Every real inductor has a tiny bit of capacitance (between its windings). Every real capacitor has a tiny bit of inductance (in its leads and plates). At some frequency, these parasitic properties resonate with the intended property — and the component stops behaving like what it's supposed to be.
An inductor below its self-resonant frequency (SRF) acts like an inductor: impedance rises with frequency. Above its SRF, it acts like a capacitor: impedance falls with frequency. At the SRF itself, impedance peaks.
This is important because many Open Energy experiments operate near or at the SRF of their components — that's where the interesting resonant behaviors emerge.
How to measure it
Use a NanoVNA or any vector network analyzer. Sweep a frequency range and look for the impedance peak (or the S11 return loss dip). The frequency at that peak is the SRF.
The Bifilar Coil Resonance Characterization experiment is specifically designed to measure SRF on two different winding geometries and compare them.
Why winding geometry changes it
A bifilar coil has much higher inter-winding capacitance than a conventional coil because the two wires run parallel and close together for the entire winding length. More capacitance means a lower SRF. This shifts where the coil resonates — and that shift is one of the key measurements in the experimental program.