Velocity factor can have a little to do with building an antenna, but not much really. The only time velocity factor comes into play is when there's a critical reason for timing or specific lengths for various reasons. With antennas their length mainly determines resonance, which can only be calculated by equations to a 'ball-park' estimate, never exact. From that point you 'trim' that antenna for best resonance then match it to the feed line. Resonance does NOT have to be anywhere close to any particular resistance value. Impedance matching (SWR) takes care of that part of it. They are two completely separate thingys.
For antennas made from wire or small tubing, the commonly found 'magic' numbers (234 and 468) will certainly take care of that velocity factor thingy. (Wire typically has a VF of about 95%. That's where those two 'magic' numbers come from they are respectively about 95% of those other numbers, 246 and 492.)
- 'Doc