Antenna Measurements Questions and Answers

Answer: A

Boresight error is the mismatch between where the antenna points mechanically and where its beam actually points, and it must be surveyed and built into the pointing model of any tracking system. Positioner uncertainty, pointing loss and beamwidth error are separate quantities measured in different ways. Distinguishing an electrical direction from a mechanical one is essential in every system where a beam must be aimed at something, and it is a frequent source of unexplained pointing offsets.

Answer: B

An optical or laser survey establishes the mechanical axis, and the measured pattern gives the electrical beam direction, with the difference between them being the error. A difference channel null gives a sharper reference than the beam peak where one is available. Nulling the whole signal, calculating from drawings and measuring impedance do not yield the quantity. Comparing an electrical measurement with an independent mechanical one is what makes the error identifiable at all.

Answer: C

Near a maximum the pattern is flat, so a large angular error produces a small change in level, whereas through a null the response falls steeply and reverses phase, giving a sensitive and signed indication. That is why monopulse tracking, direction finding loops and boresight setting all work on nulls. Signal strength does affect the null depth measured, nulls are not automatically on any mechanical axis, and peaks can be located, just less precisely.

Answer: D

Cables flex as the positioner rotates, so a loose or damaged connection changes its loss and phase with angle, and the result is a pattern with irregular ripples that repeat with the rotation. Rotary joints and careful cable management exist for this reason. A constant loss would indeed affect only the level, which is why the distinction matters. Recognising that an error correlated with the measurement variable will masquerade as real data is an important habit.

Answer: A

A rotary joint allows unlimited rotation while maintaining a stable electrical path, avoiding the flexing and eventual failure of a cable wrapped around the axis. Its own loss and its variation with angle are specified because they enter the measurement directly. Isolation, impedance transformation and angular readout are performed by other components. Any part whose properties vary with the measurement variable must be characterised, and a rotary joint is the clearest example on a range.

Answer: B

A regular ripple is the signature of two signals interfering, and in a chamber that means a reflection from a wall, the floor or the positioner structure. Its period in angle indicates how far away the reflector is. Limited dynamic range raises the noise floor without producing a periodic ripple, defocusing broadens the beam and fills the nulls, and an encoder error distorts the angle scale rather than the level. Periodic structure in data almost always points at a second path.

Answer: C

The interference pattern has a period set by the path length difference between the direct and reflected routes, so measuring it points at how far off axis the reflector lies, and placing absorber there confirms it. Amplitude indicates how strong the reflection is rather than where it comes from, and neither a vanishing frequency nor polarisation identifies the surface. Extracting geometry from the structure of an interference pattern is the same reasoning used in radio astronomy interferometry.

Answer: D

The absorber lining attenuates any wave that strikes a surface, so the antenna sees essentially only the direct path from the source. Keeping external signals out is the job of the metal shell behind the absorber, personnel safety is handled by interlocks, and the positioner holds the antenna. Separating the two functions of a chamber, shielding by the metal and anechoic behaviour by the absorber, makes clear which part of the structure does what.

Answer: A

A range yields gain by comparison, polarisation and axial ratio by rotating the source, and with a full spherical scan the directivity and hence the efficiency. Impedance is measured separately at the antenna terminals, dimensions are a mechanical check, and transmitter consumption is not an antenna quantity. Listing what a facility can and cannot measure is the first step in planning a test programme and in deciding what other equipment will be needed.

Answer: B

A pattern spans fifty or sixty decibels between the main beam and the deepest measured side lobes, and only a logarithmic scale shows both on one plot. On a linear scale everything below a few per cent of the peak would be invisible. Receivers report in whatever units are chosen, standards follow practice rather than dictating it, and the main beam is clear on either scale. Choosing a scale that reveals the features of interest is a basic part of presenting data.