Antenna Arrays Questions and Answers

Answer: C

Near the peak of a sum beam the output barely changes with angle, whereas the difference channel passes through zero and reverses phase, so a small angular error produces a large and signed output. Dividing by the sum channel removes the dependence on signal amplitude, which the difference channel alone does not have. Its gain on boresight is zero rather than high, and its lobes are wider than the sum beam. Measuring against a null rather than a peak is a recurring theme wherever precision is needed.

Answer: D

Spoiling the aperture phase deliberately widens the transmit beam so a larger volume is illuminated at once, at the cost of gain, while narrow receive beams preserve the resolution. It is a standard search technique. Side lobes are managed by amplitude taper, squint is cured by true time delay, and range resolution follows from the pulse bandwidth rather than from anything the antenna does. Trading one performance dimension for another deliberately is what makes an electronically controlled aperture valuable.

Answer: A

Adding elements at fixed spacing lengthens the aperture in proportion, and directivity follows the aperture, so gain rises roughly linearly with the count, three decibels for each doubling. The field in the beam direction rises as the count while the input power also rises, which is why the power gain is linear rather than quadratic. Confusing the field and power relationships is the usual source of the square law answer, and it is worth being explicit about which is meant.

Answer: B

A passive feed simply distributes the available power, so the sum of the element powers is the input power less network loss, and the array radiates all of it. What changes is the direction in which it goes: the same power concentrated into a narrower beam. Passive elements amplify nothing, matching affects how much enters rather than how much is shared, and coupling redistributes power between elements rather than returning it. Gain is redistribution, not creation, and an array is the clearest demonstration of the point.

Answer: C

The array factor is naturally a periodic function of a variable combining spacing and angle, and only part of that period corresponds to angles that physically exist. Grating lobes are simply repetitions of the main lobe that have moved into the visible region as the spacing or the scan angle increased. Thinking of the pattern as a periodic function partly visible makes the grating lobe condition obvious rather than something to be memorised. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: D

Steering adds a constant to the pattern variable, so the entire periodic array factor slides and a repetition that was outside the visible region can move into it. That is why an array with spacing acceptable at broadside develops a grating lobe when scanned. The lobes are not created by scanning, they always exist mathematically, and they move together rather than converging on the main beam. This is the clearest way to see why wide scan demands close spacing.

Answer: A

Grouping elements behind one phase shifter or one receiver cuts the count of the most expensive components sharply, at the cost of quantisation lobes and reduced adaptive degrees of freedom. It is a compromise between a fully populated active array and a passive one. Coupling is unaffected, the beamwidth follows the total aperture as always, and calibration remains necessary. Most large arrays are subarrayed to some degree, and the level at which it is done is a major architectural decision.

Answer: B

Real phase shifters have a finite number of bits, so the applied phase is a staircase approximation to the ideal ramp and the periodic error radiates into spurious lobes. Randomising the assignment of shifter settings across the aperture breaks up the periodicity and spreads the energy into the side lobe floor instead. Signal sampling, ordinary side lobes and scan loss are separate matters. It is a good example of a digital implementation detail with a directly visible effect on the pattern.

Answer: C

The lobes arise from a periodic error, so deliberately dithering the assignment of the quantised settings destroys that periodicity and converts a few discrete lobes into a slightly raised, spread out floor. More bits of phase control achieves the same end at higher cost. Raising the power scales everything equally, wider spacing brings grating lobes, and amplitude taper addresses a different mechanism. Trading a structured error for an unstructured one is often the cheapest available improvement.

Answer: D

Both statements are true and the second is the reason for the first. Because the pointing of the beam is determined entirely by the phase gradient across the aperture, changing that gradient electronically moves the beam as effectively as rotating the whole structure would, and far more quickly. The narrowness of the beam comes separately from the size of the aperture. Together these two facts are what make modern radar and satellite communication antennas possible in the forms they take.