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Re: Coaxial Capacitor?
Tesla List wrote:
>
> Original Poster: Bob Wroblewski <bwroblewski-at-yahoo-dot-com>
>
> Hi All,
> Has anyone considered using sections of RG-8/U coaxial cable as a
> homebrew HV capacitor? The insulating dilectric is solid polyethylene.
> The inner conductor is solid copper as is the braid shield.
> The capacitance is 29pf per linear foot from an Aplha Wire catalog.
> Then 34 one foot long bundled sections equals a .001 uf cap.
> Start with a bundle of 15 inch segments. Strip the outer braid an
> inch or so from one end. Connect all center conductors together,
> likewise connect all braid sections, rolled back an inch or so from
> the opposite end, together. Place in a 3 or 4 inch OD tube, (maybe oil
> fill?) and cap the ends (no pun intended) and extend a stud terminal
> to the conductors.
> I'm sure if this worked well someone would have tried it by now and
> it would be spun off to others. So where's the flaw? Too costly per
> capacitance value? Area of conductors? Thickness of dilectric?
> Difficulty of assembly?
> Just thinking, I've never tried it. Comments??
Should work for a small cap. The series inductance would be about 70
nH/foot, but all your lengths would be in parallel so the actual
inductance would be around 2 nH. RG-213 is good to 5 kVRMS RF with a 3:1
mismatch (practically speaking, about 30-40 kV DC). Surplus coax is
about 10-20 cents per foot, and you'd need about 50 feet, or $5-10
Now, if you needed a .05 uF cap, it starts to get a bit tedious (you'd
need 2500 feet of coax).
Actually, you could use the coax in one long length and it would
probably work ok. If your TC is running at 300 kHz, the wavelength would
be 1000 meters, so your 17 meter length is a small fraction of a
wavelength, so you wouldn't have to worry about transmission line
effects (although, I'll bet your radiated EMI might have some distinct
peaks in it). The extra 3 uH inductance might have to be taken into
account.