Antenna Arrays Questions and Answers

Q: What are random phase and amplitude errors in an array likely to produce?

Random errors scatter power out of the coherent beam, so gain falls a little and the scattered energy appears as a raised, roughly uniform side lobe level. A systematic linear phase error steers the beam instead, and grating lobes come from spacing. Nothing narrows the beam. The tolerable error is set by the side lobe specification, which is why very low side lobe arrays demand phase accuracy of a degree or two across every path.

Q: Why do very low side lobe arrays demand tight manufacturing tolerances?

However carefully the intended distribution is tapered, random errors add an incoherent floor, and once the design side lobe level approaches that floor further tapering achieves nothing. Reaching minus forty decibels therefore requires phase and amplitude accuracy across every path that ordinary construction will not give. Spacing and element matching matter for other reasons. The lesson is that a specification must be achievable in manufacture as well as on paper.

Q: What is a planar array?

A planar array places elements on a rectangular or triangular lattice in a plane, giving control of the beam in both azimuth and elevation. A linear array controls only one plane, a conformal array follows a curved surface, and printing on a board is a construction technique that any of these might use. The triangular lattice is often preferred because it needs about thirteen per cent fewer elements for the same grating lobe free scan volume. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Q: What is a conformal array?

A conformal array is fitted to a non-planar surface, so its elements point in different directions and the excitation must compensate for both position and orientation. It saves drag and volume on vehicles at the cost of much harder beamforming, since pattern multiplication no longer applies. Shaped beam synthesis, common element design and flush mounting are all different matters, though a conformal array may involve any of them.

Q: What does an adaptive array do that a fixed array cannot?

An adaptive array measures the signals arriving at its elements and computes weights that suppress interference while preserving the wanted direction, updating continuously as the environment changes. Nothing about it increases available power or element bandwidth, and it certainly requires weight control on each element. The gain over a fixed array appears only when there is interference to reject, which is why adaptive systems are specified by their interference environment.

Q: What is the principal cost of an adaptive array?

Computing weights requires the signal from each element separately, so a full receiver chain and analogue to digital converter is needed per element, which is expensive in cost, power and processing. Subarray schemes reduce it at the cost of fewer degrees of freedom. The aperture is whatever the gain requires, bandwidth is set by the elements and the sampling, and the main beam is steered as freely as in any phased array.

Q: What is digital beamforming?

In digital beamforming each element's signal is digitised and the beams are formed afterwards in software, so many beams can be made simultaneously from the same data. That is impossible with analogue phase shifters, which form one beam at a time. Digitally controlled phase shifters are still analogue beamforming, pattern display is a measurement matter, and beam switching is a much simpler technique with fixed patterns.

Q: What advantage does digital beamforming give a receiving array?

Because the element signals are stored, as many beams as the processor can compute may be formed at once, all looking in different directions, which multiplies the search rate of a radar or the capacity of a communication system. Grating lobes still depend on spacing, coupling still alters the element patterns, and calibration becomes more critical rather than less, since the processing assumes known amplitude and phase at every channel. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Q: Why must a digital beamforming array be carefully calibrated?

Beamforming weights are applied on the assumption that each receiver chain has a known gain and phase, and any unmeasured difference between channels appears exactly as an array error, raising side lobes and displacing nulls. Injecting a common calibration signal periodically is the usual answer. Element resonance and grating lobes are unrelated to channel calibration, and converter drift is one contributor among several rather than the whole story. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

41.

What are random phase and amplitude errors in an array likely to produce?

A.A raised side lobe floor and slightly reduced gain
B.A beam steered away from the intended direction
C.Grating lobes at exact multiples of the element spacing
D.A narrower main beam than intended

Answer: A

Random errors scatter power out of the coherent beam, so gain falls a little and the scattered energy appears as a raised, roughly uniform side lobe level. A systematic linear phase error steers the beam instead, and grating lobes come from spacing. Nothing narrows the beam. The tolerable error is set by the side lobe specification, which is why very low side lobe arrays demand phase accuracy of a degree or two across every path.

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42.

Why do very low side lobe arrays demand tight manufacturing tolerances?

A.The elements must be individually matched to fifty ohms
B.Random errors set a floor that cannot be tapered away
C.Tapering requires more precise amplitude control
D.Low side lobes require wider element spacing

Answer: B

However carefully the intended distribution is tapered, random errors add an incoherent floor, and once the design side lobe level approaches that floor further tapering achieves nothing. Reaching minus forty decibels therefore requires phase and amplitude accuracy across every path that ordinary construction will not give. Spacing and element matching matter for other reasons. The lesson is that a specification must be achievable in manufacture as well as on paper.

Discuss in Forum

43.

What is a planar array?

A.An array mounted on a curved surface such as a fuselage
B.An array whose elements are printed on a circuit board
C.Elements arranged in a two dimensional grid in one plane
D.An array of elements all lying along a single straight line

Answer: C

A planar array places elements on a rectangular or triangular lattice in a plane, giving control of the beam in both azimuth and elevation. A linear array controls only one plane, a conformal array follows a curved surface, and printing on a board is a construction technique that any of these might use. The triangular lattice is often preferred because it needs about thirteen per cent fewer elements for the same grating lobe free scan volume. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Discuss in Forum

44.

Why is a triangular lattice sometimes preferred to a rectangular one in a planar array?

A.It is simpler to manufacture and to feed
B.It gives lower side lobes for the same taper
C.It allows wider bandwidth in each element
D.It needs fewer elements for the same grating lobe free scan

Answer: D

The triangular arrangement packs the array's spatial frequency cells more efficiently, so about thirteen per cent fewer elements cover the same scan volume without grating lobes. On an array of thousands of modules that is a substantial saving. It is rather harder to lay out and feed than a rectangular grid, side lobe level depends on the taper, and element bandwidth is a property of the element. Efficiency in element count is the whole justification.

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45.

What is a conformal array?

A.An array following a curved surface such as an aircraft skin
B.An array whose pattern conforms to a specified shape
C.An array in which all elements conform to one design
D.An array mounted flush with a flat ground plane

Answer: A

A conformal array is fitted to a non-planar surface, so its elements point in different directions and the excitation must compensate for both position and orientation. It saves drag and volume on vehicles at the cost of much harder beamforming, since pattern multiplication no longer applies. Shaped beam synthesis, common element design and flush mounting are all different matters, though a conformal array may involve any of them.

Discuss in Forum

46.

What does an adaptive array do that a fixed array cannot?

A.Steer its beam without any phase control at all
B.Place nulls automatically on sources of interference
C.Radiate more total power than its amplifiers supply
D.Operate over a wider frequency band than its elements allow

Answer: B

An adaptive array measures the signals arriving at its elements and computes weights that suppress interference while preserving the wanted direction, updating continuously as the environment changes. Nothing about it increases available power or element bandwidth, and it certainly requires weight control on each element. The gain over a fixed array appears only when there is interference to reject, which is why adaptive systems are specified by their interference environment.

Discuss in Forum

47.

What is the principal cost of an adaptive array?

A.A far narrower operating bandwidth
B.The inability to steer the main beam at all
C.A receiver and digitiser behind every element
D.A much larger physical aperture than a fixed array

Answer: C

Computing weights requires the signal from each element separately, so a full receiver chain and analogue to digital converter is needed per element, which is expensive in cost, power and processing. Subarray schemes reduce it at the cost of fewer degrees of freedom. The aperture is whatever the gain requires, bandwidth is set by the elements and the sampling, and the main beam is steered as freely as in any phased array.

Discuss in Forum

48.

What is digital beamforming?

A.Steering the beam with digitally controlled phase shifters
B.Displaying the antenna pattern on a digital instrument
C.Switching between a set of fixed beams electronically
D.Combining digitised element signals in a processor

Answer: D

In digital beamforming each element's signal is digitised and the beams are formed afterwards in software, so many beams can be made simultaneously from the same data. That is impossible with analogue phase shifters, which form one beam at a time. Digitally controlled phase shifters are still analogue beamforming, pattern display is a measurement matter, and beam switching is a much simpler technique with fixed patterns.

Discuss in Forum

49.

What advantage does digital beamforming give a receiving array?

A.Many independent beams can be formed from one set of data
B.The elements can be spaced further apart without grating lobes
C.The array becomes immune to mutual coupling
D.No calibration of the element paths is required

Answer: A

Because the element signals are stored, as many beams as the processor can compute may be formed at once, all looking in different directions, which multiplies the search rate of a radar or the capacity of a communication system. Grating lobes still depend on spacing, coupling still alters the element patterns, and calibration becomes more critical rather than less, since the processing assumes known amplitude and phase at every channel. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

50.

Why must a digital beamforming array be carefully calibrated?

A.The analogue to digital converters drift with temperature only
B.The processing assumes known amplitude and phase in each channel
C.The elements would otherwise resonate at different frequencies
D.Calibration is required to prevent grating lobes

Answer: B

Beamforming weights are applied on the assumption that each receiver chain has a known gain and phase, and any unmeasured difference between channels appears exactly as an array error, raising side lobes and displacing nulls. Injecting a common calibration signal periodically is the usual answer. Element resonance and grating lobes are unrelated to channel calibration, and converter drift is one contributor among several rather than the whole story. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Discuss in Forum

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