Measurement and estimation of minimum antenna height of free-space antenna impedance

Multimedia equipment emission standard CISPR32 requires the use of a free-space antenna factor in radiated emission measurement of equipment under test (EUT). The calibration method for antennas to be used in radiated emission measurement is stipulated in CISPR16-1-6 and ANSI C63.5. In ANSI C63.5, because antennas used in a frequency range of 30 MHz to 1 GHz are susceptible to vertical polarization by the antenna mast and cables, they are calibrated with horizontally-polarized waves in a near-free space, at an antenna height of 2 m, and at a distance of 10 m. The free-space antenna factor for biconical antennas is calculated by using a correction factor. However, for log-periodic dipole (LPD) and hybrid antennas, no such factor has been established. Antenna impedance varies with antenna height (due to ground plane influence), which in turn affects antenna sensitivity. Therefore, to obtain the free-space antenna factor of these antennas, antenna calibration height must be higher to reduce ground plane influence. We measured impedance values of typical antennas to be used in radiated emission measurements within a frequency range of 30 MHz to 1 GHz, while varying antenna height up to 10 m. Then, we estimated the height at which antenna impedance converges to the antenna impedance in a free space. This paper describes the measurement and results of estimation.