---
title: "Reflector antenna features and feed types"
subject: "Propagation and Antenna"
chapter: 3
key: "semester_6/propagation_and_antenna:reflector-features-feeds"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/reflector-features-feeds
site: BCT Engineering
---

# Reflector antenna features and feed types

Propagation and Antenna · Chapter 3: Antenna Classification

- Asked in: 2078 Chaitra, 8 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/reflector-features-feeds
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3

## Question

Explain the special features of reflector antenna and discuss on different types of feed used with necessary diagrams.

## Short answer

A reflector antenna uses a large conducting surface to focus feed energy into a narrow high-gain beam with gain G = eta(pi D/lambda)^2 and beamwidth ≈ 70lambda/D. Feeds include dipole with reflector, horn, offset, Cassegrain (hyperboloid subreflector), and Gregorian (ellipsoid).

## Answer

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.

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

```text
      )
     )  <== |(  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.

```text
      )
     )  <==<|=== 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.

```text
     )
    )  ---->
   )   \
        [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.

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

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

| Feed | Blocking | Waveguide length | Typical use |
|---|---|---|---|
| Dipole + reflector | Small | Long | Low microwave |
| Horn (front) | Moderate | Long | Radar, links |
| Offset | None | Short | DTH TV |
| Cassegrain | Subreflector | Very short | Earth stations |
| Gregorian | Subreflector | Very short | Radio telescopes |

## Check yourself

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

1. What limits the bandwidth of a reflector antenna?
   A. The reflector, which is frequency dependent
   B. The boom length
   C. Mainly the feed, since the reflector is frequency independent
   D. The ground plane
   Answer: C. The reflector surface is frequency independent (if errors are much less than lambda/16); the feed sets bandwidth. (Key point: Bandwidth of reflector)

2. How are low side lobes obtained in a dish?
   A. Uniform illumination
   B. Removing the feed
   C. Using a smaller f/D
   D. Tapering the feed illumination (edge taper about -10 dB)
   Answer: D. Tapering illumination towards the rim reduces side lobes. (Key point: Side lobe control)

3. What is the advantage of an offset feed?
   A. No aperture blocking and lower side lobes
   B. Longer waveguide
   C. Higher spillover
   D. Needs a hyperboloid subreflector
   Answer: A. Using an off-axis dish section with the horn outside the beam removes blocking; used in DTH. (Key point: Offset feed)

4. How does a Gregorian feed differ from a Cassegrain feed?
   A. Gregorian has no subreflector
   B. Gregorian uses a concave ellipsoidal subreflector beyond the focus; Cassegrain uses a convex hyperboloid
   C. Gregorian uses a convex hyperboloid
   D. They are identical
   Answer: B. Gregorian: concave ellipsoid beyond focus; Cassegrain: convex hyperboloid near the focus. (Key point: Cassegrain vs Gregorian)

## Related in Propagation and Antenna

- [Parabolic reflector and feed systems](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aparabolic-reflector-feed) — Propagation and Antenna, chapter 3, asked 3×
- [Parabolic reflector operation and Cassegrain feed](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aparabolic-reflector-cassegrain) — Propagation and Antenna, chapter 3
- [Short note: Corner reflector antenna](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Acorner-reflector-short-note) — Propagation and Antenna, chapter 3
- [Parabolic reflector diameter: 85%, 3 GHz](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aparabolic-diameter-85-3ghz) — Propagation and Antenna, chapter 3, asked 4×
- [Parabolic reflector diameter: 75%, 3 GHz](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aparabolic-diameter-75-3ghz) — Propagation and Antenna, chapter 3, asked 4×
- [Parabolic reflector diameter: 75%, 5 GHz](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aparabolic-diameter-75-5ghz) — Propagation and Antenna, chapter 3, asked 4×

## Related in other subjects

- [Waste heat methods and feed water heating layout](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-waste-heat-feedwater) — Power Plant Equipment, chapter 8, asked 2×
- [Combined plant heating feed water with exhaust](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-feedwater-layout) — Power Plant Equipment, chapter 8, asked 2×
- [Regenerative scheme with open feed water heater](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aregenerative-open-heater-layout) — Power Plant Equipment, chapter 7
- [Feed water heating combined circuit and reasons](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-feedwater-reasons) — Power Plant Equipment, chapter 8