Antenna Measurements Questions and Answers

Answer: A

A metal room with a stirrer produces a statistically uniform, randomly polarised field, so the power received or radiated can be averaged over all directions and polarisations without measuring any pattern. That yields total radiated power and hence efficiency very quickly. Pattern and impedance are measured elsewhere, and polarisation information is deliberately averaged away, which is precisely what makes the measurement fast and orientation independent.

Answer: B

Because a phone is held at unpredictable angles against a hand and head, no single direction characterises it, and the power summed over all directions predicts performance far better. Network operators specify it for that reason. Regulators specify emissions and exposure rather than patterns, handset patterns are routinely measured on spherical scanners, and the case is closed for either measurement. A figure of merit should match how the device is actually used.

Answer: C

The Y factor method compares the receiver output with hot and cold references of known temperature and solves for the unknown, and pointing the antenna at cold sky and at an absorber provides those references. Physical temperature is not the same as noise temperature, pattern integration over sky brightness is a computation rather than a measurement, and a signal generator measures gain rather than noise. Using two known states to solve for an unknown is the same idea as the three antenna gain method.

Answer: D

Y is the measured ratio of receiver output power with a hot load and with a cold one, and from it the noise temperature of the receiver or of the antenna follows algebraically. A large Y factor means a quiet receiver and a well conditioned measurement. Gain over temperature is the station figure of merit, the split between antenna and receiver noise is what the measurement resolves, and bandwidth ratio is unrelated. One measured ratio and two known temperatures determine the unknown.

Answer: A

At microwave frequencies away from absorption lines the zenith sky is only a few kelvin, which is a convenient and repeatable cold reference against an ambient absorber at about two hundred and ninety. The difference between the two is large, which conditions the measurement well. The sky is a noise source rather than a calibrated signal, side lobes still see warm ground, and interference rejection is incidental. Nature supplying a stable reference is a convenience worth exploiting.

Answer: B

A vector network analyser sweeps the band and reports complex reflection coefficient, provided it has been calibrated with known standards at the plane where the antenna connects. The slotted line is the classical method it replaced and is still instructive. Power comparison and resistor substitution do not give phase. The essential discipline is that the calibration plane must be where the measurement is meant to apply, or the cable becomes part of the antenna.

Answer: C

Calibration removes the effect of everything between the instrument and the reference plane, so any cable beyond that plane is measured as part of the antenna and rotates the impedance on the chart. Measuring at the instrument port rather than at the antenna is a classic error that makes a bad match look acceptable. Frequency range and delivered power are set by other considerations. Knowing exactly what is included in a measurement is half of interpreting it.

Answer: D

Transforming a swept frequency measurement to the time domain separates the direct path from later reflections, which can then be windowed out before transforming back. It allows respectable measurements in rooms that are far from anechoic. Impulse response is what the transform shows rather than the purpose, and synchronisation and averaging are separate features of the instrument. Separating signals by arrival time is the same principle radar uses, applied here to a metrology problem.

Answer: A

Resolution in time is inversely proportional to the swept bandwidth, so a reflection whose extra path length is short compared with that resolution overlaps the direct arrival and cannot be gated out. Reflections from close structures are therefore the hardest to remove. The transform is routine, gating does distort the band edges but not by removing low frequencies as such, and antenna size is not a restriction. A resolution limit set by bandwidth appears wherever time and frequency are exchanged.

Answer: B

Rotating a linear source antenna about the line of sight traces out the polarisation pattern, whose maximum to minimum ratio is the axial ratio and whose orientation gives the tilt angle. A spinning dipole recorded against angle is the classical implementation. Frequency comparison, impedance measurement and pattern integration measure other things entirely. Measuring a vector property requires varying the orientation of the reference, which is what the rotation provides.