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CISPR 16-1-6: Specification for Radio Disturbance and Immunity Measuring Apparatus — EUT Antenna Calibration
Calibration methods for equipment-under-test antennas and EMC measurement uncertainty
1. Scope and Importance
CISPR 16-1-6 addresses the calibration of antennas that are integral parts of equipment under test (EUT) — for example, wireless communication devices, broadcast transmitters, and radar equipment where the antenna is an inseparable component of the product. This standard provides methods for characterizing the antenna’s contribution to the overall EMC measurement and for separating the disturbance generated by the EUT electronics from the antenna’s inherent radiation characteristics.
The standard is particularly relevant for EMC testing of wireless devices operating in the 30 MHz to 18 GHz range. When a wireless device transmits, its antenna is both an intentional radiator (for the communication signal) and an unintentional radiator (for noise generated by the device’s internal electronics). CISPR 16-1-6 provides methods to separate these two contributions, enabling accurate measurement of the noise component that must comply with emission limits.
The critical insight in EUT antenna calibration is that the antenna’s gain pattern and impedance matching affect both the wanted signal and the unwanted noise equally. By characterizing the antenna’s frequency response and efficiency, the noise contribution at the device’s output can be separated from the intentional transmission for compliance assessment.
2. Calibration Procedures and Requirements
CISPR 16-1-6 defines three levels of EUT antenna calibration. Level 1 is a basic calibration that determines the antenna’s impedance and VSWR across the frequency range of interest. Level 2 adds gain and radiation pattern measurement. Level 3 provides the most comprehensive characterization, including polarization purity, phase center location, and time-domain response.
| Calibration Level | Parameters Measured | Test Setup | Typical Application |
|---|---|---|---|
| Level 1 (Basic) | Impedance, VSWR, bandwidth | VNA + calibration kit | Pre-compliance / R&D screening |
| Level 2 (Standard) | Gain, radiation pattern (2 principal planes) | Anechoic chamber + positioner | Full compliance testing |
| Level 3 (Full) | 3D pattern, polarization, phase center, time-domain | Full anechoic chamber + 3D scanner | Reference measurements / type approval |
The standard specifies that the EUT antenna should be calibrated in its final operational configuration (including the device enclosure, battery, and any external accessories). This is because the antenna’s characteristics can be significantly affected by the surrounding structures — a phenomenon known as “platform effects.” For example, a laptop’s Wi-Fi antenna pattern changes when the lid is at different angles.
One of the most challenging aspects of EUT antenna calibration is the effect of the feed cable. A coax cable connected to the EUT for test purposes can act as a second antenna, radiating noise and distorting the measured pattern. The standard specifies the use of ferrite chokes, common-mode absorbers, and in some cases, fiber-optic links for the test signal to minimize cable effects. The cable should be routed away from the antenna and loaded with ferrite.
3. Applications in Compliance Testing
The results of EUT antenna calibration feed directly into the compliance assessment process. For wireless devices, the emission limits in the spurious domain apply to both the transmitter harmonics and the noise floor. By knowing the antenna factor and gain at harmonic frequencies, the true spurious emission level at the device output can be calculated from the measured field strength.
For example, if a 2.4 GHz Wi-Fi device has its EUT antenna calibrated with a gain of 3 dBi at 4.8 GHz (2nd harmonic), the measured radiated emission at 4.8 GHz of 80 dBµV/m at 3 m distance can be referred back to the device output as: P_out = E_field + 20log₁₀(3m) — G_antenna — 10log₁₀(120π) — correction factors, enabling comparison with the conducted spurious emission limit.
Modern 5G NR devices operating at mmWave frequencies (24–40 GHz) present new challenges for EUT antenna calibration. At these frequencies, the antenna is often an integrated array (AiP — Antenna in Package) with beamforming capability. CISPR 16-1-6 provides guidance for over-the-air (OTA) calibration methods that characterize the entire device as a radiating system, including the beam-steering effects of the phased array. This OTA approach is essential because conventional conducted test points are not available in integrated mmWave designs.
4. Frequently Asked Questions
Q: Is EUT antenna calibration required for all wireless devices?
A: It depends on the regulatory framework. For intentional radiators, the antenna’s characteristics affect both the fundamental emission and spurious emission measurements. Most regulatory schemes require antenna calibration (or equivalent OTA characterization) for devices where the antenna is not detachable and a conducted test point is not available.
A: It depends on the regulatory framework. For intentional radiators, the antenna’s characteristics affect both the fundamental emission and spurious emission measurements. Most regulatory schemes require antenna calibration (or equivalent OTA characterization) for devices where the antenna is not detachable and a conducted test point is not available.
Q: How does the EUT operating mode affect antenna calibration?
A: The antenna impedance can change with device operating mode (transmit power level, modulation scheme, frequency channel). The calibration should be performed in the mode that maximizes impedance mismatch (typically the highest power mode), as this represents the worst case for spurious emission levels.
A: The antenna impedance can change with device operating mode (transmit power level, modulation scheme, frequency channel). The calibration should be performed in the mode that maximizes impedance mismatch (typically the highest power mode), as this represents the worst case for spurious emission levels.
Q: Can EUT antenna calibration be performed in a reverberation chamber?
A: Yes, reverberation chambers offer an alternative for efficiency measurement and total radiated power (TRP) characterization. The standard recognizes reverberation chamber methods as an alternative to anechoic chamber measurements for certain calibration levels, particularly for Level 1 and some Level 2 parameters.
A: Yes, reverberation chambers offer an alternative for efficiency measurement and total radiated power (TRP) characterization. The standard recognizes reverberation chamber methods as an alternative to anechoic chamber measurements for certain calibration levels, particularly for Level 1 and some Level 2 parameters.
