The First Radio Sets: a Spark Gap and a Coherer

[Ashish] let us know about his experiments in recreating the earliest type of radio set: a spark-gap transmitter and iron-filings coherer. He goes through the historical development of the kit in great detail, so we’re just going to skip that part. Go read it yourself!

Instead, we’re going to tease you with the coolest part of the rig: the coherer. In [Ashish]’s build, it’s a piece of tubing with some iron filings between two bolts. When a sufficiently strong EM wave hits the filings, they stick together and bridge the gap between the bolts, allowing electricity to flow and light up an LED, for instance. You can see this in [Ashish]’s video below the break, along with kmore discussion of that coherer.

A coherer is a one-shot receiver — the filings have to be physically separated after each reception. Repeatedly tapping on the coherer by hand must have gotten old pretty quickly: period coherers included a “decoherer” — an electromechanical tapper that reset the coherer multiple times per second by hitting it, and contribute to a low buzzing sound when receiving with one.

Human ingenuity being what it is, there were many advances in coherer design over a few decades before crystals made them entirely obsolete. We love peering into these technological cul de sacs and finding that they were full of cleverness. We hope [Ashish] keeps playing around with coherers.

Most of us at Hackaday are licensed amateur radio operators, and we should definitely note that a spark-gap is a ridiculously broadband emitter, and you’re probably transmitting on all sorts of frequencies for which you don’t have a license. The fact that [Ashish]’s signal is strong enough to move iron filings across the room suggests that he might also be interfering with other people’s radio in the neighborhood, or further. A quick check with an AM radio, for instance, would be indicative.

It does look like it’s possible to get permission to run an experimental spark gap transmitter in the form of a Special Temporary Authority from the FCC, though, so all is not lost. If anyone else knows something about legally operating a spark-gap transmitter, please post up in the comments. The retro tech is cool enough that you know people are going to be trying this — let’s see if we can find a way to do so responsibly?

[Ashish] suggested to us that a coherer can probably make a pretty decent lightning detector. We know what our next quickie project is going to be.

32 thoughts on “The First Radio Sets: a Spark Gap and a Coherer

  1. I doubt an STA, or any other provision in the law, would allow this in the US. The STA specifies that it is a) for cases where the time needed for a regular license issue is impractical, and b) the same noninterference requirement as all other licenses

    1. By choosing his language carefully when providing a link to the information about the STA of topic Al was pretty clear such a STA isn’t going to be common occurrence. In visiting the link one would have learned that this was for one time event of limited duration using very limited power <1 watt,with efforts to minimize the possibility of interference. However the article read there may had been a second STA Many licensed hams where granted a STA to operate automatic PBBS' on HF. seems that the nature of STA is there are no set in stone rules, since the purpose of STA is to suspend rules for experimental, educational, research purposes.

      1. Yes, I understand that. Mechanically commutated Tesla coils use the the same principle to generate high frequency current pulses near the resonant frequency of the secondary coil, and because there is no way of suppressing the frequencies around the resonant frequency or harmonics thereof, the mechanically commutated Tesla coil acts as a very high power spark gap transmitter.

        1. The FCC may treat Tesla coils as incidental radiators, I wouldn’t know. I would think to operate one enough to create interference to (FCC licensed) radio services complaints the FCC would have jurisdiction and could prohibit your operating one if you can’t eliminate the interference. Something to consider before investing money and time into a Tesla Coil.

      2. Well, you can put a tuned filter on the output, that will band-limit it.

        You can also run a spark gap without an antenna, which is what the New England Wireless and Steam Museum does with theirs. I was lucky enough to be able to try it, and it is very impressive. Blue sparks and lots of noise. Real old-timey CW, just like Marconi!

      3. You do not need special license for Tesla Coils experiments and i think is not need also for Spark transmitters if you experiment with low power. The basic transmission is in VLF and low range. Using high power will be a problem.

        1. In the US, transmitting below the AM broadcast band (so-called VLF) falls under Part 15, defining ‘Low Power’ as about 100 mW and very limiting antenna restrictions as I understand it…


      1. they’re also well-shielded with an electrical ground-mass of the engine block and car frame, and magnetos are shielded by the magneto structure, which is generally grounded, including in an integral mag, where the flywheel acts as the grounding shield

    1. That was my first idea too. Try to find a local EMC test lab with a shielded anechoic chamber they use for radio interference testing. The nice part about that is that they have the generators, antennas, HF-amps and spectrum analyzers already there to possibly do some pretty extensive testing on these “old-school” devices. The downside is that these labs are usually pretty expensive to rent, at least if you go there to get your (pre-)compliance testing on paper. (As far as i remember, we pay more than 1000$/hour for the chamber and the operating engineer).
      But you might just run into another “nerd” in there that is fascinated by these projects enough to help you with your experiments on the off-times where the lab is not rented out to customers. If you don’t try and ask around, there is no way of knowing.

  2. does anyone know what the other mechanical “demodulators” are called?

    the one I keep thinking of uses a loop of ferro-magnetic wire, driven by clock work, the wire runs through a coil.

    I saw an article years ago in Electronics Australia magazine, but I can’t remember what it is called

  3. i did some extensive testing of a large 5 kw Tesla coil a few years ago with some very expensive test gear and lot of wideband antennas. the RF goes up to the gigahertz but the range is not a that far. a good grounding system really helps a lot. the only things i have seen that get regularly damaged are dmx controllers on lighting rigs. they don’t like Tesla coils at all. well grounded faraday cages help a lot.

      1. There are lots of gems like that on old tech in but it is sometimes hard to get a search by publication date to work so that you limit the books to the old ones before 1930 with the very simple DIY radio information.

        I found also 2 interesting papers on the Coherer, including how to improve it. Interestingly the description of it’s behaviour sounds very much like a memristor!

        On the Action of the Coherer
        by Wheeler, E. B.
        Published April 28, 1899

        A Self-Readjusting `Coherer.’
        by Lawrence, A. E.
        Published December 9, 1898

          1. No joke! Thanks for the early radio links y’all!

            Reading through the Work of Hertz book, Lodge guesses (p.23) that the particles in a coherer are covered with oxides and that they get welded together in tiny spots. See above for my surprise that someone published a paper in 2004 documenting this effect much more rigorously.

            I’m gonna put down my work on inventing the wheel for the evening…

  4. its only ileagal to put an antena on the spark transmiter , if you want mega high voltage then use a light dimmer switch with a triack with mains volts through , put on neutral return a cap of aound 2 or 4micro farads in series . so then conect to ac mains , Then see the high voltage come out , in the uk our supply is 240 volts , so a 12 volt ignition coil with a triac modulating it and a cap to limit curent means a 12volt coil will take only what it needs , i did it with 360 volt supply and got 55kv out . mind me it didnt last long as the big blue hallow turned to a fire ball lol. . using a triac in a simple circuit like a light dimmer swith will give you variable output and frequency . .. just beware high volts can kill . . whith 55kv you can light a floresent tube up feet away ………………………….ps you will get more from this than and 12 circuit . . details im on call sign G6zdb Gene ……………………………..

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