Common Mistakes When Specifying an Antenna Test Range

Specifying the Chamber Before Defining the Antenna

Specifying the preferred chamber size prior to specifying the antenna to be tested is probably the most frequent mistake. The antenna’s aperture, dimensions, weight, and method of mounting, operating frequency, beam width, polarization, and measurement angles influence the far field range.

For example, a high gain large antenna may require a much greater separation in the far field compared to a small antenna. The commonly known far field relation is R ≥ 2D²/λ, in which R is the separation distance, D is the largest antenna dimension, and λ is the wavelength. NASA documentation also notes that for some measurements, there may be a need for greater separation distance, depending on the aperture phase and the uncertainty.

Better approach:

• Define the largest antenna first.
• Identify the lowest operating frequency.
• Record antenna weight and mounting requirements.
• Define required angular coverage.
• Establish the required measurement accuracy.

Choosing Frequency Range Too Narrow

If future products are not considered, the antenna laboratories may not be able to support the bands of future antennas, higher frequencies, phased arrays, automotive radars, satellite systems, and other evolving wireless technologies.

Frequency affects the performance of absorbers. It influences the size of the chamber, the design of source antennas, cables and connectors used, as well as the measurement and calibration instruments.

A practical specification should therefore identify:

• Minimum frequency
• Maximum frequency
• Continuous or discrete frequency bands
• Required measurement bandwidth
• Future frequency expansion
• Instrumentation compatibility

The goal is not simply to specify the widest possible frequency range. Over-specification can increase cost without providing useful measurement capability.

Ignoring Quiet-Zone Requirements

A chamber consisting of RF absorbers is not an accurate chamber. The quality of the quiet zone must offer the desired electromagnetic environment for the measurement.

The quality of the quiet zone is measured by amplitude, phase, reflections, and uniformity in the spatial domain. The NASA range specifications illustrate a quiet zone by establishing limits of amplitude and phase, thus demonstrating that the quiet-zone criteria are measurable and not arbitrary.

Common mistakes include:

• Specifying quiet-zone size without defining the antenna under test.
• Asking for extremely tight uniformity without a measurement justification.
• Ignoring phase requirements for applications where phase accuracy matters.
• Assuming absorber coverage alone guarantees adequate quiet-zone performance.

Using the Wrong Test Method

Not all antennas require traditional far-field testing. Depending on the dimension, frequency, and scope of the coverage required, engineers may consider far-field, planar near-field, cylindrical near-field, spherical near-field, or compact antenna test range approaches.

Near-field systems use mathematical transformation techniques to obtain far-field information. The IEEE outlines the importance of calibration, probe compensation, echo reduction, positioning, and uncertainty analysis in near-field measurements.

Choosing the wrong measurement method can incur unnecessary space, cost, or measurement restrictions.

Forgetting the Complete Measurement Geometry

The measurement geometry consists of the transmitter, antenna under test, positioner, absorber boundaries, cables, support structure, and chamber walls.

A range specification should clearly establish:

Treating these items as independent results in integration issues.

Underestimating Reflections and Scattering

Unwanted reflections distort radiation patterns, gain, and sidelobe measurements. Chamber discontinuities, support structures, cables, doors, and other openings and paths for ventilation contribute to scattering.

IEEE technical notes state that chamber absorber configuration, ventilation, access paths, support structures, and discontinuities of chamber walls can affect the scattering of an anechoic chamber.

Therefore, the specification should address not just the type of chamber absorber material, but also the entire anechoic chamber.

Treating Positioning Accuracy as an Afterthought

Highly capable RF systems will not help resolve poor mechanical positioning. Poor mechanical positioning affects the final results due to factors such as Azimuth and elevation, polarization rotation, movement of the probes, and alignment of antennas.

When evaluating potential positioners, it is important to analyze the following specifications:

• Angular range
• Angular accuracy
• Repeatability
• Maximum payload
• Rotation speed
• Mechanical alignment
• Cable management
• Control-system integration

For automated antenna measurements, mechanical repeatability is most important, since small positioning discrepancies may become significant and complicate comparisons of measurement runs.

Ignoring Measurement Uncertainty

Uncertainty must be considered at the specification stage, and not after the chamber is constructed.

Some of the factors that affect uncertainty are cable losses, connector repeatability, receiver noise, calibration errors, positioning errors, reflections, alignment, dynamic range, and environmental effects. Both the IEEE and NASA material emphasize uncertainty as one of the more important aspects of antenna measurement.

Useful specifications indicate the required uncertainty or accuracy, and describe how the system will be checked.

Over-Specifying Everything

More performance is not better if the additional capability is unnecessary.

There are several examples where the performance requested may significantly increase the cost and complexity of the project, such as very large quiet zones, extremely low amplitude variation, ultra-wide frequency coverage, or extremely high positioning precision.

A good specification should connect each major performance criterion to a relevant measurement requirement.

Forgetting Future Expansion

Antenna laboratories often remain in service for many years. Designing just for the first project may make future upgrades much more difficult.

Consider:

• Larger antennas
• New frequency bands
• Additional positioners
• Automated measurement software
• Higher dynamic-range receivers
• Phased-array testing
• OTA requirements
• Additional calibration equipment

NASA has many antenna ranges with various sizes of antennas and frequencies illustrating why architecture needs to match the range and measurement application.

Pro Tip

Do not front load an antenna test range specification with chamber dimensions. Antenna test range specifications should begin with measurements requirements. Describe the antennas and frequency bands, accuracy, quiet-zone volume, angular coverage, test method, and future requirements then design the chamber and measurement system around those requirements.

Conclusion

The most expensive mistakes for an antenna test range occur before the first shovel is picked up and involve selecting a chamber without considering the quiet zone, replicating specs from other labs, and selecting a chamber based on the only available space. Limitations from these oversights often result in expensive and prolonged modifications.

A good antenna test range is a complete measurement system and not simply an absorbing room. Frequency, antenna size, test distance, quiet zone performance, mechanical positioners, RF instruments, calibration and measurement uncertainty need to work as an integrated system.

Diamond Microwave Chambers Ltd offers engineered solutions based on customer RF and antenna measurement requirements. DMC has the experience to design high confidence measurement systems that that enhance measurement capability in current domain and provide future flexibility.