These are the cores typically used in 49:1 broad band transformers for 80m-10m End Feed Half Wave Antennas, in conjunction with a 100pf capacitor. These can be used to make a tiny portable EFHW transformer for SOTA operations.
 
Use 1 core for up to 5w output power SSB and 2w CW/Digital modes.
 
Dimensions
12.70mm Wide
04.90mm Thick
07.15mm Hole
 
These smaller cores are quite tricky to use. But they do offer a really good way to build a stealthy small antenna.