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Author Topic: Build a Spark-Gap Transmitter and Coherer Receiver?  (Read 62161 times)
M0HCN
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Posts: 473




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« Reply #15 on: September 15, 2013, 02:47:06 PM »

There was that guy in IIRC India who won a prize some years ago for coming up with a very simple emergency radio link to a local medical center.

Take one piezo ignition cigarette lighter, fit a tank circuit, a ground and a wire up a tree, now in an emergency you can call for the doctor by clicking the lighter a number of times corresponding to the location.....

The thing was sufficiently broadband that a radio tuned to a quiet spot on the AM dial would clearly hear the pulses from the transmitters.
Very useful given the dangers inherent in manual subsistence farming, and the technology was cheap enough to be affordable to the people who needed it.

The other place that spark looks legal (at least according to the 1927 conference) is on a ship, over 375KHz and at less then 300W input, M0HCN/MM might have to try that on topband next time he goes sailing.....).

73 Dan.
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G3RZP
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Posts: 4466




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« Reply #16 on: September 16, 2013, 12:39:00 AM »


>The other place that spark looks legal (at least according to the 1927 conference) is on a ship, over 375KHz and at less then 300W input, M0HCN/MM might have to try that on topband next time he goes sailing.....).<


Sorry Dan - no go!

 Radio Regulation 3.15 The use of damped wave emissions is forbidden in all stations.
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KB1WSY
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Posts: 716




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« Reply #17 on: September 16, 2013, 06:50:56 PM »

Why do spark transmitters emit a damped wave? Is it because there is a certain "crossover" potential at which the spark begins, but it can then be sustained with lower and lower potential until it fades out?

What is the main objection to the damped wave: signal too wide? If so, what makes it so wide?



73 de Martin, KB1WSY


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KB4QAA
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Posts: 2332




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« Reply #18 on: September 16, 2013, 07:55:53 PM »

The damped wave itself is not a problem technically.  The term is used to differentiate spark gap sets from continuous wave sets.
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G3RZP
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Posts: 4466




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« Reply #19 on: September 17, 2013, 03:53:24 AM »

When the spark starts, it is a very steep edge waveform, so has lots of harmonics. Because the spark has to repeat itself, you get harmonics of the repetition frequency modulating the RF signal, which is why it is so wide.

You really need to read those 27 pages of the Admiralty Handbook!
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N2EY
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Posts: 3877




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« Reply #20 on: September 17, 2013, 02:41:03 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........

Or doing stuff like this:

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

73 de Jim, N2EY
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WX7G
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Posts: 5973




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« Reply #21 on: September 17, 2013, 06:55:49 PM »

I was at a hamfest many years ago when someone fired up a genuine rotary spark gap into a dummy load. The hotel management rushed in and informed the spark gap user that every TV and radio in the hotel was experiencing interference and to stop immediately.

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KB1WSY
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Posts: 716




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« Reply #22 on: September 18, 2013, 06:48:33 AM »

I was at a hamfest many years ago when someone fired up a genuine rotary spark gap into a dummy load. The hotel management rushed in and informed the spark gap user that every TV and radio in the hotel was experiencing interference and to stop immediately.

Apparently spark gap transmitters went on being used into the 1950s (and '60s?) as jammers during the Cold War. You can see why!

73 de Martin, KB1WSY
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PA1ZP
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Posts: 226




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« Reply #23 on: September 18, 2013, 09:38:38 AM »

Hi to you all

Have a look at this website.
http://www.antiqueradioreplica.com/

It is the site of Sebastian Blommaart PA0LB.

I have played around with his spark gap trx on a hammarket.
He makes very beautifull remakes of historacly important TX and RX designs.

He will even make his own nuts and bolts to make the design as accurate as possible.
And he can tell the most amusing and wonderfull stories about the past of radio and its history.

Another great historian in Morse code and telegraphy is Fons Vandenberghen.
http://www.telegraphsofeurope.net/
But he is more specialized in line telegraphy.

73 Jos
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KB1WSY
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Posts: 716




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« Reply #24 on: September 18, 2013, 03:32:05 PM »

Have a look at this website.
http://www.antiqueradioreplica.com/
It is the site of Sebastian Blommaart PA0LB.

That's lovely. What amazing workmanship!

73 de Martin, KB1WSY
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W2WDX
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Posts: 188




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« Reply #25 on: September 22, 2013, 11:40:39 AM »

One thing I have always wondered about.

When the world had no wireless (radio), and was wired for telegraph exclusively I wonder what the airwaves would sound like on a modern receiver. If one traveled back in time with one that is.
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AA4PB
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Posts: 12779




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« Reply #26 on: September 22, 2013, 12:29:37 PM »

I'll bet that before there was any electric power the bands were very, very quiet. Even in the 1950's you could turn on your AM broadcast band radio at night and pick up stations around the country. Try that same radio today and all you hear is buzz and noise if you are in a residential area.

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ZL1BDA
Member

Posts: 30




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« Reply #27 on: September 22, 2013, 05:08:04 PM »

Actually I have a reproduction, working rotary gap spark transmitter.
It puts out about 30 watts and uses a modified UPS supply for a 200 Hz frequency.
This gives 400 sparks per second and quite a readable tone.
It uses some components from a wrecked Marconi 1/4 Kw lifeboat transmitter and is
about 900mm wide 700 mm high and 450mm deep.
Unfortunately I was never able to source an old aircraft 28 vDC to 80 volt 400 Hz
rotary converter which would make it more authentic.
Would never use it on the air of course, but have demonstrated it many times at ham
conventions and NZ vintage wireless club exhibits using a bulb dummy load.
It went with a Magnetic detector and tuner that I built up as reproductions, but these are
already in our local Museum of Transport and Technology.
Looks very impressive going with a genuine "Ring of fire" as the gap spins to synchronise
with the 200 Hz supply.
You may find some video and pictures of it going in the NZART archives as it was videoed
at their ham convention a few years ago.
ZL1BDA
 
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KB1WSY
Member

Posts: 716




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« Reply #28 on: September 25, 2013, 03:13:17 AM »

Actually I have a reproduction, working rotary gap spark transmitter.

Sounds great!! Couldn't find the video/pix at the NZART website or the receiver at the MOTAT website ... but will keep trying. Would love to see what they look like.

73 de Martin, KB1WSY
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N1NQC
Member

Posts: 60




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« Reply #29 on: October 08, 2013, 11:30:43 AM »

Hey All,

 re  Spark transmitters,  I've built several over the years.  Lots of fun- but no big whoop to construct.

Coherers are ANOTHER story ! The theory of operation behind them seems SO clumsy that I can't wrap my mind around HOW they could really tell a dit from a dah. I also understand  that in general MECHANICAL coherers were relatively undependable and prone   to ("some" ?) error.

I know that the technology had start SOMEWHERE ( I think of a coherer as a  wireless TELEGRAPHY "sounder"). Even a LAND telegraph sounder seems a million times better to deal with .

Coherers   remind me of the "Mousetrap" game  a little bit.It ALL has to go right or no go at ALL !

The humble crystal detector, even with it's limitations in an UNamplified receiver  must have seemed a GODSEND to ops  compared to  having  to deal with a   mechanical coherer.

FWIW, crystal sets were used   with their "adjustment buzzers" as QRP transmitters to rag chew among tied up ships in port.Try THAT on a coherer.

K
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