I made one (not following instructions well), that is made of two 16 inch tall cones, that are 16 inches wide, separated by like 2mm with a BNC connector soldered into the bottom of one of the cones.
The other one is different, each cone is 3 inches wide and 5 inches tall. It is separated by about 1mm and has a piece of 0.86mm hardline soldered into it. Does anyone know enough about these things to estimate their input impedance?
The formula scared me.