Which matching system for Yagi antennas requires the driven element to be insulated from the boom?
Why The beta or hairpin match works on a split driven element: the element is made slightly short so it looks capacitive, and a shorted transmission line stub (the hairpin) placed across the feedpoint adds the inductive reactance that cancels it and raises the resistive part to 50 ohms. Because the feed point is a gap at the center of the element, that element cannot be bolted to a grounded metal boom, so it must be mounted on an insulator.
Watch out Gamma, T-match and other shunt-fed schemes are the opposite case: they tap onto a continuous, unbroken element that is deliberately bolted right to the boom, which is why they are popular for DC grounding and static bleed.
Hairpin = split element = insulated. Gamma/T = solid element = bolt it to the boom.