Antenna differences and their influence on radiated emission measurements

Numerous types of antenna are used for emission measurements and at present there are two main reference antennas. For the FCC a tuned dipole is used, while the CISPR requires an antenna no longer than a halfwave dipole tuned to 80 MHz. In the frequency range from 30 to 80 MHz the physical size differences between these and other commonly used antennas can result in different readings. The author explores two of the dominant effects, antenna impedance and the electric field to which the antenna is exposed. The study indicates that there are offsetting penalties: the tuned dipole cannot measure the peak field but behaves fairly well, while the shorter antennas can get into the peak field but are mismatched. Both types of antennas can be expected to give lower readings than the actual peak field strength. If a tuned dipole is adopted the antenna match will be good over the scan height range used, but greater operator intervention will be needed for an accurate measurement