Get starting-point element lengths and spacing for a Yagi beam: reflector, driven element and directors, plus boom length, for any frequency.
Starting-point Yagi dimensions: driven element (feet) = 468 / MHz, reflector about 5 percent longer (492 / MHz), directors about 4 percent shorter (450 / MHz), element spacing about 0.2 wavelength (197 / MHz). These get you close; real Yagis are optimised with antenna-modelling software. Gain is a rough typical figure.
As a starting point the driven element in feet is 468 / MHz, the reflector about 5 percent longer (492 / MHz) and each director about 4 percent shorter (450 / MHz). Spacing is roughly 0.2 wavelength, or 197 / MHz feet.
No. They are a proven starting point. A real Yagi is tuned for the best gain, front-to-back ratio and SWR using antenna-modelling software, which adjusts the lengths and spacing for the wire diameter and boom. Expect to trim from here.
Roughly 5 to 6 dBi for two elements, 7 to 8 for three, and about 1 dB more for each added director with diminishing returns. The gain shown here is a typical figure; the exact value depends on the optimised design.