Antenna Measurements Questions and Answers

Answer: A

Closer than about two D squared over lambda the phase across the aperture is not flat, so the measured pattern still changes with range and does not represent the published one. Signal is in fact stronger nearer, matching is a terminal property independent of range, and polarisation is established immediately. Everything published about an antenna implicitly refers to the far field, so a measurement that does not reach it is describing something else.

Answer: B

An elevated range puts both antennas high enough, and uses directive source antennas and fences, so that the ground reflected path is suppressed relative to the direct one. It is the classical outdoor arrangement for large antennas. Safety, atmospheric thickness and lifting equipment are not the reasons. Every range design is fundamentally an answer to the same two questions: how to obtain far field conditions and how to suppress unwanted paths.

Answer: C

Rather than fighting the reflection, this design chooses the geometry so that the direct and reflected waves combine into a smooth taper across the test aperture, giving a controlled illumination. It suits lower frequencies where absorber and elevation are impractical. Absorber covering describes a different treatment, ground level mounting would not work, and measuring the ground is a separate exercise. Turning an unavoidable second path into part of the design is an unusually neat piece of engineering.

Answer: D

Placing the source antenna on a tower and the antenna under test on the ground, or the reverse, tilts the path so that the ground reflection point moves to where it can be absorbed or where the source pattern is low. It is cheaper than two tall towers. Pattern cuts, installation geometry and path length are not what the term refers to. The range length is set by the far field criterion regardless of the direction the path takes.

Answer: A

Pyramidal or wedge absorber presents a gradual transition from air to lossy material, so a wave entering it is attenuated rather than reflected, typically by forty decibels or more. The shielding against outside signals, and the containment of radiation, are provided by the metal walls behind the absorber rather than by the absorber itself. Climate control is a separate service. Separating the shielding function from the anechoic function clarifies what each part of the chamber does.

Answer: B

A flat slab of lossy material would present an abrupt change of impedance and reflect strongly, whereas a pyramid grades the effective impedance from that of air to that of the absorber over several wavelengths. Performance therefore degrades below the frequency at which the pyramids are a wavelength or so long. Surface area, scattering and strength are incidental. Gradual transitions to avoid reflection is the same idea that shapes a horn and a tapered transmission line.

Answer: C

Absorber works by grading the impedance over a distance, so at long wavelengths the pyramids are electrically short and reflect much more. Ferrite tiles are added for the lowest frequencies because they absorb in a thin layer. Metal shielding improves rather than worsens at low frequency, chamber size is a separate constraint that also matters, and positioning accuracy is unaffected. A treatment whose thickness is fixed always has a lowest frequency at which it still works.

Answer: D

A compact range uses a large offset reflector, with a rolled or serrated edge, to collimate the feed radiation into a plane wave across a quiet zone only metres from the reflector. That gives far field conditions indoors. Portability, deliberate near field measurement and scale modelling are all different ideas, though scale modelling is a legitimate technique in its own right. Substituting optics for distance is the essential trick.

Answer: A

A sharp rim diffracts, and that diffracted field interferes with the collimated wave to produce ripples across the quiet zone, which would corrupt every measurement made in it. Rolling the edge away or serrating it spreads the diffraction harmlessly. Stiffness, assembly and spillover absorption are handled separately. Edge treatment appears wherever a finite aperture must produce a clean field, and the same reasoning applies to earth station rim shrouds.

Answer: B

A probe records the complex field at a grid of points on a plane, cylinder or sphere, and a transform yields the far field pattern. Phase is essential, so the scanner's mechanical accuracy must be a small fraction of a wavelength and the instrumentation must be phase stable. Total power is obtained by other means, impedance is a terminal quantity, and surface profiling is a mechanical measurement, though the same near field data can reveal surface errors as a by-product.