Bifilar Pancake coil for experimentation. Each winding has 40 turns of a linearly-increasing radius (Archimedean Spiral). Inner-Spiral Spacing is 12.5mils (25mils center-to-center of alternating winding and 50mils center-to-center of same winding) and the Spiral Trace Width is 12.5mils. The calculated inductance of each winding is 62.857uH and the calculated combined inductance in series is 251.428uH (due to the mutual inductance effect).

Update: After receiving and measuring the board, I was happy to see that the measured values were extremely close to the above calculated/predicted inductance values:

Measured Inductance Values:

• Inner Winding: 65.41uH

• Outer Winding: 65.16uH

• Windings in Series: 288.1uH

Measured Resistance Values (DC Impedance):

• Each winding: 10.5Ω

• Windings in series: 21.5Ω (also included wire connecting them in series)

Measured Winding to Winding capacitance: 383pF

Note: Measurements are subject to the error of my test gear, but seem somewhat reasonable and consistent with calculated/predicted values.