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Registered Member #4325
Joined: Mon Jan 09 2012, 04:54AM
Location:
Posts: 8
I'm definitely no expert in antenna theory but I realize that for a highly directional pulse there are designs with either a TEM horn or a TEM horn-fed pulse to a parabolic antenna. But what about having two electrodes placed at either end of a parabolic reflector's focus in terms of gain? I put a crudish image below to show that. The issue is gain and not having deconstructive interference in the far field (from what I learned). What'd this kind of simplistic design offer? The assumption is that the reflector is of high quality with no imperfections to affect positive gain.
A couple things about the design: aiming for 200kv marx output to a 24 inch reflector. When researching antennas I don't see this kind of simplisitic design which leads me to believe it offers nothing or is terribly inefficient somehow. The closest thing I saw was a dipole antenna centered at the focus, but not sure how a spark gap would be coupled with that. Suggestions appreciated in general!
Registered Member #4325
Joined: Mon Jan 09 2012, 04:54AM
Location:
Posts: 8
I have a set of cheap calculators and want to experiment building various shielding enclosures to test with. What you mention about not knowing about what frequencies it'd broadcast on raises a good point too. It may cause excessive interference like a spark gap transmitter, although its just intended to be one pulse and not a continuous transmission. Not sure, perhaps not going through building it is better idea... but plenty of people here have build marx generators, so is the issue not the generator but rather the antenna being a problem? The point of the antenna is partly for gain but mostly so it doesn't affect surrounding electronics by focusing the energy away from it. Although that in itself I have no idea if my idea would achieve that properly.
Is there a design I should consider like much lower voltage marx to still be able to test with but not cause problems? Sounds like naive question, actually.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Marx generators are pretty good at radiating and destroying electronics even without an antenna. The generator tower itself, and the wiring carrying the discharge current back to the tower base, acts as quite a good antenna.
To make a directional EMP source suitable for experiments without being too dangerous, I'd use a UHF TV antenna, a length of good quality coax (pulse forming line?) a car ignition coil and a spark gap. I leave it as an exercise to the reader as to how to wire it up.
Registered Member #4325
Joined: Mon Jan 09 2012, 04:54AM
Location:
Posts: 8
Yeah, the wires themselves will radiate EMI too which will be undesirable for me if I can't have a relatively controlled environment. Perhaps the best solution is to build a gigantic faraday cage. $$$ :(
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
The gain of a parabolic antenna rises with the square of the ratio of aperture width to wavelength, so parabolics are not suitable for true UWB use, though very large aperture astronomical parabolics could have a good go at it.
The biggest unresolved problem with high energy UWB antennas fed by Marx impulse generators is transmission line flashover, which has resulted in some hopelessly inefficient antenna designs, such as bow ties with terminating resistors, where a compromise match is attempted at the price of energy burnt off in the resistors.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, you quickly reach a contradiction where the antenna needs to have parts close together to work well at the high frequency end, but the same parts have to be far apart to stand off the high voltages.
Another way of saying the same thing: if you design a Marx for very high output voltage, it becomes a tall tower so the top won't just arc to the bottom. It then has high inductance as a result of its shape, so the output pulse has a slow rise time and doesn't contain much high frequency energy.
Around the same time as Nikola Tesla, the Indian physicist Bose (not the speaker guy!) was experimenting with millimetre wave generators based on small spark gaps. A google search should turn up some of his designs.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Steve Conner wrote ...
Around the same time as Nikola Tesla, the Indian physicist Bose (not the speaker guy!) was experimenting with millimetre wave generators based on small spark gaps.
I recall some very interesting experiments where metal particles suspended in a dielectric fluid were excited with an HV discharge to produce millimetric waves. The particles acted as lumped-constant tuned radiators.
Registered Member #4325
Joined: Mon Jan 09 2012, 04:54AM
Location:
Posts: 8
I'm fairly sure that I'd be faced with slow rise times with my design unless I can improve the switching somehow. Only easy thing I can do is keep the gaps aligned so that the first gap can help irradiate the other gaps. But that only goes so far.
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Simple parabolic antenna, Marx Generator driven (EMP)
