Parabolic reflector and feed systems

A reflector antenna uses a large conducting surface (plane, corner or curved) to reflect and focus the radiation of a small primary feed, producing a highly directive beam. The most important is the parabolic reflector (dish).

Parabolic reflector: principle

A paraboloid has the property that every ray leaving the focus F and reflected by the surface travels the same total distance to the aperture plane (FP + PQ = constant). So a spherical wave from a feed at F becomes a plane wave across the aperture, giving a narrow pencil beam. In reception, a plane wave is focused onto F.

          reflector
           )|
          ) |----------->
         )  |----------->  plane wavefront
  F  o  )   |----------->  (parallel rays)
  feed  )   |----------->
         )  |----------->
          ) |
           )|  aperture diameter D
  f = focal length, f/D ≈ 0.25 to 0.5

Characteristics

Gain:       G = η(πD/λ)²     η ≈ 0.55-0.7
HPBW:       θ ≈ 70 λ/D  degrees
FNBW:       ≈ 140 λ/D  degrees

• Very high gain (30–60 dBi), very narrow beam; used at microwave frequencies (above ~1 GHz).
• Efficiency is reduced by spillover, aperture blocking, non-uniform illumination and surface errors.
• Polarization follows the feed.

Radiation pattern

            main lobe (pencil)
   side  ___ /‾‾‾‾\ ___  side
   lobes     |    |       lobes
 ---<|dish>--+----+-------> axis
   small back lobe

A narrow main lobe along the axis, low side lobes (−20 to −30 dB with tapered illumination) and a small back lobe from spillover.

Feed systems

1. Dipole with reflector (front feed): a λ/2 dipole at F with a small parabolic/plane reflector behind it to send energy only towards the dish. Simple, for lower microwave.
2. Horn feed (front feed): a waveguide horn at F pointing at the dish; good illumination control; the waveguide run is long and blocks part of the aperture.
3. Offset feed: only part of the paraboloid is used, with the feed outside the beam path; no aperture blocking (DTH dishes).
4. Cassegrain feed: a hyperboloid subreflector near F reflects the wave from a horn located at the dish vertex. Short waveguide, low noise; used in satellite earth stations.
5. Gregorian feed: like Cassegrain but with an ellipsoidal subreflector placed beyond the focus.

 Front feed          Cassegrain
   )                    )
  )  <--[horn]         ) ( <- subreflector
  )   at F             )  
   )                 [horn] at vertex