Antenna Measurements Questions and Answers

Q: Which near field scanning geometry suits a high gain antenna with a narrow beam?

A planar scan captures the forward hemisphere well and is the natural choice when almost all the energy is in a narrow forward beam, since the scan area need only be a little larger than the aperture. Cylindrical scanning suits antennas with a fan beam, and spherical scanning is needed for broad patterns where energy goes in every direction. Choosing the surface to match where the energy actually goes keeps the measurement time and the truncation error acceptable.

Q: What is truncation error in planar near field scanning?

Energy leaving the antenna at wide angles misses a finite scan plane, so the computed far field is reliable only within a cone determined by the scan size and the separation. Enlarging the scan or moving closer widens the valid region. Sampling spacing, probe correction and positioner accuracy are separate error sources, each with its own treatment. Knowing over what angular region a measurement is valid is as important as the measurement itself.

Q: Why must the probe pattern be corrected for in near field scanning?

The probe has a pattern of its own, so what it records is the field weighted by that pattern, and the transform must divide it out. Open ended waveguide probes are used because their patterns are smooth and well known. Gain is a scale factor removed by calibration, probe scattering is minimised by design and by separation, and loading is negligible at normal distances. Correcting for the instrument's own response is a standard step in any careful measurement.

Q: What is the usual sample spacing in a near field scan?

Sampling theory applied to the aperture requires about half a wavelength spacing to represent the field without aliasing, which is why a large aperture takes a long time to scan. Finer spacing wastes time without adding information, and coarser spacing aliases wide angle energy into the computed pattern. The parallel with the Nyquist criterion in signal processing is exact, since the aperture to far field relationship is a Fourier transform.

Q: How is gain most commonly measured in practice?

The substitution or gain transfer method compares the received level from the antenna under test with that from a calibrated standard on the same range, so every other factor in the path cancels. Pattern integration gives directivity rather than gain, dimensional calculation works only for simple horns, and absolute field measurement is difficult and rarely accurate. Making a measurement differential so that common errors divide out is the foundation of accurate metrology.

Q: What does the three antenna method determine?

Measuring the transmission between each of the three pairs gives three equations in the three unknown gains, which can be solved absolutely. It is how a primary gain standard is established without circular reference to another standard. Relative comparison, selection testing and repeat measurement are all different exercises. A scheme that extracts absolute values from a set of relative measurements is a powerful and widely reused idea.

Q: Why is directivity obtained by integrating the measured pattern rather than measured directly?

Directivity compares the peak intensity with the average over the whole sphere, so it requires knowledge of the pattern everywhere and follows by integration. Comparing the computed directivity with a measured gain then yields the radiation efficiency, which is one of the few practical routes to that quantity. Directivity deliberately excludes losses, integration is slower rather than quicker, and gain measurement is the more straightforward of the two.

Q: How is radiation efficiency obtained from pattern and gain measurements?

Since gain is directivity multiplied by efficiency, dividing the measured gain by the directivity obtained from the integrated pattern gives the efficiency directly. The accuracy depends on integrating the pattern over the whole sphere, including the back lobes, which is why spherical scanning is used for the purpose. The other options describe unrelated calculations. Obtaining a third quantity from two measured ones is often easier than measuring it at all.

Q: What is the Wheeler cap method used for?

Enclosing the antenna in a conducting cap suppresses radiation while leaving the ohmic losses unchanged, so the difference between the input resistances with and without the cap separates the two. It works best when the two resistances are comparable, which is exactly the case for a small antenna. Patterns cannot be measured inside a cap, calibration uses known standards, and resonance is read from the impedance sweep. Suppressing one mechanism to measure the other by difference is an elegant general technique.

11.

Which near field scanning geometry suits a high gain antenna with a narrow beam?

A.Spherical, because it captures the complete pattern in every direction
B.Conical
C.Planar
D.Cylindrical, because it covers all azimuth angles around the antenna

Answer: C

A planar scan captures the forward hemisphere well and is the natural choice when almost all the energy is in a narrow forward beam, since the scan area need only be a little larger than the aperture. Cylindrical scanning suits antennas with a fan beam, and spherical scanning is needed for broad patterns where energy goes in every direction. Choosing the surface to match where the energy actually goes keeps the measurement time and the truncation error acceptable.

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12.

What is truncation error in planar near field scanning?

A.Error caused by sampling the field at too coarse a spacing
B.Error caused by the probe pattern not being perfectly isotropic
C.Error caused by mechanical inaccuracy in the scanner positioner
D.Pattern error caused by the finite size of the scan plane

Answer: D

Energy leaving the antenna at wide angles misses a finite scan plane, so the computed far field is reliable only within a cone determined by the scan size and the separation. Enlarging the scan or moving closer widens the valid region. Sampling spacing, probe correction and positioner accuracy are separate error sources, each with its own treatment. Knowing over what angular region a measurement is valid is as important as the measurement itself.

Discuss in Forum

13.

Why must the probe pattern be corrected for in near field scanning?

A.The probe is not isotropic and weights the field it samples
B.The probe adds its own gain to the measured field values
C.The probe reflects part of the field back towards the antenna
D.The probe changes the impedance of the antenna under test

Answer: A

The probe has a pattern of its own, so what it records is the field weighted by that pattern, and the transform must divide it out. Open ended waveguide probes are used because their patterns are smooth and well known. Gain is a scale factor removed by calibration, probe scattering is minimised by design and by separation, and loading is negligible at normal distances. Correcting for the instrument's own response is a standard step in any careful measurement.

Discuss in Forum

14.

What is the usual sample spacing in a near field scan?

A.About two wavelengths, which halves the measurement time required
B.About half a wavelength
C.About one wavelength
D.About a tenth of a wavelength, to capture the fine structure

Answer: B

Sampling theory applied to the aperture requires about half a wavelength spacing to represent the field without aliasing, which is why a large aperture takes a long time to scan. Finer spacing wastes time without adding information, and coarser spacing aliases wide angle energy into the computed pattern. The parallel with the Nyquist criterion in signal processing is exact, since the aperture to far field relationship is a Fourier transform.

Discuss in Forum

15.

How is gain most commonly measured in practice?

A.By computing it from the physical dimensions of the aperture
B.By measuring the input power and the field at one metre distance
C.By comparison with a calibrated standard gain antenna
D.By integrating the measured pattern over the whole sphere

Answer: C

The substitution or gain transfer method compares the received level from the antenna under test with that from a calibrated standard on the same range, so every other factor in the path cancels. Pattern integration gives directivity rather than gain, dimensional calculation works only for simple horns, and absolute field measurement is difficult and rarely accurate. Making a measurement differential so that common errors divide out is the foundation of accurate metrology.

Discuss in Forum

16.

What does the three antenna method determine?

A.The relative gains of three antennas of the same design
B.The best of three candidate antennas for a particular application
C.The gain of one antenna measured three times for repeatability
D.Absolute gains without any previously calibrated antenna

Answer: D

Measuring the transmission between each of the three pairs gives three equations in the three unknown gains, which can be solved absolutely. It is how a primary gain standard is established without circular reference to another standard. Relative comparison, selection testing and repeat measurement are all different exercises. A scheme that extracts absolute values from a set of relative measurements is a powerful and widely reused idea.

Discuss in Forum

17.

Why is directivity obtained by integrating the measured pattern rather than measured directly?

A.Directivity is defined as a ratio over all directions
B.Directivity cannot be measured because it includes the antenna losses
C.Integration is quicker than making a comparison against a standard
D.The pattern is easier to measure accurately than the gain is

Answer: A

Directivity compares the peak intensity with the average over the whole sphere, so it requires knowledge of the pattern everywhere and follows by integration. Comparing the computed directivity with a measured gain then yields the radiation efficiency, which is one of the few practical routes to that quantity. Directivity deliberately excludes losses, integration is slower rather than quicker, and gain measurement is the more straightforward of the two.

Discuss in Forum

18.

How is radiation efficiency obtained from pattern and gain measurements?

A.By measuring the antenna temperature and applying the Nyquist relation
B.By dividing the measured gain by the computed directivity
C.By subtracting the measured gain from the physical aperture gain
D.By comparing the measured gain at two different frequencies

Answer: B

Since gain is directivity multiplied by efficiency, dividing the measured gain by the directivity obtained from the integrated pattern gives the efficiency directly. The accuracy depends on integrating the pattern over the whole sphere, including the back lobes, which is why spherical scanning is used for the purpose. The other options describe unrelated calculations. Obtaining a third quantity from two measured ones is often easier than measuring it at all.

Discuss in Forum

19.

What is the Wheeler cap method used for?

A.Calibrating a network analyser before an impedance measurement
B.Determining the resonant frequency of an antenna without a signal source
C.Measuring the efficiency of an electrically small antenna
D.Measuring the pattern of an antenna inside a small metal enclosure

Answer: C

Enclosing the antenna in a conducting cap suppresses radiation while leaving the ohmic losses unchanged, so the difference between the input resistances with and without the cap separates the two. It works best when the two resistances are comparable, which is exactly the case for a small antenna. Patterns cannot be measured inside a cap, calibration uses known standards, and resonance is read from the impedance sweep. Suppressing one mechanism to measure the other by difference is an elegant general technique.

Discuss in Forum

20.

Why does the Wheeler cap method work poorly for a highly efficient antenna?

A.The cap would have to be too large to be manufactured accurately
B.An efficient antenna does not radiate inside a conducting enclosure
C.The measurement frequency shifts when the cap is fitted over the antenna
D.The difference between the two resistances becomes a small residue

Answer: D

If the loss resistance is a hundredth of the radiation resistance, the capped and uncapped readings differ by one per cent and the efficiency is the difference of two nearly equal numbers, which is a poorly conditioned measurement. A differential method works best when the difference is a substantial part of the whole. Cap size, radiation behaviour and frequency shift are practical issues rather than the fundamental limitation. Knowing when a method is well conditioned is part of choosing it.

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