THE HAWG CAPACITOR

Hawg products are homegrown Hi Tek Gadgets made from common materials. The first Hawg project, popularized by the illustrious Kip, is an 11 dB gain 1250 MHz antenna made from hardware cloth, or here in Georgia, "Hawg Fence Wire" In keeping with Hawg tradition, here is an investigation of high voltage capacitor banks suitable for use in small Tesla Coils. The high voltage dielectric of Hawg Capacitors are plastic soda pop bottles.

A capacitor is two conductors separated by an insulator (dielectric). In the Hawg capacitor, a plastic bottle separates two conductive salt solutions. Salt water capacitors are common among novice Tesla Coilers, although most experienced hands graduate to surplus capacitors or more sophisticated homegrown capacitors. The main problem with plastic soda bottles is they cannot withstand the electrical stress, and the dielectric will soon puncture, short-circuit, leak, etc.

The Dielectric :

The operating life of a capacitor is roughly inversely proportional to the 5th power of the applied voltage, and is halved for every 10 C temperature rise. Dielectric in thin layers will stand more voltage per mil than in thicker layers. Plastic soda pop bottles are usually made of Polyethylene Terpthalate ( PET or Mylar ) with a wall thickness of 0.010 +- 0.0005 inches.

"Reference Data for Radio Engineers, 4th Edition" (1957) list of Material Properties :

Polyethylene DE-3401 with 0.1% antioxidant additive :

Freq	K	TAN K
60 Hz	2.26	<.0002
1 kHz	2.26	<.0002
1 MHz	2.26	<.0002
100 MHz	2.26	<.0002
3 GHz	2.26	0.00031
25 GHz	2.26	0.0006

Dielectric Strength (0.033") is 1200 V/mil at 25C.
Softening point is 95 - 105 C
Moisture absorption 0.03%

Factors that promote failure of the dielectric are : concentration of E-field at sharp corners, corona discharge (bewaare of corona in trapped bubbles) and heating. PET bottles are blown into molds which 'orients' the molecules and improves the dielectric strength. Heating will cause the plastic to shrink and molecularly disorient, leading to dielectric rupture.

The Conductors :

A common technique using bottles is to wrap the outside with foil and put a condusctive liquid inside. With a foil wrapping, my 2 liter bottle measured 2.4 nF, I fully expected to get one or two nF more with electrolyte replacing the outer foil, but to my surprise the capacitance increased to 8.5 nF !! By putting liquid inside and out, a smooth, close contact is made, increasing capacitance, reducing field concentrations and corona. Corona is BAD NEWS for capacitors.

I dont know a lot about electrolyte conductivity, but ion concentration, ion mobility and valence will be important. However I suspect in this case the major portion of resistance is contact resistance. I normally use table salt (sodium chloride) and/or Epsom salt (magnesium sulfate). Jim Lux suggests copper sulfate for compatibility with copper electrodes. Watch out for generation of dangerous chemicals, for instance sodium chloride may decompose into chlorine. Floating a layer of oil over the top of the liquid, both inside and out, reduces corona discharge at the edges.

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