Copper Clad/Covered Steel wire (CCS wire), RF application

[WimRFP]

For mechanical stability I would like to use Copper Clad Steel wire instead of solid copper conductor. I already tried 0.6 mm copper coated MIG welding wire. However the copper coating thickness seems well below 5 um as at 150 MHz a got extremely high resistive loss compared to solid copper.

Where to get CCS wire with diameter around 0.5...0.8 mm and at least 10 um copper thickness in small quantity (say several 100 m)? My location is Europe, Netherlands.