Why The standard rule of thumb for a half-wave wire dipole is length in feet = 468 divided by frequency in MHz. At 3.550 MHz that gives 468/3.550 = 131.8 feet, about 132 feet total, or roughly 66 feet per leg. The 468 constant already accounts for the velocity factor and end effects of real wire, so it comes out about 5 percent shorter than a free-space half wavelength.
Watch out The 263-foot answer is what you get from 936/f, which is a full wavelength, not a half wave; 42 feet would suit a much higher frequency band.
468 over f(MHz) for a half-wave dipole; 234 over f for a quarter-wave vertical. 468/3.55 is about 132 ft.