Build a Spark-Gap Transmitter and Coherer Receiver?

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Is there any way one could get away with running such a dinosaur, just for a few minutes from time to time, as a demonstration?

I assume it would be illegal ... given the width of the signal and its essentially uncontrolled nature.

Or perhaps it could be kept low-power enough? So that it would only reach from one side of a lecture hall to the other?

73 de Martin, KB1WSY

Bob Lewis:
No, it would not be legal but you might get away with it, depending on who is around to be interfered with. When I was a kid out in farm country two of us kids built spark transmitters from car radio vibrators and ignition coils. We used old AM broadcast band radios for receivers. We made contact between our houses about 3/4 mile apart. No one ever said anything except our parents when we tore up their radio or TV reception  ;D

For a demo, just run the spark transmitter with no antenna connected and it won't travel very far.

Tisha Hayes:
You can make a cute little spark-gap transmitter out of a DIP relay and a nine volt battery. The power levels would be similar to what you get from a fish tank heater.

Dieter Kuespert:
And with a bit of improvement you could use a zinc negative resistance crystal oscillator on 80 m:

What got me thinking about this was re-reading the autobiography of my grandfather, Eric Marris (1890-1976). Among other things, he was a wireless telegraphy officer in the British army in France during World War One. He was in charge of a W/T unit and here's his description of the training, and the equipment:

"Here men of all ranks, who had never seen a W/T set were being trained to understand, use, and repair them.  The officers at least were supposed to become capable of dealing with long distance reception, such as the Atlantic transmissions on 2000 metres wave length for which we had a 500 yard aerial on masts, and transmission and reception on 200-500 metres with portable sets of various ranges from a mile or two up to 50 miles.  We knew about "valves" which were being used for "detection" ("rectification") in the longer range sets, and in the cumbersome 3 valve amplifiers in "listening posts", but the short range "trench" sets which were my concern were operated by sparking coils for transmission and carborundum crystals for detection.

"The Trench set, about the size of a small record player, required if possible 100 yards of aerial on two 15 feet masts and was powered by a heavy 6 volt accumulator.  The transmission coil of flat "basket" weave had tappings to the studs of a rotary turning switch in the centre of the set.  To the left was a powerful "shocking coil", with an interrupter which could be crudely adjusted to a given "pitch" of buzz.  To the right was the knob of a condenser tuning the reception coil (coupled to the transmission coil), the crystal holder, and an elaborate Morse key with "shorting" contacts to protect the crystal during transmission.  Faults were almost always due to the elaborateness of the key contacts or a dud crystal.  Effective range in open conditions was a mile or two but almost nil in wooded country.  The more powerful Wilson set, range about 5 miles, had a separate crystal receiver, quite a refined affair, but difficult to tune quickly.  In the other box containing the transmitter the necessary sparking system was worked by a rotary interrupter, resulting in a clean high pitched note.  Altogether quite effective but very bulky and using so much current that batteries only lasted a few hours.  Effective range meant that the signals were "R7" or at least "R5", that is they could be heard clearly in sensitive headphones in a quiet place.  No-one mentioned the noise of guns during action."

Thought it would be neat to try to replicate, very approximately, one of these "Trench sets" to see how it works. I am assuming that the original models are now rare enough that they are worth their weight in gold....

73 de Martin, KB1WSY


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