Antenna Measurements Questions and Answers

Q: Why is a spinning linear source used to measure axial ratio?

As the source rotates, the received level rises and falls between the two axes of the ellipse, so the peak to trough ratio read from the record is the axial ratio at that angle. Spinning it fast while the positioner turns slowly indeed gives both the pattern envelope and the axial ratio in one sweep, which is a real convenience but follows from the method rather than motivating it. Signal strength and source polarisation knowledge are not the argument.

Q: What is a polarisation efficiency?

When the incident and receiving polarisations differ, only part of the available power is captured, and that fraction is the polarisation efficiency, unity for a perfect match and zero for orthogonal linear polarisations. A linear antenna in a circular field captures half, which is the familiar three decibel loss. The other options describe cross polar discrimination and component performance. A mismatch loss that depends only on the geometry of two vectors is easy to compute once it is recognised as such.

Q: What is cross polar discrimination?

Cross polar discrimination states how far below the wanted polarisation the unwanted orthogonal component lies, usually specified on boresight and over the coverage angles. It limits how closely two polarisations can reuse a frequency. Port isolation is a related but separate component specification, and rearward rejection is the front to back ratio. Distinguishing an antenna property from a component property matters when a system specification has to be apportioned between them.

Q: What is a positioner in an antenna measurement system?

The positioner turns the antenna under test in azimuth and elevation, or in roll, and reports its angle to the acquisition system, so its accuracy and repeatability enter every measured pattern directly. Multi axis positioners allow arbitrary pattern cuts. The receiver records level, software performs the transform, and the source mount is a separate structure. Mechanical accuracy is a first order contributor to an electrical measurement, which is easy to forget when reading a pattern plot.

Q: What is an open area test site used for?

An open area test site is a flat, clear, conducting ground plane used to measure the radiated emissions of equipment against regulatory limits, with the receiving antenna scanned in height to find the maximum. Semi anechoic chambers now do much of the same work. Gain measurement, environmental testing and standard calibration are separate activities. The site is defined by the standard it serves rather than by any antenna property, which is why its validation is prescribed in such detail.

Q: Why is the receiving antenna scanned in height on an open area test site?

The ground reflection interferes with the direct wave, so the received level varies strongly with antenna height, and the standard requires the maximum over a stated height range to be reported. Without the scan the result would depend on an arbitrary choice. It is not a pattern measurement, far field conditions are set by the distance, and environmental averaging is not the aim. Making a procedure yield a defined worst case is what allows results to be compared between laboratories.

31.

What does an axial ratio of unity indicate?

A.An antenna radiating no power at all in that direction
B.Equal gain in the two principal planes of the radiation pattern
C.Perfectly circular polarisation
D.Perfectly linear polarisation aligned with the reference antenna

Answer: C

Axial ratio is the ratio of the major to the minor axis of the polarisation ellipse, so unity, or zero decibels, means the ellipse is a circle. An infinite ratio means linear polarisation. It says nothing about the amount of power radiated or about the beamwidths in the two planes, which are separate properties. Keeping the polarisation ellipse in mind, with its two axes and its tilt, makes every polarisation quantity straightforward to interpret.

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32.

Why is a spinning linear source used to measure axial ratio?

A.It provides a stronger signal than a circularly polarised source would
B.It avoids the need to know the polarisation of the source antenna
C.It allows the pattern and the axial ratio to be recorded in one sweep
D.Its rotation traces the polarisation ellipse directly on the recorder

Answer: D

As the source rotates, the received level rises and falls between the two axes of the ellipse, so the peak to trough ratio read from the record is the axial ratio at that angle. Spinning it fast while the positioner turns slowly indeed gives both the pattern envelope and the axial ratio in one sweep, which is a real convenience but follows from the method rather than motivating it. Signal strength and source polarisation knowledge are not the argument.

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33.

What is a polarisation efficiency?

A.The fraction of incident power a receiving antenna can accept given the mismatch
B.The proportion of the transmitted power radiated in the wanted polarisation
C.The ratio of the cross polar to the co-polar gain of an antenna
D.The efficiency with which a polariser converts linear to circular polarisation

Answer: A

When the incident and receiving polarisations differ, only part of the available power is captured, and that fraction is the polarisation efficiency, unity for a perfect match and zero for orthogonal linear polarisations. A linear antenna in a circular field captures half, which is the familiar three decibel loss. The other options describe cross polar discrimination and component performance. A mismatch loss that depends only on the geometry of two vectors is easy to compute once it is recognised as such.

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34.

What is cross polar discrimination?

A.The rejection an antenna offers to signals arriving from behind it
B.The ratio of co-polar to cross polar response in a given direction
C.The difference in gain between two orthogonally polarised antennas
D.The isolation between the two ports of an orthomode transducer

Answer: B

Cross polar discrimination states how far below the wanted polarisation the unwanted orthogonal component lies, usually specified on boresight and over the coverage angles. It limits how closely two polarisations can reuse a frequency. Port isolation is a related but separate component specification, and rearward rejection is the front to back ratio. Distinguishing an antenna property from a component property matters when a system specification has to be apportioned between them.

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35.

What is a positioner in an antenna measurement system?

A.The software that computes the far field from near field data
B.The device that holds the source antenna at a fixed height and angle
C.The mechanism that rotates the antenna to each measurement angle
D.The instrument that records the received amplitude against angle

Answer: C

The positioner turns the antenna under test in azimuth and elevation, or in roll, and reports its angle to the acquisition system, so its accuracy and repeatability enter every measured pattern directly. Multi axis positioners allow arbitrary pattern cuts. The receiver records level, software performs the transform, and the source mount is a separate structure. Mechanical accuracy is a first order contributor to an electrical measurement, which is easy to forget when reading a pattern plot.

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36.

Why must the antenna under test be aligned with the positioner axes?

A.The positioner cannot support an antenna that is not centred on it
B.Alignment is required for the far field distance calculation to hold
C.Misalignment would change the impedance measured at the antenna port
D.Misalignment displaces the measured pattern and blurs the boresight

Answer: D

If the antenna's phase centre and axes do not coincide with the rotation axes, the antenna translates as well as rotates, and the measured pattern is distorted and its boresight displaced. Careful optical or laser alignment is therefore part of the setup. Mechanical support, far field distance and terminal impedance are not affected in the same way. A measurement geometry that departs from the assumed one produces errors that look like antenna faults, which is why alignment is checked first.

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37.

What is an open area test site used for?

A.Radiated emission measurement for electromagnetic compatibility
B.Measuring the gain of large reflector antennas at long range
C.Testing antennas intended for use in the open rather than indoors
D.Calibrating standard gain horns against a primary reference

Answer: A

An open area test site is a flat, clear, conducting ground plane used to measure the radiated emissions of equipment against regulatory limits, with the receiving antenna scanned in height to find the maximum. Semi anechoic chambers now do much of the same work. Gain measurement, environmental testing and standard calibration are separate activities. The site is defined by the standard it serves rather than by any antenna property, which is why its validation is prescribed in such detail.

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38.

Why is the receiving antenna scanned in height on an open area test site?

A.To average out the effect of nearby buildings and vehicles
B.To find the height at which direct and reflected waves add
C.To measure the pattern of the equipment under test in elevation
D.To ensure the antenna is in the far field of the equipment

Answer: B

The ground reflection interferes with the direct wave, so the received level varies strongly with antenna height, and the standard requires the maximum over a stated height range to be reported. Without the scan the result would depend on an arbitrary choice. It is not a pattern measurement, far field conditions are set by the distance, and environmental averaging is not the aim. Making a procedure yield a defined worst case is what allows results to be compared between laboratories.

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39.

What does normalised site attenuation validate?

A.That the equipment under test meets the applicable emission limit
B.That the ground plane has sufficient conductivity for the measurement
C.That a test site behaves as an ideal site over its frequency range
D.That the receiving antenna has the gain stated on its certificate

Answer: C

Comparing the measured attenuation between two antennas with the theoretical value for an ideal site reveals reflections and other departures, and the site passes only if the difference stays within a few decibels. It validates the site rather than any antenna or product. Antenna calibration, product compliance and ground plane conductivity are checked separately. A procedure that tests the measurement environment before the product is a hallmark of a mature measurement discipline.

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40.

Why is a semi anechoic chamber used rather than a fully anechoic one for emission testing?

A.Absorber on the floor would be damaged by the equipment being placed on it
B.A fully anechoic chamber cannot be made large enough for the purpose
C.The reflecting floor increases the signal and improves the sensitivity
D.The standard is written around a reflecting ground plane

Answer: D

Emission limits were established on open area test sites with a conducting ground plane, so the chamber reproduces that geometry with absorber on the walls and ceiling only in order to give comparable results. Damage, size and sensitivity are not the reasons, although the ground reflection does raise the maximum received level at the right height. When a limit is defined by a measurement method, the method must be reproduced rather than improved upon.

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