Ferrites at coaxial wideband transformers - help

Why we need ferrites at coaxial wideband transformers?
For example here:
Broadband VHF/UHF Amplifier Design Using Coaxial Transformers:
http://www.polyfet.com/HFE0503_Leong.pdf

There are two reasons using ferrite cores in transformers.
First, is to increase the inductance for the same number of turns, and second, to increase the coupling between primary and secondary of the transformer.
Unfortunately ferrite cores are not very wideband, as for example air core coupling.