Highland Ham wrote:

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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

I disagree with this advice. It would be appropriate for a narrowband,
tuned transformer, but that's not what the schematic indicates. It's a
broadband transformer which has different requirements for a core. What
you need is high winding impedance, not the high Q and relatively low
impedance provided by the cores Frank is recommending. An appropriate
core is a "low frequency" ferrite such as Fair-Rite (and Amidon) type
70-series (72, 73, 77, etc), or type 43 which is very widely used for
EMI filtering applications. You can easily identify "low frequency"
ferrites because they're the only ones which give you any continuity
(although the R might be high) when probed at two points with an
ohmmeter. Type 43 can't be identified this way -- they'll show an open
circuit. As for core size, the number of turns specified on the diagram
will provide enough impedance with any core of size half inch diameter
or so, or larger, and with normal geometry. You could probably get by
with a core smaller than that if necessary.

High frequeny ferrites (which aren't suitable) like 61 are relatively
rare, so if you have any ferrite cores in your junk box, they're likely
to be ok. Powdered iron cores won't give you enough winding impedance.

Roy Lewallen, W7EL