On Wed, Jul 5, 2023 at 09:26 PM, Bob Ecclestone VK2ZRE wrote:
But in your 2nd iteration, I notice the coil length is significantly longer
(156.8mm vs 80mm) with the same coil diameter (37.5mm) yet the inductance
remains the same at 97.8uH.
Surely the inductance should increase.
Bob, for a given number of turns, inductance decreases as the coil is lengthened due to lower coupling between turns. The optimizer maximizes Q while keeping inductance constant. It increased the number of turns just enough to compensate for the smaller coupling in the longer coil.
And finally, the Resistance (of the winding?) for the 2nd iteration is lower
than the 1st despite using a longer length of wire (6 vs 7.39).
Resistance is the RF resistance of the coil plus leads due to all causes. It is R in the R + jX model. Mike close-wound his coil with no space between turns. This minimizes coil length but maximizes the proximity effect. This magnetic effect bunches current inside a turn due to the proximity of adjacent turns. It increases RF resistance in the same way as the skin effect, by reducing available copper. The proximity effect is strong. I've never seen a close-wound coil where spacing the turns didn't increase Q even if it required more wire. The optimizer increases coil length to the point where any additional drop in the proximity effect is offset by the increase in wire resistance.
Brian
