Horn and Reflector Antennas Questions and Answers

Answer: D

A small disc facing the open end of an axial waveguide turns the radiation around so that it illuminates the reflector from the focus, without the bulk of a horn pointing backwards. It is cheap and compact and is common on offset domestic dishes and small radars. Weather protection, mounting and polarisation conversion are all separate functions handled by other parts. Redirecting radiation with a small reflector rather than a shaped horn is a neat piece of packaging.

Answer: A

A scalar feed is a small corrugated horn with concentric rings around its mouth, giving a rotationally symmetric pattern and low cross polarisation in a compact form. It is the familiar feed on satellite receiving dishes. The other options play on other meanings of the word scalar and describe nothing standard. The rings perform the same function as the corrugations inside a larger horn, which is to control the field at the boundary and so the field at the aperture.

Answer: B

The hyperboloid makes the system behave optically as though the focal length were several times the physical one, and that ratio is the magnification. A long effective focal length improves scan performance and cross polarisation while the structure stays compact. The subreflector adds no gain of its own, the diameter ratio is a separate quantity affecting blockage, and beam narrowing follows from the aperture rather than from the feed pattern. It is the main optical reason for the arrangement.

Answer: C

The Gregorian ellipsoid sits past the focus, where the rays have already crossed, so a concave surface is needed to return them to the vertex. That makes the structure slightly longer than an equivalent Cassegrain but gives a real focus that can be used for testing or for a focal plane array. Reflection angle, material and feed size are not the reason. The two arrangements are optical duals and the choice between them turns on mechanical and cross polar detail.

Answer: D

A beam waveguide uses a train of curved mirrors to carry the focused beam down through the elevation and azimuth axes into a fixed room, so heavy cryogenic receivers need not ride on the moving structure. Large earth stations and deep space antennas use it. It is not a waveguide in the ordinary sense, is not a multibeam device, and does not shape the pattern beyond what the mirrors require. Moving the difficulty from the moving part to the fixed part is the whole idea.

Answer: A

Keeping cryostats, amplifiers and their services in a fixed room removes flexible cabling, rotating joints and the weight penalty on the mount, which is decisive for large steerable antennas with several receivers. The price is several extra mirrors, each adding a little loss and spillover, so efficiency is slightly lower rather than higher. A subreflector is still used, and weather protection is a separate question. Mechanical and operational convenience outweighs a small electrical penalty here.

Answer: B

A sphere looks the same from every direction, so moving the feed across its focal surface steers the beam without moving the enormous reflector, which is how the Arecibo and FAST telescopes work. The price is spherical aberration, since a sphere does not focus a plane wave to a point, and a line feed or a corrector is needed to fix it. Cost and blockage are not the motivation. Trading an optical imperfection for a mechanical simplification is a legitimate choice at very large scale.

Answer: C

A sphere brings rays near the axis to a different focus from those near the rim, which is spherical aberration, and it must be corrected by a line feed, a Gregorian corrector or a shaped subreflector. Astigmatism needs unequal curvatures, coma arises from off axis feeding, and chromatic aberration requires refraction and so does not occur in a mirror at all. Naming the aberration correctly points immediately at the family of corrections that will remove it.

Answer: D

Distorting the reflector surface, or using an array feed, spreads the beam into a contour matching a country or a service area, so power is not wasted on the ocean. Broadcast satellites almost all use shaped reflectors for this reason. A pencil beam comes from an unshaped aperture, electronic steering needs phase control, and tolerance is unaffected by shaping. Matching the pattern to the demand rather than maximising gain in one direction is the design philosophy throughout satellite payload work.

Answer: A

Surface shaping redistributes the radiated energy into the wanted contour while keeping almost all of it, which an absorber or a power reduction could never do. An array feed illuminating a conventional reflector achieves the same end with electrical rather than mechanical means and allows the coverage to be reconfigured in orbit. Tilting merely points the beam elsewhere. Redistributing power rather than discarding it is the difference between shaping and attenuating.