How do the lengths of a three-element Yagi reflector and director compare to that of the driven element?
Why In a Yagi, only the driven element is fed; the reflector and director are parasitic elements that re-radiate with a phase shift set by their length. An element about 5 percent longer than the driven element is inductive and acts as a reflector, pushing the pattern away from it; an element about 5 percent shorter is capacitive and acts as a director, pulling the pattern toward it. So the typical boom order is reflector (longest), driven element, director (shortest), with the main lobe going out the director end.
Watch out The idea that relative lengths depend on frequency confuses absolute size with proportion: all three elements scale with wavelength, but their length relative to each other stays the same.
Longest element is in the back: reflector long, director short, beam fires toward the short end.