Aperture Antennas Questions and Answers

Answer: A

A leaky wave antenna uses a guide whose wave loses power to radiation steadily as it travels, so the aperture is illuminated progressively and the beam points at an angle set by the phase velocity. Because that velocity depends on frequency, the beam scans as the frequency is tuned, which is used deliberately in some radars. A fault, a mismatch and a deliberately mismatched slot array all describe different situations, though the last is closely related in construction.

Answer: B

The beam points where the radiated contributions add in phase, and that angle depends on the ratio of the free space wavenumber to the guide wavenumber, which changes with frequency. Tuning therefore steers the beam without any phase shifters at all. Element resonance, feed delay and attenuation shifts are secondary effects. Frequency scanning is a cheap form of electronic steering, limited by the fact that the frequency is then no longer free to be used for anything else.

Answer: C

Tying the beam direction to the frequency means the transmitted frequency is dictated by where the beam must point, which removes frequency agility and complicates interference management. Wide scan ranges are achievable and the antenna is compact, since no phase shifters are needed. Side lobes do degrade somewhat at the extremes but that is not the governing objection. Every steering method costs something, and identifying what it costs is how the choice between them is made.

Answer: D

A reflectarray is a flat panel of printed elements, each sized or delayed to impose the phase a curved reflector would have provided, illuminated by a feed in the usual way. It combines the low loss of a reflector with the flat, foldable form of an array and can be made electronically steerable. Panelled reflectors and focal plane arrays are different things, and the second option describes no standard device. Achieving with phase what would otherwise require shape is the recurring idea.

Answer: A

Being a flat panel, it stows compactly for launch, can be made in folding sections and can follow a curved surface, which a rigid paraboloid cannot. It still has a feed and still spills power past its edge. Bandwidth is in fact its weakness, since the phase correction is exact only near the design frequency, exactly as with a zoned lens. Mechanical form rather than electrical performance is what recommends it, which is a perfectly respectable reason to choose a technology.

Answer: B

Each element supplies a phase rather than a true delay, so the correction that flattens the wavefront at the design frequency no longer does so away from it, and the aperture develops a phase error. The same limitation afflicts any phase based rather than delay based correction, including zoned lenses and phase steered arrays. Substrate loss, feed illumination and element spacing all matter but none is the fundamental restriction. Distinguishing phase from delay is the key to seeing which techniques are inherently narrowband.

Answer: C

Placing several feeds around the focus produces several beams, each squinted slightly from the axis, so one dish can serve several coverage areas or survey the sky faster. The spacing of the beams is set by how far each feed can be displaced before coma becomes serious. Aperture phase measurement is done by holography, surface correction requires an active surface or adaptive optics, and efficiency is a feed design matter. Getting more than one beam from one expensive aperture is often the whole justification.

Answer: D

Lateral displacement introduces an odd order phase error across the aperture, producing asymmetric coma lobes and a falling gain, and a few beamwidths of scan is usually the practical limit. Longer focal length reflectors tolerate more. Mechanical collision is a layout detail, coherence length is irrelevant at these scales, and polarisation is only slightly affected. Naming the aberration and the aperture error that produces it is what allows the limit to be predicted rather than discovered by experiment.

Answer: A

With a shallow dish, the geometry is closer to that of a thin optical lens, so the aberration introduced per unit of feed offset is smaller and more beamwidths of scan are available. The penalties are a physically longer structure and a feed that must have a narrower pattern. Illumination evenness, blockage and manufacturing ease all change too but do not explain the scan tolerance. The focal length to diameter ratio is one of the first choices made in any reflector design and it decides several things at once.

Answer: B

Prime focus dishes usually fall between 0.3 and 0.5, deep enough that the feed can be supported conveniently and its pattern is not required to be unusually narrow. Longer ratios give better scan performance and lower cross polarisation but need a longer support structure. The very short ratios suggested would put the feed almost on the surface, and there is no single optimum value, only a set of trades whose balance depends on the application.