On Wed, Nov 27, 2024 at 11:05 PM, Tom Lee wrote:
Thanks very much, jmr, for going to the trouble of running the numbers through Genesys. You've provided a valuable cross-check of my cheese formulas, which give L = 64uH, Q = 190, and srf = 16MHz. The inductance comes out quite close (thanks to Wheeler). The Q formula is extremely crude (assumes resistance for close-wound coils is 4x the textbook skin resistance), but seems to be in the right general ballpark, if optimistic. The srf formula comes from simplifying a helical transmission line formula. From your curves, I infer that it's giving a reasonable approximation.
Thanks. It's a long time since I wrote that software program but (to compute Q) I think the program uses a map based on empirical measurements made decades ago by Butterworth and Medhurst some time around 1947? Both noted that the AC resistance changes from the skin effect model of a straight wire and they produced tables of how it changes based on the L/D ratio of the coil and also on the wire to gap ratio. So I plotted all this data in a huge 3D map in my software and the program code then interpolates between the data points as required. I can't remember whose lookup tables I eventually used, it might even have been a later revision of the data I found in a modern text book. I do the same for the impact of the self capacitance of the coil in that I mapped all the data for this as well.
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Before this I had to do it all in an excel spreadsheet and manually navigate the tables to find the various fudge factors for Q and capacitance etc.
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The Genesys air solenoid model seems to work quite well although for wideband stuff at work I prefer to use a VNA based model or a physical model I developed at work many years ago. The Genesys model (and my software program) falls apart near the first resonance mode so it is hopeless for wideband design work. My physical model is transmission line based although it is a bit more involved than just a transmission line. I did all this at work many years ago in order to be able to understand how to make better (ultra wideband) bias tees and how to make RF filters work over wider bandwidths (especially highpass filters). The aim was to be able to model the various resonance modes in a solenoid such that the model could be used in a simulator.
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