Electronics Guide

Accurate measurement of radiated electromagnetic emissions is fundamental to ensuring electronic products comply with regulatory standards and function reliably in their intended environments. Radiated emission measurement quantifies the electromagnetic energy that electronic devices unintentionally broadcast into the surrounding space, enabling engineers to identify compliance issues, troubleshoot problems, and verify the effectiveness of mitigation strategies.

The science of radiated emission measurement has evolved significantly since the early days of electromagnetic compatibility (EMC) testing. Modern test facilities and equipment provide precise, repeatable measurements across frequency ranges spanning from tens of kilohertz to beyond 40 GHz. Understanding the various measurement methodologies, test environments, and equipment requirements enables engineers to select appropriate approaches for their specific applications and interpret results correctly.

Open Area Test Sites (OATS)

Open area test sites represent the traditional reference standard for radiated emission measurements and remain the benchmark against which alternative test methods are validated. An OATS provides a controlled outdoor environment that approximates free-space propagation conditions while incorporating a ground plane that creates predictable reflections.

OATS Construction and Requirements

A properly constructed OATS consists of a flat, unobstructed area with a conductive ground plane, typically constructed from metal mesh or solid metal sheets laid over prepared earth. The ground plane must extend beyond the measurement area by at least the measurement distance in all directions to ensure that reflections from ground plane edges do not affect measurements. For a 10-meter test site, the ground plane typically measures at least 30 meters in diameter or equivalent rectangular dimensions.

The site must be free of reflecting objects within the elliptical zone defined by the antenna and equipment under test (EUT) positions as focal points. Buildings, fences, overhead wires, trees, and even vehicles can create reflections that distort measurements. The ideal OATS location is remote from such structures and from sources of ambient electromagnetic interference.

Site attenuation measurements verify that an OATS meets performance requirements. This procedure measures the transmission between two antennas positioned as they would be during actual testing and compares results to theoretical calculations for an ideal site. Deviations indicate problems with ground plane conductivity, site geometry, or reflecting obstacles. ANSI C63.4 and CISPR 16-1-4 specify site validation procedures and acceptable tolerances.

OATS Measurement Procedure

During OATS measurements, the EUT is placed on a non-conductive turntable at a specified height above the ground plane, typically 0.8 to 1 meter for tabletop equipment or on the ground for floor-standing equipment. The receiving antenna is positioned at the specified measurement distance, commonly 3, 10, or 30 meters. The EUT rotates through 360 degrees of azimuth while the antenna scans vertically from 1 to 4 meters height to find the maximum emission level at each frequency.

This maximization procedure accounts for the complex interaction between direct radiation, ground reflection, and the EUT's directional emission pattern. At some frequencies, the direct and reflected signals add constructively; at others, they partially cancel. By varying EUT orientation and antenna height, the procedure finds the worst-case combination at each frequency.

Both horizontal and vertical antenna polarizations must be tested because emissions may be predominantly polarized in either direction depending on their source structure within the EUT. The higher emission level at each frequency determines compliance.

OATS Advantages and Limitations

OATS measurements provide excellent correlation with real-world electromagnetic environments because they replicate the ground reflection geometry that exists in actual use. The outdoor location allows testing of large equipment without facility size constraints. OATS remains the reference method specified in many standards for type approval and certification testing.

However, OATS testing has significant practical limitations. Weather dependence restricts testing to acceptable conditions, and ambient electromagnetic noise from broadcast stations, cellular systems, and other sources can mask device emissions or create false readings. Site preparation and maintenance costs are substantial, and testing is time-consuming due to the mechanical maximization procedure. These factors have driven the development of alternative test methods that provide equivalent results in more controlled environments.

Semi-Anechoic Chambers

Semi-anechoic chambers have become the predominant facilities for radiated emission compliance testing, providing controlled indoor environments that simulate OATS conditions while eliminating weather dependence and ambient interference. These chambers represent a substantial investment but offer superior convenience, reproducibility, and throughput compared to outdoor testing.

Chamber Construction

A semi-anechoic chamber consists of a shielded enclosure lined with radio frequency absorbing material on the walls and ceiling, while the floor remains a conductive surface serving as the ground plane. The shielding, typically constructed from welded steel panels or multiple layers of sheet metal, prevents external signals from entering and internal signals from escaping. Shielding effectiveness of 80-100 dB is typical for compliance-grade chambers.

The absorbing material, usually pyramidal foam or ferrite tile construction, eliminates reflections from walls and ceiling that would otherwise create standing waves and measurement errors. Pyramidal absorbers consist of carbon-loaded polyurethane foam shaped into cones that provide graduated impedance matching from free space to the lossy material. The pyramid height determines the lowest effective frequency, with taller pyramids required for lower frequencies. A chamber effective down to 30 MHz might use pyramids 1.5 to 2 meters tall.

Ferrite tile absorbers use sintered ferrite material that absorbs electromagnetic energy through magnetic losses. They are thinner than pyramidal absorbers and effective at lower frequencies but more expensive. Many chambers use hybrid approaches with ferrite tiles behind shorter pyramidal absorbers to achieve broadband performance in less space.

Chamber Qualification and Validation

Before a semi-anechoic chamber can be used for compliance testing, it must be validated to demonstrate equivalence to an ideal OATS. The normalized site attenuation (NSA) measurement procedure compares transmission between reference antennas in the chamber to theoretical values for an ideal site. CISPR 16-1-4 specifies that NSA must fall within plus or minus 4 dB of theoretical values across the frequency range of use.

Site voltage standing wave ratio (SVSWR) measurements provide additional validation for frequencies above 1 GHz where NSA measurements become less reliable. This procedure characterizes the uniformity of the electromagnetic environment by measuring how much the received signal varies as a probe antenna moves through the test volume. Lower SVSWR indicates better absorber performance and more uniform fields.

Chamber validation must be repeated periodically and after any changes to the chamber structure or absorber configuration. Many facilities perform annual revalidation to ensure continued compliance with site qualification requirements.

Semi-Anechoic Chamber Measurement Procedures

Measurement procedures in semi-anechoic chambers closely parallel OATS procedures. The EUT is placed on a non-conductive turntable at the specified height, rotated through azimuth angles while the receiving antenna scans through heights. Both polarizations are tested, and the maximum emission at each frequency determines compliance.

Modern chambers often incorporate automated systems that control turntable rotation, antenna positioning, and measurement receiver operation. These systems can perform complete emission scans in a fraction of the time required for manual testing, improving throughput and reducing labor costs. Automated pre-scans quickly identify frequencies of interest, allowing detailed maximization at specific frequencies rather than time-consuming broadband maximization.

Chamber Size Considerations

Chamber size determines the maximum measurement distance and EUT dimensions that can be accommodated. Three-meter chambers are common for testing smaller equipment and provide adequate results for many product categories. Ten-meter chambers are required when standards specify 10-meter measurements or when testing larger equipment that requires greater separation from chamber walls.

The quiet zone, the region where absorber reflections are sufficiently suppressed for accurate measurements, depends on chamber geometry and absorber performance. Larger chambers provide larger quiet zones, accommodating bigger EUTs while maintaining measurement accuracy. The EUT must fit entirely within the quiet zone for valid measurements.

Fully Anechoic Rooms (FAR)

Fully anechoic rooms extend the absorber treatment to cover all surfaces including the floor, eliminating ground reflections entirely. This configuration simulates true free-space conditions, making FARs particularly valuable for antenna measurements, radar cross-section testing, and emission measurements where ground reflection effects are undesirable.

FAR Construction and Applications

FAR construction follows similar principles to semi-anechoic chambers but requires floor absorber treatment that can support the EUT weight while maintaining electromagnetic performance. Solutions include raised floors with grating that allows absorber placement beneath, or specialized walkway materials with minimal electromagnetic impact. Some facilities use removable floor sections that allow conversion between FAR and semi-anechoic configurations.

Emission measurements in a FAR produce different results than OATS or semi-anechoic chamber measurements because the ground reflection is absent. Standards that specify OATS or semi-anechoic testing cannot be directly fulfilled in a FAR without mathematical correction. However, FARs excel for measurements where ground reflection interference is problematic, such as antenna pattern measurements where ground reflections would distort the pattern.

Above 1 GHz, many standards permit FAR measurements because the ground plane effect becomes less significant and more difficult to properly control. The simplified propagation geometry in a FAR actually improves measurement repeatability at these frequencies.

FAR versus Semi-Anechoic Selection

The choice between FAR and semi-anechoic configurations depends on the intended measurements. For compliance testing to standards specifying OATS correlation, semi-anechoic chambers are preferred. For antenna measurements, research applications, or testing where ground plane effects are undesirable, FARs provide superior performance. Many facilities maintain both configurations or convertible chambers to address diverse requirements.

Reverberation Chambers

Reverberation chambers offer a fundamentally different approach to radiated emission measurement, using mechanical stirring to create a statistically uniform electromagnetic environment rather than absorbing reflections. This technique provides several unique advantages including high field strength capability, reduced measurement time for certain applications, and potentially lower facility costs.

Operating Principles

A reverberation chamber is a shielded enclosure with highly reflective walls, creating a cavity that supports numerous electromagnetic modes at any given frequency. A rotating metal stirrer, typically a large paddle or tuner, continuously changes the boundary conditions, causing the mode structure to shift. Over time, or averaged over many stirrer positions, the electromagnetic field becomes statistically uniform throughout the chamber volume.

Unlike anechoic chambers that attempt to create free-space conditions with controlled wave propagation, reverberation chambers deliberately exploit reflections to create an isotropic, unpolarized field environment. Every orientation and polarization of emission from the EUT contributes equally to the received signal, eliminating the need for turntable rotation and antenna height variation.

Measurement Procedures

Emission measurements in reverberation chambers involve measuring the received power as the stirrer rotates through a complete revolution or over a statistically adequate number of positions. The maximum received power, or the statistical average depending on the standard, characterizes the emission level. Calibration factors convert received power to equivalent field strength for comparison with regulatory limits.

The statistical nature of reverberation chamber measurements requires careful attention to stirrer effectiveness and the number of independent stirrer positions sampled. At lower frequencies where fewer modes exist, achieving statistical uniformity becomes more difficult, limiting the usable frequency range. Most reverberation chambers become effective above their lowest usable frequency (LUF), typically in the range of 80-200 MHz depending on chamber size.

Advantages and Applications

Reverberation chambers offer significant time savings for radiated emission measurements because no mechanical maximization through rotation and antenna height variation is needed. The isotropic environment automatically samples all emission directions and polarizations simultaneously. For EUTs with unknown or complex emission patterns, this approach ensures that maximum emissions are captured without elaborate scanning procedures.

The high field strength capability of reverberation chambers makes them valuable for immunity testing, where they can generate field strengths of hundreds of volts per meter with modest input power. This same capability can be leveraged for emission testing of low-level radiators that might be difficult to measure in anechoic environments.

Reverberation chambers are recognized in several standards for radiated emission testing, including CISPR 16-2-3 and various automotive and military standards. Correlation studies have demonstrated good agreement with traditional OATS and semi-anechoic chamber measurements when appropriate procedures and correction factors are applied.

Limitations

The lower usable frequency limitation restricts reverberation chamber application for measurements below approximately 100-200 MHz. The statistical nature of measurements means that specific emission directions or polarizations cannot be isolated, which may be important for troubleshooting. Additionally, the complex electromagnetic environment can stress EUT circuitry differently than the more orderly fields in anechoic environments, potentially affecting EUT behavior during testing.

GTEM Cells

Gigahertz transverse electromagnetic (GTEM) cells provide compact, economical facilities for radiated emission measurement from sub-megahertz frequencies to several gigahertz. These tapered transmission line structures create a region of uniform electromagnetic field, enabling measurements without the facility size and cost of full-scale chambers.

GTEM Cell Structure

A GTEM cell consists of a tapered, asymmetric coaxial transmission line with a rectangular cross-section. The outer conductor forms the cell walls, while an internal septum (center conductor) creates the transverse electromagnetic field region. The cell tapers from a small coaxial connector at one end to absorber termination at the large end, providing a smooth impedance transition across a wide frequency range.

The EUT is placed in the test volume between the septum and the cell floor, where the electromagnetic field is relatively uniform. The cell dimensions determine the maximum EUT size and the upper frequency limit, with larger cells accommodating larger equipment but with reduced high-frequency performance. Cell sizes range from desktop units for small devices to walk-in configurations for complete equipment assemblies.

Measurement Principles

When the EUT radiates electromagnetic energy, this energy couples to the cell's transmission line mode and propagates to the measurement port. The received power relates to the radiated emission level through cell factors that account for geometry and propagation characteristics. Multiple orientations of the EUT within the cell are typically measured to capture emissions in different directions.

Unlike far-field measurements in anechoic chambers, GTEM cell measurements occur in the near field, requiring different interpretation of results. Correlation algorithms transform GTEM cell measurements to equivalent far-field values for comparison with regulatory limits. These algorithms have been refined through extensive correlation studies and provide good agreement with OATS measurements for many equipment types.

GTEM Cell Applications

GTEM cells excel as pre-compliance and design verification tools, providing quick feedback during product development at a fraction of the cost of full compliance testing facilities. Their compact size allows installation in engineering laboratories, enabling designers to evaluate emissions iteratively as development progresses.

For troubleshooting, GTEM cells offer rapid measurements that help identify emission sources and verify mitigation effectiveness. The immediate feedback accelerates the design cycle and reduces the risk of failures during formal compliance testing.

While GTEM cells are recognized in some standards as alternative test methods, their correlation with traditional methods depends on EUT characteristics. Equipment with external cables presents particular challenges because cable radiation in a GTEM cell differs from free-space behavior. For final compliance testing, semi-anechoic chambers or OATS generally remain preferred, with GTEM cells serving the valuable role of pre-compliance screening.

Antenna Types and Selection

Receiving antennas are critical measurement system components that capture radiated emissions for analysis. Different antenna types offer varying characteristics across frequency ranges, and appropriate selection is essential for accurate, standards-compliant measurements.

Biconical Antennas

Biconical antennas consist of two conical elements arranged point-to-point, creating a broadband dipole structure effective from approximately 20 MHz to 300 MHz. Their broad bandwidth covers the lower portion of the standard radiated emission frequency range without requiring multiple antennas or antenna changes during measurement.

Biconical antenna factors vary with frequency and must be applied to convert received voltage to field strength. Manufacturers provide calibrated antenna factor data traceable to national standards. The relatively large size of biconical antennas, typically 1-1.5 meters in length, requires adequate chamber dimensions for proper operation.

Log-Periodic Dipole Arrays

Log-periodic dipole array (LPDA) antennas provide directional reception over frequency ranges typically spanning 200 MHz to 1 GHz or beyond. The antenna consists of multiple dipole elements of progressively varying length and spacing, creating a frequency-independent design with consistent characteristics across its bandwidth.

LPDA antennas offer higher gain than biconical antennas, improving sensitivity for detecting low-level emissions. Their directional pattern focuses sensitivity in the forward direction, which can be advantageous for isolating emissions from specific sources but requires proper alignment with the EUT.

Horn Antennas

Horn antennas provide excellent performance at frequencies above 1 GHz, offering high gain, well-defined patterns, and stable characteristics. The horn structure guides electromagnetic waves from a waveguide feed to free-space radiation through a flared aperture. Different horn sizes cover different frequency bands, with smaller horns serving higher frequencies.

Double-ridged horn antennas extend the usable bandwidth by loading the horn with ridged waveguide, enabling single-antenna coverage from below 1 GHz to beyond 18 GHz. These versatile antennas have become standard equipment for broadband emission measurements at higher frequencies.

Combined and Broadband Antennas

Some antenna designs combine multiple elements to cover extended frequency ranges with a single antenna. Biconical-log-periodic combination antennas (BiLog or BiConiLog) integrate biconical and log-periodic elements, providing coverage from 30 MHz to 1 GHz or higher without antenna changes. These antennas reduce measurement time and eliminate uncertainty associated with antenna substitution.

Antenna Factors and Calibration

The antenna factor quantifies the relationship between the electric field at the antenna location and the voltage delivered to the measurement receiver. This factor varies with frequency and must be applied during measurements to convert received voltage to field strength in the units required by standards (typically dBuV/m or dBuV/m at specified distance).

Antenna calibration is typically performed at accredited calibration laboratories using reference antenna methods or standard site methods. Calibration data provides antenna factors at discrete frequency points, with interpolation used for intermediate frequencies. Calibration should be traceable to national standards (such as NIST in the United States) and repeated periodically to account for any degradation or damage.

Antenna factor uncertainty contributes to overall measurement uncertainty and must be considered when evaluating compliance margins. Well-calibrated antennas from reputable manufacturers typically provide antenna factor uncertainties of plus or minus 1-2 dB.

Measurement Receiver Specifications

The measurement receiver, typically an EMI receiver or spectrum analyzer, detects and quantifies the signals captured by the receiving antenna. EMC standards specify receiver characteristics to ensure consistent, comparable measurements across different laboratories and equipment.

EMI Receivers versus Spectrum Analyzers

EMI receivers are specialized instruments designed specifically for EMC measurements, incorporating features required by standards including precisely specified bandwidth, detector types, and preselection. Traditional EMI receivers use analog designs with stepped tuning, measuring one frequency at a time across the emission spectrum.

Modern spectrum analyzers with EMI measurement capability combine the speed advantages of swept or FFT-based spectrum analysis with standards-compliant EMI measurement functions. These instruments can perform rapid pre-scans to identify frequencies of interest, then switch to compliant measurement modes for detailed analysis. This approach dramatically reduces overall test time while maintaining measurement validity.

Bandwidth Requirements

CISPR and related standards specify measurement bandwidths that ensure consistent results regardless of the receiver used. The CISPR bandwidths are:

• Band A (9 kHz to 150 kHz): 200 Hz bandwidth
• Band B (150 kHz to 30 MHz): 9 kHz bandwidth
• Band C/D (30 MHz to 1000 MHz): 120 kHz bandwidth
• Band E (1 GHz to 18 GHz): 1 MHz bandwidth

These bandwidths represent compromise between frequency resolution and measurement sensitivity. Narrower bandwidths provide better frequency resolution but reduce sensitivity and increase measurement time. The specified bandwidths have been standardized to enable consistent comparison of results from different laboratories.

Detector Types

EMI receivers incorporate multiple detector types that respond differently to various signal characteristics. Standards specify which detector to use for different measurements:

Peak detectors capture the maximum instantaneous signal amplitude within each measurement interval. Peak detection is fastest and most sensitive but may overstate effective interference levels for intermittent or modulated signals.

Quasi-peak detectors weight signals according to their repetition rate, with higher repetition rates producing higher readings. This response approximates the subjective annoyance of interference to audio or visual reception and is specified in most commercial EMC standards for radiated emission measurements.

Average detectors measure the true average of the signal envelope, providing lower readings than quasi-peak for intermittent signals. Some standards specify average limits in addition to quasi-peak limits.

RMS-average detectors provide accurate power measurements regardless of signal characteristics and are increasingly used in modern standards, particularly for digital communication systems.

Dynamic Range and Sensitivity

Measurement receivers must provide adequate sensitivity to detect emissions at levels below regulatory limits while handling strong signals without overload or distortion. Dynamic range, the ratio between the strongest signal that can be measured accurately and the noise floor, determines the range of signal levels that can be characterized in a single measurement.

Preamplifiers increase system sensitivity for detecting weak emissions but reduce dynamic range and may introduce overload susceptibility. Attenuators reduce signal levels to prevent overload from strong emissions but raise the effective noise floor. Proper configuration of preamplifiers and attenuators optimizes the measurement system for the expected emission levels.

Preselection and Image Rejection

Preselector filters in EMI receivers reject out-of-band signals that might otherwise create spurious responses through receiver nonlinearities. This capability is essential when measuring in environments with strong ambient signals or when EUT emissions include strong narrowband components at some frequencies.

Modern receivers use multiple conversion stages with careful frequency planning to reject image responses and other spurious signals. Specifications for image rejection and spurious response levels ensure that detected signals genuinely represent EUT emissions rather than receiver artifacts.

Pre-Compliance Testing Setups

Pre-compliance testing enables engineers to evaluate radiated emissions during product development, identifying problems early when corrections are least expensive. Effective pre-compliance setups balance measurement capability against cost and convenience, providing actionable results without the expense of full compliance facilities.

Essential Pre-Compliance Equipment

A functional pre-compliance setup for radiated emissions includes:

Spectrum analyzer or EMI receiver: While a full EMI receiver with CISPR-compliant bandwidths and detectors provides the most accurate pre-compliance results, a general-purpose spectrum analyzer serves adequately for identifying emission frequencies and relative levels. Key features include adequate frequency range (typically 30 MHz to at least 1 GHz), reasonable sensitivity (better than -100 dBm), and stable, calibrated amplitude response.

Broadband antenna: A combination antenna covering 30 MHz to 1 GHz or higher enables measurements across the primary regulated frequency range without antenna changes. While precision calibration is valuable, even approximate antenna factors provide useful relative measurements for design optimization.

Near-field probes: Small loop and monopole probes enable near-field scanning to localize emission sources on PCBs and within enclosures. These inexpensive probes provide rapid feedback for troubleshooting even without a complete far-field measurement setup.

Test Environment Considerations

Full shielding and anechoic treatment are often impractical for pre-compliance testing. Alternative approaches include:

Shielded rooms without absorbers allow measurements free from ambient interference, though internal reflections affect accuracy. Results indicate emission frequencies and relative levels but may not correlate precisely with compliant test environments.

GTEM cells provide excellent pre-compliance capability in compact form, offering broadband measurements with reasonable correlation to far-field results.

Open environment testing, measuring emissions in ordinary laboratory or office spaces, can identify major emission sources and gross compliance failures. Ambient signals limit sensitivity, and reflections affect accuracy, but this approach costs nothing beyond the measurement equipment.

Interpreting Pre-Compliance Results

Pre-compliance measurements typically underestimate formal compliance test results due to differences in measurement environment, equipment calibration, and procedure. A common approach applies margin, perhaps 6-10 dB, to pre-compliance results when estimating formal test outcomes. Emissions approaching limits during pre-compliance testing warrant design modifications before formal testing.

Relative measurements during pre-compliance testing provide valuable design guidance even when absolute accuracy is limited. Comparing emissions before and after design changes quantifies improvement regardless of absolute calibration uncertainty. This approach accelerates design optimization and builds confidence that formal testing will succeed.

Correlation Between Test Methods

Different test methods each have unique characteristics that affect measurement results. Understanding correlation between methods enables appropriate interpretation of results and supports regulatory acceptance of alternative test approaches.

OATS and Semi-Anechoic Chamber Correlation

Semi-anechoic chambers are designed to replicate OATS conditions, and properly constructed and validated chambers provide excellent correlation. The ground plane and one-reflection geometry are common to both methods, ensuring similar propagation characteristics. Site validation procedures, particularly normalized site attenuation measurements, verify that chamber performance matches theoretical OATS behavior within specified tolerances.

Minor differences may arise from chamber size constraints, absorber imperfections, or differences in ground plane characteristics. These effects are typically small for properly validated chambers and are accounted for within measurement uncertainty budgets.

Reverberation Chamber Correlation

Reverberation chamber measurements correlate with OATS results through statistical relationships and conversion factors derived from chamber theory and empirical validation. The isotropic, unpolarized field environment in a reverberation chamber effectively averages all emission directions, requiring different interpretation than the directional measurements of OATS or semi-anechoic methods.

Correlation factors convert reverberation chamber maximum or average received power to equivalent field strength for comparison with limits expressed in traditional units. These factors depend on chamber characteristics and measurement procedure details. Standards such as CISPR 16-2-3 specify procedures and provide guidance on correlation.

GTEM Cell Correlation

GTEM cell correlation with far-field methods is more complex because GTEM measurements occur in the near field with different field distributions than free-space propagation. Correlation algorithms use multiple EUT orientations within the cell to estimate far-field emission levels, with accuracy depending on EUT radiation characteristics.

For small, electrically simple EUTs, GTEM correlation can be quite good. Larger equipment or devices with significant cable emissions may show poorer correlation due to cable behavior differences between GTEM and free-space environments. Correlation studies for specific product types help establish confidence levels for GTEM pre-compliance results.

Factors Affecting Correlation

Several factors influence correlation between test methods:

EUT radiation characteristics: Equipment with directional emission patterns may produce different results depending on how each method samples different directions. Omnidirectional radiators generally show better correlation.

Frequency range: Correlation typically improves at higher frequencies where test environment effects become more consistent. Lower frequencies present more challenges due to chamber mode behavior and the difficulty of creating ideal field conditions.

Cable configurations: External cables are major emission sources, and their behavior varies between test environments. Standardized cable layouts and termination impedances improve correlation but may not fully represent real installation conditions.

EUT operating mode: Emissions often depend on EUT operating conditions. Consistent operating modes across test methods are essential for meaningful correlation.

Measurement Uncertainty

All measurements include uncertainty that must be quantified and considered when evaluating compliance. Radiated emission measurements are particularly subject to uncertainty due to the complex interaction of multiple factors affecting results.

Sources of Uncertainty

Major uncertainty contributors in radiated emission measurements include:

Antenna factors: Calibration uncertainty in antenna factor data directly affects calculated field strength. Typical calibration uncertainties range from 1 to 3 dB depending on antenna type and frequency.

Receiver accuracy: Amplitude accuracy of the measurement receiver, including effects of frequency response, linearity, and detector implementation, contributes to overall uncertainty. Well-calibrated receivers provide amplitude accuracies within plus or minus 1-2 dB.

Site imperfections: Deviations from ideal OATS or chamber performance introduce variability between measurement facilities. Site validation limits these effects but does not eliminate them entirely.

Mismatch losses: Impedance mismatches between antenna, cables, and receiver cause signal reflections that affect measured levels. Mismatch effects vary with frequency and can contribute 1-2 dB of uncertainty.

Cable losses: Signal attenuation in cables between antenna and receiver must be accounted for, and calibration of cable loss contributes uncertainty. Temperature effects on cable loss introduce additional variability.

Uncertainty Budgets

Laboratories develop measurement uncertainty budgets that quantify and combine individual uncertainty contributors according to established procedures such as those in ISO/IEC Guide 98-3 (GUM). Combined uncertainty is typically expressed as an expanded uncertainty with a specified coverage factor, commonly k=2 corresponding to approximately 95% confidence.

Radiated emission measurement uncertainties typically fall in the range of 5-7 dB at lower frequencies, decreasing somewhat at higher frequencies where antenna and site effects become more predictable.

Uncertainty and Compliance Decisions

When measured emissions approach regulatory limits, uncertainty becomes critical for compliance decisions. Different approaches handle this situation:

Simple acceptance: Results below the limit are accepted regardless of uncertainty. This approach may accept equipment that would exceed limits if uncertainty were considered.

Shared risk: Results are compared to limits without uncertainty adjustment, with both manufacturer and regulator accepting that some compliant products may fail and some non-compliant products may pass.

Guard band approach: Limits are reduced by the measurement uncertainty, ensuring high confidence that products meeting adjusted limits truly comply. This conservative approach places the compliance burden on manufacturers.

Different regulatory regimes and accreditation bodies apply different philosophies, and engineers should understand the approach applicable to their specific testing requirements.

Practical Measurement Considerations

Successful radiated emission measurements require attention to numerous practical details beyond equipment selection and facility design. These considerations affect measurement quality, reproducibility, and correlation with regulatory test results.

EUT Configuration and Operating Modes

The EUT must be configured and operated in a manner representative of typical use and consistent with standards requirements. Exercise all functions that might generate emissions, including communication interfaces, display modes, and processing activities. Standards often specify that measurements be performed in worst-case operating modes identified through preliminary testing.

Cable configurations significantly affect emissions. Standards specify cable layouts including lengths, routing, and termination conditions. Cables should be arranged to represent typical installation while following standard layouts that enable reproducible measurements. Excess cable length is typically arranged in a specified bundle configuration.

Support Equipment

Equipment needed to operate the EUT, such as monitors, keyboards, or communication partners, must be positioned and configured to minimize their contribution to measured emissions. Use equipment known to have low emissions, maintain specified distances from the EUT, and orient cables to minimize coupling. Some standards specify testing with support equipment removed if the EUT can operate independently.

Measurement Procedure Optimization

Complete broadband measurements covering all frequencies, rotations, antenna heights, and polarizations can be time-consuming. Efficient approaches begin with fast pre-scans using peak detection to identify frequencies of interest, followed by detailed maximization measurements at specific frequencies using appropriate detectors.

Automated measurement systems control turntables, antenna masts, and receivers to execute measurement procedures efficiently. Software algorithms optimize scanning patterns and focus measurement effort on frequencies approaching limits. These systems can reduce measurement time from days to hours while improving reproducibility.

Documentation and Reporting

Thorough documentation supports measurement validity and enables troubleshooting if questions arise. Essential documentation includes EUT identification and serial numbers, software versions, operating modes, cable configurations, support equipment, environmental conditions, equipment calibration status, and any deviations from standard procedures.

Test reports should present results clearly, typically in tabular and graphical formats showing emission levels versus frequency compared to applicable limits. Margin to limits, measurement uncertainty, and any identified non-compliances should be clearly indicated.

Conclusion

Radiated emission measurement is a sophisticated discipline requiring understanding of electromagnetic theory, specialized facilities, precision equipment, and standardized procedures. The evolution from open area test sites to modern semi-anechoic chambers, reverberation chambers, and GTEM cells provides engineers with multiple approaches suited to different applications, from research and development through formal compliance certification.

Success in radiated emission measurement depends on proper facility design and validation, appropriate equipment selection and calibration, rigorous adherence to measurement procedures, and thorough understanding of uncertainty and correlation factors. Pre-compliance testing integrated into the development process identifies problems early and builds confidence that formal compliance testing will succeed.

As electronic products continue to increase in complexity and operating frequencies, radiated emission measurement techniques continue to evolve. New standards address higher frequencies, new test methods improve efficiency, and new equipment provides enhanced capability. Engineers who master the fundamentals of radiated emission measurement are well-prepared to adapt to these continuing developments while ensuring their products meet electromagnetic compatibility requirements in all intended markets.