The frequency response at low gain is very flat (±0.2 dB) from 200 kHz
to 35 MHz, and is only 0.4 dB down at 60 MHz. At standard high gain,
the response is very flat to 25 MHz and -3 dB at 50 MHz. The maximum
output level in all gain configurations is over 500 mV rms into a
75ohm load.

Does anyone have any idea what type of core that would work as well?
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Assuming the freq range intended is 1 - 30 MHz an Iron powder core made of
Mix No 2 material (permeability 10) ,colour code : RED , would be suitable.

Size T50 has outer diameter 0.5 inch and inner diameter 0.30 inch
hence T50-2
Size T68 has outer diameter 0.69 inch and inner diameter 0.37 inch
hence T68-2
Size T80 has outer diameter 0.795 inch and inner diameter 0.495 inch hence
T80-2

If the frequency range is to be from 0.02 - 30 MHz the above type of Iron
core can be combined with a same size core of Mix no 3 material ,colour
code : GREY eg the 2 types of ring glued together and wound together.

There is also a ferrite type core which covers the frequency range
0.02 -30MHz made from Mix No 61 material ( permeability 125) ;colour code
: GREY ; designations FT50-61 ; FT68-61 ; FT80-61.

Frank GM0CSZ / KN6WH