---
title: "Cassegrain feed for parabolic reflectors"
subject: "Propagation and Antenna"
chapter: 3
key: "semester_6/propagation_and_antenna:cassegrain-feed"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/cassegrain-feed
site: BCT Engineering
---

# Cassegrain feed for parabolic reflectors

Propagation and Antenna · Chapter 3: Antenna Classification

- Asked in: 2080 Chaitra, 3 marks; 2073 Magh, 6 marks (asked 2 times)
- 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/cassegrain-feed
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3

## Question

Describe the Cassegrain method of feeding parabolic reflectors.

## Short answer

The Cassegrain feed places a horn at the paraboloid vertex, radiating to a hyperboloidal subreflector whose one focus is the main dish focus. The subreflector makes the wave appear to come from that focus, so the dish produces a plane wave. It allows the feed behind the dish, low noise, and a long effective focal length.

## Answer

The **Cassegrain feed** is a dual-reflector method of feeding a parabolic reflector in which the primary feed (horn) is placed at the **vertex** of the main paraboloid and radiates towards a small **hyperboloidal subreflector** placed in front, which reflects the energy back onto the main dish.

```text
          main paraboloid
            )
           )      sub-reflector
          )        (hyperboloid)
         )   F'        (
  [horn]=o vertex ---> (  F = focus of paraboloid
  at F'  )   <-------- (    = one focus of hyperbola
          )             (
           )
            )
 horn feed at F' (other focus of hyperbola)
```

#### Geometry and working

1. The hyperboloid has two foci. One focus coincides with the focus F of the paraboloid; the horn's phase centre is placed at the other focus F′, near the vertex of the main dish.
2. Waves from the horn diverge as if coming from F′; after reflection from the hyperboloid they appear to come from F.
3. Since they appear to come from the focus of the paraboloid, the main reflector converts them into a plane wave, forming a narrow pencil beam.
4. The system is equivalent to a single paraboloid of much longer focal length: f_eq = M·f, with magnification M = (e + 1)/(e − 1), e = eccentricity of the hyperboloid.

#### Advantages

- Feed and receiver/transmitter can be placed **behind the main dish**; very short waveguide, so low loss and low noise temperature.
- Long effective focal length with a compact structure; better control of aperture illumination.
- Feed spillover points to the cold sky instead of the warm ground, lowering noise; useful for satellite earth stations.
- Easier maintenance of the feed and receiver.

#### Disadvantages

- The subreflector and its supports **block** part of the aperture, so it suits only large dishes (D > ~50λ).
- More complex, costlier and needs accurate alignment.

**Applications**: satellite earth stations, radio telescopes, deep-space communication, tracking radars.

## Check yourself

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

1. The Cassegrain subreflector is
   A. an ellipsoid
   B. a flat plate
   C. a paraboloid
   D. a hyperboloid
   Answer: D. The Gregorian uses an ellipsoid; Cassegrain uses a hyperboloid. (Key point: Subreflector)

2. The horn phase centre is placed at
   A. the other focus F' of the hyperboloid, near the vertex
   B. the focus of the paraboloid
   C. the edge of the dish
   D. infinity
   Answer: A. One hyperbola focus coincides with F of the paraboloid and the horn sits at the other. (Key point: Focus arrangement)

3. A key advantage is
   A. no subreflector blockage
   B. short waveguide with low loss and noise because equipment sits behind the dish
   C. small dish suitability
   D. lower cost than front feed
   Answer: B. Placing the feed at the vertex shortens the feed line. (Key point: Main advantage)

4. Cassegrain suits only large dishes (D > about 50 lambda) because
   A. it cannot work at microwave
   B. the horn is too large
   C. the subreflector blocks part of the aperture
   D. gain falls with D
   Answer: C. Blockage is significant on small dishes. (Key point: Disadvantage)

## Related in Propagation and Antenna

- [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
- [Aperture antenna and parabolic feeding](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aaperture-antenna-parabolic-feeding) — Propagation and Antenna, chapter 3
- [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×