HF ferrite core transformer, tan(delta) = 1, can I use it?

I consider making a 200 MHz transformer with a ferrite core. Tan(δ) = 1 at 200 MHz, but calculated power loss is very small. Should I believe that I can use this ferrite?

You won't hardly find a ferrite material with much better performance at 200 MHz. The parameter indicates, that you can't design ferrite inductors with considerable Q in the > 100 MHz range, neither a resonant transformer. But a wideband ferrite transformer can still work at several 100 Mhz, or even > 1 GHz. That's because the main inductance (which changes into an almost real parallel impedance at high frequencies) is sufficient high related to the transformer characteristic impedance.