high voltage dummy loads

You left out the dimensions.

Trade secrets.

He doesn't know how to do the calculations.

has got a chapter on Impedance Standards, starting at page 50 with the Thompson-Lampard calculable cross-capatitor and proceeding to slightly more practical secondary standards over the next 19 pages.

Better to go to the horse's mouth than the horse's ass.

Amazon seems to be claiming that the book is back in print, which is good news.

Maybe John could buy one. It doesn't say nice things about him - written too early (in 1984) - or about rich Republicans - not the target audience, so he may not be interested.

Sometime if I need a high voltage capacitor of somewhat larger capacitance I'm gonna try an "array" of pickle jars filled with aqueous ammonia and wrapped in aluminum foil, immersed in a mineral oil bath.

I used copperclad FR4 capacitors in an induction heater, but they got very hot and I had to change them to something less lossy.

A PTFE (teflon) substrate might be less lossy.

Rogers offer a wide choice of substrates.

LOL! Seriously, you should go into production with those, John. The market for multi KV 5pf dummy loads is seriously under-served. :-)

John Larkin wrote in news: snipped-for-privacy@4ax.com:

High voltage coax. Very specific per foot capacitance... at HV levels. Very easy to construct a precise HV capacitive load with it.

Depends on the risetime. Say 30pf is 1 ft. A 100ps risetime pulse would ring like crazy. During the initial portion, the coax would not look capacitive. It would look like the characteristic impedance of the coax. I think JL wants a lumped capacitance.

Steve Wilson wrote: ..

If you can supply kV with 00ps risetime you are a genius :-)

It certainly is less lossy.

There are ex-USSR capacitors on ebay claiming to have PTFE dielectric, which might be cheaper and thinner than the Rogers stuff, though some pictures of disassembled ones on the internet suggest that the capacitors as sold may not have extended foil nor connections with high enough current rating. Disassembling them for the PTFE film might be reasonable value though, if you don't have another source of it.

IIRC, 100ps is just possible in the lowish kV, with a liquid filled spark gap in a coaxial trigger setup.

I'm not sure what Steve is thinking about lines as capacitors, though -- as soon as you've got a time spec, if you want the line to look like a capacitor, the line length must be significantly shorter than that. At

Tim

Not my idea. snipped-for-privacy@decadence.org proposed:

"High voltage coax. Very specific per foot capacitance... at HV levels. Very easy to construct a precise HV capacitive load with it."

I was the one who pointed out the risetime problem.

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Nobody is insisting that you use just one chunk of coax. A nice radial arra ngement of shorter bits of coax would present a lower characteristic impeda nce, and shorter propagation times. Using different lengths for the various chunks of coax could smear out the voltage-doubling reflections from the u nterminated open ends.

Plenty of room for ingenuity, of the kind John Larkin seems to think he off ers.

It can be done with avalanche transistors or Grehkov's second (avalanche breakdown) diode effect.

Apparently microwave weapons use some sort of closing switch, maybe diamond films, to pump terawatts into antenna arrays.

My pulses are fast enough that I need a lumped capacitor as the dummy load. Coax looks like 50 ohms at first.

I could order a bunch of HV caps from Digikey, but the FR4 was just there.

Steve Wilson wrote in news:XnsA8B33082B814Eidtokenpost@69.16.179.22:

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I see what you are saying. The end of the segment gets insulated. That is what makes a small HV capacitor out of it. So you are saying the the shape of the "plates" makes a difference and that his FR4 spaced copper plates have no such characteristics?

It's the length of the coax, the prop delay, and the risetime of the signal.

Copper plates definitely have a resonance. They are used as microwave antennas for GPS and smartphones. But I think that's a bit above where JL is using them.

Ah, re-reading it now, I see what you meant. Cheers,

Tim

You just need more, shorter, pieces of coax. ;-)