Aperture Antennas Questions and Answers

Q: What is a Gregorian reflector?

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.

Q: What is meant by shaping a dual reflector system?

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.

Q: Why is the feed pattern of a reflector antenna designed to fall off towards the rim?

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.

Q: What is a corrugated horn used for?

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.

Q: What is the aperture field of an ideal feed for a symmetric reflector?

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.

Q: What happens if a reflector feed is displaced laterally from the focus?

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.

Q: What is the effect of moving a reflector feed along the axis away from the focus?

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.

Q: What pattern signature indicates a quadratic phase error across an aperture?

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.

Q: Which aperture error produces a beam steered away from boresight?

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.

41.

What is a Gregorian reflector?

A.A dual reflector using a concave ellipsoidal subreflector
B.A single reflector whose surface is shaped to give a flat topped beam pattern
C.A reflector fed by a waveguide running along the axis to a rear mounted horn
D.A reflector whose subreflector is placed outside the focus of the main dish

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.

Discuss in Forum

42.

What is meant by shaping a dual reflector system?

A.Adjusting the focal length so that the feed pattern just fills the reflector rim
B.Departing from the exact conic surfaces to equalise the aperture illumination
C.Cutting the reflector edge to a non-circular outline to control the beam shape
D.Adding a dielectric layer to the reflector surface to alter its phase response

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.

Discuss in Forum

43.

Why is the feed pattern of a reflector antenna designed to fall off towards the rim?

A.To keep the cross polarisation low across the whole of the aperture area
B.To match the impedance of the feed horn to that of the reflector surface
C.To limit spillover past the edge of the reflector
D.To reduce the mechanical loading on the outer part of the reflector surface

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.

Discuss in Forum

44.

What is a corrugated horn used for?

A.Increasing the gain of a horn without increasing its physical aperture size
B.Allowing a horn to operate over two widely separated frequency bands at once
C.Reducing the weight of a horn by removing metal from its walls in strips
D.Producing a symmetric pattern with very low cross polarisation

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.

Discuss in Forum

45.

What is the aperture field of an ideal feed for a symmetric reflector?

A.Rotationally symmetric with equal patterns in all planes
B.Uniform in amplitude right out to the edge of the aperture opening
C.Strongly tapered in one plane and uniform in the perpendicular plane
D.Linearly polarised with a deliberate phase gradient across the aperture

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.

Discuss in Forum

46.

What happens if a reflector feed is displaced laterally from the focus?

A.The polarisation rotates in proportion to the displacement of the feed
B.The beam is steered off axis with some pattern degradation
C.The gain falls sharply but the beam continues to point along the axis
D.The reflector becomes mismatched and reflects power back to the transmitter

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.

Discuss in Forum

47.

What is the effect of moving a reflector feed along the axis away from the focus?

A.The side lobes fall while the main beam keeps its original width and gain
B.The polarisation purity improves because the illumination becomes more even
C.The beam broadens and the gain falls as the aperture phase curves
D.The beam is steered smoothly away from the boresight direction of the dish

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.

Discuss in Forum

48.

What pattern signature indicates a quadratic phase error across an aperture?

A.A main beam displaced from boresight with the pattern otherwise unchanged
B.A pair of equal lobes symmetric about the intended beam direction
C.A pattern of grating lobes spaced regularly away from the main beam
D.A broadened main beam with the nulls filled in

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.

Discuss in Forum

49.

Which aperture error produces a beam steered away from boresight?

A.A linear phase gradient across the aperture
B.A quadratic phase variation across the aperture from centre to edge
C.A random phase error distributed uniformly over the aperture surface
D.An amplitude taper applied symmetrically about the aperture centre line

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.

Discuss in Forum

50.

Assertion (A): A larger aperture always gives a narrower beam at a given frequency.

Reason (R): Beamwidth is inversely proportional to the aperture measured in wavelengths.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

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.

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