Antenna Measurement Techniques
Accurate antenna characterization is essential for meaningful EMC measurements. Whether calibrating test antennas, evaluating shielding effectiveness, or characterizing unintentional radiators, reliable measurement techniques provide the foundation for quantitative EMC analysis. Understanding the principles, procedures, and uncertainties of antenna measurements enables engineers to obtain valid data and correctly interpret results.
Antenna measurement encompasses multiple parameters: radiation pattern, gain, polarization, impedance, and the antenna factor critical to EMC work. Each measurement requires appropriate facilities, equipment, and procedures to achieve accurate results. This article explores the techniques used to characterize these parameters, with emphasis on methods particularly relevant to EMC applications where broadband performance and calibration traceability are paramount.
Pattern Measurement
The radiation pattern describes an antenna's directional properties, showing how radiated or received power varies with angle. Pattern measurements reveal mainlobe width, sidelobe levels, null positions, and overall directional characteristics.
Far-Field Pattern Measurement
Traditional pattern measurement places the antenna under test (AUT) in the far field of a source antenna, rotating the AUT while recording received signal strength. The far-field distance (2D^2/lambda) ensures that the incident wave is sufficiently planar for accurate pattern representation. A complete pattern requires rotation about two axes: azimuth for the equatorial plane and elevation for great-circle cuts. The source antenna should be small compared to the AUT to minimize pattern distortion from source directivity.
Pattern Measurement Facilities
Open-area test sites (OATS) provide a ground-reflected environment suitable for certain pattern measurements but complicate interpretation due to ground effects. Anechoic chambers eliminate reflections through absorptive wall treatment, providing free-space-like conditions. Semi-anechoic chambers have absorber-treated walls and ceiling but a reflective floor, mimicking the ground-reflected environment of OATS measurements. The facility choice depends on the intended application and required accuracy.
Near-Field Pattern Determination
For large antennas or high frequencies where far-field distances become impractical, near-field measurements can determine far-field patterns through mathematical transformation. The near-field amplitude and phase are sampled over a surface (planar, cylindrical, or spherical), and Fourier transform techniques compute the far-field pattern. This approach requires phase-coherent measurement equipment and careful sampling to satisfy Nyquist criteria.
Pattern Parameters
Key pattern parameters include half-power beamwidth (HPBW, the angular width between -3 dB points), first sidelobe level relative to the mainlobe peak, front-to-back ratio, and cross-polarization level. For EMC antennas, pattern uniformity across the operating bandwidth is also important. Pattern specifications help engineers understand antenna directional selectivity and predict performance in specific orientations.
Gain Measurement
Antenna gain quantifies directional concentration of radiated power relative to a reference antenna. Accurate gain measurement is essential for link budget calculations and field strength determinations.
Gain Definition
Gain (G) is the ratio of radiation intensity in a given direction to the intensity that would be produced if the same total power were radiated isotropically. Gain is usually referenced to an isotropic antenna (dBi) or a half-wave dipole (dBd); dBi = dBd + 2.15. Gain includes the effects of antenna efficiency (ohmic losses), so it is less than or equal to directivity. Maximum gain corresponds to the peak of the radiation pattern.
Comparison Method
The comparison or substitution method measures the AUT gain by comparing its received signal to that of a reference antenna of known gain under identical conditions. The AUT and reference antenna are alternately placed at the same position, and the power difference directly yields gain difference. This method requires a calibrated reference antenna but is straightforward and widely used. Careful alignment ensures valid comparison.
Three-Antenna Method
The three-antenna method determines the gains of three antennas without requiring a pre-calibrated reference. Three transmission measurements using different antenna pairs yield three equations that can be solved for the three unknown gains. This self-calibrating approach establishes primary gain standards. The method assumes reciprocity (antennas have identical transmit and receive characteristics) and requires identical range conditions for all three measurements.
Gain Versus Frequency
EMC antennas must be characterized over their entire operating bandwidth. Gain typically varies with frequency, and calibration data must span the measurement range with sufficient frequency resolution. Rapid gain variations with frequency indicate resonances or impedance anomalies that may affect measurement accuracy. Broadband antennas like log-periodics show relatively smooth gain variation, while narrowband antennas exhibit sharper gain peaks.
Antenna Factor Calibration
Antenna factor (AF) relates incident field strength to received voltage, making it the critical parameter for converting EMC measurements to field strength units. Traceable antenna factor calibration is essential for compliance testing.
Antenna Factor Definition
Antenna factor is defined as AF = E/V, where E is the incident electric field strength (V/m) and V is the voltage at the antenna terminals (V). In logarithmic form: AF(dB/m) = E(dBuV/m) - V(dBuV). A higher antenna factor means less voltage for a given field strength. Antenna factor incorporates the antenna's effective aperture, impedance mismatch, and any internal losses, making it a complete characterization for EMC measurement purposes.
Standard Site Method (SSM)
The standard site method measures antenna factor using a calibrated transmit antenna and validated test site. The test site's normalized site attenuation (NSA) must be verified to be within acceptable tolerance of theoretical values. A transmit antenna of known characteristics generates a calculable field, and the AUT response determines its antenna factor. The SSM is the basis for most commercial antenna calibrations and is specified in standards like ANSI C63.5.
Reference Antenna Method
Comparison to a reference antenna of known antenna factor provides a practical calibration approach. The reference and AUT are alternately exposed to the same field, and the difference in received voltage yields the antenna factor difference. This method transfers calibration from a standard antenna to the AUT. Reference antennas must be traceable to national standards through an unbroken chain of comparisons.
Calibration Uncertainties
Antenna factor calibration uncertainties arise from multiple sources: site imperfections, measurement equipment accuracy, cable losses, alignment errors, and environmental factors. Total uncertainty is calculated by combining individual contributions, typically using root-sum-square methods for uncorrelated sources. Calibration certificates should state expanded uncertainty with coverage factor. Typical uncertainties range from 1-2 dB under ideal conditions to 3-4 dB in less controlled environments.
Impedance Measurement
Antenna impedance affects power transfer, system matching, and the relationship between terminal voltage and radiated field. Impedance measurement characterizes these properties across the operating frequency range.
Input Impedance
An antenna's input impedance is the ratio of voltage to current at its terminals. It has resistive and reactive components: Zin = Rin + jXin. The resistive part includes radiation resistance (power delivered to radiation) and loss resistance (power dissipated in the antenna structure). The reactive part represents stored energy in the antenna's near field. Impedance varies with frequency, approaching resonance (X = 0) at specific frequencies.
Network Analyzer Measurements
Vector network analyzers (VNAs) measure antenna impedance through S-parameter characterization. The reflection coefficient S11 relates to impedance by Z = Z0(1+S11)/(1-S11). VNA measurements provide magnitude and phase over frequency sweeps, enabling complete impedance characterization. Proper calibration of the VNA and measurement cables is essential for accurate results. Time-domain gating can separate antenna response from cable and connector reflections.
Voltage Standing Wave Ratio
VSWR quantifies impedance mismatch to the system characteristic impedance (typically 50 ohms). VSWR = (1+|S11|)/(1-|S11|) ranges from 1 (perfect match) to infinity (complete mismatch). EMC antennas typically specify VSWR less than 2:1 or 3:1 across their operating band. Higher VSWR causes reflection losses and can affect antenna factor calibration if not properly accounted for. Return loss in dB equals -20*log|S11|.
Impedance Bandwidth
Impedance bandwidth is the frequency range over which an antenna maintains acceptable impedance match. Broadband EMC antennas must maintain low VSWR across multiple octaves. Antenna designs trade off bandwidth against other parameters like gain and pattern stability. Understanding impedance behavior helps predict antenna performance and guides matching network design when needed.
Near-Field Scanning
Near-field scanning measures field amplitude and phase over a surface close to the AUT, enabling far-field pattern computation and detailed field distribution analysis.
Measurement Principle
A small probe antenna samples the electromagnetic field at points on a defined surface surrounding the AUT. Both amplitude and phase must be measured relative to a reference, typically a portion of the signal driving the AUT. The probe must be small enough to provide adequate spatial resolution without significantly perturbing the field. Planar, cylindrical, and spherical scanning geometries each offer advantages for different antenna types.
Planar Near-Field Scanning
Planar scanning moves the probe across a flat surface in front of the AUT. Two-dimensional Fourier transformation converts the measured near-field data to the far-field angular spectrum. Planar scanning works well for directive antennas but cannot determine patterns at angles far from boresight. The scan plane must be large enough to capture significant field contributions, typically extending beyond the antenna aperture by several wavelengths.
Spherical Near-Field Scanning
Spherical scanning acquires data over a complete sphere surrounding the AUT, enabling full three-dimensional pattern determination. Spherical harmonic expansion and transformation yield the far-field pattern at all angles. This technique is most comprehensive but requires more complex measurement systems. Spherical scanning is standard for characterizing omnidirectional and wide-beam antennas.
EMC Applications
Near-field scanning has specific EMC applications beyond antenna calibration. Scanning over a device under test can identify radiation hot spots and coupling paths. Emission source localization uses near-field data to pinpoint components or structures responsible for radiated emissions. The technique provides spatial resolution impossible with far-field measurements, making it valuable for diagnostic work.
Time-Domain Techniques
Time-domain measurements use pulsed signals to characterize antenna response, offering advantages for broadband characterization and multipath rejection.
Time-Domain Reflectometry
Time-domain reflectometry (TDR) sends a pulse into the antenna and monitors reflections. The time delay to each reflection indicates the physical location of impedance discontinuities along the feed structure and in the antenna itself. TDR helps identify connector problems, cable defects, and antenna structural issues. Modern VNAs can compute TDR from frequency-domain S-parameter data through inverse Fourier transformation.
Pulse Radiation Measurements
Time-domain antenna ranges transmit short pulses and use time gating to separate the direct signal from reflections. The direct signal arrives first; reflections from walls, floor, and other structures arrive later. Time gating passes only the direct signal, effectively creating an anechoic environment without absorber material. This technique enables pattern measurements in facilities otherwise unsuitable for antenna testing.
Frequency-Domain Conversion
Fourier transformation converts time-domain measurements to frequency domain and vice versa. A broadband pulse contains energy at all frequencies within its bandwidth; transforming the received pulse reveals the antenna's frequency response. This approach can be more efficient than stepped-frequency measurements for broadband characterization. Resolution bandwidth in one domain trades off against extent in the other domain.
Impulse Response Characterization
An antenna's impulse response reveals time-domain behavior including dispersion and ringing. Antennas with frequency-dependent characteristics (like log-periodics, where different elements respond at different frequencies) exhibit impulse spreading. Understanding time-domain behavior is important for pulsed applications and for interpreting transient measurements. Compact, wideband antennas are preferred when pulse fidelity matters.
Compact Ranges
Compact antenna test ranges (CATRs) create a plane wave test zone using reflector optics, enabling far-field measurements within an indoor facility much shorter than the conventional far-field distance.
Operating Principle
A compact range uses a large precision reflector to collimate the spherical wave from a feed antenna into a plane wave. The resulting quiet zone provides approximately uniform amplitude and phase, simulating far-field conditions at a fraction of the conventional distance. The feed is offset to prevent blockage, and shaped reflectors or serrated edges reduce edge diffraction. Compact ranges typically achieve quiet zones with amplitude taper less than 1 dB and phase variation less than 10 degrees.
Range Specifications
Compact range performance is characterized by quiet zone size, amplitude ripple, phase variation, and cross-polarization level. The quiet zone must accommodate the AUT with margin for positioning tolerance. Lower frequencies require larger reflectors for equivalent quiet zone quality. Compact range specifications should be verified through probe measurements and NSA validation. Well-designed ranges achieve performance competitive with outdoor far-field ranges.
Advantages and Limitations
Compact ranges offer controlled environment (temperature, humidity, security), all-weather capability, and reduced real estate requirements compared to outdoor ranges. They enable accurate measurements that might be impossible in electrically noisy or inclement outdoor environments. Limitations include high facility cost, lower frequency limits set by practical reflector size, and quiet zone size constraints for large antennas. Compact ranges complement rather than replace other measurement facilities.
EMC Testing Applications
Compact ranges support EMC testing by providing controlled radiated emissions measurement environments and uniform field generation for immunity testing. The plane wave characteristic eliminates ground reflection complications present in semi-anechoic chambers. Large quiet zones can accommodate complete systems rather than just components. Integration of compact range and anechoic chamber features creates versatile EMC test facilities.
Reverberation Methods
Reverberation chambers provide a statistical approach to antenna measurement and EMC testing, using mode stirring to create a spatially uniform average field environment.
Reverberation Chamber Principle
A reverberation chamber is a shielded enclosure with highly reflective walls and a mechanical or electronic stirrer that changes the field distribution. As the stirrer rotates or steps through positions, the chamber cycles through many resonant modes. Statistical averaging over stirrer positions yields spatially uniform field statistics despite the complex instantaneous field structure. This uniformity is valuable for testing equipment with unknown orientation.
Total Radiated Power Measurement
Reverberation chambers excel at measuring total radiated power (TRP), integrating emissions over all angles and polarizations. This measurement is simpler than pattern integration in anechoic chambers and provides a single figure of merit for comparing sources. TRP measurements are increasingly used for regulatory compliance and as acceptance criteria. Antenna efficiency can be determined by comparing TRP to input power.
Statistical Considerations
The statistical validity of reverberation measurements depends on the number of independent samples (stirrer positions) and the chamber's modal density. Low-frequency operation is limited by insufficient modes to establish statistical uniformity. Standard uncertainty decreases with the square root of independent samples. Chamber calibration validates statistical properties and determines usable frequency range. Typical chambers provide valid measurements above a lowest usable frequency where modal density becomes adequate.
Comparison to Anechoic Methods
Reverberation and anechoic methods provide complementary information. Anechoic measurements yield pattern details and directional characteristics; reverberation measurements yield statistical averages. Reverberation chambers are more efficient for measuring total power but cannot provide pattern data. For devices that will be used in environments where orientation is controlled, anechoic measurements are appropriate; for devices used in random orientations, reverberation testing may better represent real-world performance.
Uncertainty Analysis
Quantifying measurement uncertainty is essential for interpreting results and maintaining quality systems. Rigorous uncertainty analysis enables valid comparisons and regulatory compliance assessment.
Sources of Uncertainty
Antenna measurements involve numerous uncertainty sources: equipment accuracy (receiver, network analyzer, cables), antenna positioning, site reflections, environmental factors (temperature, humidity), calibration uncertainty of reference standards, and repeatability. Each source contributes to total measurement uncertainty. Understanding individual contributions guides efforts to improve measurement quality and helps identify dominant sources requiring attention.
Uncertainty Budgets
Uncertainty budgets systematically list all identified sources with their estimated magnitudes and probability distributions. Type A uncertainties derive from statistical analysis of repeated measurements; Type B uncertainties come from other information (calibration certificates, specifications, experience). The budget provides a framework for evaluating and documenting measurement quality. Accredited laboratories must maintain uncertainty budgets for their measurement procedures.
Combined and Expanded Uncertainty
Combined standard uncertainty combines individual components using root-sum-square addition for uncorrelated sources or appropriate covariance methods when correlation exists. Expanded uncertainty multiplies combined uncertainty by a coverage factor (typically k=2 for 95% confidence) to give an interval likely to contain the true value. Measurement results should be reported with expanded uncertainty and coverage factor to enable valid interpretation.
Uncertainty in EMC Testing
EMC test uncertainty affects compliance decisions. When measurement results are close to limits, uncertainty determines whether compliance can be confidently claimed or denied. Some standards include measurement uncertainty in their limits; others require uncertainty to be subtracted from margin. Understanding and minimizing measurement uncertainty enables more confident compliance statements and reduces over-engineering margins needed to assure passing compliance tests.
Measurement Best Practices
Achieving accurate, repeatable antenna measurements requires attention to procedure details, equipment maintenance, and quality systems beyond the basic techniques.
Equipment Calibration
All measurement equipment requires regular calibration traceable to national standards. Calibration intervals depend on equipment stability and usage intensity. Verification checks between calibrations confirm continuing accuracy. Calibration records must document results, traceability, and any adjustments made. Using out-of-tolerance equipment invalidates measurements regardless of procedural correctness.
Environmental Control
Temperature and humidity affect antenna dimensions (especially plastic elements), cable electrical length, and equipment performance. Controlled environments improve repeatability. If environmental control is not possible, conditions should be recorded with measurements for later correction or interpretation. Outdoor measurements face additional variables: wind loading, precipitation, and solar heating can all affect results.
Documentation
Complete documentation enables result reproduction and troubleshooting. Records should include equipment identification and calibration status, environmental conditions, test configuration details, raw data, and any observations that might affect interpretation. Good documentation distinguishes professional measurements from casual observations and supports legal defensibility of compliance test results.
Quality Assurance
Quality systems ensure consistent measurement processes and results. Standard operating procedures define measurement methods. Training qualifies personnel for specific measurements. Proficiency testing and round-robin comparisons verify capability relative to other laboratories. Accreditation to standards like ISO 17025 provides external validation of quality systems. Quality assurance investment pays dividends in measurement credibility and customer confidence.
Summary
Antenna measurement techniques characterize the parameters essential for EMC work: radiation pattern, gain, antenna factor, impedance, and polarization. Each measurement requires appropriate facilities, equipment, and procedures to achieve valid results. Pattern measurements determine directional characteristics; gain measurements quantify radiating efficiency; antenna factor calibration enables field strength determination; impedance measurements assess matching and bandwidth. Advanced techniques like near-field scanning, time-domain methods, compact ranges, and reverberation chambers address specific measurement challenges. Throughout all measurements, rigorous uncertainty analysis quantifies result quality and enables valid interpretation. Adherence to measurement best practices, including equipment calibration, environmental control, thorough documentation, and quality assurance systems, ensures that antenna measurements provide the reliable foundation needed for EMC analysis and compliance testing.