What's a good ferrite material for broadband transformers in the UHF band?
I believe the baluns used in UHF TV applications were actually transmission line transformers. Twisted pair transmission lines, specifically, but you can use coax as well. Bandwidth is high, but available ratios are limited. 

For example, using Ferroxcube 4S2 material, it was possible to make a 1MHz to 5GHz 1:4 transformer (Image from Horn, J., &amp; Boeck, G. (n.d.). Ultra broadband ferrite transmission line transformer. IEEE MTT-S International Microwave Symposium Digest, 2003.).