Really? When I built my OWA's (6 el 28 Mhz, also a 6 el 21 MHz) I used
EZNEC (NEC2). The SWR curve of the final antenna matched pretty well with
the modeling and I didn't need to adjust anything on the final antenna.

Hi Tor,
problems begin when the first director is too close to the dipole...
How is the distance between DE and D1 in your antennas and, could you,
please, send me your .ez files, just to see your antennas.
Thanks and 73
Guido, ik2bcp