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Author Topic: Build a Spark-Gap Transmitter and Coherer Receiver?  (Read 103425 times)
KB1WSY
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Posts: 1276




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« on: September 11, 2013, 09:13:10 AM »

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
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AA4PB
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« Reply #1 on: September 11, 2013, 09:37:58 AM »

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  Grin

For a demo, just run the spark transmitter with no antenna connected and it won't travel very far.
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AA4HA
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« Reply #2 on: September 11, 2013, 10:31:23 AM »

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.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
Free space loss (dB) = 32.4 + 20 × log10d + 20 × log10 f
KA4POL
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« Reply #3 on: September 11, 2013, 11:07:17 AM »

And with a bit of improvement you could use a zinc negative resistance crystal oscillator on 80 m: http://sparkbangbuzz.com/zinc-osc-2/zinc-osc3.htm
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KB1WSY
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« Reply #4 on: September 11, 2013, 11:22:54 AM »

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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KD0REQ
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Posts: 1998




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« Reply #5 on: September 11, 2013, 12:01:09 PM »

try to tune the fundamental to 14.212 and let us know how that works out for ya
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N2EY
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« Reply #6 on: September 11, 2013, 12:03:28 PM »

Why bother with toys?

take a look at this (operated into a dummy load)

http://www.youtube.com/watch?v=Uua5cO3_26c

btw, it's a "spark" transmitter. The gap is just one component.

73 de Jim, N2EY
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K0OD
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« Reply #7 on: September 11, 2013, 12:37:25 PM »

I made one that worked from a door buzzer connected to an antenna when I was about 10 years old.  I think you can still find buzzers in some hardware stores.
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KA4POL
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« Reply #8 on: September 11, 2013, 09:36:10 PM »

The real spark gap: http://www.youtube.com/watch?v=k8lXOnsvD80
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G3RZP
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« Reply #9 on: September 12, 2013, 02:27:46 PM »

>Here men of all ranks, who had never seen a W/T set were being trained to understand, use, and repair them.<

In 1941/2, a new RAF recruit got 6 weeks of basic training. To be a WO (Wireless Operator), he got 13 weeks to learn about basic transmitters and how to tune them, TRF and superhet receivers and how to use them to copy signals in Morse at 20wpm, and to be able to do all this while in a bomber over Germany being shot at. He may well have been a WOP/AG - as well - Wireless Operator /Air Gunner, although that at least prevented him being "Tail end Charlie".

It's fair to say that the level of understanding required was at least as great if not greater than that required for answering a "vote for Joe" amateur Extra class licence today........
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KB4QAA
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« Reply #10 on: September 12, 2013, 05:21:50 PM »

As shown, others have working spark demonstrators.  I don't have any problem doing so as long as one uses low power design, work into a dummy load and keep the demos short.   

I have long term plans to build any early spark set!
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KB1WSY
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« Reply #11 on: September 13, 2013, 02:58:44 AM »

From Wikipedia: 'Use of the spark-gap transmitter led to many radio operators being nicknamed "Sparks" long after spark transmitters ceased to be used. Even today, the German verb "funken", literally, "to spark", also means "to send a radio message/signal".'

Fun reading: "Thunderstruck" by Eric Larsen. It's a novel/true story based on the capture of notorious murderer Crippen as he sailed from the UK to America, made possible by the recent invention of shipboard wireless. It goes back and forth between the murder story and flashbacks to Marconi inventing W/T.

Another: "Electric Universe" by David Bodanis, which includes an account of Heinrich Hertz and his eventual tragic early death of a mystery disease, probably contamination from chemicals. He (and someone called David Hughes, independently) invented the spark transmitter but it was left to others, notably Tesla and of course Marconi, to develop it.

I am adding the spark transmitter to my list of projects. I am good at making the lists and amassing the components, not so good at completing the projects ... but my retirement years are going to be a lot of fun!!!

From time to time eBay comes up with old spark transmitters but usually they are very pricey (there's a "WWI" spark transmitter for auction right now, but it was actually built in 1927).

Perhaps I'll try the buzzer or relay ideas that were kindly mentioned here. My local electronics store has buzzers. The other fun bit would be building a coherer. I think there was actually such a thing in an electronics toy set that I had as a child: I remember it as being a glass cylinder filled with what looked like iron filings. However, I suppose that producing a big enough signal to activate the coherer might be a problem, legally speaking (spark transmitters were banned in the 1920s by international treaty, apart from legacy installations on ships). Presumably it would work if I put it a few inches away from the dummy load but that's not much fun.... More here: http://en.wikipedia.org/wiki/Coherer.

73 de Martin, KB1WSY
« Last Edit: September 13, 2013, 03:11:55 AM by KB1WSY » Logged
G3RZP
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Posts: 8021




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« Reply #12 on: September 13, 2013, 06:19:31 AM »

>(spark transmitters were banned in the 1920s by international treaty, apart from legacy installations on ships)<

Sort of. The 1927 Washington conference decided:

No  spark below 375 kc/s from Jan 1, 1930
No new spark installations on land after Jan 1, 1935
No new marine or airborne spark installations over 300 watts input after Jan 1, 1930
No spark transmissions after Jan 1, 1940 except for ships running less than 300 watts input

If you can get hold of the Admiralty Handbook of Wireless Telegraphy, Volume 2, of 1938, there are 27 pages on spark transmitters.....
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AE5X
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« Reply #13 on: September 15, 2013, 07:24:35 AM »

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


Not illegal at all if you keep the power down like I did:
http://www.ae5x.com/blog/2012/04/25/my-video-tribute-to-marconi-and-branly/

QRP spark gap transmitters are in everyday use - they just aren't called that ;-)

John AE5X
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G3RZP
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« Reply #14 on: September 15, 2013, 11:26:00 AM »

Spark can still, according to the Radio Regulations, be used legally in one situation:

4.9      No provision of these Regulations prevents the use by a station in distress, or by a station providing assistance to it, of any means of radiocommunication at its disposal to attract attention, make known the condition and location of the station in distress, and obtain or provide assistance.


73

Peter G3RZP
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