Cassegrain feed for parabolic reflectors

The Cassegrain feed is a dual-reflector method of feeding a parabolic reflector in which the primary feed (horn) is placed at the vertex of the main paraboloid and radiates towards a small hyperboloidal subreflector placed in front, which reflects the energy back onto the main dish.

          main paraboloid
            )
           )      sub-reflector
          )        (hyperboloid)
         )   F'        (
  [horn]=o vertex ---> (  F = focus of paraboloid
  at F'  )   <-------- (    = one focus of hyperbola
          )             (
           )
            )
 horn feed at F' (other focus of hyperbola)

Geometry and working

1. The hyperboloid has two foci. One focus coincides with the focus F of the paraboloid; the horn's phase centre is placed at the other focus F′, near the vertex of the main dish.
2. Waves from the horn diverge as if coming from F′; after reflection from the hyperboloid they appear to come from F.
3. Since they appear to come from the focus of the paraboloid, the main reflector converts them into a plane wave, forming a narrow pencil beam.
4. The system is equivalent to a single paraboloid of much longer focal length: f_eq = M·f, with magnification M = (e + 1)/(e − 1), e = eccentricity of the hyperboloid.

Advantages

• Feed and receiver/transmitter can be placed behind the main dish; very short waveguide, so low loss and low noise temperature.
• Long effective focal length with a compact structure; better control of aperture illumination.
• Feed spillover points to the cold sky instead of the warm ground, lowering noise; useful for satellite earth stations.
• Easier maintenance of the feed and receiver.

Disadvantages

• The subreflector and its supports block part of the aperture, so it suits only large dishes (D > ~50λ).
• More complex, costlier and needs accurate alignment.

Applications: satellite earth stations, radio telescopes, deep-space communication, tracking radars.