Why A quarter wavelength in feet is approximated by 234 divided by the frequency in MHz (this is the 468/f half-wave formula cut in half, already including the velocity factor of wire). For 28.5 MHz, 234/28.5 is about 8.2 feet, so roughly 8 feet of radiator above the ground plane.
Watch out The choice near 16 feet is the half-wave dipole length (468/28.5 = 16.4 feet), which is twice what a quarter-wave monopole needs.
234/f for a quarter wave, 468/f for a half wave. 234/28.5 = 8 ft.