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Aperture antenna and parabolic feeding

Short answer

An aperture antenna radiates from an opening several wavelengths wide, using field distribution (Huygens sources). A parabolic reflector places a feed at the focus of a paraboloid; the equal-path property converts spherical waves to a plane wave, producing high gain G = eta(pi D/lambda)^2. Feeds include dipole with reflector, horn, offset, Cassegrain (hyperboloid subreflector), and Gregorian (ellipsoid).

What do you mean by aperture antenna? Explain the construction, working principle and the feeding mechanism for Parabolic Antenna.

Aperture antenna

An aperture antenna radiates through an opening (aperture) that is several wavelengths in size; the field across the opening acts as a set of Huygens sources. Examples: horn, slot, lens and parabolic reflector antennas, used mainly at microwave frequencies.

Parabolic antenna: construction

            )|
           ) |---->
          )  |---->   plane wave
  feed F o   |---->   (pencil beam)
          )  |---->
           ) |---->
            )|
     <- f -> aperture plane, diameter D
  • A conducting paraboloid of revolution (solid sheet or wire mesh with gaps < λ/10).
  • Aperture diameter D (many λ); focal length f; ratio f/D ≈ 0.25–0.5.
  • A small primary antenna (feed) at the focus F, supported by struts; mounted on a steerable pedestal.

Working principle

  1. Equal-path property: for any point P on the dish, FP + PQ = constant (Q on the aperture plane), so all waves from F reach the aperture in phase.
  2. Parallel-ray property: reflected rays are all parallel to the axis.
  3. Thus the spherical wave from the feed becomes a plane wave, giving a narrow high-gain beam. In reception, incoming plane waves are focused at F.
  4. Gain G = η(πD/λ)², HPBW ≈ 70λ/D degrees; η ≈ 0.55–0.7, reduced by spillover, blocking and phase errors.

Feeding mechanisms

  1. Dipole with reflector: λ/2 dipole at F with a small reflector behind it to direct energy to the dish.
  2. Horn feed (front feed): horn at F facing the dish; good illumination control.
  3. Offset feed: only part of the paraboloid used, feed outside the beam; no blocking (DTH).
  4. Cassegrain feed: horn at the dish vertex illuminates a hyperboloid subreflector, whose one focus coincides with F; short waveguide, low noise.
  5. Gregorian feed: ellipsoidal subreflector placed beyond the focus.
 Front feed            Cassegrain
     )                     )      (
    )  <==[horn]          ) [horn]->( sub-
     )   at F              )       ( reflector

Applications: satellite earth stations, DTH, radar, microwave links, radio telescopes.

✓ Answer checked by Claude Opus 5.5 (high effort)

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