Antenna Measurements Questions and Answers

Answer: A

Features belonging to the antenna rotate with it, while reflections from the chamber and the mount stay where they are, so comparing the two records separates the two classes immediately. It is one of the most useful diagnostic tricks on a range. Noise averaging, encoder checking and polarisation measurement are separate exercises. Changing one variable to see what moves with it is the essence of experimental diagnosis, and it applies here as directly as anywhere.

Answer: B

A genuine antenna feature moves with the antenna, so anything that stays put in the room must be produced by the range, the mount or an external interferer. That observation converts a puzzling pattern into a localisable fault. A slipping positioner would show no rotation at all rather than a fixed feature on a rotating pattern, and a receiver fault would not produce an angularly stable artefact. Asking what a feature is attached to is the quickest route to its cause.

Answer: C

Every antenna quantity changes across its operating range, and a specification is written over the band rather than at a point, so band edges and centre are measured as a minimum. For a broadband antenna many more points are needed. Calibration is performed across the range, no universal three frequency rule exists, and averaging across frequency would conceal exactly the variation being sought. A quantity that varies must be characterised over the range in which it is used.

Answer: D

Sweeping records the quantity as a function of frequency in one operation, which is how impedance and return loss are normally measured and how time domain gating obtains its data. Sweeping angle at fixed frequency is a pattern cut, which is the complementary operation. Band averaging discards the variation rather than recording it. Most antenna characterisation consists of sweeping one variable while holding the other, and knowing which is which keeps the data organised.

Answer: A

A network analyser measures across frequency, and the inverse Fourier transform of that response is the time domain response, in which paths of different length appear separated. Gating and transforming back then removes the unwanted ones. Generating a true impulse is indeed difficult, which is part of why the swept method is used, but the transform relationship is the reason it works. The antenna is linear and responds identically to either excitation.

Answer: B

Removing the late arrivals suppresses the room's reflections numerically, so an ordinary laboratory can yield measurements approaching chamber quality for many purposes. It cannot help with the far field distance, which is a geometric requirement, nor with polarisation, and the receiver's own dynamic range is unchanged. A technique that substitutes signal processing for physical facilities is attractive precisely because facilities are expensive, provided its limits are respected.