On Wednesday, September 19, 2012 11:26:28 AM UTC-5, ai8o wrote:
I want to build a dipole optimized for use on 14070/7080 KHz, and 3581 if
possible. These frequencies are not exact harmonic multiples of each other.

Even multiples don't work for single-wire dipoles. Only odd multiples work. A resonant dipole cut for 3581 would also be resonant around 10.7 MHz, 17.9 MHz, and 25.1 MHz. It would be *anti-resonant* around 7.16 MHz, 14.32 MHz, 21.5 MHz, and 28.7 MHz. Anti-resonance is worst-case for coax fed antennas.

You could build a fan dipole with a separate dipole per band or you could build an 80m dipole with traps for 40m and 20m.

A standard G5RV works well on 80m, 40m, and 20m with a tuner. A ZS6BKW could be optimized for 40m/20m operation by playing with the dipole length and matching section length.