  We present the design, manufacturing, and testing of a 37-element array of
corrugated feedhorns for Cosmic Microwave Background (CMB) measurements between
$140$ and $170$ GHz. The array was designed to be coupled to Kinetic Inductance
Detector arrays, either directly (for total power measurements) or through an
orthomode transducer (for polarization measurements). We manufactured the array
in platelets by chemically etching aluminum plates of $0.3$ mm and $0.4$ mm
thickness. The process is fast, low-cost, scalable, and yields high-performance
antennas compared to other techniques in the same frequency range. Room
temperature electromagnetic measurements show excellent repeatability with an
average cross polarization level about $-20$ dB, return loss about $-25$ dB,
first sidelobes below $-25$ dB and far sidelobes below $-35$ dB. Our results
qualify this process as a valid candidate for state-of-the-art CMB experiments,
where large detector arrays with high sensitivity and polarization purity are
of paramount importance in the quest for the discovery of CMB polarization
$B$-modes.
