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Making a spark gap transmitter

  1. Sep 6, 2004 #1
    in one of my electronics classes, as a group we have to design and build a transmitter and receiver to transmit and receive morse code. we can only use a tesla coil and/or a spark gap for transmission. since we have 3 in our group... i am making the spark gap, someone else - a tesla coil, and the thrid - a receiver to receive frequencies from both the tesla coil and the spark gap. i was wondering if anyone has advice or at least a good link on how to build a spark gap with a good explanation of what is being done. i am also only supposed to use 50V. thanks for any suggestions.
  2. jcsd
  3. Sep 6, 2004 #2


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    What does the range need to be?
  4. Sep 7, 2004 #3
    The most reliable homemade spark-gap that I ever made was with a small D.C. motor.

    Make a small wooden disc flywheel and stick it on the motor shaft. Make sure you balance it properly. It should be make of something heavy enough to keep spinning for a few seconds without power applied to the motor.

    Put several brass contacts around the outside edge of the flywheel.

    Then make a pair of sweeper brushes and mount them next to the flywheel.

    Every time a contact on the flywheel goes by, it sweeps up against both brushes at once, making contact.

    Connect one brush to the motor. Connect the other brush to your power supply.

    Now, if you apply power to the motor and to the other brush and give it a spin, the motor will be running on pulses and making sparks on the flywheel contacts.

    Run positive power into your tesla primary winding.
    Connect the other end of the primary to the motor positive.
    Motor negative goes to one brush.
    The other brush goes to your code key.

    Hold the key down to get the motor up to speed. Then you can start tapping out morse code once the flywheel is up to speed.
  5. Sep 7, 2004 #4
    the range does not need to be far at all. my teacher suggested only 50 v. i would probably use a battery or dc power supply. from what i've looked at so far, this is what i understand (or think i understand) about the kind i am going to build...

    a dc power supply, a switch (morse key), and the primary side of a step-up transformer in series. the secondary coil of the transformer is connected in parallel to a tank circuit. the tank circuit has a spark gap between the C and the L and an antenna and ground.

    does this sound correct? if it is, i don't quite understand how the tank circuit would actually resonate at a certain frequency if only one spark is fired per time you close the switch (key). any help with this would be great. thanks.
  6. Sep 8, 2004 #5
    You don't need any "tank circuit" on the primary.

    The motor is spinning a flywheel,
    The flywheel has a bunch of contacts mounted around it,
    The contacts are in SERIES WITH the motor so that contact is be made and broken as the flywheel spins. This is a mechanical "Pulse Width Modulator".

    As the motor spins, it breaks it's own contact, producing sparks.

    If you wire the motor assembly (rotating spark gap) in series with your primary and your code-key, every time you press the key, the motor is whizzing along and the contacts are sparking.

    Add a 1kv Diode in series with your battery positive to prevent oscillations in the primary winding. All oscillations will be restricted to the secondary (RF output).

    The attached pic should clear things up. The flywheel has brass contacts. The two brass brushes are in series with the motor. Whenever the key is pressed, the motor is making and breaking the negative-side connection between the tesla primary and the battery.

    Tesla had TWO TYPES of coils. Some were "resonant primary", high frequency AC coils that used capacitor tank primaries.. These produce the familiar blue lightning that wriggles around.

    The second type used a UNI-DIRECTIONAL IMPULSE (D.C.) with NO CAPACITOR in parallel. He made special magnetic and hot-air spark quenchers to PREVENT OSCILLATIONS in the primary. This type make a very FLUFFY WHITE arc from the top terminal on the secondary and it's quite different from his previous coils.

    Attached Files:

  7. Sep 8, 2004 #6
    thank you, that explanation and picture helped!
  8. Sep 9, 2004 #7
    Oh, I forgot to mention something.

    The motor, itself, acts as a "current limiter" in your primary circuit.

    This will make sure your input current will never exceed a certain level which means you'll have your transmission wattage under control.

    It's probably the highest resistance point in the primary circuit so nothing else should get hot, hopefully.
  9. Sep 10, 2004 #8
  10. Mar 24, 2011 #9
    Spark gap transmitters have been illegal for a long time, but with only 50 volts, nobody will hear you. Of course, if you start with 12 volts and use an automotive ignition coil to generate the spark, you will get somewhere.

    First of all, your transmitter will not sound like the continuous tone of regular radio transmitters; it will sound like a series of ticks, like the old fashioned telegraph sounders. So you will need a start tick for each character. After a while, you can train yourself to read these.

    Diversion: a history lesson. The first Americans captured in the Vietnam war were not communication Einsteins. They did not know the morse code, and thought it would be impossible to use because a continuous tone can not be sent by tapping on the prison walls. They invented the Vietnam code, a 5 x 5 grid of letters, with K missing. (further proof of stupidity, they could have merged U with V and I with J, for only 24 letters, just like in ancient Rome). But with their code, using taps, they could communicate.

    A . . B . .. C . ... F .. . G .. .. Y ..... .... Z ..... .....

    The enemy forbade comminication, but on one occasion put the prisoners to work on a rockpile. They turned it into a giant broadcast transmitter while the Viets commented on their wonderful enthusiam and progressive socialist attitude with that great rhythm.

    So, even with ticks alone you can send morse code. It is a lot more efficient than Vietnam code because the most frequent symbls are the quickest to send. For example, the most frequent letter E is a single dit. So here is how you change the morse code for ticks alone:

    Letter Standard Symbol Ticks only

    A dit dah .. .
    B dah dit dit dit . ....
    C dah dit dah dit . .. ..

    All you do is add an initial tick to each letter. It is not hard to get used to hearing and decoding.

    Now for the receiver: I recommend an ordinary AM broadcast band receiver set between stations below 600 kHz.

    Transmitter: For the transmitter, you can use a 12 volt car battery with an automotive ignition coil. Just wire the battery in series with a telegraph key and the primary of the ignition coil. The high voltage output of the coil goes in series with TWO spark plugs or other spark gap to the antenna. The antenna will be HOT, this will discourage stray cats from messing with it. Back in the day, I was able to communicate 5 miles with a similar rig, using a 150 foot wire in a tree for an antenna, a resonant transformer circuit that tuned both the spark gap and the antenna, but at that time I did not know about the advantages of two spark gaps in series.

    It is illegal to transmit this way, so keep your antenna no larger than needed to prove your project to the teacher. Do NOT let your teacher near the antenna when you are transmitting, or you will get a very nazi surprise.

    It is good to know how to do this, because if civilization collapses, we will be able to keep some low bandwidth global communication going. I have a plan using carfully timed pulses so that you are listening for a pulse only when it is expected. I have a modified code that includes a parity pulse: all symbols have an even number of pulses. It is a little harder to learn. Also, I was considering the use of quartz crystals in series with the antenna to limit interference with other stations, but I will worry about that after civilization collapses. If you can grind a telescope mirror, you can grind a quartz crystal.

    If you want to know more, call me, K6YVL. You can find me through the radio amateur callbook. I am in southern California.
    Last edited: Mar 24, 2011
  11. Mar 25, 2011 #10


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    This thread is almost as old as spark gap.
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