Hi Dave,
Interesting stuff, as always a learning experience and as you say very much a work in progress.
Steve, G3TXQ (SK) has some additional charts on his chokes webpage , which are a good starting point for any balun constructors.
These extra charts show the optimum frequency range (black line) for popular ferrite mixes, numbers of cores and numbers of turns.
No single design can provide ideal performance from 1.8 to 30MHz, but you can either choose a design that provides the most useful frequency range, or use multiple chokes to achieve the required characteristic. You can't stack a mixture of cores to achieve this, but you can cascade them.
DX Engineering's "Maxi-Core" method of using multiple ferrite tubes to form large binocular cores, are another method of construction that could be considered.
Regards,
Martin - G8JNJ
