Here are some photos of the feed for our radio telescope that I built last month. Basically, a metal can is used as a waveguide for 21cm radiation (it supports a single TE mode at around 1420 MHz). The feed is placed lambda (guided)/4 from the back, and it itself lambda(free space)/4 long. The most important things here machining-wise (thanks to Mark at the Edgerton shop!) were the conical drill bit for drilling thin materials, and the center punch for aligning the screw holes for the coax connector. The can is to be connected to a horn antenna, and the output signal is fed to amplification stages and finally a software-defined USB dongle receiver.