A Spark Gap Transmitter, Characterized

When we think of a spark gap radio transmitter, most of us immediately imagine an early twentieth century ship’s radio room or similar. Most of us know these transmitters as the first radio systems, and from there we’ll probably also know that they were phased out when better circuits arrived, because of their wide bandwidth. So it’s rare in 2024 to find anyone characterizing a spark gap transmitter, as [Baltic Lab] has.

The circuit is simple enough, a high voltage passes through an RC network to a spark gap, the other side of which is a tuned circuit. The RC network and the spark gap form a simple low frequency relaxation oscillator, with the C being charged until the spark gap triggers, forcing the subsequent discharge of the capacitor and causing the spark to extinguish and the cycle to repeat. The resulting chain of high voltage pulses repeatedly energizes the tuned circuit, with each pulse causing a damped oscillation at its resonant frequency. The resulting RF signal is a crude AM tone which can be received fairly simply.

The mathematics behind it all is pretty interesting, revealing both the cause of the bandwidth spread in the low Q factor of the tuned circuit, and the presence of a large spurious frequency spike on an interaction with the capacitor in the RC circuit. It’s all in the video below the break, and we have to admit, it taught us something about radio we didn’t know.

Meanwhile spark gaps weren’t the only early radio transmitter technology. How about an alternator?

31 thoughts on “A Spark Gap Transmitter, Characterized

  1. In 1966 I used a 9000v neon sign transformer, it had 60ma current limiting built in. I used window glass and aluminum foil taped on both sides to make the capacitor, no antenna. Spark gap was a couple of wires. Made a lot of audio noise and radio noise, the parents thought I’d killed myself.

  2. Listen up boys and girls. Spark gap transmitters are illegal. Big problems if you are found using one.
    Harmonics even dampened harmonics are a big problem. Modern radios has excellent receivers. They hear it all. Hence the problem.

    1. Are they illegal everywhere or just in certain jurisdictions? I’m not sure the FCC, for example, could do anything about you operating one in international waters. Is there an international equivalent?

      1. Banned internationally everywhere that has RF regulations which is virtually all countries. Operating one in international waters you can still have your emitter seized if the local naval power so wishes

          1. Yeah… there’s quite a lot of history of boats with transmitter rigs operating in international waters to bypass local restrictions. Look up Radio Caroline for an example, but there are many others. The UK government couldn’t legally do anything to the boat or the transmission, so they made it illegal for people in the UK to listen to it!

          2. Practically speaking, anyone that’s annoyed enough to do something about it and capable enough to matter — Merchant marine probably won’t go out of their way but nobody in a military/enforcement capacity thinks twice about giving the business to some pendejo cabron in a yacht with a broadband jammer regardless of “jurisdiction”. Radio is pretty important in shipping

          3. Whoever has the coast guard to reach you and legislation permissive enough to do something about it. Pirate radios didn’t get treated with armed forces (in the west atleast) because it was ultimately a civil issue, operating a jammer near territorial waters might get a different response. (Some states probably reserve the right to arrest their citizens in international waters for breaking the laws of the state as well)

    2. Once you realize that there’s more to the world than just the USA and that other countries might have their own rules, you’ll realize that your second sentence has a geographically limited scope of validity :-)

      1. Except that it was an international convention in 1927 that forbid the use of damped wave (spark gap) transmitters:


        It wasn’t just the US forbidding them, it was every country that participated.

        See the list of participants starting on page 9:


        Each country implemented its own laws, but the decision was made internationally.

        Spark gap transmitters are pretty much outlawed in every country on the planet.

        1. That international agreement, however, is not a legal basis for national enforcement. Neither the FCC nor our German equivalent, the BNetzA can enforce such treaties without ratification into local law. That’s why the FCC specifically mentiones damped wave transmitters as illegal. The BNetzA, however does not have such equivalents. Possibly indirectly due to the specified spectral purity requirements. But that’s fairly irrelevant for low power, local experiments. So assuming that it is “pretty much outlawed in every country on the planet”, just because countries signalled in an international agreement their intention to ratify such restrictions into local law, is far from the truth. That said, even if that would be illegal, I personally don’t care. In Germany the harshest enforcement the BNetzA could levish onto me would be the seizure of the transmitter and a fine somewhere around 100 €. I think I’ll be fine with that.

          1. “seizure of the transmitter and a fine” – That’s about what would happen to you in the USA as well.

            Still, it’s inconvenient to have to deal with it.

        2. According to the document, I can still use it on my ship or aircraft provided the frequency is above 500 kc/s and the power is less than 300 W, but no land or fixed installations permitted. I’m sure there’s a more recent version of this… right?

      1. That’s why they are forbidden. They are essentially jammers – they create wide band “noise” on frequencies far outside of the nominal transmitter frequency range. They interfere with nearly all other forms of radio transmission.

    3. Another. Idea. The. Spark. Could. Trigger.
      A. Light. Which. Would. Be. Used. As. A
      Light. Beam. Trans. Mitter. Which. Would
      Be. Focused. On. A. Single. Point. Receiver.
      The. Signal. Would. Not. Go. Off. Into
      Atmosphere. Which. Is. The. Fcc. Problem
      Right. A. Telescope. On. Receiver. Would
      Receive. The. Light. Beam. From. Far. Away

    4. Modern radios have a much better selectivity and pulse noise suppression than the old ones. You need very wide receivers to make the best of a spark signal.
      If the spark signal is 45khz wide (realistic, when you operate at 500khz) and your receiver is has a 1khz CW filter, you’ll only receive a fraction of the power that’s in the airwaves.
      You can find out information with regards to bandwidths in old 1920s books.

    5. So what, nobody hear should care about what lawyers would want…

      But spark gap transmitters are much worse than illegal, they are anti-social. Breaking laws is fine, but p***ing on radio spectrum that someone else is trying to use is not ok.

      When you want to discourage spark gap transmitter use, don’t start by quoting legislation that no-one in their right mind would care about, quote instead the reason that spark gap transmitters are an unwelcome menace. We are hackers, makers, engineers… we don’t care about written statues, we want reasoning, there are very good reasons not to use spark gap transmitters, lead with those reasons.

    6. Use. A. Auto. Ignition. Coil. Hooked. To. A
      Battery. With. A. Spark. Plug. Attached.
      To. A. Long. Wire. Antenna when. You. Touched. The. Button. It. Activated. The
      Coil. Hooked. To. Spark. Plug. Attached. To
      Long. Wire. Sending. Out. A. 20,000. Volt
      Radio. Wave. Dot. Dot. Dot. Dash. Dash
      Dot. Dot. Dot. S. O. S. Usefull. When
      You. Are. Kidnapped. In. Woods.

  3. Playing with a small spark gap setup like this should be fine provided you use a small power source like a 9v battery or similar even smaller. Since you don’t have a dedicated antenna apart from the spark itself, your emmisions will probably be in the milliwatt range somewhere, so you won’t be radiating a lot of power. Certainly not enough to draw the attention of the regulator in your country. Having said that – keep your experimentation down to reasonable levels, and also l try to make sure you understand the scale of the possible problems you are creating by measuring and investigating with SDR devices and spectrum analysers like the TinySA. Small spark-gap transmitters like these are fabulous teaching toys.

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