Aperture Antennas Questions and Answers

Answer: A

The duality interchanges the electric and magnetic fields, so a horizontal slot radiates vertical polarisation and a vertical slot horizontal. The pattern shape is the same, which is the source of the confusion. Nothing becomes circular, and reversal of sense applies to circular polarisation rather than to linear. Remembering that the slot is the magnetic dual of the dipole predicts both this rotation and the impedance relation without any further work.

Answer: B

Cutting slots in the wall of a waveguide interrupts the wall currents, and each slot radiates with an amplitude set by its offset from the centre line and a phase set by its position along the guide. The result is a rugged, low loss, accurately controlled array widely used in radar. Dielectric loading, separate guides and tuning access are different arrangements entirely. The elegance is that feed network and radiating elements are the same piece of metal.

Answer: C

A slot radiates in proportion to how much wall current it interrupts, and that depends on its offset from the centre line where the current distribution is known. Setting the offsets therefore sets the amplitude taper across the array, which is how a low side lobe distribution is realised. Wall thickness affects the slot's resonance slightly, frequency shifts the whole design, and the termination absorbs whatever is left. Controlling the taper by geometry rather than by a power divider is what makes the arrangement so precise.

Answer: D

The wall current reverses every half guide wavelength, so placing successive slots on opposite sides of the centre line restores them all to the same radiating phase and produces a broadside beam. It is a geometric solution to a phase problem. Mechanical stress, independent tuning and bandwidth staggering are not the reason, though the last is a technique used in some non-resonant designs. Recognising that geometry can supply phase as effectively as a cable length is the general lesson.

Answer: A

The open mouth of a waveguide radiates directly, giving a broad pattern and a gain of a few decibels, and it is a common element in phased arrays because it is compact and rugged. The match is imperfect, which is why horns flare the aperture out gradually. It absorbs nothing, the cutoff behaviour belongs to the guide rather than to the opening, and mode transitions are made by tapered sections. It is the simplest aperture antenna there is and the starting point for the horn.

Answer: B

The gradual flare presents a slowly changing impedance from the guide to free space, reducing reflection, and at the same time gives a larger aperture and so more gain. Both effects follow from the same taper. Higher order modes are in fact excited by the flare rather than removed, the run length is a layout matter, and polarisation is unchanged. Gradual transitions as a way of avoiding reflection is an idea that recurs from transmission lines to optics.

Answer: C

A lens delays the central rays relative to the edge ones, or the reverse, so that a spherical wave from the feed emerges as a plane wave across the aperture. Dielectric lenses do it by refraction and metal plate lenses by the phase velocity in parallel plate guides. It has no blockage problem, since the feed is behind the aperture, but it is heavy and lossy. Reflectors, radomes and focused arrays are all different devices addressing different problems.

Answer: D

Removing whole wavelengths of path from the thickest parts cuts weight and material dramatically, since only the fractional part of the delay matters. The price is that the correction is exact only at the design frequency, so the bandwidth narrows and shadowing occurs at the steps. Tolerance, dual band operation and side lobe control are not what zoning achieves. It is a clear case of a technique whose whole justification is mechanical and whose whole cost is electrical.

Answer: A

Zoning relies on the fact that a delay differing by exactly one wavelength is equivalent to no difference at all, and that holds at only one frequency. Away from it the zones no longer align in phase and the aperture develops errors. Material dispersion is usually small over the bands concerned, step reflection is a secondary loss, and feed mismatch is a separate issue. Any technique that relies on a whole number of wavelengths is narrowband for the same reason.

Answer: B

A lens is a solid body, so doubling its diameter multiplies its mass and its absorption several fold, which is why lenses are rare above modest sizes while reflectors scale to tens of metres. Zoning helps but does not remove the problem. Focal length is a design choice, surface tolerance is comparable to a reflector's, and aperture efficiency can be very good since there is no blockage. Scaling behaviour, rather than performance at any one size, is what decides between the two technologies.