Why Spiral Inductance Matters
A spiral wire inductor gives useful inductance in a flat space. It is common in radio experiments, sensor loops, wireless power trials, and compact prototypes. Its value depends strongly on diameter, turn count, fill factor, and nearby materials. Small geometry changes can shift the final result by a visible amount.
Understanding the Geometry
The outer diameter sets the largest magnetic loop. The inner diameter controls the open center. More turns usually increase inductance by the square of the turn count. A wider spiral can improve inductance, but it also changes fill factor. Tight spacing may increase coupling between turns. Wide spacing may reduce the final inductance.
Why Multiple Formulas Are Included
Spiral inductors are not perfect textbook solenoids. Their field spreads through a flat winding area. That makes empirical formulas very useful. The modified Wheeler equation is simple and stable for many layouts. The current sheet method gives another practical estimate. The classic Wheeler estimate is also shown for comparison. The preferred value averages the suitable results, then applies spacing and correction adjustments.
Resistance and Frequency Effects
Inductance is only part of the design. Wire length creates resistance. Higher frequency creates reactance. The skin effect can raise AC resistance. That lowers the estimated Q factor. A thicker conductor can reduce resistance, but it may require wider spacing. The best winding balances inductance, size, resistance, and frequency behavior.
Practical Design Advice
Use accurate centerline dimensions for better results. Keep metal shields away unless they are part of the design. Measure the finished coil when precision matters. Air-core spirals should use relative permeability near one. Ferrite or magnetic backing can change inductance significantly. Use the manual correction field when test data shows a repeatable difference. Export the result to compare prototypes and document design choices.