I got a lot of ZCAT3035-1330 snap-on cores by TDK. What material is it, how good/bad are they for choke baluns and RF power transformers? I have already the TDK "Clamp Filters (Ferrite Core with Case)" catalog from April 2013 but there is no cross-reference to Amidon. Any info appreciated! Many thanks, 73 Martin, OK1RR