Novel near-field microwave and millimeter wave differential probe using a dual-modulated single aperture

A novel differential probe design is introduced in this paper for near-field microwave and millimeter wave non-destructive testing (NDT) and imaging applications. In such applications, the variations in the distance between the probing antenna and the structure under inspection, i.e., standoff distance, can potentially mask the signal of interest, and hence, adversely impact the detection capability of the probe. Differential near-field probes and compensation methods were developed in the past to null out the standoff distance variation effect from the measured signal. The available methods, however, suffer from some limitations such as using two balanced apertures or offering limited range of compensation. While the differential probe proposed here exhibits an excellent immunity against standoff distance variation, it overcomes the limitations of the aforementioned methods. The proposedprobe is based on electronically modulating the aperture of a rectangular waveguide using PIN diode-loaded dipoles placed symmetrically in the aperture region. It will be shown that the adverse effect of standoff distance variation can be eliminated, or otherwise, significantly reduced by non-coherently subtracting the signals measured at two diferent aperture modulation states.