Antenna Measurements Questions and Answers

Answer: A

Antenna factor, in decibels per metre, is added to the receiver reading in decibels above a microvolt to give the field strength in decibels above a microvolt per metre. It is on the calibration certificate of every measurement antenna and varies with frequency. Gain conversion, pattern correction and aperture relations are separate calculations. A single calibrated factor that turns an instrument reading into a physical quantity is what makes a measurement traceable.

Answer: B

Antenna factor depends on the effective length, which is a function of the antenna's electrical size, and on the impedance match, both of which vary across the band. That is why the certificate is a table or a curve rather than a single number. Receiver impedance is constant, calibration is performed across the range, and cable loss is accounted separately. A correction factor that varies must be applied at the frequency of the measurement, not at some nominal value.

Answer: C

An uncertainty budget enumerates every contribution, from mismatch and cable loss to site imperfection and instrument drift, and combines them, usually in quadrature, into a stated uncertainty. Compliance decisions then account for it explicitly. Cost, product tolerance and pass margin are different things, though the last is affected by the uncertainty. A result quoted without an uncertainty is an incomplete measurement, whatever the number of digits it carries.

Answer: D

Independent random contributions add as the square root of the sum of squares, which is the correct statistical treatment and reflects that they are unlikely all to be at their extremes at once. Correlated contributions must be added linearly instead, which is why the independence has to be justified rather than assumed. The result is indeed smaller, but that is a consequence rather than a reason, and unidentified contributions are not covered by any method of combination.

Answer: A

When two components are imperfectly matched, multiple reflections between them add to the signal with an unknown phase, so the measured level has an uncertainty set by the product of the two reflection coefficients. Attenuators between them reduce it. The other options describe the antenna's own impedance, a specification comparison and a calibration error. An uncertainty arising from the interaction of two components, rather than from either alone, is easy to overlook in a budget.

Answer: B

An attenuator reduces the reflected wave twice over, so the interaction between two imperfectly matched components is much diminished and the mismatch uncertainty falls. The price is a loss of signal, which is acceptable when there is margin. It does not raise the level, does not equalise a cable, and protection is a separate concern. Trading signal to noise ratio for a reduction in systematic uncertainty is a legitimate and often worthwhile bargain.

Answer: C

Its gain can be computed accurately from its dimensions and verified by the three antenna method, which makes it the working standard for gain transfer measurements. Feeds, polarisation sources and instrument calibration are separate roles, though a horn may be pressed into any of them. Predictability rather than performance is what qualifies an antenna as a standard, and the pyramidal horn is predictable because its aperture field follows from the waveguide mode.

Answer: D

Directing the source at the test antenna puts less energy on the floor, walls and range structure, so the reflected paths that corrupt the measurement are weaker. It must not be so directive that the illumination across the test aperture becomes uneven, which is the competing requirement. Field strength is a secondary benefit, polarisation is set by orientation, and the far field distance depends on the aperture sizes. Every range design balances taper against stray illumination.

Answer: A

If the source pattern falls appreciably across the width of the antenna under test, the illumination is not the uniform plane wave the far field assumption requires, and the measured gain and pattern are in error. A taper below about a quarter of a decibel is the usual criterion. The antenna's own taper is a design property, pattern roll off is the measurement itself, and range loss is accounted separately. A range must supply a plane wave in amplitude as well as in phase.

Answer: B

The two D squared over lambda criterion corresponds to a maximum path error of a sixteenth of a wavelength, that is about twenty two and a half degrees of phase, across the aperture. That is small enough that the measured gain is within about a tenth of a decibel. Zero would indeed require infinite range, which is why a tolerance is defined at all, and the larger figures would produce serious errors. Every practical criterion is a tolerance rather than an ideal.