Even with short fat coils, you can interpret the resonance condition to be the “slow wave” double transit time for the coil of wire. Which effect is most dominant depends if the wire is wound in a long skinny coil or a short fat one. The second effect is a resonance between the lumped inductance and a lumped equivalent capacitance across the coil. The first is that there is a natural resonance associated with the time it takes a speed-of-light wave to travel down the entire length of coil wire and bounce back. However, there are two properties of coils of wire that start to make for difficulties as the frequency goes up. In the low frequency limit, the traditional inductance calculations work okay and can give you a pretty good estimate of a coil’s lumped inductance. I use coils of wire or coils of coax cable as trap inductors for various antenna designs that I have built. A lowly coil of wire can be surprisingly difficult to understand and model correctly.
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