Antenna Arrays Questions and Answers

Answer: C

Power travelling along a single line reaches successive elements later, so the progressive phase depends on frequency and the beam moves as the frequency changes. Some radars use that deliberately to scan a beam by tuning. It works with any number of elements, needs fewer amplifiers rather than more, and grating lobes depend on spacing as in any array. Frequency scanned arrays turn the disadvantage into the operating principle.

Answer: D

In an active array each element has its own amplifier, phase shifter and often its own receiver, so feed losses no longer sit between the amplifier and the radiator. That gives far better noise figure and graceful degradation, since the failure of a few modules costs only a little gain. Cooling is a consequence rather than the definition, switching elements is a crude form of thinning, and a single common amplifier describes a passive array. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: A

With hundreds or thousands of independent modules, the loss of a few reduces the aperture very slightly and raises the side lobes a little, rather than disabling the system. A passive array with one central transmitter fails completely if that transmitter fails. Spare amplifiers, mechanical reconfiguration and automatic power increase are not how it works. This reliability argument is often as important as the performance one in justifying an active array.

Answer: B

The active or embedded element pattern is measured by driving one element while every other is present and match terminated, so it includes the effect of the array environment. Multiplying it by the array factor predicts the scanned pattern far better than the isolated element pattern does, and it reveals scan blindness directly as a dip. The free space pattern ignores coupling entirely and is the naive alternative that array measurements exist to replace.

Answer: C

The field from a neighbour falls with distance, so at a quarter wavelength the induced current is a large fraction of the driven one and the impedance shifts markedly. At a full wavelength it is much weaker. Feed length depends on the layout, resonant frequency is a property of the element, and grating lobes appear at wide rather than close spacing. Coupling is the reason an array cannot be designed one element at a time.

Answer: D

At certain angles a surface wave supported by the array structure is excited, the active reflection coefficient approaches unity and almost no power leaves the aperture. It appears as a sharp null in the active element pattern. Dielectric covers and thick ground planes make it worse. It is a resonance of the whole periodic structure and quite distinct from the geometric limits that grating lobes and scan loss impose.

Answer: A

Since the effect comes from a surface wave the structure supports, the cure is to design the element, ground plane and radome so that no such wave is trapped in the scan range. Baffles between elements are sometimes used. Reducing element count shrinks the aperture without curing anything, wider spacing brings grating lobes, and amplitude distribution has no bearing on the phenomenon. It is one of the effects that only a full wave simulation or a measurement will reveal.

Answer: B

Thinning omits a proportion of the elements, usually at random or by a designed pattern, keeping the aperture and so the beamwidth while cutting the number of expensive modules. The penalty is a raised side lobe floor, since the missing elements act like random errors, and a loss of gain proportional to the elements removed. Amplitude tapering is a different technique that keeps every element, and the other options describe unrelated ideas. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: C

Gain follows the power actually radiated into the beam, so removing half the elements costs three decibels even though the beamwidth is unchanged. This is the clearest demonstration that beamwidth and gain are not the same thing: a thinned array has the resolution of a large aperture and the sensitivity of a small one. Side lobes rise rather than fall, and the relationship is linear rather than quadratic.

Answer: D

The first is correct because beamwidth depends on the overall aperture, which thinning does not change. The second is wrong: gain depends on how much power is radiated coherently into the beam, and that falls with the element count. The pair separates two properties that usually rise together and so are easily confused. Thinning is worth doing precisely when resolution matters more than sensitivity, as in some radio astronomy and direction finding applications.