We present the first, complete 230 GHz feedhorn array manufactured by direct drilling of smooth–walled horns into a single plate of aluminium. The horn design process, based on a genetic algorithm, is described and the fabrication process, via direct drilling using shaped drill bits, is presented. We present cross coupling and beam pattern measurements of a close–packed pair of the smooth–walled horns fabricated in a single block of aluminium. We also present a prototype 37 horn array, again fabricated by drilling into a single block. Our measurements show that our designs and fabrication techniques will be robust when applied to large focal arrays of horns consisting of hundreds or thousands of feedhorns. We expect our smooth–walled horn designs and novel manufacturing techniques will offer an attractive, low-cost alternate to traditional horn arrays consisting of electroformed corrugated horns
