Here's what I said in posting #16 of this thread. "It appears that there exist two different kinds of ferrite materials with the designation 'K' so caution is advised." I got my FT-240-K from Amidon. Testing with an AIM4170 indicates that it is slightly superior to #52 material for HF applications. In posting #18 of this thread, G3TXQ posted a graph comparing Amidon's K-material to 52-material. Here it is again: One would think that buying the FT-240-K toroid or the AB-240-250 balun kit from Amidon would ensure the u=250 K material. Although I cannot find any Fair Rite K-material technical graphs on the web, I believe (not certain) that Amidon's K material toroids were manufactured by Fair Rite.