Radartutorial .euFeed Horn
Figure 1: Schematic picture of a horn feed
Of course you can let open a waveguide on an end simply, to emit the electromagnetic wave into the air. Since the characteristic impedance of the air, however, has another value, as the waveguide, reflections will appear on this case.
The horn is essentially a flared waveguide. It provides increased directivity if the aperture is large (i.e. the surface area of the flare) and the angle is low (i.e. the horn is long). It can be shown that the gain of a horn feed is proportional to its length times the aperture.
In most radars, the feedhorn is covered with a window of polystyrene fiberglass to prevent moisture and dirt from entering the open end of the waveguide. If a dry-air pressure exist in the waveguide system, ceramic materials or quartz glass is used as covering material.