Aperture Antennas Questions and Answers

Answer: C

Both statements are correct. Aperture efficiency is the product of several factors, of which taper and spillover are the two largest and the ones that trade directly against each other, with blockage, phase error, surface tolerance and cross polarisation loss making up the rest. Quoting a single efficiency figure without saying which factors it includes is a frequent source of disagreement between a manufacturer's specification and a measured result.

Answer: D

The first two are the standard results for the two canonical geometries. The third is the reverse of the truth: uniform illumination gives the highest side lobes and the highest directivity, and every taper trades one for the other. Confusing maximum directivity with best pattern is easy because both sound like optimality, and the correction is to ask which quantity is being optimised before calling any distribution the best.

Answer: A

Each factor has a distinct physical cause, and an efficiency budget lists them separately so that the largest can be attacked. Taper and spillover trade against one another while blockage and surface error are reduced by construction, which is why an offset shaped reflector with an accurate surface can exceed seventy per cent where a simple prime focus dish reaches little more than half.

Answer: B

The first three are correct. A larger aperture gives a narrower beam, not a wider one, since beamwidth is inversely proportional to the aperture in wavelengths. The error is worth guarding against because it inverts the single most useful relationship in aperture antenna work, the one that connects the physical size of a structure to the angular resolution it can achieve.

Answer: C

Blockage, surface error and spillover are all loss mechanisms counted in the aperture efficiency budget, whereas increasing the diameter raises the gain as the square of the diameter. The odd one out is the only entry that improves performance. Sorting the entries of an efficiency budget from the design variables that set the underlying gain is the first step in deciding where effort should be spent on an underperforming antenna.

Answer: D

The Ruze equation converts a root mean square surface error, expressed in wavelengths, into a gain loss and a scattered side lobe level. It is what turns a mechanical tolerance into an electrical specification and sets the highest frequency at which a given reflector is worth using. Friis governs a transmission link, Babinet relates slots to dipoles, and Fraunhofer names the far field region rather than any equation about surfaces.

Answer: A

At 6 GHz the wavelength is 0.05 m, so twice the square of the aperture divided by the wavelength gives:

\[ \tfrac{2 \times 9}{0.05} = 360 \]

metres, that is 360 m. A range of that length is possible outdoors but inconvenient, which is exactly why near field scanning and compact ranges were developed. The quadratic dependence on aperture means that doubling the dish quadruples the range needed, and the problem becomes impossible long before the antenna does.

Answer: B

A finite aperture cannot concentrate energy into a point, and diffraction gives a focal spot whose size is of the order of a wavelength across and rather more along the axis. That sets how accurately a feed must be positioned and how large it may be. Surface accuracy, frequency spread and angular spread all blur the focus further but the diffraction limit remains even when they are removed. A limit that survives making everything else ideal is an intrinsic one and is worth identifying as such.

Answer: C

By Babinet's principle the slot and the metal strip that would fill it are complementary, so the slot radiates the dipole pattern with electric and magnetic fields exchanged and the polarisation rotated by ninety degrees. Its impedance follows from the dipole's through the duality relation. Loops, waveguide apertures and monopoles are all different radiators with their own analyses. The value of the duality is that no fresh electromagnetic calculation is needed for the slot at all.

Answer: D

A slot is simply an opening in the skin, covered by a dielectric window, so it adds no drag and survives the mechanical and thermal environment far better than any protruding structure. Its gain is that of the equivalent dipole rather than anything higher, it certainly needs a feed behind the skin, and a plasma sheath attenuates any antenna. Choosing a radiator that suits the vehicle rather than one that suits the electrical problem alone is often the deciding consideration.