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5C) Secondary Coils sForms used for secondary coils are most often PVC, as it is cheap, works well and is easily available. Cardboard and fiberglass are sometimes used as well. ABS (black pipe) is avoided as it contains carbon, a good conductor at HVs. Clean the form well, and remove all lettering, as it may be conductive at HVs. Magnet wire is the most common wrapping wire, although Litz wire, silk covered wire and so on can be used. I prefer double polymide coated wire. Most secondaries are "close wound" meaning as little space between the wraps as possible. Some prefer "space winding" where each turn is spaced slightly away from the last one. This usually requires scribing a spiral onto the form to facilitate even winding. Don't bother with space winding is my take. Most secondary coils have between 600 and 1600 wraps, with 1200 seeming to be the most common. There is an old rule that says the aspect ratio of a secondary should be 4:1. However, I see that rule liberally broken on lots of coils. In fact, a higher ratio may be helpful, as a taller coil will be less likely to strike itself or the primary circuit with a streamer. My latest secondary has 48" of wraps (almost 1300 turns) on a 6.5" form. This far exceeds the 4:1 ratio. I haven't tested it yet, but the consensus is that it will work fine. Smaller coils can be wound pretty easily by hand. I have wound several coils by hand, and it isn't too bad on small coils, say less than 18" of wraps on whatever sized form. If one is going to make anything larger, I recommend taking the time to make a winding jig. Not only will the coil turn out much neater, but coating the coil while revolving makes for a drip and run free coil. I HATE runs and drips! For more information on winding and coating a secondary, click HERE.
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