Development of an Optically Modulated Scatterer Probe for a Near-Field Measurement System

Using near-field radiation patterns to diagnose antenna array defects.

Naval Research Laboratory, Washington, DC

Near-field radiation patterns are useful in diagnosing antenna array defects, measuring far-field antenna patterns where the far-field is prohibitively far, and locating field concentrations in high power microwave applications, which could lead to material breakdown. There are two categories of near-field measurements: direct and indirect. In a direct measurement, the field from the antenna-under-test (AUT) is directly measured by a probe whereas, in an indirect measurement, the field is inferred from the scattering off of a probe that is placed in the near-field.

A simple direct measurement method is to use an open-ended waveguide connected to a network analyzer as the receiver. The near-field is measured by spatially scanning either the AUT or the receiver. Reflections off of background objects can be filtered out in the time domain. For array applications, antennas that are smaller than the open-ended waveguide must be used to achieve spacing less than a half wavelength. An array of time-domain sensors has been demonstrated using loop antennas. The metal transmission line from the probe can perturb the very-near-field making the direct measurement approach less favorable than the indirect measurement approach.