Simple(maybe not) question about common spark gaps / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Simple(maybe not) question about common spark gaps

Move Thread

LAN_403

leviterande

Wed Dec 11 2013, 09:37PM

Registered Member #14432 Joined: Sat Apr 20 2013, 01:18PM
Location:
Posts: 18

Hi all!

If you connect any HV source , be it an ancient induction coil or a flyback or a NST to a simple spark gap w/o any physical capacitors or inductors to make an LC, the spark gap itself is still a resonant LC, right? since the tiny spheres of the spark gap have tiny capacitance and inductance ( but lets just for clarity's sake, connect a very short wire piece to each sphere for the sake of an inductor just like our old friend Hertz).

This means that this tiny " spark gap -LC circuit " at the end of the cables of a NST for example or a flyback etc, oscillates at very very very high frequencies indeed, perhaps in the high GHZ or in 100s MHZ at least ? but is this really the case? I am confused and hope that my point is somewhat clear.
I feel that I am completely forgetting something really small but vital here in my stupendous assumption.

Regards

Proud Mary

Wed Dec 11 2013, 10:49PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Don't make the mistake of thinking that the wires to the spark gap and the induction coil itself are not all part of the resonant circuit!

However, in very early experiments with sub-millimetre waves, metal particles were suspended in oil, and then excited by an electrical discharge, which made the micro particle resonate as quater-wave antennae.

X-rays are produced by ordinary sparks, but of such low energy that they are almost at once attenuated by the air, and are difficult to detect outside the well-eqipped laboratory.

Also, think about a lightning discharge, which emits electromagentic waves from a few Hz to hundreds of MHz.

leviterande

Thu Dec 12 2013, 08:03AM

Registered Member #14432 Joined: Sat Apr 20 2013, 01:18PM
Location:
Posts: 18

Proud Mary wrote ...

Don't make the mistake of thinking that the wires to the spark gap and the induction coil itself are not all part of the resonant circuit!

......
Also, think about a lightning discharge, which emits electromagentic waves from a few Hz to hundreds of MHz.

Yes indeed, but which frequency is the entire circuit going to oscillate at ?, or do you mean that in other words that there will be two oscillating frequencies depending on which part of the circuit we measure? i.e one oscillating frequency is going to be on the HV generating side and the other is on the other spark gap end, but they are all connected, yet Hertz for instance generated two frequencies, one low from the LC induction coil side and the other is from the dipole..?

Lets take for example the famous Hertz experiment where he generated 500mhz . The HV induction coil itself was LC and had much lower resonant frequncy of course, the seondary of this induction coil is conected as you kow to a tiny LC or a dipole, or a short strait piece of wire , a sparkgap and a pair of plates that resonate at 500mhz. So how does all this interact, why when how does the circuit decide to resonate at which frequency? Shouldnt this also mean that , had diploe wire were much shorter, the resonant frequency would shoot up to much higher?

If we follow the logic that the induction coil low frequency LC is the one at work, than we would never have another oscillation on that other connected tiny LC.. Hope my point is somewhat clear.

Thanks

Proud Mary

Thu Dec 12 2013, 09:41AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

If you use shock exictation to excite a resonant antenna, then the antenna acts as a filter and attenuates frequencies outside of its passband. I think it fair to say it favours the radiation of frequencies and harmonics of frequencies in the passband, because in real life there will be radiation on a very broad range of frequencies from a spark gap transmitter, which is why spark gap transmitters were banned as soon as the advance of thermionic valve technology made it possible.

leviterande

Thu Dec 12 2013, 01:18PM

Registered Member #14432 Joined: Sat Apr 20 2013, 01:18PM
Location:
Posts: 18

Hmm, but isnt the primary and secondary of the HV LC source working also as an antenna, but at much different bands than the "intentional ghz antenna" that we added via sparkgap?.

So we here basically got two antennas, 2 LCÂ´s...with 2 different bands, one is the indirect prim/sec LC feeding source supply, and the other antenna is the actual tiny LC antenna that we added at the end of the secondary.. so which antenna has its passband favoured if I put it this way? :)
Thanks

Proud Mary

Thu Dec 12 2013, 02:54PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

leviterande wrote ...

Hmm, but isnt the primary and secondary of the HV LC source working also as an antenna, but at much different bands than the "intentional ghz antenna" that we added via sparkgap?.

So we here basically got two antennas, 2 LCÂ´s...with 2 different bands, one is the indirect prim/sec LC feeding source supply, and the other antenna is the actual tiny LC antenna that we added at the end of the secondary.. so which antenna has its passband favoured if I put it this way? :)
Thanks

The whole thing will radiate EM waves, with varying degrees of antenna efficiency from extremely low to moderately high in different parts of your apparatus. Even a 50 Hz mains cable radiates energy. With the right sort of equipment, you can detect radio waves down to a few Hz coming from thousands of miles away.

leviterande

Thu Dec 12 2013, 08:08PM

Registered Member #14432 Joined: Sat Apr 20 2013, 01:18PM
Location:
Posts: 18

Not sure if my point was clear, I meant, which of the two mentioned LCÂ´s of the same circuit LC would radiate its band of frequencies, or is it just so that both the LCs radiate their bands of frequencies?

so the Hertz induction coil may radiate around the 700khz band while the dipole radiate around 500mhz , simulationsly ?

Proud Mary

Thu Dec 12 2013, 11:02PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

leviterande wrote ...

Not sure if my point was clear, I meant, which of the two mentioned LCÂ´s of the same circuit LC would radiate its band of frequencies, or is it just so that both the LCs radiate their bands of frequencies?

so the Hertz induction coil may radiate around the 700khz band while the dipole radiate around 500mhz , simulationsly ?

Yes, they might very well radiate at these two very different frequencies, and others that you haven't even thought of.

Many people who build TCs on this forum - especially VTTCs - would be astonished if they could see on a spectrum analyser the quantity and energy of the spurious emissions and harmonics their device was radiating, when they thought it was just operating at 300 kHz, or whatever.

A single tuned circuit - such as you might find in the most basic spark transmitter - is not nearly enough to ensure that undesired out-of-band frequencies are not produced and radiated. A single device such as a vacuum tube is perfectly capable of oscillating on a number of different frequencies simultaneously - indeed many will attempt to do just that unless a very definite strategy is put in place to forbid them.

Java calculators and simulators may give the impression that a circuit will oscillate on one frequency alone, but the behaviour of real world RF circuits is often very different, and the best configuration and layout often has to be determined empirically, by trial and error, to get the desired result. This is what engineering is all about.

leviterande

Fri Dec 13 2013, 07:55AM

Registered Member #14432 Joined: Sat Apr 20 2013, 01:18PM
Location:
Posts: 18

Thanks! I guess see. I know old electronics are dirty but to this degree... wow, I guess thus that the real reason old tx rx worked wasnt because of the so called single tuned frequency of the final stage of the circuit. The rx tx resonated to all the shared similar but near endless frequncies and harmonics produced by the spark gap & feeding LC & Antenna LC of each recieveng or transmitting circuit.

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob