Aperture Antennas Questions and Answers

Answer: A

Beamwidth depends on the aperture measured in wavelengths, so raising the frequency makes the same dish electrically larger and the beam narrower in proportion. Gain rises as the square of frequency for the same reason. The effect applies in both planes together for a circular aperture. This single relationship explains why high resolution imaging drives systems to ever higher frequencies, and why surface tolerance eventually stops them going further.

Answer: B

Effective aperture is an area and so is measured in square metres, being the received power divided by the incident power density. Effective length, measured in metres, is the corresponding quantity for converting field strength to voltage; steradians measure solid angle; and decibels relative to isotropic express gain. Checking that the units of a quantity match what it is supposed to do catches most misremembered formulas before any arithmetic is attempted.

Answer: C

Amplitude tapering softens the aperture edge and so lowers the side lobes, at the cost of a wider beam and reduced effective area. Beam direction is set by the phase distribution, impedance by the feed and its match, and polarisation by the orientation of the aperture fields. Sorting which aperture variable controls which pattern property makes an unfamiliar specification readable, because each requirement then points at the part of the design that must satisfy it.

Answer: D

With equal phase everywhere, all contributions add along the normal to the aperture, putting the beam on boresight. Adding a linear phase gradient steers it away. Side lobes are governed by the amplitude distribution rather than the phase, and a flat phase distribution gives a stable rather than a frequency sensitive pattern. Phase points the beam and amplitude shapes it: that division covers most of what an aperture distribution does.

Answer: A

The flare enlarges the aperture, raising the gain, and grades the impedance from the guide to free space, reducing the reflection. Higher order modes are in fact generated by the flare, the horn is itself the matching section, and its patterns in the two planes differ unless the design is corrugated or otherwise corrected. Two benefits from one geometric change is a common pattern in antenna design and is worth noticing when it occurs.

Answer: B

A radome keeps rain, snow, ice and birds off the structure and greatly reduces the wind loading the mount must carry, in exchange for a little transmission loss and some pattern distortion. It does nothing about interference, which is a pattern and filtering matter, and surface currents cause no corrosion of consequence. Every protective measure in this field has an electrical price, and quantifying that price is part of specifying the radome.

Answer: C

A layer of ice adds a lossy dielectric of uneven thickness over the surface, introducing phase errors and absorption, so gain falls and side lobes rise. Heaters or a radome are the usual defences in cold climates. A reflector has no resonant frequency in the sense implied, polarisation is not rotated by a dielectric layer, and ice is a poor conductor rather than a good one. Environmental effects on the aperture surface are as much a part of antenna engineering as the electromagnetics.

Answer: D

Provided the mesh openings are small compared with the wavelength, the surface reflects almost as well as solid metal while weighing far less and passing the wind, which matters greatly on large steerable structures. The upper frequency limit is set by the mesh size. It does not add polarisation capability or bandwidth, and while it sheds water it does not prevent icing. Knowing when a structure may be approximated as a continuous surface is a recurring judgement in this subject.

Answer: A

Once the openings become an appreciable fraction of a wavelength the mesh begins to leak and both gain and front to back ratio suffer, so a mesh good for one band is transparent at a higher one. A tenth of a wavelength is a common rule. Wire conductivity affects loss slightly, tension affects surface accuracy, and diameter sets the gain rather than the frequency limit. The criterion is always the size of a feature measured in wavelengths, which is the recurring theme of the whole subject.

Answer: B

The far field is the Fraunhofer region, borrowing the term from optical diffraction, where the pattern shape no longer depends on distance. The Fresnel region is the radiating near field between the reactive zone and the far field, and the reactive and induction names describe the very close in region where energy is stored. Using the optical vocabulary consistently is helpful, since the underlying diffraction mathematics is identical.