Although this is not a user requested antenna we decided to include it due to its usefulness when needing a feed for a prime focus reflector antenna with small focal-length to diameter (F/D) ratio.
Since the reflector geometry is fixed (parabolic), the performance of a prime focus reflector antenna depends primarily on the characteristics of its feed. To achieve good aperture efficiency the feed should illuminate the reflector uniformly. Popular feed antennas like the conical horn, dielectric rod antenna and corrugated horn do not lend themselves to optimal illumination for deep reflectors (F/D ratios less than 0.35) as their radiation patterns taper monotonically from a peak gain at broadside. The coaxial-cavity horn can provide a broader, almost-uniform illumination which is better suited to deep reflectors, as shown in the 2D and 3D radiation patterns below. ...2D radiation pattern 3D radiation pattern