---
title: "Aperture antenna and parabolic feeding"
subject: "Propagation and Antenna"
chapter: 3
key: "semester_6/propagation_and_antenna:aperture-antenna-parabolic-feeding"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/aperture-antenna-parabolic-feeding
site: BCT Engineering
---

# Aperture antenna and parabolic feeding

Propagation and Antenna · Chapter 3: Antenna Classification

- Asked in: 2075 Bhadra, 1+6 marks
- Video: [Feeding Methods of Reflector Antenna (Center Feed, Offset Feed & Cassegrain Feed) Explained](https://www.youtube.com/watch?v=XLcXpZTUcv4) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/aperture-antenna-parabolic-feeding
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3

## Question

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

## 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).

## Answer

#### 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

```text
            )|
           ) |---->
          )  |---->   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.

```text
 Front feed            Cassegrain
     )                     )      (
    )  <==[horn]          ) [horn]->( sub-
     )   at F              )       ( reflector
```

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

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What defines an aperture antenna?
   A. Radiates through an opening several wavelengths in size
   B. Radiates from thin wire currents
   C. Has no feed
   D. Works only below 1 MHz
   Answer: A. Aperture fields act as Huygens sources. (Key point: Aperture antenna)

2. What does the parabolic dish do to the feed's wave?
   A. Converts plane to spherical
   B. Converts spherical wave to plane wave through equal path and parallel ray properties
   C. Absorbs it
   D. Changes frequency
   Answer: B. Reflected rays are parallel to the axis. (Key point: Parabolic working)

3. Where is the primary feed placed in a parabolic antenna?
   A. At the vertex only
   B. At the rim
   C. At the focus
   D. Behind the dish
   Answer: C. The feed at F illuminates the dish; Cassegrain moves the horn to the vertex. (Key point: Feed location)

4. Typical f/D and wire mesh gap?
   A. f/D 10; gaps lambda
   B. f/D 5; gaps 2 lambda
   C. f/D 0.01
   D. f/D 0.25-0.5; mesh gaps less than lambda/10
   Answer: D. Mesh must look solid to waves. (Key point: Dish parameters)

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