Aperture Antennas Questions and Answers

Answer: A

The Gregorian uses an ellipsoidal subreflector placed beyond the prime focus, giving a real intermediate image and a slightly longer structure than the Cassegrain. Both fold the path to bring the feed to the vertex, and shaped versions of either can reach seventy per cent aperture efficiency. Shaped single reflectors, axial waveguide feeds and subreflector positioning conventions are all related ideas but none of them is the definition. The choice between the two is usually made on mechanical and cross polar grounds.

Answer: B

Shaping alters both surfaces slightly from the ideal hyperboloid and paraboloid so that the power is redistributed more evenly across the aperture while the phase stays flat. Aperture efficiencies above seventy per cent become possible because taper loss is nearly eliminated without increasing spillover. Edge cutting shapes the beam contour, dielectric layers are not used this way, and focal length adjustment is ordinary design rather than shaping. It is the clearest example of solving an illumination problem with geometry rather than with the feed.

Answer: C

Energy that misses the reflector is lost and, worse, arrives from directions near the horizon where it picks up ground noise, so the feed is designed to illuminate the rim about ten decibels below the centre. The taper against spillover optimum follows. Mechanical loading is unrelated, cross polarisation depends on the geometry, and there is no impedance match between a feed and a reflecting surface. Controlling where the feed power goes is the whole art of feed design.

Answer: D

Corrugations in the walls support a hybrid mode whose aperture field is nearly rotationally symmetric, so the pattern is the same in every plane and the cross polarisation is very low. That makes them the standard feed for high performance reflectors and for radiometry. They do not raise gain for a given aperture, dual band operation requires other techniques, and the metal removed is incidental. Achieving a desired aperture field by engineering the mode inside the structure is a powerful idea.

Answer: A

A feed whose pattern is the same in every plane illuminates a circular reflector evenly around its rim, which is what a corrugated horn achieves and a simple rectangular horn does not. Uniform illumination to the edge would maximise taper efficiency but spill badly, asymmetric patterns waste part of the aperture, and a phase gradient would steer the beam off boresight. Matching the symmetry of the feed to the symmetry of the reflector is the first requirement in any feed specification.

Answer: B

A lateral offset tilts the emerging wavefront, so the beam squints away from the axis, and coma lobes appear as the displacement grows. Several beamwidths of scan are possible before the degradation becomes unacceptable, which is the basis of multibeam reflector antennas serving several coverage areas from one dish. Axial displacement, by contrast, defocuses and broadens the beam without steering it. Distinguishing the two kinds of feed displacement explains most reflector misalignment symptoms.

Answer: C

Axial defocusing puts a quadratic phase error across the aperture, which broadens the beam, fills in the nulls and reduces the gain without any steering. It is sometimes done deliberately to spoil a beam for search purposes. Lateral movement is what steers, side lobes rise rather than fall, and polarisation is not improved. Recognising a quadratic phase error by its signature of a broadened beam with filled nulls is a useful diagnostic on any aperture.

Answer: D

Quadratic phase error widens the beam and, because the cancellation that produces the nulls is no longer exact, fills them to a level that rises with the error. A linear phase error steers the beam instead, an antiphase aperture gives the twin lobed difference pattern, and grating lobes come from periodic sampling. Reading the pattern back to the aperture error that caused it is a standard diagnostic skill and it rests entirely on the transform relationship.

Answer: A

A linear phase gradient is exactly what a phased array applies deliberately to steer its beam, so an accidental one produces the same result. Quadratic error broadens, random error raises the side lobe floor, and a symmetric amplitude taper widens the beam and lowers the side lobes without moving it. Classifying aperture errors by their order in position, and knowing the pattern signature of each, turns pattern measurement into a diagnostic tool.

Answer: B

Both are true and the second is the reason. Because the far field is the transform of the aperture distribution, widening the aperture narrows the transform in exact inverse proportion, which is the same relationship as between a pulse and its spectrum. The qualification is that the aperture must be illuminated with a flat phase; a large aperture with severe phase errors gives a broad beam and poor gain, which is precisely why surface tolerance matters so much on large reflectors.