Fair-Rite 2643625002 mix43 round core is larger than the T50-43 core used in many QRP EFHW transformers and tiny BALUN's. See photo of products.
 
When used in an EFHW transformer this core offers higher efficiency than the T50 core and therefore less of your transmission power is converted to heat in the core. We have added a 49:1 transformer efficiency measurement to this product using a Nano VNA.
 
Each core is only 10.3g making a great alternative to the smaller T50-43 core. Capable of 15w SSB 10w CW and 5w FT8 making it a good match for newer QRP transceivers.
 
Atonally due to the inside diameter of 7.9mm this core will slip over RG58 or Mini8 making it ideal as a sleeve common mode choke.
 
An ideal core for the home builder.