Limitations of symmetry test method for antennas as specified in ANSI C63.5-2006 standard

The ANSI C63.5-2006 standard stipulates a symmetry test to check the antenna balance. The s21 response between two similar antennas, placed at 1m height above the ground plane, is first recorded. The antenna under test is rotated 180° and the response is recorded again. The difference gives the symmetry of the antenna and should be less than 1.0 dB. This paper discusses the limitation of the symmetry test configuration. Specifically, it specifies to place both transmit and receive antennas at a fixed height of 1m above a conducting ground plane, resulting the antenna under test to situate in a null field at some frequencies due to the ground reflection. The effect from the ground plane at the nulls of response distorts the symmetry test result. A new technique is proposed wherein the receive antenna (antenna under test) is placed at 1m height above the ground plane and the transmit antenna scans from 1m to 4m at a step of 0.05 m. The scanning of the transmit antenna helps to eliminate the nulls in the s21 response. Because the antenna under test is always at the fixed height, the measurement yields valid symmetry results for the antenna under test for 1 m height.