Antenna Arrays Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.