I did a fair bit of experimenting myself in the mid-late '70s - adding extra directors to an Antiference 4-element 'standard' product (not much improvement) - ending up with 2 x Fuba UKA8s stacked vertically with a tuneable masthead pre-amp, all rotatable by means of a 'Stolle' system. I acquired a copy of 'Antenna Engineering Handbook' by Henry Jasik which has a nice mix of practical and theoretical approaches - I've often wondered about having a play with computer-aided design but without access to some form of testing 'range' it's hard to verify the success (or otherwise) of one's efforts ...
I knocked up a wooden gizmo for making folded dipoles - pic to follow
Best wishes
Guy