From: bill.sloman@ieee.org   
      
   On 7/11/2025 10:41 am, john larkin wrote:   
   > On Thu, 6 Nov 2025 23:48:45 +0100, Jeroen Belleman   
   >  wrote:   
   >   
   >> On 11/6/25 20:25, Christopher Howard wrote:   
   >>> In an coil with inductance and a distributed capacitance — neglecting   
   >>> resistance for the moment — do the inductance and distributed   
   >>> capacitance map over to the inductance and capacitance in a parallel LC   
   >>> circuit, or am I fundamentally misunderstanding something?   
   >>>   
   >>   
   >> Yes indeed. There is a frequency where the impedance reaches   
   >> a peak and then it drops. In fact, it may do that several   
   >> times over.   
   >>   
   >> The first resonance frequency of many inductors is depressingly   
   >> low.   
   >>   
   >> Jeroen Belleman   
   >   
   > A real inductor is a nightmare. Especially a long solenoid. Every turn   
   > inductively couples to every other turn with all possible coupling   
   > coefficients. Distributed capacitances will be similarly complex.   
      
   This is ignorant nonsense. The current through each turn does generate   
   field of flux, and that individual flux field threads all the adjacent   
   turns to an extend depends on the distance between the turns and the   
   extend to which the flux is concentrated by the high-permeability core   
   of the inductor.   
      
   You can almost always lump them together.   
      
   The capacitance between each turn works the same way.   
      
   > And then there's the speed of light.   
      
   When the propagation delays are long enough to be worth worrying about,   
   you model the inductor as a transmission line.   
     .   
   > In a bit of googling (I do have a day job) I haven't seen a good sim   
   > or measurement of a real inductor. Everybody keeps citing the simple   
   > LC formula as if it's true.   
      
   It mostly is. I once went to the trouble of modelling an inductor as two   
   closely coupled inductors in series, and got exactly the same result.   
      
   I'd been worried about parallel capacitance, but splitting it into two   
   capacitors of twice the size in series didn't make any perceptible   
   difference at all.   
      
   > We make some laser drivers that include a multi-section home-made bias   
   > tee, trying to synthesize a super-wideband high current inductor. Yuk.   
      
   John Larkin is very reluctant to design his own transformers and get   
   them wound, and when he posts a LTSpice simulation, his inductors rarely   
   have a parallel capacitance.   
      
   LTSpice has built-in models for a huge range of Wurth inductors and   
   those do have parallel capacitance.   
      
   John doesn't seem to willing to exploit this resource.   
      
   --   
   Bill Sloman   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)