--- title: "Parabolic reflector and feed systems" subject: "Propagation and Antenna" chapter: 3 key: "semester_6/propagation_and_antenna:parabolic-reflector-feed" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/parabolic-reflector-feed site: BCT Engineering --- # Parabolic reflector and feed systems Propagation and Antenna · Chapter 3: Antenna Classification - Asked in: 2076 Bhadra, 1+6 marks; 2075 Baisakh, 4 marks; 2073 Bhadra, 7 marks (asked 3 times) - Video: [Parabolic Reflector Antennas Feed Systems | Basic Antenna Tutorials](https://www.youtube.com/watch?v=BJlqNlI5PXk) — Professor Rekt - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/parabolic-reflector-feed - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3 ## Question What is reflector antenna? With necessary diagrams, explain parabolic reflector antenna with its characteristics, radiation pattern and different types of feeding systems. ## Short answer A parabolic reflector converts a spherical wave from a feed at the focus into a plane wave, giving a pencil beam. Gain is G = η(πD/λ)² and HPBW about 70λ/D degrees. Feeds include front dipole or horn, offset, Cassegrain, and Gregorian arrangements. ## Answer 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. ```text reflector )| ) |-----------> ) |-----------> plane wavefront F o ) |-----------> (parallel rays) feed ) |-----------> ) |-----------> ) | )| aperture diameter D f = focal length, f/D ≈ 0.25 to 0.5 ``` #### Characteristics ```text 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 ```text 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. ```text Front feed Cassegrain ) ) ) <--[horn] ) ( <- subreflector ) at F ) ) [horn] at vertex ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A feed at the focus of a paraboloid produces A. a plane wavefront across the aperture, giving a narrow beam B. a spherical wave across the aperture C. a wide fan beam D. no reflection Answer: A. Every ray from F has equal total path to the aperture plane. (Key point: Focusing principle) 2. The gain of a parabolic dish is A. G = eta D / lambda B. G = eta (pi D / lambda)^2 C. G = 70 lambda / D D. G = (lambda / pi D)^2 Answer: B. Gain rises with the square of D/lambda and aperture efficiency eta of about 0.55-0.7. (Key point: Gain formula) 3. In the Cassegrain feed, the horn is A. at the focus pointing at the dish B. behind a parabola with no subreflector C. at the dish vertex and illuminates a hyperboloid subreflector D. outside the beam only Answer: C. This shortens the waveguide, lowering loss and noise. (Key point: Cassegrain feed) 4. The advantage of the offset feed is A. higher blockage B. uses the entire paraboloid C. needs a subreflector D. no aperture blocking, since the feed is outside the beam path Answer: D. Only part of the paraboloid is used, so the feed does not shadow the beam (as in DTH dishes). (Key point: Offset feed) ## 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 - [Reflector antenna features and feed types](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Areflector-features-feeds) — 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 - [Antenna radiation pattern and lobes](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aradiation-pattern) — Propagation and Antenna, chapter 2, asked 4× - [Single wire radiation and omnidirectional antenna characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Asingle-wire-radiation-omnidirectional) — Propagation and Antenna, chapter 1 - [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×