Horn collectors
 
For frequencies GHz, horns are commonly used to act as collectors. Apertures are normally 15λ.
A basic design is depicted in figure 1(a). An alternative design might have a reflective paraboloid section attached to the horn lip to increase the effective area (see figure 1(b)). One of the features of horns is that their beams can be calculated accurately which, in turn, ensures good calibration of signal strengths and absolute measurements of bright sources. In some telescope designs, a collection of horns might be used and their signals combined.
A horn is sometimes placed at the focus of a radio dish to accept the collected radiation. Obviously it is important that the horn only ‘sees’ the dish collection aperture and not beyond its perimeter otherwise it would respond to illumination from the ground. The design is, therefore, engineered so that its beam tapers rapidly at its edges.

Figure 1. (a) A horn antenna with feed output. (b) A horn antenna with a reflection collector.