Reflector antenna features and feed types

A reflector antenna combines a small primary radiator (feed) with a large conducting reflecting surface that redirects and focuses the feed's energy into a desired direction. Common types: plane reflector, corner reflector, parabolic (dish), and dual-reflector (Cassegrain, Gregorian).

Special features

1. Very high gain and directivity: gain G = η(πD/λ)² for a dish; 30–60 dBi is common.
2. Narrow pencil beam: HPBW ≈ 70λ/D degrees, giving precise pointing and low interference.
3. Equal path property: paraboloid converts a spherical wave from the focus into a plane wave in phase across the aperture.
4. Wide bandwidth: the reflector is frequency independent (if the surface error ≪ λ/16); the bandwidth is set by the feed only.
5. Simple feed: only one small feed is needed; polarization follows the feed (linear, dual or circular).
6. Low side lobes by tapering the feed illumination (edge taper ≈ −10 dB).
7. Reduced efficiency by spillover, aperture blocking, phase errors; typical η = 55–70 %.
8. Mechanically steerable; used at microwave frequencies (above ~1 GHz) where size is practical.

              )|
             ) |--->
 feed at F  o  |---> plane wave
             ) |--->
              )|
        D = aperture diameter, f = focal length

Types of feeds

1. Dipole with parasitic reflector (front feed): a λ/2 dipole at the focus with a small plane or spherical reflector behind it so that energy goes towards the dish instead of forward directly.

      )
     )  <== |(  dipole + small reflector at F
      )

2. Horn feed (front feed): a pyramidal or conical horn at the focus facing the dish; gives good control of illumination and polarization. The waveguide must run to the focus and causes some blocking.

      )
     )  <==<|=== waveguide
      )    horn at F

3. Offset feed: only an off-axis section of a paraboloid is used, and the horn is placed outside the reflected beam; no aperture blocking, lower side lobes. Used in DTH dishes.

     )
    )  ---->
   )   \
        [horn] below the beam

4. Cassegrain feed: horn at the vertex of the main dish illuminates a convex hyperboloid subreflector, whose one focus coincides with the dish focus. Short waveguide, low noise; used in earth stations.

     )        (
    ) [horn]-> ( subreflector
     )        (

5. Gregorian feed: like Cassegrain but with a concave ellipsoidal subreflector placed beyond the focus.