How to Reduce Measurement Errors in Anechoic Chambers

Validation of the Quiet Zone of the Chamber

The quiet zone of the chamber means that the electromagnetic environment in this zone is under control. Reflectors from the chamber walls, supports, positioners, and any other items may affect antenna measurements.

Periodic validation of the chamber may help reveal unwanted reflectors and variations in the measurement volume.

These key tests should be considered:

• Quiet zone reflectivity
• Site VSWR when relevant
• Field uniformity
• Measurement volume
• Influence of positioners and supports
• Performance over the full frequency range

A chamber should be calibrated on installation and each time changes are made to the chamber or measurement setup.

Optimize Antenna Alignment and Positioning

Any slight misalignment may create differences in measurements, especially in case of testing very directional antennas with narrow beam widths. Antenna alignment, turntable positioning, and pointing errors are listed as the key factors that lead to measurement uncertainties according to standards and technical information.

To prevent them:

• Use accurate mechanical positioners.
• Check the phase center and reference point of the antenna.
• Align the transmitter and receiver antennas accurately.
• Ensure that the turntable is rotating accurately.
• Maintain consistent antenna height and orientation.
• Avoid unnecessary movement of the test setup.

Repeatability checks can also reveal whether a positioning system is introducing a systematic error.

Calibration of the Measurement Equipment

The quality of the RF measurement process is highly dependent upon the calibration and maintenance of equipment used in the measurement process. Network analyzers, receivers, signal generators, power meters, cables and antennas need to be properly calibrated.

Calibration should take into account:

• Frequency accuracy of instruments
• Amplitude accuracy
• Cable loss
• Connector repeatability
• Antenna characteristics
• Receiver dynamic range
• Impedance matching

According to NIST, cable loss, reflections, positioning factors and other sources of uncertainties associated with equipment are listed.

Elimination of Reflections and Interfering Objects

An anechoic chamber eliminates reflections; however, it cannot deal with all objects that may appear in the test environment. Metallic construction fixtures, cables, positioners and other objects can interfere with RF radiation.

In order to prevent these factors:

• Do not bring unnecessary equipment into the chamber
• Use RF absorbing material when needed
• Route cables properly.
• Keep the DUT within the validated quiet zone.
• Check that fixtures do not significantly disturb the electromagnetic field.

The chamber should be operated within the conditions under which its performance has been validated.

Maintain Correct Test Distance

Antenna measurements may require far-field conditions. It varies based on the antenna or the dimensions of the device under test and the wavelength. The ETSI guidelines state that the range length of the chamber is required to be sufficient for making far-field measurements and the DUT should stay within the quiet zone verified.

Prior to testing, verify:

• Distance between the Antenna and the DUT
• Dimensions of the DUT
• Operating Frequency
• Wavelength
• Far-field condition required
• Dimensions of the available quiet-zone

Testing beyond the required conditions for measurement may lead to increase in uncertainties.

Test Using the Same Test Method

Repeatability is the simplest way of finding out problems in measurement. The same setup, equipment configuration, cable routing, antenna orientation, and software settings should be used when comparing results.

Pro Tip:

Do not treat chamber performance and measurement-system accuracy as the same thing. A chamber can have excellent RF characteristics while the overall measurement still contains significant uncertainty from antennas, cables, instruments, positioning, or the DUT. Build an uncertainty budget that considers the complete measurement chain.

Document and Review Measurement Uncertainty

A reliable RF testing program should document the major sources of uncertainty instead of reporting only the final measurement value.Approaches based on theoretical analysis, simulations, and modifications of the measurement system are recommended by NIST in order to assess uncertainty sources.

In relation to each measurement campaign, the following should be documented:

• Equipment name and calibration status
• Antenna type and alignment
• Test distance
• Chamber setup
• DUT location;
• Frequency range;
• Environmental conditions, where applicable
• Repeatability results; and
• Uncertainty in measurement estimates.

Efficient reduction of measurement errors in anechoic chambers entails a series of measures. RF measurement reliability results from quiet zone verification, precise positioning of the antennas, equipment calibration, reflection control, correct distance for testing, repeatability, and uncertainty analysis.

In cases where a company is seeking a reliable RF and EMC measurement environment, Diamond Microwave Chambers Ltd can assist in designing and validating the chamber solution necessary for your application.