A distance-detecting method for planar near-field measurements using a printed ring resonator

In this paper, a low-cost technique to measure the distance between a near-field probe and a planar antenna under test (AUT) is presented. This technique uses an unshielded printed ring resonator whose resonant frequency is shifted by varying its proximity to the AUT. The distance is derived from measured return loss of the ring resonator. The sensitivity of the resonant frequency with respect to the distance between the ring resonator and the AUT is more than 10 MHz/mm. This is compliant with the positioning accuracy requirements in near-field antenna measurements. Thus, it provides a convenient way to obtain the bound limit of z-positioning error. Such information can be used in AUT characterization based on the extraction of equivalent magnetic currents.