I second the solid core wire. Even better is magnet wire. Insulation is the key. Stored energy in the insulation (dielectric constant) is the issue.
From Electromec
"UNWANTED ENERGY STORAGE IN CABLES – DIELECTRIC CONSTANT"
KEY TAKEAWAYS
- A wire’s dielectric constant is related to the energy stored in the insulation.
- Lower insulation dielectric constants are preferred for high-speed data applications
- This parameter must be tightly controlled for impedance-controlled systems.