EMCS and National EMC Calibration calibrate most types of antennas used for EMC testing and EMC test site validation, including Dipole, Biconical, Log-Periodic, Bilog and Horn antennas. Patterns for antennas can also be measured in our anechoic facilities.
Antennas have to be calibrated routinely in order to 1) meet legislation requirements, 2) know antenna factor for the correction of measured electric or magnetic field strength, 3) be able to claim traceability on measurements of field strength to national standards, 4) know the uncertainty contribution from the antenna used, 5) identify antenna gain when measuring the transmission loss between antennas.
Most calibrations are performed according to CISPR international standards, however, with special arrangements, SAE ARP 958 at 1 m separation distance can be performed as well. The uncertainty for measured antenna factors is usually below 1 dB over the antenna operating frequency range.
Loop antennas used for EMC radiated disturbance measurements over the frequency range 9 kHz to 30 MHz are typically up to 0.6 m in diameter. To measure the RF magnetic field strength using a loop antenna, its magnetic antenna factor in units of dB (mA/mV/m) or dB (Siemens/m) needs to be known. Several techniques have been developed for calibrating loop antennas by other National Standards Laboratories. At EMC Services, both active and passive loop antennas are calibrated using the modified standard field method by means of swept frequency.
Reference dipole antennas are calibrated using the NMIA 20m x 30m open area test site to be compliant with CISPR 16-1-5 by using a three-antenna method at frequencies between 30 MHz and 1 GHz. Free-space antenna factors, and antenna factors for horizontally polarised antennas at fixed height above a flat reflecting plane, can also be measured. Biconicals, Log-periodics and Bilog Antennas, Swept frequency measurements of free-space antenna factors, and, distance/height dependant antenna factors are conducted using the three-antenna method described in CISPR 16-1-6 at frequencies between 30 MHz and 3 GHz. All measurements are performed using the NMIA open area test site.
The free-space antenna factors of double-ridged horn antennas, in the frequency range from 1 GHz to 18 GHz, are determined by using the three-antenna method in a microwave fully anechoic chamber described in CISPR 16-1-6 and SAE ARP958. To achieve more accurate measurements, at each frequency, an averaging process to remove the chamber reflections is applied to the measured antenna factors at different separation distances over a quiet zone.
Antenna radiation patterns can be measured in the NMIA anechoic chamber with a roll-over-azimuth positioning system and a calibrated network analyser which are fully controlled by a LabView program.