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Mikek

I did say PILES, piles are produced when things ( wires) are layered on top of others, that is pile winding. Pies are baked pastry encased food. My callsign ( G8LCO) tells that I am in England so I speak English. The Piles in your picture are wave wound, that normally means cotton covered wire. I have used an automatic winding machine to wave wind inductors years back, an Uncle had a coil winding business when I was a kid. Cotton covered wire layers more readily than enamel, it is also a slightly thicker coating than enamel so has less self capacitance. Cotton covering is hard to find now, large wound parts are very much out of fashion and there are very few machines that can wind an insulator ( cotton or silk) onto thin wires. Such machines can be tempremental to run, with labour costs and demand being what it is covered wire is now hard to find. A few years back the last wire coating winder in the UK fell out of use because the lady that ran the machine retired. My Wife remembers that happening? because she used to sell double silk covered silver wire in her business.

Your line that self capacitance is always there is absolutely? correct BUT it needs to be factored into any LC calculation. That is why the multiple frequency method is used, the resonance curve is offset by the self capacitance, plotting the curve back to Cext=0 leaves the value of the self capacitance.

The self capacitance is distributed over the turns depending on the construction the distribution may vary. Pile winding allows self resonances to occur at much higher frequencies,? if an inductor is used as a Radio Frequency Choke ( RFC) then pile winding increases usable bandwidth. Dividing the inductance into several parts means that the L and C for each pile are also divided so the resonance frequencies are higher. Ideally each pile has a different number of turns so the resonances are staggered but that significantly adds to the cost of pile wave winding. The issue of RFC resonance has largely dissapeared with the replacement of? thermionic valves ( Tubes) with much lower impedance semiconductors but it was a significant problem in RF power amplifiers.