Full size (2.4") mix 31 ferrite with white identifying color and surface sealer/coating that provides excellent long term weatherproofing.  

This is low permeability (<300) custom ferrite mix used by Balun Designs for most wire wound designs.  Best permeability for most wire wound baluns and ununs. Not recommended for EFHW. Can be used bare or inserted in Teflon tubing for very...