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CISPR 16-1-4: Specification for Radio Disturbance and Immunity Measuring Apparatus — Antennas and Test Sites for Radiated Disturbance Measurements
Antenna specifications and test site validation for EMC radiated measurements
1. Scope and Antenna Requirements
CISPR 16-1-4 specifies the requirements for antennas and test sites used for radiated disturbance measurements in the frequency range of 9 kHz to 18 GHz. The standard defines the characteristics of various antenna types, their calibration methods, and the validation procedures for test sites including open-area test sites (OATS), semi-anechoic chambers (SAC), and fully-anechoic chambers (FAR).
The standard covers several antenna types: rod antennas (9 kHz – 30 MHz), loop antennas (9 kHz – 30 MHz), biconical antennas (30 – 300 MHz), log-periodic antennas (200 – 1000 MHz), hybrid broadband antennas combining biconical and log-periodic elements (30 – 1000 MHz), and horn antennas (1 – 18 GHz and above). For each antenna type, the standard specifies the required frequency range, polarization purity, gain/antenna factor tolerance, and VSWR.
The transition from biconical to log-periodic antennas at approximately 200–300 MHz creates a measurement discontinuity that can cause ±3 dB variations in the measured field strength at the crossover frequency. Modern hybrid broadband antennas (bilog antennas) eliminate this discontinuity by combining both elements in a single mechanical structure with an optimized crossover network.
2. Test Site Validation — Normalized Site Attenuation (NSA)
The normalized site attenuation (NSA) is the key metric for validating a radiated emission test site. It quantifies the deviation of the actual site from an ideal free-space or ground-plane site. CISPR 16-1-4 specifies that the NSA deviation must be within ±4 dB for frequencies up to 1000 MHz for a compliant OATS or SAC.
| Site Type | Frequency Range | NSA Tolerance | Quiet Zone Size | Reflectivity Level |
|---|---|---|---|---|
| OATS | 30 – 1000 MHz | ±4 dB | 1.5 m radius (typical) | N/A (natural ground) |
| SAC (3 m) | 30 – 1000 MHz | ±4 dB | 1.5 m radius | N/A (absorber floor) |
| SAC (10 m) | 30 – 1000 MHz | ±4 dB | 2.0 m radius | N/A (absorber floor) |
| FAR | 30 – 18000 MHz | ±4 dB | 1.5 m radius | < -15 dB (reflectivity) |
The NSA measurement procedure involves positioning a broadband transmitting antenna (typically a biconical or log-periodic) at the EUT location and a receiving antenna at the measurement antenna location. The site attenuation is measured for both horizontal and vertical polarizations at specified heights and positions. For a 3 m SAC, the NSA is measured with the receiving antenna scanning from 1 m to 4 m height, and the transmitting antenna at 1 m or 1.5 m height.
The ±4 dB NSA tolerance seems generous, but it includes all measurement uncertainties (cable loss, antenna factors, instrumentation errors). A well-constructed SAC should achieve NSA deviations of less than ±2 dB. Sites operating near the ±4 dB limit leave no margin for EUT measurement uncertainty and should be improved. Common issues include absorber aging (reduced reflectivity below 200 MHz), ground plane corrosion at seams, and ferrite tile cracking.
3. Antenna Calibration and Factors
Antenna calibration determines the antenna factor (AF), which converts the voltage measured at the receiver input to the electric field strength at the antenna. The antenna factor is defined as AF = E / V, where E is the field strength in V/m and V is the receiver input voltage. Typically expressed in dB(m⁻¹), the AF is frequency-dependent due to the antenna’s effective length and impedance characteristics.
CISPR 16-1-4 specifies three methods for antenna calibration: the standard site method (SSM), the reference antenna method, and the three-antenna method. Each method has specific advantages: SSM is the most practical for routine calibration, the reference antenna method provides direct traceability, and the three-antenna method enables self-consistent calibration without a calibrated reference antenna. The calibration uncertainty for the antenna factor must be better than ±0.5 dB (k=2).
For above 1 GHz measurements, the standard specifies that dual-ridge waveguide horn antennas should be used. These antennas provide broadband performance (1–18 GHz) with good polarization purity and gain flatness. The calibration at these frequencies requires careful consideration of the measurement distance — the far-field distance (2D²/λ) for these antennas means that 3 m is only sufficient for the lower frequencies; at 18 GHz, a 1 m distance is typically used.
4. Frequently Asked Questions
Q: What is the difference between a 3 m and 10 m test site?
A: A 10 m site provides more accurate far-field measurements (the EUT appears more like a point source) and is the preferred distance for formal compliance. A 3 m site is more compact and economical but requires near-field correction factors for larger EUTs and has higher measurement uncertainty for large equipment.
A: A 10 m site provides more accurate far-field measurements (the EUT appears more like a point source) and is the preferred distance for formal compliance. A 3 m site is more compact and economical but requires near-field correction factors for larger EUTs and has higher measurement uncertainty for large equipment.
Q: Can I use an indoor OATS for year-round testing?
A: An indoor OATS is effectively a semi-anechoic chamber. A traditional outdoor OATS is subject to weather conditions, ambient electromagnetic noise, and temperature variations that affect measurement accuracy. Most commercial test labs use indoor SAC facilities for these reasons.
A: An indoor OATS is effectively a semi-anechoic chamber. A traditional outdoor OATS is subject to weather conditions, ambient electromagnetic noise, and temperature variations that affect measurement accuracy. Most commercial test labs use indoor SAC facilities for these reasons.
Q: How often should a test site be re-validated?
A: CISPR 16-1-4 recommends annual site validation (NSA measurement). Additionally, a daily verification check using a stable noise source (comb generator) is recommended to detect any drift or degradation in the measurement system.
A: CISPR 16-1-4 recommends annual site validation (NSA measurement). Additionally, a daily verification check using a stable noise source (comb generator) is recommended to detect any drift or degradation in the measurement system.
Q: What causes NSA measurement failure?
A: Common causes include: degraded ferrite tile absorber performance (particularly below 200 MHz), damaged or missing hybrid absorber cones in the chamber corners, ground plane discontinuity at chamber door seals, and cable routing changes that alter the common-mode path.
A: Common causes include: degraded ferrite tile absorber performance (particularly below 200 MHz), damaged or missing hybrid absorber cones in the chamber corners, ground plane discontinuity at chamber door seals, and cable routing changes that alter the common-mode path.
