Big Bottle Cap


This is my new, bigger bottle cap. I'm using 98 bottles in parallel for 81nF. The cap is divided into two boxes of 49 bottles in a 7 x 7 grid. The electrolyte is 12 lbs. of salt in 10 gallons of water, and I used all of it. The frames are 1" x 4" pine with 1/4" plywood bottoms. I used short chunks of 3/4" sch40 PVC pipe for feet. It works great, except that the increased capacity has made tuning on my 6-inch coil very critical. The sweet spot is only about 1/3 of a turn wide! When I was using only 42nF, I had a tuning range of plus or minus one whole turn. A picture is worth 1000 words, so here you go…

This bottle cap is bulky and heavy. The saltwater alone weighs 92 lbs. (80 lbs. of water + 12 lbs. of salt) Add the weight of bottles and boxes, and It's over 100 lbs., (over 50 lbs. per box). On the plus side, it is cheap and durable. I've never punctured a bottle, even at pole pig power levels.

This diagram spells out the design pretty clearly. One detail is missing. The top foil-covered cardboard square is supported at each corner. The four corner bottles are plugged with plastic Champagne stoppers drilled to accept a threaded rod. The threaded rod pokes through each corner of the upper plate, which is in turn secured with washers and nuts. Thus, the cardboard is elevated a short distance above the other bottles. This allows the bottles to exhale the products of electrolysis. However, the fit is still close enough to keep dust and grit out of the bottles. The electrolysis thing isn't really a problem. I used my old salt water cap for months without a measurable drop in water level.