Re: Old school radio jammer
Dave Marshall, Thu Aug 28 2008, 03:49PM

I'll allow this thread to continue, since you don't state a purpose, legal or otherwise, for this jammer, but I should point out that spark gap transmitters are illegal in almost all ITU countries regardless of intended purpose or frequencies affected.

In addition, in the United States willful or negligent interference with licensed RF users (say the Ham bands, the most likely folks to find you) is punishable by fines up to $10,000 per instance. Interfering with public safety communications can earn you a felony conviction and significant jail time (google for KG6IRO if you're so inclined).

A spark gap transmitter offers almost no ability to apply filters to prevent interference with frequencies even 10s of Megahertz above your intended target.

-Dave

Re: Old school radio jammer
Nallsak, Thu Aug 28 2008, 04:19PM

Thank you for your reply.

I know that since 1916 spark gap transmittors have been banned for radio transmissions, for normal use. But, I know that during WWI, WWII, and during the cold war spark gap transmittors were usually used for jamming radio signals. Even if I don't really build this device, it would be fun to discuss construction and it's characteristics.

I should also point out, that I have no intention what so ever of actually using this device as a jammer. This is for my own curiosity only. (actually by a side note, every time you draw a spark, or fire off a tesla coil, you actually commit the crimes you mention. ;) )

A spark gap transmitter offers almost no ability to apply filters to prevent interference with frequencies even 10s of Megahertz above your intended target.

David, sorry for being a bit naive or stupid, but I don't quite get your statement here. For actually jamming, isn't the main point to actually use the interferance in the device?

Re: Old school radio jammer
Avalanche, Thu Aug 28 2008, 04:25PM

I'm just wondering what's your main intention here - do you want to experiment with old school spark-gap radio, or do you just want to build something that will act as a good jammer? If you just want a jammer, there are much easier ways than with all the spark gap madness. Once you have the 555 circuit up and running you're already half way there... but it's generally a bad idea to fool about with jammers

edit: oops, my reply was too late.

Re: Old school radio jammer
Dave Marshall, Thu Aug 28 2008, 04:32PM

It depends.

In today's world, the idea is typically to disable modes of communication used by your opponent while leaving your own communication infrastructure intact. A spark gap transmitter is what's known as an 'Indiscriminate' or 'Passive' Jammer. Its only purpose is to raise the noise floor across a huge section of bandwidth such that no communications, enemy or otherwise, is possible. It's is a fairly inefficient means of jamming, as your power is spread out across a very broad piece of spectrum, and there are a number of noise-tolerant communications modes that can cope with such a jammer.

Modern jammers are 'Active' or 'Targeted Jammers'. These systems typically use a receiver to monitor the traffic on the communications systems they mean to interrupt, and then target those systems specifically. Radar jammers are such an example. They monitor incoming pulses from a radar system, and at the appropriate time transmit a number of pulses spread across multiple range gates and throughout the time domain, simulating many many targets, and making it difficult to determine which targets are real, and which are false.

Yes, a tesla coil or spark gap is technically illegal if not properly shielded and filtered to prevent interference, however, the short run times and limited duty cycle of these systems, along with the very inefficient means of radiation act to limit the harm done to surrounding spectrum users. A true sparkgap transmitter, complete with an antenna system, has no such limitations and can very effectively disrupt radio communications across 10s of Megahertz and for several miles around the transmitter.

-Dave

Re: Old school radio jammer
Nallsak, Thu Aug 28 2008, 04:51PM

I think that comes in terms of "old warfare" and "new warfare". If you look at WWI and WWII it may have been the desired to blank out all radio communication in an area, and use already planned strategy. But in modern terms you want to have to flexibility of changing your plan on the fly.

Any case though, how may such a device have been constructed? Myself being quite new when it comes to radio and antennas and stuff like that. Would one have used a monopolar antenna directly connected to the spark gap, or would one have used dipole antenna? And if using a dipole antenna, would it be connected directly over the spark gap or using a coupling coil to the main resonance coil?

Re: Old school radio jammer
Dave Marshall, Thu Aug 28 2008, 05:00PM

A Spark gap transmitter, contrary to popular belief, did not use the gap itself to generate RF. They work in much the same way that a spark gap TC works. A high-Q, tuned LC tank circuit (with a very high capacitance compared to inductance for maximum energy storage) was used to excite a coil that resonated at the desired frequency, which was then fed to an impedance matched antenna. You can find a number of drawings and descriptions of the systems Tesla, Marconi, and Hertz used, which should give you a good starting point.

The broad band nature of the output signal came from the powerful harmonics associated with the desired signal.

-Dave

Re: Old school radio jammer
Nallsak, Fri Aug 29 2008, 12:44PM

Mohahaha!!
Did my homework!
Actually, you're wrong there. The first type of spark gap transmitter did actually couple the antenna directly on the spark gap. This type of transmitter was called hard coupled. A later evolvement of the system was something that Tesla came up with, the loose coupled transmitter. Which basically added another tank circuit over the spark gap, and the coil from this tank circuit coupled to the antenna coil. (Go figures, the more I look upon spark gap transmitters, the more similiar they are to tesla coils.)
The loose coupling was a significant power boost to the spark gap transmitter. In both making it more power efficient and more narrow banded.

Re: Old school radio jammer
Chris Russell, Fri Aug 29 2008, 02:30PM

The main problem is that the radio spectrum is very very big. Jamming any significant portion of it is more or less impossible. This is why modern jammers use narrow bandwidth "noise" that is difficult to filter out.

Spark gap transmitters produce an ugly waveform, consisting of a broadband impulse, and then a narrower bandwidth signal as the circuit rings down. In a loosely coupled setup, the signal ends up much cleaner, since the Q of the tank is high. A hard coupled setup will be much noisier, since the Q of the antenna alone is in play. The circuit doesn't oscillate as much, so the gap must fire more often, and the overall result is a much more broadband signal.

Regardless of the type of coupling, it's not the spark that generates the main part of the signal, it's the ringing down of the circuit, be it the LC tank, or the equivalent LC provided by an antenna.

In either case, I find that a modern noise blanker does pretty darn well against the broadband impulse type of noise. The narrower, ringing down waveform would be much more difficult to filter out, but it's relatively narrow. Changing frequencies to avoid it would be simple, while physically tuning the jamming setup would be much more complicated.


I honestly don't know of any cases of a spark gap jammer being built to deny all radio communications. The key to a transmitter is to couple energy to an antenna that is resonant, or tuned to resonance at the frequency of interest. If you just wanted to get a lot of noise out there, the best thing you could do is string up a dipole, cut or tune it to the frequency of interest, and feed it with a spark gap transmitter. A rotary spark gap operating at high speed would generate a lot of ugly hash. Even so, at tens of kilowatts of power, you're only going to deny the use of a small part of the band at any significant distance. Such a station would be pretty easy to find, so I'd imagine in wartime they would have just had the crap bombed out of them very early on.

Re: Old school radio jammer
Proud Mary, Fri Aug 29 2008, 04:35PM

Jamming during and after the Second World War has always been application specific.

What SNR will render the enemy receiver or (more usually) telemetry or radar device inoperative, and what tx power and type of modulation is required for this to be achieved at a given range and bandwidth?

Highly local broadband jamming systems usually employ UWB technology featuring ultra-fast pulse rise times. Everett Farr and others have been at the forefront of attempts to use high speed Marx generators to create such disruptive signals for military and security applications in the United States.

Interference with any radio communication is a criminal offence in the United Kingdom.

Re: Old school radio jammer
Danielle, Fri Aug 29 2008, 08:24PM

I know this is a bit off topic but its within the peramiters of interference. when I was testing running my primary with no load and increasing the frequency around 800Khz I started to hear the inturupter signal through the intercom system at home.... well I experimented with this and I was able to play music from the interrupter of my DRSSTC through all the ppeaker systems of my house, is this ieligal? because I was thinking it would be an incredible way to cupple any speaker to your signal :)

Re: Old school radio jammer
Shaun, Sat Aug 30 2008, 12:05AM

Since most common SSTC frequencies are in or very close to the AM radio band, you can often witness this effect (interrupter "playing" through speakers) if you turn on any AM radio near an SSTC.

You don't even need to tune to an empty station-an SSTC, even a very small one, will be orders of magnitude more powerful than radio stations at that distance.

Re: Old school radio jammer
Nallsak, Sat Aug 30 2008, 09:02AM

Done some more homework.
I'm not too suprised that you can hear your SSTC on your intercom. Actually, in the early days tesla coils and variants of it was used as spark gap transmitters. (Basic throw away, any kind of RF filter on a modern coil, and you got a spark gap transmitter. Also on another side note, the seconder coil can be thought of as a helical antenna)
I didn't expect to find or build a spark gap transmitter, able to transmit, on a true UWB. But, the main reason spark gap transmitters where legally banned, was that they had a very wide transmitting signal, and so it was really difficult to have multiple transmitters working within the same band.

A small analog, take an ordinary FM transmitter you can find on the net. Make the oscillator into a VCO and then put a triangular or sawtooth control signal to it. Now you got a transmitter that sweeps it's transmitting band.
In a case of a spark gap transmitter, you will try to find the center frequency of the FM band and let the nature of the circuit drift the sending frequency. (All you HAM radio enthusiasts know that the LC based oscillators tend to drift, and it's very hard to actually have a specific frequency. That's why VCO and PLL techniques are used.)

Re: Old school radio jammer
Seoul_lasers, Sat Aug 30 2008, 09:13AM

This is quite the project. And there is an interesting use for it.
Cell phones are everywhere and they become rather annoying. You don't have to build a device for these phones from not working.
-- have you ever seen what happens to cells in elevators. They stop working. No signal. Simply all you have to do is build a faradays cage, earth the cage & bingo... no signal. :). It's pretty easy to do.
Ah sometimes, silence is wonderful.