to short-wave transmitters. Strange voices in hedgerows at night were reported to the police or the Home Guard. On investigation it was found that modulated arcs on barbed-wire fences were picking up BBC World Service transmissions.”
They may sound like urban legends, but as we have seen now, they are more than plausible. Perhaps the Mythbusters should indeed reevaluate the polemic case of the talking tooth fillings. There are a lot of ways radio waves can be heard.
After all, with a really big paper clip – one, who knows, 90m long – even without a rectifier, you would just have to be near an AM transmitter at the right frequency to listen to the radio. Just a big paperclip. MacGyver would be proud.
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