Help understanding something about Spark Gap Transmitters

[honeyfairy]

Hi everyone, I've just recently become interested in learning about Spark Gap Transmitters. I have read many guides on building a small one, I even tried building my own with limited success. I'm hoping the kind ladies and gentlemen here will answer some of my questions and help me along the way.

I have watched a documentary on Marconi and his spark gap transmitter that made a transmission half way around the world, and I saw that the antennas were HUGE!

First Question, what frequency was he transmitting on? And how would he adjust his frequency and how would it benefit if he transmitted on a higher or lower frequency?

Secondly, I have built a small spark gap using an Auto ignition coil and a car battery which I managed to find laying around, and i was able to get it to spark whenever i made contact to the battery. And since i know a bit of Arduino, i hooked up a relay between the negative of the battery and the coil and coded the arduino to pulse the relay, doing this, i was able to get a steady spark, but after a few minutes the relay burnt out
Anyway, I then read that i would have to build a 555 Timer circuit to pulse the DC to the induction coil. But i havent done so yet as i have to go to the city to get the parts, ( i live a bit outside the city, on my dads farm place )
I would like to know, what frequency would i be transmitting at with that 555 timer and what is the highest frequency i can transmit at.
I read about the legal issues with this but i am on the outskirts of the city so i dont think i'll cause much harm by playing around with it for a couple of minutes, anyway these sparks that i managed to produce didn't interfere with my FM radio, or my TV, or my satellite.


Sorry for the long post, but i'm really interested in this currently and i'd like to learn more. I have many more questions, like how to build the transformer to transform the Spark gap energy into Radio Waves, and the thing that i really cannot understand is the Radio Frequency that the spark gap produces, and HOW? Some say its just decided by the length of the antenna, so for example for a frequency of 10ghz the length would probably be around 50mm... i dont understand.

Thanks for reading my post and i hope you can help me understand.
I know some of my questions...or all of it lol may be dumb....sorrryyy.. :blowkiss:

Jenny B

 

[n0nhp]

Spark gap transmitters don't transmit on a frequency. Like a lightning bolt, they transmit on all frequencies simultaneously. I don't know what the radio laws are in your country but in the USA, if your transmitter is powerful enough to be heard more than a couple of meters away, you would be looking at a very large fine.
The tuning (such as it is) on a spark gap is done with filter coils and antenna length. Detecting the spark is usually easiest at very low frequencies and with an AM demodulator. See articles on lightning detectors for hints and tips.
As for building the transmitter it's self, there are many web pages on building a "Jacobs Ladder" that will give information on creating the spark and maintaining it.

If you have any amateur radio operators or Short Wave Listener neighbors, they will not be happy with your experimentation as your "noise" will, even with whatever tuning you employ, be spread across most of the HF radio bands and interfere with the signals they are trying to listen to.

Bruce

 

[KC4RAF]

+1 what Bruce posted.

You also have to have an oscillator circuit to produce a frequency, which isn't all that complicated.
Were you controlling the spark of the coil through the secondary, or were you controlling the primary voltage going to the coil? If you were controlling the secondary, that will burn up your relay. What is the voltage of your secondary of the coil?

 

[wyShack]

A spark gap 'transmitter' is indeed a low frequency device by today's standards. Back when they were used, the thought was the longer the wavelength, the better the range. After experimenters started building transmitters, they were regulated -the beginning of the Amateur (ro Ham) radio service. Hams were 'restricted' to 200 meters or below (in frequency terms 1.5 MHz and up) as those 'wavelengths were considered useless. I remember reading '200 meters and down' covering the history as higher and higher frequencies were shown to be usable (and equipment developed) by the amateur operators, then given to other services. this is why 'shortwave' refers to the 200 meter to about 10 meter (or 1.5 to 30 MHz)

A spark gap generator generates a signal at its switching speed and nearly any multiple (or harmonic) of that frequency. As already mentioned the antenna (and sometimes other circuits) was the 'tuner'-making for a very broad banded signal.

As a word of caution, some frequencies allow for low power signals to travel great distances -I have 'worked' stations 10,000 miles away with 2-5 watts. Your 'transmitter' may be affecting receivers for a considerable distance and is likely not allowed -on the other hand many types of industrial equipment that switch large currents (like variable speed drives) is doing the same thing -which is why there is so much 'noise' in receivers today.

 

[RFI-EMI-GUY]

Today's equivalent technology is UWB or Ultra Wide Band.

 

[SCPD]

Spark Gap Transmitters

Ah, an interesting subject, and one that brings back a memory.....

Years ago a friend and I were curious about just how far we could talk on spark. We were living and working out in the middle of No-Where's and neither of us was the least bit worried about Spark Gap having been outlawed since the 1920's.

We took a common doorbell buzzer and hooked it up in series with a large coil (the primary) and wound over that a much bigger coil (the secondary.) The buzzer's 16 VAC gave a rich RF hash that when stepped up by that transformer, produced a nice hot 1 cm spark across an air gap. One end of the secondary was connected to a well pipe, the other was feed thru an LC circuit consisting of an air variable capacitor and a coil-- then thru a large value, high voltage capacitor to an ~100 foot semi vertical wire. The whole Gizmo resonated around 1000Khz. Tune up was simple. Using an AM broadcast band receiver, the LC network was tuned for maximum noise.

We then tried the receiver at 1/2 a mile and were rewarded with a RST signal of "570" (note the nice "0" for tonal quality ! Another transmitter was hastily constructed and by the end of the week, after a few modifications and improvements (secret ! We were surprised that while the signal was a "Tone 0"- it was surprisingly narrowed in bandwidth by the LC network. My friend Ana and I were now talking 3 miles on spark-morse. How much power were we achieving?.. A good question-- key-down thru a #47 pilot bulb in series with the antenna produced a faint glow- and in those days, we'd say "if you could light a #47 pilot bulb you could work the world!"

Having proved to ourselves we could do this, the system was disassembled and and we gracefully retired from Spark.

Now I do not recommend anyone try and repeat this. Spark Gap, remember, is illegal! But !..Oh , it does work quite well !......

......................................... CF