The antenna is a device for collecting the incoming mm-waves and channelling them into the radar detection box behind. There are three main parts to the antenna: the main dish, the sub-reflector and the feedhorn. The main dish is specially shaped so that the incoming mm-waves are reflected back onto the subreflector no matter which part of the dish they hit. The subrefelector has a similar job, reflecting the mm-waves again so that they are focussed at the feedhorn. The feedhorn is basically a hollow metal cone about a centimetre across at the open end. Once the mm-waves are inside the feedhorn, the cone gets slowly narrower and the mm-wave signals are concentrated into a small rectangular metal pipe inside the radar box and can then be fed to the signal detector.