Measurement of antenna factor-a case study

Radiated emission is one of the electromagnetic compatibility (EMC) compliance tests enforced by various regulatory and advisory agencies. The specifications limit for this test are expressed as field intensity in the units of volts per meter or more commonly as dB/spl mu/v/M. The emissions are measured using calibrated antennas and a matched receiver. To compute the field strength from the response of the receiver, "antenna factor" is a vital parameter. But the antenna factor used for EMC measurements, generally supplied by the manufacturer, is valid mostly for far-field conditions and many times considerable error occurs when situations other than far-field ranges are encountered. This paper describes, in detail, test set up, procedures and test results for antenna factors w.r.t. various parameters such as distance, polarisation and test area. Discussion on factors responsible for errors and their control are enumerated.