I don't seem to be able to find the data to enter a 4:1 current balun in the 4nec2 software. Has anyone found or written one?
I am building a 75 Ω fed OCFD and need to model the radiated patterns, but I have no data for 4nec to add a balun.
I don't know 4nec2, just regular NEC. A balun isn't a NEC primitive, it would have to be modelled.
For the 4:1 part of it, this is just maths done outside of the simulation itself. You simply double the applied voltage (if that matters for actual total power), and divide the impedance by 4 afterwards. Impedance makes no difference to patterns, so you could ignore that part if you're just after patterns.
For the current balun, you need to know the choke impedance of the balun. Connect a feed cable to the side of the antenna which will take the braid of the coax. Load the last segment of the coax (braid) with a load equal to the balun common mode impedance. If you don't have a figure, try about 1000 Ohms resistance.
As always with NEC, rather use long thin segments that obey the rules, than try to exactly model the shape of things. For example at 14 MHz your coax wire will have segments about 1 m long. Don't stress that the balun is really only 25 cm long, just load the last 1 m segment. Making tiny, accurate segments will just make it worse.