# Re: Re: [TCML] inductance Vs diameter Vs height

```bartb wrote:
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Guys, I have a little spreadsheet for that graph I showed. It was based on h/d and Fr. My thoughts at the time is that this maxL was always based on a low frequency condition. Thus, I thought, what about when the coil is operating at Fr? Does that max inductance ratio still apply? The answer is NO. Of course not. Les is the effective series inductance at the resonant frequency of the coil. And of course,, depending on Les for your specific coil, the maximum inductance attainable is different than the standard ratio so often referred to.
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So often, coilers forget that our coils have an operating frequency and the inductance at that AC is not 50 or 60 Hz. Inductances does change with frequency and thus, so does maximum inductance for a geometry.
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It's not very correct to say that "inductance changes with frequency". What happens is that a coil is not just an inductor. It is a combination of distributed inductance and capacitance, not counting resistance. The impedance seen across the coil is inductive at low frequency, tending to the DC value of the inductance, but at higher frequencies the distributed capacitances contribute more and more to the impedance. If you consider just the value of the reactance and attributes it to a single inductor, the equivalent inductor really appears to change with the frequency. In the simplest model, a grounded vertical coil seen from the top has a capacitor in parallel with it. As the frequency rises, the impedance grows to a maximum (infinite in the lossless case) at the resonance frequency of the combination. At frequencies below the resonance the reactance is positive, and equivalent to the reactance of an inductor larger than the one seen at DC.
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Antonio Carlos M. de Queiroz
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