Advances in measurement methods and reduction of measurement uncertainties associated with antenna calibration

A major source of uncertainty in electromagnetic-compatibility conformance testing is the antenna factor. Until recently, broadband antennas in the frequency range 30 MHz-1 GHz were calibrated typically to an uncertainty of ±2 dB. Antennas can now be calibrated to an uncertainty of less than ±0.5 dB. This has made it possible to quantify a second category of uncertainties which arise when the antenna is used for emission testing on an open-field site. Ideally, the most useful calibration is of free-space antenna factors, but when antennas are used above a ground plane mutual-coupling effects can alter the antenna factor. The directivity of the antenna affects the magnitude of the received signal; balun imbalance affects the received signal; and the phase centre of log-periodic antennas changes with frequency. On top of this there are site effects which include reflections from surrounding objects, reflections from antenna-support structures and input cables and edge diffraction from the ground plane. These effects can be quantified to uncertainties as low as ±0.1 dB using antennas with calculable antennas factors